Design of freeform optic module creating a ring-shaped laser beam profile for localized heating of sheet metals

Laser-based preheating is an energy-efficient method to locally enhance the formability of a metal sheet. A ring-shaped beam can be used for preheating in the flange forming process. According to the tailored heat principle, only the area to be formed is heated. In this paper, a method is presented to design modules with reflective freeform optics (FFO) that converts a collimated Super-Gaussian beam into a ring-shaped beam at a working distance of e.g. 170 mm and deflects the laser beam by 90° simultaneously. The FFO design is based on a 2D geometric model of the optical system and is iteratively optimized according to the process requirements. The FFO was manufactured by fast tool diamond turning and placed into an integration model within a progressive die tool. The design was verified by beam profile measurement as well as by analyzing the formed part quality.