1 Introduction

One of the emerging areas of manufacturing technology that promises great benefits for the aerospace industry is wire arc additive manufacturing (WAAM) [1,2,3,4,5,6,7]. Like traditional arc welding but with the intention of building up complex metal parts in layers, this process has become more refined and widely used in recent years. In addition to the significant design freedom and wide variety of materials provided by all additive manufacturing (AM) processes, WAAM offers the ability to deposit a large volume of material very quickly. Over the next couple of decades, the aerospace sector is expected to need about 20 million tons of raw materials, including many materials that have a high buy-to-fly (BTF) ratio such as aluminum, nickel, and titanium alloys where BTF is described as the ratio between the mass of raw material to the mass of the final product, where a low number indicates a large amount of wasted material in the processing [8,9,10,11,12,13,14,15,16]. Much of this material is wasted during traditional processing, driving the high BTF ratio commonly seen. Processing such a volume of material efficiently and sustainably will come with many challenges, many of which WAAM promises to alleviate due to its low waste and energy requirements, processing automation, and its fast material deposition rates.

Fig. 1
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Process diagram for WAAM [23] reproduced under the terms of a CC-BY license

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Fig. 2
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Flowchart of WAAM process

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From a manufacturing perspective, WAAM (Fig. 1) is a hybrid AM process combining traditional AM concepts and robotic control with arc welding, allowing selective deposition of the material at a fast rate and relatively good material properties [4,5,6,7, 9, 17,18,19]. The raw feedstock is a wire or other metal filament, while the fusion between layers is accomplished via an electric arc which melts and fuses the deposited material in a scanning pattern. Therefore, WAAM is considered a scanning-type AM process, producing anisotropic properties within each layer as well as between the layers. Shielding gas (most commonly argon or helium) is used to protect the melt pool and prevent oxidation in a similar way as traditional arc welding processes. Using helium as a shielding gas can significantly affect the cost of WAAM; while it offers benefits such as improved quality and productivity, it is more expensive in some localities than other gases such as argon or carbon dioxide. Therefore, the decision to use helium should be made after a thorough cost-benefit analysis. This assists with fusion between layers and prevents scale or oxides from producing poor fusion and adhesion between the layers [15, 19,20,21,22].

A complete WAAM process consists of three stages in total, as demonstrated in Fig. 2. First is software-based planning of target components, followed by WAAM-based deposition of metallic materials, and finally the subsequent post-treatment of the deposited components, including post-heat or post-mechanical treatment, to obtain intended mechanical properties and microstructure and dissipate residual stress [10, 24,25,26]. Due to different technical requirements and different levels of production, many different WAAM system designs have been developed, each tailored to a specific application domain. The most common variations are those based on gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW), often colloquially known as TIG and MIG welding, respectively. Other variations exist that are based on cold metal transfer (CMT) and tandem gas metal arc welding (TCMAW) (both variations of GMAW) and plasma arc welding (PAW) [3, 11, 12, 27, 28]. Table 1 shows the common types WAAM systems observed in the literature, with details about their heat sources and process characteristics. Figure 3 shows process diagrams for the major WAAM process types.

Table 1 Comparison of several types of WAAM processes
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Fig. 3
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Basic WAAM process types: aGMAW-based WAAM, b GTAW-based WAAM, and PAW-based WAAM [29] reproduced under the terms of the CC-BY license

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For WAAM, the cost of materials and investments is lower than other additive manufacturing technologies [30]. The welding wire is used as feedstock in WAAM, which offers a wide choice of materials and allows for speedier manufacturing because of its rapid deposition rate. This method can produce deposition rates of 50–130 g/min, which is far more than any other existing metal AM process [29,30,31]. It produces near-net-shape products, resulting in less machining and less waste that would be seen with parts made using casting or other common processes and helping to produce very large components [23, 29, 31]. The resulting material properties are also generally better than those seen with other AM processes such as selective laser melting (SLM), electron beam melting (EBM) and direct metal eposition (DMD) [10, 13, 27, 29].

