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https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2021.575530/full

MINI REVIEW article Front. Mater., 23 April 2021 Sec. Structural Materials Volume 8 - 2021 | https://doi.org/10.3389/fmats.2021.575530 This article is part of the Research Topic Latest Developments in the Field of Magnesium Alloys and their Applications View all 17 articles Novel Magnesium Based Materials: Are They Reliable Drone Construction Materials? A Mini Review \r\nDaniel Hoche,*Daniel Höche1,2*Wolfgang E. WeberWolfgang E. Weber3Eugen Gazenbiller,Eugen Gazenbiller1,2Sarkis Gavras,Sarkis Gavras2,4Norbert Hort,Norbert Hort2,4Hajo Dieringa,Hajo Dieringa2,5 1Institute of Surface Science, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany 2MagIC - Magnesium Innovation Centre, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany 3Chair of Structural Analysis, Faculty of Mechanical Engineering, HELMUT-SCHMIDT-University/University of the Federal Armed Forces, Hamburg, Germany 4Institute of Metallic Biomaterials, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany 5Institute of Materials and Process Design, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany Novel magnesium-based materials are ideal candidates for use in future aviation vehicles because they are extremely light and can therefore significantly increase the range of these vehicles. They show very good castability, are easy to machine and can be shaped into profiles or forgings to be used as components for next generation aerial vehicle construction. In the case of a large number of identical components, high-pressure die casting of magnesium alloys is clearly superior to high-pressure die casting of aluminum alloys. This is due to the lower solubility of iron in magnesium and thus tool/casting life is significantly longer. In addition, the die filling times for magnesium high-pressure die casting are approximately 30% shorter. This is due to the lower density: aluminum alloys are approximately 50% heavier than magnesium alloys, which is a significant disadvantage for aluminum alloys especially in the aerospace industry. There are cost-effective novel die casting alloys, besides AZ91 or AM50/60 such as DieMag633 or MRI230D, which show very good specific strength at room and elevated temperatures. In the case of magnesium-based wrought alloys, the choice is smaller, a typical representative of these materials is AZ31, but some new alloys based on Mg-Zn-Ca are currently being developed which show improved formability. However, magnesium alloys are susceptible to environmental influences, which can be eliminated by suitable coatings. Novel corrosion protection concepts for classical aerial vehicles currently under development might suitable but may need adaption to the construction constraints or to vehicle dependent exposure scenarios. Within this mini-review a paradigm change due to utilization of new magnesium materials as drone construction material is briefly introduced and future fields of applications within next-generation aerial vehicles, manned or unmanned, are discussed. Possible research topics will be addressed. Lightweight materials such as magnesium and its alloys are of great interest for the industrial sector. Potential applications can be found in the automobile industry and in civil engineering as structural components (Dieringa and Kainer, 2013), in batteries as anode material (Deng et al., 2018; Höche et al., 2018) and in medical engineering as biocompatible, resolvable implants (Kirkland, 2012; Luthringer et al., 2014). The aspects of applying magnesium based materials in vehicle concepts have been widely discussed (Dieringa et al., 2007; Dieringa and Kainer, 2009; Dieringa and Bohlen, 2016) with the result that functionalizations of magnesium alloys are feasible (Xianhua et al., 2016). Thus, it is reasonable to think about application of novel magnesium based materials to construct additional vehicle components e.g., for a quadcopter or other next generation aerial vehicles, if they meet technical and economic constraints. Typically, drones are classified according to weight or flight range (Weibel and Hansman, 2004; Arjomandi et al., 2006; Brooke-Holland, 2012; Dalamagkidis, 2015). Of these weight or flight range classifications, improvements to ultra-light-weight drones is an ultimate goal to achieve. Presently drone materials are very often composites as sandwich panels applying fiberglass, graphite fiber or aramid based systems as skin and reinforcement materials, and foams mostly based on polymers as panel core materials (Fahlstrom and Gleason, 2012). Such systems are already established within aviation sector and just need to be applied. For example, the City-Airbus, the Lilium-Jet (Lilium, 2018) or Volocopter aiming to conquer the air taxi sector construct their vehicles with CFRP concepts. Generally lightweight vehicle constructions require a high strength to weight ratio with the ability to adapt shapes onto aero- and flight dynamical requirements. In this context, morphing materials are of great interest and might bring constructional aspects to the limit (Sun et al., 2016; Goh et al., 2017). Despite this trend there is a lot of space to use new magnesium alloys, such as DieMag633, MRI230D (Gavras et al., 2019; Tu et al., 2019), Mg-Zn-Ca based (Pan et al., 2016; You et al., 2017; Tu et al., 2019), magnesium based foams (Kucharczyk et al., 2017) or high strength AM60 + 1AlN nanocomposites (Dieringa et al., 2017; Malaki et al., 2019). Particularly with regards to manned aerial vehicles (MAV’s) or even air-taxis which might become very expensive and not environmental benign if they are conventionally designed. The application of magnesium in recent aerial vehicles is a first step. For example, the commercially available DJI Inspire 2 unmanned aerial vehicle (UAV), has an AZ91 housing. The DJI Mavic Air applies brackets based on AZ91. The Phantom 4 Pro V2.0 quadcopter applies a titanium magnesium hybrid structure to achieve maximal rigidity of the airframe for excellent maneuverability1. Obviously, based on this existing experience, there is a lot of potential in conceptional design approaches using magnesium materials. Possibilities going beyond this need to be further explored. Design of unmanned aerial vehicles and manned aerial vehicles follow the well-established construction criteria of aircraft industry. However, this does not mean that there is no space for innovation and novel thinking. Considering the price developments (Q4/2020 at 2.55–2.70 $/kg Source: Platts Metals Week) but also enhanced manufacturing capabilities Mg based components might replace vehicle components and attachments (e.g., camera housings) but also on structural level. The structure of a typical UAV consists of basic subsystems. All those components typically do not mainly differ in their function from conventional aircraft concepts but might be adapted. The most important are listed: - The fuselage including stringers, the skin (coated), longerons (longitudinal structural members reinforcing the skin), and some bulkheads can be made from many lightweight engineering materials. This can be impregnated fabrics, standard AA2024 structures or many other composites (CFRP, GLARE, etc.). Components for the fuselage can be made of magnesium alloys. These are, for example, sheets for the outer skin made of AZ31 or aluminum-free ZE10. The latter shows excellent forming properties and a reduced texture. Frames and cross members can be made of AM60 by high-pressure die casting. Any further fairings (and filets) can still be made of Al alloys or respective foils. - The wings including the spars (mono and multi), ribs, stiffeners, and the skin in 2, 3, or 4 rotor systems (sheathed and unsheathed) are structural parts as well and critical loads are transferred to the fuselage by a topological designed beam or truss structure. Materials in use must be light but also stiff and limit vibrations. However, using current lightweight materials such as CFRP leads to structures being more flexible than desired. Thus, additional effort is necessary to limit the deformations—one example for that are truss braced wings (see e.g., Scott et al., 2016). Instead, extruded profiles of wrought magnesium alloys could be used here for the load-bearing structures; they show yield strengths of up to 300 MPa. Cast components for wing and spar construction made of AZ91 or AM60 could be used as ultralight structural components here as well. - The tails (horizontal and vertical) very often are made of carbon- or glass fiber materials applying resins, wax and hardeners with foam cores. - The rotor blades are mostly made of composite materials involving impregnated fabrics. Especially for UAV’s which are used for civil applications in urban areas, an outer ring housing the rotor blades is necessary in order to prevent people and animals from accidental collisions and to reduce noise. This ring might be made of magnesium or magnesium alloys because this material may better withstand collisions with both the built environment and moving objects such as other UAV’s. Additionally, the damping properties of such Mg based systems are excellent. - The engine/powertrain group consisting of for example pylons, inlet, supports, etc. does not follow the classical turbine design with Ti- or Ni based alloys. For UAV’s like quadcopters electrical engines depending on battery power are required, where it should be mentioned that battery mass is a problem. Casings or heads of piston engines are often made of cast aluminum but might be replaced by Mg based materials. Ti alloys or steels are other options if the situation (temperature, etc.) requires countermeasures. Magnesium alloys for higher temperatures are available, as well, although they are often more costly (AE-series, DieMag-series, AJ-series) (Gavras et al., 2019). - The landing gear (and if required, its bay door) can also be part of the structure especially for MAV’s. It has to take up high loads and can already be made of carbon composites or simply AISI 300M steels. - It should be noted that the dies for applying the forming processes mentioned above may also be made from ultra-high performance concrete (UHPC) (see e.g., Kleiner et al., 2008), which may lead to economic gains. This holds especially for low and medium contoured part shapes. For producing parts with small radii, the UHPC dies should be additionally confined or reinforced (see also Kleiner et al., 2008). Material selection especially for aerospace vehicles has been introduced by Arnold et al. (2012) based on Ashby (2010). The structure and its materials have to ensure the functionality of the drone. They have to keep the aerodynamics of the UAV and carry the occurring loads at any time. Thus, structural components are constructed following the lightweight design rules (e.g., high Youngs modulus/density ratio E/ρ, etc.; Arnold et al., 2012) as shown in Figure 1, whereas safe design in c) has special emphasis for manned vehicles. Here the most common material is aluminum, followed by composite materials, such as glass/epoxy and carbon fiber. Surprisingly, magnesium based materials such as magnesium based metal matrix nanocomposites (MMNC’s) are yet not in use despite their very good potential. Some of these Mg-MMNC’s show excellent properties caused by only small additions of ceramic nanoparticles. The great advantage of these materials is that the nanoparticles not only increase mechanical strength (by grain refinement or Orowan strengthening), but also increase ductility (Dieringa, 2011; Dieringa et al., 2017). Figure 1. Materials selection following Ashby for aerospace applications (based on Granta-design data). (A) Stiffness limited, (B) strength limited, and (C) safe design as proposed by standard structural design concepts, whereas the bottom-right diagram (D) shows the product of previous mechanical parameters vs. the product of non-physical properties like the material price, the carbon dioxide footprint and the reciprocal of recycling fraction. The structural members are designed to carry the flight loads or to handle stress without failure. In designing the structure, the entirety of the wing and fuselage must be considered in relation to the physical characteristics of the material of which it is made. Every part of the structure is engineered to carry the load which is applied on it. The structural designer has to determine flight loads (including bird strikes) and other loads, calculate stresses, and design the structural elements such as to allow the UAV components to perform their aerodynamic functions and durability requirements efficiently. This goal will be considered simultaneously with the objective of the lowest structural weight. The entire airframe and its components are joined by rivets, bolts, screws, and other fasteners. Welding, adhesives, and special bonding techniques are also employed. In order to enhance the delamination toughness of e.g., CFRP laminates, techniques such as z-pinning are investigated (cf. Mouritz, 2007). Additionally, composite materials with enhanced delamination toughness such as GLARE are of scientific interest (see Rittmeier et al., 2018). The most common design of UAV structure is semi- monocoque (single shell) which implies that the respective skin is stressed and thus needs to be reinforced. Structural parts of UAV’s are subjected to: (1) tension, (2) compression, (3) torsion, (4) shear, and (5) bending. Typically, a single member of the structure is often subjected to a combination of the resulting stresses. It is worth mentioning that the UAV is subjected to time-harmonic loads (i.e., due to vibrations induced by the engine) and impact loads (e.g., during the landing phase, due to bird strikes or impacts such as the Hudson River landing in 2009). Additionally, fluttering as well as limit-cycle oscillations due to wind loads is a major issue. Hence, the design principles known from e.g., aeronautical applications or wind power plants are a good starting point when designing UAV’s. A major issue in designing lightweight-structures from composite materials is that these structures tend to delaminate. Although there are substantial contributions to prevent structural elements from delamination, this problem is not yet solved. Promising approaches to increase delamination toughness can be found in the literature (e.g., Mouritz, 2007; Rittmeier et al., 2018). However, by suggesting magnesium based construction materials for e.g., quadcopter drones, the authors believe that the potential problem of delamination can be circumvented. This advantage holds for the drone’s skin and bulkhead, which are also subjected to lateral loads. Structural members such as frames and longerons are mainly loaded longitudinally and consequently can be treated as rods in most cases (cf., Rothert and Gensichen, 1987). Using lightweight configurations made of composite materials mentioned above has another effect: the respective structures are getting more flexible. Thus, scientific effort is necessary in order to reduce the resulting deformations. For example, this led to the development of the so-called truss braced wing (see Scott et al., 2016). Again, the authors of the current contribution believe that the rigidity of structural members made of magnesium or magnesium alloys is sufficient to ensure moderate deformations and might improve acoustic emission profiles. Concerns about corrosion issues are not appropriate due to the existence of novel corrosion protection concepts (Lamaka et al., 2016; Dieringa et al., 2018) and the expected uncritical corrosive in-service exposure conditions. Besides these mechanical and design aspects, the ecological footprint (Ehrenberger, 2020) and the social acceptance of the materials used in drone design is also of interest. Many contributions show the positive effect on fuel/energy consumption when using CFRP for MAV’s (see Timmis et al., 2015). The authors expect that similar conclusions hold for UAV’s and other lightweight materials such as magnesium or magnesium alloys. However, recycling of lightweight materials such as CFRP is challenging (Dong et al., 2018). In comparison, recycling of magnesium and magnesium alloys is quite well-understood. Additionally, magnesium and magnesium alloys can be separated easily from the other materials which is not the case for carbon in CFRP. Thus, from an ecological but also from an economical point of view the use of magnesium and magnesium alloys seems to be promising since the price and CO2 fingerprint are on the same level as aluminum. In a life cycle assessment study (Ehrenberger et al., 2013; Ehrenberger, 2020), it was shown that magnesium, which is primarily produced by electrolysis, has a better carbon footprint than aluminum from the production stage. Over the service life of a component, this advantage increases continuously due to its lower weight. In a comparison of aircraft components made of magnesium or aluminum, the slightly higher greenhouse gas emissions caused by the production of magnesium are compensated for after only a few flights by the fuel savings during flight. The utilization of Mg based materials generally launches a fire risk discussion. A dedicated study on flammability of the material within aircraft fire tests (Marker, 2013) indicates that for example the behavior of the Mg alloy E21 does not significantly differ to aluminum alloys. Nowadays the SAE standard AS8049D paves the way toward utilization of novel Mg based materials (Czerwinski, 2014) at commercial cabin interiors. The use of magnesium in the aircraft industry has been an important topic since the beginning of aircraft construction. Even before the Second World War, a large number of magnesium parts were used in aircraft construction, including propellers and sheet metal for the outer skin (Reed, 1925; Beck, 1939; Hallion, 2017). During the Second World War, however, a great amount of magnesium was used to build aircrafts in Germany. Both the Messerschmidt Bf 110 and the Junkers Ju 88 used both die-cast and forged magnesium parts (small bell cranks, engine cowl flaps, large dive brake forgings) (DOW, 1941). The first all-magnesium aircraft was described in 1954 (American Aviation, 1955). The F-80C was characterized by a simpler construction of the wings, because considerably fewer components were needed, the number was reduced from 1,644 to only 508, which corresponds to a decrease of 69%. The lighter construction led to an increase of the maximum speed by 5 mph. After the war, the United States built the Convair B-36 bomber, an aircraft that was made of 10% magnesium (Jenkins, 2001) large parts of the outer skin and the control parts were made of magnesium alloys. In 1950 the Sikorsky S-56 Helicopter from Westland Aircraft Ltd., applied 115 kg magnesium. Furthermore, in the Sovjet Union in 1963 the TU-134 from Tupolev already had 1,325 parts made of Mg (Ostrowsky and Henn, 2007). The operating flight loads limits on a UAV are usually presented in the form of a V-n diagram (airspeed and load variation diagram). Structural designers selecting Mg based materials will construct this diagram with the cooperation of the flight dynamics group. The diagram will determine the failure areas, and area of structural damage/failure. The UAV should not be flown out of the flight envelope, since it is not safe for the structures. The UAV structural design is out of scope of this mini-review. Detailed information can be found elsewhere (Megson, 2016). Figure 2 shows the principles and an idea to design a hypothetical quadcopter mainly based on Mg-materials. The different requirements for the respective construction parts can be fulfilled by the materials in the boxes. They are: Figure 2. Possible Mg-based alloys/hybrid materials for application in aerial vehicle construction like a quadcopter. Exemplified sources for further information about the materials are a: Naghdi et al. (2016), b: Dieringa et al. (2004), c: Gavras et al. (2019), d: Dieringa et al. (2017). • Rigid—such as MgZnCa wrought alloys with 306 MPa YS and 11% elongation (You et al., 2017). • Vibration limiting—AM60 based nano composites with high ductility of 15.4% at RT (Dieringa et al., 2017). • Creep resistant—such as novel DieMag alloys without rare-earth elements showing better creep resistance compared to commercial alloys and better high temperature yield strength than even aluminum alloy A380 (Gavras et al., 2019). • High strength and thermally resistant—such as carbon fiber reinforced Mg MMCs up to 300°C (Dieringa et al., 2004). Considering production and design aspects, it becomes obvious that besides efficient manufacturing- and surface technologies (computational based), topological design but also efficient joining technologies for Mg based parts are key enablers for this technology. It will require a concerted research effort and might also be a future market for additive manufactured parts made out of Magnesium. Magnesium based materials can become an essential part of design concepts of UAV’s, especially if flight maneuvers require high rigidity and noise pollution become critical decision criteria. Also crash-worthiness is an aspect where magnesium has benefits. The authors are convinced that the economic advantages of using magnesium based materials in drone construction may be further increased if promising techniques such as e.g., performing sheet metal hydroforming by means of UHPC dies are applied. As compared to CFRP and related composite materials, magnesium based materials can be easier separated and recycled once the UAV’s reached the end of its service-life. With respect to the title of this contribution and based on the precedent sections the authors thus are convinced that: Magnesium is—A drone construction material! 