Combined effects of heat treatment and TiB2 content on the high-temperature tensile performance of TiB2-modified Ni-based GH3230 alloy processed by laser powder bed fusion

Abstract

Laser powder bed fusion (LPBF) is widely used to manufacture advanced nickel-based superalloys for aero-engine components. LPBF-formed GH3230 nickel-based superalloy, however, suffers cracking and high residual stresses. This study systematically investigated the effects of different contents of submicron TiB2 particles on crack suppression and post-heat treatment for residual stress relief and improved microstructure anisotropy. The introduction of both 0.5 wt% and 1 wt% TiB2 achieved complete crack suppression in the LPBF-manufactured GH3230 specimens. As-built specimens with 0.5 wt% and 1 wt% TiB2 exhibited a supersaturated solid-solution microstructure, with the solid elements (Cr, Mo and W) almost fully dissolved in the matrix and only a limited number of Mo and Cr enriched carbides precipitated at the grain boundaries. Electron backscattered diffraction (EBSD) characterization demonstrated that a large number of deformed grains remained in the as-built specimens. After heat treatment, the residual stresses were released and the number of deformed grains significantly reduced. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization demonstrated the disappearance of the dense dislocation network in the as-built specimens after heat treatment, accompanied by the precipitation of a large number of W-rich M6C carbides. This paper also discusses elevated-temperature (900 °C) mechanical properties. The ultimate tensile strength (UTS) values of the as-built GH3230 with 0.5 wt% (GH-0.5) and 1 wt% TiB2 (GH-1) specimens were 255 and 290 MPa, respectively. Compared to the as-built materials, the UTS values decreased by 40 and 53.5 MPa, respectively, after heat treatment. The paper also discusses the effect weight of the second-phase, solid-solution and dislocation-strengthening mechanisms on the elevated-temperature mechanical properties. The findings indicate that the added 1 wt% TiB2 particles and the solid-solution elements contributed about 106.2 MPa and 111.5 MPa stress, respectively, in the heat-treated GH-1 specimen. The precipitated M6C carbides after heat treatment, however, were not found to significantly improve the high temperature strength. This work is expected to provide a reference for industrial applications to improve the LPBF formability of nickel-based superalloys and to acquire attractive microstructures.