| Aluminium alloys have low density, high specific strength, good formability, and can be reclaimed and reused easily, which are used widely in the national economy. As a kind of A1-Si-Cu aluminum casting alloy, ADC12 has a broaden application in automobile industry because of its superior performance. However, coarse α-Al and Si phases of the microstructure can reduce properties of materials to some extent. The study found that some in-situ reinforced phases(Al2O3, Al3 Ti particles, et al) can refine microstructures of Al-Si alloys, which can improve different properties of materials. In this paper, ADC12 aluminum matrix composites were synthesized by combined in-situ technology with high-energy ultrasonic treatment in reaction systems of Al-B2O3, Al-K2TiF6, Al-Ti. It results from acoustic cavitation and acoustic streaming effect induced by ultrasonic in the melt, which can refine particles, improve the wettability between particles and melt, and enforce the particles dispersing uniformly in the melt. Thus improving comprehensive properties of composites, meeting the requirement of composites. Besides, Semi-solid slurry of in-situ Al3Ti/ADC12 aluminum matrix composite in the system of Al-K2TiF6 was investigated by electromagnetic stirring. It results from mixed convective flow induced by electromagnetic stirring in the melt, which can improve crystallization solidification of crystals, then getting good semi-solid slurry of composites. The main work and results are as follows.In-situ Al2O3/ADC12 aluminum matrix composite was synthesized by combining melt direct reaction method with ultrasonic treatment in the system of Al-B2O3. The results of the microstructure study show that synthetic in-situ Al2O3 reinforced particles under the ultrasonic function are round, fine, and well-distributed in the matrix. Morphologies of composites have a certain degree of improvement than those of the matrix. The morphologies of Si phases have been changed into rod-like shape, particle-like shape from original bulk-like shape, long acicular-like shape, strip-like shape gradually. Si phases of rod-like shape are in the majority of the composite without ultrasonic treatment, those of particle-like shape are few. Si phases of rod-like shape in the composite with ultrasonic treatment are decreased, and fine, those of particle-like shape are increased. The results of the mechanical property test show that the tensile strength and the elongation increase by 13.5% and 25.2% than those without ultrasonic assistance, respectively.In-situ Al3Ti/ADC12 aluminum matrix composite was synthesized by combining melt direct reaction method with ultrasonic treatment in the system of Al-K2TiF6. The results of the microstructure study show that ultrasonic-chemistry reaction in the melt is much faster and more complete than traditional in-situ reaction, and synthetic Al3 Ti reinforced particles are regular, fine, the average size is 1~2μm, and well-distributed in the matrix. The cast microstructures of in-situ composites have been improved than those of the matrix. The outline of α-Al phases is more clear, the size is refined gradually, the morphology is tend to be more rounded. The morphologies of Si phases have been changed into short rod-like shape, particle-like shape from original long acicular-like shape, strip-like shape gradually. Crystal grains of the composite without ultrasonic assistance have been refined to some extent, Si phases of biggish particle-like shape are in the majority, Si phases of short rod-like shape are in the minority. Crystal grains of the composite with double-ultrasound treatment have been refined largely, the morphologies of Si phases have been changed into smaller particle-like shape, even some are nemaline. The results of the mechanical property test show that the tensile strength and the elongation reach 255.2MPa and 2.18%, increasing by 19.4% and 33.7% than those without ultrasonic assistance, respectively. The fracture morphologies indicate that the cleavage surfaces are lessened, and the dimples are increased in quantity, diminished in size, deepened in the composite synthesized under ultrasonic. The characteristic of ductile fracture is obvious.Semi-solid slurry of in-situ Al3Ti/ADC12 aluminum matrix composite was prepared by electromagnetic stirring in the system of Al-K2TiF6. Effects of different stirring parameters on semi-solid structure of the composite were investigated. The evolution mechanism of aluminum matrix composites semi-solid microstructure in the electromagnetic field was revealed. The results show that the morphologies of primary α-Al phases in semi-solid microstructure of composites have multifarious transformation with the increasing of stirring time during hypothermia stirring. The morphologies of α-Al particles are confirmed to evolution into dendritic, roseform or near-spherical. when the temperature of water quenching is 585℃, semi-solid microstructure of the sample is better, primary α-Al phases become mainly near-spherical shape, well-suspended in the liquid phase, the average diameter is 35.2μm. In an appropriate range, crystal grains are tended to be round and fine with the accumulating of stirring voltage if stirring frequency and stirring time are fixed during hypothermia stirring. The average diameter of particles reduces at first and then beginning to increase with the accumulating of stirring frequency if stirring voltage and stirring time are fixed during hypothermia stirring.In-situ Al3Ti/ADC12 aluminum matrix composite was synthesized in the system of Al-Ti. The results of the test show that Al3 Ti particles of the composite have become dispersive shape from original bond shape, conglobate shape gradually, and the average size is refined. The cooling way can have a big effect on the morphology of Al3 Ti phases in the composite. It will be lumpish or granulated after being cooled quickly. Then it will be needlelike or banded after being cooled slowly. Al3 Ti particles of the composite with ultrasonic treatment can have a great change. In an appropriate range, Al3 Ti particles will be finer, more homogeneous with the increasing of ultrasonic power if ultrasonic time, ultrasonic frequency and Ti addition are fixed. Al3 Ti particles will reunite and grow up with the increasing of Ti addition if ultrasonic power, ultrasonic time and ultrasonic frequency are fixed. Ultrasonic effect will be strengthened and then weakened with the increasing of ultrasonic time if ultrasonic power, ultrasonic frequency and Ti addition are fixed. In addition, the tensile strength of in-situ Al3Ti/ADC12 aluminum matrix composite shows a rising tendency, and the varying tendency of mechanical properties is consistent with that of microstructures. The comprehensive properties of the composite is better when Ti addition is 3%, ultrasonic power is 1.5kw, ultrasonic frequency is 20 kHz, ultrasonic time is 6min. Its tensile strength and elongation reach 247.34 MPa and 2.31%, increasing by 21.3% and 50.0% than those without ultrasonic assistance, respectively. Besides, addition of Ti can also refine α-Al phases of the matrix, changing the morphology from coarse dendritic to tiny dendritic, even roseform or near-spherical. |
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