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Eng. 45, 2269–2274. doi: 10.1016/s1875-5372(17)30015-2 CrossRef Full Text | Google Scholar You, S., Huang, Y., Kainer, K. U., and Hort, N. (2017). Recent research and developments on wrought magnesium alloys. J. Magnes. Alloys 5, 239–253. doi: 10.1016/j.jma.2017.09.001 CrossRef Full Text | Google Scholar Keywords: ultra-lightweight construction, hybrid design, magnesium alloy, aerial vehicle, urbane mobility Citation: Höche D, Weber WE, Gazenbiller E, Gavras S, Hort N and Dieringa H (2021) Novel Magnesium Based Materials: Are They Reliable Drone Construction Materials? A Mini Review. Front. Mater. 8:575530. doi: 10.3389/fmats.2021.575530 Received: 23 June 2020; Accepted: 30 March 2021;Published: 23 April 2021. Edited by: Reviewed by: Copyright © 2021 Höche, Weber, Gazenbiller, Gavras, Hort and Dieringa. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. *Correspondence: Daniel Höche, [email protected] Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

https://www2.deloitte.com/us/en/insights/industry/aerospace-defense/aerospace-and-defense-industry-outlook.html

2024 aerospace and defense industry outlook Aerospace and defense companies should embrace new technologies and innovation to help navigate upcoming challenges and capitalize on growth opportunities. Article • 19-min read • 28 November 2023 • Deloitte Research Center for Energy & Industrials Lindsey Berckman Lindsey Berckman United States Kate Hardin Kate Hardin United States Matt Sloane Matt Sloane United States Tarun Dronamraju Tarun Dronamraju India In 2023, the aerospace and defense (A&D) industry witnessed a revival in product demand. In the aerospace sector, domestic commercial aviation revenue passenger kilometers surpassed prepandemic levels in most countries.1 This surge in air travel led to an increased demand for new aircraft and aftermarket products and services. In the US defense sector, new geopolitical challenges, along with the prioritization of modernizing the military, drove robust demand in 2023, particularly for weapons and next-generation capabilities.2 The demand for A&D products and services is expected to continue into 2024. On the commercial side, travel is likely to continue its upward trajectory. In the defense segment, demand for products is expected to continue to increase as geopolitical instability grows. Furthermore, companies in emerging markets, such as advanced air mobility, are expected to advance testing and certification as they prepare for commercialization. 2024 energy, resources, & industrials outlooks Read more from the Deloitte Center for Energy & Industrials’ 2024 outlook collection Take me to 2024 energy, resources, & industrials outlooks. While these trends are likely to drive both domestic and international spending, the increased demand may cause A&D companies to deal with new challenges as they grapple with ongoing ones such as supply chain issues, longer lead times, and a talent shortage. To address these challenges, A&D companies may further embrace digitalization and adopt emerging, advanced technologies and, thus, could achieve profitability by both addressing their cost challenges and initiating the development of novel revenue streams. Such technologies may be foundational for A&D companies in creating a more resilient supply chain, mitigating logistical issues, attracting new talent, and rapidly creating new products. As A&D companies prepare for the year ahead, there are some key trends they may consider focusing on to take on the challenges and capitalize on emerging opportunities, with digitalization being the unifying theme across the trends: Talent: A&D companies are addressing workforce challenges amid rising demand and changing workforce expectations Supply chain: Global supply chain complexity is driving a multitiered solution Digital transformation: The industry can unlock growth and efficiency through digital technologies New product introduction: Evolving customer preferences and growing emphasis on sustainability are fueling product innovations Defense and commercial spending: Industry spending is helping drive growth and innovation Table of contents Talent Supply chain Digital transformation Product introductions Spending The future 1. Talent: A&D companies are addressing workforce challenges amid rising demand and changing workforce expectations The A&D industry is facing a new talent landscape that is mainly driven by increasing salary levels, increased employee mobility, a reevaluation of employee relationships with the workplace, and an intensely competitive job market. Evolving employee expectations, which have only amplified since the pandemic, have often made it challenging for A&D companies to attract, retain, and develop a skilled workforce. These workforce issues could prove to be a significant pain point for A&D companies as they begin to scale production operations to meet growing demand across the industry. Attracting talent The US A&D industry has surpassed prepandemic levels in terms of employment. In 2022, US-based A&D companies increased their workforce by 101,700 to 2.2 million, marking a 4.87% increase from 2021 and surpassing prepandemic 2019 levels of 2.18 million.3 Despite the rise in workforce numbers, A&D companies are likely to face talent shortages. These shortages may hamper production demands resulting from domestic travel increases and nations increasing their armament stockpiles. Currently, the industry is struggling to attract talent, ranging from technicians and manufacturers to engineers. One leading Asia-based engineering company noted that the shortage of technicians may lead to increased maintenance costs and impact fleet expansion forecasts.4 Furthermore, in the National Association of Manufacturers’ (NAM’s) 2023 third quarter manufacturers’ outlook survey, about three out of four surveyed US manufacturers stated that attracting and retaining a quality workforce is their primary business challenge.5 The manufacturing industry, including A&D manufacturing, is increasingly competing with other industries for skilled talent.6 A&D companies recognize this, as top talent with digital skills can be drawn to digital jobs within a multitude of areas, including automotive (for example, the battery electric vehicle and autonomous vehicle segments) and life sciences (for example, medical technology, and biotechnology). A&D companies may need to expand their strategies beyond compensation to accommodate the changing needs of the workforce. The US A&D industry has an annual average salary of US$108,900 which is approximately 55% above the national average.7 That said, many Gen Z employees are keen to have careers with broad impact and a purpose.8 These companies may attract talent with mission-focused marketing campaigns and undertake more strategic recruitment to ensure that the values of the employees and the company align. Retaining talent Despite a significant number of new hires, job openings in the manufacturing industry were still 604,000 in August 2023.9 Additionally, quits accounted for about 68% of total separations. To compete in a tightening labor market, A&D companies will likely address both talent attraction and, potentially more significantly, talent retention issues. The A&D industry has high requirements and expectations for its employees, including specific educational criteria and security clearances. This could necessitate a focus on retaining talent, especially with the increasing number of retirees. Historically, these companies have succeeded with a workforce that possesses deep experience and a strong connection to the company culture. Therefore, given the competitive state of the job market, it is important for the industry to retain highly qualified workers. A&D companies are using compensation and flexible work arrangements to enhance retention and turn the tide of employee attrition. This approach has been adopted by the manufacturing segment of the A&D industry where, according to Deloitte’s 2024 manufacturing industry outlook, there was a rise of 4% in the average hourly earnings of employees between the first quarter of fiscal year 2022 and the first quarter of fiscal year 2023. Additionally, the manufacturing industry has undergone a significant shift toward remote work over the last two years, during which there has been a 19% reduction in the average number of quits.10 The retention levels of A&D companies could plateau at a certain point if they continue to only offer flexibility benefits. According to the NAM’s 2023 second quarter manufacturers’ outlook survey, inflexible production, shift needs, and the creation of differences between workforce populations are some major challenges manufacturers face when offering such flexibility to workers.11 The industry will likely explore new avenues to both attract and retain talent. Pursuing and implementing emerging technology can help not only address profit margins, but also invigorate a workforce. Regardless of an employee’s job description, A&D companies may begin to consider initiating new programs that allow employees to engage with advanced technologies, which may include tech-based rotational programs or designated time to pursue passion projects within the company. Developing talent With an increasing number of retirements and an aging workforce (about 26% of the A&D workforce is over the age of 55),12 the industry should train younger professionals for long-term sustainable operations and security. A&D companies can upskill their workforce by developing more learning avenues in alignment with the technological evolution within the industry. These avenues may include: Working closely with colleges to create novel training and workforce development programs; Expanding internships, apprenticeships, and other career development initiatives; and Furthering cooperative research agreements with tier one research universities. Employees in the A&D sector, across all career levels, should be educated on the importance of cybersecurity and should possess a clear understanding of their specific responsibilities in upholding a secure environment.13 And in line with that, A&D companies’ workforce development strategies will not only attract and retain talent but can also help ensure a secure environment for them. Pursuing and implementing emerging technology can help not only address profit margins, but also invigorate a workforce. Regardless of an employee’s job description, A&D companies may begin to consider initiating new programs that allow employees to engage with advanced technologies. 2. Supply chain: Global supply chain complexity is driving a multitiered solution A&D companies should expect to see continued fragility of and disruption in the global supply chain, which may lead to production delays, delivery delays, and increased pricing for both raw materials and components. From raw material and tier three suppliers through to original equipment manufacturers and maintenance, repair, and overhaul providers, the issues across the supply chain include shortages of skilled labor, labor attrition, unavailability of materials and parts, and measurable inflation. Since the pandemic, these factors have been causing an increase in material lead times and supplier decommits, resulting in increased volatility in operating, program, and financial performance.14 The manufacturing industry has seen only a slight recovery in lead times―the delivery times for production materials reached 87 days in August 2023 from its peak of 100 days in July 2022; however, the average lead time has yet to reach the prepandemic level.15 Despite this improvement in 2023, the continuing shortage of raw materials, semiconductors, microelectronics, and other key components or parts (e.g., engine casting or forgings) will likely remain a key issue for A&D companies in 2024 as production demand increases across the board. Raw material sourcing, especially for “critical” minerals used to support defense, poses a unique challenge for A&D supply chains. Current production and reserves of such minerals is minimal in the United States. For instance, the United States was 100% reliant on net imports of gallium from countries such as China, the United Kingdom, Germany, and others.16 At present, China leads in the export of a majority of the 13 major critical minerals,17 with a market share of 50% to 70% for each, while the United States leads in only one.18 Reliance on these minerals may be a complicating factor for A&D companies, as replacing them is most likely unrealistic in the short term.19 As geopolitical tensions often lead to increasing trade barriers, affecting both raw materials and advanced technologies, A&D companies are likely to see increased restrictions on the availability of key imports as well as their ability to export sensitive items. A&D companies reliant on the most advanced material technologies, such as space technology, electronics, and semiconductors, are likely to monitor geopolitical events closely in the coming year and may consider maintaining strategic reserves of these critical minerals, such as gallium and germanium.20 The challenge of limited or sole sourcing for parts continues within the industry. The A&D industry has been slow in developing alternate sources of supply, as meeting industry standards and requirements through the qualification and certification of production lines is an arduous process. As such, companies often must rely on current production for the supply of both new and aftermarket parts. These companies will likely bolster their strategic inventory to build additional stock for critical items and maintain a flexible supply chain as they continue to develop new production options. In the United States, production options include cross-border manufacturing and the continued exploration of friendshoring. The US government is continuing to work with international partners to develop its defense production capabilities. Additionally, A&D companies may further invest in novel manufacturing technologies to create an eventual alternate source of supply and avoid sole-source manufacturing, long lead times, and obsolescence issues. Finally, it can be expected that A&D companies might continue turning to supply chain digitalization and automation options in 2024. In a recent Deloitte supply chain study, 78% of survey respondents agreed that digital solutions would enhance visibility and transparency throughout the supply network.21 A&D companies may develop digital twins of the supply chain to obtain a complete view of the procurement, production, and delivery processes. The A&D supply chain is a complex, globalized ecosystem of customers and original equipment manufacturers; multiple tiers of suppliers; and maintenance, repair, and overhaul providers. This complexity makes implementing diversification and transparency across the value chain extremely difficult, but imperative. By maintaining strategic raw material reserves, committing to bulk buying of long lead time items, exploring alternate sources of supply, and digitizing operations, A&D companies may position themselves well to handle any continued fragility across the entire supply chain. In a recent Deloitte supply chain study, 78% of survey respondents agreed that digital solutions would enhance visibility and transparency throughout the supply network. A&D companies may develop digital twins of the supply chain to obtain a complete view of the procurement, production, and delivery processes. 