Dynamic method for determination of coupling efficiencies in laser material processing

The knowledge about the amount of power transferred to a workpiece in relation to the applied total power of a particular heat source is crucial for every kind of thermal material processing. It allows the evaluation of process efficiencies for competing technologies and enables predictions of the thermal load of a workpiece. In case of laser material processing, in which the workpiece is heated up by laser radiation, the coupling efficiency is often strongly affected by or even corresponds to the absorptivity of the material being processed. However, documented values in literature are only valid for perfect and smooth surfaces, mostly incomplete in terms of different laser wavelengths, inconsistent or not available for relevant technical alloys. Therefore, a new method for determining absorptivity values and energy coupling efficiencies for almost arbitrary materials and processing conditions was developed. This technique relies on an adjustment of data achieved by experimental thermographic imaging and numerically computed temperature fields and provides as a result the energy coupling efficiency of the considered process. The potential of the proposed method was evaluated by performing experiments on standard type 304 stainless steel with the purpose to determine its absorptivity for solid-state laser radiation with a wavelength of 1.07 mm. The results show the absorptivity as a function of both the angle of incidence and the polarization state of the laser light.