Effect of thermal conditions on defect formation in laser metal 3D printing

Abstract

This thesis investigates the effect of thermal conditions on defect formation in laser metal 3D printing. A three-dimensional thermo-fluid model and a three-dimensional thermal model were built and simulated for SUS316L and Ti6Al4V. First, a dimensional analysis was introduced to identify defect formation regimes depending on process parameters. Normalized enthalpy h^* and dimensionless thermal penetration depth δ_th^* were used to characterize melt pool behaviors and to explain defect formation. Stable melt pool and high density were obtained for keyhole mode with high h^* and low δ_th^* and for conduction mode with low h^* and high δ_th^*. When δ_th^* increased, the melt pool became stable and uniform for the same h^* in SS316L. Lack of fusion was simulated to study its mechanism. Lack of fusion occurred due to surface melting and rapid solidification. Also, the limitation of linear energy density was investigated for cases that had the same linear energy density at different combinations of the laser power and the scan speed. The melt pool behavior differed widely for the same linear energy density. Second, the effect of surface remelting on defect formation was revealed. Surface remelting reduced internal porosity for the both directions. Heat accumulation effect was key to reducing RMS and R_p. Third, the effect of preheating on defect formation was revealed. When T_amb=200℃, the melt pool showed no difference. However, when T_amb=800℃, the keyhole threshold shifted more than 2. Also, temperature gradient was largely reduced when T_amb>400℃.