【作者】 余建波；

【导师】 任忠鸣；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 凝固组织细化是提高金属材料力学性能及使用性能的有效手段之一。大量研究表明:在金属凝固过程中施加电磁振荡能控制柱状晶,细化等轴晶,甚至形成近球状组织。但是关于电磁振荡对金属凝固组织的影响机制,缺乏系统性、规律性的研究。此外,以往的研究多采用交变/脉冲磁场,仅限于应用电磁场的振荡效应。近年来随着超导磁体的发展,强静磁场下的凝固受到人们重视,发现强磁场对金属凝固有多重效应,在强磁场背景下复合电流对金属凝固无疑将产生除振荡外更多样的作用,利用这些作用将为细化各层次组织和改善性能开辟新的途径,但至今这方面的研究十分欠缺。本文将稳恒强磁场和交流电相复合(文中称复合场),考察由此所产生的电磁振荡、金属流动等磁流体力学效应和磁化极化效应对纯铝及其合金凝固组织的影响机制,进而开展实际应用的研究。结果表明:(1)稳恒强磁场和交流电复合产生的电磁振荡对纯Al的凝固组织具有明显的细化作用。同一磁场强度下,随着电流强度的增大,晶粒变细。在同一电流强度下,晶粒的细化程度随磁场强度的增加先增加后减小,当电流为10A时,在磁场强度为6T处存在细化的最佳值。(2)在Al-6wt%Si合金凝固的过程中同时施加磁场和交流电,晶粒尺寸急剧减小,微观形貌由树枝形转变为蔷薇形。磁场一定时,当磁场强度较小时(小于4T),晶粒尺寸随着电流密度的增加而增大;磁场强度大于4T时,晶粒尺寸基本不随电流密度而变化;电流密度一定时,晶粒尺寸随着磁场强度的增加而减小。电磁振荡不仅能够细化α-Al,而且可以细化共晶硅,使共晶组织由粗大的板片状转变成细小的针片状。(3)在Al-12.6wt%Si共晶合金的凝固过程中施加单一的强磁场,共晶组织细化。而同时施加稳恒强磁场和交流电产生电磁振荡。当振荡强度较小时,共晶组织粗化;当振荡达到一定强度后,发生离异共晶,形成α-Al、共晶硅以及初生硅三种晶粒共存的特殊组织。(4)在Al-18wt%Si过共晶合金的凝固过程中施加单一的强磁场,随着磁场强度的增大,初生硅细化。而同时施加稳恒强磁场和交流电产生电磁振荡,随着振荡的增大,初生硅经历碰撞、团聚、分散及迁移等过程,最终形成均匀细化的组织。通过对合金凝固曲线的分析,发现电磁复合作用提高了初生硅的形核温度,降低了形核过冷度,延长了初生硅的生长时间。振荡促使初生硅的生长方式由连续生长转变成层状生长,形态也由粗大五星片状转变为细小块状。基于以上电磁振荡对合金凝固组织影响研究的基础上,开展采用局部施加电磁振荡,考察复合场对细晶范围影响的研究。结果表明:同一磁场强度下,电流强度越大,细化效应的传递范围也越大。同一电流强度下,晶粒的细化范围随磁场的增大先增大后略有减小,存在极值。分析认为复合场下晶粒细化的原因是由于振荡促使晶核脱离形核位置,形成游离晶,游离晶在外力的作用下下沉,形成“结晶雨”,从而引起细晶范围的变化。利用初生晶核在磁场中受磁力作用原理,提出利用梯度强磁场控制“结晶雨”方法。并建立了梯度强磁场下金属熔体中球形颗粒迁移的一般动力学模型,对模型进行了求解,得到了梯度磁场下颗粒运动速度的解析解:为了进一步研究电磁复合作用对金属凝固过程的影响,利用EBSD考察了电磁振荡对纯铝晶界结构的影响。研究发现振荡可以改变纯铝的晶界结构,使小角度晶界增多,从而提高材料的耐腐蚀性能。利用Al-4.5%Cu合金考察了电磁振荡对合金取向的影响。X射线衍射结果表明:施加复合场后,晶粒的<111>晶向沿磁场方向取向;晶粒取向度随晶粒的细化有增大趋势。从球形颗粒旋转取向和颗粒迁移角度推导了晶体粒径和取向的关系。最后,在对电磁复合场进行了大量实验研究的基础上,提出了复合交流电的EMC电磁连铸新工艺,并将这种工艺成功的应用于工业实验。通过对黄铜的实验表明:复合交流电的EMC工艺能够改善铸坯的表面和内部质量,提高铸坯的力学性能。进而把电磁复合场技术应用于焊接工艺中。利用磁镜原理,设计了电磁复合场下的TIG振荡焊接工艺。实验表明:该装置产生的磁场可以有效的控制电弧旋转频率和电弧形态,从而对TIG焊接过程产生明显影响,细化晶粒,提高焊缝性能。更多还原

【Abstract】 It is well known that the mechanical properties of a metal or metallic alloy, such as strength and toughness, are highly dependent on its grain size. Electromagnetic vibration has proven to be effective in controlling columnar dendritic structure, reducing the size of equiaxed grains, and under some conditions, producing globular non-dendritic grains. However, most of the studies are generally quantitative, limited to a small range of vibration conditions on the solidification, in addition, most of the magnetic fields used in vibration are AC magnetic fields or pulsed magnetic fields. With the development of superconductor technology, the application of strong static magnetic fields becomes more and more wide. It is found that the strong magnetic field has multiple effects on the solidification of metal, and it is became more complicated by simultaneously imposing an AC current and strong static magnetic field. However, the reports about the effects of complex fields, besides vibration on the material processing were seldom available. Therefore, it has great significance to research the influence of superposing electricity and magnetic field on the solidification structure of metals.In this paper, effects of complex field, including electromagnetic vibration, MHD and magnetization effects on the solidification structures of pure Al and alloys have been investigated experimentally. The results show that their macrostructures of pure aluminum are remarkably refined by simultaneously imposing an AC current and a high static magnetic field. At certain a magnetic field, the extent of grain refinement increased with the increasement of alternating currents. At the AC current of 10A, the refinement firstly increased with the increasement of magnetic fields and reached the optimum at a 6T magnetic field, however, as magnetic fields further increased, the refining effect diminished.With simultaneously imposing a high magnetic field and AC current during solidification of Al-6wt%Si alloy, the size of the grains was remarkably decreased and the microstructure evolved from dendrites to rosette-shaped grains. The size of grains was increased with the increase of current density when the intensity of magnetic fields was below 4T. Nevertheless, the size of grains was unchanged when the intensity of high magnetic field was above 4T. At certain a current density, the size of grains was decreased with the increase of the intensity of