Evolution of microstructure and its influence on the mechanical behavior of LPBF Inconel 625 upon direct aging

Abstract

Additive manufacturing is known for its many benefits over traditional manufacturing, however, it produces very different microstructure compared to wrought materials. Laser powder bed fusion (LPBF) produces heterogeneous microstructure that requires different heat treatment schedule than typical treatments that are applied to wrought material and it requires extensive research to understand the response of a material to different heat treatments. In this study, an approach has been made to understand the evolution of LPBF Inconel 625(IN625) microstructure upon one step direct aging at different temperatures until it gets close to full recrystallization. Microhardness after these treatments has also been reported and explained in the light of microstructure obtained by each heat treatment. Microstructure and mechanical behavior of LPBF IN625 are compared to as-received wrought IN625. Precipitates found at each temperature are confirmed by either electron diffraction pattern or energy dispersive spectroscopy. Strengthening γ′′ precipitate is found in as-printed as well as heat treated at 700 °C LPBF IN625. Detrimental δ phase is identified after heat treatment at 900 °C and carbides are found after 1050 °C. Hardness of as-printed LPBF IN625 is higher than that of wrought IN625. Similar trend is observed for both 700 °C and 900 °C heat treatment where hardness first decreases after 2 h, then increases after 5 and 10 h. For 1050 °C, hardness first decreases after 2 h and gets closest to hardness of wrought IN625, then increases after 5 h before decreasing slightly again after 10 h.