Controlling laser-powder interaction process and molten pool dynamic behaviors during laser directed energy deposition process by laser beam defocusing

The laser–powder interaction process (LPIP) and molten pool dynamic behaviors (MPDB) during laser directed energy deposition (LDED) process have significant effect on the forming quality of parts. In this paper, a LDED numerical model is established for controlling LPIP and MPDB by laser beam defocusing (LBD). Based on the calculated results from the established model, the influence of LBD on LPIP and MPDB is investigated. With the defocusing amount increasing, the laser beam power density (LBPD) and its attenuation amplitude are reduced. The uniformity of laser beam power density distribution (LBPDD) is improved and the average temperature increase of powders (ATIP) is decreased. Energy density of substrate surface is decreased due to the LBPD reduction and the uniformity of energy density distribution of substrate surface perpendicular to laser beam scan direction is increased due to the uniformity improvement of LBPDD. The decreases in ATIP and energy density of substrate surface cause the temperature reduction in the molten pool. The uniformity increase in energy density distribution of substrate surface perpendicular to laser beam scan direction leads to the temperature gradient reduction of the cross section of molten pool in most regions and hence the Marangoni force is reduced. The monitoring molten metal flow velocity in the cross section of molten pool is decreased due to the reduction in Marangoni force, which results in the stability improvement in MPDB.