Study On Electromagnetic Force And Forming Uniformity In Electromagnetic Tube Bulging With Concave Coil

The use of lightweight alloy materials is an important way to reduce the weight of materials,and can achieve energy conservation in the fields of transportation and aerospace.However,the forming process of the lightweight alloy material cannot be realized by the conventional processing technology,because its poor forming performance at room temperature,easy tearing and rebound.For this reason,the electromagnetic forming technology has been paid attention to by forming high strain rate,non-contact and single abrasive tools,and has become an important means for processing light alloy materials.At present,electromagnetic forming technology is commonly used in the forming of sheet metal materials and tube materials,and is divided into electromagnetic sheet forming technology and electromagnetic tube forming technology.Among them,electromagnetic tube forming technology usually uses a set of spiral coils to realize tube bulging or tube compression,and the radial electromagnetic force generated by the conventional coil is large in the middle and small in the side,so that the electromagnetic tube forming has an end effect,and the forming amount of the tube at the center is larger than the side,resulting in uneven forming,which is easy to cause the tube to be ruptured,instability and other phenomena.In order to improve the non-uniformity of electromagnetic tube forming,this paper proposes a new method called as electromagnetic tube forming with concave coil,that is,by designing a new coil,the radial electromagnetic force of the center is weakened,and the radial electromagnetic force distribution becomes "concave" distribution.Under the action of the radial electromagnetic force of the new "concave" distribution,the bulging process of the tube begins with the side bulging,so that the axial profile of the tube is "concave",and then the side of the tube drives the tube center whose electromagnetic force is weakened.When the amount of bulging at the center is equal to the end,the tube bulges to a uniform state.In view of the innovative technology of electromagnetic tube forming with concave coil,this paper first reviews the current research status of uniformity in electromagnetic forming technology by domestic and foreign scholars,mainly by designing the coil system to change the electromagnetic force distribution to achieve uniform forming.Secondly,an effective finite element model is established to simulate the forming system,and a new R-L criterion for measuring the uniformity of tube bulging is proposed.Meanwhile,the relationship between the electromagnetic force distribution and the bulging uniformity of the tube under different parameters is studied.The simulation results show that the electrical parameters mainly affect the peak value of the electromagnetic force,and the coil structural parameters can also affect the peak position of the electromagnetic force.Under the action of the "concave" distributed radial electromagnetic force,one parameter of some system parameters is adjusted,and the tube is radially bulged.The axially shaped profile will transition between the "concave" distribution and the "convex" distribution,and the critical point of the change is the uniform bulging of the tube;further,by comparing the traditional coil forming and concave coil forming by simulation,it is found the concave coil can increase the L value of the uniform bulging range of the tube by about 2 times.The use of the multi-layered concave coil can further increase the uniform bulging range of the tube.In this example,the three-layer concave coil is used to compare with a single concave coil,it is found the L value is increased by more than 1 time,that is,the L value is about 6 times that of the conventional coil.Finally,relying on the Wuhan National High Magnetic Field Center of Huazhong University of Science and Technology,the experiment of electromagnetic tube forming with concave coil is designed,and the discharge voltage is adjusted to make the axial profile of the tube bulging from "concave" to "convex".The axially uniform bulging range of the tube at the critical point of change is 27.22 mm,and the axially uniform bulging range used by the conventional coil is 8.87 mm,which clarifies the effectiveness of the concave coil.