| Summary: | 碩士 === 國立東華大學 === 材料科學與工程學系 === 105 === In this study, A357 and A201 aluminum alloys were used as the research materials, prepared by sand casting process with different intensity AC magnetic field. The effects of external magnetic field on the mechanical properties and grain refinement of A357 and A201 were investigated by microstructure observation and mechanical testing.
The minimum grain size of A357 and A201 can be obtained at 5.55 mT, and the grain refinement of A357 is more significant than A201, The reason is due to the different shape of primary crystals for both alloys. The results showed that the uneven distribution of the grain size of A357 and A201 alloys cast under magnetic field can be improved.
Rockwell hardness test results showed that the hardnesses of A201 and A357 alloys increased when increasing the external magnetic field intensity. According to the tensile test results, the ultimate tensile strength, yield strength and elongation were improved when the external magnetic field intensity was increased. The optimal mechanical properties appeared when cast at magnetic field intensity of 5.55 mT. A357 alloy cast at the higher magnetic field intensity, the lager shaking during cast could be more serious. Therefore, the holes were formed around the eutectic phases. The optimal mechanical properties appeared at magnetic field intensity of 1.45 mT. Because the grain size have been refined and therefore the numbers of holes in eutectic phases were less than the alloy cast under high magnetic intensity.
According to the XRD results, both A357 and A201 alloys displayed the higher preferred orientation of (111) α-Al phase along the direction of parallel magnetic field. The reason is proposed that the nucleation of grains were rotated by applying the magnetic field.
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