# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.1 Steel
Figure 16 shows this and how the occurring microstructure changes with an increase of the interpass temperature. The beginning of perlite dissolution at a higher interpass temperature can be seen. Normally, however, for their used material, a welding-related microstructure has to be anticipated [53,189]. [PERSON] et al. and [PERSON] et al. showed that an isotropic microstructure is achievable for WAAM parts, and their impact toughness is high enough to have ductile behavior at room temperature [191,192]. Furthermore, [PERSON] et al. showed that the additive-manufactured ER70S has similar corrosion behavior to St37 (S235) [192]. The mechanical properties of WAAM ER70S are highly anisotropic. Figure 17 shows this anisotropy and the found explanations from [PERSON] et al. [193,198]. Figure 16: Micrographs of WAAM material for the interlayer [53]. Figure 17: Stress–strain curve for WAAM ER70S [193]. The state of the vertical tensile test specimen across from the building direction has mixed mode fracture behavior of a brittle and ductile fracture, and it was proposed that the found anisotropy is related to welding defects. This anisotropic behavior has also been found in various other studies (e.g., [203]). For high-strength steels like ER120S, [PERSON] et al. and [PERSON] et al. showed that the mechanical properties are related to the used heat input [62; 196]. The heat input is related to the cooling times and the decrease in hardness due to larger cooling times, which is commonly known for high-strength steel welding metal and high-strength low-alloy steels as well. This is also the case for newly developed materials [199]. The dependence of the mechanical properties on the heat input has been shown by [PERSON] et al. for a cross-process study of GMA welding and plasma arc welding [200]. A detailed model for the cooling times and the geometrical weld seam characteristics has been developed by [PERSON] et al. for a high-strength low-alloy steel (Figure 18) [237]. Furthermore, they showed that the residual stresses within WAAM-made parts increase with increasing heat input [237]. For low-alloy steels, toughness is one critical aspect, and the toughness needs to be kept above certain application-dependent ranges. For ER70S, [PERSON] et al. showed the experimental results for Charpy tests (Figure 19) [207]. They showed that the achievable toughness was in the range of the comparable X65 steel and, in some case, above it. Figure 19: Charpy impact data and ductile-to-brittle transition curve fit for carbon steel arc-AM specimens [207]. Figure 18: Average layer width (**a**), layer height (**b**) and \(\mathrm{t_{8/5}}\) cooling times (**c**) as a function of the wire feeding and welding speed in the investigated parameter range (contour plots of the regression models) [237]. The relatively high impact toughness was closely related to the goals for additive manufacturing of pressure vessels in the last century. The toughness of the weld metal can overcome the toughness values for correspondent steels in many cases, especially in the beginning of industrial steel production. Aside from the toughness and strength, the fatigue strength of the additive-manufactured low-alloy steel is one point regarding the usability of the additively manufactured parts. [PERSON] et al. looked closely at the material behavior under fatigue. They used a tensile specimen cross-wise and lengthwise to the welding direction (Figure 20), welded with two different interlayer temperatures. These specimens have undergone different strain-controlled fatigue testing. The stress-strain curves for the four used specimen types are shown in Figure 21. They showed that the Young's modulus from those curves differed between the different states but were more or less in the range of a normal scatter of those values [189]. Figure 21: Cyclic stress–strain curves from the strain-controlled tests [189].