# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.1 Steel
This leads to known relationships between the heat input, cooling regimes and microstructure and the mechanical properties [30]. In addition, the influence of shielding gasses is mostly known. Furthermore, welding-related microstructural issues like the formation of sigma phase or cracking phenomena [240; 241] can be transferred from the knowledge of joint welding. The geometrical correctness of WAAM-made stainless steel parts has been evaluated by [PERSON] et al. [242] and others [30]. [PERSON] et al. summarized the different studies and stated that the geometrical correctness is closely related to the welding speed, wire feed speed, welding mode (e.g., impulse or short arc), cooling time and interpass temperature. The welding speed and wire feed can be combined with the energy input. In combination with the part geometry, the energy input and the interpass temperature define the cooling rate. One crucial point for geometrical correctness is the proper assessment of distortion. [PERSON] et al. presented a model for distortion prediction of 316L steel [243]. They showed that a nonlinear approach shows good results in predicting the distortion of 316L steel. They furthermore showed that current state-of-the-art heat source models can be used to successfully build that prediction model [243]. Aside from geometrical correctness, the mechanical properties of the parts are essential for the use of the parts. [PERSON] et al. stated that the reported mechanical properties of additive-manufactured high-alloy steels match those of machined components [30]. This is  
Figure 23: Power and welding speed for different additive manufacturing processes for 316L stainless steel [30] (DED-L = direct energy deposition laser, PBF-L = laser-based powder bed fusion). supported by works from [PERSON] et al. [2011], [PERSON] et al. [2012] and others [215; 217] as well as work on maraging steels [218] and for other materials due to the highly oriented microstructural growing direction where the anisotropic material properties occur [221; 222; 223; 64]. For quasi-static loads, this behavior is also present for specimens with as-welded surfaces [63]. This anisotropy shows up for the fatigue crack growth behavior as well [214]. Other studies state that additive-manufactured high-alloy steels show a homogeneous distribution of the material properties [216]. This difference can be influenced by the chosen energy input and manufacturing strategy and needs to be addressed in further research. To cover those effects, [PERSON] et al. developed an orthotropic elastic model for WAAM parts which included the microstructural orientation and calibrated this model with experiments. They stated that this model allowed the prediction of the elastic material behavior [244]. A special group within high-alloy steels are duplex stainless steels, which derive their properties from a defined austenite-to-ferrite ratio. This ratio is highly dependent on the cooling conditions and therefore on the welding parameters, but different welding parameters can also result in acceptable microstructures [222; 223; 225; 228; 245]. Furthermore, the repeated reheating during WAAM has an influence on the phase fractions of duplex steels and needs to be considered [246]. [PERSON] et al. showed that the additive manufacturing of heat-resistant martensitic chrome steels is possible as well, and they show proper mechanical properties, aside from the fact that long-term creep results were missing and not expected for the next few years [224]. Furthermore, [PERSON] et al. showed strong texturing of the microstructure [247]. In addition, [PERSON] et al. showed that post-weld heat treatment can have a significant and positive effect on the microstructure and mechanical properties of these types of steels, especially to reduce the amount of retained austenite and generate a fully martensitic microstructure [248]. Aside from the basic mechanical properties, the properties of the materials under wear is in the focus of the research and can be essential for technical applications.