Contents lists available at ScienceDirect Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro Determination of magnetic coupling and its influence on the electromagnetic tube forming and discharge circuit parameters S.K. Dond a, *, Tanmay Kolge b , Hitesh Choudhary b , Archana Sharma b , G.K. Dey b a Homi Bhabha National Institute, Mumbai, 400094, India b Bhabha Atomic Research Centre, Mumbai, 400085, India ARTICLE INFO Keywords: Electromagnetic forming Tube expansion Magnetic coupling Numerical simulation ABSTRACT The coil-workpiece magnetic coupling coefficient K is one of the factors that influence the efficiency of elec- tromagnetic forming (EMF) process. Along with this, an evaluation of the magnetic coupling coefficient is im- portant in simplifying the electrical representation of the process. The first aim of this study is to analyse the effect of K on the EMF process efficiency, and second is to obtain a simplified relation between K and coil- workpiece gap (h) and thereby study the variation of discharge circuit parameters with h. The analysis is carried out with the help of experiments, and the finite element based numerical simulation. Aluminum tube of 100 mm length and 1.5 mm thick is electromagnetically expanded using a 7 turn helical coil. Trials are taken for the different gaps between the coil and tube keeping the discharge energy constant. The coupling coefficient (K) variation with the coil-tube gap (h) is related with curve fitting, and this K-h relation is then used to analyze the discharge circuit parameter dependency on h. Simulation results, as well as K-h relation based analytical study, show a good match with the experimental observations. The EMF process efficiency and tube displacement are found to be improved exponentially with the increase in K. Among the discharge circuit parameters, with the increase in h, exponential rise in inductance and exponential decay in peak current is observed with a marginal decrease in the resistance. 1. Introduction The electromagnetic forming (EMF) technology has the capability of forming lightweight materials like aluminum. The technology can be employed to deform the workpiece with several hundred meters per second and has shown improvement in formability, wrinkling, and spring back in some materials [1]. Such advantages of EMF process lends itself for use in aerospace, automobile and power industries [1–3]. A typical EMF system (as shown in Fig. 1) consists of a constant current power supply, capacitor bank, discharge switch and tool coil along with a workpiece. First, the energy is stored in capacitors in electrostatic form and then it is converted into electromagnetic energy by discharging the capacitors into the tool coil. An under-damped coil current generates a magnetic field that links with the workpiece and causes an eddy current to flow into it. The Lorentz force responsible for material forming is generated because of eddy current in the workpiece and magnetic field between the coil and workpiece. In EMF, electrical efficiency denotes the amount of transfer of electrostatic energy stored in capacitor banks to the electromagnetic energy in the tool coil whereas mechanical efficiency is the ratio of energy discharged from the capacitor bank to forming energy. The overall process efficiency in the electromagnetic forming applications is very less. For tube forming applications, it is 10–25 % [4,5]. One of the factors for less efficiency is the leakage flux, and that is related to the coil and workpiece magnetic coupling. EMF is a transient phenomenon consisting of electromagnetic- structural coupled physics. Numerical simulation and analytical ap- proaches are being used to analyze the process in the past. The coil discharge current is used as input excitation to the numerical and analytical models. The accuracy of these models depends on the ex- actness of the discharge circuit parameters of EMF system, as the cur- rent flow in the coil is a function of discharge circuit parameters. Yu et al. [6] have studied the dynamic behavior of the tube during the forming from circular to a square shape. In the developed numerical model, discharge current is used as excitation to the coil. Rajak et al. [7] have developed a numerical model to analyze the EMF wire crimping application. In this, estimated discharge current waveform is used as input to the software. Likewise, the numerical model-based EMF study in the past for electromagnetic forming [8–10] and welding applica- tions [11] have considered discharge current as input to the coil. https://doi.org/10.1016/j.jmapro.2020.02.034 Received 10 January 2019; Received in revised form 20 February 2020; Accepted 23 February 2020 ⁎ Corresponding author. E-mail address: shandond12@gmail.com (S.K. Dond). Journal of Manufacturing Processes 54 (2020) 19–27 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. T