Tailoring the microstructure and mechanical properties of laser metal-deposited Hastelloy X superalloy sheets via post heat-treatment

Abstract

Nickel-based superalloys are subject to severe microsegregation and exhibit anisotropic properties during laser metal deposition (LMD), which restricts their wide application in the aerospace field. Thus, post heat-treatments must be investigated to further regulate the microstructure and mechanical properties of such components. The aim is to provide a comprehensive understanding of the effects of double-stage aging (DA) and solution plus double-stage aging (SDA) heat treatments on microstructure and mechanical properties of Hastelloy X superalloy sheets fabricated via LMD. The effects of DA and SDA heat treatments on the density, residual stress, phase composition, carbides precipitation behavior, dislocation structure, grain structure, and mechanical properties of the sheets were studied and in comparison with the as-deposited (AD) samples. The results showed that the DA heat treatment yielded a significant amount of lamellar and granular phases in the interdendrites and grain boundaries, whereas the anisotropic microstructure between the transverse and vertical planes was similar to that observed in the AD samples. These phases are mainly composed of Laves phase and carbides. However, the needle-like carbide phase was dispersedly precipitated within the grains and at grain boundaries, and exhibited full recrystallization and reduced dislocation density after SDA heat treatment. Consequently, the SDA samples presented superior strength-ductility balance to those of the AD and DA samples with irregularly shaped Laves phase and carbides owing to microstructural changes. For instance, the SDA heat treatment improved the ultimate tensile strength and fracture elongation of the vertical samples by up to 10.9% and 30.7%, respectively, whereas the microhardness decreased by 13.3% compared to the AD samples. The compressive residual stress of 98 MPa and tensile residual stress of 147 MPa were also observed on the surfaces of the SDA and AD samples, respectively. In addition, the SDA samples exhibited an almost isotropic tensile behavior in the vertical and horizontal directions as the epitaxially grown columnar grains transforms into equiaxed grains, as well as exhibited a prominent overall crystallographic texture. The effect of DA and SDA heat treatments on the mechanical properties were analyzed on the basis of stress–strain curves, hardening behavior, and multiple strengthening mechanisms.