Development of high strength Al-Mg2Si-Mg based alloy for high pressure diecasting process

Abstract

Aluminium alloys are the most promising lightweight materials used in the automotive industry to achieve weight reduction for improving fuel efficiency and reducing CO2 emissions. High pressure diecasting (HPDC) is a fast and economical near-net shape manufacturing method to produce engineering components. About 80% of cast aluminium alloys are currently manufactured by HPDC. The increased demands of manufacturing structural components by HPDC process require high strength Al-alloys for the automotive industry. However, the currently available die cast Al-alloys are unable to fulfil this requirement. Al-Mg2Si alloy is known as an alloy capable of providing superior high strength by Mg2Si particles. However, Al-Mg2Si alloy is not applicable in the HPDC process because of the severe die soldering problem. This has limited its applications throughout industries. Moreover, the existing studies on the Al-Mg2Si alloy are mainly focused on the hyper-eutectic alloys and limited information is available for hypo-eutectic alloys. Generally, the mechanical properties of Al-alloys are determined by the alloy composition, the defect levels in the components, the microstructure which is mainly controlled by the casting process and heat treatment process. Due to the high cooling rate provided by the die block in the HPDC process, the refined microstructure in the die cast Al-Mg2Si alloys can be obtained to improve the mechanical properties. Therefore, the development of high strength Al-Mg2Si based alloys for the HPDC process is significant for manufacturing quality automotive components. The present study mainly focuses on the alloy development for the HPDC process. In order to make die castable Al-Mg2Si based alloys, the effect of excess Mg has been investigated to modify the hypo-eutectic Al-Mg2Si system for improving the mechanical properties. The effect of excess Mg on the solidification and microstructural evolution, and the mechanical properties of Al-Mg2Si alloys, has also been investigated by the combination of thermodynamic calculation and the experimental validation. The excess Mg in the hypo-eutectic Al-Mg2Si alloys has been found to be able to shift the eutectic composition to a lower Mg2Si content, which means that the hypo-eutectic composition of Al-Mg2Si alloy can be at eutectic or hyper-eutectic compositions after adding different levels of excess Mg. The experimental trials have also found that Al-8Mg2Si-6Mg alloy provides the best combination of strength and ductility in the as-cast castings made by the HPDC process. This can be further enhanced by adding 0.6wt.% Mn, which exhibits yield strength of 189MPa, UTS of 350MPa, and elongation of 6.5%. Investigations have also revealed that the Al-8Mg2Si-6Mg alloy exhibits a relatively high tolerance to the Fe impurity because of the insignificant reduction of ductility of the alloy. The elongation is still at a level of 5% when Fe is at 1.6wt.% in the alloy. Furthermore, Cu and Zn can further enhance the mechanical properties of the Al-8Mg2Si-6Mg-0.6Mn alloy. Cu contents between 0.31wt.% and 0.92wt.% in the Al-8Mg2Si-6Mg-0.6Mn alloy can increase the yield strength from 193MPa to 207MPa, but decrease the UTS from 343MPa to 311MPa, and the elongation from 4.8% to 3.8% under as-cast condition. This can be attributed to the formation of hot tearing defects in castings. Therefore, the Cu content in the alloy should be limited to a low level. On the other hand, zinc can be controlled to a level of 4.3wt.%, which will dramatically increase the tensile strength of the alloy. More importantly, Zn can significantly increase the mechanical properties of the alloy after a quick T6 heat treatment under a condition of solution at 490oC for 15 mins and ageing at 180oC for 90 mins, at which the yield strength is 345MPa, UTS is 425MPa, and elongation is 3.2 %. In the present study, the solidification and microstructural evolution, the relationship between the microstructure and mechanical properties, and the strengthening mechanisms in the developed alloy are discussed on the basis of the experimental results. A two stage solidification has been recognised to be responsible for the microstructure formation in the HPDC process. The primary α-Al phase is formed as prior phase for the hypo-eutectic alloy and the primary Mg2Si phase is formed as prior phase for the hyper-eutectic alloy. The solute elements including Mg, Mn, Fe, Cu, and Zn can enhance the solution strengthening and/or the precipitation strengthening in the alloys, but alter the solidification ranges, which will affect the formation of defects in the castings. In the quick T6 heat treatment, the AlMgZn phase is dissolved into the Al phase during solution treatment and precipitated during ageing treatment. The quick heat treatment is also found to be able to spheroidise the Mg2Si phase. Only η′ MgZn phase is precipitated during aging in Zn containing alloys. The alloy with 4.3wt.% Zn provides the best combination of the mechanical properties because of the high density of MgZn precipitates in the α-Al phase.