However, there are some potential issues that must be mitigated or at least accounted for during the use of WAAM. Since it is a scanning-type AM process, it is more likely to have defects, pores, and inclusions. In addition, the use of an electric arc to complete the fusion opens up the possibility of introducing carbon or slag into the welds, as well as the typical welding concerns such as residual stresses and distortion. One of the main techniques for preventing these issues when using WAAM is to use a construction strategy known as symmetrical building. This usually involves depositing material on both sides of the build plate, helping to balance the residual stresses. Once the build is started, the geometry is built from the inside toward the boundaries to prevent material defects and gaps. Once the part is completed, it can be post-processed to reduce the anisotropy and remove any remaining destructive residual stresses [4,5,6,7, 14, 15, 29, 32,33,34].

This article reviews the current state-of-the-art and progress made in applying WAAM to the aerospace industry to solve some of the many manufacturing problems in that sector. Of particular interest during this review were the modeling and simulation approaches, major application areas, WAAM materials, and comparisons with other AM processes employed by the aerospace industry. This material collected during this review is presented in several sections, beginning with Sect. 2 where the research questions and outline of the review is shown. Section 3 explored application sectors and problems that WAAM can help solve, followed by a discussion of modeling and simulation work in Sect. 4. From here, Sect. 5 discusses progress on developing and characterizing materials processed using WAAM. Section 6 presents a discussion on the comparison of WAAM with other AM (both metal and non-metal) processes used in aerospace design. Considering the previous sections, Sects. 7 and 8 provides some discussion and future research directions for WAAM in the aerospace sector.

2 Review approach and major research questions

Once the topic and scope of the review were established, a set of search keywords (approximately 30) was compiled, which were then used by the authors to search for relevant literature in English on Google Scholar, Scopus, EI Compendex, Web of Science, and the major AM-related journals and conference proceedings. A list of the search keywords and journals searched is available upon request from the corresponding author. After this search was done, the reference lists of each paper were analyzed to find additional relevant references that may have been missed during the main search. The full-text versions of each paper were collected and screened for relevance and credibility, with any papers in obviously predatory journals being removed from the set being considered. In addition, all the conference papers, book chapters, and theses for which a journal version was found by the same authors were removed from the set in favor of the journal version. The final check for new papers was completed on December 17, 2022; the authors made every effort to identify and include every relevant paper published and distributed before that date. Copyright permissions were obtained for all figures not unique to this paper, with the exception of figures that were published using a CC-BY license. In all cases, the sources of the figures were cited and acknowledged. The major research questions explored in the course of this review were:

  • In brief, what is WAAM and how does the process work? (Sect. 1).

  • How can WAAM be used to solve some of the major manufacturing problems encountered in the aerospace industry? (Sects. 1 and 3).

  • What specific sectors within the aerospace industry has WAAM been used for and how successful were those applications? (Sect. 3).

  • What modelling and simulation work has been done to better understand and control WAAM for applications relevant to the aerospace industry? (Sect. 4).

  • What progress has been made on WAAM for common materials used in the aerospace industry? (Sect. 5).

  • Within the aerospace sector, how does WAAM compare in terms of usefulness and applicability to other AM (metal and non-metal) processes? (Sect. 6).

3 WAAM in the aerospace industry

The aerospace industry is continuously looking for reliable lightweight aerospace parts with good mechanical properties and high strength-to-weight ratios. These materials will help advance the aerospace industry by improving fuel efficiency, reducing carbon emissions, improve the size and safety of payloads, and to meet industry safety and reliability targets. Designers currently have two major options for accomplishing this, namely (1) reducing the amount of material used or (2) using lighter materials. In both cases, concerns come up about reliability, safety, and cost since old and established materials and design methods are discarded in favor of new and emerging ones. The use of AM in general and WAAM in particular can aid greatly in this when the manufacturing characteristics are taken into account.

