Technical And Theoretical Research Of Semi-solid Al-30%Si Slurry Prepared By Low Superheat Pouring Low Intensity Electromagnetism Stirring

Technical and Theoretical Research of Semi-solid Al-30%Si Slurry Prepared by Low Superheat Pouring Low Intensity Electromagnetism StirringAbstractThis text aimed at hypereutectic Al-30%Si alloy with high content Si, semi-solid slurry of which was prepared by low superheat pouring and low intensity electromagnetism stirring. This is an exploring job to extend application field of Al-Si alloy material. The method preparing semi-solid slurry in this text can't only fully exert the technical advantages of two techniques, but also can economically prepare ideal semi-solid slurry. So it is provided with certain practical value. Because it is fresh to prepare semi-solid slurry by low superheat pouring and low intensity electromagnetism stirring and there is no report on related theory research, it is of great significance to material exploitation and production application to lucubrate systemically process and theory.Based on the slurry preparing assembly and rheometer designed and factured by the author, this text studied the experiment process of Al-30%Si alloy poured at low superheat temperature, preparation process of Al-30%Si alloy slurry poured at low superheat temperature and stirred with low intensity electromagnetism, and the transient rheological behavior, steady state rheological behavior and thixotropism properties of hypereutectic Al-30%Si alloy. The theory of low superheat pouring and low intensity electromagnetism stirring was studied in the general solidification theory. Based on the semi-solid slurry microstructure equation and viscosity model modeled in poly-dispersions, the rheological behavior of semi-solid Al-30%Si alloy was calculated and analyzed in theory by the numbers. Then the following primary conclusions were received:1) The structure grain size of semi-solid Al-30%Si alloy turned to finer as a result of the pouring temperature approached gradually to the liquidus line and the superheat minished, and distributed more equably in the substrate with a shape of nearly rotundity. Diminishing the grain and controlling the growth of primary Si during low superheat pouring incarnated mostly in changing of melt characteristic, nucleation and growth.2) The relative perfect semi-solid Al-30%Si alloy slurry could be prepared by low superheat pouring and low intensity electromagnetism stirring, which improved the figure, the size and the distributing of primary a lot. So the synthetical application properties of slurry heightened. Pouring temperature, stirring power and time affected the slurry structure and properties. In the condition of the lab, both good structure and properties semi-solid slurry was prepared when the pouring temperature was 840℃and stirred by 4．8kW for 20s～25s.3) This text brought forward that the melt flowed mostly along the tangent whensuffered from electromagnetism stirring, and corresponding movement equation including interfacial concentration distributing, concentration gradient and effective partition coefficients was calculated. The result indicated that this model could reflect more about the influence of melt agitation.4) Primary Si in hypereutectic Al-Si alloy melt stirred by low intensity electromagnetism power moved generally with dauby liquid. Mechanical force, restraining anisotropic growth and aging effect will accelerate primary Si to be fine or rotundity. Integrating the low intensity electromagnetism process, the absolute and relative stability criterion of spherical surface growth was derived. Using relative stability parameter(S), calculated result indicated that the relative instability radius could arrived at 2000 times of critical core formation radius of spherical grain, which assured that spherical grains could reversely grow steadily to a bigger size.5) The steady state and transient rheological behavior of semi-solid Al-30%Si alloy were studied by experiment. The result indicated that the effect of solid fraction, shear rate and cooling rate on slurry viscosity. The thixotropism behavior studying indicated that the of the steady state apparent viscosity of hypereutectic Al-Si alloy depended a little upon time, and mechanical shearing dilution on semi-solid hypereutectic Al-Si alloy weakened a lot,too.6) Microstructure and viscosity models of Semi-solid slurry were modeled based onpoly-dispersions. Specific expressions of microstructure parameters and viscosity being the same with semi-solid Al-30%Si alloy slurry were concluded.7) The calculated result of the viscosity of semi-solid Al-30%Si alloy slurry based on the model matched well with experimental result. During the isothermal and continuous cooling stirring, the effect of solid fraction, shear rate and cooling rate on the apparent viscosity of semi-solid Al-30%Si alloy posts the rheological property of slurry, which will provide theoretic direction for preparation of semi-solid hypereutectic Al-30%Si alloy slurry.