Laser beam absorption measurement at molten metal surfaces

Laser light absorption is one of the elementary effects of laser material processing. Absorption values are relevant to calculate the process efficiency and predict the impact on the material for the increasingly used laser pro-cesses. However, absorption measurement can be a complex task. At high temperatures of metals, only limited experimental data is available due to the dynamic surfaces and the often unknown emissivity needed for the temperature measurement. Models were created to predict the absorption at different temperatures, which are successful with assumptions in some regimes, but often fail in others. For improving the theoretical models, an experimental measurement of high-temperature metal surfaces is desired. Therefore, a radiometric measurement method is proposed in this work using a heating laser to create a metal melt pool, while measuring temperature and reflection of its surface by a second measuring laser beam. General tendencies known from literature could be confirmed by the measurements, while absorption values tend to scatter at increasing temperature. However, trends could be observed. Between melting and boiling temperature, a slight absorption increase was seen in the range between 35% and 38%. Those values indicate that both interband and intraband absorption must be considered to explain the absorption in this regime. At increased temperatures, the intraband absorption be-comes the dominating absorption mechanism, reaching absorption values above 45% at very high temperatures.