The effect of ultra-low frequency pulsations on tearing during deep drawing of cylindrical cups

Abstract

In sheet metal forming there is an increasing demand for deep drawing of light weight components with complex shapes. However, thinner and more ductile materials are more susceptible to tearing during the process. It is known that greater draw depths can be achieved without tearing if the blankholder is subjected to a pulsating force. The present paper describes experimental investigations in which cylindrical-shaped cups are drawn with a pulsed blankholder force (PBHF) at ultra-low (<1 Hz) and low (1–10 Hz) frequencies. The tests were performed with blanks of steel (DC04) and of aluminium (5754-H111) over a range of draw ratios. The tests showed that tearing occurred within a well-defined part of the punch stroke. Of the two parameters, amplitude and frequency of the pulsed force, the latter had a more pronounced effect. For draw ratios resulting in a narrow working range of the static blankholder force, the process can be made more robust if the PBHF is applied at ultra-low frequencies. In addition to this, the PBHF must be synchronised in such a way that, during the identified critical tearing part of the stroke, the instantaneous PBHF is kept below the static tearing limit by synchronising the punch force and PBHF. Feasible values of the frequency depend on the length of the critical tearing part of the stroke and the upper and lower BHF limits of the process.