【作者】 张忠涛；

【导师】 李廷举；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2009， 博士

【摘要】 铝是用量仅次于钢铁的第二大金属材料,其具有密度小、比强度高、耐腐蚀、易于回收等优异的性能,在汽车、建筑、食品及包装、电力、交通基础设施、航空航天、机械、电子信息等部门的传统或新型产品的制造中,具有广泛的应用。但是,铝合金生产中主要存在着易于吸气、细小夹杂物不易去除的问题,严重阻碍了铝工业的发展,同时,铸坯的表面质量粗糙、合金元素偏析、凝固组织粗大等缺陷也使其不能满足现代工业对优质材料的需求。目前外场如电磁场、超声场在冶金及材料加工领域得到了一定的研究及应用,为制备洁净、高性能铝合金提供了良好的基础和契机。本文以去除金属熔体中的气体和夹杂物粒子及细化金属液凝固组织、改善铸坯性能为目的。首先分别进行了电磁场、超声场作用下铝熔体中夹杂物和气体运动行为的研究,利用研究结果制备了组织和性能梯度分布的铝硅梯度材料,进而将超声场与旋转电磁场结合起来组成复合场对铝合金凝固行为进行研究,考察其对铸坯凝固过程、凝固组织、合金元素分布及力学性能的影响,并结合试验结果对复合场作用下金属凝固过程中形核及长大过程进行分析,对复合场下熔体形核和长大的机理提出了较为科学的理论解释。最后,开发了一种新型的复合场水平连铸工艺,并制备了表面和内部质量好、力学性能高的Al-1%Si合金线材。上述研究为电磁场和超声场技术应用到铝合金的实际生产中奠定了一定的试验和理论基础,相信随着复合场技术的推广和应用,生产洁净、高性能的铝合金材料将成为现实。论文的研究结果表明:1.利用高频磁场对铝合金中的氧化铝夹杂物进行分离,对于30-200μm的氧化铝颗粒施加表面磁感应强度为0.04T的磁场1s时,在熔体的边部就有明显的氧化铝颗粒偏聚层,分离时间大于3s时,在铝熔体内部已经很少有直径在30μm以上的氧化铝颗粒;当金属熔体表面磁感应强度为0.06T时,仅施加电磁场1 s即可对氧化铝颗粒实现有效的分离。2.利用功率超声场对铝合金中的异相粒子进行了分离研究,超声场对液态金属中的夹杂物和气体粒子有较好的去除效果,600W超声作用120s后,A356合金中的气体基本上被去除;利用酒精水溶液中的聚乙烯颗粒作为夹杂物的模拟物进行超声除杂模拟试验,结果表明,超声对粒径为96-109μm的聚乙烯颗粒的去除率可达90%以上;利用Al-18%Si中的初生硅作为铝合金中夹杂物的模拟物进行试验,结果表明超声场可对初生硅的运动行为进行有效控制;通过采取内部施加超声场配以外部强制冷却的方法制备了组织和性能梯度分布的梯度组织铝合金材料。3.将旋转磁场和超声场组成复合场,考察其对Sn-3.5%Pb合金及A356合金凝固过程的影响,结果表明;复合场可以提高合金初始凝固温度、抑制元素偏析、细化合金凝固组织并改善共晶相形貌,初生铝平均晶粒度由原始的250μm降低到复合场作用时的30μm,相应的抗拉强度也由163MPa提高到了268MPa。一方面,功率超声空化作用使合金在其液相线温度以上就可以爆发生核,这些初生核被超声的声流效应和电磁搅拌共同产生的强制对流传递到整个熔体而使合金凝固组织大大细化:另一方面,超声场和电磁场引起的熔体的温度场和浓度场的起伏也有利于少数残存枝晶生长过程中在枝晶臂发生熔断,复合场产生的上述两方面共同作用使得最终铸坯具有细小的凝固组织和优良的力学性能。4.将行波磁场与超声场组成复合场应用到Al-1%Si合金键合线材的水平连铸过程中,开发出了一种利用复合场水平连铸制备铝硅键合线材的新技术。复合场可以均衡保温炉内温度场、抑制硅元素偏析、提高连铸坯表面质量,同时还可以改善铸坯内部组织、提高铸坯力学性能,铸坯的抗拉强度和延伸率明显提高,分别由110MPa和13%提高到160MPa和40%。更多还原

