Microstructural evolution and defect formation in a powder metallurgy nickel-based superalloy processed by selective laser melting

Abstract

In this study, the selective laser melting (SLM) technology has been employed to manufacture a nickel-based superalloy which was conventionally prepared through powder metallurgy (PM) route. The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart. Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point (γ + γ’) eutectic played a vital role in their formation mechanism. Columnar grains oriented along building direction were ubiquitous, corresponding to strong <001> fiber texture. Solidification cell structures and melt pools are pervasive and no γ’ precipitates were detected at about 10 nm scale before heat treatment. After super-solvus solution and two-step aging treatments, high volume fraction γ’ precipitates emerged and their sizes and morphologies were comparable to those in PM alloy. <001> texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events. Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization. Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.