# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.6 Nickel  
Wire arc additive manufacturing is also performed for nickel alloys, and various working groups have realized the WAAM of nickel base alloys successfully [70; 81; 103; 327; 328; 329; 330; 331; 332; 333; 334; 335; 336; 337; 338; 339; 340; 33; 337; 338; 339; 341]. Different common nickel base alloys are being used for additive manufacturing like Inconel 625 [337; 337; 340], Hastelloy C276 [335] or Inconel 718 [328; 333; 329; 332]. [PERSON] et.al. studied the process response of Inconel 718 and CMT-GMA welding and determined correlations between the process parameters and the build-up height and seam width [333].  
The mechanical properties of nickel-based alloys are mostly achieved due to post-veld heat treatment. The knowledge of post-veld heat treatment is not only driven by WAAM-related research, especially when new alloys are developed. [PERSON] et al. showed that heat treatment of an additive-manufactured, newly developed alloy changes its material properties through post-veld heat treatment [342]. However, many nickel-based welded alloys undergo additional post-veld heat treatment (PWHT) [112; 333]. It has been shown that the post-veld heat treatment parameter can differ from the recommended ones for bulk material [333] and has its most impact on the formed secondary phases [334]. In addition to the properties under quasi-static loading as well as under fatigue, the behavior of materials under increased strain rates (increased loading rates) is becoming more and more interesting for applications. The current focus is on modeling crash behavior. The characteristic values for the modeling of high strain rates have also been determined for WAAM-manufactured nickel-based alloys [329; 331].  
The milling of a WAAM-made nickel-based alloy has also been investigated [112]. Most nickel base alloys tend to form so-called \"white layers\" in the process of milling, which will be the most used post-process for WAAM to achieve good surface quality [112].  
These surface layers have undesirable properties for technical applications. In addition, milling induces undesirable plastic deformations and residual stresses below the surface (Figure 25). To reduce these effects, ultrasonic assisted milling could be used. Figure 26 shows the occurring residual stresses due to milling of the additively manufactured alloy 725. By using ultrasonic assisted machining, the residual stresses that occur can be significantly reduced, and this is independent of other peripheral variables such as the stepover or the use of cooling lubricants. Figure 26 shows that this effect is also maintained down to the depth of the material. White layers have also been found for additively manufactured alloy 725 (Figure 27).  
Figure 25: Surface morphology of a white layer [112].  
Figure 27: SEM image of cross sections of milled specimens of test 3 (**a**) and test 4 (**b**) and detail of the surface plastic deformation zone and white layer of test 3, showing the SEM analysis (**c**) and EBSD mapping (**d**) [112].  
By ultrasonic assisted milling, the occurrence of these can be significantly reduced (Figure 27) for the used WAAM-made alloy 725 [188]. On the left, an REM image of a cross-section of the surface of the additive-manufactured alloy can be seen. In the surface region, a homogeneous phase without any detectable substructure is visible. This brittle white layer can lead to faster damage to the part and needs to be avoided. On the right of Figure 27, this layer is not visible, and no subsurface deformation can be detected. Aside from that, the needed cutting forces could have been reduced by 20%.  
Overall, the presented works show that wire arc additive manufacturing of nickel base alloys is possible and gives suitable results.