Numerical Simulation of Meltpool Evolution During Laser Powder Bed Fusion (LPBF) Processing of Ti6Al4V
J-Y. Deng, D-Y. Zhang, Y-X. Xu, X-P. Wu, L-S. LI, Y-K. Xie, R. Poprawe, J.H. Schleifenbaum and S. Ziegler
A physical model is developed with the volume of fluid (VOF) method to track the free interface in the meltpool to understand the meltpool evolution, fluid flowing and the behaviour in gas and liquid interface. The model is characterized with a combined heat source model consisting of a parabolic rotating and a cylindrical distribution, and a stochastic distributed model of powder bed with powder particle size. The heat source is only loaded in the unit interface between metallic and gas phase in the laser-powder interaction zone. The simulation is carried out only in first and second laser scanning track due to the reduction in computing time. Simulation results exhibits the fluid flowing and surface morphology in the meltpool is determined significantly by different laser parameters such as laser power and scanning velocity. The simulated meltpool geometry has good agreement with the experimental ones.
The meltpool geometry of the first track changes from the approximate circle, ellipse to ‘comet’ due to the heat accumulation during continuous laser scanning. The meltpool geometry of the second track becomes much larger, its temperature is higher and the fluid flowing is faster than that of the first track. The melt metal flows violently in the initial position due to the high flowing velocity in a limit melt space. The second track has enough space for metal flowing and it is easy to obtain a smooth surface morphology than the first track; thus, a long enough meltpool helps to get a steady fluid and smooth surface morphology. The temperature of the meltpool at beam centre fluctuates during laser scanning process, which depend on the determined effects of the interaction between thermal conduction and violent fluid flowing or the interaction between heat accumulation and violent fluid flowing. The information about temperature distribution and fluid flowing in the meltpool provided by the simulation helps to analyse and understand the formation mechanism of the meltpool geometry, surface morphology and meltpool evolution as well, benefiting to further control the laser powder bed fusion (LPBF) processing of Ti6Al4V alloy powder.
Keywords: Laser powder bed fusion (LPBF), Ti6Al4V, numerical simulation, finite volume method (FVM), volume of fluid (VOF) method, meltpool evolution, temperature distribution