Forming limit diagram of an ultra-thin commercially pure titanium sheet: Combined experimental-numerical approach

Abstract

This study presents a methodology for determining the forming limit diagram (FLD) of an ultra-thin (0.1 mm) commercially pure titanium (CP-Ti) sheet developed for use in metallic bipolar plates for fuel cells. Due to its extreme thinness and pronounced planar anisotropy, the standard specimen geometries specified in ISO 12004-2 were found to be inadequate for FLD evaluation. To address this, optimized specimen geometries were designed using finite element (FE) simulations incorporating the anisotropic evolutionary Yld2000-2d yield function. Additional simulations using actual bipolar plate geometries revealed that major strain direction during forming may align with either the rolling or transverse direction. Based on these findings, a new evaluation method was proposed to derive direction-dependent FLDs, while maintaining full compliance with the ISO 12004-2 procedure. Experimental validation confirmed that the proposed methodology is suitable for assessing forming limits, revealing a strong dependence of FLD shape on the major strain direction. This work offers a practical and accurate approach for evaluating directional formability in ultra-thin CP-Ti sheets, providing valuable insight into their deformation behavior driven by hexagonal close-packed (HCP) crystal structure and strong crystallographic texture.