Aluminum alloys are widely used in modern industries, transportation, aerospace, and other engineering applications. Process ability and mechanical properties of aluminum alloys can be improved via the addition of trace scandium. In the current production process, aluminum alloy containing Sc was prepared by adding Al-Sc master alloy, which required pure scandium metal and aluminum remelting and recasting, which could make large burning loss, uneven composition, and high production cost. In molten salt electrolysis method, the target aluminum alloy containing Sc could be directly obtained by using Sc-containing chemical compounds as raw materials, so that it could shorten the alloy production process. In this study, Al-M-Sc alloys were prepared by aluminum cathode method, and further expanded into the microstructure and mechanical properties of different aluminum alloys after adding rare earth metal Sc. In the experimental part, Na3AlF6-15%KF-41%AlF3-9%LiF (mass fraction) fluoride system was prepared by the analytically pure anhydrous fluorides (KF, AlF3, CaF2, LiF and cryolite), and scandium oxide (99.99%, mass fraction) was used as raw material for electrolysis. High purity Al and binary alloys (Al-5Mg, Al-7Zn, Al-5Li, Al-4Cu and Al-7Si) were used as cathodes. The laboratory electrolytic cell was composed of anode (graphite, Ф15 mm×35 mm), graphite crucible (Ф47 mm×88 mm), corundum lining of Ф41 mm×63 mm and cathode metal (Ф20 mm×15 mm), molten cryolitic electrolyte containing 2% Sc2O3, where argon gas and cooling water were continuously supplied during electrolysis. The electrical current for electrolysis process was provided by a DC power supply (TRADEX MPS 302). The operating temperature, testing time and current density were 1073 K, 60 min and 1 A·cm-2, respectively. The microstructure and mechanical properties of Al-M-Sc alloys were investigated through metallographic observation, scanning electron microscope (SEM) observation. X-ray diffraction (XRD) was used to further determine the phase composition of the alloy produced. The content of Sc was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). It was found that under the same electrolysis conditions, the Sc content of alloys which prepared by Al-liquid electrolysis with binary alloy as cathode was higher than that of the high-purity aluminum cathode electrolysis alloy (0.41%), and the Sc content of Al-5Li cathode electrolysis alloy was up to 1.25%. This should be related to the enlarged superheating degree and enhanced diffusion rate after alloying addition. At 1073 K, the diffusion coefficient of Sc solute in binary alloy cathode was greater than that in pure aluminum, and its migration rate from the reaction boundary to the alloy melt interior was faster. The main forms of Sc in Al-M-Sc alloys were binary Al3Sc phase, ternary W-(Al, Cu, Sc) phase and AlSi2Sc2 phase, respectively. In Al-Sc alloy, Al3Sc phase was uniformly distributed in the Al matrix and was presented as a thin layer (~150 μm). The phase morphology of Al3Sc was square and triangle and the average particle size of Al3Sc was (46±13) μm. There were Al3Sc phases in Al-5Mg-Sc, Al-7Zn-Sc and Al-5Li-Sc alloy, and AlSi2Sc2 phase in Al-7Si-Sc alloy, Al3Sc and W-(Al, Cu, Sc) phases in Al-4Cu-Sc alloy, respectively. In Al-5Mg-Sc, Al-7Zn-Sc and Al-5Li-Sc alloys, the number of Al3Sc particles increased significantly and distributed more widely in the matrix. The corresponding average particle sizes of Al3Sc was (67±26), (59±24) and (73±30) μm in the above three alloys, respectively. The morphology of Al3Sc phase was cube, triangular cone and star-like in Al-Sc alloy. W-(Al, Cu, Sc) phase was flocculent, which was formed by the cluster of rods connected with each other, its thickness was ~15 μm, and the cluster W-(Al, Cu, Sc) phases were connected with each other by clustered rods. There were needle-like AlSi2Sc2 phases around massive AlSi2Sc2 phase, and the bulk AlSi2Sc2 phase was 210 μm in length. The test results of mechanical properties showed that the hardness of Al-Sc alloy matrix was 33.8 kg·mm-2, and the hardness of Al-5Mg-Sc, Al-7Zn-Sc, Al-5Li-Sc, Al-4Cu-Sc and Al-7Si-Sc alloys matrix was higher than that of Al-Sc alloy matrix. The matrix hardness of Al-5Li-Sc ternary alloy reached 51.4 kg·mm-2. In comparison, the hardness of Al3Sc phase, ternary W-(Al, Cu, Sc) phase and AlSi2Sc2 phase were significantly higher than that of the alloy matrix, and their hardness reached 289.9, 114.2 and 162.0 kg·mm-2, respectively. This implied that these second phases could be used as effective strengthening phases to improve the mechanical properties of Al alloys. The results demonstrated the excellent strengthening effects of Sc-containing second phases in aluminum alloy directly prepared by molten salt electrolysis, which could be a potential shorten, low cost process for improving the current industrial practice in making aluminum alloys. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.
