# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Laser-Assisted Welding
#### 2.1.7 WAAM Process-Specific Challenges  
The WAAM process has very specific challenges which need to be mastered for defect-free and geometrically correct manufacturing of the part. Three of them are the prediction of the layer height in the path planning, the distance between the welding beads and the direction and order of the single beads. Commonly, the choice of welding parameters is made by trial and error, but as the state of the art advances, the prediction of welding parameters becomes more and more model-based (Figure 13) [115].  
[PERSON] et al. showed the classical and more knowledge-based way for the determination of the welding parameters, and they derived a model to determine the distance of the weld beads [115; 116]. This model shows that an optimal distance between the weld seams lays in the region of 0.63-0.77 times the weld bead's width. This correlates with the commonly known distances of multi-run welds, where the new weld seam is set into the notch between the previous weld seam and the substrate. The proposed models need to be modified for each combination of filler and shielding gas. A similar approach was taken by [PERSON] et al. [62] and [PERSON] et al. [117] with comparable results. Due to process-imminent surroundings, the order and direction of the welding process has a great influence on the geometrical correctness of the part. An alternating strategy has shown the geometrical best practice [118]. A more detailed description of the challenges regarding adequate path planning can be found in Section 2.2.  
The energy input is, like for welding in general, the key point for achieving certain properties and a proper part. In welding, the energy per unit length is considered a sufficient value to describe most phenomena. This has been transferred to WAAM as well from many working groups. However, this approach can be modified to suit the needs of additive manufacturing with a more desired material build. Here, volumetric energy density or volumetric energy was introduced by [PERSON] et al. [119].  
Figure 13: Parameter determination in welding arc additive manufacturing [116].  
Another critical point in wire arc additive manufacturing is a phenomenon called humping [120]. This phenomenon is known from high-speed welding, and [PERSON] and [PERSON] showed that the effect occurs due to the momentum of the back flow of the melt and that the effect limits the range of usable welding travel speeds [121]. For wire arc additive manufacturing with GMAW, the starting point when humping occurs is determined by the wire feed speed and the travel speed of the torch. For a common CMT process, [PERSON] et al. showed that the maximum travel speed without humping is around 60 cm/min [120]. [PERSON] et al. observed the humping for the TIG welding process [69], as did [PERSON] et al. [77]. To overcome this issue, the travel speed, energy input and filler feed rate need to be balanced with each other in a certain range. [PERSON] et al. developed a shape-driven control of the layer height to overcome uneven substrates or uneven previous welding paths [122]. They chose the deposition rate as the signal and the travel speed as the variable value.  
The layer height and layer width are directly related to the welding process conditions [123]. [PERSON] et al. proposed a mixed heat input (MHI) strategy to overcome geometrical and microstructural imperfections [123]. Aside from the heat input, the temperature of the substrate and additional heating or cooling of it will influence the shape of the weld bead as well, as [PERSON] et al. showed [124].  
For automated manufacturing with WAAM, a suitable process of surveillance and control is needed to maintain all relevant parameters in a certain range and overcome any faults.