Study On The Technology And Theory Of Air Film Continuous Casting Of Al Alloys

This project is a part work of "Foundational research on the high quality aluminum alloy products and high efficient use of aluminum resource", which is supported by the Key Fundamental Research Program of China (973 ) . This work is aimed to study the effects of air film and electromagnetic field on ingot quality, and to develop an economical casting technology of aluminum alloy with high surface quality.The work in this thesis includes that effects of air pressure on the shape and size of meniscus, effects of effective cooling length and cooling mode on surface quality and microstructure of ingots, effects of casting parameters on surface quality and microstructure in air film casting (AFC) process, effects of static magnetic field on surface quality and microstructure of ingots, effects of low frequency electromagnetic field (LFEF) on surface quality and microstructure of AFC ingots, effects of flow divider and air film on the temperature field and surface of shaped-blank. The main results are presented as follows:The relationship between meniscus shape, height and air pressure was established in AFC process using Laplace equation. It is found that the meniscus shape, size and stability are decided by air pressure. An appropriate gas pressure can separate the melt from the inner mold, which decreases the first cooling intensity of ingot significantly, and thus improves the ingot surface quality.In AFC process, the primary cooling intensity is very low, so that the ingot is cooled mainly by the second cooling water. According to the results of heat transfer calculation, a good surface quality can be gained by adjusting the upstream conduction distance (UCD) caused by the reverse heat transfer by second cooling, and the height of meniscus, which obeys the following equation:B (the height of meniscus) + UCD = D(the effective cooling length of the mold) When B+UCDD, the meniscus overlaps with UCD, the shell can extend into the meniscus, so that the cold shuts will occur.Effects of hot top shape and secondary cooling mode on the ingot surface quality during AFC were detailed investigated. The funnel-shaped hot top and double-layers water cooling can ensure the AFC process. A new AFC mold was designed to produce high surface quality ingots. Effects of casting parameters, such as casting temperature, casting speed, cooling water, air flow, lubricants and gas type, etc., on surface quality, microstructure, and macrosegregation ofΦ174mm aluminum alloy ingots using the new mold were studied. When the casting temperature, casting speed, the flux of cooling water, air flow and oil flow are 710℃, 120mm/min, 0.07m~3/min, 21/min and 6ml/min, respectively, a smooth ingot surface of 6063 alloy ingot is obtained with few cold shuts and thin surface segregations about 100μm, which is same as the ingots using air-slip casting with porous graphite ring.Effects of static magnetic field on the surface quality and microstructure of AFC ingots were investigated also. It is well known that when electro-conducting melt moves cross the static magnetic field, flux variations are generated which leads to the local electro-motive forces. The interaction of induced electric currents with the applied static magnetic field gives rise to Lorentz force in the direction against melt flow, which brakes the flow of melt. Thus, the magnetic field can effectively stabilize meniscus by suppressing the melt flow which brushes meniscus. All these conditions are beneficial to increase the casting speed of AFC. The casting speed of AFCΦ174mm 6063 alloy is increased by 50% to 180mm/min, and the ingot surface is smooth with few casting defects when 100000At static electromagnetic field was applied.Effects of LFEF on the surface quality and microstructure of the AFC ingots were investigated. Both the flow field and temperature field are changed significantly in LFEF casting. The melt flow direction is reverse to that without LFEF, the melt flow speed increases obviously, and the temperature field also changes evidently. LFEF influenced the stability of meniscus.However,increasing the frequency could decrease this impact. When the magnetic frequency is 30Hz, a good surface ingot is gained with few casting defects.A new technology of air-film casting for shaped-blank (triangle ingot) was developed due to low first cooling intensity in AFC. It is found that the meniscus could be stabilized with applied air-film, so the contact height of the melt with mold could be reduced effectively. The uneven cooling intensity by first cooling which causes corner bending is eliminated. The friction between shell and mould is reduced and the scratches and cracks on surface disappear, so that the smooth surface of ingots can be obtained. The optimum casting parameters of triangle ingots using air film casting are that the casting temperature and velocity are 730℃and 120mm/min, respectively, and the flux of cooling water, air flow and oil flow are 0.05 m~3/min, 2L/min and 5ml/min, respectively. Under the optimal conditions the ingot surface is smooth and has no cold shuts and surface segregations. The subsurface segregation zone of the ingot is thin about 100μm, and the as-cast microstructures are fine and uniform.