Effects of laser scanning speeds on different states of the molten pool during selective laser melting: Simulation and experiment

Using numerical simulations and experimental tests, the temperature and velocity fields of the molten pool during selective laser melting (SLM) were investigated, where the laser scanning speed ranging from 2.5 m/s to 0.3 m/s was employed. Experiments for single tracks and part samples were conducted for verification. Three kinds of molten pool states were identified and investigated: unstable state, transition state and stable state. The unstable state is characterized by numerous balling defects, where the bulk density is severely deteriorated. The transition state is featured by the transition region where the melt velocity is relatively lower, and the molten pool is vulnerable to the necking defect. The molten pool with a depression region is identified as the stable state. A small depression is favorable for improving the surface quality of single track and the bulk density. However, exorbitant energy input will convert the depression into a keyhole. Additionally, a threshold of the scanning speed was found, where the bulk density peaked. Over the threshold, the density decreased continuously with the speed increasing. However, the density slightly decreased by 1.5% when the speed was below the threshold; this anomaly was ascribed to the residual pores induced by the recoil pressure.