From a general engineering perspective, aircraft and spacecraft designers need to be able to maximize performance of the selected materials and design geometry, while using the minimum amount of material. The material must be minimized both to minimize the mass of the aircraft but also to preserve material (since many aerospace-grade materials are rare and expensive) and reduce waste as much as possible. In many cases, this need to minimize the use of material can create design adversity, where performance and reliability requirements struggle to be met using the available materials. In addition, it is also vital that aerospace components be maintainable and replaceable, as these systems often have long lifespans and operate in some very extreme environments. WAAM in particular can aid in solving these problems, as it allows very durable and strong materials to be processed by selective deposition. This not only allows stronger and more durable materials to be used, but also provides the ability to optimize the placement of it [35].

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a and b Aluminum airframe designs from STELIA Aerospace, with c close-up example of a section manufactured using WAAM [36] Figures reproduced under the terms of a CC-BY license. The sections shown in (a)-(b) are approximately 3.5 m in diameter, while the panel in (c) is approximately 1 m square

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Since a wide range of excellent manufacturing processes (both additive and traditional) exist to produce smaller components without the disadvantages of WAAM (thermal shock, residual stresses, welding inclusions, and others), the main emerging area of application for the aerospace industry is in creating large parts, tanks, trusses/spars, and airframes. STELIA Aerospace (Toulouse, France) designs aluminum airframes and frame sections (Fig. 4a and b) for several different Airbus aircraft. They have successfully used WAAM to create some of the sections, as shown in Fig. 4c [37, 38]. The demonstration part measures 1\(m^2\) and was manufactured by depositing layers of aluminum wire welded by electric arc.

While the creation of frames and whole spars (Fig. 5a), Cranfield University and Thales Alenia Space) are the most obvious applications of WAAM for these design problems, it has also been used successfully for some other structures as well. The fuel head tank (Fig. 5b) and another unspecified part (Fig. 5c) for a Lockheed Martin aircraft were also successfully manufactured using WAAM. Some other applications that have been explored were the manufacturing of frame stiffening ribs (Fig. 5d) and satellite breadboards (Fig. 5e). In order to explore the feasibility of using WAAM to create an entire aircraft using WAAM, a large section (nose cone) was manufactured by Aircraft Research Association Ltd (ARA), Bedford, and WAAM3D. The dimensions were 190 mm in diameter and 350 mm in length. In order to be tested in ARA’s own transonic wind tunnel in the summer of 2022, the component has been successfully integrated into a wind tunnel model [36]. More information about these technologies and case studies can be found on the respective companies’ websites [37,38,39,40] if the reader is interested.

Fig. 5
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a 6-m aircraft spar, bfuel tank, cconical part, d panel stiffeners, and e satellite breadboards manufactured using WAAM [36]. Figures reproduced under the terms of a CC-BY license. The parts shown vary in size from up to 6 m long in (a) to only  200 mm in diameter for (c). The part shown in (b) is approximately 1 m in diameter, while the builds in (d)–(e) are about 2 m\(\times\)3 m in size

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A major emerging area of concern and study within WAAM, particularly for high-risk or high-precision applications such as those encountered in the aerospace, automotive, and medical industries, is testing and evaluation of the parts. While beyond the main focus of this review, it is an important consideration that could affect how the technology is used and perceived by potential users. The solutions proposed so far fall into three main categories: (1) in-process defect detection [41], (2) non-destructive evaluation of finished parts [42], and (3) application of manufacturability constraints during design to ensure effect post-processing [43, 44]. For process monitoring, a sensor network method was developed by Xu et al. [12] and used to monitor for process errors during WAAM of titanium alloys. Hauser et al. [45] developed a similar but more simple method using cameras and computer vision software to complete the process monitoring during aluminum WAAM. A larger-scale monitoring method for WAAM systems using robotic arms was presented by Dharmawan et al. [46], where reinforcement learning was used to correct errors during the process. Finally, a predictive control model for WAAM based on system mechanics was developed and refined by Xia et al. [47]. While most of the existing studies are not related directly to aerospace applications, the primary non-destructive evaluation methods for finished WAAM parts has been X-ray techniques, as shown by Raguvaran et al. [42], Rodrigues et al. [48, 49], Shen et al. [50], and Morais et al. [51]. In terms of post-processing, the main manufacturability constraints for WAAM parts have been related to machinability of the finished parts. Works by Hoye et al. [52], Ceritbinmez et al. [53], Veiga et al. [54], Feier et al. [55], and Gou et al. [56] have explored this extensively.

