Study On Preparation And Mechanism Of RE-Al-Cu Intermediate Alloy By Molten Salt Electrolysis In Fluoride

On account of RE-Al-Cu alloy with the advantages of high specific strength,high temperature resistance,significant corrosion resistance and good ductility properties etc.,it has been widely applicated in aerospace,nuclear industry,wire and cable field,new special material.In order to obtain RE-Al-Cu alloy with stable composition and reliable quality,RE-Al-Cu intermediate alloy having a density similar to that of the substrate is firstly generally obtained in industrial production.At present,with the disadvantage of high cost,large energy consumption,long cycle in RE-Al-Cu intermediate alloy preparation method,thus it is of great practical significance to find a preparation method with high efficiency,environmental protection,energy saving,economy and mass production for the preparation of RE-Al-Cu intermediate alloy.So,this paper took Na₃AlF₆-AlF₃-Li F-MgF₂ fluoride foundation molten salt as electrolyte,used RE₂O₃?La₂O₃ or Sm₂O₃?,Al₂O₃,CuO etc.,metal oxide MeO as electrolytic raw materials,prepared RE?La or Sm?-Al-Cu intermediate alloy by molten salt electrolysis.Based on the study of dissolution behavior of molten salt system,the structure of molten salt,and the basic physical and chemical properties,electrochemical characteristics,and electrolytic preparation technology process,it is possible to set up theoretical foundation and technical prototype for molten salt electrolytic preparation of RE-Al-Cu intermediate alloy with fluoride-oxide system by analyzing dissolving mechanism,dissolution kinetics process,the thermodynamics-dynamic mechanism of electrode process and studying the control mechanism of components and impurities.First of all,It had been studied dissolving behavior of single or mixed of MeO in Na₃AlF₆-AlF₃-LiF-MgF₂ basic molten salt by using isothermal saturated method,and found that after a certain time La₂O₃,Sm₂O₃,Al₂O₃ or CuO could reach to dissolving balance and saturated solubility were 5.389 wt%,5.447 wt%,8.45 wt%and 6.488 wt%,respectively.MeO solubility increased with the rising of temperature,and determined the numerical of?35?_fusG⁰_MeO,?_MeOin dissolving process.When NaF-AlF3 mole ratio increased,RE2O3 and CuO solubility decreased,while Al₂O₃ solubility increased.It illustrated that the dissolving mechanism of RE₂O₃,CuO and Al₂O₃ was different.RE₂O₃ and CuO were alkaline oxides in molten salt,but Al₂O₃ was acidic oxides.Therefore NaF-AlF₃ mole ratio was 2.3.MeO dissolution rate was mainly determined by mass diffusion,which is caused by the MeO concentration difference between particles surface and molten salt.Secondly,by high resolution Raman spectrometer,Na₃AlF₆-AlF₃-RE₂O₃?La₂O₃,Sm₂O₃?molten salt microstructure was able to be measured through high temperature situ Raman spectrometry.Comparing with Raman spectrum of pure and mixture substance in high or normal temperature,it was found that the original single material structure characteristics of Na₃AlF₆,AlF₃ and RE₂O₃ generally disappeared while new Raman peaks emerged.The result confirmed that RE₂O₃ could dissolve in Na₃AlF₆ and generated new substances.Na₃AlF₆-AlF₃-La₂O₃ molten salt Raman characteristic peaks were located in 270 cm^-1,319cm^-1,462 cm^-1,500 cm^-1,590 cm^-1?552 for shoulder peak?,740 cm^-1,2328 cm^-1.Na₃AlF₆-AlF₃-Sm₂O₃ molten salt Raman characteristic peaks were located in 270 cm^-1,288cm^-1,319 cm^-1,474 cm^-1,500 cm^-1,554 cm^-1,590 cm^-1,750 cm^-1,2328 cm^-1.It indicated that Al-F,Al-O-F,RE-F,RE-O-F complex anion structure existed in molten salt,which might be AlF₄^-,AlF₆^3-,AlOF₂^-,Al₂OF₈^4-,AlOF₅^4-,AlOF₃^2-,REF₄^-,REOF₂^-etc.Then,Na₃AlF₆-AlF₃-LiF-MgF₂-MeO molten salt density,viscosity,conductivity,surface tension were measured respectively by Archimedes method,rotation method,continuously vary cell constant method,pull cylinder method,influence of temperature,single or mixed MeO amount on physical and chemical properties was studied,and the regression mathematical model between density,viscosity,conductivity,surface tension and temperature,the dosage of MeO?La₂O₃,Sm₂O₃,Al₂O₃,CuO?was established.Fourthly,the electrochemical process of La?III?,Sm?III?,Al?III?,Cu?II?in Na₃AlF₆-AlF₃?12 wt%?-LiF?5 wt%?-MgF₂?5 wt%?-MeO molten salt system was studied mainly by cyclic voltammetry method and auxiliary by square wave voltammetry,chronoamperometry and chronopotentiometery method.It determined that La?III?reduction process was La?III??La?I??La?0?,the reduction potential were-1.0^-0.8 V,-1.6^-1.3 V respectively and reduction process was a reversible process on carbon electrode.Sm?III?reduction process was Sm?III??Sm?0?and reduction potential was more negative to Na₃AlF₆-AlF₃-MgF₂-LiF molten salt system steady potential.Al?III?reduction process was Al?III??Al?0?with the reduction potential between-1.2 V and-1.0 V,reduction process was a reversible process on carbon electrode.Cu?II?reduction process was Cu?II??Cu?0?with the reduction potential between-0.3 V and-0.1 V,reduction process was a reversible process on carbon electrode.Finally,RE-Al binary or RE-Al-Cu ternary intermediate alloy was prepared by electrolysis in Na₃AlF₆-Al F₃?12 wt%?-Li F?5 wt%?-MgF₂?5 wt%?-RE₂O₃?3 wt%?-Al₂O₃?6wt%?-CuO?3 wt%?molten salt system.Effect of electrolytic temperature,cathodic current density on cell voltage,back electromotive force,intermediate alloy composition,current efficiency was investigated and the optimal process conditions of preparation intermediate alloy was found.The microstructure of intermediate alloy,prepared under better technological conditions,was characterized by SEM,EDS,XRD.The results indicated that RE-Al binary alloy was mainly composed of Al matrix,RE₃Al₁₁1 intermetallic compounds and RE-Al-Cu ternary alloy was mainly composed of Al matrix,Al₂Cu,LaAl₆Cu₆ or SmAl₈Cu₄ intermetallic compounds.The impurity in intermediate alloy only was Si or O,of which content was rather low.Therefore,it was a great superiority and development prospect to prepare RE-Al-Cu intermediate alloy by molten salt electrolysis in Na₃AlF₆-AlF₃-Li F-MgF₂-RE₂O₃-Al₂O₃-CuO system.