Effect of heat treatment on the microstructure and recrystallization of an SLM nickel-based superalloy

Abstract

Heat treatment is essential for selective laser melting (SLM) of nickel-based superalloys. However, the conventional solution heat treatment method of cast superalloys usually leads to serious recrystallization in the samples. The recrystallized grain boundaries under high-temperature conditions reduce the strength of the alloy, which hinders their widespread applications in aerospace. In this work, a nickel-based superalloy with high γ′ content fabricated by SLM was heat-treated at different solution temperatures. The microstructure of the alloy in different heat-treated states was also characterised. A sub-solvus heat treatment was proposed, which can effectively reduce residual stress and inhibit recrystallization. The microstructure showed that the dislocation density was significantly reduced and recrystallized after the solution heat treatment at 1285 ℃/2 h+1290 ℃/2 h+1295 ℃/4 h and 1250 ℃/0.5 h, and the proportion of recrystallization were 60.0 % and 42.9 %, respectively. However, after sub-solvus heat treatment at 1240 °C/0.5 h, the sample dislocation density decreased and the volume fraction of recrystallization (3.2 %) was slightly higher than that of the as-SLM sample (2.5 %). The sample maintained a strong cubic texture in the build direction. After sub-solvus + aging heat treatment, the microstructure maintained a columnar grain structure. The γ/γ′ interfacial dislocation networks and dislocations pile-up were the main dislocation configurations.