Study On Refining Process Of Cast Al-2Li-2Cu Alloy

Al-Li alloys have many advantages,such as low density,high specific strength and specific stiffness,good corrosion resistance and anti-fatigue performance,and has broad application prospects in aerospace and other fields.However,at present,the researches of Al-Li alloys are mostly concentrated in wrought Al-Li alloys,while there is still lack of data on cast Al-Li alloys.Lithium has low melting point and very lively chemical properties,and it is easy to inhale and oxidize during the preparation of casting aluminum lithium alloy smelting,forming a large number of stomata and oxidation inclusions,which seriously damages the mechanical properties of the castings.At present,the casting of wrought Al-Li alloys alloy is mostly carried out under the protection of vacuum or closed argon,which is not suitable for the production of large complex structure Al-Li alloy castings.Therefore,the development of smelting preparation and high efficiency melt refining process,is of great value to the industrial application of cast Al-Li alloys.In this paper,Al-2Li-2Cu-0.2Zr alloy was used as the object to do the research on the refining process.According to the different state of the melt before and after the addition of Li in Al-Li alloy smelting process,the refining process was carried out in two stages.Through OM,XRD,SEM,EDS and other analytical methods,combined with density,tensile and other performance test experiments,two-stage C₂Cl₆ refining,two-stage gas bubbling refining,two-stage composite refining were tested.The effects of different refining processes were characterized by the Li loss rate,density,fluidity,inclusion volume fraction and composition,and mechanical properties.The refining process of cast Al-Li alloys was systematically studied,the principle of different refining processes was clarified,and the optimal refining process of two-stage composite refining process was developed.As for the unrefined alloy,the inclusions were mainly composed of Al_xO_y,Li₂O,LiH.After two-stage C₂Cl₆ refining proccess,the gas content and inclusions of the alloy were greatly reduced,because the Cl₂ produced by C₂Cl₆refining proccess will react with Li and produce LiCl,may lead to the formation of new inclusions.After two-stage gas bubbling refining proccess,the gas content and inclusions in the alloy were also reduced,but Argon does not react with aluminum melt and Li,and no new inclusions were introduced in the refining process.After the two-stage composite refining process,the purity of the alloy was the best,and the content of gas and inclusions was greatly reduced.Compared with the unrefined alloy,the loss of Li element is reduced,the density of alloy increases,the fluidity of melt and the mechanical properties of the alloy are greatly improved after different refining processes.C₂Cl₆ refining proccess and gas bubbling refining proccess are the floating method in adsorption refining,the difference is that C₂Cl₆ produce a large number of bubbles to carry out gas and discharge slag through the reaction with high temperature aluminum liquid,the reaction is severe efficient,but the refining process will react with a part of Li,increase the loss rate of Li and introduce new inclusion LiCl.The gas bubbling refining proccess is carried out by rotating the nozzle and blowing argon into the melt to carry out the gas and removal slag,which is less efficient than C₂Cl₆ refining proccess,but the argon does not react with the aluminum melt and Li and can provide a protective atmosphere and reduce the loss rate of Li.The three refining processes studied in this paper are divided into two stages:first-stage refining in order to reduce the content of gas and inclusions in the melt before Li addition,and the second stage refining is aimed at removing the gas and inclusions content in the melt after Li addition,especially the easily produced Li₂O inclusions,to ensure the melt quality.In the process of refining the two-stage composite refining process,the first stage C₂Cl₆ refining proccess can efficiently remove the gas discharge slag,compared with the argon rotary jet blowing,reduce the refining time,improve the efficiency of degasification andslag removal;the secondary gas bubbling refining proccess can carry out gas discharge slag for the melt without reacting with Li and providing a protective atmosphere.Therefore,compared with three kinds of refining processes,two-stage composite refining process gets best effect.Through two-stage composite refining process,the loss rate of Li in cast Al-Li alloy decreased from 23.7%to 14.4%,the average number of integral objects of alloy was reduced to less than 0.125%,and the average flow length of alloy fluidity specimen reached 1240mm.After aged treatment,the yield strength,ultimate tensile strength and elongation of the alloy reached 225MPa,345MPa and 9.2%respectively,and the strong toughness was greatly improved.