Modelling of the capillary in laser beam penetration welding with the Smoothed Particle Hydrodynamics Method

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The development of laser beam sources led laser based processing to essential improvements regarding beam quality und power. This promoted the industrial distribution of the laser welding process. In combination with beam propagation, further development steps can be expected concerning e.g. beam geometry, polarization, wave length, and modulation. Nevertheless, weld bead imperfections caused by spattering, blow holes, pores, cracks, and humping restrict the scope of applications or even prevent the use of lasers. New results from investigations lead to the conclusion that wavy structures at the capillary front and protrusions at the rear side of the capillary play a decisive role in the generation process of pores and spatters. New diagnostic tools like the online X-Ray technique and high speed recording systems in combination with new optics components, which allow to use the polarization state of the light coming from the capillary to reconstruct the 3D geometry of the capillary, have already contributed to attain the above-mentioned findings and shall be used in this project to clarify the processes. Since, however, also these techniques are limited both in their accessibility and in the local and temporal resolution, it will be indispensable to analyse the identified processes with numerical methods. The core problem is the simulation of the melt flow, whereby the free surface to the gas phase plays a decisive role. For such applications, where large deformations of the surface have to be expected, grid-free methods have the huge advantage that calculation grids have not to be recalculated repeatedly. The Smoothed Particle Hydrodynamics (SPH) method is especially well suited for such problems, because the state variables of the continuum are discretized at points, which are moved according to the velocity field and are, therefore, not bound to a grid. The SPH program Pasimodo, which is available at the applicants site, has to be extended with some modules and combined with a ray-tracing program, which is also available and which calculates the beam propagation in the capillary and the spatial distribution of the energy transfer into the melt layer. A further module has to be developed, which calculates the pressure acting on the melt surface resulting from the evaporation and gas flow. The final tool shall be used to calculate waves generated at the capillary front as well as protrusions at the rear side of the capillary together with the resulting processes, which lead, according to present information, to pore generation and spattering. The physical mechanisms shall be investigated with three significantly different materials, steel, aluminum, and ice. The aim in a second project phase will be to evaluate the influence of the process parameters and to use the resulting model to optimize the laser welding process also concerning its stability.