Effect of Thermal Cycling on Grain Evolution and Micro-Segregation in Selective Laser Melting of FGH96 Superalloy

Abstract

Extremely rapid heating and cooling rates during the additive manufacturing (AM) process generate complicated thermal cycles, which affect the microstructure evolution and ultimate mechanical properties of the alloy. In this paper, FGH96 blocks with a height of 6 mm were prepared by selective laser melting (SLM) and the microstructure was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Comparing specimens of varying heights, it was found that subsequent thermal cycles (STC) coarsened some solidified grains and accelerated the grain growth along the build direction, together with an increase in texture intensity and high-angle grain boundaries (HAGBs). After coarsening the grains in the middle portion of the built block, finer grains were observed near the top area due to a faster cooling rate. There were numerous dislocations in the grain because of the occurrence of unequal internal tension. In the middle of the sample with stable thermal cycles, the dislocations were both perpendicular to the grain growth direction and 45° off it. In spite of the texture characteristics, the segregation of elements was also found to be influenced by thermal cycling. Inherent reheating leads to the increase in the Laves phase and the decrease in the γ’ phase as subsequent deposition. This was also one of the reasons why the microhardness of the sample decreased as the building height and the other reason being the decrease in the solution treatment of the later sediments.