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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.

Structuring of multilayer stacks with ns-laser pulses is widely used in the industrial production of LEDs. These stacks are built up from several different materials each one layered upon each other. In order to thoroughly understand the physics of the structuring process a multiphysical simulation model has been developed. This model is capable to handle the beam propagation and the energy coupling into the work piece both for transparent and for absorbing materials as well as phase transitions (melting, solidifying, evaporation, condensation) of multiple different materials within the calculation domain. Furthermore a modified volume of fluid approach has been developed to calculate the material and energy flow within both the liquid and vapor phases and thus to track the free surface of the material during laser ablation. The paper gives an insight into this model illuminating the physical background of the ablation process and shows the excellent correspondence of the simulations with experimentally obtained results.