Phase formation and microstructure evolution in laser rapid forming of graded SS316L/Rene88DT alloy

The graded material of the SS316L stainless steel/Rene88DT superalloy was fabricated using laser rapid forming. A linear compositional gradient, from 100% SS316L stainless steel to 100% Rene88DT superalloy was achieved. The dendritic growth of the γ phase, the precipitation of the γ′ precipitate, as well as the formation of the η phase and the carbides along the compositional gradient zone were investigated. The growth morphology of primary γ-dendrites can be predicted by the microstructure selection models based on the maximum interface temperature criterion. The precipitation of the γ′ phase and η phase was to a large extent governed by the concentration of Ti and Al elements as well as by the Ti/Al ratio in the gradient zone. The size of the γ′ phase along the gradient zone can be explained by the LSW theory. In the deposit, apart from MC carbides, some M6C carbides were obtained, and there was no evidence of M23C6 carbides. This differed from the predictions of the phase diagram. It was interesting to find that the MC carbides exhibited a cored structure. The formation of the cored structure as well as the M6C carbides was discussed.