Study On Continuous Casting Of Aluminum Alloys Under Low-frequency Electromagnetic Field

This research is supported by the program of quality improvement of aluminum products, which is a part work of Major State Basic Research Projects of China. The ultimate aim is to develop a highly efficient technique by which the quality of the continuous casting ingot of aluminum alloys can be substantially improved. Influences of low-frequency electromagnetic field on solidification process of aluminum alloys are experimentally and theoretically studied in this paper. For the first time, make a point that the low-frequency electromagnetic field can effectively promote the solution of alloying elements, and refine the microstructures. A new technique for continuous casting of aluminum alloys by application of low-frequency electromagnetic field is developed, by which 7075 aluminum alloy ingot is produced, the fine equiaxed as-cast structures is obtained and surface quality is remarkably improved. The magnetohydrodynamic equations describing the behavior of interdendritic liquid in directional solidification process of aluminum alloy in the presence of low-frequency electromagnetic field is achieved. The equations indicate the existence of eigenfrequency determined by dendritic arm spacing and temperature gradient. When oscillatory component of Lorentz force in the range of eigenfrequency, resonance effect occurs and local compositional convection of interdendritic liquid is greatly enhanced. The effect of low-frequency electromagnetic field on solute distribution in directionally solidified Al-Cu binary alloy is investigated experimentally. It is found that excluding the influence of rotary component of Lorentz force, electromagnetic field substantially modified the distribution of alloying element, when frequency is in the range of 10 Hz-15 Hz, solution of alloying element in matrix is greatly promoted, and fine dispersed precipitates is obtained. The results reveal that the resonance effect plays a significant role in interaction between Lorentz force and compositional convection of interdendritic liquid, adopting eigenfrequency, the electromagnetic field with relative low intensity can effectively improve the microstructure of materials. Effect of electromagnetic field on flow pattern, temperature field and distribution of thermal stress in continuous casting process of aluminum alloy is numerically studied. The results show that the distribution of magnetic flux density vector in the air and melt is determined by density and frequency of coil current, shapes and locations of mold and melt. Intensity of magnetic field in the melt and ingot rapidly declines with increase of depth because of skin effect, and cannot be efficiently strengthened by increase the intensity of coil current. On the other hand, frequency greatly influences the pattern of magnetic flux density in melt and ingot. Alternating current generates a time varying magnetic field in the melt, which, in turn, gives rise to an induced current in melt and ingot. Therefore, the melt is subjected to electromagnetic body forces caused by the interaction of the induced current and the magnetic field. Under the effect of Lorentz force, the ideal flow pattern in the melt will be achieved, when the overheated melt can be driven from center region of the melt to the periphery more effective, and the depth of the sump will be decreased under the same casting speed. When frequency is higher than 50 Hz, vortex core position of forced convection mainly near the periphery of ingot, intensity of convection at center region or melt is relatively weak, and cannot be enhanced by increase of the intensity of coil current. Under the certain intensity of electromagnetic field, the flow pattern of the melt and therefore, the temperature field in the sump can be effectively modified by application of various frequencies, and whenfrequency reach the range of 25 Hz to 30 Hz, minimum depth of the sump is achieved. Thermal stresses in solidifying ingot present complex situation, because of reduction of temperature gradient and the decrease of sump depth in the presence of electromagnetic field, the stress intensity substantially decreases at the same time, the crack possibility greatly reduces accordingly. 7075 aluminum alloy ingot with diameter of 100 mm and 200 mm respectively are prepared by application of newly developed electromagnetic casting technique. Constrained effect of electromagnetic force results in the formation of a convex surface meniscus, and reduces the height of contact line between the mold and the melt, which, in turn, reduces the primary cooling intensity. Electromagnetic forced convection carries the detached side branch into the melt, broadens the mushy zone and promote the heterogeneous nucleation, decreases the height and the depth of the sump, reduced the temperature gradient. Low-frequency electromagnetic field is more effective in grain refinement and, at the same time significantly promotes the solution of alloying elements. When frequency is in the range of 15 to 20 Hz, fine-grained equiaxed microstructure is obtained. Frequency, as a principal parameter in electromagnetic casting process, greatly influences large-scale distribution of alloying elements. When frequency of alternating current reaches 30 Hz, surface exudation and center region segregation are inhibited effectively; distribution of alloying elements over the cross-section of ingot is relatively homogeneous. The result reveals that this new technique is a significant advancement for those aluminum alloys that are difficult to be...