Research On Method Of Grain Refinement By High-Speed Impact

The deformation rate of magnetic pulse forming is 100-1000 times as that of the Quasi-static forming, and magnetic pulse forming can expand the forming limit of metallic materials effectively and lead to a series of improvements in materials'mechanical properties and microstructure. In this paper, the punching deformation and the refining process of pure Cu and Q235 steel under the magnetic pulse forming process are studied from the aspects of simulation, test and microstructure analysis.The load-stroke Curves of high-speed upsetting of magnetic pulse and high-speed impact of magnetic pulse are obtained by DEFORM-3D. The effective strain of Q235 plate's high-speed impact of magnetic pulse is analyzed and 3 different strain parts are found. Firstly both the Curved and extended areas and the unreformed parts have a compatible deformation during the deformation process; secondly, the distribution of effective strain in the plate is relatively well-proportioned in the direction of thickness and width orientation while it is seriously uneven in the length direction, whereas, there is a distorted area with the width of one tooth in the right and left end of the plate respectively. Thirdly, the fluctuation of effective strain in the length direction becomes greater with the increase of pressing numbers.Based on metallographic observation, the laws of grain refinement of high-speed upsetting of magnetic pulse and high-speed impact of magnetic pulse are obtained in this paper. Under Electromagnetic high-speed upsetting, the pure Cu is ultimately subdivided into a large number of submicron grains and coarse-grains with sub-micron structures. Under high-speed impact of magnetic pulse, Q235 steel refines the average grain size from 20.7μm to 5.15μm. Based on the observation, the hardness of Cu bar in vertical section and in horizontal section will increase as the pressure increases, but the change is small. The phenomenon indicates the hardness improved little and the central section is harder than other areas. Under high-speed impact of magnetic pulse, the average hardness of section A and section B will increase with the upsetting times increasing. The maximum augment happened in the 1st term. Although the initial hardness of the section A and section B are different, after four terms, both sections could achieve the same hardness at last. However, the non-uniformity of hardness in section B is more serious than that in section A.Based on the comparison of the High-speed impact of magnetic pulse and the constrained groove pressing, the following results is obtained: 1) high-speed impact of magnetic pulse can delay the cracks as the number of upsetting times increases. And under the same condition, the high-speed impact of magnetic pulse's deformation is more uniform.2) the high-speed impact of magnetic pulse elucidates the stronger refinement ability and higher refining rate. Meanwhile grains'bending phenomenon is not obvious. The angle of the direction of the grain and the board will be smaller .3) In the 1st deformation term, the hardness of the sheet under two methods will rapidly increase, but under high-speed impact of magnetic pulse are greater and surprisingly get a lower final hardness value.4) under high-speed impact of magnetic pulse the I.F. values of section A and section B are smaller than those under constrained groove pressing, so the high-speed impact method can improve material's hardness uniformity.