Microstructure evolution and mechanical properties of the dissimilar joint between IN718 and STS304

Modern fuel injectors for gasoline, diesel, and gaseous fuel use dissimilar joints of Inconel 718 and STS 304. To better understand the microstructural evolution and mechanical properties of these types of dissimilar joints, the joints of these alloys were fabricated by employing laser beam welding process. The joint was characterized by electron microscopy and hardness measurement techniques. By employing the laser beam welding, a crack-free deep penetrating joint was obtained. The weld metal was primarily composed of an austenite phase with a distribution of Laves and Ti/Nb carbides in the interdendritic regions. The low heat inputs and fast cooling rates suppressed the Laves precipitation, as its volume fraction was less than that of most IN718 joints fabricated by traditional welding techniques. Partially melted zones (PMZ) were formed on both sides of the joints. The PMZ of IN718 base metal was complex with Nb-rich precipitates detected as δ-Ni₃Nb, while that of STS 304 was free of precipitates. No significant grain coarsening or negative metallurgical effects were observed at the heat-affected zones. The fusion zone (FZ) recorded a lower hardness, compared to the parent metal, which is mainly due to the depletion of strengthening elements (Nb, Mo, and Ti) and lack of hardening γ′′ particles. The joint showed a low strength with failure occurring at the FZ. Post-weld solution treatment at 980 °C resulted in the partial dissolution of the Laves phase and complete disappearance of the dendrites, followed by the precipitation of plate- and needle-like δ phase. Post weld heat treatment increased the hardness of the joint. The solution treated + double aged joint showed much higher strength than the directly aged joint. The increased hardness reflects the precipitation of γ′′ in the weld.