Research On DCEN A-TIG Welding Mechanism Of 2219 Aluminum Alloy

2219 Aluminum alloy has a series of advantages such as good mechanical properties,stress corrosion resistance,welding performance,low thermal crack tendency and so on.It is widely used in aerospace field.During the process of evaluating the welding process performance of 2219 Aluminum alloy,it was found that the high porosity of AC TIG welding was proved to be the main weakness of welding joint quality.The welding arc of2219 Aluminum alloy by AC TIG welding is unstable,the welding seam is easy to clip tungsten,and the material itself has high sensitivity to porosity,so the porosity is serious,especially in the fusion zone prone to dense micro-porosity,in addition,the AC TIG of low welding depth and low production efficiency restricts its development.In order to solve the problem of high porosity in 2219 Aluminum alloy AC TIG welding seam,DCENA-TIG welding method was proposed.In view of the characteristics of stable arc,high heat production in DCEN TIG welding and fluoride removal of oxide film,the advantages of fluoride and DCEN TIG welding were combined,and the fluoride activating flux was used to remove oxide film on the surface of Aluminum alloy to realize DCEN A-TIG welding process of 2219 Aluminum alloy.The outstanding characteristics of DCEN A-TIG welding were that the internal porosity of TIG welding seam was effectively reduced,the penetration of welding seam was increased,the welding quality was improved,and the grade I welding seam quality requirements of medium thickness structure of carrier rocket storage tank were met.DCEN A--TIG welding of 2219 Aluminum alloy has been successfully carried out.however,the optimization of activating flux formulation,removal of oxide film on the surface of 2219 Aluminum alloy and internal porosity of weld seam and increase of penetration of weld seam still need to be clarified through in-depth and systematic research.Therefore,the activating flux formulation for 2219 Aluminum alloy DCEN A--TIG welding was designed and optimized,and the mechanism of removing oxide film on 2219Aluminum alloy surface and porosity in TIG welding seam and increasing penetration of welding seam was discussed.In this paper,2219 Aluminum alloy surface oxide film was removed by the development of activating flux formula,and 2219 Aluminum alloy DCENA-TIG welding technology was realized.The mechanism of activating flux to remove 2219 Aluminum alloy surface oxide film,inhibit TIG weld internal porosity,increase weld penetration was discussed,which provided theoretical support for promoting2219 Aluminum alloy DCEN A-TIG welding technology in the carrier rocket tank structure application.?1?The formulation of mixed component activating flux for 2219 Aluminum alloy DCEN A-TIG welding was studied.The development of activating flux is the core content of 2219 Aluminum alloy DCEN A-TIG welding technology.In this paper,the mixed component activating flux formula based on fluoride was obtained by the method of uniform mix design and quadratic polynomial stepwise regression.This formula can not only remove the oxide film on 2219 Aluminum alloy surface,inhibit the porosity in TIG welding,but also increase the penetration of welding seam.?2?The mechanism of removing the oxide film on 2219 Aluminum alloy surface and inhibiting the porosity in TIG weld was studied.Thermodynamic calculation and criterion of chemical reaction between fluoride activating flux and Al₂O₃ oxide film were carried out by using Gibbs free energy function method.The surface morphology of TIG weld pool before and after arc quenching was collected by high-speed photography system,and the change of oxide film on the surface of pool with and without activating flux was studied.XRD was used to analyze the phase of the slag.It was found that the difference of thermal expansion coefficient between Al and Al₂O₃ oxide film that under the action of welding high temperature arc,the Al₂O₃ oxide film will crack,but the Al₂O₃ oxide film will still cover the surface of the molten pool firmly.The coating of fluoride activating flux increases the defect of Al/Al₂O₃ interface and reduces the adhesion of Al₂O₃ oxide film to Al matrix.Therefore,the oxide film on the weld surface can be peeled off.The removal mechanism of oxide film by different fluoride ZnF₂,LiF,Nocolok and mixed component activating flux is analyzed.The infrared thermal imager was used to measure the temperature field of the welded joint.It was found that the application of activating flux could increase the heat input of DCEN TIG welding,decrease the solidification rate of the molten pool,increase the existence time of the molten pool,and promote the hydrogen bubble to escape from the molten pool.Through the analysis of the stress on the surface of the molten pool,it can be seen that the application of activating flux causes the increase of welding heat input,changes the original stress state of the molten pool,and the flow direction of the molten pool is conducive to bubble emerging.Thereby these measure inhibit that generation of weld porosity.?3?The mechanism of activating flux increasing the penetration of 2219 Aluminum alloy DCEN A-TIG weld was studied.The weld penetration and weld width were measured.Compared with the DCEN TIG weld without activating flux,the weld penetration and weld width were increased by AlF₃,ZnF₂ and mixed component activating flux.The weld penetration and weld width decreased with the increase of welding speed and increased with the increase of welding current.It was found that the arc morphology coated with AlF₃activating flux appeared tailing and the arc morphology coated with ZnF₂ activating flux expanded to some extent,while the arc morphology coated with LiF,Nocolok and K₂SiF₆did not change.Through double tungsten sheet test,weld bead migration test and surface scanning of weld element,fluoride activating flux will not enter the weld pool along with the flow of metal,fluoride is mainly through the influence of welding arc to increase penetration.The ionization and recombination of fluoride activating flux in welding arc can increase weld penetration.Coating fluoride activating flux causes the increase of the heat input of welded joint.The weld penetration increases with increasing heat input,at the same time,the heat input makes the weld width increases.?4?The effects of DCENA-TIG welding and VPTIG welding on the microstructure and properties of 2219 Aluminum alloy joints were studied.DCENA-TIG welding reduces the occurrence of porosity in the weld fusion zone and chain porosity in joint.Through tensile strength and stress corrosion test,DCENA-TIG welded joint has the same performance as VPTIG welded joint.The effect of process parameters on mechanical properties of 2219Aluminum alloy DCENA-TIG welded joint was studied.Proper concentration of activating flux is beneficial to reduce weld porosity,cleaning process before welding has little effect on joint strength after welding,and welding current and welding speed have great effect on joint strength.Too small welding current and Too large welding speed will cause the phenomenon of incomplete fusion on the back of weld,greatly reduce the mechanical properties,wire feeding speed mainly has an effect on the residual height and width of weld.The microstructure and phase of DCENA-TIG welded joint were investigated by scanning electron microscopy,transmission electron microscopy and XRD.The relationship between microstructure and hardness distribution and corrosion properties of welded joints in different regions was established.