【作者】 李桂荣；

【导师】 戴起勋； 赵玉涛；

【作者基本信息】 江苏大学 ， 材料学， 2007， 博士

【摘要】 原位制备颗粒增强铝基复合材料是复合材料研究领域的热点,前期研究集中在开发不同的原位合成工艺和体系。如何通过过程优化来控制颗粒在基体中的形貌和分布特征对促进复合材料发展非常关键,所以通过优化原位合成体系、制备和成型工艺来控制复合材料的组织和性能具有重要的理论意义和现实意义。论文的研究基于两个学术思想:一是对原位合成的复合材料实现组织控制,包括颗粒种类、形貌、尺寸和分布以及基体晶粒度和硅相等,为复合材料的实践应用提供有章可循的理论依据;二是挖掘电磁技术在复合材料制备领域的应用,在优化复合材料组织的同时,体现电磁技术在复合材料制备中的优越性和前景。首先用Al-Zr（CO₃）₂组元通过熔体直接反应法原位合成了Al-Zr-O系颗粒增强铝基复合材料,颗粒强化相是Al₂O₃和Al₃Zr;用Al-Zr（CO₃）₂-KBF₄组元和Al-Zr（CO₃）₂-H₃BO₃组元合成了Al-Zr-O-B系复合材料,颗粒强化相是Al₂O₃、Al₃Zr和ZrB₂,颗粒粒度2～3μm。为了促使颗粒细化和在基体中弥散分布,通过外加电磁场来控制复合材料组织。用电磁搅拌法原位合成（Al₂O₃+Al₃Zr）_p/Al复合材料,当磁感应强度0.025T时,颗粒在基体中弥散均匀分布,且尺寸细化,粒度1～2μm。原因是切向电磁力造成熔体涡流,反应物颗粒更易进入铝液,增加了反应物之间的接触几率,电磁力加大了体系的混合对流运动,提高了传热传质和物质扩散速度,并促进增强颗粒在铝基体中的弥散均匀分布。并且当B值为0.04T时,电磁搅拌力增大,在离心作用下制备出颗粒分布呈现梯度特征的（Al₂O₃+Al₃Zr）_p/Al复合材料。高频磁场叠加低频正弦调制信号后可产生高频正弦调制磁场,当高频电流30A,外加正弦波信号频率10Hz,电压幅值7V时,磁感应强度为0.029T,系统研究了线圈内磁场分布规律。调制磁场产生的扰动、搅拌、振荡等效应,改善了固液相体系铝热反应的动力学条件,合成复合材料中颗粒体积分数成倍增加,颗粒细化至1～2μm,且在基体中均匀弥散分布。磁化学分析认为熔体接收了磁场能量,改善了原位反应体系的熵增和能量状态,促进了原位反应的进行。制备好的复合材料熔体如果采用金属模成型,存在气孔和缩松等浇注缺陷,而采用“挤压铸造”成型可提高组织致密度并细化基体晶粒,颗粒形貌没有明显变化,适于生产复合材料铸件。为了大规模生产铸锭,进行熔体的“半连铸”和“半连铸电磁搅拌”成型,浇注温度710～720℃,拉坯速度9cm/min,冷却水量为0.8t/h,颗粒特征没有大的变化,但分布均匀化程度提高。并当电磁搅拌磁感应强度0.025T时,基体晶粒度细化,出现大面积等轴晶,硅相形貌随搅拌强度增加由片状初晶硅向针状、细棒状共晶硅转变。又采用“高频电磁连铸”改善“半连铸电磁搅拌”铸锭表面质量,选用特制闭路水冷割缝结晶器,实现了铸锭与结晶器之间的软接触。当线圈内电流80A时,铸锭表面光洁,无明显褶皱;高频方波调幅磁场的铸锭表面存在周期性振痕,调幅波频率越高,振痕的间隔就越小。基于合成成型工艺的研究结论,提出复合材料的“联合电磁制备”概念,即熔炼过程中施加低频磁场,连铸过程中施加低频高频磁场的复合材料铸锭制备方法,充分利用电磁场在复合材料制备中的优越性。凝固组织观察发现,复合材料中颗粒弥散均匀分布,粒度在1～2μm,适于工业规模制备高性能高表面质量的复合材料铸锭。对于基体合金、15vol%（Al₂O₃+Al₃Zr）_p/A356复合材料熔体金属模成型、挤压铸造、半连铸电磁搅拌以及联合电磁连铸时制备的复合材料抗拉强度分别为184.6、263.2、345.7、355.8、340.6MPa,相应的延伸率分别为7.1%、4.7%、15.1%、14.2%、10.1%。断裂机制多是脆性和塑性混合断裂。材料耐磨性研究结果显示,并非反应物加入量越多,复合材料的耐磨性越好,因为颗粒在其中起到支撑减磨和脱落后加剧磨损的双重效应。对于（Al₂O₃+Al₃Zr）_p/A356复合材料的最佳颗粒理论体积分数值是10%。本次研究外延了原位制备复合材料的体系内容,并拓展了电磁技术在材料制备中的应用领域,将电磁搅拌和高频调制磁场成功运用到颗粒增强铝基复合材料的原位合成和成型工艺中来,并优化了磁场参数,电磁力改善了原位合成过程的热力学和动力学条件,促使颗粒细化和在基体中的弥散均匀分布。提出了复合材料熔体挤压铸造、半连铸电磁搅拌和联合电磁制备的复合材料成型过程机制,控制了复合材料铸件和铸锭的内部组织和表面质量,并在电磁力作用下制备出颗粒增强梯度复合材料。更多还原

