# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 3 Microstructure
[89] mixed two different filler metals to decrease the nickel equivalent, thus increased the ferrite content in the specimens. The sample fabricated with 100% of the filler metal G 22 9 3 (see Figure 6(a)) contains large austenitic side plates. From Figure 6(a)-\(f\), as the percentage of the filler metal GZ 22 5 3 increases, the nickel content reduces, and the austenite plates become smaller in size and less in content. The sample fabricated with 100% of the filler metal GZ 22 5 3 (see Figure 6(f)) contains relatively small austenite platelets. However, the investigations carried out by [PERSON] et al. did not involve any post processing heat treatments. Figure 6: Microstructures in the adjacent area of the middle section of the deposited walls [100] (H00 Cr21 Ni10 steel). [PERSON] et al. [91] researched the microstructures of CMT 17-4 PH stainless steel samples under four different heat treatment conditions, including as-deposited (AD), as-deposited plus H900 ageing treatment (AD + H900), and as-deposited followed by solution treatment and H900 ageing (ST + H900; solution treatment: 1040 \({}^{\circ}\)C for 30 min; H900 ageing treatment: 480 \({}^{\circ}\)C for 1 h), as presented in Figure 8. The shielding gas was comprised of 38% He and 2% CO\({}_{2}\) in argon (SG1). The Cr\({}_{\text{eq}}\)/Ni\({}_{\text{eq}}\) ratio of 17-4 PH stainless steel is greater than 1.55, which yields a primary ferrite solidification mode. Under equilibrium cooling, phase transformation sequence of alloy 17-4 PH is as follows: L \(\rightarrow\)\(\delta\)-ferrite \(\rightarrow\)\(\gamma\)-austenite + \(\delta\)-ferrite \(\rightarrow\) martensite + \(\delta\)-ferrite [106]. However, the high cooling rates during WAAM processing limit the amount of \(\delta\)-ferrite that can convert to \(\gamma\)-austenite. Thus, some of the \(\delta\)-ferrite will remain at room temperature. This is why a dendritic martensite-\(\delta\) ferrite microstructure can be observed in the AD condition sample (Figure 8a). Additionally, most \(\delta\)-ferite is transformed into austenite which thereafter transformed into martensite due to the slower cooling rate when temperatures are above A\({}_{3}\). As a result, the microstructure is mainly composed of martensite, as shown in Figure 8c (sample under ST+H900 condition) [91]. Figure 8: Micrographs [91]: **(a)** Sample fabricated with SG1 (Shielding gas 1: 38% He + 2% CO\({}_{2}\) + Ar), As-deposited condition; **(b)** Sample for tensile testing, AD + H900 condition; **(c)** Sample for tensile testing, ST+H900 condition (Material: 17-4 steel). Figure 7: Micrographs of the structures fabricated with different mixing ratios at 500 x magnification, taken in layer 10, etching: Beraha II, white: austenite, blue/brown: ferrite [89]. (WAAMed duplex stainless steel components by mixing two different filler metals)  
[PERSON] et al. [91] also reported the influence of shielding gas (SG) composition on the final microstructure. They found that a higher percentage of retained austenite was observed when using SG1 (Shielding gas 1: 38% He + 2% CO\({}_{2}\) + Ar) compared with using SG2 (Shielding gas 2: 2.5% CO\({}_{2}\)+Ar). Different heating cycles in different areas produce an inconsistent solid phase transformation, leading to different microstructures of H00 Cr21 Ni10 stainless steel part [100]. Cooling rates and temperatures of different heat treatments have a significant impact on phase transformation of alloy 17-4 PH [91]. For duplex stainless steels, the ratio of austenite to ferrite phase crucially affects the properties in strength and corrosion resistance [107]. The high content of austenite and the formation of secondary austenite can significantly lower corrosion resistance and strength of stainless steel [89]. In summary, thermal history in WAAM processes play an important role in controlling the microstructure, such as the ratio of austenite and ferrite phases.