Modelling of droplet detachment in the laser droplet brazing process

• Droplet detachment phase of laser droplet brazing process is theoretically modelled.
• Algebraic expression for maximum gas overpressure before droplet detachment.
• Numerical model based description of droplet detachment dynamics.
• Model results provide guidance for selection of proper process parameters.

The laser droplet brazing process has been recently experimentally considered for the electrical contacting of thermally sensitive components. In this process, a spherical brazing preform, placed in a tapering nozzle, is melted by a laser pulse, detached from the nozzle by a shielding gas overpressure, and deposited on the brazing spot. The detachment of the brazing droplet from the nozzle has been studied theoretically in this paper with the aim of providing guidance for the selection of the main process parameters, i.e. the gas overpressure and the droplet contact angle. The droplet detachment is described by two models: an algebraic droplet force balance model and a numerical isothermal two-phase fluid flow model. Using the droplet force balance based model, an algebraic expression defining the dependence of the maximum gas overpressure before droplet detachment on the droplet contact angle was obtained. The numerical model was used to determine the droplet detachment occurrence in terms of the main process parameters. Additionally, the nonlinear dependencies of the time of droplet detachment, the detached droplet velocity and vertical position, and the droplet shape on the gas overpressure and the brazing droplet contact angle were defined, and can be used for process parameter selection. It was also found that the detached droplet shape is influenced, beside the gas overpressure and the droplet contact angle, by oscillation of the droplet, which can be significant at droplet contact angle values of less than 75°. Based on comparisons of the modelled and experimental results of droplet detachment time, vertical position of the detached droplet, and its shape, it was concluded that the contact angle of the CuSn11 brazing material on the WC/Co nozzle was, in the experiments, near 105°. Furthermore, comparisons of the results indicated that the laser melting phase of the preform significantly influences droplet detachment, and should therefore be taken into consideration for the improvement of the numerical model.