# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.1 Steel
Figure 20: Wire arc additive manufacturing (WAAM) material: (**a**) semi-finished block products, (**b**) sampling plan for tensile specimens with directional designation related to the weld layers [189]. Furthermore, they derived stress-strain curves for the four different states and showed that the fatigue strength had no significant anisotropic behavior (Figure 22 and Table 2) [189]. This behavior can be explained by the intense reheating and heat input they chose. This led, as shown in Figure 16, to a non-welding related microstructure which had different mechanical properties and a less pronounced anisotropy. Aside from the fatigue properties, crack growth rates become more important to avoid brittle instantaneous fractures. [PERSON] et al. showed that the crack growth rates did not differ between different specimen orientations in accordance to the welding direction, and they furthermore showed that the commonly used determination methods for fatigue crack growth rates can be applied to WAM-made parts as well [238]. To conclude, for the subsection regarding the wire arc additive manufacturing of low-alloy steels, it can be stated that low-alloy steels are processable with WAM and reach a wide range of material properties, which can be in correspondence to known properties from the used welding filler. High-Alloy Steels in Additive Manufacturing  
High-alloy steels are usually grouped with stainless steels and heat-resistant steels, and this group contains other steels as well. Within these classes, high-alloy steels can also be differentiated due to the microstructure (e.g., ferritic stainless steels, austenitic stainless steel, austenitic-ferritic duplex stainless steel, ferritic chrome steels and martensitic chrome steels). One large group within high-alloy steels is stainless steels. [PERSON] et al. gave a good overview of the work up to January 2020 in the wire arc additive manufacturing of stainless steels [30]. They pointed out that the macroscopic characteristics-the geometry-are closely related to the process values, such as wire feed and travel speed and also the energy input \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Interlayer** & **Extraction** & **Fatigue Limit** & **Fatigue Limit** \\
**Tem./\({}^{\circ}\)C** & **Direction** & \(\sigma_{\mathbf{a,E}}\)/MPa & **Converted to Strain \(\epsilon_{\mathbf{a,E}}\)** \\ \hline
150 & lengthwise & 256 & 0.137\% \\
150 & crosswise & 254 & 0.129\% \\
300 & lengthwise & 263 & 0.154\% \\
300 & crosswise & 267 & 0.148\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fatigue limits determined by staircase tests at \(5\times 10^{6}\) cycles with a probability of failure Pf = 50% [189]. Figure 22: Strain–life curves (logarithmic scale) from the strain-controlled tests, including the fatigue limits from the stress-controlled tests [189]. and cooling times. Furthermore, they stated that understanding of the solidification behavior is crucial for an adequate manufacturing process and the correlates with the common view in joint welding for high-alloy steels, and it is correspondent with the common knowledge of welding processes, where each solidification mode (e.g., ferritic, ferritic-austenitic or austenitic-ferritic) has its own challenges. Aside from the microstructural implications, the effect of residual stresses and distortion, which occur due to thermal cycles, has to be considered, and thus it will increase with the increasing part size [239]. Furthermore, [PERSON] et al. pointed out that the WAAM techniques, material composition, shielding gas composition, post-heat treatments, microstructure and welding-related defects have a significant influence on the material properties of the stainless steel parts [30]. The main difference of WAAM in comparison with other additive manufacturing processes is the higher energy input (Figure 23) [30]. This main difference allows the usage of established welding consumables for the welding of high-alloy steels.