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WAAM simulation model [57] Figure reproduced under the terms of a CC-BY license

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4 Modeling and simulation of WAAM

Simulations at different length scales are being performed to improve the quality of the final WAAM products. Given the needed precision and reliability of aerospace applications, it is vital that the mechanics of the process be well-understood and easily controllable. The defects found in WAAM products, pores, cracks, distortion, non-homogeneous chemistry, and large columnar grain structure, all are related to the process variables and composition of the alloys used. Pores can be formed because of gas dissolution in molten metals, i.e., hydrogen in aluminum alloys. Cracks can be associated with shrinkage in solidification and the internal stress that is developed due to contraction during solidification and cooling. One category of simulations is focused on optimizing the process by understanding the effects of thermal cycles, deposition rate, travel speed, molten pool size, etc. Wu et al. [57] and Aldarur et al. [58] used finite element method simulations (Fig. 6) to study the effect of thermal cycle and pool size on the microstructure and properties of Ti-6Al-4V. They showed an increase in the width of the molten pool results in columnar grains. In the same way, Ogino et al. [59] used numerical methods to predict the shape of the molten pool and show managing the torch temperature and speed affects the shape of the pool.

Rolling ultra-high-frequency pulsed (UHFP) arc and ultrasonic vibration are examples of processes that are used along with WAAM to enhance the mechanical properties of printed parts. These hybrid processes are also investigated computationally as some of their effects cannot be measured experimentally. Computational fluid dynamic (CFD) was used to study the effect of ultrasonic vibration on the pool size [60]. The study showed that the flow rate and high-temperature diffusion are improved; hence, a more uniform temperature distribution is formed. It also significantly changes the pressure field in the molten pool which promotes equiaxed grain structure formation. The predictions of the simulations were confirmed by analyzing an AISI 1045 steel WAAM part. The effect of UHFP on NiTi WAAM product was also investigated using CFD [61]. It was shown that the velocity fluctuation frequency of different points in a molten pool is higher when UHFP is used, and it leads to an equiaxed grain structure. It also showed a deeper penetration of the pool is possible when UHFP is used. Figure 7 shows the setup of the simulation.

CALPHAD-based simulations (calculation of phase diagrams) that use thermodynamic databases are used to calculate equilibrium phase diagrams to predict transition temperatures and phases. It is an especially important method when using multi-element alloys. Information extracted from these diagrams can be used to determine the alloy’s behavior during solidification and whether it is susceptible to crack formation or segregation [62,63,64,65,66,67]. The CALPHAD method predicts the cooling rates at which precipitate phases are formed, based on which an appropriate heat treatment is determined. The Scheil-Gulliver solidification model and diffusion-controlled transformation simulations are used to understand the non-equilibrium solidification in AM. In Al-Zn-Mg-Cu system development [68], the heat treatment process was determined by Scheil-Gulliver solidification simulation and CALPHAD. The presence of phases such as Mg2Si and Al3Zr are predicted by these models. Post-WAAM heat treatment for grade 91 steel was also designed based on the prediction of precipitate formations during non-equilibrium solidification. The heat treatment, 2 h of homogenization at 1200 \({}^{\circ }\)C, and 2 h of aging at 760 \({}^{\circ }\)C resulted in an ultimate tensile strength 200 MPa higher than wrought products. A two-step heat treatment was developed for Al-Zn-Mg-Cu alloy using CALPHAD simulations [51]. Welk et al. used the CALPHAD method to investigate the effect of different alloying elements on the freezing range of Ti-6Al-4V [69]. By increasing the freezing range of an alloy, a more significant undercooling can be created that leads to the formation of smaller and equiaxed grains.

