Solidification Condition And Alloying On Microstructure And Mechanical Properties Of Mg-Zn-Sn Alloys

Magnesium alloys, with a number of desirable features including low density,the highest strength to weight ratio, the highest specific toughness, better dampingcharacteristics and shielding properties, are thus very attractive for applications inthe automotive, electronic and aeronautical industries. However, conventionalmagnesium alloys exhibit the relatively low strength, poor creep resistance andinferior plasticity and can not be used for the manufacture of key structuralcomponents. Therefore, it is of great significance to develop novel magnesium alloyswith high strength and good toughness. In the recent years, more and more attentionhas been paid to the Mg-Zn-Sn system for the development of low cast highperformance magnesium alloys. Effects of cooling rate and pressure onmicrostructure and mechanical properties of the as-cast Mg-6Zn-3Sn-2Al-0.2Caalloy are systematically investigated and the optimal solidification condition isdetermined. On the basis of this, effects of the mass ratio of Zn to Sn, the Caaddition, the minor Ti addition and the combined Ti and B addition onmicrostructure, mechanical properties and creep resistance of the as-castMg-Zn-Sn-Al based alloys are systematically investigated and the actionmechanisms of different alloying are explored. The aim is to improve thecombination of mechanical properties and creep resistance of the Mg-Zn-Sn alloysystem by the combination of increasing cooling rate, the exertion of pressure andalloying and thus to offer a guidance for the development of low-costhigh-performance magnesium alloys. The research results are given below:（1）Effects of solidification conditions including steel mould with no appliedpressure, the water-cooling steel mould with no applied pressure, the water-coolingcopper mould with no applied pressure, the water-cooling steel mould casting underpressure and the water-cooling copper mould casting under pressure on themicrostructure and mechanical properties of the as-cast Mg-6Zn-3Sn-2Al-0.2Caalloy are systematically studied. The cooling rate has an obvious influence on thephase constitutions of the as-studied alloy. The as-cast alloy prepared by steel mouldwith no applied pressure is mainly composed of four phases i.e.-Mg, Mg₂Sn,Mg₃₂（Al, Zn）₄₉and MgZn, while those prepared by the water-cooling steel mouldcasting under pressure and the water-cooling copper mould casting under pressuremainly consist of-Mg, Mg₂Sn, Mg₃₂（Al, Zn）₄₉and Mg51Zn20. The volume fraction of the intermetallics in the as-cast alloy increases with a higher cooling rate, whilethe solid solubility of alloying elements in the-Mg matrix decreases. The volumefraction of intermetallics in the as-cast alloy prepared by the water-cooling steelmould casting under pressure is69%higher than that prepared by the steel mouldwith no applied pressure. The as-cast alloy prepared by the water-cooling coppermould casting under pressure exhibits the finest microstructure and the bestcomprehensive mechanical properties. In addition, the solidification condition has aninfluence on the ambient tensile fracture mode. The mixed mode of cleavage fractureand void coalescence is dominant for the alloy prepared by the steel mould with noapplied pressure, while the ductile fracture characteristics of void coalescence isdominant for the alloy prepared by the water-cooling copper mould casting underpressure.（2） Effects of the Zn/Sn mass ratio （0.5,1and2） on microstructure andmechanical properties of the as-cast Mg-xZn-ySn-2Al-0.2Ca （x+y=9wt%） alloys areare systematically studied. The Zn/Sn mass ratio has no obvious effect on the phaseconstitutions of the as-cast alloy, but has an obvious influence on the morphologyand the volume fraction of the intermetallics in the alloys. With a higher Zn/Sn ratio,the volume fraction of the intermetallics in the as-cast alloy decreases. The amountof the Mg2Sn phase increases, but that of the Zn-containing intermetallics decreases.The alloy with the Zn/Sn mass ratio of1exhibits the highest tensile strength andyield strength at ambient temperature, while the alloy with the Zn/Sn ratio of0.5posesses the highest strength at200℃. The change of the Zn/Sn mass ratio leads tothe variation of the ambient tensile fracture mode. The ductile fracture characteristicof void coalescence is dominant for the alloy with the Zn/Sn ratio of2, while themixed fracture is dominant for the alloy with the Zn/Sn ratio of1. In addition, theductile fracture is dominant for both alloys tested at elevated temperature. Therefore,the Mg-4.5Zn-4.5Sn-2Al-0.2Ca alloy is the better candidate for further alloying.（3） Effects of the Ca addition （0.2%,0.4%and0.6%） on microstructure andmechanical properties of the as-cast Mg-4.5Zn-4.5Sn-2Al-0.2Ca alloys aresystematically studied. The Ca addition can obviously refine the microstructure,improve the RT and elevated temperature strength and enhance the compressivecreep resistance of the as-cast alloy. The ternary CaMgSn phase can be formed in thealloys with the Ca addition higher than0.4%. The alloy with0.2%Ca has the highestRT strength, while the alloy with0.4%Ca exhibits the highest tensile and yieldstrength at200℃. The initial strain and the steady creep rate （200℃/55MPa） of these alloys are inversely proportional to the Ca content from0to0.4%. The initialstrain and the steady creep rate （200℃/55MPa） of the alloy with0.4%Ca are77%and71%higher than those of the Ca-free alloy respectively. The Ca addition also has aninfluence on the tensile fracture mode. The transition from cleavage fracture toquasi-cleavage fracture at ambient temperature is observed with a higher Caaddition, while the transition from ductile fracture to quasi-cleavage fracture isdetected at elevated temperature.（4） Effects of the Ti addition and the combined Ti and B addition onmicrostructure, mechanical properties and creep resistance of the as-castMg-4.5Zn-4.5Sn-2Al-0.2Ca alloys are studied. The0.1%Ti addition and thecombined0.1%Ti and0.02%B addition refine the microstructure and the secondphases of the as-cast alloy, and promote the formation and the segregation of thegrain boundary compounds. Trace Ti addition improves the RT yield strength andultimate tensile strength at200℃of the as-cast alloy, but reduces the elongation atRT. The combined Ti and B addition brings about the improvement of theelevated-temperature tensile strength, which is30%higher than the matrix alloy.The initial strain and the steady creep rate （200℃/55MPa） of the alloy with0.1%Tiare lower than those of the matrix alloy respectively. The steady creep rate （200℃/55MPa） of the alloy with the combined0.1%Ti and0.02%B addition is also lowerthan that of the matrix alloy, while the initial strain is higher.