Laser polishing of metallic freeform surfaces by using a dynamic laser beam preforming system

Laser polishing is an automated polishing solution for polishing metallic freeform surfaces, which is hoped to support or replace the time-consuming and cost-intensive manual polishing in the near future. This technique is based on remelting a thin surface layer by laser radiation. The surface tension leads to a material flow from the peaks to the valleys; thus, the surface resolidifies in a smoother way. Laser polishing of 3D surfaces often leads to a nonperpendicular angle of laser incidence. This results in a projection of the focused circular laser beam on the surface of the workpiece that is primarily approximated with an elliptical shape. The beam geometry in the projection depends on the surface geometry of the workpiece and the actual spatial working position of the laser beam. To ensure a constant polishing quality, the circular symmetry of the laser spot on the workpiece has to be maintained. A method is developed and discussed, which enables the compensation of the elliptical deformation, called elliptical preshaping. This is realized by a combination of an anamorphic zoom lens with a beam rotating device comprising a Pechan prism. It is demonstrated that constant surface qualities up to inclination angles of 60° can be achieved on the tool steel X37CrMoV5-1 using elliptical preshaping.