Correlation of microstructure, mechanical properties, and residual stress of 17-4 PH stainless steel fabricated by laser powder bed fusion

Abstract

17-4 precipitation hardening (PH) stainless steel is a multi-purpose engineering alloy offering an excellent trade-off between strength, toughness, and corrosion properties. It is commonly employed in additive manufacturing via laser powder bed fusion owing to its good weldability. However, there are remaining gaps in the processing-structure-property relationships for AM 17-4 PH that need to be addressed. For instance, discrepancies in literature regarding the as-built microstructure, subsequent development of the matrix phase upon heat treatment, as well as the as-built residual stress should be addressed to enable reproducible printing of 17-4 builds with superior properties. As such, this work applies a comprehensive characterisation and testing approach to 17-4 PH builds fabricated with different processing parameters, both in the as-built state and after standard heat treatments. Tensile properties in as-built samples both along and normal to the build direction were benchmarked against standard wrought samples in the solution annealed and quenched condition (CA). When testing along the build direction, higher ductility was observed for samples produced with a higher laser power (energy density) due to the promotion of interlayer cohesion and, hence, reduction of interlayer defects. Following the CA heat treatment, the austenite volume fraction increased to ∼35%, resulting in a lower yield stress and greater work hardening capacity than the as-built specimens due to the transformation induced plasticity effect. Neutron diffraction revealed a slight reduction in the magnitude of residual stress with laser power. A concentric scanning strategy led to a higher magnitude of residual stress than a bidirectional raster pattern.