The development of high pressure die-cast magnesium alloys for the applications at high temperatures

Abstract

The applications of magnesium (Mg) alloys have been growing significantly in the past 30 years, with which high pressure die cast (HPDC) Mg alloys account for a large proportion of industrial applications. One of the attractive areas is the application of Mg alloys at elevated temperatures because Mg alloys usually show poor mechanical properties in the operating environment with high temperatures. In particular, the high creep rate and poor thermal conductivity are recognised as the main factors to limit the applications of Mg alloys at high temperatures (200-300℃). Therefore, the development of Mg alloys with excellent creep resistance and thermal conductivity is attractive for both academic and industrial applications as a thorough understanding of fundamentals can bring a step change in industrial applications of new Mg alloys. In this study, through alloying and heat treatment, heat-resistant Mg-RE-Al alloys and high thermal conductivity Mg-Al-RE alloys are developed for the applications for the engine parts of handheld powered tools working at 200 ℃ to 300 ℃. The alloys were comprehensively studied using XRD, SEM and TEM for the detailed microstructure. The mechanical properties at room temperature and at elevated temperatures were obtained by tensile, hardness and creep tests. The relationship between the alloying elements, casting process, microstructure characteristics and mechanical properties are explored and set up for insight into strengthening mechanisms in the newly developed alloys. The effects of element Nd, Gd and Y contents on the microstructure and mechanical properties of the HPDC Mg-2.6RE alloy were first investigated. The major intermetallic phases in the alloys with Nd, Gd, and Y additions were identified as continuous networked Mg12RE phases, and multiple Mg-RE phases were found at the grain boundaries when elements were added at higher levels. It is also found that Nd has the strongest ability to refine the grain size compared to Gd and Y in the die cast alloys. The order of the ability to increase the yield strength of the alloys at room and high temperatures is Y>Gd>Nd. Importantly, the addition of Y can simultaneously increase the yield strength and elongation of the alloys. Second-phase strengthening was found as the main mechanism in the alloys containing Nd, Gd, and Y elements. In addition, the microstructure, and mechanical properties of the HPDC Mg-3.5RE-1.5Gd alloy were investigated with Al addition. The main intermetallic phase in the alloy was the reticulated Mg12RE phase, and the Al2RE3 phase formed on the grain boundaries at 0.5 wt.% Al addition and the lamellar Al11RE3 and massive Al2RE phases at 1 wt.% Al addition. The reduced strengthening effect of the second phase produced by the Al addition is responsible for the reduction of yield strength of the alloy at room and high temperatures, while the Al addition significantly increased the elongation of the alloy. A new HPDCMg-3.5RE-1.5Gd-Al alloy has been developed with optimised content of Nd, Gd, Y and Al. The intermetallic phases in the alloy mainly consisted of the continuous networked Mg12RE phase and the petal-like Al2RE3 phase. The yield strength at room and high temperatures of the new alloy was significantly better than that of commercial AE44 and HP2+ alloys. The creep properties of the new Mg-3.5RE-1.5Gd-Al alloy were investigated at 300 ℃. It is shown that, compared to the as-cast state, the Al2RE phase was generated within the grain of the alloy after creep, while there was no significant change in the morphology of the main intermetallic phase Mg12RE and the petal-like Al2RE3 phase at the grain boundaries. The main creep mechanisms of the alloy are GBS and GBD at 200-250 ℃ 50-80 MPa, and SRO/clusters drag dislocation gliding associated with GBD at 300 ℃ and 50-80 MPa. The improvement of the microstructure and mechanical properties of the new HPDC Mg-3.5RE-1.5Gd-Al alloy by T5, T4 and T6 was further investigated. The yield strength of the alloy at room temperature and the high temperature was improved under artificially aged conditions. The precipitates of the Al2RE phase are formed within grains. The second phase strengthening is the main contribution to the enhancement of the mechanical properties of the alloy. The discrete Mg12RE phase and the blocky Al2RE phase are mainly present at the grain boundaries of the alloy after solid solution treatment, while the striped Mg41RE5 phase is formed at the grain boundaries after ageing. The yield strength of the T6 alloy is increased in comparison with the alloy under the T4 condition but is much lower than that of the alloys under as-cast and T5 conditions, mainly due to the influence of the second phase strengthening. The thermal conductivity of HPDC Mg2.8Al3.8La0.4Nd alloy was studied for the effect of alloying elements through CALPHAD calculations and experimental verification. The optimised Mg2.8Al3.8La0.4Nd alloy can substantially increase the yield strength at room and high temperatures and show better thermal conductivity compared to the commercial AE44 alloy. The alloy contains lamellar (Al,Mg)3RE and Al11RE3 phases at the grain boundaries, the needle-like Al2.12RE0.88 phase and the blocky Al10RE2Mn7 phase as intermetallic phases. The optimised alloying resulting in lower solid solution content of Al and RE compared to the AE44 alloy is the main reason for the high thermal conductivity of the newly developed HPDC Mg2.8Al3.8La0.4Nd alloy. The main contributions of the present study include: (1) The influence of various rare earth elements and Al elements on the microstructure and mechanical properties in the room and high temperature of the alloying of HPDC magnesium alloys has been systematically investigated. (2) Based on the alloying means, a highly creep-resistant HPDC Mg-RE-Al alloy that can be applied in the working environment of 200 ℃-300 ℃ has been creatively developed, and the creep mechanism has been analysed. (3) The high thermal conductivity HPDC Mg-Al-RE alloy was developed based on the quantitative strategy to provide new ideas for developing thermally conductive alloys. These are studied for the first time over the world. The findings of the development study of creep-resistant HPDC Mg-RE-Al alloys and quantitative development strategy for high thermal conductivity HPDC Mg-Al-RE alloys open a window for the Mg alloys working at high temperatures.