Transformation shear bands can be observed at the cutting edges of high-speed blanked samples. Compared to conventional blanking, high-speed blanking increases punch velocities by an order of magnitude, up to 11m/s, producing extremely high strain rates (> 10⁴ 1/s). Such high strain rates have been demonstrated to cause highly localised deformation in micrometre-sized bands with an altered microstructure. Due to the short process time, almost adiabatic conditions prevail in the forming zone, resulting in a significantly elevated temperature within the narrow forming zone. Depending on the blanked materials, process kinematics, and the tool design, a localised deformation shear band without or a transformation shear band with microstructural changes occurs.
The high-speed blanking of the Aluminium and high-strength steel at different process parameters is analysed concerning the shear band formation and its properties. After blanking the cutting surface's geometrical properties, fracture mechanics is analysed by visual inspection, surface profilometer and electron microscopy. Cross-section images and light-microscopic images detect the occurrence of shear bands. The shear-band's properties are analysed regarding microstructure, dimensions and hardness. FEM simulations accompany the experimental investigations to determine the stress state, strain rate and temperature resulting from the process-specific influencing factors (e.g. clearance).
No transformation shear bands are observed in aluminium alloys, but a deformation shear band with a width of 20 micrometres is formed, covering all plastic deformation during the cutting process. In high-strength steel, an approximately 50-micrometre width transformation shear band with nanocrystalline microstructures is induced; its width reduces width at higher punch velocities and strain rates, respectively. The shear band length increases at higher punch velocities, and a characteristic S-shape of the cutting edge is formed, indicating a shear-dominated transformation behaviour of the microstructure.