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WAAM simulation model [57]. Figure reproduced under the terms of a CC-BY license

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Table 2 Comparison of types of titanium alloys processable using WAAM [83,84,85,86]
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5 WAAM aerospace-grade materials

Many studies on the WAAM mechanism and performance on various materials, focusing on titanium alloys, aluminum alloys, and nickel-base alloys have been conducted. The deposited material undergoes multiple thermal cycles during the construction of a WAAM workpiece. During these heat cycles, various grain patterns occur in distinct layers along the deposition direction. These grain structures determine the mechanical properties of the workpiece. Many column particles have grown significantly from the substrate and are compact towards the boundary section with a high gradient temperature. Often these metals seem to be appropriate for WAAM forming workpieces, but residual stress, deformation, and precision concerns persist [58, 70,71,72,73,74]. Alloys of other materials may be widely used with WAAM in the future, particularly alloys based on magnesium and copper, for the aerospace industry. During this review, it was found that WAAM has been used successfully for various kinds of steel and some magnesium alloys, but these applications were not related to aerospace problems and therefore were out of the scope of this review. The alloys available for WAAM are limited, so it is necessary to design new ones as new applications and problems arise. Much of the modeling and simulation work for WAAM has focused on the behavior of the alloys and developing new alloys that are not susceptible to the formation of common defects in the WAAM process. For example, aluminum alloys in the 7000 series which have high strength and are important in the aerospace industry are not weldable because of the formation of cracks, hence not appropriate for WAAM. Many groups are trying to design new aluminum alloys for WAAM [36, 62,63,64,65,66,67, 75, 76].

5.1 WAAM-processed titanium alloys

Titanium alloys generally have excellent natural characteristics such as a high strength-to-weight ratio and good toughness. Its fatigue strength, growth resistance, oxidization rate, and corrosion resistance are high as well. Molding, forging, and casting are the most common methods of processing titanium alloys, but these methods present many problems with aerospace-grade materials [77]. Naturally, titanium and its alloys have low thermal conductivity, very high viscosity (i.e., low fluidity), and an expandable modulus, which can result in the material being difficult to form even when heated [28, 58, 78, 79] and encourages defects in the material structure. Titanium is also subject to surface oxidation during processing (especially during machining), which can insulate the material bulk and prevent the even distribution of heat [80, 81]. In addition to a higher energy requirement to process the material, it also causes accelerated tool and mold wear. Special equipment is generally required to keep these risks at an acceptable level. In contrast, the hardware and processing methods used by WAAM are low-cost and straight-forward. No special equipment or tools/molds are needed with WAAM, as the method is the same as used with other easier alloys (such as aluminum) [57, 82].

Table 3 Example aerospace titanium alloys processed using WAAM. Yield and ultimate stress values shown are averages unless otherwise specified
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There are two common forms of titanium alloys used in the aerospace industry and can be processed easily using WAAM [83, 89]: \(\alpha\) phase and \(\beta\) phase depending on the metal’s crystal structure. The properties of the \(\beta\) phase are that its ductility is high and tensile strength is moderate. The lattice structure of \(\beta\) phase is body-centered cubic (BCC), whereas that of \(\alpha\)-phase is hexagonal close-packed (HCP). The strength of \(\alpha\) phase is high but is also quite brittle. Various titanium alloys are divided into 5 types based on their phases, \(\alpha\)-Ti alloys, near \(\alpha\)-Ti alloys, \(\alpha + \beta\) Ti alloys, near \(\beta\)-Ti (metastable \(\beta\)) alloys, and \(\beta\)-Ti alloys. Table 2 shows a comparison of the characteristics for each one. The benefits of WAAM are most apparent for the applications requiring large or unusually shaped parts. Due to the special lightweighting requirements, the aerospace industry may cater to most WAAM titanium applications, even over other common materials such as aluminum and stainless steel [31, 82, 83, 87, 88, 90]. Table 3 shows the major studies completed using WAAM and titanium alloys observed in the literature; the WAAM type and mechanical properties after WAAM were collected from each for the table.

