A new modelling technique for prestrained heat treated aluminium sheets

This presentation deals with a new technique to model the effect of Intermediate induction heat treatment (IIHT) in prestrained aluminium sheets. IIHT is applied between two conventional cold forming steps, which eventually lead to enhanced formability of aluminium alloys. The aim of IIHT is to alleviate the strain hardening of the material which is introduced in the first cold forming step and there by reducing the yield limit and increasing the hardening modulus for subsequent forming steps. As a result, a remarkable increase in formability can be achieved in the subsequent forming steps at room temperature for AA5182. The scientific aspect of the IIHT process is demonstrated by defined pre-strained tensile test specimens at different object temperatures to establish a process window. To accurately model the effect of IHTT in simulations, it is necessary for the material model to consider the change in yield behaviour and plastic recovery that the material undergoes during heat treatment. To this effect, material model Mat133 (Barlat_YLD2000) in LS-Dyna has been enhanced to account for the effect of intermediate heat treatment. Since the non heat treated areas cannot be adequately protected by any cooling method, the neighbouring areas also get affected by the heat to some extent. Hence it is inevitable to consider heat affected zone as well in the model along with heated zone and non heated zone. For this, a thermo-mechanical coupled simulation is carried out to exactly define the heat affected zone in the model. In summary this approach is carried out in four steps namely pre-forming simulation, springback simulation to account for residual stresses, thermo-mechanical coupled simulation for heat treatment, and final forming simulation with enhanced material model. Finally, experimental investigations have been carried out on a simple cross-die deep drawn cup and the numerical simulations have been validated. In conclusion, the potential and limitations of this method is discussed.