# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
###### Abstract
showed an example of the utilization of new design rules for a part of a crane arm [15] with less weight than a conventionally manufactured part. [PERSON] et al. showed the possibility to manufacture compressor impellers with WAAM and described the whole process chain and the benefit of WAAM in comparison with conventional impeller manufacturing in a decent way [16]. In addition, [PERSON] et al. showed this for the manufacturing of a steel knot for construction applications [17]. The advantages of WAAM in comparison with other additive manufacturing processes have been evaluated by [PERSON] et al. and are shown in a well-arranged diagram in Figure 6[18]. [PERSON] et al. stated that the crucial advantages of wire arc additive manufacturing are a shortened lead time, less material waste, improved functionality, customized tooling for lower volume parts and the possibility of multi-material design [19]. Figure 5: WAAM made bridge [14]. Figure 6: Advantages of WAAM and other additive manufacturing processes [18]. WAAM is not only used for the manufacturing of new parts; it also can be used for the modification or repurposing of existing parts. One example for modifications is given by [PERSON] and [PERSON] for the reinforcement of car parts [20], and one example study on the repurposing of steel parts was conducted by [PERSON] et al. [21]. Furthermore, WAAM can be used for the localized repair of molds and other parts, like [PERSON] et al. showed exemplarily [22] and [PERSON] et al. demonstrated with the repair of a sprocket [23]. The previous examples illustrate the variety of possible applications of wire arc additive manufacturing. Nevertheless, the wide range of different applications, processes, materials and used surrounding technologies, such as path planning software and process surveillance methods, are presented in a very specific way, and each focus on an interesting part of the WAAM process, but a general consolidation of the knowledge is missing. Therefore, this review has two objectives. First, it is intended to provide an overview of the international research activities in the field of WAAM, and secondly, it is intended to summarize and evaluate the results achieved and point out where there is a demand for further intensive research. The review is divided into two big parts. The first will focus on the materials, welding process, methods and strategies for welding arc additive manufacturing. The second part will focus on the modeling of welding arc additive manufacturing processes. Modeling is the key to achieve the best possible additive-manufactured parts, aside from a deep understanding of the welding processes and a closed loop process of surveillance and control. This paper will be concluded by the identification of future research interests. A common method for how additive manufacturing with welding processes is conducted these days is described by [PERSON] in [24]. First, the CAD model is drawn. Second, the path planning with the related parameters of the welding process will be performed, and afterward, the welding itself will be performed, in most cases, with a welding robot. Figure 7 shows a basic process chain for WAAM-made parts, from the product developments and simulation drawn along the information flow to the manufacturing process (e.g., welding), followed by process surveillance and the application along the material flow. Between all steps of the process chain, interactions can be seen. The key for nearly all WAAM-made parts is the parts' properties, especially when the parts are designed for structural applications. In each step, different methods can be used. While welding, the welding process and the cooling or geometry can be observed and controlled in a closed loop. Figure 8 shows the detailed process path of WAAM parts exemplarily. In the presented literature, the tendency towards closed loop control of the weld bead and material property orientated path planning is visible [25; 26].