# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 4 Residual Stresses and Distortion  
Similar to welding and other additive manufacturing process, residual stress and distortion are inevitable to WAAM processes [33; 108; 109]. [PERSON] et al. [108] reported that the stresses generated during the WAAM process led to large distortion of the parts after clamp removal. Post heat treatment is usually necessary to relieve these stresses. Besides, high residual stresses can have a substantial effect on mechanical properties of the manufactured structures. The residual stresses higher than the local UTS (ultimate tensile strength) of the material can lead to cracking, while the ones between the local YS (yield strength) and UTS can cause warpage or plastic deformation [110]. According to [PERSON] et al. [108], the residual stress distribution across the WAAM fabricated wall was uniform and the residual stress in the former layer rarely affected the subsequent layers. However, after the clamping was released, the value of the rebalanced internal stress at the top of the integral component became much lower than that at the interface to the substrate because of bending deformation of the sample. Apart from the above-mentioned residual stresses caused by the thermal history of WAAM process, for WAAM of steels, solid-state phase transformation is an important factor that one must consider in studying stress evolution [111]. In multi-pass laser metal powder deposition (LMPD) process of martensitic stainless steel (16 wt % Cr-4.5 wt % Ni-1.6 wt % Mo-0.9 wt % B-0.6 wt % Mn-0.12 wt % C), [PERSON] et al. [111] studied the influences of solid-state phase transformation on residual stress by the numerical simulation and XRD residual stress measurement. When phase transformation is ignored, the simulated residual longitudinal stress is around 1100 MPa. However, it is just about 253 MPa when phase transformation is considered. The residual longitudinal stress measured by XRD method is 390 MPa. With phase transformation and stress influence considered, the longitudinal residual stress is about 296 MPa, which is closer to the measured data (390 MPa). The results show that phase transformation can result in lower residual stresses. The austenite (high temperature phase) is the weaker phase. As the temperature decreases, the strain induced or stresses assisted martensitic transformation occurs. Transformation induced plasticity (TRIP) and the density change can result in the large stress reduction [111]. There are several types of distortion in WAAM parts, including bending deformation, shrinkage in the longitudinal and transverse directions [112]. The repeated heating and cooling can cause thermal expansion and shrinkage of the deposited parts, leading to deformation, especially in large thin-walled components [109]. Residual stresses and distortion are closely related to deposition paths in WAAM [113; 114]. [PERSON] et al. [114] reported that a bidirectional tool path with 180\({}^{\circ}\) rotation decreased the residual stress by around 50% at the bottom corner of the sample, which potentially reduces the cracking susceptibility. When fabricating martensitic stainless steel samples by the oscillation pass (Figure 9) [115], high distortion occurred in the substrate because of high heat input, as shown in Figure 10. [PERSON] et al. [116] reported that when deposition path was one-direction from left to right, the residual stresses resulted in a distinctive curvature of a single-bead wall, which can be seen in Figure 1 of [116]. The cooler left-end at the beginning makes the sample beginning a little higher than desired. However, the right-end of the build is lower than anticipated, because residual heat from the previous deposited layers causes the metal to flow more [116]. In general, proper deposition paths can notably reduce residual stresses and deformation, particularly in large scale WAAM fabrication [33; 114].