Numerical prediction and measurement of the weld depth dependence on the beam focal position during laser welding

The effect of laser beam focus position on penetration depth in CW laser welding was studied numerically and experimentally. Calculations were performed using a transient hydrodynamic model, which assumes front keyhole wall propagation due to the melt expulsion induced by evaporation recoil pressure. The calculation results are compared with experimental welding data obtained using a 1650W CO₂ laser. The simulations predict that beam absorption in the plasma has a significant effect on the relationship between penetration and focus position. When the process parameters result in strong beam absorption in the near surface plasma, the maximum penetration will occur when the laser focus is at or above the sample surface. In a case of weak absorption however, the penetration depth reaches its maximum value when the beam focus is located below the sample surface.