# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 3 Microstructure  
The microstructures of WAAM stainless steel are largely determined by the thermal history during the processes. During WAAM processing, the thermal cycle--including repeated heating and cooling [102; 103]--creates non-equilibrium microstructures in the deposited parts [104]. Table 7 summarizes the microstructure, tensile strength, yield strength, and elongation of stainless steels manufactured by different WAAM technologies. The process factors affecting microstructure and mechanical properties are listed in Table 7. The microstructures of a H00 Cr21 Ni10 stainless steel part prepared by DWF-PAM are illustrated Figure 5[100]. Owing to different heating state and thermal cycles in different areas of the deposited samples, the microstructures are characterized by three different regions: the top, middle, and bottom sections. The macrostructure, microstructure, and mechanical properties of the middle section are critical to the manufactured components. As presented in Figure 5b, the ferrite (8-Fe) phase are embedded on the fine austenite matrix and each layer consists of a sequence of fine grain (C), columnar grain (B), and equiaxed grain  
\begin{table}
\begin{tabular}{l l l l l l l l l l} \hline \hline
**Process** & **Materials** & **Process Factors** & **Microstructure** & **YS (MPa)** & **UTS (MPa)** & **EL (\%)** & **Hardness (HV)** & **Reported by** \\ \hline DWF-PAM & HOQC21N101 & Scanning speed & A lot of complete grown & / & & About 550, similar in the vertical and horizontal directions & 44.7-58.7 & \(\sim\)193 & [PERSON] et al. [100] \\  & (sutentile) & Scanning speed & & & & horizontal directions & & \\  & stainless steel & & & & & & & \\ \cline{3-8} SWF-PAM & & & A number of incomplete & & Around 510, similar & & & \\  & & & grown equaised ferrite (ICEF) & / & in the vertical and horizontal directions & 20.5-35.4 & \(\sim\)187 & [PERSON] et al. [100] \\ \hline SpeedPulse WAM & 316 (tausticnit) & Heat input, cooling & Corner secondary dendrite & Horizontal 418.0 & Horizontal 550 \(\pm\) 6 & / & / & [PERSON] et al. [90] \\ SpeedArc & & & & Smaller secondary dendrite & & & & Higher than the hardness of \\ WAAM & & & & & & & \\ \hline GMAW-CMT & Duplex Stainless Steel & Chemical composition of feedstock & As the nickel content reduces, the austenite plates become smaller in size and less in content & / & / & / & / & [PERSON] et al. [99] \\ \hline CMT & 17-4 PH & As-built, Shielding & Higher percentage of retained & & & & \\  & (martenstic) & As-built, Shielding & (\(\approx\)0.85), area & percentage. Mainly a dendritic & / & Average 994 & Average 11.9 & \(\sim\)330 & [PERSON] et al. [91] \\  & stainless steel & & & & & & \\ \cline{3-8}  & & & & & & & \\ \cline{3  
Similarly, [PERSON] et al. used GMA-WAAM to fabricate 2209 duplex stainless steel (DSS) walls and tubes, and indicated that calculation of cooling rates in deposited layers could help forecast the formation of the phases [105]. Their numerical and experimental results revealed that austenite formation could be significantly promoted by slow cooling rates in the layers at elevated temperatures. [PERSON] et al. reported that cooling rate significantly affected the scale of the solidification structure [90]. The relationship between cooling rate (GR, the cooling rate in K/s) and secondary dendrite arm spacing (\(\lambda_{2}\)) is given in Equation (1). \[\lambda_{2}=50(\text{GR})^{-0.4} \tag{1}\]  
According to Equation (1), SpeedArc WAAM could produce smaller secondary dendrite arm spacing in 316L stainless steel components than SpeedPulse WAAM because of the higher cooling rate [90]. (SpeedArc: the primary metal transfer mode is the short-circuited transfer mode. SpeedPulse: the primary metal transfer mode is a projected spray transfer mode.) Other factors also have specific effects on phase balance, including material composition [105] and heat treatments [91]. For instance, in the investigation of adjusting the ferrite-austenite ratio of WAAM duplex stainless steel components, [PERSON] et al.