Metal additive manufacturing in aircraft: current application, opportunities and challenges

Additive Manufacturing (AM) has huge advantages in prototyping and rapid manufacturing, also potentials in integrated design, cost and weight reduction, which are suitable for applications in aircraft. In recent years, aircraft manufactures as well as aviation authorities have made significant progress in Metal Additive Manufacturing components and structures, from materials and processes, to authority approval. Also, new AM technologies are still under rigid development, many of which have even more potentials than current favorite processes like selected laser melting (SLM), wire-arcing additive manufacturing (WAAM). However, as the first and overall requirement for aircraft, security level of AM components are still in question. Stability in mechanical properties, researches in fatigue and defect and issues in AM certification are major concerns for manufactures before AM technologies are really and widely used in aircraft design and manufacturing.


Introduction
Additive Manufacturing (AM) [1], or a public-friendly name, 3D printing, is challenging traditional industries with a revolution in full production life time, from design [2], prototyping, and even now mass production and quick repairing [3][4][5].
As for aircraft manufacturers, AM can shorten developing period for a new model, reduce weight and total cost for a single plane and increase economic efficiency and ECO performance. Metal Additive Manufacturing, referring to technologies using laser [6], electron beam [7], arcing [8] for metallic nearnet-shape fabrications, are even more important for aircraft, since metals like aluminum, titanium are major components for most aircraft models.
This review shows current applications, introducing new opportunities as well as analyzing challenges for AM technologies in aircraft manufacturing industry.

Metal additive manufacturing in aircraft: current applications
Traditional metals that are widely used in aircraft is aluminium alloys as well as titanium alloys, due to their relatively low density, high resistance to fatigue and corrosion and high strength. The most popular material for AM processes is Titanium 6Al-4V alloy, which is already the most important titanium alloy in aviation industry. However, talking about metal additive manufacturing, due to the unfriendly nature of poor casting performance, aluminium alloys suitable for AM are still suffering from low strength and fatigue resistance [9][10][11]. Due to the safety regulations for aircraft, capability to fabricate full density metal structure is the priority consideration for the choice of AM processes. A lot of researches have been made on popular processes, like Selected Laser Melting (SLM), also named as Directly Metal Laser Sinter (DMLS), Electron Beam Melting (EBM), Wire-Arcing Additive Manufacturing (WAAM) are well studied, as well as commercially produced. Major machine suppliers and solution providers, like EOM, Renishaw, and GE Additive have developing a whole series of combines of different materials and machines. Some of the published mechanical properties of AMed Ti-6Al-4V are listed in Table 1. Tests in Table 1 show that additive manufactured Ti-6Al-4V specimens generally have a better tensile performance in tensile properties than minimum requirement for those through casting. In aircraft industry, additive manufacturing, especially SLM, is considered to be one of the candidates replacing casting or wrought. Some of metal AM applications in certificated aircraft are listed below.

Boeing 787 structural components provided by Norsk Titanium
In 2017, Norsk Titanium AS reported their first delivery of Federal Aviation Administration (FAA) approved structural components for installation on the Boeing 787 [21], which is also the first FAAcertified AM Ti-6Al-4V components [21]. Norsk Titanium uses a relatively new technology, the Rapid Plasma Deposition (RPD), which is believed to have better mechanical performance and less defects than other AM processes [22].

3D printing fuel nozzles in GE9X Engine
In 2017, GE started certification testing plan for its newest high-bypass turbofan aircraft engine, GE9X [24]. Designed for Boeing 777X-8/9, the world's largest jet engine has shocked the industry with its 3D printing fuel nozzles [23]. Certificated by FAA in middle of 2015, this nozzle is only a part of GE's effort in bringing AM to jet engines. Reports shows GE is also pushing AM technology to its new turboprop engine and other models[23]. APWorks had announced an aluminium-scandium alloy Scalmalloy, providing a better strength-weight ratio than titanium alloys [25,26]. APWorks is also playing the leading role in the industrial revolution of "Design for Additive Manufacturing", concerning a full innovation from design to fabrication.

