Electrodynamic simulation of energy absorption in laser keyhole welding of zinc-coated and uncoated steel sheets

Laser beam absorption phenomena during laser welding of zinc-coated and uncoated DP 590 steel sheets were simulated using the finite-difference time-domain (FDTD) method for the Maxwell equations. For accurate simulations, the keyhole shapes that were measured using a coaxial observation method during multi-mode fiber laser welding of the above steels were used, and the beam divergence and focusing characteristics of the laser were accurately matched. Also, the wavelength enlargement method was used to deal with a large computational domain required for laser welding simulation. Absorption mechanisms, multiple reflection patterns, and heating patterns were found to be significantly different for the two steels because of the different keyhole shapes. Zinc-coated steel has a much lower keyhole absorptance than uncoated steel at the same experimental condition, which was experimentally validated by melt pool size measurement. Ray tracing simulations were also performed for comparison purposes.