Contribution of hot isostatic pressing on densification, microstructure evolution, and mechanical anisotropy of additively manufactured IN718 Ni-based superalloy

Abstract

Microstructural defects and mechanical anisotropy are regarded as the main obstacles in additively manufactured, laser powder bed fusion (LPBF) IN718 superalloy, decelerating industrial development of the related components. Hot isostatic pressing (HIP) is an effective solution to overcome these challenges. In this study, standard heat treatment (HT) with and without prior HIPing was applied to the LPBF IN718 and the impacts of the HIP operation on microstructures and room- and high-temperature mechanical anisotropy were studied. Observations showed that intrinsic pores and large contour defects were significantly closed by HIPing and consequently the relative density enhanced from 99.50% to 99.96%. The highly distorted columnar structure of the HT specimen was broken down due to recrystallization, and an equiaxed morphology with about 5-time coarser grain size appeared in the HIP + HT specimens. Although too many transgranular precipitates were observed in the HT microstructure, they were just formed at grain boundaries when HIP was carried out before HT. Furthermore, the formation of nano-scale γʺ/γʹ/γʺ co-precipitates was favored by HIPing which had a positive effect on strengthening. It was also found that prior HIPing effectively mitigated mechanical anisotropy both at room- and high-temperatures by influencing several factors. The degree of room-temperature mechanical anisotropy was reduced from 11.6% (HT) to 3.5% (HIP + HT) by HIPing and mitigated from 4% to 2% at 650 °C. While the fracture mechanism was orientation-dependent in the HT samples, it was completely ductile for the HIP + HT samples in all circumstances.