# Monotonic and Fatigue Properties of Steel Material Manufactured by Wire Arc Additive Manufacturing
## 4 Discussion
### Findings from the Conducted Experiments
In total, the layers with normalized material (measuring points D2 and D3) in the near-surface area show residual stress states that tend towards the compression side at both interlayer temperatures--the maximum principal stress is less far in the tensile range than the minimum principal stress in the compression range, Figure 6. This can be explained by the cooling conditions of the cuboid and is attributed to the cooling from the interlayer temperature level to room temperature. Since the cooling takes place from the surface inwards, the material is kept at a higher temperature level there for a longer time than at the surface. The yield strength, which is dependent from the temperature, is therefore lower in the core than at the cooled surface. This leads to a compression of the warmer core material compared to the colder surface layer during cooling and to the formation of residual compressive stresses when the entire body has cooled down [26]. Figure 6 shows the magnitude of the two principal stresses and their directions. The maximum principal stress (tensile residual stress) is present in a plane parallel to the layer structure, the minimum principal stress (compressive residual stress) under 90\({}^{\circ}\) to it, i.e., in crosswise direction. This also seems plausible. The compressive residual stresses are caused along the largest dimension of the cuboid by the cooling processes described above, and the tensile residual stresses by transverse contraction. This would also explain the ratios of the two principal stresses. If, on the other hand, the residual stress states in the upper non-normalized layers (measuring point D1) are considered, different amounts of residual stress are noticed, but also a different orientation of the principal axis systems. These are rotated by 90\({}^{\circ}\) compared to those in the normalized layers. The residual stresses appear to be generated by cooling conditions of a different nature than those described above for larger cuboid areas. The cooling of material that has just been deposited and is now solidifying is initially carried out at a higher speed and locally more restricted. The internal stresses that are initially apparent in this way remain unchanged in the upper layers. In the lower layers, however, they are released by the subsequent layers applied later and the renewed energy input. The cooling from interlayer temperature to room temperature then follows the processes described above. If the differences in the residual stress states for the two interlayer temperatures are now considered, it is noticeable that the principal stresses in the normalized layers (measuring points D2 and D3) are less far apart at 300 \({}^{\circ}\)C than at 150 \({}^{\circ}\)C interlayer temperature. The amounts of the two principal stresses are greater for 150 \({}^{\circ}\)C and are thus located further in both the tensile and compressive range. At measuring point D1 in the non-normalized range, significantly greater residual stresses can be detected for 150 \({}^{\circ}\)C than for 300 \({}^{\circ}\)C. The amounts of the latter are in the same range as the values in the normalized material. The higher interlayer temperature seems to significantly reduce the influence of the local cooling of the solidifying material due to longer holding at a higher temperature level. The effects found here and the influence of the interlayer temperature can certainly be qualitatively transferred to a more complex WAAM geometry, i.e., a real component, but such a component will show a more complex thermal conductivity and cooling behavior due to the more complex geometry. The residual stress states in a real WAAM component will therefore be difficult to predict without simulation tools for this problem. Overall, however, the positive effect of heat treatment by subsequent layers is noticeable, which tends to lead to compression-dominated residual stress states at the surface of a WAAM component.