Fabrication of functionally graded material of 304L stainless steel and Inconel625 by twin-wire plasma arc additive manufacturing

Functionally graded materials (FGMs) are novel composite materials characterized by gradual changes in compositions and/or microstructures along at least one direction and hence locally tailored properties. In this paper, an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process was used to fabricate the thin-walled stainless steel 304 L/Inconel625 compositionally graded materials. The chemical composition, microstructure, phases, and microhardness of the as-fabricated FGMs were investigated. The results reveal that sharp changes in composition between 304 L and In625 in non-graded sample resulted in large variations in microstructural morphology and hardness around the interface between these two materials. With the increase of the gradient layers, the microstructural morphology displayed a smooth transition from 304 L to In625. However, cracks were found in the 25%In625 region due to MC carbides distributed at the grain boundary and A solidification mode. The grain growth at the interfaces between adjacent layers with different compositions follows a typical epitaxial growth. As the mixing ratio of In625 increased, the secondary phases changed from MC carbide to Laves phase and their content increased. Furthermore, the microhardness decreased in the 21%In625 region, then increased with the increase in mixing ratio of In625. The absence of δ-ferrite and low content of secondary phases were the main factors contributing to the decrease in microhardness in the 21%625 region. The study provides a guideline for the wire arc additive manufacturing of 304 L/In625 FGMs with the consideration of gradient composition to avoid cracks and weakening of properties.