# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
###### Abstract  
Wire arc additive manufacturing is currently rising as the main focus of research groups around the world. This is directly visible in the huge number of new papers published in recent years concerning a lot of different topics. This review is intended to give a proper summary of the international state of research in the area of wire arc additive manufacturing. The addressed topics in this review include but are not limited to materials (e.g., steels, aluminum, copper and titanium), the processes and methods of WAAM, process surveillance and the path planning and modeling of WAAM. The consolidation of the findings of various authors into a unified picture is a core aspect of this review. Furthermore, it intends to identify areas in which work is missing and how different topics can be synergetically combined. A critical evaluation of the presented research with a focus on commonly known mechanisms in welding research and without a focus on additive manufacturing will complete the review. wire arc additive manufacturing; welding; steel; aluminum; titanium; nickel; magnesium; copper; path planning; process surveillance; multi-material; intermetallic 20  
and process behavior is widespread, and the known fundamentals can be transferred [6; 7]. The difference between normal joint welding with arc welding processes and additive manufacturing was nicely illustrated by [PERSON] et al., [PERSON] et al. and others, and it can be summarized as a difference in the thermal conduction of the material [7; 8; 9]. In joint welding, the thermal conductivity takes place in more dimensions (Figure 2a) than in WAAM processes, where the heat has to be deduced, in most cases, in one direction: to bottom of the part (Figure 2b). Furthermore, thanks to new developments in the robotics. computer-aided design and manufacturing along with new welding processes and process control possibilities, WAAM is more promising now than it was in the 1970s and 1980s, when WAAM was called \"shape welding\" (in German: \"Formgebendes Schweissen\") and was performed for large parts [10]. Figure 3 shows a large piece of equipment for shape welding up to 80 t on the left and a shape welded reservoir on the right side, which showed superior material properties in  
Figure 1: Comparison between wire-based and powder bed-based additive manufacturing [1]. Figure 2: Heat transfer in joint welding and WAAM [8]. (**a**) thermal conductivity takes place in more dimensions; (**b**) thermal conductivity takes place in one direction. comparison with normal manufacturing processes of this time [11]. Furthermore, the first trials of shape welding were carried out in the 1930s [11]. Nevertheless, this manufacturing process has had its niche market throughout the last few decades. Today, the variety of manufactured and tested parts has increased. Figure 4 shows two different parts which have been tested for use in the industry. On the left is a ship propeller, and on the right are Pelton turbines for waterpower plants shown. The bandwidth of different applications and different used materials is currently being exaggerated by different working groups for uses such as naval applications [12]. Figure 5 shows the first WAAM-made bridge realized in four parts and combined with non-welded parts [14]. It has a weight of 7.8 t and a welded weight of 4.6 t.  
Figure 4: **(Left)** WAAM-manufactured propeller (Ramlab) (Right) WAAM-manufactured water turbine blades (Hydroworld) [13]. Figure 3: **(Left)** Welding equipment for shape welding up to 80 t (Thyssen factory picture) [11]. **(Right)** Shape welded reservoir of 72 t. Material: 10 MnMoNi55 (Thyssen factory picture) [11]. The bridge shows the possibility of organic-looking parts nicely, which usually are not easy to manufacture with conventional manufacturing processes. To utilize this, new design rules are currently being developed [15] and implemented in teaching at universities around the world. [PERSON] et al.