Effects of combined pre/post-weld heat treatments on microstructural evolution and corrosion resistance of linear friction welded GH4169 superalloy joints

This study investigates linear friction welding of GH4169 alloy for aero-engine integrally blisks, with particular focus on elucidating the mechanisms by which combined pre- and post-weld heat treatments influence microstructural evolution and corrosion behavior of welded joints. Microstructural characterization reveals that composite heat treatment promotes the formation of large-scale spherical γ′ and disc-shaped γ′′ phases in the Base Material (BM), while in the Thermo-Mechanically Affected Zone (TMAZ), original precipitates coarsen and fine γ′ and γ′′ phases reprecipitate. Additionally, needle-like δ phases precipitate along grain boundaries. The synergistic effect of grain refinement and precipitation strengthening results in superior joint mechanical properties, including a microhardness of 540 HV_0.5, a tensile strength of 1400 MPa, and a fracture elongation of 18%, with the joint strength comparable to that of the BM. Electrochemical analysis shows that the joint exhibits significantly lower corrosion resistance than the BM, due to enhanced micro-galvanic coupling between the γ-matrix and precipitated γ′ or γ′′ phases. This is evidenced by an increase in corrosion current density from 1.62×10^-6 A/cm² of BM to 3×10^-6 A/cm² of joint. High-temperature molten salt corrosion tests indicate that corrosion mainly occurs through the combined action of oxides and soluble salts. The joint shows accelerated corrosion, reaching a peak value in the average corrosion rate of 269.9 g/m²/h, characterized by a fine-grained microstructure and loosely oxide films. The electrochemical impedance of these is measured at 1.83 kΩ ·cm², attributed to thermo-mechanical effects. In contrast, the coarse-grained BM forms dense and protective oxide layers, with a higher impedance of 14.20 kΩ ·cm² and a lower peak corrosion rate of 134.9 g/m², reflecting more stable corrosion behavior. This work deciphers how heat treatment controls corrosion resistance through the regulation of precipitate distribution in welded joints, providing valuable guidelines for the optimization of integrated welding and heat treatment process in blisk production.