Off-axis laser beams for improved welding efficiency

Most metallic laser welding processes use a laser beam incident perpendicular to the work surface, which maximises power density and gives a symmetrical heat profile. For absorptivity however, a perpendicular laser beam is not the most energy efficient processing angle. Absorption increases with incidence angle, from a minimum at 0° up to a maximum at Brewster’s angle. For a 1070 nm fibre laser welding 316L stainless steel, this predicts an improvement from approximately 21% to a potential maximum of 67%. Potential benefits include reduced-power processing, with smaller, cheaper lasers, with the attendant benefits of reduced equipment costs, running costs, and reduced cooling requirements (and hence reductions in running and equipment costs). Previous work on tilted laser welding has generally involved simply altering the angle of the welding head to create an off-axis beam. This method improves absorptivity, but also alters the spot shape, distorts the thermal profile, reduces power density, and alters the velocity and cooling effects of the shield gas. The effects on absorptivity alone were impossible to isolate, and the energy improvements negligible. In this investigation, a welding process was designed to isolate and control the incidence angle absorptivity effects, with all other variables, that were also altered in previous work, left unchanged. Firstly, the reflectivity principles underpinning the work are presented and justified. Creation of the required laser beam shapes is then evaluated via ray optics simulations. This is concluded with the design and testing of a physical optical array with beam test results presented.