Research On Microstructure And Properties Of Low Temperature Aged Mg-6%al-4%Zn And Mg-6%Al-2%Cu Magnesium Alloy

The characteristics, application status, molding method of Mg alloys were summarized, and the research progress on Mg-Al-Zn series and Mg-Al-Cu magnesium alloys were introduced. By used optical micro- scopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM) and microhardness sclerometer, effects of different aging methods (100℃) on microstructure and property of Mg-6Al-4Zn and Mg-6Al-2Cu magnesium alloy were revealed.The microstructure of the as-cast Mg-6Al-4Zn was made of Mg matrix, intragranular precipitates(γ-Mgl₇Al_l2), equilibrium and non-equilibrium eutectic (α-Mg +γ-Mgl₇Al_l2 +Ï„-Mg₃₂(Al,Zn)₄₉). Compared with the microhardness of as-cast, the microhardness increased only by 13.7% when as-cast magnesium alloys were directly aged at 100℃. The reason is that equilibrium and non-equilibrium eutectic, mostly segregated in grain boundaries, consumed most of the alloying elements. So age-hardening by directly aging was not strong due to less intragranular precipitates. After alloy was solid solution treated at 380℃for 24h, most of non-equilibrium eutectic on grain boundary integrated into the matrix, resulting in a strong solid solution strengthening, so the microhardness increased by 7.0%. Compared as-cast, the microhardness of AZ64 magnesium alloy significantly increased by 65.7% after it was T6 heat-treated for 10 days. And the precipitates, most of which wasγ-Mg17Al12 phase, dispersed in the grain. During the heat- treatment, there were not transition phase. So,γ-Mg17Al12 phase andÏ„-Mg₃₂(Al,Zn)₄₉ phase were the main strengthen phase for the AZ64 magnesium alloy.The microstructure of the as-cast Mg-6Al-2Cu was made of Mg matrix, intragranular precipitates(γ-Mgl₇Al_l2), equilibrium and very little non-equilibrium eutectic (α-Mg +γ-Mgl₇Al_l2 + U-AlCuMg). Compared with the microhardness of as-cast, the microhardness increased only by 8.9% when as-cast magnesium alloys were direct aged at 100℃. The reason is that equilibrium and non-equilibrium eutectic, mostly segregated in grain boundaries, consumed most of the alloying elements. After alloy was solid solution treated at 420℃for 16h, most of non-equilibrium eutectic on grain boundary integrated into the matrix, but non-equilibrium eutectic were very little formed during the cooling process. So the microhardness increased by 12.8% but not best. Compared as-cast, the microhardness of AC62 magnesium alloy only increased by 13.8% after it was T6 heat-treated. Because of the low content of alloying elements and low aging temperature, significant strengthening effect was not found in this study. Most ofγ-Mgl₇Al_l2 phase and U-AlCuMg phase were on the boundaries of grain in the form of divorce eutectic. Meanwhile, a little ofγ-Mgl₇Al_l2 phase precipitated near grain boundary.Compared AZ64 magnesium alloys were T6 heat-treated at 200℃or 150℃, the peak of microhardness appear later and hardening time was longer when they were T6 heat- treated at 100℃. The microhardness was hignest when alloy was aged for 10 days and remain the peak to 60 days. The main precipitates isγ-Mg17Al12 andÏ„-Mg32(Al, Zn)49 instead ofγ-Mg17A112 andη-MgZn2.η-MgZn2 was not observed by TEM.Compared AC62 magnesium alloys were T6 heat-treated at 200℃, the peak of microhardness appear later when they were T6 heat- treated at 100℃. And the main precipitates also isγ-Mg17Al12 and U-AlCuMg.η-MgCu2 was not observed by TEM.