【作者】 李庆林；

【导师】 夏天东；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2014， 博士

【摘要】 随着汽车发动机向着高速、大功率和低排放等方向的发展,对材料耐磨性能、机械性能以及尺寸稳定性的要求越来越高。过共晶Al-Si合金由于具有优良的铸造性能、低的密度、低的热膨胀系数、优良的导热性能、高的强度、较好的耐磨和耐腐蚀性能被广泛地应用在航空航天和汽车领域。在汽车领域,过共晶A1-Si合金作为传统铸铁的理想替代材料用以制作汽车发动机用活塞和缸套等耐磨零部件,不仅可以提高发动机的工作效率,而且可以减轻汽车重量、提高燃油利用率和减少尾气排放。然而,过共晶Al-Si合金铸造组织中,初生Si为粗大不规则的块状、五瓣星状和板片状,共晶Si为粗大的针片状,它们在基体上的分布严重割裂了基体的连续性,并且尖端导致局部的应力集中,明显降低了合金的力学性能和耐磨性能,限制了过共晶A1-Si合金的工业化应用。为进一步开发过共晶A1-Si合金的性能潜力,扩大工业化的应用范围,开展过共晶A1-Si合金中初生Si和共晶Si形态演变的研究具有现实意义。目前,国内外常用的过共晶Al-Si活塞合金的含Si量为17%-22%。该范围的合金相比较高Si含量的Al-Si合金具有窄的结晶温度间隔,具有优良的流动性,从而使合金具有良好的铸造性能。因此,本论文选用Al-20%Si合金为研究对象,研究了能够同时细化和变质初生Si和共晶Si的单一变质剂或细化处理技术,包括稀土变质剂、添加合金元素、原位自生γ-A1203颗粒、熔体的热速处理技术和热扩散处理技术,为过共晶Al-Si合金广泛地工业化应用提供理论依据和科学指导。过共晶A1-20%Si合金中加入单一的稀土元素Ce和Er后,发现稀土Ce和Er不仅能够细化初生Si,而且能够变质共晶Si组织。随着稀土Ce含量从0.3%增加到1.0%,初生Si的形态从粗大不规则块状、板片状和五瓣星状转变成边缘和端部钝化的块状,且较均匀地分布在合金基体上,其平均尺寸从192μm减小到大约为33μm；共晶Si由原始铸态时的粗大针片状细化为细小的纤维状组织和粒状组织；当加入0.5%Er时,初生Si细化为端部和边缘钝化的细小块状,平均尺寸减小到41μm,共晶Si变质为细小的珊瑚状纤维组织。但是,当添加量为0.8%时出现了初生Si和共晶Si的粗化现象；α-Al的二次枝晶间距发生了先减小后增大的现象。已有的研究都集中在合金元素的添加对Al-Si合金中析出相以及对合金性能的影响,而没有研究合金化处理对过共晶Al-Si合金中Si相形貌和尺寸的影响。本论文通过向过共晶Al-20%Si合金中加入Mg、Ni和Mn元素进行合金化处理,发现加入2.0%Mg使初生Si细化成平均尺寸大约为29μm的细小规则块状,共晶Si细化为具有多分支的细小纤维组织；添加1.5%Ni可以使初生Si细化成细小的规则块状组织,分布趋于更加的均匀,平均尺寸减小到42μm,共晶Si细化为细小多分枝的纤维组织；加入0.1%Mn时,初生Si的形貌和尺寸发生了突变,其形貌由尺寸较大的块状和板片状转变成了细小块状和少部分长度小于50μm,宽度小于10μm的片状初生Si,其平均尺寸减小到39.7μm,共晶Si细化为细小的粒状和纤维状组织。将去结晶水的NH4Al(SO4)2无机盐加入到Al-20%Si合金熔体中,通过搅拌和保温后能够原位反应生成微米尺度的γ-Al2O3颗粒。原位自生的γ-Al2O3颗粒通过非均质形核和抑制生长有效地细化Al-20%Si合金中的初生Si和共晶Si组织。原位自生γ-Al2O3质量分数达到0.8%时,初生Si被细化成边缘钝化的细小块状组织,其平均尺寸减小为40μm。然而,进一步使生原位自生的γ-Al2O3颗粒质量分数到1.0%时,出现了粗化现象,其平均尺寸为41μm。随着原位生成γ-A1203颗粒质量分数的增加,共晶Si的片层间距减小,长度变短,尖端和边缘钝化,由原来的粗大针片状向细小的棒状和粒状转变,而非细小纤维状组织。熔体热速处理技术既能细化初生Si相,也能细化共晶Si组织。当熔体经过1050℃的过热处理后,初生Si的平均尺寸减小到38μm,共晶Si以细小的粒状均匀分布在金属基体上。合理的热扩散处理工艺条件下能够获得端部和边缘钝化的初生Si和细小的纤维状共晶Si。610℃经过15min的热扩散处理能够获得边缘钝化的小块状和细小的近球状初生Si,平均尺寸大约为11μm；共晶Si以细小的颗粒状和纤维状从液相中重新析出并包围了球状的a-A1组织。580℃经过40min的热扩散处理获得了平均尺寸为35μm的近球状初生Si；共晶Si以细小的纤维状从熔化的液相中重新析出。550℃经过90min的热扩散处理后,初生Si未发生明显的钝化和球化,形态以相互交叉的类树枝状存在；共晶Si细化为平均粒径为1.8μm的细小粒状和球状。室温力学性能和摩擦磨损性能的测试表明,Al-20%Si合金中初生Si和共晶Si组织的细化处理够使合金的抗拉强度从92MPa提高到变质后的159MPa,提高了42%；断后延伸率从变质前的0.49%提高到变质后的1.72%,提高了72%；合金的摩擦系数和体积磨损率减小,分别减小了27%和45%,该结果揭示Si相的细化能够提高合金的抗拉强度和耐磨性能。更多还原

【Abstract】 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.更多还原