An Investigation of Splaying in Redrawing

Abstract

Experimental and theoretical studies were made of splaying behaviour in the direct redrawing process. Cup diameter/thickness ratio governs whether redrawing is conducted with or without a hold-down punch to prevent wrinkling during deformation. Both situations, (termed constrained redrawing and unconstrained redrawing respectively), were investigated from a splaying aspect. Splaying was related to that deformation in the cup which precedes die contact during steady-state redrawing conditions, and was associated with a reduction in redraw load. The phenomenon was explained by the principle of minimum energy of deformation. In unconstrained redrawing splaying increased as cup diameter/thickness ratio diminished, whereas in the constrained situation severity of bending around the hold-down punch profile radius was the predominant parameter, diameter/thickness ratio having only a weak influence. For constrained redrawing die geometry did not directly influence the potential for splaying but dies of small angle prematurely terminated the splaying phase. For unconstrained redrawing die geometry both influenced splaying potential and restricted maximum splaying. Dies of large angle will therefor generally promote greatest splaying. Experimental results implied that splaying decreases as the rate of work-hardening increases. Greatest splaying was 4 - 4.5% and occurred in constrained redrawing with the combination of a small hold-down punch profile radius, large die angle and work-hardened cups. Theory correctly predicted all experimental trends. Two expressions for maximum splaying are proposed, one providing an over-estimate and the other an under-estimate. Experimental results were spread between the limits of these expressions. The use of a guide to prevent splaying did not produce a reduction in redraw load. There was evidence to suggest that the contrary was true. Results of research should be equally applicable to the reverse redrawing process.