# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.1 Steel  
Steel is one of the most used materials in construction and many other sectors of engineering. Therefore, steel is one of the most studied materials for WAAM as well. Due to the intense use in all areas of the industry, the mechanisms regarding welding-related microstructural and property changes are widely understood but still under investigation. For most applications and commonly used steel grades, the knowledge base of welding-related issues is huge. This knowledge includes single- and multi-run welds. These results for multi-run welds can be transferred to additive manufacturing with few changes. For example, the influence of different alloying elements such as titanium niobium and vanadium on the microstructure and properties of low-alloy steels are currently under investigation for joint welding [179, 180, 181, 182]. In particular, work regarding nucleation for grain growth and the interaction between the alloying elements can be transferred to WAAM [179, 183, 184, 185]. Aside from microstructural changes, the building-mechanisms of welding-related residual stresses can be transferred to WAAM as well since they are driven by microstructural phase transformations and restraints [186, 187, 188]. However, the restraints between joint welding and WAAM are different. The main difference between WAAM and joint welding is that the heat dissipation and some related microstructural phenomena become more pronounced in WAAM than in joint welding. For example, [PERSON] et al. has shown that the microstructure of a low-alloy steel can change from a welding-related one into a non-welding-related one due to repeated reheating and the chosen process values [53, 189]. Commonly, steels can be divided into two groups: high- and low-alloy steels. In most cases, they differ in the microstructure and the welding-related effects that occur. In both cases, the properties of the deposited material depend on the thermal history of the material and are correlated to the microstructure, toughness, fatigue strength material and the used low-alloy steel [53, 62, 118, 189, 190, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 191, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230]. The microstructure after manufacturing largely determines the properties of the additively manufactured component, and different steels can be combined in multi-material structures to tailor the material properties to the needs of the application [52, 231, 232]. The current state of knowledge for the two steel groups is summarized below. Low-Alloy Steels in Additive Manufacturing  
Low-alloy steels are widely used in construction and industry applications (e.g., offshore construction) [233]. Currently, there are different grades of steel under investigation. The most studied grade is ER70S [118, 191, 192, 193, 194, 195, 196, 197, 198, 198, 201, 202, 203, 204, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 223, 224, 225, 226, 227, 228, 229, 230]. This trend in research leads to the usage of more advanced high-strength materials like ER90S [53, 189, 201], ER110S-G [209, 236] or ER120S [62, 196, 201], but other grades are studied as well, such as ER55-Ni2 [210]. All works regarding the microstructure and the mechanical properties are related to the energy input. This behavior can be anticipated by the common knowledge about steel and its microstructure, and these relationships are known for most welding consumables and obtained for single- or multi-run welds. These differ little in most cases from that one found for additively manufactured materials, although the dilution and the number of heating cycles differ from joint welding. If the energy input into low-alloy steels is too large, and if they are repeated too often, the microstructure changes from a weld microstructure to a structure of \"normal\" heat-treated steel. [PERSON] et al. showed that a ferritic-periltic microstructure can be achieved due to WAAM.