【Abstract】 Aluminium is the second largest metal material in the world and has been widly used in mobile, vehicle, food, electricity, transportation, aerospace, mechanical and other new styles of production owing to its virtues of small density, high specific strength, corrosion resistance and easy recovery. Well, in the production of aluminium alloys, the problems such as gas porosity, fine impurities hinder its improvement a lot. In addition, the poor surface condition, segregation of alloy elements and coarse microstructures also can’t meet the demand of modern industry. Owing to the development of electromagnetic field and ultrasonic field on the metallurgy and materials processing, it’s a good opportunity to find a new clean and high efficiency method to produce higher quality aluminum alloy.The aim of this thesis is to find a new method to produce aluminum casting billets with higher property through removing gas and impurity from the liquid metal and refining the solidification microstructures. Researches of high frequency magnetic field and power ultrasonic field on the movement behavior of gas and impurity heterogeneous particles are carried out. Based on the results, Al-Si graded material is successfully fabricated. In addition, the study of compound field (combination of electromagnetic field and power ultrasonic field) on the solidification process and the effects of compound field on the solidification microstructure, mechanical property are also studied. The mechanism of the solidification under compound field is also discussed theoretically according to the experimental results. At last, a new technique of continuous casting of Al-1%Si bonding wire under compound field is developed which is favorable for the improvement of surface quality, casting microstructure, element segregation and mechanical property. All the researches above lay a good foundation of both experiments and theory to the future use of electromagnetic field and ultrasonic field to the fabrication of high purity and high quality aluminum alloys.The results of the study are showed below:1. High frequency magnetic field is used to separate alumina particles and the results show that: to the alumina particles of 30-200μm with 0.04T magnetic field 1s, there are no particles gathering in the inner, when the separation time prolongs to 3s, there is nearly no particles more than 30μm in the inner; when the magnetic density increases to 0.06T, 1s is enough for the particle in the inner to migrate out.2. Power ultrasonic filed is used to separate heterogeneous particles in liquid, which shows that it is very efficient for purification of gas and impurity particles. After using 600W ultrasonic 120s, the gas porosity in vacuum reduced pressure A356 alloy samples is nearly eliminated. To alcohol water system, the polyethylene particle can be eliminated and the removal rate can reach to more than 90%. Taking primary Si in Al18%Si alloy as the mimics of alumina particles, ultrasonic field can cause the upwards migration of them, based on the results of this, Al-Si graded materials are fabricated.3. The study of compound field (combination of stirring magnetic field and power ultrasnic field) on the solidification of Sn-3.5%Pb and A356 are carried out and the results show that compound field can increase the beginning temperature of solidification, suppress element segregation, refine solidification microstructure and modify the morphology of eutectic silicon. The size of primary Al changes from 250μm to 30μm with the application of compound field and the tensile strength changes from 163MPa to 268MPa. On the one hand, ultrasonic field can generate numerous nuclei even when the temperature is above the liquidus, after that the nuclei are dispersed to the whole melt by the forced convention caused by acoustic streaming and electromagnetic stirring; on the other hand, the concentration and temperature fluctuation caused by ultrasonic and electromagnetic field is facilitated to the remelting of dendrities on the root, which can also refine the solidification microstructure.4. Compound field (combination of traveling magnetic field and power ultrasonic field) is applied on the continuous casting of Al-1%Si bonding wire and a new technique is obtained. The compound field can even the temperature field in the tundish, suppress Si element segregation, improve surface quality and refine microstructure thus improve the mechanical property of the casting billets. With the application of compound field, the tensile strength and elongation rate changes from 110MPa and 13% to 160MPa and 40%, respectively.更多还原