3. Digital transformation: A&D companies can unlock growth and efficiency by embracing digital technologies A&D companies are continuing to advance in their digital transformation journey and can accelerate it further in 2024. The frontrunners in the industry are leading the charge on digital technologies such as model-based enterprise and digital twins. Digitalization can enhance product development, improve operational efficiencies, and help capitalize on growth opportunities. However, before adopting advanced technologies, A&D manufacturers might consider modernizing their processes, technologies, and tools. This may allow them to increase throughput with existing infrastructure and to effectively manage demand fluctuations and costs, despite persistent labor and supply chain challenges. Digital transformation of the production process A&D companies operate in a highly complex environment that is defined by regulatory requirements and specific customer, contract, and product specifications. Modernizing and integrating processes and enabling technologies are some significant steps for the A&D industry to improve production throughput and cost efficiency. Embracing digital transformation can reshape A&D production processes at all stages, significantly reducing industrialization cycle times, improving efficiencies, increasing production yield, and elevating quality standards. At the design stage, model-based systems engineering22 approaches can allow for the virtual design, analysis, and verification and validation of systems, potentially shortening the time to qualify and certify a new manufacturing line. Similarly, employing Industrial Internet of Things (IIoT) technologies and integrating both operational technology (OT)23 and information technology (IT)24 systems can help generate better insights in the manufacturing environment. These types of end-to-end visibility solutions can help improve production yield and elevate product quality. The implications of digital transformations and emerging digital technologies, such as digital twins, do not end with producing a final product. A&D companies can implement digital twin technology to track parts throughout their life span to improve maintenance protocols. It can also open opportunities for aftermarket service provider readiness. By leveraging the vast amounts of data produced by A&D systems, aftermarket companies may implement AI techniques to predict how and when an aircraft or system will need to be serviced. This data, including information on the parts and labor required, could reduce repair times, improve turnaround times, and create competitive advantage. The pace of digital transformation among A&D companies varies and is often dependent on a company’s unique requirements, priorities, and available resources. Nonetheless, digitalization is expected to be imperative in the coming year as A&D companies continue to compete for business. Digital enhancement The demand for sustainability, product innovations, and wartime defense is expected to result in rapid technological evolution. A&D companies are exploring and applying digital technologies, specifically artificial intelligence and generative AI (a subset of AI), in an array of scenarios. AI can streamline operations, enhance productivity, enable real-time data synchronization, and simplify customization processes. AI solutions have benefited the A&D industry for various applications, from improving cockpit avionics technology for surveillance and decision-making to optimizing maintenance and defect monitoring for manufacturers. Generative AI, through generative design, can be critical in delivering more opportunities for creating energy-efficient designs and low-carbon products. Beyond product development, generative AI–enabled virtual field assistants can support engineers by improving their problem-solving capabilities and productivity.25 Such a use case of AI technology may have further workforce implications, as worker productivity enhancements may prove to be one approach for A&D companies to ease the effects of a talent shortage. As A&D companies continue to enhance their digital capabilities, they will likely move to promote trust in their technologies, manage risks associated with data security, and monitor potential federal or state regulatory activity. To promote trust in their digital technologies, such as AI, A&D companies will likely strive to ensure that they are using the proper data and possess a complete understanding of the implemented algorithms.26 To protect sensitive data against growing cyber risks, A&D companies will likely make substantial investments in robust cybersecurity risk management measures and strengthened digital infrastructure systems. Finally, to ensure lawful practices, A&D companies may more closely monitor the increasing regulatory focus on the responsible use of these digital technologies, including national security implications. A&D companies should continue exploring the major challenges impacting digital transformation and move from viewing it as optional to treating it as a requirement for achieving a competitive advantage and long-term success. Generative AI, through generative design, can be critical in delivering more opportunities for creating energy-efficient designs and low-carbon products. Beyond product development, generative AI–enabled virtual field assistants can support engineers by improving their problem-solving capabilities and productivity. 4. New product introductions: Evolving customer preferences and growing emphasis on sustainability are fueling product innovations Developing a new product in the A&D industry can, by itself, be a difficult task. And when you add in the complex operating environment, various regulatory requirements, and extensive testing demands associated with product certification, the task may seem more daunting. Despite these hurdles, evolving consumer demands for enhanced technology, greater sustainability, reduced emissions, higher performance systems, and lower costs are driving the A&D industry toward new product innovations and introductions in 2024. In the coming year, A&D companies are likely to continue developing environmentally friendly propulsion alternatives to reduce emissions and prepare for any future emissions regulations. Concurrently, commercial and government entities will likely continue to push for development in supersonic and hypersonic flight, respectively. While these are major areas for product innovation, research and development is likely to continue in an array of areas in the A&D industry, such as materials development and advanced manufacturing incorporation. Sustainability and emissions reductions With the goal of achieving net-zero CO2 emissions by 2050 (since the United States is a signatory of the Paris Agreement),27 the aerospace industry is actively developing innovative designs and sustainable propulsion technologies. A&D companies are setting targets to reduce greenhouse gas emissions, water waste, and energy use as they progress toward meeting their interim 2030 sustainability targets. A&D companies are even more likely to continue their push for sustainability and reduced emissions as they acknowledge the broader implications of California’s Climate Accountability Package and await the release of the final version of the US Securities and Exchange Commission’s climate disclosure rule. The demand for reduced and zero-emission aviation is likely to drive the growth momentum for electric, hybrid, hydrogen, and even solar-powered propulsion technologies in the coming year. For instance, a prominent US defense contractor has strategically invested in an electric propulsion solutions provider to drive the development of electric aviation solutions.28 The A&D industry should anticipate further opportunities to apply these technologies in the defense sector, as well as in maritime and aviation applications. Beyond traditional aircraft, the advanced air mobility (AAM) industry is trending upward. The AAM industry is expected to advance design, testing, and certification in 2024 while preparing for commercialization in 2025. The industry has witnessed substantial orders from airlines for operating AAM aircraft in 2024 and beyond.29 In the short term, AAM manufacturers are focused on establishing robust manufacturing capabilities, transitioning from the proof-of-concept phase to scaled production. One leader in the AAM industry has already revealed their plans to construct manufacturing plants for large-scale aircraft production.30 Currently, the AAM industry seems to be considering electric propulsion for five- to six-seater aircraft. Moving forward, as the A&D industry moves to develop sustainable aircraft with 20 to 100 seats, companies may begin to experiment with and further develop hybrid electric or hydrogen fuel systems. Regardless of the application, the development of these propulsion technologies takes time and can demand significant changes in aircraft design. The A&D industry will likely continue to see sustainable aviation fuel as an interim option to bridge the gap between the propulsion systems of the past and the future. The International Air Transport Association (IATA) anticipates that sustainable aviation fuel could contribute 62% of the carbon mitigation required to achieve the 2050 emissions reduction goals.31 Finally, as A&D companies seek an edge in sustainability, both aircraft and engines are expected to undergo significant redesigns. These companies are actively engaged in producing lightweight aircraft components using advanced materials. This can help not only reduce fuel consumption but also bolster the overall structural strength of an aircraft. In commercial aviation, engine manufacturers are dedicated to developing engines that incorporate cutting-edge technologies. Such integrations are aimed at enhancing fuel efficiency, reducing noise emissions, and minimizing environmental impact. On the defense front, the US Air Force is making strides in developing new generation fighter aircraft as part of the Next Generation Air Dominance program. This program is poised to leverage adaptive engine technology, which has been under development as part of the Next Generation Adaptive Propulsion (NGAP) program. Under the Air Force’s proposed fiscal year 2024 budget, the NGAP is fully funded with US$595 million.32 Supersonic and hypersonic technologies NASA’s X-59 QueSST program aims to demonstrate quiet supersonic travel by reducing sonic boom intensity.33 Despite technological evolution, significant hurdles remain. Manufacturers claim that it is possible to develop sustainable and economically viable supersonic aircraft, but it is still at a nascent stage of product development. That said, a leading supersonic aircraft maker is securing investments and a collaboration with one of the major airlines in the United States, thereby increasing optimism in the overall promise of such a technology.34 Beyond supersonic aircraft, there is a growing demand for both offensive and defensive hypersonic technology in 2024. In fiscal year 2024, the US Department of Defense (DOD) and NASA have allocated budgets for developing and testing hypersonic capabilities.35 With the influx of funding from the DOD in the hypersonic space, A&D companies have been and will likely continue to work toward operationalizing hypersonic weapons, including glide vehicles and cruise missiles. The A&D industry invests substantially in R&D to create new products, enhance existing ones, and drive technological progress. Each of these programs will likely rely on speed to market with a qualified and certified system. To truly reap the benefits of the most cutting-edge technologies, A&D companies are expected to prioritize the digital transformation of their entire supply chain and production processes. The digital thread—digital tools and representations that connect engineering, supply chain, manufacturing, and aftermarket activities and services to enable a model-based enterprise—will likely play a pivotal role in new product introductions throughout the industry. The A&D industry invests substantially in R&D to create new products, enhance existing ones, and drive technological progress. Each of these programs will likely rely on speed to market with a qualified and certified system. … The digital thread … will likely play a pivotal role in new product introduction throughout the industry. 5. Defense and commercial spending: Industry spending is helping drive growth and innovation The A&D industry should expect an increase in spending, from both the defense and commercial segments, in the year ahead. Defense spending will likely increase to meet the demand generated by escalating geopolitical tensions and the necessity for future capabilities. Commercial spending will likely stem from the increased demand for passenger flights and advancements in emerging markets. Defense spending The outlook for the global defense sector remains robust, with defense expenditures surpassing US$2.24 trillion in 2022 (most recent data at the time of publication).36 Geopolitical events across the world have driven, and are likely to continue to drive, an increase in defense funding. The US DOD requested a US$842 billion budget for fiscal year 2024, representing a 3.2% increase compared to fiscal year 2023 at the enacted base level. This budget will prioritize fostering innovation and countering potential adversaries from other countries.37 In 2024, the rise in military spending can drive innovation and digital infrastructure by implementing commercial technologies in defense applications and maximizing the use of private capital for emerging defense technologies and manufacturing. The US DOD has requested approximately US$1.8 billion for AI applications (a 63.6% increase in comparison with FY23), US$9.3 billion for advanced technologies, and US$0.69 billion for the experimentation and evaluation of advanced technologies for joint warfare in fiscal year 2024.38 The lion’s share of funding on technology is expected to focus on enhancing capabilities and improving readiness. To do this, the United States is concentrating its investments on procuring next-generation air, ground, and naval vehicles and modernizing existing ones to help meet defense requirements.39 Additionally, the US DOD is committed to investing in defense supply chain resiliency to help ensure the stability of the industrial base and meet future demand. Additionally, they are embracing and emphasizing AI as a technology that can support the United States in maintaining its strategic advantage in future conflicts.40 The DOD is also investing in several technology initiatives such as microelectronics, advanced propulsion, quantum technologies, and advanced computing.41 In response to global advancements, the US DOD is funding development from an array of entities. The DoD has established various mechanisms to engage the private sector (particularly small businesses and startups) and academia, promote innovation, and accelerate the integration of emerging technologies. These include, but are not limited to, the Defense Innovation Unit (DIU), Accelerating the Procurement and Fielding of Innovative Technologies (APFIT) program, National Security Innovation Network (NSIN), and AFWERX.42 US spending in the space sector has increased to keep pace with the growing demand for space capabilities. According to the Space Foundation, total government spending on space programs increased by 8% in 2022, with the United States holding a majority of the total spending share at approximately 60%.43 The United States Space Force (USSF) has requested funding of US$30.1 billion for fiscal year 2024,44 a 15% increase from fiscal year 2023 at the enacted level. The USSF is focused on building resilient, ready, combat-credible space forces, with an emphasis on cybersecurity, by using modern talent management processes. For this purpose, the USSF has allocated about 60% of the total budget for research, development, testing, and evaluation (RDT&E).45 Furthermore, NASA has requested a budget of US$27.1 billion for fiscal year 2024, a 7% increase from fiscal year 2023, with a focus on continued research on the International Space Station (ISS) and space exploration to create scientific and economic opportunities.46 Despite the rise in defense spending, growing inflation rates could be causing concerns among defense authorities. In real terms, the growth in defense spending is not substantial—the defense budget has increased year-over-year by 3.18% which is significantly lower than the current inflation rate of 6%.47 Also, the US defense spending as a percentage of GDP has decreased over the years and currently stands at 3.1%. This percentage is expected to decrease further to 2.8% by 2033.48 Such a situation could lead to future challenges for the US DOD in executing planned activities, as inflation may limit flexibility in launching new missions or require fund reallocation in order to prioritize essential activities. Nevertheless, in 2024, A&D companies can still capitalize on the DOD’s interest in R&D of cutting-edge defense equipment and advanced technologies. Commercial spending The international and domestic revenue passenger kilometers are expected to recover to prepandemic levels in 2024. This could result in increased demand for new aircraft orders. Furthermore, continued investments from emerging markets like AAM and space are expected to drive increased commercial spending. This spending may become a strategic capability that can aid the sustainable growth of A&D companies and shape new business opportunities. Commercial spending is likely to be focused on digitalization, new product development, and more broadly on future-facing subsectors such as AAM and space. The AAM sector is poised for continued spending in 2024 to establish full-scale manufacturing and operational capabilities. AAM aircraft manufacturers are working to achieve type certification and are anticipated to enter into service and commercialization phases in 2025. Substantial investments are likely required to create facilities capable of accommodating future aircraft demand. To support operations, the AAM industry is expected to invest in developing new vertiports, modernizing underutilized airports, establishing maintenance, repair, and overhaul facilities, expanding charging and battery infrastructure, building pilot training facilities, and fostering public awareness and acceptance. AAM manufacturers face an interesting road ahead as they continue to scale for commercialization while carefully managing spending. The commercial space industry witnessed a shift in funding dynamics in 2023. While government investments contributed to the space industry’s growth, albeit at a slower pace than in 2022, commercial investment experienced a notable decline in 2023.49 In 2024, it is anticipated that there will be an increase in acquisition and consolidation within the industry. Commercial investors are likely to exercise more caution as the revenue projections of previous Special Purpose Acquisition Companies (SPACs) for space companies are not being met.50 As a result, although the space industry is expected to grow, the investments and growth observed in recent years are not likely to return. Growth is expected to align with more conservative trends in space activities. The future: Technology and innovation will likely drive growth in the A&D industry In 2024, the A&D industry may be shaped by various factors, from geopolitical challenges to innovation and market demands, to technological leaps. As the industry navigates this uncertain landscape, it may need to prioritize adaptability and agility and commit to embracing technological advancements. The A&D industry is often characterized by long product development cycles and complex supply chains, which can demand constant innovation, technology integration, and continuous improvement. A&D companies that can leverage these areas may thrive in an ever-evolving global landscape. The speed-to-market factor remains a significant challenge for the development of new products. Quick prototyping, certification, and commercialization can be crucial. Therefore, A&D companies should consider adopting digital technologies, such as the digital thread, to help accelerate product development by connecting value chain phases and yielding results in less time. Finally, increased demand from both the defense and commercial markets could lead to stronger revenue streams for A&D companies in 2024. To capitalize on the potential growth opportunities in the industry, A&D companies may consider positioning themselves strategically in the following areas: Customer demands: A&D companies should remain agile and innovative in a growing number of areas to meet evolving customer needs while upholding their standard of delivering high-quality products and services. Technological advancements: The top A&D companies will likely strive to stay at the forefront of technological advancements in an effort to maintain a competitive edge, enhance business and operational processes, and attract top talent. Cyber resiliency: As the A&D industry becomes more digitally interconnected, cyberattack risk grows. Thus, establishing a robust security culture and investing in cyber-resiliency measures may prove to be vital in ensuring a secure digital future.

