Influence of Heat-Treatment Temperature on Microstructure and Mechanical Properties of Selective-Laser-Melted GH3536 Superalloy

For laser additive manufacturing components, subsequent heat treatment is crucial for regulating the material microstructure and properties. This paper investigates the evolution of microstructure and precipitates in GH3536 nickel-based superalloy fabricated by selective laser melting (SLM), subjected to heat treatment at temperatures ranging from 980 °C to 1280 °C for a duration of 1 h, and further explores the underlying mechanisms governing the resultant effects on mechanical properties. In the 980 °C-1 h specimen, a large number of Mo-rich M6C type carbides are distributed along the grain boundaries and between the dendrites. Exposure to 1080 °C results in the coarsening of grain-boundary carbide particles. For the 1180 °C-1 h specimen, the carbides are completely dissolved into the matrix. At 1080 °C and below, the SLMed columnar grain morphology persists. At 1180 °C, the alloy attains a recrystallized fraction of 49.7%, accompanied by a subgrain proportion of 49.8%. The recrystallized grains exhibit a polygonal morphology, with an average aspect ratio of 2.51, which contains a large number of annealing twins. The formation of numerous subgrains could be attributed to the microstructural inhomogeneity and the solute drag effect. At 1280 °C, the recrystallized fraction increases to 89.8%, and the grains coarsen to 40.8 µm in diameter. The grain aspect ratio and texture intensity do not monotonically decrease with temperature. As the temperature increases, the hardness gradually decreases from 251.8 ± 13.5 HV (as-SLMed) to 176.3 ± 5.5 HV. The sequential occurrence of recovery and recrystallization leads to the progressive decrease in yield strength and increase in elongation. The influence of back stress, the Hall–Petch relationship and solid solution strengthening on the yield strength is discussed. For the 1280 °C-1 h specimen, elongation increases to 67.4%, indicating the excellent grain boundary thermal stability and interfacial bonding strength.