Effect Of Modification On Primary Silicon In Hypereutectic Al-Si Alloys And Modified Mechanism

Recently, there is growing interest in hypereutectic Al-Si alloys because of their interesting properties such as high wear resistance, low thermal-expansion coefficient, good corrosion resistance and castability. Under normal cast condition, primary silicon crystals in unmodified hypereutectic Al-Si alloys exhibit a variety of shapes such as polygonal, star-like (fivefolded), coarse platelet, etc. Generally, the machinability of hypereutectic Al-Si alloys is worse due to the presence of coarse primary silicon. On the other hand, the presence of the sharp corners and ledges of coarse primary silicon aggravate the localized stress concentration at the sharp corners and ledges, which promotes cracks to be initiated at the sharp corners and ledges and propagate along the silicon phase/ the matrix interface. Therefore, it is believed that coarse primary silicon in hypereutectic Al-Si alloys usually result in the inferior strength and surface roughness of machine parts. In addition, when the materials of hypereutectic Al-Si alloys was processed by the high speed cutting tool, the coarse primary silicon in hypereutectic Al-Si alloys will destroy the cutting tools and degrade the life of cutting tools. Therefore, the research of the effect of modification on hypereutectic Al-Si alloys is very important.In the present study, the effect of modification on the microstructure and mechanical properties is researched. The major research efforts of the present study are as follows:(1) Tetrahedral primary silicon in Al-20wt.%Si alloy was obtained by using rapidly solidified technique, which can confirm the assumption, proposed by Kobayashi et. al, that nucleation of five-folded primary silicon will be preceded by the presence in the liquid of tetrahedral groups of silicon atoms. The morphological features of sub-octahedral primary silicon at the free surface of the ribbon were preserved by using melt-spinning technique. If V 1 00 V111=1.5, the primary silicon crystal will grow as a perfect octahedron. If V 1 00 V111> 1 .5, the primary silicon crystal will grow as a imperfect octahedron.(2) If Vg V111≥5 and V g = Vr, the flat-plate primary silicon will become hexagonal shape with higher aspect ratio. If Vg V?111? < 5 and V g> V r, the flat-plate primary silicon will become twinned primary silicon with lower aspect ratio.(3) Primary silicon in unmodified hypereutectic Al-20wt.%Si alloy exhibits coarse platelet, star-like, octahedral and other irregular morphologies (approximately 200μm). When the new modifier was added, the morphologies of primary silicon were drastically changed to fine blocky shape, which should show the significant modification effect of the new modifier. Statistical analysis shows that the average sizes of primary silicon in modified hypereutectic Al-20wt.%Si alloy is approximately 20μm. The new modifier has better resistant fading behavior. When the holding time was up to 240min, the primary silicon size was still lower than 21μm. The RExOy play an important role in resistant fading. In addition, the modified hypereutectic Al-20wt.%Si alloy with the new modifier has better re-molten behavior. When the re-molten times of the modified alloy were the first time and the second times, the average sizes of primary silicon were 22μm and 26μm, respectively. When the re-molten time was ninth times, the average size of primary silicon was still 28μm.(4) According to the present experimental results, TiC has significant effect on the morphologies and size of primary silicon in hypereutectic Al-Si alloys. When TiC was added into the molten alloys, the TiC decomposed completely to form a complex compound of Ti-Al-Si. Primary Si can not nucleate heterogeneously on the complex compound of Ti-Al-Si. Therefore, it can be conclude that: when TiC was added into hypereutectic Al-Si alloys, the modification of primary silicon should be mainly poison mechanism by the TiC.(5) According to the present experimental results, RExOy has significant effect on the morphologies of primary silicon in hypereutectic Al-Si alloys. When RExOy was added into the molten alloys, the RExOy decomposed completely to form a compound of Al-RE-Si. Primary silicon can not nucleate heterogeneously on the complex compound of Al-RE-Si. Therefore, it can be conclude that: when RExOy was added into hypereutectic Al-Si alloys, the modification of primary silicon should be mainly poison mechanism by the RExOy.(6) The shifting distance of eutectic point under non-equilibrium solidification condition (in horizontal direction toward higher silicon contents) can be expressed as following:(7) According to the present results, with the elevation of melt overheating temperature, primary silicon will change from star-like and other irregular primary silicon to octahedral primary silicon and the size of primary silicon will gradually decrease. For example, the majority of primary silicon with a pouring temperature of 1050℃should be octahedral primary silicon, and primary silicon with pouring temperature of 750℃, 850℃and 950℃exhibit a variety of morphologies such as coarser polygonal, star-like and other irregular morphologies. There is a critical cooling rate of unmodified hypereutectic Al-20wt.%Si alloy between 375℃/s and 1.11×106℃/s. If the cooling rate is lower than a critical cooling rate, the cooling rate will have hardly effect on the morphologies of primary silicon in the solid state, and primary silicon exhibit coarser polygonal, star-like and other irregular morphologies. If the cooling rate is higher than a critical cooling rate, the cooling rate will play an important role in determining the morphologies of primary silicon in the solid state, and primary silicon exhibits finer octahedral and other regular morphologies.(8) Compared with unmodified and heat-treated Al-20wt.%Si alloy (264.5MPa), the tensile strength of modified with the new modifier and heat-treated Al-20wt.%Si alloy (317.5MPa) increases by 20%. Compared with the wear rate of unmodified and heat-treated hypereutectic Al-Si alloys (23,26,28 and 31wt.%Si), the wear rate of modified hypereutectic Al-Si alloys increase by 14%,25%,15% and 8%, respectively, and the wear rate of modified and heat-treated hypereutectic Al-Si alloys increase by 20%,18%,20% and 17%, respectively.