【作者】 苏斌；

【导师】 严红革；

【作者基本信息】 湖南大学 ， 材料加工工程， 2013， 博士

【摘要】 颗粒增强铝基梯度复合材料具有密度低、比强度及比刚度高、耐磨性能优异和材料内部热应力过渡平稳等一系列优点，非常适合于制造高速制动部件和耐磨部件，在生物材料、机械工程材料、耐磨耐蚀的表面涂层以及航空航天材料等领域有广泛的应用前景。国内外对梯度复合材料的成分设计和制备技术等方面已有大量研究，但精确控制增强颗粒浓度梯度分布以及大尺寸、高性能连续梯度复合材料的制备技术仍然没有突破性进展。本论文通过对自动控制陶瓷颗粒输送装置的探索，发明了连续梯度复合材料的喷射沉积装置，通过对喷射沉积工艺参数的优化，制备出了SiC颗粒沿沉积坯高度方向呈连续梯度分布的圆柱锭坯、大尺寸圆盘件和环件。通过对热模压致密化工艺研究，得到了最佳热模压工艺参数；同时考察了温度、应变速率对SiCp/Al-20Si-3Cu梯度复合材料的流变应力影响，通过多阶梯形阳模结合平模致密化技术大幅度降低了压制压力，在此基础上自行研制出了超塑性致密化技术；采用此项技术对不同尺寸的圆柱锭坯和大尺寸圆盘件进行了致密化研究，检测并分析了材料的力学性能、断裂机制和强韧化机制；探讨了超塑性致密化过程中组织演变规律，分析了超塑性致密化机理，建立了Al2O3颗粒增强铝基体条带组织形成模型和超塑性致密化的孔洞演变模型。另外，研究了SiCp/Al-20Si-3Cu连续梯度复合材料的摩擦磨损性能，并对其在制动部件中的应用进行了考察。论文得到的主要结论如下：(1)自行研制出了自动控制的陶瓷颗粒输送装置，解决了喷射沉积过程中对SiC颗粒呈梯度分布的控制问题；通过对液流直径、雾化气体压力和喷射高度等工艺参数探索，得到了喷射沉积制备连续梯度复合材料最佳工艺参数。在最佳工艺下制备了SiC颗粒质量分数沿沉积坯高度方向呈030%的连续梯度分布的圆柱锭坯和呈015%的大尺寸600mm150mm圆盘件和外径达1200mm环件。对沉积坯微观组织分析表明：基体组织细小，晶粒尺寸在5μm以下；初晶Si细小、尺寸仅1~3μm；析出相为-Al2Cu和AlCuMg，细小、弥散分布在晶界或晶内。孔隙率随SiC质量分数的增加其体积分数逐渐增加；沉积坯的显微硬度随SiC含量增加而增加。(2)研究了SiCp/Al-20Si-3Cu连续梯度复合材料的热模压致密化工艺，通过对热模压的温度、压制压力和保压时间等工艺参数探索，得到热模压最佳工艺参数。为了降低压制压力和扩大材料在致密化过程中横向剪切流动，自行设计了多阶梯形阳模结合平阳模热压致密化工艺，此工艺致密化结果表明：在没有改变梯度复合材料组织特点前提下，该工艺可以大幅度降低压制压力，并取得了良好的致密化效果。探讨了温度和应变速率对喷射沉积SiCp/Al-20Si-3Cu连续梯度复合材料流变应力影响，分析了该材料高温高应变速率下的超塑性，并自行研制出了超塑性致密化技术。（3）采用超塑性致密化技术对小尺寸锭坯进行了致密化研究，通过对压制压力、应变速率等工艺参数摸索，得到了最佳超塑性致密化技术工艺参数和较好的致密化效果，在此最佳工艺条件下对大尺寸锭坯进行致密化研究，结果表明：315吨的夜压机可实现对300×140mm圆柱锭坯的致密化，630吨的液压机可实现600×150mm厚盘件的致密化，材料的致密度均在99%以上，实现了小吨位设备对大尺寸坯料的全致密化，压制压力最大可降到同等条件下普通热模压的1/8。（4）对超塑性致密化后的SiCp/Al-20Si-3Cu梯度复合材料的性能研究表明：材料的整体性能在300MPa以上，较沉积坯的强度提高了23倍，随梯度层中SiC含量的增加，材料的抗拉强度和屈服强度呈先增后减变化规律、伸长率呈下降的变化趋势。SiC含量为15.2%梯度层的抗拉强度达到最大值388MPa，经热处理后的抗拉强度可达430MPa。性能大幅提高是细晶强化、SiC颗粒增强相强化、沉淀相强化和良好的致密化效果综合作用结果。（5）对超塑性致密化后SiCp/Al-20Si-3Cu梯度复合材料的微观组织分析结果表明：超塑性致密化没有改变SiC的梯度分布特征，梯度层内SiC分布更加均匀；保持了合金基体原有晶粒细小的组织特点，-Al基体、Si颗粒及SiC颗粒三者间界面结合达到了冶金结合；-Al2Cu和AlCuMg析出相细小、弥散分布并和位错交互作用明显；喷射沉积制备和致密化加工过程中在沉积颗粒边界上形成的Al2O3薄膜经超塑性致密化后，形成了以Al2O3薄膜碎片和MgAl2O4颗粒为增强相的宽度为50100nm的铝基体条带组织；通过界面反应，此条带状组织把沉积颗粒紧紧地连接在一起，提高了材料的强度和塑性，并建立了Al2O3增强铝基体条带组织形成模型；对超塑性致密化机理进行了分析，建立了超塑性致密化的孔洞演变模型。（6）研究了SiCp/Al-20Si-3Cu连续梯度复合材料的摩擦磨损行为，结果表明：摩擦系数随载荷和转速的增加而减小，随SiC含量增加，摩擦系数增加，摩擦系数在0.35~0.42之间变化，对比HT250，摩擦系数变化幅度小；磨损率随载荷或转速增加呈先增后减再增变化趋势，同等条件下其磨损率为HT250的1/10。在摩擦磨损过程中，材料的机械混合层厚度随SiC含量的增加而减小，当SiC颗粒含量由2.86%~15.21%变化时，机械混合层的厚度由25μm~5μm变化。随载荷或滑动速度增加，材料的磨损机制转变顺序为：磨粒磨损→氧化磨损→剥层磨损。更多还原

【Abstract】 Particulate-reinforced aluminum matrix gradient composites are of low density,high strength, stiffness, excellent wear properties, smooth thermal stress transitionand a series of advantages, are very suitable for high speed brake parts andwear-resistant workpieces. They are of broad application prospects in the fields ofbiomaterials，mechanical engineering materials, corrosion resistant coating aerospacematerials and etc. Many studies have been conducted on design and preparationtechnology of gradient composites at home and abroad. However, there is still nobreakthrough in precise control of gradient distribution of particles concentration andthe preparation technology of the large size and high-performance continuous gradientcomposites.Process parameters of the automatic control ceramic particle conveyingequipment are explored and thus the continuous gradient composite spray device isinvented. through the optimization of spray deposition process parameters, cylindricalingots, large size discs and rings are prepared successfully. The densification processof traditional