# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 5 Summary  
The great economic and scientific interest in arc-based additive manufacturing has resulted in a significant increase in scientific interest in this manufacturing process. This is also shown by the larger number of literature studies conducted so far from all parts of the world [9; 10; 13; 30; 31; 147; 233; 417; 418; 419; 42; 42; 430; 42; 431; 432; 433; 434; 435; 436; 437]. All the literature sees great potential in this manufacturing process and attributes this potential to new developments in welding processes as well as robotics, sensors and CAX technologies. The current literature presents a wide range of used welding processes for every metal-based material group, including upcoming material groups like intermetallic alloys or high-entropy alloys. For every material group presented in this review, it was stated in the cited research that WAAM is a suitable manufacturing process which gives a lot more degrees of freedom. However, in comparison with some other market-available, direct energy deposition processes, most scientific experimental set-ups do not utilize the promised degrees of freedom, while for laser powder direct energy deposition, there are existing machines with five axes to weld only in the PA position, and the path planning can be performed in an three-dimensional way. For WAAM, very limited machinery with these state-of-the-art moving and path planning systems are scientifically used. In WAAM, currently, most studies focus on the interdependencies of process-related values, the geometrical correctness (including residual stresses and distortion) and the microstructural behavior of the used materials. These studies already give a good basis to move on to more complex scientific questions, like whether to use a multi-material approach with increased process surveillance. However, aside from all the presented knowledge and advances in the last few years, wire arc additive manufacturing is not understood completely, and more topics need to be addressed even further. [PERSON] et al. identified four main research areas for the next steps on the road to WAAM series production [438]. These steps are as follows: process, materials, modeling and automation. These main topics should be addressed with regard to the residual stresses, geometrical correctness, mechanical properties and cost sensitivity [438]. This is supported by [PERSON] et al. They identified the residual stresses and distortion as the limiting factors in wire and arc additive manufacturing [436]. Within this this field of issues that needs to be addressed, one crucial point is the control of the heat input due to its direct influence on the microstructure, the building strategy and the geometrical correctness of the manufactured part regarding distortion and residual stresses [435]. [PERSON] et al. stated that the addition of another wire as a cold wire feed to a gas metal arc welding process and using the doubled filler material for in situ alloying (and the effect in the occurring residual stresses) can widen the field of applications of WAAM [68]. [PERSON] et al. showed that a multi-material approach can be crucial to tailor the mechanical properties of a WAAM part to the occurring loads [52; 232]. Here too, the existing knowledge of mixed joints from the welding technology can be partially transferred and thus facilitates the entry into this new field of research. Since science aims not only at industrial application but also at a complete understanding of the interrelationships and mechanisms involved in the production of and within the materials, detailed and fully understood correlations between the process, material, environmental variables and occurring process instabilities like humping, among others, are indispensable for a fundamentally oriented and scientifically complete description. For example, the influence of the melt surface tension, melt viscosity, process variables and humping start speed are missing.