Improving high-temperature mechanical properties of laser powder bed-fused Inconel 738 alloy by hot isostatic pressing: Tailoring precipitates and healing defects

Abstract

Microscopic defects and inferior high-temperature mechanical properties that occur during laser powder bed fusion (LPBF) of Inconel 738 alloys can severely hinder their industrial applications. Hot isostatic pressing (HIP) is a promising method to overcome these issues. In this study, the effects of HIP on the defects, grain structure, precipitated phase behavior, room- and high-temperature mechanical properties of an as-built Inconel 738 alloy were investigated. The results showed that, after HIP treatment, the defect densities in the XY and YZ planes of the as-built Inconel 738 alloy decreased from 0.537% and 0.292% to 0.251% and 0.123%, respectively. Moreover, the grain morphology of the as-built Inconel 738 alloy changed from elongated columnar grains to coarse equiaxed grains after HIP treatment, and the grain size became six times larger than that of the original sample, which was attributed to recrystallization and grain growth. Moreover, HIP treatment eliminated cellular dendritic segregation and significantly reduced dislocation densities in the as-built Inconel 738 alloy. Consequently, fewer M23C6 carbides were formed, but block-like M23C6 carbides were uniformly distributed at the grain boundaries after a combination of HIP and standard heat treatments. The HIP treatment reduced the room-temperature tensile properties of the as-built Inconel 738 alloy because of grain growth, low dislocation density, and coarser precipitates. However, the high-temperature (800 °C) tensile properties of the HIP sample with standard heat treatment (σuts = 827.8 MPa, σy = 744.3 MPa, εf = 2.2%) increased significantly compared to those of the as-built sample with standard heat treatment (σuts = 715.6 MPa, σy = 653.3 MPa, εf = 0.6%) and alleviated the embrittlement fracture owing to the formation of uniform carbides and lower defect densities. These results indicate that HIP treatment can significantly improve the high-temperature mechanical properties of LPBF-built nickel-based superalloys with high Al and Ti contents.