A review on additive manufacturing for aerospace application

Additive manufacturing, a cutting-edge technology often colloquially known as 3D printing, is a transformative process used to meticulously fabricate complex components by adding material layer upon layer. This revolutionary manufacturing method allows for precise control and customization, making it a go-to choice in various industries, from aerospace to healthcare. The adroitness of additive manufacturing in creating a complex geometry as a whole is very much harnessed by the aerospace Industry. Generating a component using additive manufacturing involves optimal design, methods, and processes. This review gives a broad knowledge in developing a part or product by choosing the appropriate design, method, and processes. The end-to-end flow process (from scratch to finished model) for developing a component by additive manufacturing is described with a detailed flow diagram. The flow process proposed in this review will act as a primary source for manufacturing any component as per the industry standards. Also, the role of additive manufacturing in the aerospace industry is the need of the hour and greatly in demand of innovative ideas. But as an infant technology, AM for aerospace has its fair share of issues The paper discusses issues and challenges of AM for aerospace applications to enable the widespread adoption of additively manufactured components in the aerospace industry.


Introduction
3D printing, also known as Additive Manufacturing (AM) [1] , is rapidly growing in recent decades and its applications in the aerospace industry are drawing more attention in developing critical parts [2][3][4].The aerospace industry, characterized by its relentless pursuit of innovation and precision, has witnessed a transformative shift with the advent of Additive Manufacturing (AM).This revolutionary technology, often referred to as 3D printing, has emerged as a cornerstone in the realm of aerospace applications.By allowing the creation of intricate, customized, and high-performance components, AM is redefining the way aircraft and spacecraft are designed, manufactured, and operated.AM is an attractive tool for the aerospace industry in building parts or components from expensive materials [5] at minimal cost compared with the conventional manufacturing process with the ability to fabricate parts with unbounded geometric freedom [6].Additive manufacturing is limited to prototyping, and the success can be improved by developing relevant software, embedding the hardware, and knowledge source [7].Apart from manufacturing critical parts, the maintenance time and cost can be saved through the factors such as prototyping, tooling, fixtures, jigs, part repair, and spare part production [8][9][10].The challenges faced by the aerospace industry in AM include limited materials, high cost, and inconsistent quality of printed parts [11].When parts are manufactured through AM for the aerospace industry, quality check [12][13][14][15] is an important factor to be considered during the AM process.All the parts of aircraft need to undergo qualification and certification processes by fulfilling the quality and safety goals [16,17].The qualification, certification, and quality control termed as Q&C [18] is an essential tool that must be considered for producing parts by additive manufacturing for the aerospace industry.However, the implementation of AM in aerospace is not without its challenges.Material selection, quality assurance, regulatory compliance, and the need for highly skilled personnel are among the intricacies that must be navigated.Furthermore, safety and reliability remain paramount in an industry where the consequences of failure can be catastrophic.Aerospace-grade materials, advanced processes, and rigorous quality control procedures are essential components in addressing these concerns.Overall the need for AM for aerospace applications consist of the following main phases.

Design, methods, and processes
Additive manufacturing (AM), often referred to as 3D printing, has become a transformative technology in the aerospace industry.It has revolutionized the way aerospace components and structures are designed, produced, and maintained.AM methods, including powder bed fusion (e.g., Selective Laser Sintering and Selective Laser Melting), material extrusion (e.g., Fused Deposition Modeling), and directed energy deposition, offer aerospace engineers the ability to create complex, lightweight, and customized parts that were previously impossible or impractical to manufacture using traditional methods.This technology has found extensive use in producing engine components like turbine blades and combustion chambers, airframe structures, landing gear, interiors, propulsion systems, and even spacecraft components.The aerospace sector has benefited from the weight reduction achieved through topology optimization and generative design, resulting in improved fuel efficiency and performance.While AM presents immense opportunities, it also poses challenges, such as material certification, process control, and post-processing, all of which need to be addressed for wider adoption.Nonetheless, with ongoing research and development, additive manufacturing continues to reshape the aerospace industry, offering innovative solutions to complex engineering challenges and pushing the boundaries of what is possible in the world of aviation and space exploration.The additive manufacturing procedure is split into three separate sub-phases like Design, Method, and Processes in sequential order.Each phase is linked to one another with the various individual processes involved in it.

