Computer simulated and experimentally verified isothermal extrusion of 7075 aluminium through continuous ram speed variation

Abstract

The conventional aluminium extrusion process is run at constant ram speed, leading to quality inconsistency along the length of the extruded product and even to hot shortness as a result of continued temperature evolution. In the present work, computer simulation of the process at varying ram speed was performed in order to determine the conditions to prevent the extrudate temperature from rising excessively. To maintain the maximum workpiece temperature around 500 and 480 °C corresponding to two initial microstructural states of 7075 aluminium billets, two ram speed profiles were derived from the simulation results of a series of conventional extrusion runs. The predetermined ram speed profile commenced at a relatively high value at the beginning of an extrusion cycle and decreased exponentially with ram displacement as soon as the maximum workpiece temperature reached the target value. The simulations showed that with these ram speed profiles the continued temperature increase normally occurring during conventional extrusion could be effectively inhibited. This was verified experimentally by measuring the extrudate temperature continuously using a thermocouple in the die close to the bearing and also using a multi-wavelength pyrometer behind the die. With the predetermined ram speed profiles, the fluctuations of the maximum workpiece temperature could be controlled within a range of 10 °C. The time taken to extrude each billet could be significantly shortened. In addition, the die face pressure remained stable, which would also favour the consistency of the quality of the extruded product.