| Recycling aluminum is more energy saving and environmental than primary aluminum and it is an inevitable trend to developing recycled aluminum.But at the same time it is important to remove inclusions and impurities in Al melt and improve the qualities of recycled aluminum.The behaviors of hydrogen in Al melt and the purification mechanism of recycled aluminum were studied in this paper by several materials analysis technologies,such as molecular simulation based on classical mechanics,numerical simulations of computational fluid dynamics(CFD),auger electron spectrometer,DSC,XRD,SEM,EDS etc.The research contents include the diffusion behaviors of hydrogen in A1 melt and its interaction with Al2O3,the wettabilities and solubilities between fluxes and Al2O3 inclusions,the interfacial tension between fluxes and A1 melt,the diffusion dynamics of fluxes in Al melt and their influences on the purification effects of recycled aluminum.Furthermore,a high effective flux was designed and the purification of recycled aluminum were carried out.The main results are summarized as follows.(1)The behaviors of hydrogen in Al melt were studied by molecular simulation and the nature of hydrogen diffusion and its interaction with Al2O3 were discussed on atomic level.Furthermore,Lennard-Jones potential function was used to reveal the diffusion mechanism of different types of hydrogen.The verifying tests of interaction between H and Al2O3 were carried out in recycled Al melt,and the tests results were consistent with that of simulation.The molecular simulation results showed that general H atoms(Hh)were hard to diffuse,but H atoms of H2(Hhlh)and H+ ions of H2O(Hhlo)diffused fully.Hhlh accumulated around Al2O3 and its diffusion velocity decreased,as a result,porosities formed on Al2O3.But the influences of Al2O3 on Hh and Hhlo were weak.The results of experiments on H/Al2O3 interaction in recycled aluminum showed that foils scraps and chips increased the inclusion content and porosities in Al melt,which were direct proportion.That was the higher the inclusion content,the higher the porosity was.Moreover,porosities developed on inclusions,which was consistent with the simulation results.The results proved the parasitic relationship between hydrogen and Al2O3 inclusions.So the removal of inclusions was the base of porosities deletion.The validity and practical value of aluminum purification principle developed by our team previously were also verified by present work,which was that inclusions removal being primary and degassing being auxiliary.(2)A flux component consisted of equimolar NaCl-KCl and alkali fluorides was established based on the studies of physical and chemical properties of several alkali fluorides and chlorides.The chlorides were basic components for higher chemical stability and lower melting point,and the fluorides were active components.The physical and chemical properties of this sort of fluxes and their reactivities with main elements of A1 alloys were studied.The results showed that alkali fluorides decreased fluxes melting point.NaF and KF decreased it most significantly,which were followed by AIF3 and Na3AlF6,and the influences of MgF2,CaF2,SrF2,BaF2 on fluxes melting point were weak because of the higher melting point of themselves.Moreover,some alkali fluorides reacted with main elements of A1 melt and changed the structures and constituent of Al alloys.NaF,SrF2 and BaF2 can react with Al or Mg and generated trace alkali metals,such as Na,Ca,Sr etc,which modified the eutectic silicon significantly.Na3AlF6 also generated trace Na in Al melt and modified eutectic silicon to some degree,but there were not influence of A1F3,MgF2 or CaF2 on the structures and constituent of Al alloys.(3)The thermodynamics of removing inclusions was studied by molecular dynamics simulation and experiments.The effects of alkali fluorides on the wettabilities and solubilities between fluxes and Al2O3 films and the fluxes/Al melt interfacial tension and their interaction mechanism were revealed.The coalescence experiments results showed that most of the fluorides improved the dissolve of Al2O3 films and the coalescence of A356 chips in molten fluxes at 750?.NaF,KF,Na3AlF6 and CaF2 were the most effective,which were followed by AIF3,BaF2 and SrF2,and MgF2 was the worst one.The analysis of coalescence mechanism indicated that free F-ions were essential to dissolve Al2O3 films and better wettability improved the dissolving velocity.The experimental results of interfacial tension indicated that NaF and KF decreased the interfacial tension most significantly and with the increasing of fluoride contents,it decreased steadily.CaF2,BaF2,SrF2 and MgF2 also decreased the interfacial tension but the influences weakened with the increasing of addition amount of fluorides.However,AlF3 and Na3AlF3 increased interfacial tension because they did not generate Na in Al melt.(4)The dispensability and separability of molten fluxes in A1 melt were studied by numerical simulation results of CFD,which would provide scientific basis for improving feeding methods of fluxes and dynamics conditions of removing inclusions.The results showed that it is difficult for molten fluxes to disperse uniformly in Al melt and the dispensability decreased with the increasing of interfacial tension,moreover,the improvement of dispensability was tiny by stirring.So it is necessary to disperse the fluxes in solid state when charged into Al melt and improve the contacting probability.(5)The influences of the solubility of fluxes for oxide films and fluxes/Al melt interfacial tension on removing inclusions were researched by the purification experiments of aluminum foil scraps.The results showed that good solubility of fluxes on oxide films and big interfacial tension were helpful to remove inclusions.Salt fluxes containing NaF was difficult to separate from Al melt because NaF decreased interfacial tension.The purification effect of fluxes containing AlF3 were not good because AlF3 cannot dissolve oxide films effectively.Salt fluxes containing Na3AlF6 displayed the best purification effects because of its good wettability and solubility for oxide films and higher interfacial tension.(6)New principles of fluxes component design were proposed based on the research of thermodynamics and dynamics of removing inclusions,which broke through traditional methods of fluxes component design by experiences and numerous tests.The component ratios were optimized by molten salt phase diagrams.Three different fluxes components,feeding methods and addition amounts were designed and optimized by orthogonal tests.The purification experiments were carried out in A3 5 6 melt containing aluminum chips.The results showed that the optimal flux component was equimolar NaCl-KCl(85wt%)+ Na3AlF3(7.8t%)+ AlF3(5.7wt%)+ CaF2(1.5t%).The best feeding method was mixing aluminum chips and fluxes before charging into melt.The optimal addition amount was lwt%of Al melt weight.The inclusion content,porosity removal rate and tensile strength of Al alloy purified by optimal process were better than that of commercial fluxes. |
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