Felix Zaiß

Zusammenfassung

Double-core fibers that deliver laser beams with adjustable amounts of power to the central core and the surrounding ring component of the fiber are of major interest for optimizing laser welding processes. However, the central focussed laser spot and the ring diameters are fixed by the fiber dimensions, the focusing optics, and the laser systems involved. This work investigates the influence of different ring beam diameters, with a constant central spot diameter, on the capillary dynamics and the formation of pores in the resulting welds in 1.4301 stainless steel (X5CrNi18-10, AISI 304). The different core-ring configurations were achieved using specially designed beam-shaping optics. The generation of pores during the welding process was examined by means of synchrotron X-ray imaging. The results show that different ring diameters have a profound effect on the melt pool geometry, the capillary shape and the location of the capillary collapse which gives rise to pore formation in the weld. In the examples presented here, larger ring diameters extended the melt pool in the direction of welding without contributing directly to the capillary evaporation. In this case the narrow, deep capillaries which are created by the core beam alone can bulge and collapse, trapping vapor and gas to create large pores. If, however, the ring beam has a small enough diameter, it can help to produce a wider capillary which does not trap gas and create pores in the same way. The mechanisms by which large bubbles in the melt can result in non-spherical pores in the weld are also explained.

Zusammenfassung

Laser welding was carried out using a co-axial core-ring beam, with independent control of the power to the core and the ring. The welding process was observed using high-power X-rays and high-speed imaging equipment. Adjusting the relative powers of the core and the ring beams was found to have profound effects on the shape of both the capillary created and the weld melt pool. Moderate laser power densities in the ring were found to increase the top diameter of the weld pool and reduce fluctuations in the welding process. High laser power densities in the ring led to the creation of a second capillary which largely acted independently of the capillary generated by the laser power in the core. At high power densities and welding speeds the two capillaries were almost completely separated in the direction of travel.

Zusammenfassung

Many laser processes include melting of material. Often, the material is overheated and reaches temperatures close or even above boiling temperature. However, many material properties at such high temperatures are often not known. A very important parameter that defines wetting and melt flow behavior is the surface tension coefficient, which is temperature dependent. However, the high temperature environment poses challenges to the measurement systems. This work suggests a surface tension measurement method based on surface wave analysis induced by an intense laser pulse. The results show lower surface tension values than expected from extrapolations just below boiling temperature. In addition, the surface tension is stagnating and not decreasing as typically expected. Based on a simplified estimation of broken bonds in the surface at different temperatures, the results showed that extensive vaporization can lead to increased surface tension coefficients.

Zusammenfassung

The electric vehicle (EV) market is experiencing rapid growth, leading to a significant push towards innovation. One of the critical challenges is Copper and Aluminum laser welding. These materials are inherently more difficult to weld and the welding criteria are more stringent (limited porosity and spatters). An advanced beam shaping modules have been developed using Multi-Plane Light Conversion (MPLC) technology. The beam shaping is accomplished using two integrated mirrors positioned between the collimation block and either a focusing block or a scanner. MPLC technology enables highly flexible beam shaping. Notably, this technology offers a depth of field four times greater than dual-core technologies, significantly simplifying its application. The presentation will showcase the benefits achieved in welding real components: battery modules, busbars, and hairpins. Additionally, an X-ray analysis of the welds will be presented. Simulations of numerous beam shapes and their effects on the welding process will also be provided.

Zusammenfassung

During laser welding, the formation of spatter results from instabilities of the capillary, which are associated with adverse conditions in the melt flow surrounding the capillary and in the vapor flow at the capillary opening. A change of the characteristics of these flows in the melt pool and in the capillary requires a change of the shape of the capillary and/or the surrounding melt pool. In order to influence the stabilize the capillary the intensity distribution was varied by means of a ring-core fiber system. Analysis of the resulting welds shows a significant effect of the intensity distribution on the stability of the capillary shape and considerable changes in the melt flow direction and velocity. These findings identified an optimum capillary shape, which results from a specific power share in the ring-core fiber combination.