Addressing the increasing global energy consumption and environmental pollution caused by fossil-fuel energy usage has spurred worldwide growth in renewable and eco-friendly energy solutions. A hydrogen economy based on renewables-including hydrogen production, hydrogen storage and conversion of hydrogen to electricity is widely considered as a promising solution for the future of energy [1]. In the hydrogen economy, the emissions of proton-exchange membrane fuel cell (PEMFC) are only water, and the greenhouse gas emissions are expected to be reduced to near zero when hydrogen is produced from renewables [1]. PEMFC with eminent character of environmental-friendly features, high-energy efficiency and low operating temperatures etc., are getting tremendous attention with their bright application prospects in the fields of spacecraft, automobiles, bicycle and stationary power supply, etc. [2]. Bipolar plates, graphite or metallic, are one of the key components of PEMFC stacks, and affect the performance of fuel cell significantly by the electric conductivity, corrosion resistance related with the life, power density related with weight, etc. Metallic bipolar plates accounts for about 50% of the market share especially for which is sensitive to the volume and weight, for its ultra-low thickness, easy-to-fabricate, outstanding mechanical strength, and low cost, and can be considered a reasonable alternative material to graphite [3]. However, to reduce the weight and volume of the PEM fuel cell, the thickness of metallic bipolar plates should be as thin as possible, and fabrication of metallic bipolar plates becomes more difficult for the flow channels of high depth-to-width ratio, ultra-thin sheet, array structures, etc.
In recent years, lots of investigations on the forming processes of metallic bipolar plate have been carried out including stamping, hydroforming, and rubber forming etc. The channel design was 0.8×0.75 mm for the use of a rigid punch on a 50 µm-thick stainless steel sheet (SUS 304) for micro-channel stamping processes, the scale factor modified material model were used for finite element simulation [4]. The plate of TA0 titanium with 0.15 mm in thickness was punched by rigid punch with maximum punching force of 2000kN [5]. Koç et al [6] studied the formability and surface topography issues of metallic bipolar plates using stamping and hydroforming technologies. They found that the formability of stainless steels, i.e. SS304 and SS316L, were better than those of SS430 and Ti grade 2, which was greatly affected by the hydroforming pressure. Increasing hydroforming pressure and stamping force yielded higher surface roughness values at channel peaks. High aspect-ratio micro-channels on bipolar plates have the advantage of improving fuel cell performance, which is difficult to fabricate. Different parameters such as sheet direction, load condition, and heat treatment were studied, and results showed that the channel of a bipolar plate formed with a heat-treated sheet was more than twice as deep as the one formed from a sheet without heat treatment and was more uniform. The formed channel was deeper with a dynamic load than a static load and in the transverse to rolling direction than in the rolling direction of sheet [7]. And, the relationship between the formability and stamping process, dimensions of micro-channels were investigated, and results indicated that formability was better without wrinkles and cracks at the speed of 0.1-1.1m/s, and the radius of punch (die) and the draft angle α also play an important role in the metal bipolar plates stamping process. The larger dimension of channel width, channel depth and rib width within 0.5–1.5 mm contributed to better formability of ss304 sheets [8]. Karacan et al. studied the effect of materials and depth of flow channels, and found CP-Ti material was only suitable for the flow channel of small depth considering the weight and volume of the PEM fuel cell stacks, which was a very important issue particularly for mobile applications [9]. The influence of the various factors of die sets on the forming behavior of the wave-like channel are explored, and a relationship between the thickness reduction and the pressing depth is described as an empirical exponential formula. Then, guidance is provided for the die set design of wave-like channels [10].
