by AM technology and its expertise that is post-processed to augment geometry, motorized properties and the exhibition of obligatory enactment characteristics in service of the component (Dhinakaran 2019). The contemporary expansion emphasis of AM is to yield multifaceted shaped serviceable metallic components, comprising metals, and non-metals such as plastics, metal alloys and other composites, to meet the challenging requirements of aerospace, automotive, defense and biomedical engineering.
1.4.1. Titanium and its alloys
The most essential progressive material that acts as a key to improving the achievements in spacecraft systems is titanium and its alloys. Tremendous amalgamations of definite mechanical possessions controlled by density result in exceptional corrosion resistance behavior. The assortment of the precise AM technology, along with appropriate design optimization, leads to substantial investments through significantly condensed buy-to-fly ratios and global reduction of weight, as well as reduces scrap. AM technology is growing towards the fabrication of entire functional parts using titanium and their alloys. The facility to construct hollow and curved shapes with better quality resolution adds topographies on existing parts and makes it possible to remanufacture or patch-up damaged parts, besides building parts precisely from the CAD (input) data. Ti-6Al-4V, known as the “work horse” of titanium, is considered as an active alloy among the (α+β) alloys, and it is the widely used titanium alloy. The examination of Ti-6Al-4V reveals its sensible properties such as low ductility and density with virtuous corrosion and oxidation resistance, and hence it is used in high operating temperatures and high stress apparatuses (Imam 2016). The microstructure and enumerated tomography ensures that the hot isostatic process leads to a standardized microstructure and reduces the absorbency of titanium alloy parts. Ti-3AI-2S-V has a momentous application of constructing hydraulic tubing in the aerospace industry. It is used for high pressure hydraulic outlines with working pressures up to 28 MPa. It is also used in the hardened state for cryogenic applications as it preserves better fracture toughness and ductility even at cryogenic temperatures. The Ti-10V-2Fe-3Al alloy has tremendous hardenability, high tensile strength and high fatigue strength. It is primarily used for manufacturing landing gears. Titanium alloys are widely used in aircraft industries, and each suitable alloy is designated according to its applications (John 2016). Commercially pure titanium is used in airframes where formability is measured as an essential factor. It is also used in engine components where heat resistance and strength are crucial.
1.4.2. Inconel
Inconel is one of the traits of an austenitic nickel-chromium-based super alloy. The alloys of Inconel are well known for their appropriate services in extreme locations exposed at high pressure and heat due to their oxidation–corrosion-resistant properties. When heated, Inconel forms an abundant, unchanging, protective oxide layer, which protects the external surface from further attack. The widespread temperature range of Inconel maintains its strength, which is found to be adaptable to high-temperature components of high thermal valency. In the aeronautical field, a variety of aircraft mechanisms and engines for jets, such as lock panels, exhaust linings and blades for turbine stamps, are made of Inconel 600 alloy. It is an accustomed engineering material for solicitations, providing protection from corrosion and heat. It has admirable mechanical properties such as high strength and better workability (Wei 2019). Inconel-713C has been widely used as a super alloy due to its specialized characteristics consisting of intrinsic capability of casting, stabilizing and higher level of strength, and it also acts as a ductile material at increasing temperatures, which are used for blading jet engines. The additional grouping of Inconel is the Inconel 718 which accounts for an up to 50% reduction of the mass of turbojet engines involved in aircrafts.
1.4.3. Aluminum
Metallurgy has a vital role in the enlargement of aviation. Aluminum plays a major role in aircraft construction due to its excellent strength-to-weight ratio and cost ratio. It is especially suitable for aircraft manufacture because it is considered as one-third the weight of steel, allowing an aircraft to import maximum weight or renovate the efficiency of fuel consumption. Aluminum alloys have remarkable quality for use in the fuselage, wing and the erection of commercial airliners, military cargo and aircraft. The fabricated and fashioned aluminum are used in structural components of aircraft for the United States Navy. Aluminum can be used as a crucial propellant for motors of space shuttle as its volumetric energy density is great and very hard to burn. The strength of aluminum enables it to substitute heavy metals without loss of strength along supplementary metals (Aboulkhair 2019). Moreover, load-carrying assemblies can yield the benefit of the strength of aluminum to promote aircraft fabrication more consistent with reduced cost. Aluminum is extremely resistant to corrosion as well as to chemical reactions; hence, it is unambiguously suitable for aircraft operations in extremely corrosive environments. The 6063 aluminum is known for promoting prototypical quality attributes, and it is the most suitable alloy for anodizing solicitations. The recipient choice for aerospace applications is Al-7050, as it preserves its strength and properties in wider segments and provides protection against fractures and corrosion.
1.4.4. Stainless steel
Stainless steel or inox steel (steel alloy) comprises 11% of the mass of chromium and an extreme 1.2% of the mass of carbon. It encompasses unpredictable amounts of carbon, silicon, manganese and some additional metallic materials such as nickel and molybdenum that are added to inculcate other expedient properties such as improved formability and resistance to corrosion. Its excellent corrosion-resistant property and tolerance to withstand high temperature in fabricating spacecraft components lead to longer durability of the component. Its high tensile strength and shear modulus make it more suitable for absorbing the impact stresses that landing gear equipment should endure (Martina 2019). The most abundantly recycled stainless steel worldwide is T-304, which can be weldable and machinable, and has better resistance to many chemical corrodents as well as industrial atmospheres and has well-known formability. Its austenitic structure offers excellent ductility and formability, which condenses work-hardening tendency and enhances suitability for deep drawing.
1.5. Aerospace applications of additive manufacturing
Over the last five years, AM has expanded from tooling and prototyping to explicit part production, especially to enhance industrialized fields such as aerospace, architecture, automotive and medical science. AM stands as a global technology to bring rapid developments in designing and modeling. AM technology is economically viable and provides the complete potential to process individualized products with complex geometries. Its flexibility to use any kind of raw material such as both metals and non-metals provides better replacement of materials, thereby reducing the cost of production. AM contributes its foremost capability to serving the aerospace industry, because this industry faced a great challenge to reduce the weight and size of components in order to avoid dangers and promote safety and durability. Designing and manufacturing flight-worthy components from high-performance materials are needed for commercial and military aerospace platforms, which can be achieved by AM. 3D printing (3DP) offers multifaceted, amalgamated parts with extraordinary strength. The involvement of less material and consolidated designs results in the overall reduction in weight, which acts as the most important factor in manufacturing for aerospace (Russell 2019). AM is often used for aerodynamic analysis, which has acute prominence for aerospace. The selective laser melting and binder jetting are used to create strong-featured, plane scale prototypes of aerospace designs to yield exact models that allow design indentation and feasibility. 3DP is generally used to produce structural, low-volume metal supports (with DMSL/SLM), which have intricate lifesaving systems, for the internal barricade of a plane. The extremely appropriate technique of prototyping is also ultimate for aerodynamic test results and analysis, as the stable surface finish factor is able to meet the appropriate final part through the 3DP technology. AM has an extraordinary degree of design freedom that assists in optimizing the parts and incorporating multiple features in a single component (Kamal 2019). AM greatly supports the aerospace industry as it is mainly focused
