Multi-scale modeling of ultra-thin commercially pure titanium sheet for fuel cell bipolar plates: Plastic anisotropy and distortional strain hardening

Abstract

This study presents a multi-scale modeling framework to describe the mechanical behavior of a 0.1 mm-thick commercially pure titanium (CP-Ti) sheet developed for fuel cell bipolar plates. Since standardized methods for characterizing ultra-thin sheets under complex stress states are lacking, a virtual modeling approach was employed. At the grain scale, a crystal plasticity finite element (CPFE) model was constructed to incorporate the relevant slip and twinning systems, enabling prediction of responses under diverse loading conditions. Extending to the continuum scale, the CPFE results, combined with tensile data, were used to calibrate an advanced constitutive model based on the evolutionary Yld2000-2d yield function, capable of capturing anisotropic behavior. Validation against independent limiting dome height tests confirmed the predictive accuracy of the framework. The proposed approach provides a basis for simulating the forming behavior of ultra-thin CP-Ti sheets and supports precise manufacturing of bipolar plates in fuel cell systems.

1. Introduction

Titanium and titanium alloys have gained renewed attention as candidate materials for metallic bipolar plates in fuel cells, which are essential components in hydrogen-powered vehicles, drones, and urban air mobility systems [1–2]. These applications demand improved fuel cell efficiency and reduced system weight. Traditionally, bipolar plates have been fabricated using graphite due to its chemical stability and electrical conductivity [3]. However, the inherent brittleness and hydrogen permeability of graphite limit its ability to be manufactured at reduced thicknesses, thereby restricting weight minimization of the fuel cell stack. To address these limitations, researchers have increasingly focused on metallic bipolar plates [4–6]. These alternatives offer superior mechanical strength and can be fabricated to thicknesses below 0.1 mm, contributing to significant weight reduction. Among various candidate metals, titanium and its alloys are particularly promising due to their excellent corrosion resistance and mechanical robustness [7–9]. In addition, titanium has a density approximately 60% lower than that of stainless steel, which is the most widely used metal in current bipolar plate applications [10–11].

Commercially pure titanium (CP-Ti) sheets are especially attractive as next-generation materials for metallic bipolar plates. Their benefits include high ductility, sufficient mechanical strength, and relatively low production costs enabled by the absence of alloying elements and simplified processing routes [12–13].

Bipolar plates require the fabrication of micro-scale flow channels to guide reactant gases and coolant. In metallic bipolar plates, these channels are typically formed through stamping using mechanical presses. During this process, the sheet is subjected to multiaxial stress conditions, necessitating an accurate understanding of its mechanical response. However, standardized methods for evaluating the mechanical properties of ultra-thin sheets are not yet established. For example, ISO 12004-2, which defines forming limit measurement procedures, applies only to sheet thicknesses between 0.3 and 4 mm [14], making it unsuitable for sheets thinner than 0.1 mm.