Energy Coupling Efficiency in Laser Surface Treatment

Numerous publications demonstrate the great potential of lasers for surface treatment (hardening, cladding, alloying). Nevertheless, substitution of classical techniques has turned out to be difficult mainly due to high costs. The paper discusses ways to reduce laser-related costs by increasing the energy coupling efficiency. Following theoretical considerations, experimental results are presented.A calculation of energy coupling requires reliable values of optical constants at process temperature. For materials of technical interest like steels those data are very rare if not lacking completely. An attempt was made, therefore, to calculate optical constants by an extrapolation based on well-known room temperature values of iron and on the electrical resistivity of the alloy. The results fit the existing experimental data satisfactorily in the wavelength range 0.5 to 15 μm.A method is presented allowing to measure the coupling efficiency under process conditions. Results show additional contributions to the coupling rate from oxidation and additive materials. As expected from the theoretical results, a strong influence of the laser parameters wavelength and polarization is observed. e.g., reducing the wavelength from 10 to 1 μm increases the coupling rate by a factor two to three.