【作者】 徐严谨；

【导师】 苏彦庆；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2012， 博士

【摘要】 铝合金由于其低密度、高比强度等显著特点，在交通运输工具、日常生活器具、传统装备领域等材料的选择上占有举足轻重的地位。然而铝合金铸件中存在着凝固组织粗大、气孔、疏松、偏析等缺陷使其越来越难以满足某些领域的需求，而外加电磁场在材料加工领域的应用，为制备新型的高性能铝合金提供了新的思路。本文以致密化并细化组织,同时去除熔体中气体及夹杂物为目的。基于对平面行波磁场感应器磁场分布的研究设计并制造了立式筒形行波磁场感应器。结合数值模拟与实验测量的方法研究了筒形行波磁场感应器内部磁感应强度分布、金属熔体受力和流动情况；最后通过了高能X射线断层扫描技术研究了电磁力和金属熔体的流动对铝合金凝固组织及缺陷的三维形貌的影响。上述研究为行波磁场在工业生产中的应用奠定了理论基础。筒形行波磁场感应器内部磁感应强度在面距较小时起伏剧烈，反之分布变得均匀；磁感应强度随着面距的增大呈指数规律衰减；磁感应强度随着输入电流安匝数的增加呈线性关系递增；磁感应强度各方向上的分量Br和Bz受磁感应器结构所决定，Br的极大值始终位于齿冠上，而Bz的极大值始终位于齿槽上；添加回磁铁芯可以有效地增强Br，衰弱Bz。通过数值模拟得到电磁力以及金属熔体流动的规律。金属熔体所受电磁力和流速均随着输入电流安匝数的增大而呈抛物线规律增大，两者均受电源频率的影响，但两者有着不同的规律。回磁铁芯可以影响金属熔体所受电磁力的分布，使得Fr减弱，Fz增强，而在本实验所用铸型情况下，回磁铁芯对金属熔体的流动影响甚微。通过对行波磁场作用下纯铝凝固组织的研究发现，施加行波磁场促使纯铝凝固凝固组织由柱状晶逐渐转变为等轴晶，并且有效地细化纯铝凝固组织。晶粒尺寸随着磁感应强度的增加先减小再增大，说明行波磁场存在一个临界值，当磁感应强度达到临界值时细化效果最明显。同时并不是行波磁场作用时间越长细化效果越好，综合考虑电磁场热效应，本实验条件下最佳的作用时间为液态降温阶段后期和凝固阶段前期。在行波磁场对于凝固组织缺陷影响方面，施加行波磁场可以有效地去除熔体中所含气体并致密化凝固组织。在铝合金凝固过程中施加行波磁场理论上可以有效地减小气泡的临界形核半径，促进气泡的非均质形核，同时加剧了熔体中气泡的排出。施加行波磁场可以有效地改善铝合金凝固组织中的孔洞，促使微观孔洞的形貌由复杂的树枝状转变为等轴状，并且微观孔洞的体积分数随着磁感应强度的增大而减小，使得试样从上而下致密度越来越高，且微观孔洞曲面的平均曲率随磁感应强度的增大而增大。实验研究了行波磁场作用对铝硅合金凝固组织的影响。在Al-10.3wt.%Si合金中，施加行波磁场可以使初生铝形貌由树枝晶向等轴晶转变，且等轴晶的尺寸随着磁感应强度的增大而减小，分布也越发均匀。通过高能X射线断层扫描再现了共晶硅的三维片状结构，片状共晶硅在行波磁场作用下得到了细化，并且磁感应强度存在一个临界值，共晶硅的面体比随着磁感应强度的增大先陡增然后平缓增大，当磁感应强度为临界值时此时面体比值最大，过了临界值后，面体比有所降低；此外共晶硅的长度、宽度以及厚度均随着磁感应强度的增大先陡减然后平稳减小至临界值时急剧减小，最后反弹。在Al-22.9wt.%Si合金中，施加行波磁场可以有效地控制初生硅的分布，细化初生硅颗粒，初生硅富集层的厚度随着磁感应强度的增大而减小，初生硅颗粒由原始的板状向类似球状结构发展。由于行波磁场可以影响金属熔体中不同导电率颗粒的分布，行波磁场不仅可以用作除杂，还可以用作制备自生梯度功能复合材料。同时结合数值模拟和实验结果，得出了制备铝合金筒形件的最佳工艺参数。更多还原

