CALCULATION OF CO2-LASER BEAM ABSORPTANCE AS A FUNCTION OF TEMPERATURE FOR STEELS BY THE NUMERICAL-METHOD

Temperature distributions were measured during the irradiation of a CO2 laser beam at one end of a rod-shaped specimen and at the centre of a thin plate-shaped specimen. Regarding the measured temperature distributions as one-dimensional and two-dimensional unsteady state heat transfer solutions, CO2 laser beam absorptances were calculated using a modified finite difference method. Temperature dependence of thermal properties, heat loss due to convection and latent heat during melting of the specimen were taken into account in this numerical calculation. Increasing the specimen temperature from room temperature to melting point, absorptances of STS304 stainless steel and SM45C steel were calculated as 8 similar to 40% and 6 similar to 41% for the one-dimensional calculation, and as 9.3 similar to 41% and 5 similar to 41% for the two-dimensional calculation, respectively. These calculated absortances were very close to theoretical values at relatively low temperature, which were calculated from the electrical resistivities of the specimens. Increasing the temperature of the specimens, absorptances increased considerably due to oxidation of the specimens. Regardless of specimen composition and specimen dimension, both absorptances showed nearly the same value of 41% at their melting points, in which the structures of both metals became amorphous phases.