<title>Theoretical and experimental investigations on the role of oxidation and gas flow in the machining of steel molds due to melt removal with high-power CO2 lasers</title>

Abstract

For the machining of metal cavities and reliefs with high power lasers, both the gas mixture and the actual formation of the flow is essential. High ablation rates with an appropriate surface quality can be reached only with an optimized gas flow and a sound process understanding. The impinging gas flow results in a shear force and a pressure gradient over the melt pool. These values also depend on the actual geometry of the erosion front and can not be predicted in advance. To investigate the gas flow with different nozzle arrangements numeric simulations are carried out. Calculations and experiments show that working with an inclined gas flow -- the so called laser planing process -- improves the material removal rate significantly. An optimum angle for the gas- nozzle can be shown. By using a mixture of oxygen and nitrogen as working gas, additional heat can be supplied to the process. Knowing the fraction of material which can be oxidized per time unit is important to calculate the additional heat from the chemical oxidation. Furthermore, the thickness of the formed oxide layer influences the absorption of the laser power effectively. In this paper it will be shown that the change of the absorptivity is the key parameter for a high ablation rate. The chemical reaction also changes the properties of the melt pool. These effects influence the process significantly and they are of fundamental importance to achieve a high material removal rate. This will be shown in various experiments. A simplified process model for melt removal will be presented and compared with experimental results. The model includes two important oxidation effects, which are improved absorptivity and the additional heat transfer. Furthermore the shear force and the pressure gradient over the melt pool will be taken into account to calculate the thickness of the melt film.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.