# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Laser-Assisted Welding  
Laser-assisted welding differs from laser hybrid welding and typically uses a laser power less than 1 kW and has some known benefits, like possible geometrical control of the weld seam with the laser's position [96]. Pre-ionization of the laser can be used to direct the arc in a certain direction. For metal inert gas (MIG) welding, stabilization of the arc is also proposed. For additive manufacturing, [PERSON] et al. showed that an additional laser led to a reduction in the width of an aluminum wall and a higher layer height in certain ranges. Furthermore, the surface quality could be improved [97]. Figure 9: An example of skeleton WAAM [87]. Combined Rolling and Additive Manufacturing Processes  
The combination of different additive and forming manufacturing processes can improve the microstructure and mechanical properties of additive-manufactured parts. Basically, two different forming processes are currently being investigated deeply. The first group is a combination of a WSAM process with a rolling process [98; 99; 100]. Here, forming-induced recrystallization takes place, and a finer and tougher microstructure with increased strength can be created. [PERSON] et al. realized strong recrystallization and mitigated columnar grain growth [103]. [PERSON] et al. showed that the application of rolling can lead to significant grain refinement and a reduction in welding-related residual stresses, depending on the rolling direction [101]. These results are supported by the findings of [PERSON] et al. [104], who showed that a vertical rolling process can increase the beneficial comprehensive residual stresses in a WSAM part with an FEM model validated with experimental results. Furthermore, [PERSON] et al. showed that additional rolling can have a beneficial influence on the fatigue performance of WSAM-made parts [102], and [PERSON] et al. showed that cold working has a beneficial influence on the materials' strength [105]. Figure 10 shows different possible rolling methods. [PERSON] et al. stated that rolling processes with the prevention of lateral deformation have the greatest influence on the residual stresses and have been applied for titanium-based alloys and aluminum-based alloys successfully [101]. Basically, ally processes that increase a weld's mechanical properties, like high-frequency impact treatment or shot peening, should improve the properties of WSAM-made parts as well. [PERSON] et al. showed for different materials that a combined additive and forming process for tubes can be realized [106]. A comparable effect to additional rolling can be achieved due to other forming-based processes, like hammer peening and shot peening [107]. For the second process group for a combined process of forming and WSAM [108], [PERSON] et al. showed that subsequent additive manufacturing, milling and forming is possible and can be realized in one hybrid manufacturing machine [108]. Combined Machining, Heat Treatments and Additive Manufacturing Processes  
For the combination of WSAM and machining, optimization of the process control and process values for the milling process is crucial for thin-walled parts as well [109]. [PERSON] et al. developed a method that allowed for adjusting the machining parameters to avoid resonance in the work piece [109]. An overview of the main challenges of hybrid  
Figure 10: Schematics of possible rolling methods: (**a**) vertical with a profiled roller; (**b**) in situ rolling; (**c**) pinch rolling and (**d**) rolling with an inverted profile roller [101]. additive and subtractive manufacturing is given by [PERSON] et al. [110]. They summarized the necessary post-processing to overcome the limitations of DED processes in Figure 11. The reduction of pores in WAAM is mostly necessary for aluminum. Furthermore, they derived the gains and challenges of hybrid manufacturing (Figure 12).