https://www.techbriefs.com/component/content/article/33914-lightweighting-in-aerospace-component-and-system-design

Lightweighting in Aerospace Component and System Design Lightweighting design is an extensively explored and utilized concept in many industries, especially in aerospace applications, and is associated with the green aviation concept. The contribution of aviation to global warming phenomena and environmental pollution has led to ongoing efforts for the reduction of aviation emissions. Approaches to achieve this target include increasing energy efficiency. An effective way to increase energy efficiency and reduce fuel consumption is reducing the mass of aircraft, as a lower mass requires less lift force and thrust during flight. For example, for the Boeing 787, a 20% weight savings resulted in 10 to 12% improvement in fuel efficiency. In addition to reduction of carbon footprint, flight performance improvements such as better acceleration, higher structural strength and stiffness, and better safety performance could also be achieved by lightweight design. Lightweighting optimization of a solar-powered unmanned aerial vehicle (UAV) is an example of using both clean energy and lightweight structures to achieve green aviation operation. Current solar-powered UAV designs face challenges such as insufficient energy density and wing stiffness. Lightweight design is essential for ultralight aviation, enabling longer flight duration. The principle of lightweight design is to use less material with lower density while ensuring the same or enhanced technical performance. A typical approach to achieve lightweight design for aerospace components is to apply advanced lightweight materials on numerically optimized structures, which can be fabricated with appropriate manufacturing methods. As such, the application of lightweight materials can effectively achieve both weight reduction and performance improvement. Although metal materials — especially aluminum alloys — are still the dominant materials in aerospace application, composite materials have received increasing interest and compete with aluminum alloys in many new aircraft applications. Structural optimization is another effective way to achieve lightweighting, by distributing materials to reduce materials use, and enhance the structural performance such as higher strength and stiffness and better vibration performance. Conventional structural optimization methods are size, shape, and topology. Manufacturability is a crucial constraint in both material selection and structural optimization. The development of advanced manufacturing technologies such as additive manufacturing, foam metal, and advanced metal forming not only enable the application of advanced materials, but relax constraints, enhancing the flexibility of multiscale structural optimization. Many examples of lightweight design have been successfully applied in the design of lightweight aircraft. Figure 1(a) illustrates the SAW Revo concept aircraft (produced by Orange Aircraft), which is an ultralight aerobatic airplane with carbon fiber-reinforced composite wings and a topologically optimized truss-like fuselage. The empty weight of this 6-meter-wingspan aircraft is 177 kg. Figure 1(b) shows a high-altitude, pseudo-satellite, solar-powered UAV from Airbus. The Zephyr 7 currently holds the world record for the longest absolute flight duration (336 hours, 22 minutes, 8 seconds) and highest flight altitude (21,562 m) for UAVs, partly from increased energy efficiency by lightweighting. Figure 1(c) shows a model of a future concept lightweight airplane for 2050 from Airbus, inspired by a bird skeleton. Figure 1(d) demonstrates a concept of a box wing aircraft where shape optimization is employed in the wing design. Structural efficiency could be increased by using a box wing structure; higher stiffness and lower induced drag force result from the box wing compared with conventional wing structures. The selection of aerospace materials is crucial in aerospace component design since it affects many aspects of aircraft performance, from the design phase to disposal, including structural efficiency, flight performance, payload, energy consumption, safety and reliability, lifecycle cost, recyclability, and disposability. Critical requirements for aerospace structural materials include mechanical, physical, and chemical properties such as high strength, stiffness, fatigue durability, damage tolerance, low density, high thermal stability, high corrosion and oxide resistance, and commercial criteria such as cost, servicing, and manufacturability. Studies have indicated that the most effective way to improve structural efficiency is reducing density (around 3 to 5 times more effective compared with increasing stiffness or strength), i.e. using lightweight materials. The most commonly used commercial aerospace structural materials are aluminum alloys, titanium alloys, high-strength steels, and composites, generally accounting for more than 90% of the weight of airframes. From the 1920s until the end of the century, metal — because of its high strength and stiffness, especially aluminum alloy — has been the dominant material in airframe fabrication, with safety and other flight performance measures driving aircraft design decisions. Lightweight aluminum alloys were the leading aviation structural materials — accounting for 70%–80% of the weight of most civil aircraft airframes before 2000 — and still play an important role. Since the mid-1960s and 1970s, the proportion of composites used in aerospace structures has increased due to the development of high-performance composites. Figure 2 illustrates the materials distributions for some Boeing products. Aluminum Alloys. Although high-performance composites such as carbon fiber are receiving increasing interest, aluminum alloys still make up a significant proportion of aerospace structural weight. The relatively high specific strength and stiffness, good ductility and corrosion resistance, low price, and excellent manufacturability and reliability make advanced aluminum alloys a popular choice of lightweight materials in many aerospace structural applications, e.g. fuselage skin, upper and lower wing skins, and wing stringers. The development of heat-treatment technology provides high-strength aluminum alloys that remain competitive with advanced composites in many aerospace applications. Aluminum alloys can offer a wide range of material properties meeting diverse application requirements, by adjusting compositions and heat treatment methods. Titanium Alloys. Titanium alloys have many advantages over other metals, such as high specific strength, heat resistance, cryogenic embrittlement resistance, and low thermal expansion. These advantages make titanium alloys an excellent alternative to steels and aluminum alloys in airframe and engine applications; however, the poor manufacturability and high cost (usually about 8 times higher than commercial aluminum alloys) result in the restriction of titanium alloys being used extensively. Hence, titanium alloys are used where high strength is required but limited space is available, as well as where high corrosion resistance is required. The current applications of titanium alloys in aerospace are mainly in airframe and engine components, overall comprising 7% and 36% of the weight, respectively. High-Strength Steel. Steel is the most commonly used structural material in many industry applications due to good manufacturability and availability, extremely high strength and stiffness in the form of high-strength steels, good dimensional properties at high temperatures as well as the lowest cost among commercial aerospace materials. But high density and other disadvantages, such as relatively high susceptibility to corrosion and embrittlement, restrict the application of high-strength steels in aerospace components and systems. Steel normally accounts for approximately 5% to 15% of structural weight of commercial airplanes, with the percentage steadily decreasing. Despite the limitations, high-strength steels are still the choice for safety-critical components where extremely high strength and stiffness are required. The major applications for high-strength steels in aerospace are gearing, bearings, and undercarriage applications. Aerospace Composites. High-performance composites such as fiber reinforced polymer and fiber metal laminates (FML) have received increased attention in aerospace applications, competing with the major lightweight aerospace materials such as aluminum alloys. In general, aerospace composites have higher specific strength and specific stiffness than most metals at moderate temperatures. Other advantages of composites include improved fatigue resistance, corrosion resistance, and moisture resistance as well as the ability to tailor layups for optimal strength and stiffness in required directions; however, the higher cost of composites in comparison to metals is one of the major obstacles for the application of composites. Carbon fiber reinforced polymer (CFRP) represents the most extensively used aerospace structural material apart from aluminum alloys, with the major applications being structural components of the wing box, empennage, and fuselage as well as control surfaces (e.g. rudder, elevator, and ailerons). Glass fiber reinforced polymer (GFRP) is used in radomes and semi-structural components such as fairings. Aramid fiber polymers are used where high impact resistance is required. Fiber metal laminates, especially glass fiber reinforced aluminum (GLARE), are other types of composites that have applications in aerospace (especially in the Airbus A380) due to enhanced mechanical properties such as reduced density, high strength, stiffness, and fatigue resistance compared with monolithic metals. The main applications of GLARE are the fuselage skin and empennage. Shape memory polymer composites (SMPC) are smart materials that can change their form as a result of a certain stimulus such as change of temperature, an electric or magnetic field, particular light wavelengths, etc. by releasing the internal stress stored in the material. The applications of SMPCs in aerospace components and systems include the wing skin of morphing-wing aircraft, and the solar array and reflector antenna of satellites. The advantages of SMPCs over shape memory alloys (SMAs) includes lower density, higher shape deformability and recoverability, better processing, and lower relative cost. The development of nanotechnology provides an opportunity to improve multifunctional properties (physical, chemical, mechanical properties, etc.) at the nanoscale. Unlike conventional composites, nanocomposites offer the opportunity to improve properties without too much tradeoff of density increase by only adding a small amount of nanoparticles (e.g. layered silicate, functionalized carbon nanotubes (CNTs), and graphite flakes). To increase the oxidation resistance of composites, for example, nanoparticles could be included such as silicate, CNTs, or polyhedral oligomeric silsesquioxane (POSS) that could form passivation layers. The addition of CNTs, silica, and layered silicate into composite matrix could promote energy dissipation on structural failure, increasing the toughness of the composite and resulting in the potential application to high-damage-tolerance structures. In addition to high modulus, high-strength nanoparticles such as continuous CNT could improve the stiffness and strength of the composite. The development of nanocomposites offers the opportunity for redundancy elimination and weight reduction, which provides significant potential in promoting the properties of aerospace components, especially in lightweighting. Manufacturability is a crucial constraint throughout the design process, governing the possibility of whether a design can be fabricated into a real product. Manufacturing constraints must be taken into consideration during materials selection, structure design, and optimization. Topological optimized designs tend to result in a complex geometry that cannot be fabricated by conventional manufacturing methods, such as casting and forming, without modification. Hence, manufacturing methods have significant effect on lightweighting design. The development of advanced manufacturing technology, such as additive manufacturing (AM), foam metal manufacturing, and advanced metal forming, could significantly expand the flexibility of lightweighting design, both in material selection and in structural optimization. AM was initially developed to produce prototypes rapidly and has now become a standard manufacturing tool. Although the advantages of AM attract much attention, challenges exist for AM to compete with conventional manufacturing methods, including quality of fabricated components, time-consuming processes, relatively expensive raw materials, and establishment of standards, qualification requirements, and certification. Selection of materials for an aerospace system is based on the operating conditions of the specific component or system — such as loading conditions, operating temperatures, moisture, corrosion conditions, and noise — in combination with economic and regulatory factors; for example, wings mainly sustain bending during service as well as tension, torsion, vibration, and fatigue. Hence, the main constraints for wing materials are stiffness, tensile strength, compressive strength, buckling strength, and vibration. Composites such as CFRPs and GLAREs usually have much higher specific strength and stiffness than metals, which makes composites an attractive choice for lightweighting design for many aerospace components and systems; however, metals have the advantages of ease of manufacture and availability as well as much lower cost, making them still extensively used in many aerospace applications. Lightweighting represents an effective way to achieve energy consumption reduction and performance enhancement. This concept has been well accepted and utilized in many industries, especially in aerospace component and system design. Lightweighting design involves the use of advanced lightweight material and numerical structural optimization, enabled by advanced manufacturing methods. This article was written by L. Zhu, N. Li, and P.R.N. Childs of the Imperial College London, UK. Learn more here . This article first appeared in the March, 2019 issue of Tech Briefs Magazine. Read more articles from this issue here. Read more articles from the archives here. SUBSCRIBE

https://www.sciencedirect.com/science/article/abs/pii/S1526612523009180