5.2 WAAM-processed aluminum alloys

Aluminum alloys are low-cost, have a high strength-to-weight ratio, have good fatigue and fracture properties, and are generally easy to process with existing manufacturing processes. Most alloys also have good weldability. Therefore, it has been widely used for structures from the beginning of aircraft and spacecraft development. With the advent of modern manufacturing techniques, such as CNC machining and friction stir welding, aluminum alloys can be used to make both very small and very large parts easily [13, 91]. Aluminum alloys are also generally very stable and do not present themselves in drastically different chemical structures like titanium alloys (aluminum has a face-centered cubic (FCC) crystal structure), making specific studies with WAAM much easier to complete. Table 4 shows some examples of WAAM-processable aluminum alloys that have been used successfully for aerospace applications.

Table 4 Example aerospace aluminum alloys processed using WAAM
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Table 5 Example properties of aerospace nickel alloys examined after WAAM processing
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It has been observed that WAAM-processed aluminum alloys have similar problems and concerns, such as cracking, porosity, cracking, and inconsistent grain structure, seen in cast and welded aluminum [16, 94, 98]. Therefore, it is safe to assume that the same mitigation strategies developed for such processing would also be relevant for WAAM-processed aluminum; however, not much has been explored on this question for WAAM, and therefore, it is an important point for future research projects to consider. As with traditional arc welding, most of the WAAM processing of aluminum alloys is done using a GTAW-based (i.e., TIG) method [91, 92, 99,100,101]. However, the CMT technique has been shown to work for aluminum alloys under some conditions [93, 102]. The deposition rate for aluminum alloys has been found to be about one kg/hour using WAAM, which is extremely fast compared even to other alloys processed using this method [92, 99, 100].

5.3 WAAM-processed nickel alloys

Because of their structural stability at high temperatures and extreme environments, their toughness, and corrosion resistance, nickel alloys are widely used in the aerospace industry. Due to this and the generally high cost of processing, nickel alloys are an idea choice for WAAM [79, 103, 109], particularly Inconel 718 and Inconel 625 [110, 111]. Table 5 shows the major studies completed using WAAM and nickel alloys observed in the literature during this review; the WAAM type and mechanical properties after WAAM were collected from each for the table.

6 WAAM versus other AM processes in the aerospace industry

The aerospace market for AM components comprises metal and non-metallic materials (especially polymers), which are highly connected to both critical and standard aircraft parts [9, 35, 112,113,114]. The common metal and non-metal AM technologies are listed in Table 6, with examples of applications in the aerospace industry. In the aircraft industry, premium metals such as titanium alloys, aluminum alloys, nickel-based superalloys, and special steels are in great demand [35, 52, 114,115,116,117]. Aerospace components’ complexity in structure, functioning, and properties often leads to various manufacturing challenges and higher costs. Traditionally, complicated aerospace components comprise multiple simple parts attached with various welds, screws, and bolts, which further needed a greater inspection, whereas aerospace components manufactured by AM technology enables part consolidation [118,119,120], resulting in a decrease in overall inventory cost. As the number of components in any assembly decreases, it reduces the number of tools and writing, testing, and production costs [114, 121].

One of the main reasons for the high cost of manufacturing aerospace parts is the BTF ratio. BTF is described as the ratio between the mass of raw material to the mass of the final product, where a low number indicates a large amount of wasted material in the processing. According to current scenarios, block-shaped raw material is milled to obtain large structural components for the aerospace industry. This results in a high amount of wastage, leading to a high BTF ratio and hence the high cost of tooling. Metals such as alloys of titanium, aluminum, nickel-based superalloys, and special metals are highly demanded in the aerospace industry. In 1970, 15 tons of titanium alloy were used in the Boeing 737, but in 2009, 100 tons were used in the Boeing 787. The demand for these metals is growing all the time. However, over 90% of the metal is machined away during fabrication, resulting in significant waste. Titanium made up 93

