Study On The Preparation Of In Situ Particulate Reinforced Al Matrix Composites By Using High-intensity Ultrasound Assisted Technique At Low Melt Temperature

In situ particulate reinforced Al-MMCs have an extensive prospect application in theaerospace and automobile industries due to their excellent properties, such as high specificstrength and modulus, good wear resistance and high temperature stability. In situtechnique combined with stir casting is able to improve the efficiency, and prepare thecastings with complex shapes as well. As a result, it has attained a great deal of attention.However, some issues such as clusters of reinforced particles and high porosity levelusually associate with the fabrication of in situ particulate reinforced Al matrix compositesvia conventional stir casting technique, which influence the mechanical properties of thecomposites severely. Moreover, long rod-like Al₃Ti particles are usually presented in thematrix by using Al-Ti-C （Ti-C） and Al-Ti-B （Ti-B） systems as the reactants, which alsoleads to a reduction in mechanical properties of the composites. Resolving the aboveissues is the key of improving the properties of the kinds of materials.In addition, the temperature of aluminum melt is usually too high during thefabricating process of in situ particulate reinforced Al-MMCs by using the conventionalmethods. High temperature of melt results in some issues, such as the high manufacturingcost, burning loss of some alloying elements （Mg and Zn et. al） and absorbing gas, as wellas oxidation of aluminum melt. For the in situ TiC_p/Al and Al₃Tip/Al composites, thelowest temperatures of aluminum melt are900℃and950℃, respectively.In order to optimize the fabricating process and microstructure of in situ particulatereinforced Al-MMCs, high-intensity ultrasonic treatment is introduced in this work.High-intensity ultrasonic vibration has been extensively used in the purifying, degassing,refinement of metallic melt, as well as the fabrication of particulate reinforced metalmatrix composites, for the injection of ultrasonic vibration in a melt gives rise to nonlineareffects, such as cavitation and acoustic streaming. High-intensity ultrasound assisted in situ technique is a novel method for preparing particulate reinforced Al-MMCs. In which,ultrasonic vibration can be used to affect the process of in situ reaction and optimize themicrostructure of the composites.Two main contents are focused on in this work. One is the optimization ofmicrostructure of in situ particulate reinforced Al-MMCs; the other one is the fabricationof in situ TiC_p/Al and Al₃Ti/Al composites at low temperature. Firstly, the influence ofhigh-intensity ultrasound on the microstructure of composites by usingremelting-ultrasonic treatment experiments is investigated. Secondly, using Al-Ti-Csystem, Ti-C system and Ti powders as the reactants, the fabrications of in situ TiC_p/Al,（Al₃Ti+TiC）p/Al and Al₃Tip/Al composites by using high-intensity ultrasound assisted insitu technique at low temperatures are explored, respectively.Clustering of reinforced particles, existence of long rod-like Al₃Ti particles with largesize, as well as high porosity level are present in the in situ TiB2p/Al-12Si-4Cu andTiC_p/Al-12Si composites, which were prepared by conventional stir casting techniqueusing Al-Ti-B and Ti-C as the reactants, respectively. The two composites were remeltedat850℃, and then ultrasonic vibration was applied to the melts with an ultrasonic radiator.Owing to the effects of high-intensity ultrasonic vibration, the clusters were eliminatedeffectively and the particles were distributed uniformly in the matrix; the long rod-likeAl₃Ti particles were turned into blocky ones with the size smaller than10μm, which wereconverted from harmful reinforcements into useful ones, and with the volume fraction ofreinforcements increasing, the fragmentation of Al₃Ti particles became more easily; theporosity of the composites could be controlled below1.0%, much lower than the originalones. It is clearly that the ultrasonic vibration can be used to optimize the microstructureof particulate reinforced Al-MMCs.Al-Ti-C system was used as the reactants. In situ TiC particles were synthesized inthe aluminum melt at750℃by using quick preheating treatment （QPHT） to the Al-Ti-Cpellets. The synthesizing temperature was decreased significantly, at least150℃lowerthan those used in the conventional methods. In situ formed TiC particles were spherical inmorphology and smaller that2μm in size, and most of which were submicron particles.The content of Al₃Ti phase was rather low. In this work, the effect of QPHT was discussed in detail. By which, the reactive diffusion between liquid aluminum and solidtitanium could be realized. In the meantime, plenty of heat could be accumulated in theAl-Ti-C pellets. In addition, owing to the effect of ultrasonic vibration, the reinforcedparticles were dispersed uniformly in the matrix, without clusters; in situ formed Al₃Tiparticles were blocky in morphology and smaller than10μm in size; the porosity level inthe composites was about0.86%.In this part, Ti-C mixed powders were added into the aluminum melt at850℃. In themeantime, high intensity ultrasonic vibration was applied into the melt to disperse in situformed particles and degas the melt as well. It was found that in situ formed Al₃Tiparticles were blocky in morphology, the size of which was smaller than10μm, and mostof which were in the range of2-7μm. Moreover, in situ formed TiC particles were smallerthan1μm. Owing to the effect of ultrasonic vibration, the issues such as clustering ofparticles and oxide inclusions could be eliminated effectively; the porosity level in thecomposites was lower than1.0%.Using Ti powders as the reactants, adding which into the aluminum melt at lowtemperature, blocky Al₃Ti particles with small sizes could be formed in the melt via thedirect reaction between solid Ti powders and liquid Al. A diffusion reaction-peeling modelwas suggested to illustrate the formation mechanism of Al₃Ti particles. It was found thatthe size of in situ formed Al₃Ti particles was closely related to the temperature of the melt.With the temperature increasing, the size of particles became larger. When thetemperatures of Al melt were730℃and780℃, the sizes of Al₃Ti particles were in therange of1-6μm and1-10μm, respectively. Furthermore, the lower temperature of the melt,the greater effect of ultrasonic vibration on the melt directs reaction. When thetemperature was730℃, the amount of Al₃Ti particles with the size of2-3μm was largestin the ultrasonic treated sample, which became the size of3-4μm in the sample withoutultrasonic treatment. When the temperature was780℃, the effect of ultrasonic vibrationon the reaction was not obvious, for the size distributions of the Al₃Ti particles in thesamples with and without ultrasonic treatment were almost the same.Based on the result from the fabrication of in situ Al₃Tip/Al composites, Al₃Tip/A356composites were produced by using high intensity ultrasound assisted in situ technique. It was found that the growth rate of Al₃Ti grains was decreased due to the silicon element inthe melt. The existence of in situ Al₃Ti particles could change α-Al crystalline morphologyand size obviously. After T6heat treatment, with the amount of Al₃Ti particles increasing,the form of α-Al crystal in A356transferred from columnar dendrite to equiaxial dendrite.When the amount of Al₃Ti was5wt.%, the form of α-Al in the matrix was typicalequiaxial dendrite. The size of which was in the range of100-150μm in diameter. Whenthe amount of Al₃Ti was10wt.%, more-or-less spherical shape of α-Al was obtained. Thesize of which was in the range of50-100μm in diameter. In situ Al₃Ti particles couldimprove the mechanical properties of A356obviously, such as Brinell hardness and tensileproperties.