hot-pressing and the best hot molding parameters are determined. At thesame time, the effects of temperature and strain rate on flow stress of theSiCp/Al-20Si-3Cu gradient composite are investigated. The densification technologyof the multi-step type mold in combination with flatdie can significantly reduce thepressing pressure. On this basis, superplastic densification technologies is developedand is used for densification for different sizes of cylindrical ingots and large-sizediscs. Moreover the mechanical properties, fracture mechanism and the tougheningmechanism of the composite are studied on microstructure evolution duringsuperplastic densification, analysis of superplasticity densification mechanism, theestablishment of the Al2O3stripe formation model and the hole evolution model ofsuperplastic densification. In addition, the friction and wear properties of theSiCp/Al-20Si-3Cu continuous gradient composite are studied and its application in thebrake parts is also previewed.The main conclusions are as follows:(1) The ceramic particle conveying equipment of automatic control is developedindependently, which can solve the control problems of SiC particles gradientdistribution in the spray-deposition ingots. Through probing into process parametersof the flow diameter, atomization air pressure and the jetting height and so on, the best process parameters are obtained in the preparation of the continuous gradientcomposites by spray-deposition technology. Cylindrical ingots of different sizes areprepared, in which the SiC particles exhibit a continuous gradient distribution alongthe height direction of deposition from0to30%. Furthermore, the600mm disc oflarge size and the ring with the outer diameter of1200mm ring were prepared, inwhich the SiC particles exhibit a continuous gradient distribution from0to15%.Microstructure analysis of the deposited preforms shows that the alloy matrix ischaracteristic of a refined microstructure, and grain size below5μm, liquidus Si sizeof only1~3μm, the precipitatious of-Al2Cu and AlCuMg. The two precipitatiousare distributed at grain boundaries or within grains. The volume fraction of the poresincreases with the gradually increasing SiC concentration, and the micro-hardness ofthe deposited preform increases with SiC concentration increasing.(2) The heat molding densification process of SiCp/Al-20Si-3Cu continuousgradient composite is studied. Throught exploring the process parameters ofhot-pressing temperature, pressing pressure holding time and etc., the optimumprocess parameters were obtained finally. In order to lower the pressing pressure andexpand the shear flows in horizontal direction during the densification process, thedensification technology of the multi-step type male mold combining flatdie isdesigned. The densification effect reveals that the process can significantly reduce forthe pressing pressure without changing the characteristics of graded compositeorganization, and evelate the densification effectiveness. On the other hand, theeffects of temperature and strain rate on the flow stress of the spray-depositedSiCp/Al-20Si-3Cu continuous gradient composite are studied, to analyse of thematerial superplasticity at high temperature and high strain rates, and develope to thesuperplasticity densification technology.