Mathematical model to study the keyhole formation in pulsed Nd:YAG laser welding of SS 316L material and its experimental verification

A mathematical model to study keyhole formation and its propagation in the material is developed for laser welding performed in an open atmosphere. The present model overcomes the limitations of existing models in assuming sonic vapor jet velocity to calculate vaporization-induced recoil pressure responsible for keyhole formation. In the present model, the exact value of vapor jet velocity is calculated using gas dynamics equations. The minimum threshold value of absorbed laser beam intensity required to perform keyhole welding irrespective of laser pulse duration for laser beam radius of 0.6 mm has been found to be 0.8 x 10(5) W/cm(2) and is in good agreement with the experimental value. In between conduction mode welding and keyhole mode welding, a transition mode exists where a keyhole mechanism develops itself and melt displacement is not considerable in this zone. Weld penetration occurs mainly through heat diffusion in this transition mode. The predicted values for keyhole penetration velocity are also in good agreement with the experimental values. At a longer pulse duration, the model over-predicts the keyhole penetration velocity as compared to the experimental value due to nonconsideration of vapor plasma absorption of the laser beam.