A Uniform Pressure Electromagnetic Forming Method And Process On Titanium Bipolar Plate Of Hydrogen Fuel Cell

Hydrogen fuel cell was considered as one of the important new energy technologies in the future due to its advantages of high efficiency and near-zero pollution.Bipolar plate was one of the important parts of fuel cell,which plays an important role in introducing and separating reaction gas,collecting and conducting current.The bipolar plates account for 11%-45% of the total cost and 80% of the mass of the cell,which has become the main factor restricting the commercialization of fuel cell.Titanium and its alloys have high specific strength and good corrosion resistance,which have many advantages as metal bipolar plate substrate.However,the formability of titanium was poor and the springback was serious.The channel depth was too shallow and the channel was easy to fracture when the traditional quasi-static forming technology was used to form the titanium.Compared with the traditional quasi-static forming technology,electromagnetic forming,as a high-speed forming technology based on pulse high magnetic field,has many advantages,such as improving the formability of material,reducing rebound,reducing cost and so on,which makes it one of the most potential methods to solve the forming difficulty of titanium bipolar plate.In the present research of forming titanium bipolar plate by electromagnetic forming,the machined uniform pressure actuator(UPA)was often used.But on the one hand,the forming efficiency of the machined UPA was low,on the other hand,due to the complexity of the forming process,the current research was lack of sufficient control methods to improve the forming depth of bipolar plate,resulting in the final bipolar plate with shallow channel depth.In addition,the driver sheet with high conductivity was needed to drive the workpiece with low conductivity in the process of forming,which makes the driver sheet consumed in each forming process and the economic benefit was low.Aiming at these problems,this paper carries out the work from two aspects: improving the strength and efficiency of the UPA and optimizing the the electromagnetic forming process parameters.Based on the theoretical analysis and experimental verification,the feasibility and advantages of electromagnetic forming applied to the manufacture of titanium bipolar plate were demonstrated.On this basis,the discharge current-based forming method with a background magnetic field was proposed.The specific contents were as follows:Firstly,based on the magnetic shielding assumption,the force on the workpiece and on the wire derived,the forming efficiency and the strength of the UPA were analyzed in detail.Aiming at the problem that the traditional coil has lower forming efficiency,a wire-wound UPA with biaxial winding of Zylon fiber is proposed and wound.The experimental results show that the new coil has higher strength and forming efficiency,and improved the forming margin.Secondly,according to the size of the multi-channel serpentine bipolar plate die used in this paper,the optimization and design of the wire-wound UPA were carried out.the threedimensional finite element model was used to analyze the the titanium bipolar plate forming process.It is found that the impact velocity of workpiece is directly proportional to the depth of the channel.According to the simulation results,the channel depth of titanium bipolar plate can reach 0.4 mm when the impact velocity reaches about 309 m/s,providing a clear and intuitive indicator for titanium bipolar forming.Considering the influence of various parameters(standoff,discharge voltage,discharge capacitance and so on)on the impact velocity in the discharge process,a forming window was established to guide the experiment.Then,the experiment of forming titanium bipolar plate with UPA was carried out.To ensure the impact velocity and surface smoothness of the workpiece,the air pressure in the process of the experiment is regulated to minimize the impact of air resistance without the gap breakdown based on the Paschen's law.The electromagnetic forming device of titanium bipolar plate was designed and developed,under the discharge condition of 150 ?F and 12 k V,and with a standoff of 4 mm,the titanium bipolar plate with a channel depth of 0.4 mm and a depth/width ratio of 0.67 was fabricated.The ratio was nearly 97.53% higher than the current research.The results showed that the maximum fluctuation rate of the channel was not more than 2.18%,The filling rate of channel was higher than 90%,the maximum surface roughness was 0.819 ?m,and the maximum thinning rate of channel section was only 15.89%.Finally,aiming at the consumption of driving plate in traditional electromagnetic forming,a discharge current-based forming method with a background magnetic field was proposed.A background magnetic field magnet was designed based on simulation analysis,and the the depth of the channel and the temperature rise of the workpiece were numerically simulated.The experiment of discharge current-based forming titanium bipolar plate was carried out with the magnet.The titanium bipolar plate with a channel depth of 0.2 mm was obtained.The forming ability of this method was verified preliminarily,which provides a new way for the manufacture of titanium bipolar plate.