A concept for a hydrodynamic model of keyhole formation and support during laser welding

A concept for keyhole formation during laser welding is proposed. The hydrodynamic motion of the melt is taken into account, and growth of the keyhole is attributed to the melt displaced from the laser focus due to vaporization recoil pressure. It is assumed that the top and middle keyhole regions are held open by vapor pressure equal to the sum of surface tension and hydrostatic pressures. Near the bottom of the keyhole, the value of recoil pressure during keyhole growth is estimated to be 10–100 times higher than that from surface tension or hydrostatic forces. The estimated melt velocity of 10 m/s is consistent with previous experimental data. It is assumed that absorption in near-surface and keyhole plasmas limits the keyhole length, and when maximum keyhole length is reached, the recoil pressure equals the sum of surface tension pressure and hydrostatic pressure at the keyhole bottom. Thus, a description of laser induced plasma must be an essential part of the welding model. This recoil pressure-based theoretical model will allow the creation of adequate numerical models capable of realistic predictions.