Extended forming depth and thickness uniformity of metallic bipolar plate under variable magnetic field path in magnetorheological elastomer assisted forming

Abstract

Abstract During the flexible-die forming of metallic bipolar plates, metal flow is generally constrained by the progressively increased frictional resistance at the die cavity edges, resulting in the limited formability. Accordingly, in this study, a forming method using magnetorheological elastomers (MREs) as the pressure-carrying media with adjustable magnetic field path was proposed to achieve the staged-regulation of plastic flow. The experimental results demonstrate that compared with those formed by the conventional rubber-assist forming technique, the forming depth limit and effective flow area of microchannel on bipolar plate are enhanced by 12.8% and 17.6%, respectively, when using MREs with ferromagnetic particle content ( φ ) of 20% as the pressure-carrying media under the magnetic field with flux intensity ( B ) of 1850 Gs. Meanwhile, the fracture surfaces of bipolar plates exhibit more pronounced ductile fracture characteristics. Besides, by modifying the magnetic field path in which the magnetic flux density is switched from B 1 = 800 Gs to B 2 = 1850 Gs at a piston force of 15 kN, the two-stage forming load is differentially controlled, resulting in a 9.9% increase in forming depth limit and a 21% increase in shell thickness uniformity relative to the constant magnetic field condition of 1850 Gs. That enhancement is attributed to the inhibition of the continuous increase of metal flow resistance and the improved metal flow. Furthermore, the magnetic field switching load facilitates the increase of normal pressure on sheet metal, which causes the sheet material redistribution within the microchannel cavities, leading to the improved shell thickness uniformity.