Research Of External Fields On The Movement Behavior Of Heterogeneous Particles And The Solidification Of Aluminium Melts

Aluminium is the second largest metal material in the world and has been widly used in mobile, vehicle, food, electricity, transportation, aerospace, mechanical and other new styles of production owing to its virtues of small density, high specific strength, corrosion resistance and easy recovery. Well, in the production of aluminium alloys, the problems such as gas porosity, fine impurities hinder its improvement a lot. In addition, the poor surface condition, segregation of alloy elements and coarse microstructures also can't meet the demand of modern industry. Owing to the development of electromagnetic field and ultrasonic field on the metallurgy and materials processing, it's a good opportunity to find a new clean and high efficiency method to produce higher quality aluminum alloy.The aim of this thesis is to find a new method to produce aluminum casting billets with higher property through removing gas and impurity from the liquid metal and refining the solidification microstructures. Researches of high frequency magnetic field and power ultrasonic field on the movement behavior of gas and impurity heterogeneous particles are carried out. Based on the results, Al-Si graded material is successfully fabricated. In addition, the study of compound field (combination of electromagnetic field and power ultrasonic field) on the solidification process and the effects of compound field on the solidification microstructure, mechanical property are also studied. The mechanism of the solidification under compound field is also discussed theoretically according to the experimental results. At last, a new technique of continuous casting of Al-1%Si bonding wire under compound field is developed which is favorable for the improvement of surface quality, casting microstructure, element segregation and mechanical property. All the researches above lay a good foundation of both experiments and theory to the future use of electromagnetic field and ultrasonic field to the fabrication of high purity and high quality aluminum alloys.The results of the study are showed below:1. High frequency magnetic field is used to separate alumina particles and the results show that: to the alumina particles of 30-200μm with 0.04T magnetic field 1s, there are no particles gathering in the inner, when the separation time prolongs to 3s, there is nearly no particles more than 30μm in the inner; when the magnetic density increases to 0.06T, 1s is enough for the particle in the inner to migrate out.2. Power ultrasonic filed is used to separate heterogeneous particles in liquid, which shows that it is very efficient for purification of gas and impurity particles. After using 600W ultrasonic 120s, the gas porosity in vacuum reduced pressure A356 alloy samples is nearly eliminated. To alcohol water system, the polyethylene particle can be eliminated and the removal rate can reach to more than 90%. Taking primary Si in Al18%Si alloy as the mimics of alumina particles, ultrasonic field can cause the upwards migration of them, based on the results of this, Al-Si graded materials are fabricated.3. The study of compound field (combination of stirring magnetic field and power ultrasnic field) on the solidification of Sn-3.5%Pb and A356 are carried out and the results show that compound field can increase the beginning temperature of solidification, suppress element segregation, refine solidification microstructure and modify the morphology of eutectic silicon. The size of primary Al changes from 250μm to 30μm with the application of compound field and the tensile strength changes from 163MPa to 268MPa. On the one hand, ultrasonic field can generate numerous nuclei even when the temperature is above the liquidus, after that the nuclei are dispersed to the whole melt by the forced convention caused by acoustic streaming and electromagnetic stirring; on the other hand, the concentration and temperature fluctuation caused by ultrasonic and electromagnetic field is facilitated to the remelting of dendrities on the root, which can also refine the solidification microstructure.4. Compound field (combination of traveling magnetic field and power ultrasonic field) is applied on the continuous casting of Al-1%Si bonding wire and a new technique is obtained. The compound field can even the temperature field in the tundish, suppress Si element segregation, improve surface quality and refine microstructure thus improve the mechanical property of the casting billets. With the application of compound field, the tensile strength and elongation rate changes from 110MPa and 13% to 160MPa and 40%, respectively.