Research On Process And Mechanism Of Magnetic Field Assisted MIG Welding-brazing For Ti/Al Dissimilar Alloys

The hybrid structure of Ti/Al dissimilar alloys can take full advantage of the two materials and achieve the balance of cost and performance,which has been widely used in aviation and transport industries.However,joining of these couples and their alloys remains a difficult technological task because of the important mismatch in chemical affinity and metallurgical compatibility such as melting point,heat conductivity and coefficient of linear expansion,and the formation of brittle Ti-Al intermetallic compounds.Recently,the application of arc welding is usually restrained to joining dissimilar metals because of its high heat input and arc temperature,although it offers economic advantages and convenience.However,the external magnetic field can modify the weld arc behavior and droplet transfer process,which can effectively control the heat input and improve the joint quality.Therefore,the combination of cold metal transfer(CMT)process and the external magnetic field is proposed to break through the deficiency of traditional methods.In this paper,6061-T6 aluminum alloy with thickness of 2 mm and TA2 pure titanium with thickness of 1 mm were lap welded by the external magnetic field hybrid CMT welding process.The weld appearances,mechanical properties and fracture mechanism of Ti/Al dissimilar joints were studied.The wettability of the molten metal on Ti plate enhanced and the reliable bonding area increased with the applied of the magnetic field.The tensile shear strength of the joints indicated obvious improvements with the applied of magnetic field.The fracture position was occurred at Ti base metal with the increase of coil current and presented ductile fracture mode.The microstructure and interfacial structure of Ti/Al dissimilar joint were investigated.The joints could be divided into the fusion joint at the Al side,weld seam,and the brazed joint at the Ti side.Grains of the weld metal zone were more refined and the microhardness of the weld zone and HAZ zone increased with the applied magnetic field.Moreover,the magnetic field could suppress the formation of brittle intermetallic compounds.The thickness of interfacial layer decreased with the increase of magnetic flux intensity.The effects of magnetic field on weld arc behavior,droplet transfer process and weld thermal cycle were investigated.The arc plasma exhibited a spiral motion and the weld arc changed to rotating bell shape with the application of magnetic field,which increased the spreading ability of molten pool.Besides that,the droplet changed to rotating ellipse shape and rotated around the wire,thus redistributed the heat input.The droplet transfer frequency decreased with the increase of magnetic flux intensity.Finally,the effects of the external magnetic field on formation mechanism of Ti/Al dissimilar joint and evolution of interfacial microstructure were clarified.