An analytical model for pressure distribution in automated fiber placement on irregular surfaces and its application in aeronautical manufacturing Author links open overlay panelLei Miao a b, Weidong Zhu a b, Yingjie Guo a b , Wei Liang a b, Xiaokang Xu a b, Shubin Zhao c, Yinglin Ke a b https://doi.org/10.1016/j.jmapro.2023.09.057 Automated fiber placement (AFP) is a key technology in the fabrication of composite aeronautical components, but the layup defects associated with uneven and insufficient contact pressure limit its application on irregular surfaces. Existing methods for pressure optimization in AFP are mostly based on extensive compaction experiments, which lacks theoretical basis and is not applicable to complex surfaces. In this paper, an analytical model of pressure distribution in AFP is proposed, which is suitable for any irregular surfaces. The influence of mechanical properties of contact materials and the mold curvatures on the calculation of contact pressure is taken into consideration. The Hertz theory is also extended to the three-dimensional contact area to suit the practical needs. Furthermore, an algorithm for pressure calculation along any layup path is proposed to rapid construct the pressure field. Then three sets of experiments are carried out on two aeronautical molds to validate the proposed model. The results show that the model can accurately calculate the contact pressure distribution in AFP, and compared with the previous model, the comprehensive prediction accuracy of the peak pressure is improved by 27.25 %. Combined with the layup path information, this model can be used to analyze the mechanical mechanism of layup defects, which provides a theoretical basis for the optimization of AFP parameters based on pressure distribution. Introduction Carbon fiber reinforced composites are widely used in modern aviation manufacturing because of their light weight and high strength [[1], [2], [3], [4]]. With the demand of excellent performance, the application of composites has gradually changed from secondary load-bearing components to main load-bearing components, which puts forward a higher standard for the forming quality of composite components. Compared with traditional labor-intensive methods, automated fiber placement (AFP) is more efficient and accurate, which is suitable for forming large and complex aeronautical components [5]. There are many kinds of AFP machines and each has its own characteristics. The flexibility of the robotic one (Fig. 1. (a)) is suitable for complex parts but the stiffness is poor [6,7]. The accuracy of horizontal one (Fig. 1. (b)) is relatively high but the processing range is limited. The gantry type (Fig. 1. (c)) has high accuracy and wide processing range, which is suitable for laying large aeronautical components such as wings. Despite the variety, the technological processes of AFP are basically the same: several resin-based carbon fibers are dragged separately with a controlled tension and pressed against the mold or the previous layer at appropriate pressure by a rubber-covered roller, the heat source works simultaneously to enhance the adhesion of the prepreg fibers, as shown in Fig. 1. (d). When layup at a constant speed, tension and heating power are constant, the layup quality is closely related to contact pressure. Sufficient and even contact pressure is beneficial to reduce many layup defects such as bulges and bridgings [8,9]. For thermoplastic prepreg, the increasing contact pressure is conducive to eliminate the voids in and between layers and improve the mechanical properties of composite components [[10], [11], [12]]. Therefore, it is essential to control the contact pressure of AFP within a reasonable range for the improvement of layup quality. The contact pressure is generated by the compaction force applied to the rigid shaft of the roller. The bearing seats at both ends of the rigid shaft are usually equipped with two force sensors, and the contact pressure can be controlled by maintaining the resultant force on the shaft through an output cylinder. For easy control, the resultant force in the process of AFP is constant, which is obviously unreasonable. For the same resultant force, the pressure distribution on the plane, convex surface and concave surface is different, and its influence on the layup quality is also different. Therefore, in order to find the appropriate compaction force and contact pressure, it is often necessary to conduct a large number of compaction tests [[13], [14], [15]]. Bakhshi et al. [16] studied the influence of the roller hardness and geometric parameters on the contact pressure and layup quality through a method of controlling variables. The mold analyzed in this research is plane, which is different from the irregular mold commonly used in practice. Considering the influence of mold curvature on contact pressure, Qu et al. [17] explored the contact mechanical properties of plane, convex and concave molds, defined the placement suitability based on the contact state, and used it for path optimization before AFP. The curved surface molds used in their research were simplified into fixed curvature molds, which is closer to the actual mold than the plane one, but the accuracy is not enough. The data obtained from extensive experiments provide valuable references for the optimization of AFP process parameters under similar working conditions. Due to the limitations of experimental methods, a few scholars began to study the general AFP contact mechanics model which can accurately reflect the contact pressure distribution. Jiang et al. [18] established a model of pressure distribution for laying irregular curved mold, which is based on the relationship between the combined deformation and the contact pressure. This numerical model can accurately predict the contact pressure distribution on irregular surfaces. The complexity of this method lies in that it requires a lot of information of point positions on the mold to calculate the combined deformation, which is not convenient for engineering application. In order to realize the rapid calculation of contact pressure distribution with high precision, the analytical model based on classical contact theory is an important breakthrough point. In practical applications, analytical modeling of rolling contact is a challenging problem [19,20]. Complex boundary conditions such as multi-material contact, nonlinear mechanical properties of materials and geometric constraints of irregular surfaces increase the difficulty of analysis. Hertz theory, as a classical theory of contact mechanics, is often used to analyze the contact pressure of elastomers under small deformation conditions [[21], [22], [23], [24]]. Generally, the hardness of the rubber-covered roller in AFP is about 35 HA (shore hardness), the contact pressure is not more than 0.6 MPa, and the deformation of the roller along the radius is within 3 mm [18]. This deformation is much smaller than the radius of the roller and the mold surface. Therefore, Hertz theory can be used to approximate the solution of the contact pressure in AFP [25]. It should be noted that Hertz contact is usually presented in the form of two-dimensional solutions [26], which is not applicable to the uneven pressure distribution caused by axial deformation in AFP [27,28], so it needs to be extended in three dimensions. In addition, the mechanical properties of roller and prepregs, as well as the variation of mold curvatures during AFP cannot be ignored in the calculation of pressure distribution. In the present work, a 3D analytical model of the contact pressure during AFP is established for irregular curved molds. Based on Hertz theory, this model unifies the constitutive equations of prepreg layers and rubber-covered roller in the calculation of pressure distribution. A discretization method is proposed to realize the rapid extraction of mold geometric information, and an algorithm is further proposed for pressure calculation along any layup path. Several contact points on two aeronautical molds are selected to validate the model. Additional AFP experiment is carried out to demonstrate the validity of the model in predicting layup defects. The purpose of this work is to provide an accurate analytical model for the rapid calculation of pressure distribution on irregular surfaces during AFP process. The model can predict layup defects with the cooperation of layup path information, which has not only theoretical value but also engineering application prospect. Access through your organization In this section, a novel modeling approach of contact pressure is proposed, which can be applied to the fiber placement of curved components. Firstly, the mechanical properties of two contact materials, rubber and resin-based prepreg fiber, are investigated. The influence of mold curvatures and end-effector pose on the contact pressure calculation is simultaneously analyzed. The model is based on the classical contact mechanics theory and integrates the constitutive equations of different Pressure calculation along the layup path Automated fiber placement is carried out path by path and layer by layer according to the preset procedure. Each path is composed of a series of discrete path points, each containing information about the position, orientation, and speed of the end-effector during fiber placement. In order to analyze the pressure distribution within the contact area, discretization of the contact area is performed as a prerequisite step. Taking the i-th path point on the current path as the reference, m points Experimental setup In this study, the six-axis gantry AFP machine independently developed by Zhejiang University was used as the experimental execution equipment, as shown in Fig. 8(a). The pressure distribution of typical positions in the fiber placement of winglet mold (Fig. 8(b)) and hyperboloid mold (Fig. 8(c)) was taken as the analysis object. By comparing and analyzing the calculated results with the experimental results, the proposed model can be validated. The prepreg used in the experiment was supplied Conclusion In this paper, a theoretical model of pressure distribution for fiber placement on irregular curved surfaces is proposed by considering the mechanical properties of the contact objects and the mold curvatures. The elastic moduli and the constitutive equations of the roller and the prepreg fibers are obtained by finite element simulation and compaction tests. According to the discretization method proposed in this paper, the mold curvatures around different path points are extracted. Combing CRediT authorship contribution statement Lei Miao: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Visualization, Writing - Original draft. Weidong Zhu: Writing - Review and Editing, Project administration, Resources, Supervision. Yingjie Guo: Writing - Review and Editing, Data curation, Validation, Resources, Supervision. Wei Liang: Experiments, data analysis. Xiaokang Xu: Experiments, data analysis. Shubin Zhao: Experiments, data analysis. Yinglin Ke: Resources, Supervision. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgment This research was supported by the “Pioneer” and “Leading Goose” R&D Program of Zhejiang (No. 2022C01134), the National Natural Science Foundation of China (No. 52105535) and the Major Research Plan of the National Natural Science Foundation of China (No. 91948301). Lei Miao received the M.S. degree from China University of Mining and Technology, Xuzhou, China in 2020. He is currently working toward the Ph.D. degree in the School of Mechanical Engineering, Zhejiang University. His current research interests include automated fiber placement in aeronautical manufacturing, and robot processing technology. References (36) J.H. Ma et al. Translaminar enveloping ply for CFRP interlaminar toughening Compos B Eng (2023) E.G. Koricho et al. Innovative tailored fiber placement technique for enhanced damage resistance in notched composite laminate Compos Struct (2015) D.H.J.A. Lukaszewicz et al. The engineering aspects of automated prepreg layup: history, present and future Compos B Eng (2012) W.Z. Zhou et al. Deformation and fracture mechanisms of automated fiber placement prepreg laminates under out-of-plane tensile loading Compos Struct (2021) L. Miao et al. A two-step method for kinematic parameters calibration based on complete pose measurement—verification on a heavy-duty robot Robot Comput Integr Manuf (2023) Y.J. Guo et al. Stiffness-oriented posture optimization in robotic machining applications Robot Comput Integr Manuf (2015) R.J. Crossley et al. The experimental determination of prepreg tack and dynamic stiffness Compos Part A Appl Sci Manuf (2012) B. Denkena et al. Thermographic online monitoring system for automated fiber placement processes Compos B Eng (2016) Z. Qureshi et al. In situ consolidation of thermoplastic prepreg tape using automated tape placement technology: potential and possibilities Compos B Eng (2014) F.O. Sonmez et al. Process optimization of tape placement for thermoplastic composites Compos Part A Appl Sci Manuf (2007)

https://www.mdpi.com/2075-4701/9/6/662

Alloys for Aeronautic Applications: State of the Art and Perspectives by Antonio Gloria 1, Roberto Montanari 2,* [ORCID] , Maria Richetta 2 [ORCID] and Alessandra Varone Institute of Polymers, Composites and Biomaterials, National Research Council of Italy, V.le J.F. Kennedy 54-Mostra d’Oltremare Pad. 20, 80125 Naples, Italy 2 Department of Industrial Engineering, University of Rome “Tor Vergata”, 00133 Rome, Italy Author to whom correspondence should be addressed. Metals 2019, 9(6), 662; https://doi.org/10.3390/met9060662 Submission received: 16 May 2019 / Revised: 3 June 2019 / Accepted: 4 June 2019 / Published: 6 June 2019 (This article belongs to the Special Issue Processing-Structure-Property Relationships in Metals) Abstract In recent years, a great effort has been devoted to developing a new generation of materials for aeronautic applications. The driving force behind this effort is the reduction of costs, by extending the service life of aircraft parts (structural and engine components) and increasing fuel efficiency, load capacity and flight range. The present paper examines the most important classes of metallic materials including Al alloys, Ti alloys, Mg alloys, steels, Ni superalloys and metal matrix composites (MMC), with the scope to provide an overview of recent advancements and to highlight current problems and perspectives related to metals for aeronautics. Keywords: alloys; aeronautic applications; mechanical properties; corrosion resistance 1. Introduction The strong competition in the industrial aeronautic sector pushes towards the production of aircrafts with reduced operating costs, namely, extended service life, better fuel efficiency, increased payload and flight range. From this perspective, the development of new materials and/or materials with improved characteristics is one of the key factors; the principal targets are weight reduction and service life extension of aircraft components and structures [1]. In addition, to reduce the weight, advanced materials should guarantee improved fatigue and wear behavior, damage tolerance and corrosion resistance [2,3,4]. In the last decade, a lot of research work has been devoted to materials for aeronautic applications and relevant results have been achieved in preparing structural and engine metal alloys with optimized properties. The choice of the material depends on the type of component, owing to specific stress conditions, geometric limits, environment, production and maintenance. Table 1 reports the typical load conditions of structural sections of a transport aircraft and the specific engineering property requirements, such as elastic modulus, compressive yield strength, tensile strength, damage tolerance (fatigue, fatigue crack growth, fracture toughness) and corrosion resistance. Table 1. Typical load conditions and required engineering properties for the main structural sections in an aircraft. Elastic modulus (E); compressive yield strength (CYS); tensile strength (YS); damage tolerance (DT); corrosion resistance (CR). Table This work describes the state of the art and perspectives on aeronautic structural and engine materials. Structural materials must bear the static weight of the aircraft and the additional loads related to taxing, take-off, landing, manoeuvres, turbulence etc. They should have relatively low densities for weight reduction and adequate mechanical properties for the specific application. Another important requirement is the damage tolerance to withstand extreme conditions of temperature, humidity and ultraviolet radiation [5]. Figure 1 shows a transport aircraft (Boeing 747), and Table 1 lists the typical load conditions together with the required engineering properties for its main structural sections. Figure 1. The transport aircraft (Boeing 747) and its main structural sections. As shown in Figure 2, engines consist of cold (fan, compressor and casing) and hot (combustion chamber and turbine) sections. The material choice depends on the working temperature. The components of cold sections require materials with high specific strength and corrosion resistance. Ti and Al alloys are very good for these applications. For instance, the working temperature of the compressor is in the range of 500–600 °C, and the Ti-6Al-2Sn-4Zr-6Mo alloy (YS = 640 MPa at 450 °C; excellent corrosion resistance) is the most commonly used material. Figure 2. Schematic view of a turbofan engine. For the hot sections, materials with good creep resistance, mechanical properties at high temperature and high-temperature corrosion resistance are required, and Ni-base superalloys are the optimal choice. 