The generation of a low surface roughness of the cut edge during laser beam cutting is a challenge, especially when cutting thick metal sheets. The striations, which determine the surface roughness are caused by the local melt flow inside the cutting kerf. The melt flow and the geometry of the cutting kerf was analysed with a high-speed X-ray imaging system that recorded the fusion cutting process. A local bulge of the cutting kerf is present in case of interrupted striations on the surface of the cut edge. The occurrence of a bulge coincides with a melt flow, which has a significant flow direction against the cutting direction. The absorbed irradiance and temperature on the cutting front present a maximum at the cutting depth where the bulge is localized. In case of regular striations on the surface of the cut edge, no bulge is visible. Furthermore, the values of the absorbed irradiance and the temperature are almost constant over the cutting depth and the maximum values are lower compared to a cut edge surface with interrupted striations.
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