Table 6 Metal and non-metal AM technologies relevant to aerospace manufacturing
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Table 7 Comparison of different types of AM technologies used in the aerospace industry
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A process that combines additive manufacturing with machining is a positive approach to overcoming these traditional manufacturing method issues. Table 7 compares various types of AM used in the aerospace industry [52, 112, 113] based on some major relevant process parameters. Aerospace metals are very expensive, often fragile, and sometimes unsuitable for manufacturing with one of the powder-based metal AM processes [116], restricting the types of AM processes that can be used to produce aerospace components. AM processes in general have the potential to reduce the BTF ratio from about 20:1 (for traditional machining) to around 5:1, with WAAM being the most flexible and rapid process [12, 83, 104, 122]. In addition to being far more efficient and sustainable and making better use of exotic or rare materials, this will greatly reduce the cost and difficulty of making aerospace-grade parts [30, 52]. WAAM is promising because, in addition to offering the design freedom and material customization offered by all AM processes, it has a very rapid material deposition rates, is safer to use than many other AM processes, and can make very large and awkwardly shaped parts easily [83, 122,123,124]. A good example was presented by Norsk titanium, who developed the first titanium WAAM component for an aircraft, specifically the 787 Dreamliner. After WAAM was integrated into the design and fabrication process, the overall cost of the part was reduced by 30% and the processing time was reduced by 75% [122,123,124,125].

7 Discussion

This review explored the current uses, modeling and simulation methods, and materials for the wire arc additive manufacturing process relevant to the aerospace industry. For the major problems encountered in this sector, the major differences between WAAM and other AM processes were also briefly examined. While WAAM is useful in a wide variety of production and design sectors (for example, as discussed by Li et al. [2], Vimal et al. [13], Rosli et al. [17], Jin et al. [23], Chen et al. [34], Najmon et al. [35], Langelandsvik et al. [70], Treutler et al. [71], Raut et al. [72], Liu et al. [79], Meiners et al. [90], Dhinakaran et al. [9, 109], Wu et al. [111], Uriondo et al. [114], and Cunningham et al. [123]), it offers some specific benefits to the aerospace industry that no other major process currently existing can offer in such a combination. For example,

  • WAAM can provide the excellent design freedom of AM, while also offering a very fast material deposition rate. This dramatically improves the buy-to-fly ratio for the exotic aluminum alloys and titanium and nickel alloys commonly used for aircraft and spacecraft due to the low amount of waste.

  • WAAM can produce a very wide range of part scales (from small parts to potentially entire aircraft). While the precision is less than can be gained from other AM and traditional processes, WAAM parts are relatively easy to post-process to bring them to the required precision.

  • WAAM does not require any special tooling during or after the process, making it suitable for the custom one-off parts often needed in the aerospace industry.

  • WAAM also does not require any special enclosed environment or vacuum to operate successfully, making it extremely flexible and easy to apply in most cases. Similarly to traditional GMAW and GTAW welding, the shielding gas (argon, helium, CO\({}_2\)) can be applied directly through the torch or wire feed.

  • While WAAM does have some issues with incomplete fusion, residual stresses, cracking, and similar, it is generally far less than what is encountered with other AM processes and many traditional processes such as casting.

  • Essentially any weldable metal can be processed using WAAM.

  • The hardware for WAAM and the raw materials (filler wire and shielding gas) can be obtained easily for a relatively low cost.

  • WAAM has the potential to be a good open-source AM process as all of the components needed can be obtained at any good hardware store. Unlike most AM processes, this allows it to be used in much less-than-ideal conditions and places.

  • The process is conceptually simple (unlike many other AM processes) and is relatively easy to control.

  • WAAM can be easily combined with other processes (such as milling or turning) to create synergistic hybrid processes.

Despite the benefits and potential applications of WAAM, there are some challenges and limitations associated with this process that can limit its use for some applications.

  • The primary challenges is related to the process parameters, which can affect the mechanical properties of the final product significantly. These parameters include the wire feed rate, arc voltage, welding speed, and the distance between the electrode and the work piece. While these parameters are not dificult to understand, controlling them in real-time during processing can be a major challenge.