2. Aluminum Alloys For many years, Al alloys have been the most widely used materials in aeronautics; however, the scenario is rapidly evolving, as shown by Table 2, which reports the approximate primary structure materials used by weight in Boeing aircrafts. From these data, it is evident that an increasing role is being played by composites [4]. Table 2. Materials used in Boeing aircrafts (weight %). The term “Others” refers to materials present in very small amounts, including metal alloys (Mg, refractory metals etc.) and carbon. Table Anyway, in spite of the rising use of composites, Al alloys still remain materials of fundamental importance for structural applications owing to their light weight, workability and relative low cost, and relevant improvements have been achieved especially for 2XXX, 7XXX and Al-Li alloys. In general, the 2XXX series alloys are used for fatigue critical applications because they are highly damage tolerant; those of the 7000 series are used where strength is the main requirement, while Al–Li alloys are chosen for components which need high stiffness and very low density. 2.1. 2XXX Series—(Al-Cu) Where damage tolerance is the primary criterion for structural applications, Al-Cu alloys (2XXX series) are the most used materials. The alloys of the 2XXX series containing Mg have: (i) higher strength due to the precipitation of the Al2Cu and Al2CuMg phases; (ii) better resistance to damage; (iii) better resistance to fatigue crack growth compared to other series of Al alloys. For these reasons, 2024-T3 is still one of the most widely used alloys in fuselage construction. Nevertheless, it is worth noting that the 2XXX series alloys present some drawbacks: (i) the relatively low YS limits their use in components subject to very high stresses; (ii) the phase Al2CuMg can act as an anodic site, drastically reducing the corrosion resistance. Improvements can be achieved by a suitable tailoring of the composition and a strict control of the impurities. In particular, the addition of some alloying elements such as Sn, In, Cd and Ag can be useful to refine the microstructure, thus improving the mechanical properties, e.g., an increase in hardness, YS and UTS was found by increasing Sn content up to 0.06 wt% [6]. A further increase in mechanical properties can be obtained by controlling the level of impurities such as Fe and Si. For example, the alloy 2024-T39, which has a content of Fe+Si equal to 0.22 wt%, much lower than that of the 2024 alloy (0.50 wt%), exhibits an ultimate tensile strength (UTS) value of 476 MPa, while that of a conventional 2024 alloy is 428 MPa. 2.2. 7XXX Series—(Al-Zn) Among all metals, Zn has the highest solubility in Al, and the strength results improved by increasing Zn content. The 7XXX series alloys represent the strongest Al alloys, and are used for high-stressed aeronautic components; for example, upper wing skins, stringers and stabilizers are manufactured with the alloy 7075 (YS = 510 MPa). Mg and Cu are often used in combination with Zn to form MgZn2, Al2CuMg and AlCuMgZn precipitates which lead to a significant strengthening of the alloy [7]. However, there are also some drawbacks to the 7XXX series. Specifically, the low fracture toughness, damage tolerance and corrosion resistance limit the use of the 7075 alloy in the aeronautic industry. Anyway, the composition can be varied to improve their properties. The optimal properties of the 7XXX series are obtained when the Zn/Mg and Zn/Cu ratios are approximately equal to 3 and 4, respectively. Alloy 7085 is a possible alternative to 7075 for aerospace applications due to its excellent mechanical properties (YS = 504 MPa, elongation = 14%) and better damage tolerance (44 MPa m1/2). Zr and Mn can be added up to 1% as they refine the grain and consequently improve the mechanical properties. Another important issue related to the specific applications of 7XXX series alloys is the fatigue behavior, and a lot of work has been devoted to the matter, taking into consideration different parameters [8,9,10,11,12,13]. Material discontinuities are often associated with crack nucleation. On a micro-scale, roughness and precipitate particles may act as preferred nucleation sites; however, the most serious problems arise at macro-scale level. Coating layers due to cladding and/or anodizing, and defects (machining marks, scratches etc.) induced by the manufacturing process have been found to be the principal sources of failure [12]. The fatigue performance of the 7075-T6 alloy is significantly reduced by the anodic oxidation process, and the degrading effect of the oxidation increases with the coating layer thickness. Such a detrimental effect is mainly ascribed to deep micro-cracks which form during the anodizing process. Moreover, the brittle nature of the oxide layer and the irregularities beneath the coating contribute to degradation [13]. Components with complex geometrical shapes, made of Al alloys, are usually obtained by closed-die forging of a billet, and are manufactured to obtain a good combination of strength, fatigue resistance and toughness. Some forging experiments have been performed on the 7050 alloy in agreement with AMS4333 requirements, and an alternative process [14], involving an intermediate warm deformation step at 200 °C between the quenching and ageing steps, showed the possibility to improve fracture toughness without effects on YS and UTS. Results showed a more homogeneous and finer grain structure, after warm deformation, which can explain the increase in fracture toughness. 2.3. Al-Li Alloys The density of Li is very low (0.54 g/cm3); thus, it reduces that of Al alloys (~3% for every 1% of Li added). Moreover, Li is the unique alloying element that determines a drastic increase in the elastic modulus (~6% for every 1% of added Li). Al alloys containing Li can be hardened by aging, and Cu is often used in combination with Li to form Al2CuLi and improve the mechanical properties [15]. In ternary Al-Cu-Li alloys, six ternary compounds have been identified; the most important among them are T1 (Al2CuLi), T2 (Al6CuLi3), and TB (Al15Cu8Li3). The phases precipitating from the supersaturated solid solution depend on the Cu/Li ratio [16,17]; the precipitation sequence has been described in ref. [18]. Al-Li alloys exhibit lower density and better specific mechanical properties than those of the 2XXX and 7XXX series; thus, they are excellent materials for aeronautic applications [19,20]. For example, the use of the 2060-T8 Al-Li alloy for fuselage panels and wing upper skin results in 7% and 14% weight reduction if compared to the more conventional 2524 and 2014 alloys, respectively. However, Li content higher than 1.8 wt% results in a strong anisotropy of mechanical properties resulting from texture, grain shape, grain size, and precipitates [21]. This was a serious drawback in the first two generations (GEN1 and (GEN2) of Al-Li alloys, which also had low toughness and corrosion resistance. Al-Li alloys were first developed in the 1920s, and the 2020 alloy (GEN1) started to be produced in 1958 for the wing skins and empennage of the Northrop RA-5C Vigilante aircraft. The deep understanding of the relations between the microstructure and mechanical characteristics of these materials matured much later in the 1990s, leading to the production of the third generation (GEN3), a family of alloys with an outstanding combination of properties for aeronautic applications. The former generations of Al-Li alloys had a higher Li content and a lower density than GEN3 alloys, but suffered from high anisotropy associated with the precipitation of coarse Li phases, such as T2 [22,23]. Anisotropy has been partially reduced in GEN2 alloys through a suitable recrystallization texture and the tailoring of composition [24]. In GEN3 alloys with Li content between 1 and 1.8 wt%, the anisotropy problem has been substantially overcome. These materials exhibit excellent mechanical properties; in particular, the specific stiffness ranges from 28.9 to 31.2 GPa g−1 cm3 and is much better than that of the 2XXX (26.1–27.1) and 7XXX (25.9–26.4) series. The phase δ’(Al3Li) is not present, and strengthening is mainly due to the precipitation of the T1 phase forming platelets on {111} Al planes [25,26,27]. The typical morphology of T1 precipitates is shown in Figure 3. In the first stage of precipitation up to the aging peak, T1 platelets have a constant thickness (one unit cell), then, it increases with a consequent decrease in mechanical performances [26]. Although some years ago T1 precipitates were believed to be unshearable by dislocations, more recent investigations through high-resolution electron microscopy evidenced sheared precipitates in deformed samples [27,28]. T1 precipitates are sheared in a single-step shearing event. The transition between shearing and by-passing is progressive, connected to the increase in T1 plate thickness, and takes place after peak ageing. The by-passing mechanism favours the homogenization of plasticity up to the macroscopic scale. Strain localization within the matrix can be minimized by changing the deformation mode from dislocation shearing to dislocation by-passing of the precipitates. Metals 09 00662 g003 550 Figure 3. Typical morphology of T1 precipitates in Al-Cu-Li alloys. A great variety of T1 microstructures can be been obtained, operating under different conditions of deformation and aging. The parameters of the T1 precipitate distributions have been systematically characterized and modelled by Dorin et al. [29]. In the conventional manufacturing route for producing aeronautic plates, stretching is carried out after solution heat treatment for relieving residual stresses due to quenching. The operation, which involves a plastic strain of about 5%, also allows the obtainment of a homogeneous distribution of T1 precipitates after aging, since dislocations represent preferred sites for precipitate nucleation. Such homogeneous distribution is the key factor for the excellent mechanical properties of Al-Li alloys. Increasing the pre-strain induces a higher density of dislocations, i.e., the preferred T1 nucleation sites; thus, the average diffusion distance of alloying elements is reduced and the aging kinetics is accelerated. The benefits of stretching in Al-Cu-Li alloys saturate at pre-strains of 6–9% [25]. Moreover, stretching prior to ageing is connected to a relevant technological problem: today, advances in rolling technology enable the production of plates with desired thickness, which is of great interest for manufacturing near-net-shape sections (e.g., tapered wing skins). The stretching of a tapered plate leads to a strain gradient, and it is necessary to know the maximum strain that can be achieved without fracture. Recently, Rodgers and Prangnell [30] have investigated the effect of increasing the pre-stretching of the Al-Cu-Li alloy AA2195 to higher levels than those currently used in industrial practice, focussing the attention on the behavior of the T1 phase. At the maximum pre-strain level before plastic instability (15%), YS increased to ~670 MPa and ductility decreased to 7.5% in the T8 condition. In fact, increasing the pre-strain prior to ageing leads to a reduction in the strengthening provided by the T1 phase, in favour of an increase in the strain hardening contribution. In recent years, Al-Li alloys have experienced a great development, mainly based on the tailoring of composition and the knowledge/control of the precipitation sequence of stable and metastable phases [19,31,32,33,34,35,36]. In the alloy compositions of major interest, Cu content is around 3 wt%, Li is always below 1.8 wt% (in most recent alloys it does not exceeds 1.5 wt%), Mg content varies in an extended range, and other elements, in particular Ag and Zn, can be also added. Of course, the precipitation sequence depends on the specific composition. For example, a high Li content favors the formation of the metastable δ’ phase [27], while Mg leads to the precipitates typically present in the Al-Cu-Mg alloys, namely Guinier–Preston–Bagaryatsky (GPB) zones, S’/S [37,38]. In spite of strength increase, the presence of the δ’ phase is generally undesired because it is prone to shear localization, leading to poor toughness and ductility. In a recent paper, Deschamps et al. [36] described the microstructural and strength evolution during long-term ageing (3000 h at 85 °C) of Al-Cu-Mg alloys with different contents of Cu, Li and Mg. They found that T1 is always the dominating phase in T8 condition, S phase is also present and, in the case of a high Li content, δ’ precipitates are observed. The examined alloys exhibit a very different level of microstructural stability during long-term ageing. Although the high Li alloy originally (T8 condition) has the lowest strength, its evolution leads to mechanical properties comparable with those of the other alloys after 3000 h of treatment. This is due to the precipitation of an additional fraction (~10%) of δ’ precipitates, whereas the two other alloys form a limited amount of metastable phases. Another significant drawback to the GEN2 alloys is poor fracture toughness and ductility. Delamination cracking, which is a complex fracture mechanism involving initial transverse cracks with length comparable to grain size, is of great relevance in the fracture process. Delamination cracks along grain boundaries have been observed and described by many investigators [39,40,41,42,43]; Kalyanam et al. [44] systematically investigated the phenomenon in the 2099-T87 alloy, described the locations, sizes and shapes of delamination cracks and the extension of the primary macro-crack, and found that an isotropic hardening model with an anisotropic yield surface describes the constitutive behavior of the alloy. In conclusion, GEN3 alloys have low density, excellent corrosion resistance, an optimal combination of fatigue strength and toughness, and are also advantageous in terms of cost in comparison to Carbon Fiber Reinforced Polymers (CFRP), which are considered as competitor materials to replace the traditional alloys of the 2XXX and 7XXX series in the design of new aircrafts. 