| Al-Zn-Mg-Cu alloys belong to ultra-high-strength aluminum alloys,which are usually prepared by deformation method to obtain high strength.Compared with direct casting,this preparation method is obviously more expensive.However,when Al-Zn-Mg-Cu alloys are prepared by traditional casting methods such as gravity casting and pressure casting,casting defects such as shrinkage porosity,shrinkage cavities and hot tears are prone to occur,which will deteriorate the mechanical properties of the alloys.The semisolid rheo-diecasting has many advantages,such as good fluidity and less solidification shrinkage of semi-solid metal,which can reduce casting defects.Therefore,the semisolid rheo-diecasting makes it possible to directly cast Al-Zn-Mg-Cu alloy,and the casting can be directly subjected to subsequent heat treatment.Based on the Al-6Zn-2Mg-2Cu alloy,the mechanical properties of the alloy were improved by microalloying Ti,Sc and Sc+Zr and optimizing the content of the major elements Cu and Zn in this work.The effects of Ti,Sc and Sc+Zr microalloying and the content of major elements Cu and Zn on the microstructure and tensile mechanical properties of the alloy were studied.The effects of Ti,Sc and Sc+Zr microalloying on the microstructure and mechanical properties of semisolid rheo-diecasting Al-6Zn-2Mg-2Cu alloy were investigated.The results show that Ti,Sc and Sc+Zr microalloying can refine grains,improve shape factor and inhibit the growth of α-Al grains during solution treatment.Due to the slow cooling rate of the melt during the preparation of the semi-solid slurry,the primary Al3Sc phase,primary Al3（Sc,Zr）phase and Al3Zr phase are coarse,reducing the alloy elongation,from 10.6%of Al-6Zn-2Mg-2Cu alloy to about 7%.However,the grains of the Ti microalloyed alloys were refined and no coarse primary phase was observed.After solution and aging heat treatment,the yield strength,tensile strength and elongation reached 463 MPa,553 MPa and 13.4%.Based on Al-6Zn-2Mg-2Cu-0.1Ti alloy,the effect of Cu content on the microstructure and mechanical properties of semisolid rheo-diecasting Al-6Zn-2Mg-xCu-0.1Ti alloy was studied.It is found that the Cu content has little effect on the grain size and shape factor ofα-Al.With the increase of Cu content,the volume fraction of non-equilibrium eutectic phases increase from 3%of 0Cu alloy to 6.5%of 2Cu alloy.The as-cast non-equilibrium phases of 0Cu and 0.5Cu alloys with low Cu content consist of Mg（Al,Cu,Zn）2 and Mg32（AlZn）49,and as the Cu content increases,the non-equilibrium phase only contains Mg（Al,Cu,Zn）2.When the Cu content reaches 2Cu,the non-equilibrium phase forms a small amount of Al2CuMg phase.When the Cu content is less than 2 wt.%,most of the non-equilibrium eutectic phase Mg（Al,Cu,Zn）2 dissolves into the α-Al matrix after solution treatment,but the Cu content reaches 2 wt.%,more Al2CuMg did not dissolves.As the Cu content increases,the yield strength of the alloy in the aging state（470℃ for 8 h+120℃ for 24 h）increases gradually,from 405±10 MPa for 0Cu alloy to 459±2 MPa for 2Cu alloy.The presence of the residual phase Al2CuMg in the 2Cu alloy leads to the elongation decreases sharply,from 18.3±1%of the 1.5Cu alloy to 6.5±2%of the 2Cu alloy.Based on Al-6Zn-2Mg-1.5Cu-0.1Ti alloy,the effect of Zn content on the microstructure and mechanical properties of semisolid rheo-diecasting Al-xZn-2Mg-1.5Cu-0.1Ti alloy was studied.The results show that the Zn content has little effect on the grain size and shape factor of α-Al.The non-equilibrium eutectic phase in the as-cast microstructure of alloys with different Zn contents is mainly Mg（Al,Cu,Zn）2.With the increase of Zn content,the volume fraction of non-equilibrium phase increases from 4.2%of 6Zn alloy to 7.4%of 12Zn alloy.With the increase of Zn content,the yield strength of the alloy in the aging state（470℃ for 8 h+120℃ for 24 h）increases from 420±9 MPa for 6Zn alloy to 529±5 MPa for 12Zn alloy,while the elongation decreases sharply,from 18±1%of the 6Zn alloy to 2±1%of the 12Zn alloy. |
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