Heat Transport During Low Frequency Electromagnetic Casting Of Aluminum Alloys

This subject is part work of "The energy transport in large size ingot and formation and control of its macroscopic and microscopic defects", which is the subject of "The fundamental research of high property aluminium and high efficiency utilization of aluminium resources" supported by the Key Foundational Research Program of China (973). The aim is to study study the fluid flow and temperature field in the liquid pool during the DC casting of aluminum alloys, the effects of low frequency electromagnetic field on heat transport during the solidification process, the effects of low frequency electromagnetic field on ingot surface quality and solidification structure of the ingot, and provide theoretical and experimental evidence for establishing technological parameters of industrial run of large size aluminum alloys ingots.In this study, a mathematic model was built to simulate DC and low frequency electromagnetic casting processes, and the mathematic model was verified by experimental results. The effects of low frequency electromagnetic field on the fluid flow and heat transport were studied by numerical simulation during the DC casting of aluminum alloys process, and the effects of low frequency electromagnetic field on the ingot surface quality (cold fold, exudation on the surface and the thickness of segregation layer) and solidification structure of the ingot were studied by the experiments.The temperature of the graphite ring and ingot during the hot-top and low frequency electromagnetic hot-top casting ofΦ200mm 6063 aluminum alloy was measured, and the effects of low frequency electromagnetic field on heat transport of ingot surface in the primary cooling of mold and secondary cooling of water regions during the hot-top and low frequency electromagnetic hot-top casting processes are analyzed. The results showed that when the low frequency electromagnetic field was applied during the steady state of conventional hot-top casting process, the heat transport near the primary cooling zone of mold and solidification front of ingot surface was intensified, the height of effective contact zone between the melt and the primary cooling zone of mold was reduced, and the initial solidification point on the wall of the mold moved up; the depth of sump decreased; the largest heat flux and the temperature gradient in casting direction on the ingot surface decreased; the temperature field in the ingot was modified, so the stress in the ingot was reduced. When the low frequency electromagnetic field was applied during the start-up phase of conventional hot-top casting process, the heat transport between the ingot, the mold and the starting block was intensified greatly, the temperature field in the ingot became more uniform; the sump during the start-up phase of low frequency electromagnetic casting was very flat, which is caused by the forced convection, the speed of sump becoming deeper was reduced, and the thermal stress in the ingot decreased.Numerical simulation was made to study the hot-top and low frequency electromagnetic hot-top casting of 6063 aluminum alloy ingot with a diameter of 200mm. The effects of low frequency electromagnetic field on the temperature profile, fluid flow, heat transport, upstream conduction distance (UCD), and mold along length (MAL) were studied. The results showed that the temperature profile of the melt pool was nonuniform during the conventional hot-top casting process, when the low frequency electromagnetic field was applied, the temperature profile of the melt pool became uniform, and the temperature was near the liquidus temperature of 6063 aluminum alloy; near the ingot surface, the low frequency electromagnetic field can intensify the fluid flow, made the flow and thermal boundary layer on the liquid/solid interface thinner, and cause the temperature gradient on liquid/solid interface larger; near the ingot center, the effect of low frequency electromagnetic field on the velocity of melt flow is not obvious, the difference of the flow and thermal boundary layer on the liquid/solid interface during the conventional and low frequency electromagnetic hot-top casting process is not obvious. During the conventional hot-top casting process, the velocity of the melt flow at the inlet is fast, the melt from the inlet with higher temperature flows to the bottom of sump directly, so the temperature gradient on the liquid/solid interface near the ingot center in the conventional hot-top casting process is a little higher than that in the low frequency electromagnetic hot-top casting process.During the conventional hot-top casting of 6063 aluminum alloy with a diameter of 200mm process, when the low frequency electromagnetic field is applied, the upstream conduction distance caused by the secondary cooling water increases; when the casting speed is constant, UCD increases with the increase of current frequency and intensity; The effect of low frequency electromagnetic field enlarging the UCD is obvious when the casting speed is slow; the effect of low frequency electromagnetic field enlarging the UCD is not obvious when the casting speed is high. Additionally, when the low frequency electromagnetic field is applied during the conventional hot-top casting process the mold along length (MAL) can be changed; the MAL increases when the casting speed is high, and the MAL decreases when the casting speed is slow; when the casting speed is constant, MAL increases with the increase of current frequency and intensity; the effect of low frequency electromagnetic field enlarging the MAL is not obvious when the casting speed is slow, but the effect of low frequency electromagnetic field enlarging the UCD is obvious when the casting speed is high.The experiment and numerical simulation were made to study the effects of low frequency electromagnetic field on the process of conventional level pour casting of Al-4.5wt%Cu aluminum alloy with a diameter of 200mm. The results show that for Al-4.5wt%Cu aluminum alloy with a broad two-phase region(about 100 degrees centigrade), the effects of low frequency electromagnetic field promoting the evacuation of superheat and making the temperature profile in the liquid pool more homogeneous are also obvious. During the conventional level pour casting process, the fluid flow in the liquid pool is slow, which is mastered by inertia and thermal buoyancy of the liquid, and the temperature profile of the melt in the liquid pool is layered, the temperature of the melt is high in the top of refractory, and the temperature of the melt is low in the base of refractory. During the low frequency electromagnetic level pour casting process, the melt in the liquid pool is driven by the electromagnetic force, the fluid flow is intensified, the distributary of melt in the refractory on the horizontal plane is more reasonable, there is a strong eddy flow in the refractory on the vertical plane, and the temperature profile of melt in the liquid pool is very homogeneous.The effects of low frequency electromagnetic field on the ingot surface quality and solidification structure were studied by experiments and the reasons of the effects were analyzed. The results show that when low frequency electromagnetic field is applied during the conventional hot-top casting of 7050 aluminum alloy with a diameter of 500mm, the size of meniscus under the overhang is reduced, the cold fold near the ingot surface is eliminated, the length of effective contact zone between the melt and the primary cooling of mold is reduced, and the thickness of shell zone near the ingot surface is reduced; when low frequency electromagnetic field is applied during the conventional hot-top casting of 6063 aluminum alloy with a diameter of 200mm. the macrostructure and microstructure of the ingot border and center are refined greatly; when low frequency electromagnetic field is applied during the conventional level pour casting of Al-4.5wt%Cu aluminum alloy with a diameter of 200mm, the number of exudations on the ingot surface is decreased, the thickness of segregation layer on the ingot surface is reduced, and the surface quality of ingot is improved.