【作者】 屈福；

【导师】 崔建忠；

【作者基本信息】 东北大学 ， 材料加工工程， 2009， 博士

【摘要】 本课题是国家重点基础研究发展规划(973)资助项目“高性能铝材与铝资源高效利用的基础研究”中的部分内容,主要研究气膜及磁场对铝合金连铸坯坯质量的影响,旨在开发高表面质量,经济的铝合金连铸新技术。本文的研究内容包括:气体压力及磁场对弯月面形状的影响;结晶器有效冷却长度及结晶器冷却模式对铸锭表面质量及内部组织的影响;气膜连铸过程中工艺参数对铸锭表面质量及铸态组织的影响;静磁场对气膜连铸坯表面质量及内部组织的影响;低频磁场对气膜连铸坯表面质量及内部组织的影响;设计一种异形坯结晶器,研究了分流器及气膜对异形坯连铸时的温度场及铸锭质量的影响。取得的主要成果如下:首先根据拉普拉斯方程,建立了气膜连铸(Air film casting,简称AFC)过程中弯月面形状、高度与气体压力的关系。施加的气体压力决定弯月面的形状、尺寸和稳定性,适当的气体压力可以使熔体与结晶器内壁脱离,大幅度降低铸坯的一次冷却强度,为铸锭表面质量的提高奠定了基础。由于气膜连铸过程中一次冷却(结晶器壁)很小,铸锭的热量主要由二次冷却水带走。所以,根据热传导公式导出由于二次冷却水的逆向传热所产生的从二次冷却水击水点向上的凝固高度(UCD),发现UCD必须与弯月面高度良好配合,才能获得高表面质量的铸坯。配合原则是:B(弯月面高度)+UCD=D(结晶器的有效冷却长度)当B+UCD＜D时,弯月面与UCD之间有间距,导致液体金属与结晶器内壁直接接触,可能导致表面偏析瘤的产生;当B+UCD＞D时,弯月面与UCD重叠,凝壳伸入弯月面,可能导致表面冷隔的产生。详细考察了热顶形状和二次冷却方式对气膜连铸铸锭表面质量的影响,发现采用漏斗型热顶和双排水冷有利于保证气膜铸造的顺利进行。在此基础上,设计了可以获得高表面质量的铸锭的新型气膜连铸结晶器。采用此种结晶器详细考察了铸造温度、铸造速度、冷却水量、气体流量及润滑油流量和气体类型等工艺参数对Φ174mm铝合金铸锭表面质量、铸态组织和合金元素宏观偏析的影响,确定了6063铝合金的最佳工艺参数为:浇注温度710℃,铸造速度120mm/min,冷却水流量0.07m~3/min,气体流量为2L/min,润滑油流量为6ml/min。在最佳工艺条件下,铸锭表面光滑,消除了冷隔和偏析瘤等表面缺陷;皮下偏析层较薄,约为100μm,铸锭内部组织均匀,完全达到了采用多孔石墨的气滑铸造的水平。系统地研究了静磁场对气膜铸造铸锭的表面质量和内部铸造组织的影响。在气膜连铸时施加静磁场,由于熔体切割磁力线可以产生与熔体流动方向相反的洛仑磁力,产生电磁制动,所以施加静磁场可以减少熔体的横向流动,能有效地遏制熔体的流动对弯月面的冲刷,稳定弯月面,因此可以提高气膜连铸的铸造速度。在Φ174mm6063铝合金铸锭气膜连铸最佳工艺条件的基础上,施加10000At的静磁场,铸造速度可以提高到180mm/min,提高50%;铸锭表面光滑,消除了高速铸造下的偏析瘤;没有明显的皮下偏析层,铸锭内部组织均匀。考察了低频电磁场对气膜连铸铸锭表面质量和内部铸造组织的影响。施加低频磁场后熔体的流场显著改变,熔体流动方向与无磁场时相反,流动速度显著提高,温度场也发生了显著变化。低频磁场影响弯月面的稳定性,但是提高磁场频率可以降低这种影响。在Φ174mm铸锭气膜连铸最佳工艺条件的基础上,施加30Hz低频磁场,铸锭表面光滑,消除了冷隔,基本消除了表面偏析瘤;铸锭皮下偏析层较薄,铸锭内部组织均匀细小。利用气膜铸造一次冷却强度低的特点,开发了异性(三角形)坯的气膜连铸新工艺。发现施加气膜后,可以有效地减小熔体的接触高度和消除一次冷却不均匀的影响,有效地消除冷却不均导致的角部弯曲,减少凝固壳与结晶器内壁的摩擦,消除表面划痕及裂纹,铸锭表面光滑。重点考察了三角形锭坯气膜连铸时的分流特点,获得了异性坯气膜连铸的最佳工艺条件为:浇注温度730℃,铸造速度120mm/min,冷却水流量0.05m~3/min,气体流量为2L/min,润滑油流量为5ml/min。在最佳工艺条件下,铸造工艺稳定,铸锭表面光滑,消除了冷隔和偏析瘤等表面缺陷;皮下偏析层较薄,约为100μm,铸锭内部组织均匀。更多还原

【Abstract】 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+UCD<D, there is a space between meniscus and UCD , which can make liquid metal contact with the mold directly, and thus surface segregations will occur; If B+ UCD>D, 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.更多还原