Preparation Of Al-Zr Alloys By Molten Salt Electrolysis

Al-Zr alloys have numerous excellent properties, such as high toughness, high ductility, high strength and good corrosion, etc. Therefore, new Al-Zr alloy structural materials are used in aerospace, electric power, automobile, and watercraft applications. However, current methods for the preparation of Al-Zr alloys are expensive and provide products with unstable alloy compositions, which limit the applicability of these alloys. Thus, it is important to identify improved, low-cost methods for the large-scale production of Al-Zr alloys with good quality control. Herein, results of our investigation of physicochemical properties and ionic structure of cryolite-zirconia melts and their use in the aluminothermic reaction and electrolysis of Al-Zr alloys are presented. Specifically, the solubility of ZrO2in cryolite-based melts was evaluated, and the ionic structure of the nNaF·AlF3-ZrO2(n=2.2,3) system was explored using Raman spectroscopy. The effect of ZrO2on the physicochemical properties (liquidus temperature, density, and electrical conductivity) of a cryolite-based electrolyte was also determined in order to optimize the electrolyte composition for the electrolysis of Al-Zr alloys. In addition, the electrochemical behavior of Zr ion in the electrolyte and its dependence on the electrolysis process was investigated in order to optimize the electrolysis process. Finally, the phase composition and Zr concentration of Al-Zr alloys prepared via aluminum-molten salt reduction and molten salt electrolysis was investigated and the precipitation mechanism of Zr was analyzed. The results of the above investigations will be used to realize a method for the mass production of Al-Zr alloys via molten salt electrolysis using cryolite-zirconia melts.The solubility of Zrd in2.2NaF·AlF3-3mass%Al2O3-3mass%CaF2melts was studied using the isothermal saturation method, and it was found that the ZrO2solubility in this cryolite-based molten salt increased significantly as the temperature increased, reaching approximately5mass%at980℃. An increase in the CaF2content and a decrease in the Al2O3content also led to an increase in the solubility of ZrO2in the electrolyte.The ionic structure of different nNaF·AlF3(n=2.2,3)-ZrO2melts was then studied using Raman spectroscopy. It was found that the O in the ZrO2was substituted by free F, leading to the formation of Zr-O-F ionic complexes during the dissolution of ZrO2.Next, the liquidus temperature of the nNaF·AlF3(n=2.2-2.6)-3mass%Al2O3-3mass%CaF2-ZrO2molten salts was determined using a thermal analysis method. Notably, the liquidus temperature clearly decreased with increasing ZrO2content over the range of0-2mass%. However, the decreasing potential of the liquidus temperature decreased when the ZrO2concentration was greater than2%. The following empirical equation was thus obtained:T=920.2-16n-1.36w(ZrO2)-1.8n·w(ZrO2)+15n2+0.58w(ZrO2)2, in which T(℃) is the liquidus temperature, n is the molar ratios of NaF and AIF3, and w(ZrO2) is the weight percent of ZrO2.The density of the nNaF-AlF3(n=2.2～2.6)-3mass%Al2O3-3mass%CaF2-ZrO2molten salt system was then determined using the Archimedes method. The results indicated that the electrolyte density decreased with increasing temperature and decreasing molar ratio of NaF/AlF3. Specifically, the density of the molten salts increased by approximately0.02g·cm-3for every1mass%dissolved ZrO2.Next, the electrical conductivity of the nNaF·AlF3(n=2.2～2.6)-3mass%Al2O3-3mass%CaF2-ZrO2system was investigated using a tube-type cell with a fixed cell constant and an error of less than±1.5%. A1mass%ZrO2increase was found to result in a corresponding decrease in the electrical conductivity of0.02S·cm-1. On the other hand, the electrical conductivity increased as the molar ratio of NaF/AlF3and the operating temperature(0.004S·cm-1per1℃)increased.Next, the reduction mechanism of Zr4+in the2.4NaF-AlF3-4mass%CaF2-3mass%Al2O3-nZrO2(n=0-3mass%) system was evaluated using cyclic voltammetry, potentiostatic electrolysis, and chronopotentiometry techniques. The results indicate that the reduction of Zr4+was done two steps to form Al3Zr was formed in the cryolite-zirconia melts. A reaction occurred at a potential of-0.95V (vs. Pt), which was assigned to the reaction of Zr4++2eâ†'Zr2+at the electrode. In addition, a second cathodic reaction occurred at-1.2V (vs. Pt), indicating the electrochemical formation of Al3Zr.Finally,Al-Zr alloys were then prepared via the aluminum-molten salt reduction method and the direct reduction of zirconia using the molten aluminum method. It was found that the maximum Zr content in an Al-Zr alloy obtained via the direct reduction of zirconia with molten aluminum was2.29mass%, while the Zr content reached12.4mass%in an alloy obtained via the molten aluminum-molten salt reduction method. These results suggested that the molten salt increased the activity of Zr and dissolved the Al2O3reaction product, driving the reaction in the reduction direction. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed that Zr was present in the12.4mass%alloy in the form of Al3Zr crystal whiskers and granules. An Al-Zr alloy was prepared via molten salt electrolysis, and it was found that the Zr content of the bottom layer of the alloy was12.9mass%, while the Zr content in the upper layer was1.17mass%after electrolysis in a2.3NaF·AlF3molten salt system at1000℃. In addition, the maximum Zr content in the alloy depended on the operating temperature and was found to be approximately15mass%at1000℃.