  • Another challenge is the limited range of materials that can be used in WAAM. The process is typically restricted to materials that can be wire-fed easily, which limits the range of metals that can be deposited. Unlike other DED processes (such as laser engineered net shaping (LENS), WAAM cannot easily process brittle metals or metal/ceramic composite materials.

  • The hardware needed for processing is relatively inexpensive and simple, but can become expensive very quickly for any increase in control and precision needed.

  • The material properties are generally good for WAAM (assuming the process parameters are set correctly), the natural surface quality it typically quite bad without post-processing.

WAAM is an extremely versatile AM process, being able to use several different approaches to welding the material (GTAW, GMAW, CMT, PAW) to expand the variety of materials that it can process and reduce the cost and material investment to use it. As demonstrated during this review, WAAM in several different forms has been well-validated by both the aerospace industry and academic researchers. While not able to produce small detailed parts with the precision of powder bed fusion or metal injection molding and being subject to the typical AM issues (anisotropy, cracks, residual stresses, voids), it is emerging as one of the go-to metal AM processes. For the time being, it appears to be best suited for large parts with low tolerances (or which will be machined to spec later) but as shown in several sections of this review, it is steadily improving and expanding the boundaries of its usefulness. While not much aerospace-related modeling and simulation work has yet been done, the studies reviewed show that much is being learned about how to manage and control the process, which in turn is making the process more useful and precise.

The great majority of the studies reviewed were concerned with the materials that WAAM can process. Within the papers that were concerned with aerospace applications or that studied materials commonly used in the aerospace industry, the three major types of materials were aluminum alloys, titanium alloys (mostly limited to Ti-6Al-4V), and nickel alloys (primarily Inconel 718 and Inconel 625). At the current time, the titanium and nickel alloys studied for WAAM are largely the same as those used in other AM processes (particularly MPBF). While WAAM was successful in processing these (and showing promise to process other similar alloys), it is a stand-out among metal AM processes for its ability to work well with aluminum alloys [36]. Aluminum is often a difficult material to process additively, but WAAM appears to do it very well. It also shows great potential for the processing of titanium alloys; while all of the studies found used Ti-6Al-4V (the most common titanium alloy processed by all AM processes), WAAM offers the potential to work equally well with other titanium alloys which may provide great benefits for use in extreme environments.

In addition to its ability to provide good factory parts for aircraft and spacecraft, WAAM offers two more major benefits for the aerospace sector: (1) on-demand low-cost spare parts and (2) field repair. These will be particularly useful in the future for military applications and space missions, where aircraft and spacecraft can be repaired and maintained in the field without the need to carry and store spare parts. In terms of repair, WAAM offers a safer and less complex alternative to the powder-based directed energy deposition (DED) process normally used for additive repair. While a more “quick and dirty” option than DED, it definitely has great potential in applications where a significant volume of material needs to be deposited or the conditions do not work well for DED. For both WAAM and DED, post-processing and inspection would need to be done to bring the parts into spec and ensure they are ready to be used again.

8 Conclusions and future work

In conclusion, the WAAM process is a very promising tool for the design, production, and maintenance of aircraft and spacecraft in the future. It is a very simple and low-cost alternative to many other metal AM processes, while offering most of the same benefits. It can provide good results with a wide variety of materials (beyond the aerospace materials discussed in this review), showing particular promise with aluminum and titanium alloys. Its very high deposition rate and low waste provide a good tool to dramatically reduce the potential buy-to-fly ratio for aerospace applications, while providing good properties and relatively reliable products. Future work on WAAM in the aerospace domain should focus on more detailed modeling and simulation of the process, more experimental characterization of the materials produced with WAAM, mixing of materials to make functionally-graded materials and alloys, and on developing design rules for applying WAAM to the aerospace industry. Academic researchers should also focus on completing replicable and scholarly studies of the process beyond what has been done and demonstrated by the various companies discussed in this review.