The effect of the calibration method on the critical forming strains and fracture limits in the roll forming of 7075 series aluminium

There is increasing interest in roll forming high strength 7075 Aluminium alloys to reduce vehicle weight while maintaining vehicle safety. For this, the roll forming limits of these new Aluminium systems need to be tested and understood depending on material type and process conditions. This study investigates if a general test method can be applied to analyse the fracture limits of 7075 Aluminium in roll forming. Using a laboratory V-roll forming tool the effect of the two possible roll forming calibration methods, constant radius and constant length roll forming on the fracture limit is studied for two high strength 7075 tempers. The results are compared to the reduction in area and the bend fracture limit determined in tensile and fracture bending tests performed at different strain rates. There is a clear effect of the roll forming calibration method on the fracture limit for one of the two Al7075 tempers. This is linked to differences in the evolution of the maximum outer fibre strain that lead to early strain localisation by kinking in the constant radius roll forming case. The bend fracture limits of both Al7075 tempers are affected by the strain rate which must be considered when accessing material limits in industrial roll forming. Overall, the results of this study suggest that generalised material limits such as the reduction in area and the bend fracture limit cannot give a reliable representation of the fracture limit of high strength 7075 Aluminium in roll forming.