Impact of Heat Treatment and Building Direction on Selective Laser-Melted 316L Stainless Steel: Microstructure and Mechanical Properties

Variations in the microstructural morphology with building direction during selective laser melting (SLM) result in the anisotropic mechanical properties of the specimens, while heat treatment effectively reduces this anisotropy. The degree of anisotropy of the material can be assessed by calculating the variance (σ) of the mechanical properties (strength, hardness) at different building directions at different temperatures. In this work, the effects of heat treatment temperatures (450°, 750 °C, and 1050 °C) and building directions (0°, 45°, 60°, and 90°) on the microstructure, hardness, and tensile properties of selective laser melting (SLM) SS316L were investigated. Unheated SLM SS316L specimens exhibit significant anisotropy (σ_UTS = 16.67, σ_UE = 9.60, and σ_HV = 9.60), while heat treatment effectively reduces this anisotropy. As the heat treatment temperature increases, the melt pool boundaries disappear and grains become uniform at 750 °C, significantly reducing the anisotropy of the mechanical properties (σ_UTS = 14.65, σ_UE = 4.33, σ_HV = 6.72). At 1050 °C, the grain size increases slightly, resulting in a minor rise in anisotropy (σ_UTS = 14.93, σ_UE = 4.97, σ_HV = 8.39) compared to that seen at 750 °C. After heat treatment, the SLM SS316L specimen heated at 450 °C shows the lowest anisotropy. Notably, the specimens built in the 0° direction and heated at 450 °C exhibit finer sub-grains and enhanced mechanical properties, achieving a hardness of 244.43 HV, a tensile strength of 655.85 MPa, and an elongation of 21.25%.