Fracture of ultrathin coating during micro-channel forming process of coated metallic sheet: Experiments and numerical prediction

Abstract

The coating-then-forming process offers a promising route to improve the manufacturing efficiency of metallic bipolar plates in proton exchange membrane fuel cells. However, fracture of ultrathin coatings during forming presents a formidable challenge to the corrosion resistance of coated metallic sheets. To provide theoretical guidance for addressing this issue, this work investigates the coating fracture behavior during the micro-channel forming process through experiments and numerical analysis. Coating cracks were revealed to be concentrated within the fillet and flat regions of the channel ridge, highly sensitive to local strain levels governed by both channel geometry and substrate microstructure. Therefore, a full-field simulation methodology was developed to predict the coating fracture during forming, balancing between computational efficiency with accuracy. Within this numerical framework, hybrid modeling of the metallic sheet substrate was employed, combining local implementation of crystal plasticity in the critical region with homogeneous materials elsewhere. For coating on the critical region, varying failure parameters correlated with substrate microstructure were determined by the microscopic fracture mechanism. Experimental validation confirms that the developed model can precisely predict both coating fractures and formed profiles, notably achieving an 80 % accuracy improvement in crack density compared to conventional approaches. Based on the simulation results, process windows were established to correlate coating fracture and channel geometry with forming parameters, thereby guiding optimization toward the minimization of crack density under geometrical constraints. Following parameter optimization, coating cracks were successfully eliminated at the flat region, demonstrating an effective strategy for maintaining the corrosion resistance after forming.