Design of AM
The design sub-phase comprises an initial stage and till the selection of material.The design phase includes identifying the part, collecting required data, creating the 3D model, converting the model to STL format, and material selection.Wiberg et al proposed a new design process for additive manufacturing which has got three connected activities (system, part, and process) [19].In the first step of the design phase, the part to be manufactured is identified (new design or existing design).For an existing design, repair and replacement of the particular part will be sufficient [20].Once the part is identified, collecting all the design data will be the next step.Design data includes all the information of the part such as size, shape, dimension, tolerance, and other necessary information.The required new design can be modeled using CAD software or 3D scan technology can be used for reworking the existing model [21].In case the model requires weight reduction, the material removal process can be done using the topology optimization method.The topology optimization process is applied when the component requires weight reduction [22].Integrating topology optimization with additive manufacturing helps to produce lightweight structures with high performance [23][24][25].TO is the process of minimizing the volume of a component in a given space with an improvement in its mechanical response [26].TO is done using finite element analysis software (FEA) [27,28] and for aerospace parts, factors such as aerodynamics, fatigue, and thermal loads must be considered during the design phase [29] without affecting the FOS [30].The flywheel designed for earth observation satellite is optimized obtaining 16% high energy density with lesser mass [31].To minimize the material volume, turbine blades of the T106C and C3X models are done using a topology optimization module [32].The heavy-loaded aerospace bracket design is optimized by reducing 18% of its structural mass [33].Topology optimization is a versatile and valuable tool in the aerospace industry, enabling the creation of lightweight, high-performance, and customized components.By optimizing material distribution, aerospace products become more efficient, environmentally friendly, and economically viable, making topology optimization a cornerstone of innovation in aerospace engineering.The aerospace components optimized using TO are listed in table 1.

Methods
The second sub-phase of AM is the selection of methods.The methods involved in additive manufacturing are categorized based on the raw materials such as Liquid, solid sheet, powder, and filament/paste [34].The selection of a 3D printer machine depends on the material to be fabricated in it [35].Based on binder agent and heat source the metal AM is classified into different types as represented in figure 1 [36].
For aerospace application, the additive manufacturing technique is classified based on metal and non-metal parts that helps in the reduction of cost and lead time [37,38].The metal parts for Direct metal parts fabrication, rapid tooling, and metal repair parts can be manufactured using Direct energy deposition (DED) and Powder Bed Fusion (PBF) methods.The metal materials such as titanium alloy, nickel-based superalloy and other materials which are difficult to machine can be additively manufactured using the metal AM technique [39].Direct metal laser melting (DMLM) is an accepted production solution for manufacturing parts of ceramic, polymeric, and metal materials [40].The non-metallic parts for rapid prototyping, fixture, and interiors can be manufactured using Stereolithography (SLA), Fused deposition melting (FDM), and 3D printing (3DP) [38].Among the most common methods used for manufacturing aerospace components are stereolithography (SLA), fused deposition modeling (FDM), direct energy deposition (DED), and powder bed fusion (PBF) as shown in figures 2-5.

Stereolithography
Stereolithography is a type of Vat polymerization technique that solidifies photoresist material using a light source [41].The process of 3D printing a part using the stereolithography method was first patented by Charles W Hull in the year 1984 [42].Stereolithography (SLA) is a 3D printing method to create a part using a UV laser beam as the source.The laser beam solidifies the liquid material depositing layer by layer process to attain the  complete part.Using Stereolithography only materials like resins and plastics can be processed whereas metals cannot be processed [43].The highly complex EAC-1A lunar simulant structures are fabricated using lithography-based ceramic manufacturing (LCM) , a vat polymerization technology [44].

