Heat treatments for microstructure modification and mechanical properties enhancement of wire-arc additively manufactured tungsten-inconel bimetallic structures

This study investigates the effects of three heat treatment conditions—stress relief, recrystallization annealing, and solution treatment with aging—on the microstructure, tensile properties, and high-cycle fatigue (HCF) behavior of wire-arc additively manufactured 90WNiFe–Inconel 625 bimetallic structures. After heat treatment grain coarsening was observed in Inconel 625 (95–200  µm) side, while 90WNiFe grains experienced slight reduction in size (12–17  µm). Stress relief and recrystallization annealing promoted δ-Ni₃Nb, modified Laves phases, and carbide precipitates at the interface, whereas δ-Ni₃Nb was dissolved after solution treatment with aging. The as-built solidification texture of WAAM-Inconel 625 was mostly preserved, with grain growth partially disrupting the strong 〈001〉 texture following solution treatment and aging. The highest tensile strength was achieved for stress relief condition (758  MPa, 14 % above as-built), while solution treatment with aging provided the highest ductility (35 % elongation, 28 % increase). High cycle fatigue testing (R = 0.1) after stress relief demonstrated extended fatigue life from 10⁶ to beyond 10⁸ cycles, with most specimens surviving >20 million cycles at 150  MPa. Improvements in fatigue performance were attributed to increased ductility and reduced residual stress, as no interfacial failures were observed below 90 % yield strength. Fracture occurred in bulk Inconel 625 or 90WNiFe regions, with crack initiation dominated by surface defects. Fractography revealed mixed ductile–brittle failure in Inconel 625 and brittle fracture in 90WNiFe while crack propagation was influenced by the presence of secondary δ-Ni₃Nb particles.