Study On The Solidification Structure Of Aluminum Alloys Under Vertical Cylinder Traveling Magnetic Field

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.