Preparation Of Al-Sc Master Alloy In K₃AlF₆ Molten Salt

Scandium is the most effective alloying element discovered so far.With only a few thousands the mass percent of scandium was added to the aluminum alloy,the strength of the alloy can be increased by 1-2 times,the elongation rate can be enhanced by 4-5 times,while the welding performance can be improved significantly as well.The scandium-containing aluminum alloy has been widely applied in manned space-flight,nuclear power and other fields.Russia intends to use the aluminum-scandium alloy in the future space station.Al-Sc master alloy is the key material in the production process of the scandium containing aluminum alloy.At present,the Al-Sc master alloy is prepared by mixing the pure aluminum metal with scandium.The high cost of this method restricts its extensive application.Attracted by the low cost,more and more people are paying attention to the thermal reduction method and electrolysis method for Al-Sc master alloy preparation.In this thesis,we proposed a method for preparing the Al-Sc master alloy in K3AIF6-Sc2O3 salt.This thesis firstly studied the dissolution behavior of Sc2O3 in different molten salts,then explored the optimal technological parameters for Al-Sc master alloy preparation with aliminothermic process,and finally analyzed the differences between the thermal reduction method and electrolysis method.The dissolution processes of Sc2O3 in LiF-CaF2,KF-MgF2 and KF-AIF3 molten salts were studied by isothermal saturation method.The results show that the dissolution equilibrium time of Sc2O3 before equilibrium in KF-MgF2 and KF-AIF3 molten salts is approximetely 3-4h;The solubility of Sc2O3 in LiF-CaF2(80mol%-20mol%),KF-MgF2(85mol%-15mol%)and KF-AlF3(60mol%-40mol%)molten salts is 0.14%,0.60%and 9.75%,respectively.The solubility of Sc2O3 in KF-AlF3(60mol%-40mol%)molten salt increases with increasing temperature.The dissolution enthalpy ?H is 2452.22J·mol-1 and the entropy ?S is 10.47J·mol-1·K-1 in the process of dissolution.Sc element exists in the form of ScOF after the dissolving in the melts.Dynamic analysis of the dissolution process shows that the activation energy Ea is 1.62×105 J·mol-1,pre-exponential factor A is 3.06×105mol·cm-2-s-1 in KF-MgF2(85mol%-15mol%)molten salt.The influences of the amount of reductant,temperature and reduction time on the results of aluminum thermal reduction were studied with single factor analysis method.The results showed that the preparation of Al-Sc master alloy in KF-AlF3 molten salt is feasible with Sc2O3 as the raw material using aluminothermic reduction method.The optimal proportion of aluminum to Sc2O3is 3:1.Reduction temperature and time have significant influences on the Sc content in the alloy.The scandium in the Al-Sc master alloy obtained through aluminothermic reduction method is not uniformly distributed.The Sc content is 0.5%-1%in the upper side of the alloy,while Sc content in the bottom is 4%-9%.Sc element exists in the form of Al3Sc in the master alloy,which is in the shape of square and triangle,while electrolysis preparation process has little effects on the distribution of Sc in the alloy.However,aluminothermic reaction still occurs during electrolysis process.