Numerical and experimental investigation on thermal behavior and microstructure during selective laser melting of high strength steel

During selective laser melting (SLM) process, the correlation between thermal behavior and complex microstructure must be clarified. The present study investigates the effect of laser power and scan speed on thermal behavior and solidification structures of SLM-produced high strength tool steel. A three-dimensional finite element model was built to simulate the thermal behavior in SLM process. The simulation results show that correlativity exists between the process parameters (laser energy density) and the maximum temperature, cooling rate, temperature gradient and solidification rate. Scan speed has more significant impact on the cooling rate and solidification rate than laser power. The sub-grain patterns of the part presents predominantly cellular, elongated cellular and columnar structure. The formation of these sub-grain microstructures in the molten pool was controlled by the gradient cooling rate (from 5.6×105 to 3.4×106 °C/s) and temperature gradient (from 5.0 to 30 °C/μm). The columnar structure are elongated with the increment of the temperature gradient. The grain size of the microstructure become finer with the increase of laser power or scan speed. And the gradient microstructure has a significant impact on its microhardness properties.