TP4: Interactions of thermal phenomena with the formation of adiabatic shear bands in high-speed blanking

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The research hypothesis of this subproject within the research group is based on the assumption of a dependence of the formation of adiabatic shear bands on thermal boundary conditions. This is the maximum process temperatures (T) and the thermal shock (T_punkt), i.e. the time derivative of the temperature profile, in interaction with the melting temperature and the coefficient of thermal expansion of the sheet material (T_S and α_s). The aim is to develop a phenomenological model that describes the interrelationships of the material-specific, lower limit deformation speed, above which the formation of an adiabatic shear band occurs, combined with the parameters T_S, T and T_punkt with α_s. This model also allows interpolation or extrapolation to other materials and sheet properties, such as sheet metal thickness. The results are also used to investigate the theory of dynamic recrystallization, in which shear band formation is directly related to the recrystallization temperature, which correlates with the respective melting temperature. An innovative method consisting of an experimental tool for use on a mechanical stamping press and a measurement technique based on thermoelectricity, allows the determination of these parameters. The use of thermoelectricity in the measurement setup enables instantaneous, in-situ temperature measurement at the cutting edge of the active elements. This instantaneous measurement method makes it possible to determine the thermal shock (T_punkt) during a high-speed blanking process. Preheated sheets allow influencing the thermal shock by reducing the temperature difference between the start of deformation and the time of the adiabatic shear band formation. By using special inserts that provide optical access to the forming zone, a high-speed camera can be used to record crack propagation, as well as strains and strain rates, and thus evaluate shear band formation. The melting temperature parameter (T_S) can be influenced by varying the sheet material. Different alloys are used for this purpose – from low melting metals such as tin and zinc to the aluminium alloy EN AW-5754 and steel materials such as 22MnB5.