2.4. Aluminum Composites Composites with a metal, ceramic and polymer matrix are increasingly used in the aeronautic industry, replacing other materials (see Table 2). They are of relevant interest for applications in both structural and engine parts of aircrafts. Metal matrix composites of light alloys (Al, Ti, Mg) are usually reinforced by ceramics (SiC, Al2O3, TiC, B4C), in the form of long fibers, short fibers, whiskers or particles. Typically, these composites are prepared using SiC or Al2O3 particles instead of fibers, which are used only for special applications, such as some parts of Space Shuttle Orbiter [45]. In addition, the nature of reinforcement is a relevant factor to the production costs, and whiskers and ceramic particles seem to be a good compromise in terms of mechanical properties and costs [46,47]. Al matrix composites, prepared with SiC and Al2O3 particle reinforcement, exhibit higher specific strength and modulus, fracture toughness, fatigue behavior, wear and corrosion resistance than the corresponding monolithic alloys. To further improve their mechanical properties, other types of reinforcements such as carbon nanotubes (CNTs) and graphene nano-sheets have been recently investigated [48,49,50]. If compared to the conventional reinforcements, CNTs and graphene are stronger and provide better damping and lower thermal expansion. A critical aspect is the optimization of reinforcement content because the properties of Al matrix composites strongly depend on such a parameter. For example, Liao et al. [49] found that the best characteristics are achieved with 0.5 wt% of multi-walled nanotubes. In addition to high mechanical properties, good corrosion resistance is a requirement of Al composites. The topic has been extensively investigated for many years (e.g., see [51,52,53]); however, it is not yet completely clear how the presence of reinforcing phases influences the corrosion resistance and mechanisms. It is a common opinion that galvanic corrosion may take place due to the contact between reinforcement particles and the matrix: galvanic coupling between Al and ceramic particles has been detected, with the reinforcement acting as an inert electrode upon which O2 and/or H+ reductions occur [54]. Anyway, composites are more susceptible to pitting corrosion than the corresponding monolithic alloys, and preferential attack occurs at the reinforcement–matrix interface [55,56]. The phenomenon is enhanced by the presence of precipitates, in particular when they are located at the junction between the reinforcement particles and the metal matrix [57]. 2.5. Advanced Joining Techniques for Aluminum Alloys The development of innovative joining techniques is a relevant aspect for the aeronautic applications of Al alloys. Recently, Friction Stir Welding (FSW) gained increasing attention in the aerospace industry (e.g., airframes, wings, fuselages, fuel tanks), and a lot of research efforts have been devoted to one of its variants, namely, Friction Stir Spot Welding (FSSW) [58,59,60,61,62,63,64,65]. This method is an alternative to resistance welding, riveting, and adhesive bonding in the fabrication of aircraft structures, and allows the joining of components made of Al alloys with lower costs and better strength than conventional techniques. Welding time, tool rotation speed, tool delve depth, tool plunge speed and tool exit time are crucial parameters which should be properly optimized [63,64,65]. A serious problem is represented by the hole resulting from the welding process, which strongly weakens the joint strength. A novel technique, Refill Friction Stir Spot Welding (RFSSW) [66,67], allows us to overcome this drawback through the filling of the hole. RFSSW employs a tool made of a pin and a sleeve, and its procedure is described in detail in the paper of Kluz et al. [68]. RFSSW is very useful for joining materials whose microstructure can be remarkably changed by conventional welding processes, especially the alloys of 2XXX and 7XXX series. Many advantages are related to spot welding, causing a decline in the riveting and gluing of Al alloys: (i) the drilling of parts and the use of rivets as additional fasteners are not required; (ii) a great resistance to corrosion can be achieved for welded joints; (iii) the possibility to perform simple repairs of joints; (iv) no part of the joint extends beyond the surface of the joined elements. The optimization of the RFSSW parameters to get the best mechanical performances of joints has been studied by many authors [66,67,68,69,70]. 3. Titanium Alloys Owing to their excellent specific strength and corrosion resistance, Ti alloys are increasingly used for manufacturing structural parts of aircrafts. They are also employed in engine sections operating at intermediate temperature (500–600 °C). Ti alloys can be divided into three main classes (α, β and α-β). Independently of the specific class, the mechanical properties of Ti alloys depend on O and N in solid solution [71,72]. The solubility of these interstitial elements in both α and β phases is high, increases with temperature and the part of the gas absorbed at high temperature remains entrapped in the metal after cooling, causing lattice distortion. In addition to modifying the mechanical properties, this phenomenon plays a role also in manufacturing processes and stress relieving heat treatments. X-ray diffraction experiments on the Ti6Al4V alloy carried out up to 600 °C in vacuum or different atmospheres demonstrated that the effects of O and N are synergic with the intrinsic anisotropic thermal expansion in determining the distortion of the hexagonal lattice [73,74,75,76]. The surface integrity of machined aeronautical components made of Ti alloys [77,78,79] also represents a critical problem. The cutting of Ti alloys generates an enormous amount of heat at the chip–tool interface, which is not suitably dissipated owing to the low thermal conductivity; this causes surface damage and residual stresses [80]. 3.1. α-Ti In general, α-Ti alloys have better creep behavior and corrosion resistance than β-Ti alloys [81], therefore, some of them (e.g., Ti-3Al-2.5V, Cp-Ti, Ti-5-2.5, Ti-8-1-1, Ti-6-2-4-2S, IMI829) are commonly used to make compressor disks and blades of aeronautic engines. In order to improve the microstructural stability of α-Ti alloys at increasing temperature, and, consequently, their mechanical performances, different compositions have been studied by adding Al, Sn, Zr and Si. The results are not completely satisfying because the achievement of some advantages is often accompanied by drawbacks. For instance, Jiang et al. [82] modified the composition of a Ti-25Zr alloy by adding Al up to 15% and found that the YS increase is accompanied by a reduction of ductility. 3.2. β-Ti β-Ti alloys exhibit higher strength and fatigue behavior than the α-Ti alloys, thus they are employed for high-stressed aircraft components, e.g., landing gear and springs are currently manufactured using Ti-15V-3Cr-3Al-3Sn and Ti-3Al-8V-6Cr-4Mo-4Zr alloys [83], while Ti-10V-2Fe-3Al, Ti-15Mo-2.7Nb-3Al-0.2Si, Ti-5Al5V5Mo3Cr0.5Fe and Ti-35V-35Cr are applied in airframe parts [84]. A drawback of these materials is the relatively low ductility, which can be mitigated through tailoring the composition (Ti-1300 [85]) and suitable heat treatments (Ti–6Al–2Sn–2Zr–2Cr–2Mo–Si [86]). 3.3. α-β–Ti Ti-6Al-4V is the most used Ti alloy owing to its excellent combination of mechanical properties (strength, fracture toughness and ductility) and corrosion resistance [87]. Moreover, Zr addition further improves its strength through the solid solution hardening mechanism; Jing et al. [88] showed that hardness is increased to 420 HV and YS to 1317MPa by adding 20 wt % Zr at the expense of ductility (elongation ratio drops to ~8%). Ti-6Al-2Zr-2Sn-3Mo-1Cr-2Nb, Ti-6Al-2Sn-2Zr-2Cr-2Mo-Si and ATI 425 are other α-β–Ti alloys widely used for manufacturing aircraft parts such as fuselage, landing gear and compressor disks. 3.4. Ti Composites Reinforced with SiC Fibers Ti composites are materials of great interest for aeronautic applications and, in particular, attention has been focused on those reinforced with long ceramic fibers [89,90,91,92,93,94,95,96,97,98,99,100,101,102]. Among them, the Ti6Al4V-SiCf composite is a promising material for turbine components and structural high-stressed parts. Figure 4a shows the stratified structure of the SiC fibers: a C layer of about 3 µm separates the SiC fiber from the Ti6Al4V matrix. The composite is commonly prepared by Hot Isostatic Pressing (HIP) or Roll Diffusion Bonding (RDB) of Ti6Al4V sheets alternated with SiC fiber layers [99,102]; the resulting structure is displayed in Figure 4b. Figure 4. Stratified structure of SiC fibers (a). Typical morphology of Ti6Al4V-SiCf composite (b). The Ti6Al4V-SiCf composite is a promising material for mechanical components operating at medium temperatures, especially turbine blades and structural high-stressed parts of aeronautic engines. The performances mainly depend on the fiber–matrix interface and chemical reactions occurring during the manufacturing process and in-service life, when it is exposed for a long time to temperatures around 600 °C. Direct contact of the Ti6Al4V matrix with SiC induces the formation of brittle compounds like Ti5Si3, which deteriorate the mechanical behavior of the composite [103,104], therefore the fibers are coated with a thin C layer. This coating hinders chemical reactions, preserves the fiber integrity, reduces the interfacial debonding and deflects the propagation of micro-cracks along the fiber. However, when the composite is operating for a long time at medium-high temperatures, C diffuses into the matrix, forming TiC. The TEM micrograph in Figure 5a displays TiC particles of ~200 nm forming an irregular layer around a fiber which has grown during the fabrication process at high temperature. Figure 5. The TEM micrograph shows TiC particles forming an irregular layer around a fiber (a). AFM (Atomic Force Microscopy) evidences a groove all around the fiber, in correspondence to the C coating after a treatment of 1 h at 600 °C in air (b). The depth profile measured along the line in (b) is displayed in (c). AFM observations were carried out using a Multimode III of Digital Instruments in contact mode and a Si3N4 tip. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning photoemission microscopy (SPEM) analyses, carried out before and after heat treatments at 600 °C of up to 1000 h, evidenced the stability of the fiber–matrix interface [94,95,96,97,101]. The fiber–matrix interface is also stable after prolonged heat treatments because a thin TiC layer forms all around the C coating during the fabrication process, hindering further C diffusion towards the matrix and delaying interface degradation. This is due to the fact that C diffusion through TiC is much slower than through Ti (for C in Ti, the diffusion parameters are D0 = 5.1 × 10−4 m2 s−1 and Q = 182 kJ mole−1 [105], whereas for C in TiC, D0 = 4.1 × 10−8 m2 s−1 and Q = 207 kJ mole−1 [106]). If the material is heated in air, C atoms present in the core and the coating of fibers may react with O, forming CO and CO2; in this case, a groove is observed all around the fibers (Figure 5b,c) and in the fibers’ core. Therefore, the surface of the composite during in-service life must be protected to avoid direct contact with O in the air. The stability of the fiber–matrix interface leads to the stability of the mechanical properties. YS, UTS and Young’s modulus E, determined from tensile tests carried out at room temperature on the Ti6Al4V-SiCf composite in as-manufactured condition and after different heat treatments, are reported in Figure 6a [96]. It is clear that the mechanical properties are scarcely affected by heat treatments, even in the most severe conditions. Moreover, fracture surfaces show plastic deformation of the matrix and pull-out of the fibers, i.e., a correct load transfer from the matrix to the fibers (Figure 6b). Figure 6. Yield stress (YS), ultimate tensile strength (UTS) and Young’s modulus E determined from tensile tests at room temperature carried out on the Ti6Al4V-SiCf composite in as-manufactured condition and after the indicated heat treatments (a). Figure 6a was redrawn from data reported in [94]. Fracture surface of a sample exposed at 600 °C for 1000 h (b). Figure 7 compares the results obtained from fatigue tests carried out on the material in as-manufactured condition and after 1000 h at 600 °C. Also, these data confirm the good stability of composite mechanical properties after long-term exposure to high temperature. Figure 7. Curves determined from fatigue tests carried out on the material in as-manufactured condition and after 1000 h at 600 °C. 4. Magnesium Alloys Mg is the lightest metal used in structural applications and exhibits excellent castability [107], with good fluidity and less susceptibility to hydrogen porosity than other cast metals such as Al alloys [108]. In fact, wrought Mg alloys have better mechanical properties than casting alloys; however, the higher asymmetry in plastic deformation represents a serious problem [109]. For this reason, casting is the principal way for manufacturing Mg components, and various processes are currently used for producing castings (a literature overview can be found in ref. [107]). Other relevant advantages of Mg are its abundance and recyclability [110]. On the other hand, the poor mechanical properties and low corrosion resistance of Mg alloys limit their use in manufacturing parts of aircrafts, even if some alloys (AZ91, ZE41, WE43A and ZE41) are commonly used for gear boxes of helicopters. For commercial casting alloys, the tensile yield strength is in the range 100–250 MPa and the ductility at room temperature is limited (elongation in the range 2–8%) [111,112]. The strategies for strengthening Mg alloys mainly rely on: (i) grain refinement, (ii) precipitation of second phases, and (iii) control of microstructural features on a nano-scale. The first approach is based on techniques for obtaining ultrafine grains, smaller than 1 μm [113,114,115]. The numerous grain boundaries represent obstacles for dislocation motion, thus Mg alloys can reach YS values of about 400 MPa, but strength is reduced when grain growth occurs at relatively low temperature (0.32 Tm) [114]. Another drawback is the fact that grain refining tends to suppress deformation twinning, which is an important strengthening mechanism together with dislocation slip [116,117,118,119]. The precipitation of second phases involves the composition tailoring of Mg alloys by adding elements like Al, Zn, Zr and rare earths. Moreover, the increase in Al content also remarkably improves the corrosion resistance [120,121]. As shown in Figure 8, YS and UTS of Mg alloys increase with Zn content up to 4 wt%, while for higher values they remain constant or slightly decrease [122]. The precipitation of MgxZny phases induces relevant hardening and guarantees an interesting combination of good strength and ductility. However, for the corrosion resistance of Mg–Zn based curves determined from fatigue tests carried out on the material in as-manufactured condition and after 1000 h at 600 °C, the alloy decreases as Zn content in the alloy increases [123] due to the cathodic effect of the MgxZny phases, whose volume increases with Zn. Figure 8. YS and UTS of Mg alloys vs. Zn content. Data are taken from [122]. Homma et al. [124] developed Mg–6Zn–0.2Ca–0.8Zr alloys, and found that Zr addition is beneficial to refine the grain size as well as to disperse fine and dense MgZn2 precipitates containing Ca and Zr. Mechanical properties are enhanced through the combination of texture strengthening, grain size refinement and precipitation strengthening. Mg alloys containing rare earths can exhibit high strength at both room and elevated temperatures; the effects of Y on mechanical properties have been extensively investigated by Xu et al. [125], who observed that Mg–Zn–Y phases are formed at the grain boundaries. The phases vary with Y content: when Y is 1.08 wt.%, the alloy mainly contains I-phase, whereas for higher Y content (1.97–3.08 wt.%) W-phase is also present. The alloy with 1.08 wt.% of Y has the highest strength because I-phase is closely bonded with the Mg matrix and retards the basal slip. Since W-phase easily cracks under deformation, Y contents in the range 1.97–3.08 wt.