| Magnesium alloy as a kind of nonferrous metal was used in aeronautics, astronautics and automobile industry extensively. It is even lighter than aluminum alloy. It has the advantages of high special strength, high special rigidity, high damping property and electromagnetism shielding. Magnesium alloy is increasingly used in automobile and electric industry as a lightweight material in recent years. However, conventional magnesium alloy will be oxidized severely during melting and pouring process because of high activity of magnesium element. Fluxes and protective gases are usually used as burning prohibitors in commercial manufacture. Beryllium element is also used sometimes to slow down the oxidation reaction of magnesium and air. However, both flux and protective gas would lead to some problems such as inclusion, environmental pollution and complication of melting process and equipment. Beryllium has tremendous toxicity and it can't restrain oxidation reaction absolutely.In the present dissertation, calcium was selected as alloying element and its effect on ignition-proof and tensile properties was investigated. AZ91 alloy was selected as master alloy since it is the most widely used magnesium alloy at the present time. Yttrium was also added into AZ91 alloy to optimize the microstructure of oxidation film and matrix of it. The research aspects and results of the paper are as follows:(1) Thermal analysis method was used to study the effect of calcium on ignition-proof property of AZ91 alloy. The ignition-proof of AZ91 was increased significantly after calcium was added. As the content of calcium was increased, the ignition point of AZ91 was raised. By the determination of ignition point, the relationship between the addition amount of calcium and ignition point of the alloy was obtained as following:T=179x+524Since yttrium is prone to reaction with aluminum to form Al2Y, so single-element yttrium is limited in Mg-Al alloy (<0.25wt%), the ignition point of AZ91 will onlyincrease 10~30℃ after yttrium is added. However, the content of calcium will decreased from 1.26wt% to 1.18wt% when the ignition point of AZ91 reaches 750℃ (melting temperature) after a little amount of yttrium is added.(2) X-ray diffraction (XRD), scanning electron microscope (SEM&EDS) and thermalgravitational analysis (TGA) was used to research the morphology and structure of oxide film, the distribution of ignition-proof element and oxidation kinetics at elevated temperature. The oxidation film of AZ91 composed of MgO, CaO, Al2Ca, MgAl2O4 and Mg3N2 was formed with the calcium addition. The density of it was enhanced. But it was thick and brittle and was prone to crack in the process of elevated temperature distortion. With calcium addition, the density of oxide film formed on AZ91 would be increased because of the formation of Y2O3 after yttrium was added. The oxide film was thin and the plasticity was higher than that of alloy with single calcium addition.(3) Optical microscope, XRD, SEM and tensile property test were employed to study the effect of calcium and yttrium on the structure and the tensile property of AZ91 alloy. The room-temperature tensile strength of AZ91 alloy would be enhanced due to the reduction of β phase and grain size in AZ91 alloy if the addition amount of calcium was 1.0~1.5wt%. With a little amount of yttrium addition, the tensile strength would further enhanced because of the disperse distribution of Al2Y formed in grain and grain boundary by the reaction of Al and Y. However, the tensile strength of AZ91 would reduce when the size of Al2Y phase was increased and the matrix was disserved along with the increment of yttrium content. The appropriate amount of yttrium addition is 0.11 wt% in this investigation.(4) According to above studies, the composition of ignition-proof magnesium alloy with excellent ignition-proof property and tensile strength was AZ91-1.18wt%Ca-0.11wt%Y. |
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