SIMPLIFIED ANALYTICAL MODELING OF DYNAMIC BEHAVIOR OF THE KEYHOLE FOR DIFFERENT SPATIAL LASER INTENSITY DISTRIBUTIONS DURING LASER DEEP PENETRATION WELDING

摘要

During laser deep penetration welding a characteristic keyhole is created, when the intensity of laser beam exceeds material depending limit. The generated system of keyhole and surrounding melt pool is highly dynamic. Dynamics in the weld pool and in keyhole are mainly responsible for keyhole instabilities that can cause keyhole collapses during the welding process. This can lead to unwanted enclosures or pores that reduce the quality of welded joint. For better understanding of the complex system, a simplified analytical model of the keyhole is used providing a description of the keyhole geometry. It also calculates the influence of different spatial laser intensity distributions on keyhole dynamics and resultant tendency to form pores. The model is used to calculate the temperature on the keyhole wall from energy equation containing laser beam energy absorption, heat conduction and evaporation losses. The surface temperature is needed to calculate the keyhole radius by solving the pressure equilibrium equation. This contains the recoil pressure at the end of the Knudsen layer on the keyhole surface, which keeps the keyhole open against the surface tension pressure of the surrounding liquid material. In the second step, a dynamic equation that describes the keyhole behavior is used. The dynamic calculation is based on the force balance in the keyhole. To observe the influence of different spatial laser intensity distributions the Gaussian and top hat distribution are implemented in calculation. It can be found that the keyhole geometry is influenced by different laser intensity distributions and pressure gradient changes significantly leading to highly different dynamic behaviors.