Numerical Simulation During Aluminum Twin-roll Casting And Influence Of Technology Parameter On Microstructural Of Purity Aluminum Strip

In recent ten years, there has been growing interest in the research of twin-roll rapid casting of thin-strip near end product shape in the world, which is thought to be one of the most potential technologies in the metallurgy and material field. The process has many advantages over the conventional casting technique. The primary advantages are of enhancing production efficiency, saving energy and reducing production cost, at the same time, it is potential to improve quality of casting strip. However, its practical application in industry is still hindered because some key technology problems haven't been solved. In order to industrialize the technology, it is very necessary to research on theory of twin-roll casting and mechanism of microstructural evolution during casting process.Based on analysis on the characteristic of the heat transfer during the casting process, one-dimension transient heat transfer mathematics model was established to predict the shape of solid shell and temperature field of the melt pool. Solidifying dynamics condition at the solid-liquid interface was considered for first time in the presented model. On the basis of model, effect of roll material and technology parameters (such as casting speed, pouring temperature, size of casting roll and surroundings temperature) on solidifying rate is investigated systematically. The numerical simulation shows that the main factors influencing on solidify-cation are the thermophysical property of roll ring, casting speed and roll diameter. The conclusion provides theory basis for controlling of the casting process. At the same time, the model provides help to understand further solidification behavior of liquid metal in the freezing zone.Due to the characteristic of pure aluminum deformation with visco-plasticity in high temperature, along with the theory of fluid mechanics, a approximate analytical mathematic model has been developed to calculate the velocity and strain of metal fluid in the deformed zone. It is helpful to know the law of the metal flow, to calculate flow stress, and to understand mechanism of microstructural evolution during pure aluminumcasting.A large number of experiments have been carried out using laborator-scale and industrial twin-roll casters for producing pure aluminum strips of 2-6mm thickness. The structures of pure aluminum strips were investigated by means of metallographic microscope and TEM, and its mechanical properties were evaluated by tensile and drawing test. The conclusions are as follows.1. To steel or copper roll, the solidification structure of aluminum strip is consists of coarse columnar grains with growth direction normal to the surface of roll at lower casting speed; with increasing casting speed and reducing gauge of the strips, solidification behavior of pure aluminum turns from columnar grain to equiaxed grain, and the grain becomes smaller; so as to the thin aluminum strip reveals a much finer equiaxed structure , a good mechanical and deep-drawing property.2. By adding Al-Ti-B grain refiner in the melt, equiaxed grains are observed from the surface towards the center of the strip under various casting speed, the grains in the thin strip are smaller than ones in the thick strip.3. Owing to outer friction and geometry factor of the casting zone, deformation through the thickness is quite inhomogeneous during the thin-strip casting process, surface layer metal deformed heavy than the center of strips. This leads to the inhomogeneous of substructure and recrystallization across the strip thickness, that is, incomplete recrystallization is occurred in the surface layer metal of strips.In this research, solidification behavior of pure aluminum melt during the casting process has been studied systematically. It is indicated that heterogeneous nucleation on the surface of the roll is the main pattern of nucleation when grain refiner isn't added in the melt, whereas nucleation rate is correlated with surface structure and cooling rate of roll; Due to copper having the same crystal lattice and proximate lattice parameter with aluminum, the crystal nuclei is easier to be generated on surface of copper roll, so that the fine grains are formed in the strip compared to steel roll; Then the solidification structure of pure aluminum stripdepends on not only nucleation ability on the surface of roll, but also outer field related to technology factors .Owing to the metal fluid flowing steadily in the melt pool at lower casting speed, the nuclei are difficult to separate from the surface of roll, then developed rapid into solidified shell, as a consequence, solidification is apparently completed by columnar growth. With increasing casting speed, the temperature gradient in the front of melt/solid interface increases, the interface become more steadier, curvature radius of the columnar head increases, so as to developed into segregate face in center of the strips; As casting speed is increased further, flow of metal fluid is changed from laminar flow to turbulence, a circulation zone is formed in the melt pool, the temperature distribution in the circulation zone becomes disorder, this provides environment condition for the nuclei divorcing from the surface of roll and the nuclei growing in form of equiaxed grain in the circulation zone; In addition, the undercooling region in front of the melt/solid shrinks when casting temperature of the melt is high, superheat region in the center of the melt enlarges respectively, it isn't helpful to be suited to equiaxed grain growth.Combined with the known temperature field and strain rate field, the hot-compression experiment of pure aluminum has been carried out on Gleeble 1500 machine, the microstructures of the deformed specimens is similar to the casting aluminum strip with the same thermomechanical parameters. Base on experiment data, the flow stress equation was modeled, and microstructural evolution mechanism of pure aluminum has been studied during hot deformation. The study shows that dynamic recrystallization depends on Zener-Hollomon parameter which is correlated with strain rate and temperature, the critical condition that dynamic recrystallization is generated in pure aluminum strip during casting process is inZ > 26.54.