| With increasingly application in the aerospace, automotive, high-speed trains, electronics and other fields, aluminum alloys with higher properties are required. Compounding is another important strengthening method expect for alloying. In-situ particle reinforced aluminum matrix composites due to its own merit, have become a hot research in the composites field. How to control the size, morphology and distribution of in-situ reinforced particles in the matrix is the key techniques in the preparation of composites and also is currently one of the bottlenecks for industrialization application. Therefore, the control of the microstructure and properties of the composites by selecting the reaction systems and optimizing processing conditons have important theoretical and practical significance. The frame of the paper is based on the following three ideas:1. refine the grain of the matrix alloy in order to improve its comprehensive properties by adding rare earth yttrium; 2. control the microstructure of the composites, including the size, distribution and morphology of the particles by optimizing the reaction system or applying ultrasonic and magnetic fields; 3. achieve high strength and high englation by combining with above-mentioned methods.The effects of rare earth yttrium on mircrostructure of 7055A1,6070A1 matrix alloys were investigated. The results show that the addition of the rare earth yttrium can effectively refine the grains and the optimal addition amount of rare earth is 0.25% (mass fraction). When the yttrium addition is more than 0.25%, grain coarsening takes place. Moreover, yttrium-containing compound also coarsen and the morphology become long needle-like.The process optimization for Al-K2TiF6, Al-K2ZrF6-Na2B4O7 and Al-K2TiF6-KBF4 component were studied, and Al3Ti/6070Al, A12O3/Al, TiB2/7055Al composites were fabricated. The results show that the reasonable process for Al-K2TiF6 component are synthesis first then alloying with Mg, reaction time less than 10 min and temperature 850℃. For Al-K2ZrF6-Na2B4O7 component, the proper reaction temperature is 850℃, and its reaction mechanism is reaction-solution-precipitation. For Al-K2TiF6-KBF4 component, the best process are alloying with Mg first then synthesis, reaction temperature 850℃and reaction time less than 20 min.Al3Ti/6070Al, Al2O3/Al, TiB2/7055Al composites were successfully prepared under ultrasonic field. The experiment results show that as the ultrasonic power and reaction time increase, the number of particles in Al3Ti/6070Al and Al2O3/Al composites is significantly reduced, which can be explained by standing wave mechanism and cavitation mechanism. For Al3Ti/6070Al composites, the optimal ultrasonic power is 1.6 kW, and the best reaction time is 3 min; The Al3Ti particles size is reduced from 2-5μm to about 1μm, and the morphology becomes smaller and granular; For Al2O3/Al composites, the optimal ultrasonic power and reaction time are 0.60 kW,2 min, respectively; The Al2O3 recoveries of the composites can be significantly improved under ultrasonic field. The preparation of TiB2/7055Al composite process mechanism was studied by water quenching. The whole reaction process can be divided into four stages, and the second stage of the reaction process under the ultrasonic field has played a significant role in reaction promoting. TiB2 particles gradually refined with the increase of ultrasonic power. Especially, when the ultrasonic power reaches 1.6 kW, a large number of nano-scale TiB2 particles are observed.The preparations of Al3Ti/6070Al, Al2O3/Al, TiB2/7055Al composites under low frequency magnetic field were studied. The results indicate that for Al3Ti/6070Al composites, two different type composites including uniform distribution and gradient distribution can be obtained with the change of excitation current and frequency. When the excitation current is 200 A, the frequency is less than 5Hz, the reaction time is 3-5 min, small sized particles and uniform distribution Al3Ti/6070Al composites can be prepared; meanwhile, gradient distribution composites can be prepared by increasing frequency or current and correspondingly reducing reaction time. Based on the above experiments, it can be conjectured that secondary flow exists in the melt while preparing composites by the continuous action mode under magnetic field. Through water quenching in the intermittent action mode, the process mechanism of TiB2/ 7055Al composites under low-frequency magnetic stirring was studied. The in situ reaction is accelerated at the early stage with assistance of magnetic filed. When the frequency is small (<5 Hz), the size of TiB2 particle size has no significant change. As long as the frequency is over 5Hz, the size of particle tends to be the same, and morphology is passivated.The Al3Ti/6070Al and TiB2/7055Al were synthesized under coupled field by using the optimized parameters from single field. The results show that the Al3Ti particles have tiny sizes and well distributed under the coupled field. To be more precisely, when the power of ultrasonic is 1.6 kW, the size of Al3Ti in range of 0.2-0.5μm is about 72.4% to total number. At the same time, nonetheless, the prercentage of particles in range of 0.8-1.2μm have a considerablely decrease to about 1.6%. The size of Al3Ti grows larger as the amount of powder addition increases. Further, morphologies of the particles are changed from small block and granular to large block, piece or short rodlike with assistance of coupled field. When TiB2/7055Al composite fabricated under the coupled field with continuously action mode, a large number of nanoscale TiB2 particles can be observed with 0.8 kW of ultrasonic. The size of inhomogeneous distribution particles is in range of 80-100 nm. While TiB2/7055Al fabricated under the coupled field with intermittently action mode, submicron scale TiB2 particles with uniformly distributed can be obtained.The mechanical properties of 7055A1 alloy show that the tensile strength and hardness are decreased with increase of yttrium addition, while elongation is increased. The tensile strength and elongation of 6070A1 are increased at first then decreased with increase of yttrium additon. The tensile strength and elongation of Al2O3/Al composites synthesized under ultrasonic field is increased and then decreased with increase of ultrasonic power and action time, and their optimal parameter are 0.6 kW,2 min, respectively. The tensile properties of Al3Ti/6070Al,TiB2/7055Al composites fabricated under coupled field are better than that single field. The decreased elongation of composites is significantly restrained by interaction of coupled field and rare earth with increase of volume fraction.
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