(3) Superplasticity densification is adopted for studying the densificationbehavior of small ingots. The best process parameters of superplastic densification areobtained. Moreover the densification effect is very good. Under the best processparameters, the densification behavior of the large-size ingots are studied, and revealsthat the315T and630T hydraulic press machine can realize the densification of theФ300140mm cylindrical ingots and the Ф600150mm cylindrical ingotsrespectively, the density is above99%. It is possible to densify the large size ingotscompletely with the small-tonnage equipment under the same conditions. Themaximum pressure is reduced to1/8of the ordinary hot-pressing.(4) The properties of SiCp/Al-20Si-3Cu gradient composites after superplasticdensification are examined and the results show that the overall tensil strength is above300MPa, and is23times higher than the deposited perform. With the SiCcontent increasing, the tensile strength and yield strength of the composite increasefirstly and then decrease, while the elongation exhibits a degressive trends. Tensilestrength of the gradient layer with15.2%SiC content is up to388MPa, and it is up to430MPa after heat treatment. Property inprovement is a combination of fine-grainstrengthening, SiC particles reinforcing, precipitation hardening and gooddensification effect.(5) Microstructure analysis of the gradient composites after superplasticdensification show that superplasticity densification don′t change the gradientdistribution of SiC, and SiC particles are more evenly distributed, the original graincharacteristics are refined and interfacial bonding between Al substrates, Si and SiCparticles reach metallurgical combination, the precipitates of AlCuMg and-Al2Cuare small, diffuse distribution and dislocation interaction between the precipitates andAl substrates are obvious. The Al2O3films on the boundary of the sedimentaryparticles, derived from the oxide in the process of preparation and densification, aftersuperplastic densification, are formed in the stripe of50-100nm wide. By the interfacereaction, the stripes tightly connect the sedimentary particles together, and thestrength and plasticity are increased. On the basis of microstructure analysis, theformation model of the Al2O3stripe organization is established. Through the analysisof superplasticity densification mechanism, the hole evolution model ofsuperplasticity densification is established.(6) Friction and wear behavior of the SiCp/Al-20Si-3Cu continuous gradientcomposite are studied. The results reveal that the friction coefficient decreases withthe load and speed increasing, the friction coefficient increases with SiC contentincreasing, the friction coefficient changes between0.35~0.42, in contrast withHT250, the variation of friction coefficient is smaller. Wear rate increase with load orspeed increasing firstly, then decreases and increase lastly. Under the sameexperiment conditions, the wear rate of SiCp/Al-20Si-3Cu continuous gradientcomposite is only1/10of HT250. In the process of friction and wear, the thickness ofthe mechanical mixing layer decreases with SiC content increasing. When the contentof SiC particles changes from2.86%to15.21%, the thickness of mechanical mixedlayer decreases from25μm to5μm. With the load or the sliding speed increasing, thewear mechanisms changes from abrasive wear, oxidation wear to delamination wear.更多还原