Fused deposition modeling (FDM)
Fused Deposition Modeling (FDM) is one of the most widely used and accessible additive manufacturing techniques.The FDM involves the layer-by-layer deposition of a thermoplastic material to create threedimensional objects.The complex geometries made of composite materials [45] are manufactured using the Fused deposition modeling method [46].In FDM the filament is fed onto the print bed through the heated nozzle [47].A scaled 3D prototype of exemplary rocket equipment was produced using the FDM method [9].The FDM additive manufacturing technique reduces the tooling cost and lead time compared to traditional CNC machined parts [48].FDM is versatile in terms of the materials it can handle, accommodating a variety of thermoplastics, engineering-grade polymers, and even composite materials with enhanced properties.Its costefficiency is another compelling advantage, as FDM is often more affordable compared to other 3D printing methods, making it an attractive choice for rapid prototyping and the production of functional parts.Despite its advantages, FDM has limitations, including layer lines on the printed object's surface and limitations in achieving extremely fine details or high-resolution prints compared to other 3D printing methods.However, its accessibility, versatility, and cost-effectiveness have solidified its place as a prominent technology in the world of additive manufacturing.

Direct energy deposition (DED)
Directed Energy Deposition (DED) method is used for both the repair of high-value components in the aerospace industry as well as freeform fabrication of large metallic components [49].Direct energy deposition using lasers as the source is the best process to repair aerospace components.In this process, the metal powder is directly fed onto the damaged portion of the part and laser cured, restoring the original strength of the part [50].The Wire and Arc Additive Manufacturing (WAAM) typically used in the modern aerospace industry [51,52] is a direct energy deposition process (DED) [53].WAAM uses an electric arc as a heat source and wire as feedstock material [54].Application of magnetic field during the cold metal transfer of WAAM process improves the mechanical properties (hardness, yield strength ultimate strength, and elongation) of Inconel 625 alloy [55].Two dissimilar metals TA15 and IN718 are joined using Nb/Cu bilayer by using laser additive manufacturing technique thereby avoiding the macroscopic cracking occurring between them [56].The tool wear mechanism developed during the direct energy deposition process of IN718 material is adhesive, abrasive, and chipping [57].Apart from manufacturing AM parts, also repair of aircraft components is done using the DED method [58].

Powder bed fusion (PBF)
The conventional method employed in powder bed additive manufacturing systems includes several prominent techniques, notably Selective Laser Sintering (SLS), Selective Laser Melting (SLM), and Electron Beam Melting (EBM).These techniques are instrumental in the layer-by-layer construction of three-dimensional objects and have widespread applications in various industries, including aerospace, where they are used to produce intricate, high-strength components with precision and reliability.The energy source such as laser beam, electron beam, and plasma arc is used to selectively fuse the grains of the powder in powder bed technologies [59].An image-based measurement procedure is developed to quantify the characteristics of the powder spread layer [60].Laser powder bed fusion (L-PBF), also known as Selective Laser Melting (SLM) or Direct Metal Laser Sintering (DMLS) is the commonly used metal AM technique [61].Selective laser melting is an additive manufacturing technique where the powdered metal is melted and fused by using the laser beam to create the part layer by layer process.In the selective laser melting technique, the various thermomechanical response is achieved by varying the parameters like laser power, hatch space, and scanning speed [62].The DMLS is known for its ability to produce highly intricate and complex metal parts with excellent mechanical properties.It's widely used in various industries, including aerospace, automotive, and healthcare, where precision and highstrength components are required.The 3D printing process allows for the creation of parts with complex geometries and internal structures that would be difficult or impossible to achieve using traditional manufacturing methods.The thermo-elastic topology optimization is insight implemented to design an all movable rudder structure.The optimized model is then manufactured using the selective laser melting technique [63].A multifunctional panel is manufactured using lattice material for the aerospace industry using the SLM technique [64].A resistojet design is manufactured by using the Selective laser melting technique [65].
In metal additive manufacturing among other methods, SLM has got a low-quality finish and requires a postprocessing tool for good surface finishing [66].Compared to Fused Deposition Modeling (FDM), the Selective Laser Sintering (SLS) technique gives a better surface finish with no leaks when tested [67].In the selective laser method, the laser source selectively heats and fuses the particles [68].The critical challenge in PBF is the removal of excess material powder in the post processing processes [70].Choosing the appropriate method for manufacturing a component is a challenging process as each method has its own merits and demerits.This process of selection involves considering all the parameters like material, cost, time, and source of energy.These techniques offer various advantages in terms of design flexibility, reduced material waste, and the ability to create complex geometries, making them particularly valuable in aerospace applications where lightweight, highperformance, and customized components are essential.The choice of which method to use depends on the specific application and the material requirements.

