Effect of heat treatment on the microstructure and surface damage evolution of selective-laser-melted IN718 alloy

The microstructure and damage evolution behavior of IN718 alloy prepared by selective laser melting (SLM, referred to as SLM IN718 alloy) under different heat treatment processes were studied using optical microscopy, scanning electron microscopy, electron backscatter diffraction, microhardness tests, and other material tests, combined with digital image correlation. The heat treatment of the SLM IN718 alloy achieved different degrees of recrystallization and transformed the microstructure from dendritic crystals to bulk crystals with the precipitation of a large number of γ′ and γ′′ phases. As the heat-treatment temperature was increased, the δ phase of the SLM IN718 alloy was precipitated and then dissolved, the microhardness and strength of SLM IN718 alloy increased and then decreased, the elongation decreased and then increased, and toughness fracture was the main fracture mode. Therefore, heat treatment can be used to alter the relative proportions of recrystallized and substructured grains in the SLM IN718 alloy, thereby modulating its overall mechanical properties. After heat treatment, the damage factor entered the rapid damage stage earlier, and the critical damage factor increased as the critical plastic strain was increased. Further, the damage evolution equations of the SLM IN718 alloy under different heat treatment processes were established.

Keywords:
Selective-laser-melted IN718 alloy; heat treatment; damage evolution; digital image correlation; microstructure