| A-Zn-Mg-Cu series alloys are highly-important light structural materials in the aerospace industry due to its high specific strength, good toughness, high fatigue properties and excellent corrosion resistance. In recent thirty years, many material scientists from different countries have developed a series of high strength aluminium alloy by constantly adjusting and optimizing the main alloy elements content, changing the kinds and content of micro-alloying elements, and lowering content of impurity elements, such as 7050 s 7150s 7055、7449、7136、7056, ect.. Among them, the 7056 aluminum alloy that was developed by Canada in 2004 is a very promising alloy. Compared with the 7055 aluminum alloy, alloy 7055 significantly increased the content of zinc element, appropriately reduced the content of Mg and Cu element; Compared with 7449 alloy, the fracture toughness of 7056-T7951 alloy increased by 40～50%; Compared with the 7150 aluminum alloy, it has higher corrosion resistance. In this paper, we further adjusted the ratio of the main alloying elements content on the basis of 7056 alloy to get a new alloy named A|, then by increasing the amount of Cu. Mg and Zn elements respectively we get another three alloys, namely, A1, A2, A3 In this paper, the as-cast and homogenization microstructure and the effects of alloying element content on the as-cast and homogenization microstructure have been studied by OM, SEM, XRD, DSC combined with the Pandat thermodynamic calculation, the main study results are as follows:Pandat thermodynamic calculation results show that there mainly exist a(Al) and MgZn2 phasein the four alloys with different compositions. Meanwhile, there are also a little Cu-riched phases found in them. The content of MgZn2 phase in the high-containing Zn 7000 aluminum alloy was mainly affected by content of Zn and Mg element (especially the Mg). Cu didn’t influence the content of MgZn2 phase significantly, but it will delay the precipitation process of MgZn2 phase in the solidification; Non-equilibrium solidification simulation gives initial precipitation temperature of low melting eutectic phase:475.0℃ for alloy Al-9.2Zn-2.0Mg-1.8Cu, for alloy 473.8℃(Al-9.2Zn-2.0Mg-2.1Cu), for alloy 476.7℃(Al-9.2Zn-2.5Mg-l.8Cu), and for alloy 474.7℃ (Al-9.8Zn-2.0Mg-1.8Cu).The as-cast microstuctures of actual alloys are typically exquiaxial dendritic grain which were mainly composed of a (Al) solid solution and bulk of non-equilibrium eutectic phase, and all alloys had very serious dendritic segregation; EDS results analysis showed that the second phases in the as-cast alloy mainly includes the non-equilibrium eutectic quarternary phase (AlZnMgCu), amounts of short rodlike MgZn2 phase and a little few Cu-riched phase;the number and thickness of non-equilibrium eutectic phase had been calculated statistically, it found that improving the content of Zn and Mg element would lead to augmentation of the number and thickness of non-equilibrium eutectic phase.After single stage homogenization heat treatment at 470℃/18h,470℃/12h,470℃ /48h for alloy Al-9.2Zn-2.0Mg-1.8Cu, Al-9.2Zn-2.0Mg-2.1Cu, and A1-9.8Zn-2.0Mg-1.8Cu respectively, non-equilibrium eutectic phase had dissolved into matrix thoroughly and only a small amount of residual Al7Cu2Fe phase exsisted,while alloy Al-9.2Zn-2.5Mg-1.8Cu can just obtain relatively good homogenization result with a small amount of (AlZnMgCu) quaternary phase remaining after homogenization with 470℃/72h; Alloy A1-9.2Zn-2.0Mg-1.8Cu, Al-9.2Zn-2.0Mg-2.1Cu, Al-9.2Zn-2.5Mg-1.8Cu, and A1-9.8Zn-2.0Mg-1.8Cu choosed suitable two-stage homogenization system of 440℃/12h+ 470℃/18h,440℃/12h+470℃/12h,440℃/12h+472℃/72h,440℃/12h+472℃/72h, respectively. And also, there were still a small amount of (AlZnMgCu) quaternary phase remaining in alloy Al-9.2Zn-2.5Mg-1.8Cu after two-stage homogenization with 440℃ /12h+472℃/72.By exploring the single-stage and two-stage homogenization system of these alloys, we found that improving the content of Zn and Mg in the high zinc-containing 7000 series aluminum alloys will greatly increase the homogenization difficulty. |
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