Processes
The processes is the AM sub-phase where the entire manufacturing process is done.The process involved in AM is categorized into three steps, 1. Pre-processing, 2. Processing and, 3. Post-processing.Pre-processing is the initial step where the input parameters for the process to be processed is set up.Pre-processing step includes layer slicing, support structures, deposition size, and feed rate.To develop certified parts for aerospace applications, the process variables such as laser power, scan velocity, preheating temperatures, and scan strategy are to be monitored [69].High-intensity ultrasound is used to control the process of solidification of 316L stainless steel material [70].Varying the AM processing parameters such as toolpath angle and extrusion speed will change the properties of AM materials [71].For attaining stable deposition of material, the process parameters like laser power, traverse speed, and wire feed should be taken care of [72].To process reactive materials like titanium, the wire feed DED beam technique is suitable [73].The support structures are the important parameter in holding the overhanging surface of the part in position.Having minimum support structures [74] will thereby reduce the material wastage and also makes the process easy when removing the support during post-processing.The amount of support material is calculated by rotating the part 3600 with 100 intervals and minimum support generation for orientation is chosen for processing [75].Once the input variables are set up, the 3D printing process is done using any of the suitable AM methods selected.The AM parts have to undergo an extensive post-processing process to ensure whether the surface and dimensional requirements together with their respective mechanical properties are met [36].The aerospace component is prolonged to have too many surface defects in as-built conditions [76].Hence it is necessary for a postprocessing process Like the surface finishing technique.For improved fatigue life, the as-built part is postprocessed using surface finish methods such as centrifugal finishing, linishing, shot peening, or laser shock peening [77].The interlayer hammering process is performed on WAAM processed 2319 aluminium alloy to improve its mechanical properties [78].

