Electromagnetic Expansion and Fragmentation of Hollow Aluminium 5052 Tube

Abstract

Electromagnetic forming is a high-speed forming technology by which hollow profiles can be compressed or expanded. It is done with a pulsed magnetic field to apply Lorentz' forces at electrically conductive material. Electromagnetic hollow tube expansion is limited by the fragmentation tendency. This work attempts to use a combination of analytical and computational approach to compute the net tangential stress during tube expansion. A simplified analytical framework to estimate the temporal evolution of plastic stresses present in aluminium alloy AA5052 at low and high applied magnetic pressures is developed based upon dynamic imaging. The time resolved images captured using current synchronised high speed camera record the overall dimensional changes of the tube that is validated by multi-physics simulation of expansion process. Imaging of hollow tube expansions at two selected peak currents has been carried out at various current levels in the range 76-160 kA. The direct visualisation of the increase in the tube diameter at two current levels provided a comparison of the developing net tangential stresses in the hollow tube during the undamaged and fragmented expansion. Imaging of tube expansion also facilitates the estimation of the strain rate experienced by the tube and was in the range of ~1700 s −1 to ~1200 s −1. The propensity of fragmentation was found to be due to the level and duration of generated tangential stresses above the yield stress during expansion of the aluminium tubes. Presented study provides a mean of exploiting the enhanced formability of aluminium alloys using electromagnetic forming.

The Numerical Analysis of Force and Comparison of Pulse Magnet and Electromagnetic Forming Coil

“…The forming coil adopts the reinforcement technology of the pulse magnet to improve the overall strength. However, unlike pulsed magnets, forming coils are placed in different positions inside and outside the tube when used for tube bulging and necking [9,10]. It is placed at the end of the workpiece during electromagnetic flanging [11].…”

Eddy current measurement in hollow conducting tube in electromagnetic forming process

“…In electromagnetic expansion, electrostatic energy stored in capacitor bank is discharged into a pulsed magnetic toolcoil, whichgenerates electromagnetic energy that results in a pulsed time varying magnetic field causing eddy current to induce in the conducting tube [1]. A cylindrical pulsed magnetic tool coil is used to generate magnetic field between tool coil and inner surface of tube [2].…”

“…The magnetic field of the coil current and the induced eddy current in the conducting tube results in a Lorentz force creating a radial outward pressure resulting in the expansion of the tube [2]. The electromagnetic forming process occurs within the order of few hundred micro-seconds.…”