炭酸ガスレーザによる穿孔加工時の吸収率変化の測定

Abstract

This paper describes the measurement of the change in the absorption factor with extension of the depth of the hole pierced with a laser beam by means of the inverse problem solution. First, the temperature-rise at a certain point on a workpiece is measured by a thermocouple when a focused laser beam is irradiated on the workpiece for a preset pulse duration t1. Assuming that the absorption factor is η1, the temperature-rise is calculated by the finite-difference analysis. The calculation is repeated until the η1, which produces coincidence between the calculated and measured temperatures is obtained. The workpiece is then replaced with a new one and the temperature-rise is measured for a pulse duration t2=t1+Δt. Assuming that the absorption factor for t1<t<t2 is η2, the calculation of the temperature change at the point is iterated until η2, where the calculated and measured temperature changes coincide at the same point, is obtained. Repeating this process, the change of the absorption factor with the increase of the depth of the processed hole can be determined. It was found that the absorption factor increases with increasing the depth of the processed hole and reaches 96% at the moment when penetration occurs, but subsequently it decreases rapidly after the penetration. When oxygen is used as the process gas in place of nitrogen, the maximum absorption factor reaches 133% of the incident laser power as a result of the generation of heat by the oxidation of the workpiece material.