Preliminary Investigation On Design Criteria And Preparation Of Mg-Gd Master Alloy By Molten Salt Electrolysis

As the upstream products in the field of rare earth magnesium alloys,the composition stability and microstructure uniformity of rare earth master alloys have great influence on the properties of terminal magnesium alloys under the strong influence of microstructure heritability of magnesium alloys.The existing preparation technologies of master alloys are mainly divided into fusion method and molten salt electrolysis method.Among them,the rare earth master alloys prepared by molten salt electrolysis method have the advantages of uniform composition,fine structure and low cost,so they are more widely used,but the contents of impurities in molten salt electrolysis products are high.Molten salt electrolytic system is complex,there is no corresponding screening and control criteria,intermediate alloy composition control is difficult and the current production equipment automation degree is low,electrolytic process adjustment precision is poor,high labor intensity,and the production of master alloy products by these equipment iron,silicon,aluminum and other impurities content is easy to exceed the standard.In view of the above problems,this work studies Mg-Gd master alloy for molten salt electrolysis,and systematically probes into the preparation of Mg-Gd by molten salt electrolytic fine control from the aspects of molten salt and alloy composition screening,process parameter regulation,construction of molten salt electrolytic device,multi-physical field simulation and experimental verification of electrolytic cell.Based on multi-physical field simulation,the characteristics of temperature field,flow field and electric field inside the electrolytic cell were studied.The formation rules and phase structure characteristics of the master alloys with different compositions were verified by experiments.The main research conclusions are as follows:1.The design criteria of molten salt composition and alloy composition were obtained,and the molten salt composition was determined as Gd Cl₃:Mg Cl₂:KCl=22:13:65.The optimum interval of density difference of Mg-x Gd alloy is designed:x≤10%;x≥65%to make the alloy separation effect is better.2.Cathodic eutectosis can be achieved by controlling the potential in the cathode region with cyclic voltammetry and avoiding the precipitation of potassium ions with high reduction potential.At the same time,the cathode current density is controlled to be less than 2A/cm²,so as to avoid anode effect.In this case,the alloy composition can be controlled by adjusting the proportion of materials.3.Through finite element simulation,multiple physical fields in molten salt electrolyzer were coupled.According to data collection and analysis,the electrode depth was 100mm and the electrode spacing was 75mm.The experimental verification process is consistent with the simulation results.For Mg-10Gd alloy,the master alloy product with relatively stable composition can be obtained.However,due to the intensified surface fluid disturbance caused by the anode gas,the suspended master alloy is difficult to accumulate,and the inclusion phenomenon with molten salt is more significant.The experimental verification of Mg-90Gd alloy found that the actual content of rare earth is only about 40wt.%,and the eutectoid process is not carried out according to the ideal state.4.By discussing the microscopic mechanism and the evolution law of the structure composition in the electrolytic reaction,and aiming at the problems existing in the electrolytic verification experiment,the process was optimized and the electrolytic test of the rare-earth rich master alloy was carried out with the optimized steps,and the master alloy product with relatively uniform structure and about 79.3%rare earth content was obtained.It is proved that the stability control of the master alloy composition can be achieved by improving the process to approximate the ideal electrolytic state.