| With the development of automotive engine toward high speed, high power and low emission, the requirement of wear resistance, mechanical properties and dimensional stability of materials is higher and higher. Hypereutectic aluminum-silicon (Al-Si) casting alloys have been widely employed in aerospace, automotive and general engineering industries due to their excellent casting and mechanical properties, such as reduced density, low thermal expansion coefficient, good thermal conductivity, high strength, high wear resistance and good corrosion resistance. In automotive field, the hypereutectic Al-Si alloys are recognized as attractive candidate materials, which would replace the traditional use of cast iron for manufacturing pistons, cylinder liners and other wear resistance parts. These components not only increase the efficiency of engines, but also play an important significance in reducing the weight of the automobile, improving the utilization of fuel oil and cutting exhaust emission. However, primary Si crystals reveal several morphologies, such as star-like(five-folded), polygonal and plate-like shape. Moreover, the morphology of eutectic Si is also complex, including coarse acicular and flake shape during conventional casting processes. Generally, the mechanical properties and friction performance of hypereutectic Al-Si alloys is worse due to the presence of coarse primary Si and eutectic Si. Additionally, the presence of the sharp corners and ledges of coarse primary Si and eutectic Si aggravate the localized stress concentration at the sharp corners and ledges, which limits the further application in industries. Therefore, it is very useful to study modifiers or the refinement methods in order to expand the application field of hypereutectic Al-Si alloy.In present, the typical hypereutectic Al-Si alloys containing17-22%Si are used to produce poiston in the world include. In contrast, the thermal expansion coefficient is lower and solid-liquid region is narrower on hypereutectic Al-20%Si alloy than Al-Si alloy of high Si content. In addition, it well known that the morphology of primary and eutectic Si in unmodified hypereutectic Al-Si is similar. In this paper, investigating a modifier (refiner) or refining technology which can simultaneously refine or modify primary Si and eutectic Si of hypereutectic Al-20%Si and provides some theoretical basis for industrial applications. The major research efforts of the present study are as follows:The pure Ce and Er can not only refine primary Si phase, but also can modify eutectic Si structure. The size of primary Si decreases from192μm to33μm and the morphology transfers from star-like(five-folded), polygonal and plate-like shape to fine blocky shape with increasing the concentration of Ce from0.3to1.0%. Moreover, Ce can obviously modify eutectic Si structure and make a transition from the coarse flake-like and acicular shape to fine fibrous structure and discrete particles with increasing the concentration of Ce. Element Er can significantly refine primary Si that the morphology transfers from the coarse star-like, platelet-like and polygonal shape to fine blocky shape and its size decreases to41μm when the addition of Er is0.5%. However, a further increase in the amount of addition Er up to0.8%leads to coarsening of primary Si. Er can obviously modify eutectic Si structure and make a transition from the coarse flake-like and acicular shape to fine fibrous structure with increasing Er content to0.5%. The eutectic Si structure becomes coarser when the concentration of Er further increases up to0.8%.A number of articles have been published in the literature which reported addition alloying elements can lead to the formation of different eutectic phases and improve the mechanical properties of Al-Si alloy. However, a few studies in the literature have reported the effects on the morphology of primary Si and eutectic Si in Al-20%Si alloy. In the present works, the effects on addition elements of Mg、Ni and Mn are investigated. When2.0%Mg are added into Al-20%Si alloy, the primary Si can be significantly refined from coarse irregular five-folded, polygonal and plate-like shape to fine blocky shape, and the average size of primary Si is about29μm. The eutectic can be modified into fine fibrous structure. In addition,1.5%Ni can effectively refine the primary into fine blocky shape and tend to uniform distribution. The average size of primary Si decreases to42μm. The eutectic Si transform into fine fibrous structure. After0.1%Mn is added into Al-20%Si alloy, the morphology of most primary Si significantly refine into fine blocky shape, and some primary Si transform into flake-like shape of50μm in length and10μm in width. The average size is39.7μm. The eutectic Si refine into fine granular and fibrous structure. The results show that the alloying element Mg is the best effective modifier in Al-20%Si alloy.The in-situ chemical reaction can happen and generate γ-Al2O3particles in Al-20%Si alloy by adding inorganic salt NH4Al(SO4)2powder. The result shows in-situ γ-Al2O3particles can refine both primary Si from coarse polygon, platelet-like and star-like shape to fine blocky shape with smooth edges and corners and eutectic Si structure from the coarse acicular structure to fine flake-like eutectic Si structure in hypereutectic Al-20%Si alloy. The size of primary Si crystals significantly reduces with increasing concentration of γ-Al2O3particles and reaches to the smallest value when the content of γ-Al2O3particles is0.8%. However, in-situ γ-Al2O3has no further effect on the size of primary Si crystals when the concentration increases up to1.0%. In addition, the size and interflake spacing of eutectic Si apparently reduce with increasing concentration of in-situ γ-Al2O3particles.Microstructure analysis shows increasing melt superheat temperature can not only remarkably refine size, but also can change morphology of primary and eutectic Si phases, and can decrease wear rate of the alloy. By using thermal rate treatment technology on Al-20%Si alloy melt at1050℃, the average size of primary Si is decreased from192μm to38μm, and the morphology of eutectic Si is transformed from coarse needle-like plate shape to fine grnaular structure.The edges and angles of primary Si are passivated and the eutectic Si is fine fine coral-like fibrous structure using the reasonable technological parameters of heat diffusion treatment. The edges and angles of primary Si are passivated when Al-20%Si alloy is treated at610℃for15min, and the average size of primary is about11μm. The granular and fibrous eutectic Si precipitates from liquid of low point eutectic structure and surround the spherical a-Al. After Al-20%Si alloy is treated at580℃for40min, the primary Si transform into near-spheroidal sturcture, and the diameter is about35μm. The eutectic Si precipitates from remelting liquid and the morphology is fine fibre. However, if the emperature of heat diffusion treatment reduces to550℃holding for90min, the edges and angles of primary Si is no obvious passivation and the morphology is similar to cross dendrite. However, the eutectic Si happens to fuse and granulate during the process of heat diffusion treatment at550℃for90min. The morphology of eutectic Si refine into fine granular and spherical structure, and the diameter of eutectic is about1.8μm.The properties tests show that the plasticity and strength of hypereutectic Al-20%Si alloy are improved with the refinement of Si phases. The ultimate tensile strength increases by42%from92MPa to159MPa and the elongation increases by72%from0.49%to1.72%, respectively. Wear resistance test shows that friction efficient decreases by27%and wear rate decreases by45%. The results reveal that refinement or modification primary and eutectic Si can obviously improve mechanical properties and wear resistance. |
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