% induce the degradation of mechanical properties. An effective method to produce ultra-strong Mg alloys (YS = 575 MPa, UTS = 600 MPA) with uniform elongation (~5.2%) has been proposed by Jian et al. [126], and is based on stacking faults (SFs) with nanometric spacings, induced by hot rolling. These investigators studied a T4 treated Mg–8.5Gd–2.3Y–1.8Ag–0.4Zr (wt%) alloy which was subjected to increasing deformation up to 88% of reduction and observed that the mean distance between SFs decreases with thickness reduction. Since SFs act as barriers for dislocation motion, a higher SFs density leads to an increase in strength. In addition to grain refinement, the precipitation of second phases and the control of microstructural features on a nano-scale, non-traditional approaches have been also considered to obtain high strength in Mg alloys, for instance rapid solidification and powder metallurgy achieved YS ≅ 600 MPa in a Mg–Zn–Y alloy with uniform distribution of ordered structures [127]. However, the process involves a significant loss of ductility and is difficult to transfer from a lab scale to an industrial scale. 5. Steels Ultra-High Strength Steels (UHSS) are commonly used for manufacturing aircraft parts such as landing gears, airframes, turbine components, fasteners, shafts, springs, bolts, propeller cones and axles. Some of them exhibit very high YS values, e.g., 300M (1689 MPa), AERMET100 (1700 MPa), 4340 (2020 MPa); however, there is a tendency to progressively replace these materials by composites. The reason is related to their low specific strength and corrosion resistance. Moreover, UHSS are weakened by H atoms, which favor crack growth and micro-void formation with consequent localized deformation and failure [128]. Recently, Oxide Dispersion Strengthened (ODS) steels have attracted the attention of aeronautic industries. A lot of scientific work has been devoted to ODS ferritic steels, because they are promising candidate materials for applications in nuclear reactors [129,130]. ODS steels are strengthened through a uniform dispersion of fine (1–50 nm) oxide particles which hinder dislocation motion and inhibit recrystallization. High-temperature performances are also improved by refining the ferritic grain in combination with oxide dispersion strengthening [131,132]. Usually, ODS steels are prepared by high-energy mechanical alloying (MA) of steel powders mixed with Y2O3 particles, followed by hot isostatic pressing (HIP) or hot extrusion (HE) [133,134] and annealing at ~1100 °C for 1–2 h. A drawback of the aforesaid procedure is that the final high temperature annealing causes the equiaxed nanometric grains obtained by MA to transform into grains with a bimodal grain size distribution, involving anisotropic mechanical properties and a remarkable decrease in hardness, YS and UTS [135]. Nano-ODS steels were also produced via Spark Plasma Sintering (SPS), by exploiting the high heating rate, low sintering temperature and short isothermal time at sintering temperature [136,137]; however, the difficulty in manufacturing large mechanical parts by SPS is a clear shortcoming of the technique. Some of present authors [138,139] prepared a nano-ODS steel by means of low-energy MA without the annealing stage at high temperature, obtaining a microstructure of fine equiaxed grains. As shown in Figure 9 taken from ref. [138], ODS steel exhibits higher YS and UTS than the unreinforced one, even if the difference progressively decreases above 400 °C because dislocations can easier get free from nano-precipitates. Mechanical properties are also better up to 500 °C than those of ODS steel prepared through the conventional route. The data of a conventional ODS steel (ODS*) reported in Figure 9 are taken from [140]; L and T indicate samples taken along longitudinal and transverse direction, respectively. Since precipitate distribution is not homogeneous in ODS steel prepared by low-energy MA, YS and UTS remarkably decrease when the strengthening role played by precipitates becomes dominant (above 500 °C). Figure 9. YS (a) and UTS (b) of ODS steel prepared by low-energy MA are compared with those of the unreinforced steel (steel matrix) and of another material (ODS*), prepared through the conventional route (high-energy MA, hot extrusion at 1100 °C, annealing of 1.5 h at 1050 °C). L and T indicate samples taken along longitudinal and transverse direction, respectively. The figure is taken from [138]. 6. Ni-Based Superalloys Ni-based superalloys with a biphasic structure (γ + γ‘) are usually employed to manufacture parts of aeronautic engines such as blades and rotors operating in the highest temperature range (1100–1250 °C). Three topics of great industrial relevance will be discussed: (i) microstructural stability; (ii) manufacturing parts of complex geometry; (iii) welding of superalloys. 6.1. Microstructural Stability In order to increase the working efficiency of aero-engines, they must operate at higher temperature, thus the high temperature properties of superalloys are very important, especially the microstructural and mechanical stability. Recently, some authors have evidenced an early stage of microstructural instability in both single crystal (PWA1482) [141,142] and directionally solidified (IN792 DS) [143] Ni-based superalloys, connected to the re-arrangement of dislocation structures induced by heating to moderate temperature (~500 °C). Dislocation cells present in the precipitate free (PF) zones of the matrix (Figure 10a,b) grow to form cells of larger size; the process proceeds by steps modifying dislocation density and average distance of pinning points; finally the growth stops when cells reach a size comparable to that of the corresponding PF zone. Figure 10. PWA 1483 superalloy. Precipitate free zones (PFZ) are indicated by red circles (a). The TEM micrograph in (b) displays a network of dislocations inside a PFZ. Figure is taken from [142]. In general, the coarsening of the ordered γ‘ phase and changes in its morphology (rafting) are the most relevant phenomena leading to the degradation of mechanical performances at high temperature. As shown in Figure 11a,b, at high temperature and under an applied stress, the γ’ particles, which usually have a cuboidal shape (a), tend to coalesce, forming layers known as rafts (b). At very high temperatures (above 1050 °C), rafting takes place during the initial part (1–3%) of the creep life, while at lower temperature (~900 °C) it only completely develops during the tertiary creep. Figure 11. The typical morphology of the γ’ phase in Ni-based superalloys (a). Results changed by rafting (b). At the beginning of creep, dislocations are forced to bow in the narrow matrix channels where all the plastic strain occurs, while γ’ phase deforms elastically [144]. The progressive increase in plastic deformation in the γ phase enhances internal stresses, leading to dislocation shearing of γ’ particles during the tertiary creep. Of course, γ’ particle coarsening involves the degradation of creep properties. Refractory elements, such as Re, Ta, Ru and W, are today added to Ni-based superalloys to improve their high temperature properties [145,146,147,148]. These elements provide good creep strength because their low atomic mobility retards dislocation climb in both γ and γ’ phases. Re concentrates mostly in the γ matrix, forming nanometric atomic clusters with short-range order, which reduce rafting during creep and hinder dislocation movement. Moreover, Re promotes the precipitation of topologically close-packed phases (TCP) [149]. The partition of refractory metals between the γ and γ’ phases occurs and is dependent on their relative contents in the alloy composition. For instance, the amount of Re in the γ’ phase increases by increasing W content in the alloy. In general, these alloys have more than seven alloying elements in their composition, and the addition of further elements may strongly alter segregation profiles in casting, thus solidification has been extensively investigated, focusing the attention on the partition of elements in solid and liquid during cooling [150,151,152]. Guan et al. [151] reported that liquidus and solidus decrease by increasing Cr in Re-containing alloys, and changes of these critical lines induced by Ru, were observed by Zheng et al. [153]. The addition of B and N to superalloys containing refractory metals affects solidification defects. For instance, N has been proven to increase the micro-porosity [154], while B retards grain boundary cracking and reduces the size of carbides with consequent improvement in mechanical properties [155]. Today, grain boundary engineering (GBE) represents an interesting field of research that could contribute to the reduction of inter-crystalline damage to superalloys and, in general, to the improvement of their mechanical properties [156]. Annealing twin boundaries are very important for GBE owing to their low energy. Recently, Jin et al. [157] reported an interesting result about the correlation of the annealing twin density in Inconel 718 with grain size and annealing temperature. These investigators showed that twin density mainly depends on the original one in the growing grains, but not on the temperature at which they grow, namely no new twin boundaries form during the grain growth process. 6.2. Manufacturing Parts of Complex Geometry An aspect of relevant importance for these materials is the possibility of manufacturing aeronautic components of complex geometry. Owing to their high hardness and poor thermal conductivity, the machining of superalloys is challenging and novel techniques (e.g., see [158,159,160,161,162]) have been investigated. For example, laser drilling and electrical discharge machining are used to produce effusion cooling holes in turbines blades and nozzle guide vanes [158]. Even though the description of such novel techniques goes beyond the scope of the current paper, it is worth noting that recently there is an increasing interest of aeronautic industry in the use of Additive Manufacturing (AM) for the production of Ni-based high-temperature components. Among the different AM technologies selective laser melting (SLM) and selective electron beam melting (SEBM) are the most interesting as they enable the preparation of almost fully dense metal parts of complex shape, starting from a computer-aided design (CAD) model [163,164,165,166,167,168,169]. Components manufactured through SLM exhibit excellent mechanical properties and a strong anisotropy. The directional heat flow during the process leads to columnar grain growth with consequent crystalline texture, which especially affects creep resistance and fatigue life [170,171,172,173,174]. The (001) crystallographic direction has the lowest stiffness involving better creep resistance and longer fatigue lives, thus it is optimal for the upward direction in gas turbine blades. Recent experiments by Popovich et al. [174] on Inconel 718 demonstrated that suitable SLM process parameters and laser sources allow material anisotropy to be controlled with great design freedom (either single component texture or random oriented grains, or a combination of both of them in a specific gradient). The same approach can be also applied to design functional gradients with selected properties and/or heterogeneous composition depending on the specific application. SEBM is characterized by very high solidification rates and thermal gradients, leading to relevant microstructure refinement with primary dendrite arm spacings two orders of magnitude smaller than as-cast single crystals. Moreover, in samples of the CMSX-4 superalloy prepared with high cooling rates, Parsa et al. [175] observed a high dislocation density, indicating the presence of internal stresses which could lead to crack formation. 6.3. Welding of Superalloys Cracks may form in Ni-based superalloys during both the production process and service life under severe conditions of high temperature and stress in an extremely aggressive environment. Such defects are generally repaired through welding [176], with significant economic saving. Welding should preserve, as far as possible, the original microstructure without relevant residual stresses in the molten (MZ) and heat affected (HAZ) zones, and chemical segregation changing the composition of γ and γ’ phases. During the solidification the microstructure, the MZ is affected by dendritic growth and solute partitioning, with the consequent formation of metallic compounds such as carbides, borides etc. Another critical aspect is connected to the presence of low melting compounds which could lead to micro-cracks after post-welding heat treatments (PWHTs) [177] and local residual stresses in the MZ [178,179]. Some welding technologies are already mature, such as Transient Liquid Phase (TLP) bonding, developed by Pratt & Whitney Aircraft and based on the spread with Ni-Cr-B or Ni-Cr-B-Si fillers; Activated Diffusion Bonding (ADB) developed by General Electric with fillers of composition close to that of the reference superalloy and with the addition of B and/or B+Si; Brazing Diffusion Re-metalling (BDR) developed by SNEMECA with fillers with two components: one of a composition close to that of the alloy, and the other, in small quantities, containing elements such as B and Si which lower the melting point. The advantage of BDR is the slow isothermal solidification that makes the interdiffusion of the elements easier, and guarantees a composition of the joint similar to that of the bulk superalloy. Unfortunately, the costs of the above techniques are very high, particularly BDR. In recent years, research has been focused on high energy density welding techniques such as Laser Welding (LW) [180,181,182] and Electron Beam Welding (EBW) [183,184,185,186], which provide greater penetration depth, reduced HAZ and minimal distortion, if carried out with a high speed of passes. These techniques seem to be promising, as they represent simpler and cheaper solutions for repairing cracks in Ni-based superalloys. Thanks to a reduced thermal input, high energy density welding techniques, can realize joints with narrower seams and HAZ. By using LW and EBW techniques, the superalloy microstructure is changed at little extent, so that residual stresses, micro-cracks, porosity and other defects in the junction are limited. In addition, for each welded superalloy, the optimization of the process parameters, such as pass speed and pre-heating of the workpiece, clearly plays a crucial role (e.g., see ref. [186,187,188]). 7. Conclusions The work provides an overview of recent advances in alloys for aeronautic applications, describing current problems and perspectives. The needs of the aeronautic industry stem from the strong competition to manufacture aircrafts with improved technical features and reduced costs (i.e., extended service life, better fuel efficiency, increased payload). In this challenge, materials play a crucial role. Advanced structural materials should guarantee reduced weight, improved fatigue and wear behaviour, damage tolerance and corrosion resistance, while for the hot engine sections alloys with better creep resistance, mechanical properties at high temperature and high-temperature corrosion resistance are required. A critical analysis has been carried out on different kinds of materials including Al alloys, Ti alloys, Mg alloys, steels, Ni superalloys and metal matrix composites (MMC), emphasizing the structure–property relationships. The development of new materials involves new technologies, and some of great relevance for the aeronautic sector have been briefly examined. The attention has been focused on Refill Friction Stir Spot Welding (RFSSW) for joining structural parts made of Al alloys, high energy density techniques for welding Ni superalloys, and Additive Manufacturing (AM) for fabricating components of complex geometry. In the future, the microstructural and mechanical stability of Ni superalloys will be further investigated and improved through careful tailoring of the composition to get higher operative temperatures of aero-engines. Important advancements in structural materials are also expected. The competition with Carbon Fiber Reinforced Polymers (CFRP) will drive new efforts to improve the mechanical properties, especially fatigue strength and toughness, and corrosion resistance of Al alloys. 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