Process zone morphology and melt dynamics in laser remote fusion cutting (RFC) revealed by high-speed X-ray imaging

Laser material processing procedures yield numerous benefits, as non-contact manipulation of the workpiece, high precision, and extensive automation capabilities. For metal joining, laser beam welding is a widely process employed in industry, for example in automotive body construction and in the production of electronic components. With the same optical setups as usually used for laser beam welding, a melt ejection can be induced in sheet metal, resulting in the formation of a cut. This enables laser remote fusion cutting (RFC) based on a ejection without the need for tools near the process zone. A comprehensive understanding of the conditions mechanisms causing the melt ejection or preventing it is yet to be achieved. In this study, in-situ observations of the process zone in RFC were performed using high-speed X-ray imaging synchrotron radiation, achieving frame rates up to 18 kHz for steel samples and 28 kHz for AlMg3 samples. features of the process zone morphology are extracted by means of image processing from the recordings, such the angle of front wall inclination or the ejection direction for different process parameters. The front wall angle for RFC is in line with an established model for the front wall angle in laser beam welding. Propagation-based phase-contrast imaging reveals the melt film at the cutting front, showing a decrease of the melt film thickness with increasing feed velocities. Melt dynamics at transitions between process states of cutting and not cutting could be observed. The temporal resolution was insufficient to capture humps in the melt film at the front wall steel samples. These were resolved for AlMg3 samples, confirming that humps at the front wall play an important role in the melt dynamics.