Processing of Light Weight Alloys by Molten Salt Technology

There is an ever-growing demand for the use of aluminum-lithium alloys in the aerospace industry, because of their lightweight and improved modulus. Presently these alloys are produced by the melting and casting method involving the use of pure metallic lithium which is added to the molten aluminum. The production of Al-Li alloys by mixing the pure metal is an energy intensive and expensive process. Considering the relatively high prices of high purity lithium metal, a new process is investigated in the present research to prepare lithium by in situ electrolysis of lithium compounds to Al-Li alloys in fluoride electrolytes.

To study the viability of a process for obtaining a metal from its compound, it is necessary to obtain sufficient information on the solubility of the oxide in fluoride melts. Two lithium compounds, Li2O and Li2CO3, are tried in the present investigation. Their solubilities in LiF-CaF2 melts at various temperatures and also the liquidus temperature of the eutectic mixtures are measured. Using the solubility data, other thermodynamic data such as integral molar enthalpy, entropy, and Gibbs energy of formation have been calculated.

Cathodic current efficiency and anodic overvoltage are the critical parameters to evaluate the electrolysis process efficiency. Anodic overvoltage measurements on graphite anode during electrolysis of Li2O in LiF-CaF2 were made using steady state current voltage method and limiting current density in the melt is determined. Work is in progress to evaluate other parameters and to compare the process efficiency obtained with different lithium compounds.