Soft die forming can uniformly apply the pressure to the thin sheet, and improve the formability of micro channels. Hung et al [11] studied the hydroforming process of micro-channels with aspect ratio of 0.468 with the developed high pressure container. Compared with the maximum aspect ratio of 0.31 formed using the traditional hydroforming process, the aspect ratio of micro-flow channels in this study was 51% higher. The effects manufacturing processes, stamping and hydroforming, on contact resistance were investigated [12]. Stamped BPP samples demonstrated higher conductivity than the hydroformed BPP samples, and increasing maximum pressure levels in hydroforming resulted in decreasing contact resistance values. Hybrid hydroforming–stamping methods have been used to investigate the forming capability of multi-array pin-type pattern. Results have shown desirable filling percentage and thickness distribution [13]. Rubber as a kind of soft media is used to fabricate titanium bipolar plates, and the parameters, such as thickness, hardness and pressure, etc., were studied. A 0.1mm thick bipolar plate of SS304 stainless steel is fabricated with high surface quality, dimension precision and low cost [14]. In forming of titanium bipolar plate, experimental results showed that the maximum channel depth was 0.270mm (68% of punch channel depth), and current density for titanium bipolar plate was 396mA cm-2 at a voltage of 0.6V, which was smaller than 1156 mA cm-2 of graphite bipolar plate and 799 mA cm-2 of TiN-coated titanium bipolar plate [15]. Design of experiment was used to find the key parameters of the rubber forming process and their optimal levels. The design of experiments showed that the outer corner radius, draft angle, and the friction coefficient were the most critical parameters in the forming process, and the optimal values were found, respectively [16]. However, maximum filling percentage, thickness distribution, and dimensional accuracy in convectional rubber forming process were not satisfying. A novel method was developed using a semi-stamp rubber forming, and results showed that 11.7, 9, and 1.057% improvement, respectively. In addition, the maximum percentage of error uniformity of channel depth was about 2.9% indicating the high accuracy of the stamping process, which meant that the method could produce a perfect bipolar plate by one-step forming [17, 18]. Gong et al. studied the flexible forming process using polymer powder medium with annealed pure copper of 0.1mm in thickness. Then, greater forming depth, more uniform wall thickness distribution, and low cost of the die were obtained [19]. In the flexible forming, the formability is limited by the fluidity of the rubber pad, which does not guarantee the shape accuracy, especially around the micro-channel corner. The part accuracy of the hydroforming is even worse than rubber forming, and the manufacturing time is too long, and cost of equipment is high. Considering these challenges, stamping process has been regarded as a promising candidate for the forming of fuel cell bipolar plates [20].
Multi-stage forming technology utilizing the stamping process has been investigated in various field to obtain high formability [21], small radius of corner [22], uniform thickness distribution [23], and high shape accuracy of parts made by ultra-thin sheet [20]. A double-step hydroforming process is proposed, an initial hydroforming step on a concave die and a final hydroforming step on a convex die. The results indicated that the forming depth and the filling percentage of the die cavity are significantly improved [24]. Two-stage forming by stamping was performed for ultra-thin stainless steel sheets with thickness of 0.1mm and 0.075mm. A forming depth at the first stage was chosen as a process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. Based on the simulation results, a mathematical model was established to identify the dominant factor needed for formability improvement and to propose a methodology for the process optimization of the multi-stage forming. The uniformity of thinning at Stage I and the ratio of the deformation at stage I to that at the second forming stage (Stage II) were considered to be two major factors that significantly influence the formability in the two-stage forming [20]. Xu et al. studied the effect of anisotropy of titanium sheet on the limit forming depth of micro channels, and the greatest limit forming depth can be obtained when the channel direction is parallel to the rolling direction of sample. Multistage forming process can effectively improve the aspect ratio, for example, the limit forming depth of TD samples increases for over 40% with the maximum aspect ratio reaching 0.67, and it has a little effect on the contact resistivity [3]. In fact, the quality of final product mostly depends on the parameters of several sequentially forming stages. Therefore, it is essential to design the most optimum forming process.
In the investigation, the three-stage stamping process of micro channels was studied for its high productivity and low-cost using titanium ultra-thin sheet with perfect properties, such as high corrosion resistance and conductivity, lower density, etc. FE simulation and optimization of three-stage stamping was carried out to fabricate the micro channels with high depth-width ratio used for bipolar plates in PEMFC. The effects of radius of punch and female die, ratio between width of channel and rib, punch displacement, etc. were analyzed considering the reduction of thickness. In the first stage, punch end is designed as an arc to form wave-like channels with high uniform thickness. Then, parameters for every stage are obtained, and used in design of mold and three-stage stamping process. Experiments were performed with the developed servo drive equipment, and the thickness, surface quality and microstructure of formed parts were investigated. The results show that the three-stage stamping process is a practical method to fabricate bipolar plates to meet the requirement of massive production.