# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 7 Summary and Outlook  
WAAM has been proved to be a low-cost, high efficiency AM process (compared with PBF AM processes) for producing large-scale stainless steel parts. Recent studies on WAAM of various stainless steels have been reviewed from the aspects of macroscopic characteristics, microstructure evolution, post heat treatments, residual stress and distortion, defects, and mechanical properties. Macroscopic characteristics of WAAMed stainless steel parts are closely related to wire feeding speed and scanning speed, welding current mode, cooling time, and interlay temperature. Further studies on optimizing the above-mentioned process parameters are needed for achieving better dimensional accuracy and surface quality of WAAM parts. Thermal history in WAAM processes plays an important role in controlling the microstructure, such as the ratio of austenite and ferrite phases. Thus, it is feasible to control the microstructure by controlling the process parameters. Modifying the material compositions, such as using double wires, is another effective approach to achieve desirable final microstructures. Post heat treatments can also significantly influence and change the final microstructure. Therefore, in order to obtain desirable microstructures in WAAM stainless steel parts, it is very necessary to thoroughly understand the solidification behaviors [104] and phase transformations in complex WAAM thermal cycles, and the correlation between material compositions, process parameters, heat treatment parameters, and the final microstructure. Residual stresses and thermal history have a significant impact on the distortion of WAAM parts. The effect of both becomes more pronounced as the component volume increases [116]. The residual stresses can cause deformation, decrease in dimensional accuracy, defects, as well as worsening of mechanical properties of the components. Phase transformation (especially martensitic phase transformation) has a significant impact on reducing residual stress [111]. Proper deposition path planning can also significantly reduce residual stresses [114]. Therefore, further efforts should be made to optimize the WAAM deposition path, the thermal history, and control the phase transformation during WAAM to reduce the residual stress and distortion through both modelling and experimental studies. Defects, such as lack of fusion, cracks, and porosity, need to be controlled to a minimum level to achieve sound mechanical properties. Precise control of heat input and thermal history, proper shielding gas and tight gas seals, high quality feedstock, and clean substrate surfaces are helpful to reduce defects during stainless steel WAAM process. WAAM techniques, material compositions, process parameters, shielding gas composition, post heat treatments, microstructure, and defects can significantly influence the mechanical properties of WAAM stainless steels. Further understanding on the correlation between these factors and precise control of them, especially the final microstructure, are important for better controlling the mechanical properties of WAAM stainless steels. WAAM samples often show considerable anisotropy in terms of tensile strength and microstructure due to the directionality of layer wise deposition process. It is useful to develop new processes, such as in-situ rolling + WAAM, to solve the anisotropy problems caused by the coarse columnar crystals in the deposition direction. In-situ rolling + WAAM also can reduce the residual stress in WAAM parts. The main limitation of the paper is that the numerical modelling of the WAAM process was not sufficiently discussed in this paper. For future industrial applications, fatigue properties, and corrosion behaviors of WAAMed stainless steels need to be deeply studied. Additionally, further efforts should be made to improve the WAAM process to achieve faster deposition rates and better quality control.