Laser beam welding of overlap joints is widely used in different applications, e.g., white goods industry, automotive industry, and fuel cells. However, welding of overlap joints often results in the formation of hidden defects, where the top and root of the weld appear to be properly formed, but a lack of fusion is hidden in the interface between the two sheets - so called 'false friends'. It is known that the gap height between the two overlapping sheets and the welding speed have an influence on the formation of such 'false friends' on the one hand. On the other hand, keyhole behavior and melt pool dynamics affect the occurrence of weld seam defects in general. Nevertheless, systematic knowledge that consistently describes the formation of 'false friends' and their correlation to the aforementioned factors is not available at the current state of the art.

The posters summarise the fundamental mechanisms that lead to the formation of 'false friends' based on a novel experimental setup that provides insight into the processing zone based on high speed synchrotron x-ray imaging. The setup enabled the welding of 0.5 mm thick plates to the underlying material (both joining partners: X5CrNi18-10/AISI 304/1.4301), with the ability to adjust different gap heights and to acquire high-speed X-ray images simultaneously with the welding process. It should be noted that the realization of high frame rates in X-ray imaging requires the transmission of low material thicknesses, which is why a sheet width of 1 mm was chosen. Therefore, the sheet width requires the formation of a small weld seam width which is why a fiber laser IPG YLM-450/4500-QCW with a focal diameter of approx. 60 µm was used allowing to study of welding speeds from 4 up to 12 m/min. This enabled the acquisition of frame rates up to 100,000 kHz in high-speed synchrotron X-ray imaging. The high recording frequency of the synchrotron X-ray imaging allowed the systematic study of the formation of false friends depending on keyhole and melt pool behavior by a stepwise adjustment of the gap height from 0 to 0.2 mm in 0.05 mm increments. This provided new insights into the processing zone for a systematic description of the interactions that lead to gap bridging on the one hand and to the formation of false friends on the other.