Influences Of Zn And Sn Contents On Microstructures And Mechanical Properties Of Casting Magnesium Alloys Containing Sn

As the lightest metallic structural material, magnesium alloys are supposed to havegreat potential application in lightweight industries. But compared with traditional materials,steels and aluminum alloys, the research and development for magnesium alloys are still notsufficient. Mg–Al based alloys are the most widely used commercial alloys but haverelatively poor mechanical properties and creep resistance at elevated temperature. Whilebased on their own advantages, it is feasible to develop Mg–Sn based alloys with excellentproperties at high temperature. Consequently, domestic and overseas researchers, on the onehand, are trying to combine the superiorities of these two alloys in order to developMg–Al–Sn based alloys with excellent strength and toughness; on the other hand, they areattempting to develop favourable Mg–Sn based alloys without Al to partly replacecreep-resistant magnesium alloys containing Al. In these studies, we investigated theinfluence of Zn and Sn contents on microstructures and mechanical properties ofMg–6Al–xSn （x=0–3.5wt.%）, Mg–6Al–3Sn （AT63） and Mg–5Sn alloys, and introduced acentrifugal casting method into the improvement of microstructures and mechanicalproperties in magnesium alloys. Results of the present studies are as follows:（1） Influence of Sn contents on microstructures and mechanical properties ofMg–6Al–xSn （x=0–3.5wt.%） alloys. Besides refining primary α-Mg grains, the additions of1.5–3.5wt.%Sn to the Mg–6Al alloy can also decrease the solid solubility of Al in α-Mgmatrix, increase the fraction of eutectic phases Mg₂Sn and Mg₁₇Al₁₂which often coexistamong-Mg and reduce the onset temperature for solidification of primary-Mg. At roomtemperature, with Sn content increasing from0to3.5wt.%, tensile strength σ_band0.2%offset yield strength σ_0.2evidently increase from244.2to263.8MPa and93.7to108.5MParespectively, while elongation-to-failuref（plastic elongationp） obviously drops from39.1%（28.6%） to25.2%（20.0%）. However, at200oC, with the addition of3.5wt.%Sn,σ_band σ_0.2obviously increase from103.9to123.8MPa and70.9to92.8MPa respectively, butf（p） slightly decreases from51.8%（49.8%） to49.8%（46.6%）. Moreover, theadditions of Sn can weaken the adverse effect of high temperature on yield strength, andobviously increase work-hardening rates at room temperature but decrease those at200oC.（2） Effects of minor Zn additions on microstructures and mechanical properties of aMg–6Al–3Sn alloy. The additions of0.1–0.7wt.%Zn to the AT63alloy can induce therefinement of primary-Mg grains and the homogeneous distribution and refinement fordivorced eutectic phases of Mg₂Sn and Mg₁₇Al₁₂which frequently coexist at grainboundaries. With Zn content increasing from0to0.7wt.%, σ_bandf（p） evidently increasefrom251.0to282.9MPa and20.2%（12.9%） to31.7%（21.4%） respectively, while σ_0.2steeply ascends from105.7MPa without Zn content to134.3MPa with0.3wt.%Zn but thenbasically remains stable with high Zn （0.5and0.7wt.%） contents. Simultaneously improvedstrength and ductility for Zn-containing AT63alloys are attributed to the grain refinementand the homogeneous distribution and refinement of these two eutectic phases.（3） Improved microstructure and mechanical properties of Mg–5Sn alloy produced by acentrifugal casting method. The microstructure primarily consists of dendrite crystals in thegravity casting alloy, while equiaxed grains play a predominant role in the microstructure ofthe centrifugal casting alloy. The values of σ_b, σ_0.2and σ_fin the gravity casting alloy stand at181.6MPa,38.7MPa and14.9%, resepectively. However, σ_b, σ_0.2and σ_fmarkedly increaseto257.5MPa,52.3MPa and21.2%in the gravity casting alloy on position1（r1=0.05m）and to255.8MPa,54.7MPa,17.6%on position2（r2=0.10m）. Besides, the centrifugalcasting method can significantly increase the work-hardening ability of the alloy. The crystaltransformation from dendrite crystals to equiaxed grains results in simultaneously improvedstrength, ductility and work-hardening ability.（4） Double-peak aging behavior of Mg–5Sn–xZn （x=0–1.0wt.%） alloys. Mg–5Sn–xZnalloys exhibit double-peak aging behavior. The additions of0.5–1.0wt.%Zn to Mg–5Snalloy can gradully shorten the hours to the aging peaks and improve the microhardness inaging peaks. The frst aging peak in Mg–5Sn alloy is attributed to the refined and distributedprecipitates, while the second aging peak arises from the formation of new precipitates. Precipitates lying on non-basal planes can evidently increase the yield strength but decreasethe elongation of the alloys.