Effect of preheating condition on strength of AA6060 Aluminium Alloy for extrusion

Abstract

Fletcher Aluminium is a New Zealand company that manufactures aluminium extrusions for the building market. Their extrusion process involves using a hydraulic ram to force heated raw supplied material, in the form of large diameter cylindrical logs, through a die land that is the desired product shape. The final quality of the extruded product is influenced by the extrusion temperature, ram pressure and extrusion speed. The speed of extrusion at Fletcher Aluminium is limited by the extrusion ram pressure because the extrusion machine is operating at the pressure limit of the compressor. Currently the company requires an increase in the extrusion speed, while maintaining quality requirements where these improvements are desired without upgrading the compressor. The press pressure is required to overcome friction and the flow stress of the billet, which is dependent on the precipitates and solutes conditions and their strengthening effects in the billet. However, the preheating temperature and extrusion speeds need to be sufficiently low enough to avoid hot tearing. This research starts to increase the ram speed by decreasing the Mg2Si content for precipitation strengthening. Heat treatment may be used to dissolve Mg2Si content by billet preheat to a temperature at or exceeds solvus temperature (TSolvus). However, a higher (than TSolvus) billet temperature may have two conflicting effects. First, a higher temperature in general reduces the flow stress thus lowering the required pressure. On the other hand, at temperature higher than Tsolvus, Mg2Si should completely dissolve, resulting in solid solution strengthening thus increasing the flow stress. The objective of the study is to quantify the strengthening contributions from solute atoms (Mg / Si) and precipitates (Mg2Si) on the as-received and heat treated test samples using room temperature mechanical testing and phase diagram analysis. The selected heat treatment conditions fully enclose the preheating temperature and time range currently used in Fletcher Aluminium production. Simulations of the extrusion were conducted by hot compression testing (Gleeble test), to measure the combination of strengthening effects during deformation at elevated temperatures. The room temperature tests have shown that hardness, yield strength (YS) and ultimate tensile strength (UTS) are minimised at approximately 405°C. The minimum point is shown to be caused by the combination of strengthening and softening due to solid solution and precipitation strengthening. The higher values of hardness, YS and UTS at lower and higher preheating temperatures are mainly due to precipitation and solution strengthening respectively. The findings show that, the solution of Mg2Si gradually increases as the temperature increases at temperatures exceeding 405°C. Hardness, YS and UTS were compared in order to determine the correlation between each mechanical property. A correlation exists, but it is not simple to translate between each mechanical property. The relationship between hardness and stress observed for 8% permanent strain was also investigated; there was no improvement in correlation. The elevated temperature tests show that initial flow stress and peak flow stress reduces linearly with temperature increases, which means higher testing temperature results lower precipitation and solid solution strengthening effects. Additionally, the constants of the Gleeble – Holomon equation were numerically calculated and are similar to published values. The Gleeble – Holomon equation was combined with Felthams equation in order to provide a relationship between extrusion velocity and extrusion temperature.