Role of HIPing and heat treatment on properties of alloy 718 fabricated by electron beam melting

Abstract

Although additive manufacturing (AM) of difficult-to-machine materials can open new design possibilities for production of parts with complex geometries, AM builds are characterized by presence of detrimental defects like shrinkage/ gas porosity and lack-of-fusion. Microstructural inhomogeneity, anisotropy in properties and formation of undesirable phases are added concerns. Motivated by the need for investigation of appropriate thermal post-treatments to eliminate defects, and to achieve desired microstructural and mechanical properties, the influence of hot isostatic pressing (HIPing) and heat treatment on electron beam melted (EBM) Alloy 718 specimens built at different heights from the base-plate has been studied. While HIPing led to orders of magnitude reduction in defect content, conventional heat treatment without applied pressure was found to have no effect. Thermal post-treatment was found to eliminate inherent variability in phase constitution and hardness of as-built specimens. HIPing at high temperature was accompanied by grain coarsening, suggesting modification of the HIPing protocol.