Microstructural evolution and strengthening mechanisms of gas tungsten arc welded nickel-based superalloy joints processed by laser powder bed fusion: Effects of welding direction and heat input

Laser powder bed fusion (LPBF) imparts nickel-based superalloys with unique non-equilibrium microstructural features; however, their influence on the welding microstructure evolution and mechanical performance remains insufficiently understood. In this study, autogenous gas tungsten arc welding was applied to precipitation-strengthened ZGH4142 superalloy fabricated by LPBF. Microstructural characterization combined with finite element simulation was used to systematically clarify the effects of heat input and welding direction on the structure and properties of the welded joint. The results revealed that the weld morphology and pool geometry enlarged with increasing heat input, and fine equiaxed grains in the weld center transformed to coarse equiaxed ones. With decreasing cooling rate, the MC carbides coarsened, while the γ′ fraction increased from 14 % to 33 %, shifting the dominant strengthening mechanism from grain refinement to dislocation strengthening. The joints exhibited a typical “M-shaped” hardness profile, with the heat-affected zone (HAZ) and weld zone (WZ) showing the highest and lowest hardness, respectively. Quantitative strengthening analysis indicated that the base metal and WZ were mainly governed by solid-solution strengthening, and the HAZ exhibited predominantly dislocation strengthening. Electron backscattered diffraction analysis further revealed that the grain orientation and low-angle grain boundary fractions were sensitive to the welding direction, reflecting the intrinsic anisotropy of the LPBF-processed alloys. These findings provide fundamental insights and practical guidance for welding evaluation and process optimization of additively manufactured nickel-based superalloys.