| In recent decades, the rapid development of automobile industry and rail transit result in resource shortage environment pollution and so on become more and more serious; The lightweight, high-speed vehicles has become an inevitable trend. Vehicle's parts which use lightweight materials can save energy and reduce the cost, which is a good foundation for realizing high-speed vehicles. The most important part of automobile engine is cylinder, which directly affect the engine's life and performance; it is used in the worst working enviroment, which should bear high temperature, thigh pressure and reciprocating friction with the piston, so the material of the cylinder should have hardness, wear resistance and high conductive and thermal stability. Currently the widely used cylinder material is hypereutectic Al-Si, which have lower-density compare with traditional cylinder; so it has lightweight,high specific rigidty, high wear-resistance and the match between Aluminum alloy piston increased, while reduce the clearance between the piston and engine noise and increase the efficiency of engines. However, the study about hypereutectic Al-Si alloy is limit in the 23% Si, because alloy liquidity become worse, the flaw in casting increase with the increase of the mass% of Si, which result in the decrease of strength. In others words, it has a great limitation to fabricate high volume fraction Al-Si alloy materials in the existing technology and condition.To add certain Mg to hypereutectic Al-Si alloy, can form Mg2Si between Si and Mg; Mg2Si have high melt point, low density, high hardness, low thermal expansion coefficient and high elastic module; which is suitable for aluminum matrix composites reinforcement. In-situ Mg2Si/Al composites not only have high hardness, high strength and low expansion coefficient but also thermodynamic steady and interface stability; which performance has been greatly improved compared with Al-Si alloy. The exsitment of Mg2Si/Al composites provides a new hope for cylinder material researchers, which has the huge market potential and wide application prospect and will be realize compound components, low formation cost and integrated manufacturing and promote the development of auto lightweight.Based on the experimental, this paper study about primary particles motion in solidification process of Mg2Si/Al gradient composite by centrifugal casting, and the influence factors of force and motion of primary particles in centrifugal field; also study the effect of rotational velocity and mould temperature in organization gradient distribution and hardness of Mg2Si/Al gradient composites and analyze the mechanism of gradient distribution and strengthening; The wear resistant of the inner layer of the gradient composites under different casting parameters was also studied. The specific research results as follows:(1) Through centrifugal casting fabricated the Mg2Si/Al gradient composites which the primary particles Mg2Si, Si exist in the external zones and internal zones, respectively, the middle zones only exist eutectic organization. The primary Mg2Si are contained in the primary Si, belong to half coherency.(2) Before the melt was poured, we add the P to modify,the size of primary Mg2Si is 10-20μm and Si is 30-50μm, and the image of primary Mg2Si is ploygon or irregular square, the image of primary Si is smooth block.(3) By changing the rotational speed of mould and mould temperature which both influence the distribution of primary phase,nine different cylindrical parts was prepared to study their morphology.the results show:in the same mould temperature, with the rotational speed of mould increasing, the centrifugal force suffered was more greater, the trend of primary Mg2Si and Si that they segregates the inner of parts was larger, but the outer and middle has a small amount of primary Mg2Si and primary Si or nearly none; in the same rotational speed of mould, with the mold temperature increased, primary Mg2Si and primary Si particles segregates enrich in the inner layer more easily, at the same time, we also theoretically analyzed the mechanism of the effect of mold temperature on the particle distribution. In the experiment, the mould temperature of 300℃is the best, the enrichment in inner layer of primary Mg2Si and primary Si is the largest and the distribution of primary Mg2Si and primary Si was the most obvious under this temperature.(4) The volume fraction of the primary phase from outer to inner layer of gradient composites under different casting parameters changed in gradient:the volume fraction of outer and inner layer is higher than that of the middle, corresponding to the hardness also showed a clear gradient:the hardness of the outer and inner layer is higher than that of the middle. In the same mould temperature, with the rotational speed of mould increased, the hardness of the outer and middle lower decreased while the hardness of the inner layer increased, at the same time, the middle zone of eutectic and hypereutectic more deviated from the inner and the zone width increased. The impact of the mold temperature on gradient composites was restricted by the rotational speed of mould. The main reinforcement mechanism of Mg2Si/Al gradient composites was reinforced phase dispersion strengthened. The optimum centrifugal casting parameter for the experiment was that the mould temperature was 300℃and the rotational speed of mould is 1000r/min.(5) The wear resistant of the inner layer of the gradient composites under different casting parameters was studied. The results showed:the wear resistant of the parts prepared in the condition that the mould temperature was 300℃and the rotational speed of mould is 1000r/min is the best. The volume fraction of the primary Mg2Si played an important role in the wear resistant of the composites. Compared with HT200, the wear resistant of Mg2Si/Al gradient composites is higher by an order due to metal intersolubility. |
PDF Full Text Request