Microstructure and mechanical properties of selective laser melted 17–4 PH stainless steel; Build direction and heat treatment processes

Abstract

The performance of the additive manufactured of 17–4 PH stainless steel parts is driven by microstructure properties. The microstructure profile is affected by the thermal history of the 17–4 PH which could be manipulated by altering the build direction and heat treatment process. This study investigates the effects of build direction and heat treatment on the microstructure and mechanical properties of 17–4 PH stainless steel produced via selective laser melting (SLM). The 17–4 PH samples were fabricated in two build directions (0 ° and 90 °) and two heat treatment processes of H750 and H1100. The XRD analysis indicates that the 0 ° build direction has the highest volume fraction of the martensite phase in the structure due to a higher cooling rate. For 0 ° build direction, the hardness of the as-built, H750 and H1100 indicate an increment from 27.2 HRC to 34.8 HRC and 39.5 HRC, respectively. Based on the build direction, the tensile strength is higher for the 0 ° build direction since the direction of the melt pool is parallel with the direction of the load. The tensile strength of the 0 ° build direction is the highest for H750 compared with H1100 due to the conversion of austenite to martensite at 750 °C. For both build directions, heat treatment at H1100 reduces the yield strength drastically due to the homogenisation of microstructure and removal of the strengthening mechanism by SLM boundary layers. Therefore, the highest strength is contributed by three main factors, i.e., martensite volume fraction, the direction of the melt pool with respect to load direction, and conservation of melt pool boundaries as seen in sample 0 ° build direction with H750 heat treatment.