Research For Microstructure And Properties Of Magnesium Alloy TIG Welded Joint Under The Combined Action Of Magnetic Field And Activated Flux

Due to its characteristics of small density,large specific strength,good processing performance,and recyclability,magnesium alloy has met the current requirements of energy conservation and environmental protection.It is known as"21^st-century green material"and has been widely used in aerospace,automobile,and 3C fields.However,due to the unique physical characteristics of magnesium alloy,its weldability is relatively poor,which seriously limits its application and promotion.Therefore,the development of an efficient and high-quality welding method has a positive effect on the application of magnesium alloy.The introduction of activated flux in A-TIG welding greatly improves the welding efficiency,while the magnetron welding technology can realize the contactless intervention of the welding process and improve the performance of the welding joint.Therefore,in this paper,a longitudinal alternating magnetic field and NiCl₂/TiO₂activated flux were applied in the TIG welding process of AZ91 magnesium alloy.During the welding process,the magnetic field parameters,welding parameters,types of activated flux,coating methods,and coating amounts were changed.The formation,phase composition,microstructure,and mechanical properties of welded joints were tested under different parameters.At the same time,the motion pattern of arc and molten pool was studied through simulation calculation and a high-speed camera.The crystallization nucleation process and crystal growth characteristics of magnesium alloy under the combined action of the magnetic field and activated flux were investigated.Through the relevant test and analysis,some conclusions were obtained as follows:（1）When no magnetic field was applied,the performance of the welded joints obtained under the activated flux coated on double-side was better than that of which obtained under the activated flux coated on single-side.For NiCl₂activated flux,the best coating amount for double-side coating was 2mg/cm².At this time,the forming coefficient of the welded joint was2.32,the hardness of the weld zone was 62.3HV,the tensile strength was 312MPa,and the elongation rate was 12.3%.For TiO₂activated flux,the best coating amount for double-side coating was 3mg/cm².At this time,the forming coefficient of the welded joint was 3.29,the hardness of the weld zone was 78.9HV,the tensile strength was 265MPa,and the elongation rate was 10.05%.（2）Under the combined action of the magnetic field and NiCl₂flux,the performance of the welded joint coated with activated flux on the double-side was better than that of which coated on the single-side.The optimal coating amount of activated flux was 3mg/cm².At this time,the forming coefficient of the welded joint was 2.38,the hardness of the weld zone was67HV,the tensile strength was 338MPa,and the elongation was 13.3%.The introduction of the magnetic field could refine the grain size（the grain size was 55.5%finer than that without the magnetic field,when the activated flux was coated on double sides）,increase the number of twins in the microstructure,inhibit crack propagation,thus improving the performance of the welded joint.Furthermore,it could promote the preferential growth of the crystal along the（0001）crystal plane.（3）Under the combined action of the magnetic field and TiO₂flux,the performance of magnesium alloy welded joints was slightly improved when the activated flux was coated on double-side.The best performance was obtained at the flux coating amount of 3mg/cm².At this time,the forming coefficient was 2.79,the hardness of the weld zone was 75.6HV,the hardness of the heat-affected zone was 77.8HV,the tensile strength was 292MPa,and the elongation was 11.2%.The introduction of the magnetic field did not affect the preferential growth of wed metal,but it had a more obvious effect on the grain refinement.When the coating amount of activated flux was 3mg/cm²,the grain size of the weld seam applied the magnetic field was refined 8.4%compared to that of which applied no magnetic field.（4）When the magnetic field and NiCl₂ activated flux was applied,the morphology of the welding joints and the nucleation of the molten pool was mainly changed by their combined effect on the morphology of welding arc.Under suitable parameters of the magnetic field and activated flux,the arc stiffness would be improved,and the radius of arc rotation would be reduced.However,if the parameter of the magnetic field was too large,the phenomenon of"magnetic suction"occurred,which reduced the stability of the arc and affected the weld formation.（5）When the magnetic field and TiO₂ activated flux was applied,the morphology of the welding joints and the nucleation of the molten pool was mainly changed by their combined effect on the morphology of welding molten pool.Under suitable magnetic field and activated flux parameters,the moving speed of the molten pool was increased and the stirring effect was enhanced.However,if the magnetic field parameter was too large,the phenomenon of"tailing"appeared,which affected the weld formation and comprehensive mechanical properties.