Analytical model of the laser welding of zinc-coated steel sheets by the aid of videography

Despite several approaches from different research groups, currently the best solution for welding of zinc-coated steel sheets in an overlap configuration is the use of a joining gap between the sheets. However, the introduction of the gap is prone to errors due to dirt or thermal distortion. A too small gap leads to the explosive zinc ejections. In contrast, a too large gap causes false friends where the melt is not able to bridge the gap due to an erroneous clamping of the sheets. Currently, there is no direct measurement of the joining gap available. The reason therefore is the lack in the understanding of the fluid dynamics inside the keyhole in laser welding. To increase the process of understanding, the authors analyzed the correlation between the well visible keyhole opening, the joining gap size, and the welding speed for uncoated and zinc-coated sheets. To scrutinize our findings, the authors present a model of the zinc degassing inside the keyhole which obtains its input parameters directly from our comprehensive high-speed videography analysis of the process. To get a deep view inside the keyhole without the use of x-rays, the authors use a glass plate flanking the keyhole which allows us to look inside the keyhole during overlap welding of steel sheets with very high spatial and temporal resolution. By the aid of tracer particles, the authors measured and modeled the pressure due to the evaporation of the zinc layer without the use of unknown thermodynamic coefficients. Our results show that the joining gap and the welding speed have an influence on the pressure balance inside the keyhole. Hence, our work can be used as a foundation for building a control system for the joining gap for the welding of zinc-coated steel sheets.