Effects of heat treatments on microstructure and mechanical properties of laser melting multi-layer materials

Abstract

The effects of different heat treatments on the microstructure and mechanical properties of the laser melting nickel-based superalloys were systematically investigated. The strength improvement of the heat-treated material was mainly derived from the aging treatment, which converted the γ matrix together with Niobium to the strengthening phases γ′′ and γ′. The solution treatment dissolved partial Laves phases to form the short-acicular δ phases in the remelting zone (RZ), whereas the homogenization treatment dissolved most precipitates including the Laves phase and δ phase, leading to significant grain coarsening in the multi-layer material. Moreover, based on the instrumented indentation reverse analysis, RZ shows consistent stress–strain relationships after the heat treatments, reaching the same level as the heat-treated base material (BM). The remelting-affected zone (RAZ) reveals more outstanding mechanical properties after the direct aging (DA) treatment for the work hardening contributed by dislocation strengthening. Additionally, the residual stresses and deformations in RAZ provide an extra thermodynamic driving force for the grain recrystallization and the formation of the short-clavate δ phase in the solution-treated RAZ. The remaining large grains and small Taylor factor in RZ are considered to lead to a reduction of the mesoscopic yield strength, which cannot be identified in microscopic indentation analysis. The DA heat treatment is recommended for the strength improvement of the laser melting materials.