Research On Purification Treatment Of Molten Al And Al-Si Alloys And Its Mechanism

With a demand of higher purification degree for the aluminum and aluminum-silicon alloys, the on-line purification technique of inert gas-flux injection has more and more being used in the past 10 years. Thus, the refining flux should posses the following qualities. It is with higher capacity of removing inclusion and gas, and does not block the injection pipeline. It is with higher flow-ability and fast diffusion into the melt, and does not aggregate and deposit nearby the pipeline outlet. It is with a capacity to build dry slag, which will not absorb the aluminum melt and not stick onto the Heat. If there is a slight reaction between the flux component and aluminum melt, the reaction products should be removed with the slag.In response to these requirements, A RE-included flux has been successfully synthesized through selection and optimization of proper flux compound. Finally, Al-Si alloys were purified with this optimal RE-included flux in comparison with commercially available 2# flux. By means of hydrogen tester, ICP-OES, contact angle meter, OM, materials mechanical testing machine, XRD, DSC, SEM/EDS and other equipment, the hydrogen content in the melt, the microstructure mechanical properties of cast alloys were studied to evaluate the purification efficiency and purification mechanism. The conclusions can be made as following.1) Among those fluxes containing compounds of NaCl, KCl, Na3AlF6, Na2SO4, CaCO3 and REF3, The flux with NaCl/KCl 80g, Na3AlF620g, Na2SO45g, CaCO35g, REF34g has the best purification performance for pure Al and Al-Si alloys. When A356 and ADC 12 alloys were purified by this flux, they incorporated residual inclusion and void with volume fraction of 0.068 vol.%, and 0.058 vol.%, respectively; hydrogen contents of 0.086 ml/100g Al,0.081 ml/100g Al respectively. The size of non-metal inclusion (NMI) was less than 10μm after purified with optimal RE-included flux. Compared with the commercial 2# flux produced by Jiangxi Yongte non-ferrous alloy Ltd,. this RE-included flux could reduce the inclusion and the void by 3 times, and hydrogen content in the melt by 2 times.2) Treated with RE-included flux, the A356 aluminum alloy microstructures could be improved. Compared with treatment with the commercial 2# flux, the average size of primary a(Al) cell, the secondary dendrite arm spacing and the aspect ratio of silicon phase were decreased by 38%,36% and 39% respectively. Strength and elongation were increased by 29%,76% respectively.3) The RE-included flux has three eutectic system:the multiple-component eutectic system comprising Na2SO4, the multiple-component eutectic system comprising CaCO3, and the main salt eutectic system comprising NaCl-KCl-Na3AlF6. Their lowest melting temperature are 560℃,600℃,620℃, respectively. The Multiple-composite system can reduce the melting point of Na3AlF6, Na2SO4 and CaCO3, which had been proved a positive influence in melt purification.4) Molten Na2SO4 or CaCO3 has a chemical reaction with aluminum powder (aluminum melt), respectively.3Na2SO4(l)+2Al(l)=3Na2O(s)+Al2O3(s)+3SO2(g)↑(1)3CaCO3(l)+2Al(l)=3CaO(s)+Al2O3(s)+3CO(g)↑(2)The activation energy was estimated at 1859.5 kJ/mol, reaction order 2.42696 and the released heat 917.8 kJ/mol in reaction(1). The heat released by this reaction effectively promoted the flow and diffusion of the molten flux, increased the solubility of Cryolite and reduction of rare earth compounds to the alumina. The productive gas is conducive to degas and remove inclusion.5) The addition of a small amount of REF3 can not only promote the reduction of alumina, especially those with the small size, but also significantly reduce the wetting angle between the molten flux and the alumina from 26.5°to 16.1°.The distinctive features of this RE-included flux are:1) making use of the characteristic of cryolite component capturing alumina, and the heat released by reaction of sulfate to hold back the formation of large size of colloid-liked substance comprising Na3AlF6-Al2O3-Al; 2) the flux has properties of low melting point, high mobility, high diffusion and low viscosity, low surface tension, which greatly facilitate to remove large size of inclusion; 3) making use of RE atoms to reduce alumina, while taking advantage of reaction heat to promote the mobility of Al-RE, thus reaching the purpose of decreasing oxide inclusions with small size efficiently.