Effect of Al content on microstructure and microhardness of Inconel 718 superalloy fabricated by laser additive manufacturing

In this study, the effect of Al addition on the microstructure and microhardness of Inconel 718 superalloy fabricated by laser additive manufacturing (LAM) was studied. The addition of Al had little effect on the grain morphology and orientation, but the segregation of Nb element intensified with the increase of Al content, which promoted the formation of Laves phase in the inter-dendritic region. After HSDA (Homogenization + Solution + Double aging) heat treatment, the precipitation of δ phase at grain boundaries changed from cross needle-like to the rod-like, and at the same time, the more content of Al was added, the more rod-like δ phase formed inside the grains. Besides, the addition of Al can promote the precipitation of γ′ phase, and inhibit the precipitation of γ" phase through promoting the segregation of Nb element. When 5% wt.% Al was added, only γ′ phase was precipitated in the matrix. With the increase of Al content, the microhardness of as-deposited LAMed Inconel 718 increased and reached 325 HV when 5% wt.% Al was added. For microhardness of heat-treated samples, it showed the similar evolution trend with the increase of Al content.