SAE standardization in AM specifications.
Among international specifications, Aerospace Material Specifications (AMS) issued by SAE international are the most important ones, admit by aviation authorities. SAE has been working for years on their AMS on AM technologies, that is, AMS 700X series. In June 2018, first four specifications were issued, as well as many others developed. SAE standardization in AM specification will have a huge pushing impact towards AM applications in aerospace and aviation.
3. Metal additive manufacturing in aircraft: opportunities AM technologies are still improving, innovating and changing. There is no answer to "Which AM process is best for aircraft". There is neither no definitive conclusion on "Which component cannot be fabricated by AM after all". Researchers, aircraft manufacturer and authorities are all making effort on opportunities lies in the future of AM.
3.1. FAA additive manufacturing roadmap FAA has been the leading force in applying AM into aircraft industry. They introduce a workshop towards AM parts every year since 2015 as a joint of FAA and U.S. Air Force [27]. Recently, FAA announced an eight-year roadmap for AM is under development, covering training and education, regulations and documentations, R&D plans and interagency communication [28]. This roadmap is believed to give guidance on AM non-standardized situation and its application on aircraft. Furthermore, the roadmap will set up examples on how authorities should approve and certificate AM parts.

New technologies
Technology is improving every day. Take Norsk Titanium AS mentioned above as an example, they choose RPD rather than popular SLM or WAAM. There is no perfect process now, opportunities and possibilities are still waiting for researchers. New materials and processes, like nanoparticle reinforced pounder [29][30][31], layered metal-polymer composites [32], micro-forging [33], are under robust development, and may one day be applied on a commercial aircraft.

Trend of mass customization
Aviation is changing. In 1950s, it is common that one manufacturer only provides limited models of aircraft. But nowadays, the need for customization is becoming more and more urgent, from capacity, weight, speed, engine options of a single model, to cabin arrangement, specialized decorations, or even military and business models. The cost and time advantages for AM parts are considered to be one of the best solutions to handle mass the upcoming mass customization trend.

Metal additive manufacturing in aircraft: challenges
Although in other industries like automobile, AM technologies has been applied much widely, in aircraft manufacturing however, things are different. Security is always the first red line for aviation. AM still has a long way to go before proved to be safe enough.

Stability of mechanical properties
In aircraft design and manufacturing, stability is the first requirement on mechanical properties. A material with fair but stable strength is considered much better than one with outstanding but fluctuating strength. In Table 1 for example, it can be seen UTS performance varies at most 20% for a particular combination of material and process, which is not acceptable for any approved part. Also, the anisotropy nature of AM decides the fluctuations of mechanical performance in different directions. Further works must be done before the stability of the mechanical properties of AM part fit the requirement of aviation authorities.

Studies on defects and fatigue
There is no AM technology that can promise a 100% building. Defects like porosity, voids, cracks and inclusions are still troubling the industry, limiting some crucial performances like fatigue. Though there have been a lot researching works on defects [34], fatigue [11,35] and their relationships [16,36], there is still no conclusion on inspection methods or acceptance standards of defected parts. Fatigue performance is the key item for a process to be applied on an aircraft, since it directly limits the flight life and possibility of failure for a single part.

Approval and certification
Any part that is installed on a commercial aircraft need to be approved and certificated by authorities like FAA, EASA. However, to get certificated, especially for the applications on bearing components or main structure of an aircraft, AM process needs much more testing items other than tensile and fatigue, like compress, rupture, shear, bearing, impact and creep, and all testing items need to fulfil number requirement to setup aircraft design allowable [37]. Anisotropic issue for AM also decides tests should be taken according to different directions, making the certification expense 2-3 times larger than those for traditional processes.

Paradox between safety margin and weight reduction
It is generally believed that mechanical properties for tested specimens do not always represent the performance of real components and structures. Process details for mass production cannot stay the same as those during specimen fabrication. To make things worse, mechanical properties of AM parts also varies according to dimensions of design and building scenario [38]. To promise the safety of components and structures, sufficient safety margin must be held beyond specification limits, especially for new technologies like AM. Thought these margins are necessary and part of requirements by aviation regulations, they do add up to total weight of AM parts, reducing their potential in weight reduction. Further works need to be done to reduce minimum margin before fully liberate possibilities for AM materials and processes.

Conclusion
Metal additive manufacturing has been considered a new solution in the aim of weight, cost and time reductions for aircraft manufacturing industry. Main manufacturers like Boeing and Airbus, as well as main suppliers like GE aviation have made great progress according to their own plans; authorities like FAA have also put much efforts to bring the whole AM area into the world of aviation. International standardization organizations like SAE has been issuing aerospace specifications for AM materials and processes. With new technologies improved, AM is expected to anticipate much more in aircraft manufacturing. However, much more works is still needed to be done to improve production stability and understanding of every aspect of mechanical properties required by aircraft components before certificated by authorities. Future for AM is bright, yet the possibility is still beyond our imagination.