Development and application of analytical and numerical models for characterization of thermal fields during surface laser treatment

摘要

Temperature fields in low carbon rimming steel with 2.5 and 0.45 mm thickness during surface treatment with pulsed Nd:Glass laser have been simulated. Two models namely analytical and finite elements method (FEM) have been applied for solving the one dimensional differential heat transfer equation. The analytical model is assuming constant thermophysical properties, semi-infinite size of the treated material and no heat transfer with ambient atmosphere. For the FEM the influence of the thermal dependence of the thermophysical properties and the finite size of the treated material has been investigated. It has been shown that the one-dimensional analytical model could be successfully used for the estimation of the temperature on the surface of both the thicker and the thinner steels, but is not suitable for the characterization of the thermal field in the depth of the thinner material. The oxidation kinetics during laser treatment has been simulated and the influence of the formed oxide film on the coefficient of absorption and thus on the temperature field has been analyzed. For this purpose two models, namely the "smooth surface" and the "rough surface" model have been applied. It has been found that the kinetics of the oxide film growth is defined only by the rate of the oxygen supply to the treated surface and within the laser pulse duration (7ms) is linear in time. According to the "smooth surface" model the consideration of the surface oxidation increases the optical absorption coefficient from 0.4 to 0.9 while according to the "rough surface" model this increase is up to 1.0. The absorption coefficient increase is accompanied with the same ratio increase of the temperature in respect to that when the oxidation has not been taken into account.