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Study On The Bi-Added Hypereutectic Al-Si Alloys

Posted on:2008-09-08Degree:MasterType:Thesis
Country:ChinaCandidate:N HanFull Text:PDF
GTID:2121360212493257Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
The Bi-added hypereutectic Al-Si alloys have a series of advantages such as low thermal expansion coefficient, high wear-resisting ability, excellent elongation ration and machinability, while maintaining mechanical properties that are similar to conventional A390 alloy, and thus can be applied to abrasion resistance-requiring applications, such as piston and swash plates of compressors for air conditioners in automobiles. However, nowadays there is little study on Bi-added Al-Si alloys in China. In the present paper, a Bi-added hypereutectic Al-Si alloy with excellent mechanical properties and thermal expansion property has been developed by optimizing of melting, casting, hot extrusion and heat treatment, etc. Its properties have been equal to the counterpart abroad.The microstructure of Bi-added hypereutectic Al-Si alloy has been investigated using HSVM, EPMA, and SEM, etc. It has been found that the element Bi distributes on the matrix in the form of pure Bi phase. In addition, the mechanical properties of the alloy were improved, due to the modification effect of Bi on eutectic silicon. The tensile strength was increased from 207Mpa to 223MPa, and the elongation rate was increased from 0.8% to 1.0%.Cooling rate is one of the most important factors that decide the microstructure of metals. In present work, the microstructures of Bi-added hypereutectic Al-Si alloys under different cooling rates have been investigated, and the results show that: with the increase of cooling rate, the sizes of primary and eutectic silicon decrease. Meanwhile, the cooling rate changes the distribution of Bi phase. In the microstructures of theΦ6mm andΦ12mm samples under a high cooling rate, most of the Bi particles are enwrapped by the primary silicon particles. While in theΦ150mm sample, due to the low growth velocity of primary silicon in the solidification process, the Bi droplet is gradually pushed to the solidification front of the primary silicon, and finally solidifies in the Al matrix.The cooling curves of Bi-added hypereutectic Al-Si alloys under different cooling rates have been studied by differential scanning calorimeter (DSC).It has been found that with the increase of cooling rate, the temperature at which the primary silicon begins to precipitate decreases. In theΦ150mm sample, the primary silicon begins to precipitate at 680.4℃, while in theΦ12mm andΦ6mm samples, the primary silicon begins to precipitate at 598.8℃and 585.5℃, respectively.The effect of hot extrusion on the microstructure and properties of hypereutectic Al-Si alloy has been studied. The results show that hot-extruded processing reduces the size and blunts the edges of primary silicon, and transforms the coarse flake-like silicon into a short rod-like shape, resulting in a noticeable improvement in strength, hardness and elongation of the alloy. Compared with the unextruded alloy, the tensile strength of the extruded alloy was increased from 240Mpa to 338Mpa, the elongation rate was increased from 0.7% to 2.8%, and the HBS hardness was increased from 87.6 to 131, subjected to the same T6 heat treatment.The thermal expansion coefficients of experimental alloys have been investigated using dilatometer. The results show that the addition of Bi decreases the thermal expansion coefficient, when the temperature is under the melt point of Bi, and the more the Bi is added, the lower the thermal expansion coefficient is. However, when the temperature is over the melt point of Bi, the addition of Bi increases the thermal expansion coefficient of Al-Si alloy, and the more the Bi is added, the higher the thermal expansion coefficient is. The thermal expansion property of the AlSiCuBi alloys with and without hot extrusion was measured, and it has been found that hot extrusion decreases the thermal expansion coefficient of the AlSiCuBi alloy.
Keywords/Search Tags:hypereutectic Al-Si, Bi, Cooling rate, Hot extrusion, Thermal expansion
PDF Full Text Request
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