Laser-induced thermochemical polishing of metals

Abstract

This work investigates material removal by laser-induced thermochemical machining (LCM) as a method for selectively polishing passive metals in applications where conventional finishing technologies induce unacceptable mechanical or thermal stresses. The main objectives are to identify reliable process conditions, to understand the effects of various process parameters on polishing and the corresponding mechanisms of action and to develop a model to predict surface roughness. A multipass LCM method is developed, which allows controlled polishing of different materials by adjusting laser power, spot diameter, scan passes, line spacing and scan velocity. The results show that polishing characteristics are determined by the laser-induced average thermal load and exposure time. Polishing can be understood as a geometric levelling process and can be predicted by a model based on local differences of the thermal gradient. The stable lower limit of roughness of the finished surface is mainly defined at the microstructural level by inhomogeneous etching between the face and boundaries of the grains. These findings present opportunities for flexible and selective adjustment of LCM process parameters to achieve the desired roughness level for surfaces of micro- and macroscopic dimensions.