| With the rapid development of new energy vehicles,titanium bipolar plates,as the core components of fuel cells,have gradually attracted wide attention for their high specific strength and corrosion resistance.However,the formability of titanium is poor and the springback phenomenon is serious.When the traditional quasi-static forming technology is used,the problems such as the shallow flow channel depth and the large thinning of the fillet area of the flow channel are easy to appear.Compared with the traditional quasi-static forming technology,magnetic pulse forming is one of the most promising methods to solve the difficulties of titanium bipolar plate forming,which can significantly improve the forming limit of the material,reduce the springback and restrain the wrinkle due to the characteristics of non-contact high rate forming.However,as two typical driving coils in the field of magnetic pulse forming,the distribution of electromagnetic force is not uniform in the flat coil,while the uniform coil has the problems of short coil life and contact arc,so it is difficult to form high precision bipolar plates with large areas.To solve the above problems,a uniform pressure magnetic pulse incremental forming process is adopted in this paper,that is,a small uniform pressure coil is adopted and distributed multi-step forming is carried out by moving the internal primary coil to realize the forming of a large area.Based on this process,the application of this technology in the forming of TA1 titanium bipolar plate is studied systematically.Firstly,the sequential coupling model of discharge circuit,electromagnetic field and deformation field was established based on the principle of equivalent circuit,and the analytical modeling of uniform pressure magnetic pulse forming process was realized.In this coupling process,the expressions of coil current and workpiece current are obtained for electric field analysis.For electromagnetic field analysis,the analytical expressions of workpiece surface magnetic pressure and coil wire magnetic pressure are obtained.For the deformation field analysis,the analytical expressions of workpiece deformation velocity and displacement are obtained.Through three methods,namely the analytical prediction of a coil structure size,the measured current curve of loading,and the establishment of LS-DYNA three-dimensional sequential coupling numerical model,the ability of the proposed analytical model to predict the equivalent circuit parameters and dynamic phenomena,the validity of the magnetic shielding hypothesis and the rationality of the assumption of uniform distribution of magnetic flux were compared and analyzed.The results show that the equivalent resistance is more sensitive to the materials with small thickness and high resistivity,and the maximum deviation is nearly 50%.The influence of the mass density on the motion velocity is much greater than that of the conductivity.ω/β=9.5 magnetic shielding hypothesis criterion may be more suitable as the suitability boundary of coil.On the premise of ensuring electrical insulation,the turn distance of the coil wire should be reduced as much as possible,and the width of the wire should be increased to improve the uniformity of the magnetic field distribution of the coil conductor on the magnetic circuit.Based on the analytical expressions of workpiece surface magnetic pressure and coil wire magnetic pressure,from the forming efficiency of workpiece and coil durability,the analytical expressions guiding coil height,wire thickness and the design of primary and secondary side clearance were derived,the optimal geometric structure of the uniform pressure coil was determined,and the electrical contact between the outer channel and the workpiece was evaluated.Mathematical representation of the critical clamping force to avoid welding problems and contact opening.According to the size of titanium bipolar plate,the uniform pressure coil for forming large titanium bipolar plate was designed to meet the forming requirements and obtain sufficient service life.It is found that there is an optimal solution for the initial coil height,wire thickness and primary and secondary side clearance,respectively,which is 18.3mm,2.7mm and 3.2mm.To meet the forming requirements of large titanium bipolar plates,the uniform pressure magnetic pulse incremental forming process was adopted,and the corresponding simulation was carried out based on the designed uniform pressure coil.The simulation results show that the titanium bipolar plate can be formed with single pass of 15 k V or three pass of 9k V under the conditions of discharge capacitance of 100μF,acceleration distance of 2mm and coil overlap rate of 15%.The analysis of magnetic fields acting on the coil and the driver sheet shows that the electromagnetic force on the primary coil wire develops towards the inner diameter and is compressed inward during the progressive forming process.The electromagnetic force on the conductive channel mainly acts on the inner wall which is in contact with the driver sheet and tends to repel the driver sheet.The overall direction of the electromagnetic force on the driver sheet is opposite to that of the primary coil,and the electromagnetic force distribution changes from wavy to uniform.The analysis of the velocity and equivalent strain in different regions of the bipolar plate runner shows that the slab velocity propagates in the form of stress wave,which reflects the oscillating process of the workpiece.The maximum deformation velocity is 195m/s at the bottom of the runner.The highest equivalent strain is found at the outer wall of the fillet R zone,which is the easiest place for the wall thickness reduction of bipolar plate parts.On this basis,the traditional quasi-static drive soft film forming and uniform pressure magnetic pulse incremental forming experiments were developed,and a comparative study of 0.1mm thick large titanium bipolar plate forming was carried out.The results show that under the conditions of discharge capacitance of 100μF,acceleration distance of 2mm,coil overlap rate of 15% and Cu110 thickness of 0.3mm,each discharge area is used to discharge9 k V continuously for three times.The titanium bipolar plate with channel depth of 0.4mm,channel depth-width ratio of 0.53 and size of 485mm×195mm×0.1mm was successfully fabricated.Compared with the traditional quasi-static driven soft film forming,the bipolar plate formed under three discharge voltage 9k V with uniform pressure magnetic pulse can effectively reduce the thinning rate of the flow channel,the maximum thinning rate of 18.2%,and further improve the filling rate of the flow channel,the maximum filling rate of 96.3%. |
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