Thermal Variables Evolution Inside Melt Pool During LPBF of 316L Stainless Steel: A Numerical Approach

Abstract

The anisotropy of LPBF fabricated components is a serious concern and often increases the overall production cost by creating the necessity for secondary thermal homogenization processes. The microstructural features are the main driving force behind these anisotropic behaviors. Whereas the unique and distinctive thermal history inside a melt pool and its transient transformation is the reason for the characteristic microstructural features of LPBF fabricated components. Therefore, this paper investigates the prominent thermal variables such as heating rate, cooling rate, solidification rate etc., and their evolution inside the melt pool of 316 L stainless steel during LPBF process to provide a reference for further exploring the generation of various microstructural features. A numerical model for macroscale investigation of thermal behavior inside melt pool was established. A 3D Gaussian heat source model coupled with temperature and density dependent properties of powder and solid phase 316 L stainless steel was used. The variation and evolution of significant thermal variables inside the melt pool were then investigated with the established numerical model. The study found that the Gaussian profile of a laser beam influences the thermal variables inside a melt-pool, including cooling rates, solidification rates, and thermal gradients. The nodes lying under the laser edge receive less heat, resulting in higher cooling effects, which shapes the grain morphology. Finer grains can be formed near the bottom melt front as well as at the center of the melt-pool surface. However, reheating adjacent tracks can result in grain coarsening. Since the generation of microstructural features is dominantly dependent on the thermal behavior inside the melt pool, an assessment of these variables is important and provides basics for the understating of different features generated in the LPBF processed components.