Relation of thermal behavior and microstructure evolution during multi-track laser melting deposition of Ni-based material

A three-dimension finite element model was proposed to understand thermal behavior and microstructure evolution in multi-track laser melting deposition (LMD) of Inconel 625. The latent heat of phase change, multiple heat transfer, temperature dependent thermal physical properties were considered to ensure the accuracy of the simulation. Based on the simulated results, solidification characteristics, including temperature gradient (G), solidification growth rate (R), cooling rate (G × R) and G/R, could be obtained to predict the morphology and scale of the solidification microstructure. The results showed that the G/R was increased from 7.3 × 105 °C s/m2 at the top of the molten pool to 1.22 × 107 °C s/m2 at the bottom. As a result, columnar dendrites were generated at the bottom of the molten pool, while equiaxed dendrites were formed at the top. Simultaneously, columnar dendrites were observed at the edge of the molten pool, which was attributed to the high G/R (1.18 × 108 °C s/m2) at the edge of the molten pool. Furthermore, due to the lower cooling rate at the overlapping region than that at the bottom region, columnar dendrites generated at the overlapping region were coarser compared with those at the bottom of the molten pool. Specially, it should be noted that as increasing the number of deposited track, the G/R at the top of the molten pool exhibited with a slight increase and the G/R at the bottom presented with no obvious change. However, the G/R at the edge of the molten pool had an apparent decrease. The above simulation results showed a good agreement with the experimental results.