Forming of Ultra-Thin Ferritic Stainless Steel Sheets

Abstract

The present thesis investigates the forming of ultra-thin ferritic stainless steel (FSS) sheets developed for the bipolar plates (BPs) of proton exchange membrane fuel cells (PEMFCs). Due to a lack of standard practices and research on the forming of ultra-thin materials, forming such thin sheets is challenging. Current works cover topics from the forming limit diagram (FLD), the most basic concept to evaluate the formability of sheet materials, to the application of forming technologies for the forming of ultra-thin FSS sheets.

Chapter 1 presents an extensive literature review describing the historical development of FLD determinations and predictions. It also describes the forming technologies that can be adopted for use in ultra-thin materials.

In Chapter 2, the materials investigated in the present thesis are summarized to show them all in a single view. The materials are labeled according to their sheet thickness and the year they were produced.

In Chapter 3, a robust test method to determine FLD of ultra-thin FSS sheets is proposed based on rigorous experiments, and a theoretical FLD prediction is provided. The FLDs of two FSS sheets of thicknesses 1 and 0.1 mm were determined experimentally. For the 0.1 mm sheet, the modified Marciniak test and the conventional ASTM standard test were used for the FLD determination. The advantages of the Marciniak test compared with the ASTM standard test for assessing ultra-thin FSS sheets are described. In this section, the FLD was also predicted theoretically using a modification of the Parmar-Mellor-Chakrabarty (PMC) model, which incorporates the effects of surface roughness. A non-quadratic anisotropic yield function, YLD2000-2d, was implemented in this model to represent the anisotropy of the sheet metals. The FLDs predicted with the conventional Marciniak-Kuczyński (M-K) and the modified PMC models were compared with the FLDs determined experimentally. The FLD calculated with this modified model was in better agreement with the measured data than that computed with the M-K model for both the thin and the thick sheets.

In Chapter 4, the experimental and finite element (FE) simulation results for forming an ultra-thin FSS sheet using servo-press technology are given. Forming experiments on a 0.15 mm thick FSS sheet sample were conducted using a direct-drive digital servo-press. Four different slide motions available in the servo-press were proposed: V-shaped (V), holding (H), W-shaped (W) and oscillating (O) motions. The effects of the type of slide motion on the micro-channel depth and shape accuracy were investigated. In addition to these experiments, FE simulations of forming for the ultra-thin FSS sheets were performed. The FE simulation results validated the influence of the slide motion. In particular, the proposed FE model successfully predicted the maximum thinning as well as the thickness profile across the micro-channel of the BP.

In Chapter 5, the two-stage micro-channel forming results are presented. The two-stage micro-channel forming experiments were performed using 0.1 and 0.075 mm ultra-thin FSS sheets. A forming depth at the first forming stage was chosen as the process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. In addition to these experiments, FE simulations for the two-stage forming process were conducted to optimize the punch radius and forming depth at the first forming stage for improving the formability. The comparative study between the FE simulations and the experimental results could validate improvements in the formability by using the two-stage forming approach. This work could also support the existence of an optimum forming depth at the first forming stage. Based on the FE simulation results, mathematical modeling was used to identify the dominant factor needed for formability improvements and to propose a methodology for the process optimization of the particular multi-stage forming.

Chapter 6 describes the optimization of the two-stage forming using a mathematical and statistical algorithm, central composite design (CCD). Three factors, the punch radius, die radius, and forming depth, were optimized by CCD in the first stage. The optimized results from Chapter 6 are consistent with the results presented in Chapter 5.