Microstructural and Mechanical Characterization of Selective Laser Melted 17-4 PH Stainless Steel: Effect of Laser Scan Strategy and Heat Treatment

17-4 PH stainless steel (SS) is a precipitation-hardening (PH) type of steel manufactured by selective laser melting (SLM) and known for high strength and corrosion resistance. The selection of process parameters plays an important role to reduce defects and improve mechanical properties. This study investigated the effect of hexagonal and chessboard scan strategies on relative density and residual stress of SLM-printed 17-4 PH SS samples. Relative density decreased as the scan strategy changed from hexagonal to chessboard due to insufficient melting, which causes pores in the samples and the residual stress was lower in the chessboard scan strategy due to lower thermal gradient. In this study, the hexagonal scan strategy was chosen based on higher relative density. The microstructure and mechanical properties of as-built and heat-treated (SA, SA + H900, SA + H1150) samples for optimized scan strategy were studied. The microstructure of 17-4 PH SS contains austenite and martensite. Austenite in as-built samples was retained due to nitrogen inert atmosphere, which acts as an austenite stabilizer and this can be further reduced by heat treatments, which is confirmed by EBSD and XRD. Precipitates were formed during aging heat treatments, which effects the mechanical properties. Mechanical properties depended on martensitic phase fraction and grain size. Grain size decreased from as-built to heat treatments. The increase in martensitic phase fraction and decrease in grain size resulted in high hardness and high strength. In this study, SA + H1150 samples were completely converted into martensite and showed higher mechanical properties.