Study On The Manufacture Of Al-Mg-Sc Master Alloy And Its Application

Two new preparation methods of AI-Mg-Sc master alloy were researched based on thermodynamics calculation as well as experimental study. The master alloys were prepared by ScCI3-KCI-NaCI-Mg(AI) and ScF3-MX-Mg(AI) fusion metallic thermoreduction. Sheets of AI-5Mg and AI-4.8Zn-2.4Mg alloys with and without scandium were manufactured by mixing AI-Mg-Sc master alloy with metals needed. Effects of grain fining and strengthening of scandium added in form of AI-Mg-Sc master alloy were studied contrastively. The main conclusions include:1. The results of thermodynamics calculation indicated that ScCI3 and ScF3 can be reduced to scandium by magnesium, but Sc2O3 cannot be. If reduction reactions are proceeded in aluminum bath, entropy of system will be increased because of the formation of stable intermetallic compound AI3Sc. The standard Gibbs formation free energy of AI3Sc was estimated to be about -138KJ/mol. As a result of sequent formation of AI3Sc, thermodynamics tendency of reduction will be enhanced, equilibrium constants and conversion ratio of reduction will be greatly enlarged. It is suggested that AI-Mg-Sc master can be gained well by reducing ScCI3 and ScF3 using magnesium in aluminum bath. At 1100 C, the empirical equilibrium constants of ScF3-MX-Mg(AI) reduction system is 1.82x108, and that of ScCI3-KCI-NaCI-Mg(AI) is at level of 103-105.2. Scandium chloride molten salt is the key raw materials for preparation of AI-Mg-Sc master alloys. Aqueous scandium chloride are easily obtainable by dissolving Sc2O3 in hydrochloride acid, but it is unavailable to obtain anhydrous ScCI3 by heating sample simply because the effect of a serials of thermal decompositions leading to form ScOCI or ScaOs finally. Thermal analysis (TGA, DTA) results of aqueous scandium chloride samples adding various kinds of inorganic salts indicated that addition of ammonium chloride can suppress the vaporous hydrolyze of scandium chloride effectively. ScCI3-NaCI-KCI fused salt was successful obtained by heating complex salt of aqueous scandium chloride containing ammonium chloride, potassium chloride and sodium chloride with protection of argon gas. The temperature of dehydration and deammonium was 206 C and 338 C respectively. The ignition point of fused salt was raised to 940 C.3. The optimum reduction temperature of ScCI3-NaCI-KCI-Mg(AI) tomanufacture AI-Mg-Sc alloys is 850 C, time is 40min, ScCI3 concentration in fusion is about 25%, the greatest recovery of scandium is higher then 90%.4. ScF3-MX molten salt was made from scandium oxide and ammonium hydrogen fluoride by solid state fluoridation under heating and melting. The optimum reduction temperature of ScF3-MX-Mg(AI) to manufacture AI-Mg-Sc alloys is 1100 C, time 40min, the total recovery of scandium after twice fluoridating is higher than 80%. ScF3-Mg system starts ScF3(s)-Mg(s) solid-solid phase reduction at 590 C, ScF3(s)-Mg(l) solid-liquid phase reduction at 740 C. ScF3- LiF-Mg system starts ScLiF4(s)-Mg(s) solid-solid phase reduction at 496 C, (ScF3-LiF)-Mg(l) at 650 C.5. AI-5Mg-0.2Sc-0.1Zr and AI-4.8Zn-2.4Mg-0.25Sc-0.12Zr sheets were manufactured by mixing AI-Mg-Sc master alloy with metals needed. Effect of grain fining of master alloy prepared was remarkably. The finest grain size of AI-5Mg-0.2Sc-0.1Zr and AI-4.8Zn-2.4Mg-0.25Sc-0.12Zr is low to 42-45um. Comparing with corresponding alloy without scandium and zirconium, the tension strength increment of AI-4.8Zn-2.4Mg-0.25Sc-0.12Zr sheets made from AI-Mg-Sc master alloy prepared was more than 100MPa. The tension strength and yielding strength of AI-4.8Zn-2.4Mg-0.25Sc-0.12Zr is up to 500MPa and 465 MPa respectively after 465 C/30min solidation followed 120 C/24h aging heat treatment.6. Most part of minor scandium in AI-5Mg-0.2Sc-0.1Zr and AI-4.8Zn-2.4Mg-0.25Sc-0.12Zr alloys exists in form of primary or secondly AI3Sc, only a few of them stayed in cc(AI) based solid solution. The primary AI3(Sc,Zr) particles precipitating from melt of alloy dur...