Surface Topography Evolution During Long-Run Micro-Stamping of Bipolar Plates (BPPs) and Effects on Corrosion and Contact Resistance Characteristics

Abstract

Micro-stamping, as a promising sheet metal forming process for mass production of small parts, can meet the expectations such as durability, strength, surface finish, and low cost for miniaturized metal products and features. The purpose of this research was set to investigate surface interactions during mass manufacturing of micro-stamped sheets, and its consequences; then establish correlations (if any) between surface interactions vs. corrosion and contact resistance of bipolar plates (BPPs) to be used in proton exchange membrane fuel cells (PEMFC). In experimental part of this study, 10,000 SS316L sheet blanks were micro-stamped using a stamping die set with 750 μm-deep micro-channels under 200 kN stamping force, and with a constant stamping speed of 1mm/s. Surface inspections (surface roughness and micro-channel height measurements), corrosion and contact resistance tests were carried out on BPPs. Analysis of variance (ANOVA) technique was utilized to investigate the significance of surface roughness, channel heights, corrosion and contact resistance variations for BPPs. Moreover, three-dimensional (3D) finite element models of micro-stamping process were established to approximate the stress and strain levels as well as coefficient of friction value experienced at contact interface. The results revealed that the roughness values for micro-stamping dies and BPPs followed similar trends during 10,000 micro-stampings. Since surface defects trigger corrosion, the correlation between surface roughness and corrosion resistance of BPPs was found to be direct. Increasing number of surface irregularities (asperities) lowered contact surface area that resulted in increased contact resistance. Finally, comparison of experimental and numerical channel height values showed that the coefficient of friction did not change considerably during the mass production of BPPs, at least within the 10,000 stamping cycle.