【Abstract】 Aluminum alloys is one of the major concern light metal in the word and hasbeen widely used in aerospace, aviation, vehicle, mechanical and other styles ofproduction owing to its virtues of low density, high specific strength etc.. And nowaluminum alloy is more and more difficult to meet the needs in some special areabecause of the bulk grain and defects such as pores, microporosities and segregations.Owing to the development of the electromagnetic field applied on the materialsprocessing, a new clean and environmental protection method was applied to producehigh quality aluminum alloys.In this paper, the purpose of the dissertation is to purify molten aluminum,eliminate microporosities and refine structure. We have designed and manufactured anew cylindrical traveling magnetic field generator (CTMF) which could change thestructure if need, based on the analysis on the plane traveling magnetic fieldgenerator (PTMF). Then distribution of the magnetic flux density, theelectromagnetic force, the forced convection induced by TMF and the effect of TMFon the solidification microstructure of aluminum alloys have been investigated. Threedimensions of microstructure and microporosities have been reconstructured bysynchrotron-based X-ray tomography technique.Magnetic flux density in the CTMF varies violently when close to the magneticgenerator surface, but it is the opposite. The magnetic flux density decreasesaccording to the exponential law with the increasing L, and increases with theincreasing input ampere turns linearly. The relation between the Br and the Bz aredetermined by the position in the CTMF, the maximum points of the Br locate in theabove of the teeth, when the maximum points of the Bz locate in the above of thegrooves. The application of the iron core can enhance Br and weaken Bz.The distribution of the electromagnetic force and forced convection induced bythe TMF are obtained by simulations. Both of them increases with the increasinginput ampere turns according to the parabolic law, and influenced by the currentfrequency. The addition of the iron core can enhance Fz and weaken Fr, but very littleto the forced convection.The influence of TMF on the solidification macrostructure of pure Al has beeninvestigated. The results show that the macrostructure of the pure Al transforms fromcoarse columnar crystal into fine equiaxed crystal, and the grain size of the equiaxedcrystal decreases firstly and then increases with the increasing input ampere turns, theoptimum value of the magnetic field is32mT in this situation. Because of the electromagnetic heat effect, the TMF used in the last stage of liquid cooling and theearly stage of the solidification lead to the best refinement effect.The using of TMF is an effective way that can purify the aluminum alloy melts.The critical nucleation radius of the pore decreases with the increasing magnetic fluxdensity. And the TMF promotes the heterogeneous nucleation of the pores. After thegas dissolved in the metal liquid accumulated to form large bubbles, the travelingmagnetic field forced the bubbles to the surface of the molten metal, the gas is easy tobe separated from the melt in the liquid stage, and the number of the porosities in thesample decreases with the increasing magnetic flux density, until the magnetic fluxdensity equal to45mT, the porosity disappears. The3-D microstructure of themicroporosities was reconstructured by the X-ray tomography technique. The usingof TMF can eliminate the microporosities of the sample, the morphology ofmicroporosities are changed from dendritic to equiaxed, and the volume fraction ofthe microporosities decrease with the increasing magnetic field density, the bottompart of the sample is more compact than the upper, and the mean curvature of themicroporosities increases with the increasing magnetic flux density.Effect of TMF on microstructure of Al-10.3wt%Si alloys has been investigated,the results show that, the morphology of the primary α-Al transforms from dendriticto equiaxed, and the grain size decreases with the increasing B. The3-Dmicrostructure of the eutectic Si has been characterized by X-ray tomographytechnique. The results show that the eutectic Si is a kind of plate like structure, andeutectic Si could be refined when the TMF applied. The value of the surface/volumeincreases with the increasing magnetic flux density until the magnetic flux densityequal to the optimum value and then decreases, when the length, width and thicknessof the eutectic vary with a contrary tendency. In Al-22.9wt%Si alloys, the applicationof TMF can control the distribution of the primary Si phase and eutectic Si and refinethe primary Si particles, the thickness of the enriched primary Si particles layerdecreases with the increasing B, the microstructure of the sample changes fromhypereutectic to eutectic and then to hypoeutectic from outer to inner. A novelapproach was developed to prepare in situ Al–Si functionally graded materials(FGMs) using TMF. At last, the optimal parameters were obtained by the simulationand experimental results.更多还原