Investigation Of Preparation, Densification And Properties Of Spray Deposition Continuous Aluminum Matrix Gradient Composite

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.