Materials
The process involved in the selection of appropriate material for an aerospace application involves specified standards such as ASTM, ASM, SAE, and ISO [79,18].The materials used in the aerospace industry are expensive and hence AM process helps in manufacturing parts with minimal waste material compared to the conventional methods [80][81][82].Metal additive manufacturing is seeking more attention in the aerospace industry for its weight reduction technique [83].The metal-based AM can build parts with high precision and density compared with other AM processes [84].The bracket connector for Airbus A350 XWB is manufactured using laser AM of titanium material [85].Ti-6Al-4V and Inconel 718 are the most common alloy material used in the aerospace industry [86,87].The three additively manufactured aerospace materials are Ti-6Al-4V, 316L stainless steel, and AerMet 100 steel [88].Among other alloys, Ti-6Al-4V is the most used alloy in the aerospace industry for its high strength to weight ratio [89][90][91][92][93]. Titanium alloy is the best choice for manufacturing components in the aerospace industry [94].Though Ti alloys help to reduce manufacturing cost and time besides it suffers from low-temperature ductility issues [95].NiTiHf alloys are the most known shape memory alloy used in biomedical and aerospace applications [62].the largest 3D printed component (TrentXWB) made of nickel is additively manufactured by Rolls-Royce via the electron beam method.The 17-4 PH Stainless steel metal powder (aka Laser Form ® 17-4PH) is extensively used in the aerospace, petrochemical, and chemical industry for its corrosion resistance and good mechanical properties [96].The low boiling point elements like zinc alloy are manufactured by using a wire Laser Arc Additive Manufacture (WLAAM) system [53].The composite material (Long Fibre Reinforced Nylon) are widely used in aerospace, automobile, and marine application for its high strength to density characteristics [97].The composite material is attracted towards aerospace applications for its thermal, mechanical, and environmental properties [98].In the aerospace industry, composite materials are used in rocket engine castings, radomes, stabilizers, antenna dishes, and pressure bulkheads [99].Ceramics acts as an insulator material that attracts attention in the additive industry [100].The ceramic-based materials of complex geometries can be manufactured using additive manufacturing technology thereby reducing the product cost and fabrication time [101].A combination of ceramics (reinforcement phase) and metals (binder phase) called cermets is an engineered composite material that has a high hardness and toughness factor [102].Printing advanced materials like grapheme and carbon fiber will save fabrication costs and time in the aerospace industry [103].Safran Aircraft manufactures have already produced over 500 experimental parts using AM technique [104].Boeing is extensively printing a large number of aircraft parts using AM [105].Yet swapping more components in aircraft by AM, further research exertion needs to be executed on different materials.
Materials like Inconel 625, Haynes 230, JBK-75, and NASA HR-1 are commonly used in the aerospace industry for Additive Manufacturing (AM) due to their high strength, corrosion resistance, and heat resistance properties [106].The materials for AM in aerospace industry is listed in the table 2.

Testing, standards, and certification
Standards and certification are integral components of additive manufacturing (AM), particularly in high-stakes industries like aerospace.The development of industry-specific standards by organizations such as ASTM International and ISO ensures that the processes and materials used in AM meet stringent quality and safety criteria.Material certification involves rigorous testing and validation, assuring that raw materials are of the highest quality.Process standards define the parameters for machine calibration, printing, and post-processing, ensuring consistency and reliability.Quality control standards, along with non-destructive testing methods, such as x-ray and ultrasound, allow for the comprehensive inspection of components, critical in the aerospace industry for ensuring safety.Once the manufacturing process is done, the finished part is tested for its quality.The defects that occurred can be due to powder agglomeration, balling, porosity, cracks, thermal/internal stress, poor surface finish, and chemical degradation and oxidation which could affect the quality, mechanical properties, and safety of the component [112][113][114].The sample can be tested by an experimental method to study the material behavior.The experimental testing methods of tensile, fatigue and three-point bend test are done on Long Fibre Reinforced Nylon a composite material [97].The samples with high interpass temperature exposure for a longer time resulted in less porosity [115].The pores formed during AM can be eliminated by hot Isostatic Pressing (HIP) [116].The porosity evaluation of components manufactured using SLM technology is examined using the non-destructive method (CT machine METROTOM 1500) [117].Different NDT techniques will be required for detecting the flaws in various locations of a component [118].To predict the fatigue life of additive manufactured aerospace alloy (Z Zhan and H Li) proposed a combined model of continuum damage mechanics (CDM) and machine learning (ML) theory [119].For flaw detection, both surface morphology and flaw morphology are evaluated [120].To predict the failure behaviour, the additively manufactured composite material is analysed using MSC PATRAN/Nastran FEM software [46].

