Authors:
Koji Hirano
PIMM Laboratory (Arts et Me´tiers ParisTech-CNRS), 151 Boulevard de l’Hoˆpital 75013 Paris, France and Nippon Steel Corporation, Marunouchi Park Building, 2-6-1 Marunouchi, Chiyoda Ward, Tokyo 100-8071, Japan

Remy Fabbro
PIMM Laboratory (Arts et Me´tiers ParisTech-CNRS), 151 Boulevard de l’Hoˆpital 75013 Paris, France

In laser cutting of thick steel sheets, quality difference is observed between cut surfaces obtained with 1 and 10 μm laser beams. This paper investigates physical mechanisms for this interesting and important problem of the wavelength dependence. First, striation generation process is described, based on a 3D structure of melt flow on a kerf front, which was revealed for the first time by our recent experimental observations. Two fundamental processes are suggested to explain the difference in the cut surface quality: destabilization of the melt flow in the central part of the kerf front and downward displacement of discrete melt accumulations along the side parts of the front. Then each of the processes is analyzed using a simplified analytical model. The results show that in both processes, different angular dependence of the absorptivity of the laser beam can result in the quality difference. Finally, the authors propose the use of radial polarization to improve the quality with the 1 μm wavelength.