In the conventional electromagnetic tube expansion, the end effects generated by the conventional helix coil may lead to inhomogeneous tube deformation in the axial direction. This paper is aimed at overcoming this issue by proposing a new concave coil structure to replace the helix coil currently used by the industry practice to generate a radial electromagnetic force on the tube. The proposed concave coil is expected to reinforce the electromagnetic force distribution profile and, hence, improving the axial inhomogeneous deformation of the tube. In this context, a new R-L criterion of deformation uniformity is first proposed. Second, an electromagnetic-structural coupling finite element model is established to investigate the relationship between the distribution of electromagnetic force generated by the concave coil and the uniformity of the tube under various voltage levels. The effectiveness of the proposed method is validated through a series of experimental and simulation analyses. Furthermore, based on the characteristics of the electromagnetic tube expansion, a modified multilayer concave coil structure is proposed to overcome the axial inhomogeneous deformation of long tubes. INDEX TERMSConcave coil, deformation homogeneity, electromagnetic force distribution, electromagnetic tube expansion.

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Non-uniform axial deformation and tube wall thinning are the main issues restricting the further development of the tube electromagnetic bulging technology. Aiming at overcoming these issues, a tube electromagnetic bulging method based on convex driving coil is proposed in this paper. An electromagneticstructure coupling model of the tube electromagnetic expansion process is established and the influence of detailed convex coil parameters on radial and axial Lorentz forces are analyzed. Results show that with the new proposed method, the homogeneous length of the tube axial deformation is 4.7 times that of the expansion loading with conventional driving coil. Meanwhile, the issue of tube wall thinning in the conventional electromagnetic bulging method is restrained due to the produced axial electromagnetic force using the proposed convex coil. The proposed tube electromagnetic bulging based on convex coil can effectively overcome the issues currently exist in the conventional tube electromagnetic bulging and promote wide industrial application of the electromagnetic forming technology. INDEX TERMS Electromagnetic forming, tube wall thickness, non-uniform axial deformation.

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