Training and education
The demand for AM in aerospace industry is widely growing for its demand for manufacturing light weight materials with high strength [122].Lightweight engine parts are produced using AM in the aerospace and defense industry [121].Manufacturing jet engine turbine blades gained popularity for their manufacturing possibilities in lesser time and cost [118].The turbine blades over frequent processes are subjected to failure mechanisms such as fatigue, creep, corrosion, and erosion factors [122].The usual maintenance work of gas turbines includes broken and worn-out turbine blades [123].The repair of high-value parts like turbine blades results in an economic incentive [124].Hence the research on turbine blade repair and replacement can be further focused on improving the mechanical properties.There is always a high demand for producing spare parts for the aerospace sector [125].Producing or replacing parts using additive manufacturing techniques requires proficient design skills with appropriate training [126].As there is a lag in supervision and employing AM in industry, add-on training in education perhaps will support [127].The education aspect can be enhanced by implementing the additive manufacturing training program in the curriculum.Training session for students must involves handling AM technology with proper health and safety measures [130].Creating AM tutorials supports engineering graduates to study more effectively.This process of training graduates can be achieved by using both online and offline modes [128].

Conclusion
3D printing or additive manufacturing technologies is a major industrial revolution [129,130] in the aircraft industry.The successful integration of Additive Manufacturing (AM) in aerospace applications unfolds through a series of distinct phases, each instrumental in harnessing the technology's full potential.The phases of additive manufactured aerospace components is listed in the table 3. The first phase of the need for additive manufacturing for aerospace applications is divided into three sub-phases which consist of design, methods, and processes.Each sub-phase of the section is conferred in a separate section.This review gives a method of executing the additive manufacturing process in a defined way.The parameters involved during the entire process of additive manufacturing are discussed.The application of additively manufacturing in the aerospace industry is listed with the parameters mentioned.The aerospace component manufactured additively based on the design phase proposed is given in the tabular form.More attention is needed on the turbine blade for its requirement in additively manufacturing/repairing in lesser time and cost.The future exertion can be focused on the following domains.Though additive manufacturing gives geometrical design freedom, the factors which cause fatigue and fracture strength have to be investigated [131].Also, the parts produced by different AM methods have their dimensional limitations [132].The features of large and complex parts made of titanium alloy Ti-6Al-4V are difficult to access and remain a major challenge for surface finishing and metrology operations [133].For manufacturing complex and overhanging geometries, more support structures are necessary for AM.Hence minimizing the support structures and choosing the optimized number of the support structure is still a tedious process.For the aerospace industry, the products produced by AM are used as loadcarrying members [88].This might lead to the propagation of crack growth which further results in structural damage.Hence quality and certification [69] is an important aspect to be considered for every design phase.The use of industrial robots in the post-processing process of AM is successfully evaluated by removing the support structures in different shaped objects.The process can still be improved by minimizing the time required during the path planning particularly in the collision check routine [134].By developing or modifying the AM machines will support the fabrication of new materials [135].The use of locally mined minerals and chemical components, as well as recycled materials, is suggested for 3D printing in lunar exploration [136].Various AM methods, including Powder Bed Fusion (PBF) techniques like Selective Laser Sintering (SLS), Selective Laser Melting (SLM), and Electron Beam Melting (EBM), are instrumental in creating highly complex and customized aerospace parts.These methods enable the selective fusion of metal or polymer powders to produce intricate, high-strength, and lightweight components.Materials play a pivotal role, with aerospace-grade metals, polymers, composites, and ceramics employed based on their unique properties, such as strength, heat resistance, and weight considerations.The AM process involves initial design and CAD modeling, followed by precise layer-by-layer printing.Post-processing steps, including heat treatment, machining, and quality control methods, ensure that aerospace components meet the industry's stringent quality and safety standards.In essence, AM methods, materials, and processes in aerospace applications have revolutionized component manufacturing, offering an innovative approach to create intricate, efficient, and tailor-made parts for the aerospace industry.Overall this review gives an idea of performing the AM process in an appropriate way of selecting individual parameters throughout the manufacturing process for aerospace applications.
i. Design, Methods, and Processes ii.Materials iii.Testing, Standards and Certification iv.Training and Education

Table 1 .
Topology optimization of aerospace components.

Table 2 .
Additively manufactured components in aerospace application.

Table 3 .
The phases of additively manufactured aerospace component.