【Abstract】 Much attention has been paid to the particulate reinforced aluminum matrix composites.Prior studies focused on exploring the new in situ fabrication processes and systems.But it is essential to control the feature and distribution of particulates.It has important theoretical and current meanings to optimize the in situ system,fabrication and solidifying processes so as to control the microstructure and properties of composites.The paper is based on two academic theories.One is to control the microstructure of in situ composites,such as kinds,feature,and distribution of particulates and grain size of matrix et al.It may offer the theoretical basement to produce composites on an industrial scale.The other is to explore the appliances of electromagnetic technologies in composite materials process.The in situ（Al₂O₃+Al₃Zr）_p/Al composites is fabricated composites using Al-Zr（CO₃）₂ components by direct melt reaction method.The in situ（Al₂O₃+Al₃Zr+ZrB₂）_p/Al composites are synthesized using Al-Zr（CO₃）₂-KBF₄ and Al-Zr（CO₃）₂-H₃BO₃ components.The sizes of particulates are in the range of 2～3μm.In order to refine and promote the uniform distribution of particulates the foreign electromagnetic fields are introduced to control the microstructure of composites.The （Al₂O₃+Al₃Zr）_p/Al composites are synthesized by electromagnetic stirring method with 0.025T magnetic induced intensity.The size of particulates is refined to 1～2μm.They are well distributed in matrix.The vortex flow occurs due to tangential electromagnetic force,which makes it easy for reactant particles to enter into the molten aluminum.The contact opportunity between reactants is increased.Meanwhile the electromagnetic forces accelerate the mix convection moving and add up the velocity of heat,mass transfer and diffusion.When the magnetic induced intensity is 0.04T the electromagnetic force is raised largely.The gradient（Al₂O₃+Al₃Zr）_p/Al composites are produced due to the centrifugal force.The high frequency sinusoid modified electromagnetic field is combined when the low frequency sinusoid modification signal imposed on the high frequency carrier wave. The magnetic induced intensity is 0.029T when the frequency and voltage of sinusoid signal is 10Hz and 7V separately.Meanwhile the current of high frequency magnetic field is 30A.The distribution rules in coils are studied systematically.Turbulence, stirring and oscillation phenomena take place due to the modified electromagnetic field, which improve the kinetic conditions of in situ fabrication.The volume fraction of particulates in composites is increased largely.The size of particulates are refined to 0.5～1μm and the particulates distribute uniformly in matrix.Magnet chemistry analysis illustrates that the melt receive the energy of electromagnetic field.The entropy and energy status of in situ reaction system are improved,which promote the in situ reaction.Conventional pinhole and shrinkage would appear in the cast composites with permanent mold.When the squeeze casting technique is utilized to cast composites,the tightness of microstructure is increased and the grain size of matrix is refined.The feature of particulates changes slightly.So it is suitable to produce composite parts. Semi continuous casting and electromagnetic stirring semi continuous casting are practiced to cast the composite.The casting temperature is in the range of 710～720℃,the casting speed is 9cm/min and the cooling water amount is 0.8t/h.The features of particulates almost do not change whilst the distribution state is more uniform than ever.When the magnetic induced intensity is 0.025T the grain size is refined and large areas with equiaxed grain appeare.With the increase of stirring intensity the pattern of silicon phases change from plate to needle and slender rod.Later high frequency electromagnetic continuous casting processes are used to increase the surface quality of slab.The special closed water cooling slit mold is introduced.The soft contact between slab and mold is reached.The surface of slab is smooth when the current is 80A.There are apparent oscillation marks on the surface of slab when using square wave modified the high frequency electromagnetic field.Higher frequency of modified wave,smaller space of oscillation marks.Basing on the above conclusions an "integrate electromagnetic fabrication of composites"is brought forward,which combines the low frequency melting with high frequency continuous casting.Microstructure observation shows that the size of particulates is 1～2μm and they are well distributed in matrix.The technology is suitable to produce composite slab with high properties and excellent surface quality.The tensile property and wear performance of composites are further studied.For A356,15vol%（Al₂O₃+Al₃Zr）_p/A356 cast by permanent mold,squeeze casting,semi continuous casting and integrate electromagnetic fabrication,the tensile strengthes are 184.6,263.2,345.7,355.8 and 340.6MPa,the elongationa are 7.1%,4.7%, 15.1%,14.2%and 10.1%individually.The fracture mechanisms are the mixed forms of brittle and plastic ones.The abradability study shows that it is not the case that high adding amount of reactants has high wear resistance property.Because the particulates would sustain the wear before falling and aggregate the wear after falling.For （Al₂O₃+Al₃Zr）_p/A356 composites the best value of theoretical volume fraction is 10%.The paper study extends the system of in situ composites and expands the appliance domain.Electromagnetic stirring and modified high frequency electromagnetic field are successfully introduced into the fabrication of particulate reinforced aluminum matrix composites.The electromagnetic forces enhance the thermodynamic and kinetic conditions.Hence the sizes of particulates are refined and they are well distributed in matrix.Meanwhile the parameters of electromagnetic field are optimized.The process mechanisms about squeeze casting,semi continuous casting with electromagnetic stirring and integrate electromagnetic fabrication are put forward.The aim of control the microstructure and surface quality of composite slab are realized.Furthermore the gradient particulate reinforced composites are produced under electromagnetic field.更多还原