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

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 (ε_f) were analysed and explained based on the microstructure evolution. Dendrites and molten pool boundaries are mainly responsible for the observed anisotropy in ε_f of horizontal and vertical samples in the as-fabricated condition. After their dissolution by HT an increase in ε_f was observed. The columnar grain structure also contributes to the observed anisotropy in ε_f, 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 ε_f. 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 M_xC_y were partially segregated at the grain boundaries after HIP with detrimental effect on ε_f.