The Reaserch On Mg-Li And Mg-Zr Alloys Preparation By Electrolysis In Molten Salt

As a kind of green engineering material,Mg alloy have recently attracted great attention in academic research and industry application.Mg-Li and Mg-Zr alloys were extensively applied in the aviation and aerospace field.In this thesis,the Mg-Li and Mg-Zr alloys were prepared through electrolysis method in the molten LiCI-KCl eutectic.The electrodepositi.on mechanism of Li and Zr as well as the electrolysis process parameters of Mg-Li and Mg-Zr alloys were studied.The electrochemical behaviour of Li⁺ was studied at the molybdenum and magnesium electrodes in the molten LiCl-KCl eutectic.Transient electrochemical techniques,such as cyclic voltammetry,chronoamperometry and chronopotentiometry were used in order to explore the deposition mechanism of Li.The results showed that the electroreduction of Li⁺ proceeds via a single step at 450℃and the reaction is irreversible.The diffusion coefficient of Li⁺ was determined to be 1.86×10^-6cm²/s.Nucleation polarization was observed during the electrodeposition of Li.This nucleation is an instantaneous process.At Mg electrode,the electroreduction of Li⁺ took place at a less cathodic potential values than that at Mo electrode for the depolarization of the formation of Mg-Li alloy. Two kinds of Mg-Li alloy with different phase were prepared through the controlling of electrolysis potential.Mg-Li alloy was prepared in the molten LiCl-KCl electrolyte at low temperature.The electrolysis process parameters such as current density, temperature and electrolysis time were optimized.The stability of the electrolysis equipment were also investigated.The results showed that the optimized electrolysis process was a practicable manufacturing scheme.The addition of zirconium element can significantly refine the microstructure of magnesium alloy.Molten salt electrolysis method was used to produce Mg-Zr alloy.The electroreduction process of Zr(â…£) was studied in LiCl-KCl-K₂ZrF₆ melt at molybdenum,tungsten and liquid magnesium electrodes.Transient eletrochemical techniques,such as cyclic volmmetry,chronoamperometry and chronopotentiometry were used.The results showed that Zr(â…£) was reduced to Zr metal by a two-step mechanism corresponding to the Zr(â…£)/Zr(â…¡) and Zr(â…¡)/Zr transitions.The intermediate product was identified as Zr(â…¡) by X-ray diffraction. Nucleation polarization was also observed during the electroreduction process of Zr at molybdenum electrode.Two formation mechanism of Mg-Zr alloy were promoted:(â…°) Zirconium is electrodeposited on the magnesium electrode,and (â…±) Zr(â…£) and Zr(â…¡) ions are reduced by Mg metal.The factors which might affect the Zr content were investigated,such as current density,temperature,electrolysis time and the K2ZrF6 concentration.The optimized electrolysis process was determined as follows:electrolyte composition: 53%KCl-42%LiCl-5%K₂ZrF₆(mass%),electrolysis temperature:998 K, electrolytic time:1 h,cathode current density:5.66 mA cm^-2.The Zr content of Mg-Zr alloy could be as high as 1.55 wt%.