Local laser softening of high-strength steel with an adapted intensity

Abstract

Today, manufacturers in the automotive industry have to find a compromise between ensuring maximum safety for passengers, which is achieved by the use of high stiffness materials, and a minimum CO2 footprint. These aims are achieved primarily by the usage of lightweight designs. With respect to the car body, high-strength steels—such as the manganese boron steel MBW® 1500 (22MnB5)—fulfil these requirements. Workpieces made of 22MnB5 have a high tensile strength of about 1500 MPa, respectively, enabling reduction of the sheet thickness. However, this high strength is not favorable in all sections of a part, especially those which are relevant for safety. In order to locally adjust the mechanical properties of MBW 1500, a laser-based heat treatment process has been developed to meet the requirements of high ductility in the deformation zones for better crash performance and in joining areas. By means of laser radiation, the brittle martensitic microstructure of MBW 1500 is either tempered or transformed into a ferrite/perlite-dominated microstructure. While the feasibility of the laser softening of high-strength steels with diode laser systems using a tophat intensity profile at a maximum feed rate of 1.0 m/min has already been shown, the present work focuses on the increase in the feed rate of laser softening using an adapted intensity distribution. This distribution is generated using a free-form optics and is designed in such a way that a homogeneous temperature profile on the workpiece's surface is induced. As a result, the feed rate can be increased maintaining homogeneous softening across the sheet thickness.