Journal of Power Sources, Vol.168, No.2, 408-417, 2007
Niobium-clad 304L stainless steel PEMFC bipolar plate material Tensile and bend properties
Niobium (Nb)-clad 304L stainless steel (SS) is currently under consideration for use as a bipolar plate material in polymer electrolyte membrane fuel cell (PEMFC) stacks. Because metal bipolar plates will likely be formed by stamping, the sheet-metal properties of this material were characterized in both the as-rolled and an optimized annealed condition via a series of bend and quasi-static tensile tests. Results from tensile testing demonstrate that annealing significantly softens and thereby improves the ductility of the material. Bend test results indicate that springback is nearly independent of the bend direction relative to rolling direction for both the as-rolled and annealed conditions. In the as-rolled condition, springback is also nearly independent of specimen orientation (i.e. whether the cladding layer is on the inside or outside of the bend). However, in the annealed condition, springback does depend on the cladding orientation relative to bending and was found in all cases to be substantially lower than that observed in the as-rolled condition. Microstructural analysis of the specimens indicates that two failure conditions can potentially arise, dependent on the thermomechanical condition of the material. In the as-rolled condition, failure initiates via fracture through the Nb cladding. In the annealed specimens, failure can occur by brittle fracture of an interfacial intermetallic layer that forms during the annealing treatment. This generates a series of crack-induced pores along the interface between the Nb cladding and the SS core, which eventually leads to ductile failure of the Nb cladding via localized necking. However, the conditions required for this phenomenon to take place are fairly extreme and can be readily avoided in practice. In general, the results suggest that to achieve acceptable stamping tolerances, the material should be annealed prior to forming and the bipolar plate flow channel pattern should be designed such that extreme levels of strain at the cladding/core interface are avoided to mitigate the potential for partial delamination within the material. Published by Elsevier B.V.