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.