Effects of hot processes on microstructure evolution and tensile properties of FGH4096 Ni-based superalloy processed by Laser Powder Bed Fusion

Abstract

Hot Isostatic Pressing (HIP) is one of the most important manufacturing processes to improve the performance of powder metallurgy nickel-based superalloys produced by Laser Powder Bed Fusion (LPBF), which can effectively reduce the pores and cracks in the LPBF alloys. However, there are limited studies on the effects of tuning HIP parameters on the LPBF alloys microstructure and mechanical performance. In this paper, we systematically studied the effects of hot processes on the microstructural evolution and tensile properties of an LPBF prepared typical second generation of powder metallurgy (PM) nickel-based superalloy which was designated as FGH4096. The as-deposited alloy was HIP treated at sub-solvus, solvus, and super-solvus temperature, respectively, followed by an optimized heat treatment (HT). The effects of HIP temperature on the microstructures and mechanical properties of the LPBF alloys were studied by Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and Electron Backscattered Diffraction (EBSD). The results indicated that both HIP and HT processes led to recovery and recrystallization in the as-deposited alloy. During HIP, the increase of HIP temperature gradually eliminated the dendritic and equiaxed structures in the alloy while refined the grains along the long axis simultaneously. However, the columnar crystal morphology in the vertical section, together with straight grain boundaries, was still maintained during HIP. In all the HIPed alloys, the secondary γ′ precipitates were sparsely distributed, and due to the existence of continuous pressure, some grains with large strains remained. After HIP + HT, the dendritic and equiaxed structures basically disappeared. The grains with more curved grain boundaries tended to be equiaxed and irregularly shaped. Compared with other state alloys, the LPBF + HIP (solvus) + HT alloy has a more uniform and dense distribution of secondary γ′ precipitates, together with a chain distribution of block primary γ′ precipitates at the grain boundaries, which contributes mostly to the highest room and high-temperature tensile strengths. Moreover, the LPBF + HIP (solvus) + HT alloy has the lowest internal strain among the treated alloys, leading to the highest elongation.