Effects Of Addition Element On Microstructure And Mechanical Properties Of As-Cast AZ91and Mg-5%Sn Alloy

Magnesium alloys are becoming ever more prevalent in automotive and aerospaceindustries as energy conservation and performance demands increase. However, it is limitedon further application in the automotive field for its low strength compared with steel andaluminum alloys. For most cast light alloys, the addition of a grain refiner to the molten metalprior to casting is one of the most important ways to reduce the final grain size of commercialcastings.Effects of AlN addition on the microstructure and mechanical properties of as-cast AZ91magnesium alloy were investigated using scanning electron microscope (SEM), X-raydiffraction (XRD) and tensile testing. The results showed that the phases of as-cast alloy werecomposed of α-Mg, β-Mg17Al12. The addition of AlN slowed down the aging kinetics of AZ91alloy and suppressed the precipitation of the β-phase. And, with the increase of AlN content,the microstructure of β-phase was changed from the reticulum to fine grains and thesuppression of the aging processes of the AZ91alloys became more apparent. When AlNcontent was up to0.48%in the alloy, the β-phase became most uniform distribution. Afteradding0.3%AlN to AZ91alloy, the average alloy grain size reduced from100μm to50μm,the tensile strength of alloy was the highest. The average tensile strength increased from168MPa to193MPa. However, when more amount of AlN was added, the average alloy grainsize did not reduce sequentially and increases to70μm by adding0.6%AlN and the β-phasebecame a little more.Effects of Si addition on the microstructure and mechanical properties of as-castMg-5%Sn magnesium alloy were also investigated using SEM, XRD and tensile testing. Theresults showed that the phases of as-cast alloy were composed of α-Mg, Mg2Sn; Mg2Si. Theaddition of Si produced Chinese character shape phases of Mg2Si. And, with the increase of Sicontent, Clarify the effect becomes more apparent. When Si content was up to1.25%in thealloy, the average alloy grain size reduced from90μm to40μm, the tensile strength of alloywas the highest. The average tensile strength increased from108MPa to156MPa. When theSi added at1.5%or less, the hardness strength of the alloy was improved with the increase ofSi. The hardness strength of Mg-5%Sn alloy was94N/mm2. When1.25%Si was added, the hardness increased strength to123N/mm2, adding30.85%. Then, grain became furtherrefinement while Si content continued to increase, and it began to become thick. Butmechanical properties decreased. The Elongation was decreased by degrees with increasing Siaddition.Effects of Al-3.5%P intermediate alloy addition on the microstructure and mechanicalproperties of as-cast Mg-5%Sn-1.25%Si magnesium alloy were also investigated using SEM,XRD and tensile testing. P was added to Mg-5%Sn-1.25%Si alloy in the form of Al-3.5%Pintermediate alloy. The results showed that the phases of as-cast alloy were composed of α-Mg,Mg2Sn, Mg2Siand little P and AlP. The Chinese character shape phases of Mg2Si becamegrainy after the Al-3.5%P intermediate alloy was added. The P consisted in the grainy Mg2Siphase by the solid solution forms. AlP was a heterogeneous nucleation of Mg2Si. These werethe cause that the Chinese character shape of Mg2Si phases became grainy after the Al-3.5%Pintermediate alloy was added, and the grainy Mg2Si phases became smaller and smaller withmore and more Al-3.5%P intermediate alloy adding. And, with the increase of Al-3.5%Pcontent, the average tensile strength increased from156MPa to191MPa. When Al-3.5%Pcontent was up to0.225%in the alloy, the mechanical properties of as-cast Mg-5%Sn-1.25%Simagnesium alloy was the best. The grainy Mg2Si phases had segregation phenomenon Occursby the addition of0.3%Al-3.5%P, and the average tensile strength was reduced. The hardnessof the alloy generally showed a slightly downward trend. But the elongation was just increase.