Influence of post heat treatments on anisotropy of mechanical behaviour and microstructure of Hastelloy-X parts produced by selective laser melting

Abstract

Selective laser melting (SLM) was used to fabricate tensile specimens using Hastelloy-X pre-alloyed powder. Mechanical behaviour at room temperature, normal and parallel to building direction, was investigated. Furthermore, as-fabricated tensile samples were compared with ones post processed by heat treatments (HT), hot isostatic pressing (HIP) and a combination of both (HIP+HT).Yield strength (YS), ultimate tensile strength (UTS) and elongation to failure (εf) were analysed and explained based on the microstructure evolution. Dendrites and molten pool boundaries are mainly responsible for the observed anisotropy in εf of horizontal and vertical samples in the as-fabricated condition. After their dissolution by HT an increase in εf was observed. The columnar grain structure also contributes to the observed anisotropy in εf, inducing more ductile and cleavage like fracture surfaces in vertical and horizontal samples, respectively. The removal of porosity after HIP and HIP+HT yields a positive effect on εf. HIP or HT after SLM reduces the YS due to recovery processes such as dislocation density reduction and rearrangement of these dislocations in subgrain boundaries. Carbides of the type MxCy were partially segregated at the grain boundaries after HIP with detrimental effect on εf.