| Magnesium alloys have specialities of low density, high specific rigidity, high specific strength and good electromagnetic shielding effectiveness. They are used in the fields of automobile, electronics, aeronautics and astronautics industries. However, the application of magnesium alloys is limited by their low strength, poor plasticity and bad corrosion resistance. Grain refinement is an important method to improve the strength, plasticity and corrosion resistance of magnesium alloys through adding master alloy refiners. Based on the previous research, it has a good grain refinement effect on Mg-Al based alloys by applying Al-C, Al-C-Si or Al-C-B master alloys. Therefore, new Al-C-based master alloys have been developed and analyzed by controlling the composition and process, which have large numbers of small heterogeneous nucleation particles. Al-C-based master alloys can decrease the grain size and improve the mechanical properties of Mg-Al based alloys.The major research results are as follows:(1) Microstructure of Al-C master alloy and its grain refinement of AZ63 alloyIn this paper, A1-5C master alloys have been prepared at different temperatures. It is found that Al4C3 is the main phase in A1-5C master alloys. With the temperature increasing, the x-ray diffraction peak corresponding to Al4C3 be higher, the number be more. The size of Al4C3 is 2-4μm at 750℃ and grows up to 5-10μm at 850℃. When held for 0.25h,0.5h and 1h, the grain size of AZ63 alloy can be decreased from approximately 410μm to 200μm,170μm and 180μm with addition of 1.0wt.% Al-5C master alloys at 730℃.(2) Microstructure of Al-C-Si master alloy and its grain refinement of AZ63 alloyAl-5SiC master alloy grain refiners have been prepared through melt reaction process in this paper. X-ray diffraction spectrum shows that the Al-5SiC master alloys are composed of primary Al phase, SiC, Al4C3 and Si. Most of Al4C3 phases with block shape or hexagonal plate shape distribute around SiC particles, which are dispersed homogeneously in the matrix alloy. With 0.5wt.% Al-5SiC master alloy addition at 730℃, the grain size of AZ63 alloy can be decreased from approximately 400μm to 120μm, and holding for 0.25h or 0.5h, there is no difference for the grain refinement. The coexistence of Al4C3 and SiC in the Al-C-Si master alloy can exhibit much better grain refinement effect on AZ63 alloy.(3) Microstructure of Al-C-B master alloy and its grain refinement of AZ63 alloyAl-C-B master alloy grain refiner was prepared for the grain refinement of Mg-Al based alloys through melt reaction process in this paper, which is composed of a-Al, Al3BC and AlB2. With temperature increasing, part of AlB2 particles will transform to AlB12 phases, and the rest of AlB2 and Al3BC particles will become bigger than that of low temperature. For A1-0.4C-1.6B master alloy, AlB2 particles whose size is 2-15μm distribute homogeneously in the matrix, and there are many Al3BC particles whose size is about 1μm distribute around the AlB2 phases. With addition of 0.5wt.%,1.0 wt.% and 2.0 wt.% A1-0.4C-1.6B master alloy at 730℃, the grain size of AZ63 alloy can be decreased from approximately 350μm to 230μm, 110μm and 60μm. What is more, the hardness (HB) of AZ63 alloy as-cast can be enhanced from 56.5HBW to 65.2HBW, and the hardness (HB) of AZ63 alloy can be enhanced from 62.0HBW to 71.7HBW after T6 heat treatment, which is increased by 15.6%. And the tensile strength of AZ63 alloy also be increased through the grain refinement.(4) Comparison of grain refinement effect of Al-C-based master alloys in AZ63 alloyCompared with the grain refinement effect of the three master alloys in AZ63 alloy, it can be found that the grain refinement of Al-C or Al-C-Si master alloy has better effect of recession than Al-C-B master alloy. This is because Al4C3 is more stable than Al3BC in magnesium melt. Considering the addition amount of master alloys and holding time, Al-C-Si master alloy will be a grain refiner with potential industrial application for Mg-Al based alloy. |
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