Maximizing the Expansion Ratio through Multi-stage Shear-cutting Process during Collar Forming

Previous investigations showed that a two-stage punching process reduced the edge crack sensitivity on high-strength multi-phase steels significantly compared to a single-stage process. This is caused by an alteration of the state of stress in the shear-affected zone during the second stage, which results in not only higher expansion ratios, but also higher collars during the collar forming test. Another preceding research project at the Chair of Metal Forming and Casting, Technical University of Munich, investigated the optimized values for cutting offset, and die clearance for both pre-cutting and re-cutting steps for a single-stage punching process.

The aim of this research project was to combine those previous findings by increasing the number of stages in a punching process with each using the optimized parameters. The behavior of milled edges served as a reference for optimum performance. The number of stages influenced material properties, such as surface characteristics, material deformation in the shear-affected zone, and roughness of the cutting surface. The quality of the cutting surface increases drastically due to various factors, such as a changed surface hardness, which affects fatigue resistance and, therefore, the component's lifetime. The validation of this punching strategy not only achieves an even higher expansion ratio in common sheet metals, but also makes highly edge-crack sensitive materials available for industrial applications.