Mechanism on the increased sheet metal formability in flexible-die forming process using elastic composites with magnetic-field-dependence as the pressure-carrying medium

Magnetic smart materials were employed as pressure-carrying medium in sheet metal flexible die forming process to achieve the real-time control of sheet formability in recent years. Yet, few studies involved the mechanism behind this phenomenon, which restricts the improvement of process design. Thus, elastic composites with magnetic-field-dependence (e.g., magnetorheological elastomer, MRE) were taken as an example to investigate their effects on sheet formability and the reason behind that in this work. MREs with different contents of hydroxy iron particles (φ) were fabricated, and their mechanical properties in the presence of external magnetic field with different flux density (B) were determined and introduced into finite element analysis (FEA). Bulge tests of Al1060 sheet employing MREs as the flexible dies were carried out under external magnetic field with the monitoring of digital image correlation (DIC) system. The results showed that, in the case of B = 1950 Gs, limit bulge height (H^*) and critical strain (ε^*) of deformed sheet respectively increased by 6.7% and 16.4% when adopting appropriate φ (i.e., φ = 20%), and, in the case of φ = 10%, they separately increased by 13.8% and 35.2% when adopting appropriate B (i.e., B = 1650 Gs). Compared with silicone rubber, forming pressure transmitted by MREs decreased under external magnetic field, which reduced equivalent stress of sheet metal and increased stress ratio of two in-plane stresses. By controlling MREs’ mechanical properties, their pressure-transmitting properties could be adjusted. On the basis of stress state analysis, mechanism on the increased sheet formability when MREs served as the flexible dies was clarified.