Fluid dynamical simulation of high speed micro welding

High speed micro welding with mono mode fiber lasers allows for excellent machining qualities especially concerning the joining of thin metal sheets and foils, if appropriate machining parameters are chosen. However, just slightly changing these parameters may lead to welding failures, e.g., due to the humping effect or even due to a transition from welding to cutting. To analyze and understand the underlying mechanisms for these failures the authors made use of a multiphysical, fluid dynamical model for the simulation of laser material processing that has been developed over the past few years. Within the first part of the paper, the authors will give some insight into the model that is capable of simulating the whole process including the laser beam propagation and absorption at the workpiece, convective and conductive heat transfer, phase changes from solid to liquid and vapor, respectively, back from vapor to liquid and solid. Furthermore, fluid dynamics of the melt and vapor are taken into consideration the free surface of the condensed phases, compressibility, and temperature-dependent material properties. The second part of the paper will deal with the analysis of the aforementioned welding failures. The simulation results show an excellent agreement with experiments both concerning the humping effect and the transition from welding to cutting. They also reveal the reasons for these welding defects, which will be explained in detail.