Optimisation of Punch Pressure in Containerless Backward Warm Extrusion of Al-Zn-Mg alloys

Abstract

Open-die warm extrusion is an advanced forming technology which can not only reduce the machining time and material waste but also improves the properties of the products. Container wall – billet friction is eliminated in containerless extrusion, which leads to a large reduction in the total force required for extrusion. It is suitable for Al-Zn-Mg alloys, which is difficult to extrude compared to other aluminum alloys. The high specific strength of Al-Zn-Mg alloys and the lower punch pressure for containerless extrusion promises to be a vital combination, which needs to be further explored upon, hence present investigation was to determine theoretically and experimentally the effect of containerless backward extrusion of Al-Zn-Mg alloys namely Al-5Zn-1Mg, Al-10Zn-1Mg, Al-15Zn-1Mg for different geometries under a lubrication condition. Theoretical analysis was carried out using elementary plasticity theory – Dipper model and experimental analysis by conducting containerless backward extrusion experiments. Friction was minimized by lubricating the billets with graphite powder. Experiments were carried out using a 40 T universal testing machine with ram velocity of 3.3 × 10-4 m/s. The effect of specimen geometry on the extrusion pressure at various temperatures after containerless backward extrusion were investigated and presented. Finally, an optimum ratio of lengths to diameters of billets was determined by finite element analysis software called LS. Dyna: Levermore Software Dynamic Non Linear Analysis. Flow and frictional properties of Al-Zn-Mg were determined at different temperatures from 303 K to 673 K by compression and ring compression tests. Force-stroke data were generated at temperatures 303 K to 673 K for strains 0.1, 0.15, 0.2, 0.29, 0.37 and 0.45 for conventional backward extrusion and containerless backward extrusion for theoretical analysis whereas experimentation was restricted only to containerless backward extrusion. During experimentation force stroke data were generated at temperatures 303 K to 623 K for the same strains. Variation of theoretical punch pressure against extrusion strain at selected temperatures was studied. From the results of flow stress and frictional analysis of Al-Zn-Mg alloys it was found that the flow stress and friction for forming all the three chosen Al-Zn-Mg alloys were least between 373 K and 573 K. From the theoretical investigation it was found that containerless backward extrusion requires less than half of the force compared to conventional container extrusion and the optimum warm extrusion temperature for Al-5Zn-1Mg and Al-15Zn-1Mg is 373 K and for Al- 10Zn-1Mg is 473 K. From the experimental investigation it was found that optimum warm extrusion temperature for Al-5Zn-1Mg is 423 K and for Al-10Zn- 1Mg and Al-15Zn-1Mg is 523 K which is in accordance with theoretical analysis. From the experimental investigation it is also found that initial height ho, of billet, should be as small as possible to make upsetting difficult and diameter (do) of the billet should be large in comparison to punch diameter (dp) to make extrusion strain smaller and extrusion process easier. From the finite element analysis it was found that for a billet of ho/ do ratio of 0.5 shows an absence of lateral displacement of material, which is a characteristic of containerless backward extrusion. In the present study dynamic strain aging was observed to occur in the temperature range of 573-673K. The domain where dynamic strain ageing exists must be considered when selecting operating conditions like working temperature and extrusion strain.