Reliable detection of stick welds at resistance spot welding

Resistance spot welding (RSW) of galvanized steel sheets brings a risk of faulty welds in the form of stick-welds. These differ from high-quality spot welds in the way that only the coatings are joined with a material bond, but the base materials are not fused together. Stick-welds can occur, among other things, in critical material combinations with greatly differing sheet thicknesses or with increased wear of the welding electrodes. Due to the significantly different material-specific strengths of zinc and steel, stick-welds do not meet the required weld strengths. Therefore, it is important to reliably prevent or detect the stick-welds. However, the established and most frequently used non-destructive (NDT) method for spot welds by manual ultrasonic testing reaches its limits in this respect, as the fused zinc layers are almost as sound-conductive as proper spot welds. A promising alternative is the method of passive magnetic flux density testing (pMFT) developed at the Technische Universität Dresden, Chair of Joining Technology and Assembly. With this NDT method, the spot welds are first magnetized and their residual flux density on the surface is measured. Due to the different magnetic properties of the used steel alloys and their zinc-coating, clear differences can be measured between correct and stick-welds. This paper shows the experimental design and the developed procedure for evaluating the spot welds on different galvanized steel sheet alloys. In addition, the high potential of integrating this NDT method into fully automated production lines is shown in order to bridge the gap between the highly automated RSW process and NDT, which can only be carried out manually up to now.