magnetic fields. Moreover, with the increase of electromagnetic vibration intensity, not only the morphology of primaryα-Al phase transformed from dendrite to equiaxed crystal, but also the eutectic Si phase was modified from a mainly lamellar morphology to a mainly fully divorced needle.For Al-12.6wt%Si eutectic alloy, it was found that the eutectic structure had been refined by only imposing a magnetic field while it coarsened under the electromagnetic vibration. Furthermore, divorced eutectic structure formed when the electromagnetic vibration was strong enough.For Al-18wt%Si alloy, the primary silicon particles were refined by solely imposing a magnetic field. However, when simultaneously imposing a magnetic field and AC current, many primary single silicon particles were congregated under lower vibration intensity. However, at higher vibration intensity, the primary silicon particles dispersed, and migrated at a critical size. Finally homogenous refining primary silicon particles formed at high vibration intensity. Furthermore, thermal analysis experiments were performed. The results show that the nucleation temperature have been elevated which indicated a smaller undercooling, and the steady growth time of primary silicon increases under electromagnetic vibration. Experimental results also show that the primary silicon refinement may be attributed to the change of growth pattern from continue growth to laminar growth under electromagnetic vibration.Based on the above experiments, a new process was proposed by simultaeous imposition of AC current and high magnetic field at the top of specimen. Effects of complex field on the distribution of refinement grains have been investigated. At certain a magnetic field, the size of Al grains decreased and the depth of refinement region increased with the increase of alternating currents. At certain an AC current, the refinement region firstly increased, reached an optimum and then decreased with the increase of magnetic fields. The analysis indicates that the nuclei dissociate from the top surface and migrate under gravity is the dominant factor for grains refinement.Owing to the difference in susceptibility between the liquid and the solid of pure aluminum ( 0 <χl <χs), an upward effective magnetic force is applied on the solid aluminum, when the specimen are laid in a positive gradient magnetic field, the magnetic force may restrain crystal shower and improve the distribution of the refining grains on the specimen. Dynamics of grains migrating in molten metal under high gradient magnetic field has been studied theoretically. The equation of above model was solved, and the theoretical migrating velocity is expressed asIn order to further study the effects of electromagnetic complex field on the characteristic of the structure, the EBSD analysis technology is adopted. The results show that the fraction of low-angle grain boundaries (LAGB) is increasing with the increase of vibration intensity, this modification is benefical to the corrosion resistance. Moreover, the Al-4.5%Cu alloy was chosen to investigate the influence of electromagnetic complex field on the crystal orientation. X-ray diffraction indicated that the refining grains were oriented with <111> towards the magnetic field direction. The orientation degree was increased with the decreasing of grain size. The relation between orientation and grain size was analyzed from crystal orientation and migration theory.Finally, A new method (AC+EMCC) was supposed by applying alternating current (50Hz) directly to the melt during electromagnetic continuous casting (EMCC) process. Results show that the application of AC current during EMCC is capable of improving the the surface quality and physical properties of copper billets significantly. Moreover, the application of electromagnetic complex field has also been explored and attempted to welding. A new Tig welding process with tailoring magnetic field was supposed according to magnetism len technique. The result indicates that the introducing of proper magnetic field not only can control the shape and rotational frequency of arc shape, but also can refine grains and improve mechanical properties of welding joint remarkably.更多还原