Hydro-rim deep-drawing processes of hardening and rate-sensitive materials

Abstract

The conventional deep drawing process is limited to a certain limit drawing ratio (LDR) beyond which rupture will ensue. An asymptotic solution of the complete governing equations of this process indicates that this relatively low LDR results from the steep build-up of radial tensile stress with maximum value at the die lip. This tensile stress is significantly enhanced by interfacial friction along the die/flange and by high speed of the operating load and thus holds responsible for premature ruptures. The intention of this work is to examine the possibilities of relaxing the above limitation, aiming towards a process with an ‘unlimited drawing ratio’. The ideas which may lead to this goal are: (a) exerting an external fluid pressure on the outside rim of the blank (“Hydro-rim process”) to reduce radial stress and to decrease, in parallel, the interfacial friction, (b) increasing the blank temperature to a level at which the material is more rate sensitive, and thus less prone to early failure. The benefits of these ideas are examined via parametric analysis of the solution and with experiments in deep drawing processes. A clear outcome from the solution is that if changes in the material properties (strain hardening, strain-rate sensitivity, yield stress, etc.) can be controlled, say, by controlling the temperature and/or the operating speed, the process can reach higher drawing ratios with substantially less assisted fluid pressure.