The collapse of the keyhole in the laser welding of materials

Abstract

The collapse time of a keyhole following the extinction of a continuous CO2 laser beam during the process of laser welding is calculated for a variety of processing conditions. It is assumed that in a period of time much shorter than the collapse time of the keyhole the pressure in the keyhole drops to the ambient atmospheric pressure outside. The initial keyhole geometry is calculated using the integrated model of Ducharme et al (1992). It is argued that the keyhole collapses as a result of the considerable surface tension forces operating in the keyhole wall. The boundary condition for the pressure on the keyhole wall at the mouth of the keyhole is the ambient atmospheric pressure and an additional tangential contribution arises from the surface tension forces operating on the keyhole wall. The radial velocity of the keyhole wall is calculated as a function of time and the collapse time of the keyhole is then deduced knowing the initial keyhole radius from the model of Ducharme et al. (1993). In calculating the collapse time of the keyhole a full numerical solution for the three-dimensional time-dependent velocity field in the weld pool is obtained using the finite difference method. This work represents an extension of previous work in this area carried out by Kroos, Gratzke, Vicanek and Simon (1993). The results of the calculation are discussed in relation to the analysis of infrared and ultraviolet sensor data from the keyhole and the weld pool given by Kapadia et al. (1992). The understanding so gained will be of value in clarifying the radiative spectrum produced by the keyhole and will be of additional practical help in the use of radiative sensors to monitor laser welding. It will, furthermore, also provide insight into the behaviour of the keyhole when pulsed lasers are used to carry out laser welding.