Research Of BC-MIG Welding-brazing Of Marine Steel/Aluminum Hybrid Structures

Hybrid structures of dissimilar materials between aluminum and steel can make use of the inherent advantages of two materials,such as light weight,high strength and high corrosion-resistance,etc.They are used widely in the fields of automobile manufacture and aerospace vehicle.However,the joining of aluminum and steel is very difficult due to the low solid solubility,the great difference of thermal-physical and mechanical properties,as well as the formation of brittle intermetallic compounds in the interfacial layer.According to the characteristics of shipbuilding welding structure,the new welding method of bypass-current MIG welding-brazed?BC-MIG?was applied in this thesis to joining aluminum/steel dissimilar metals.The different welding parameters on influence of mechanical properties and the morphology of aluminum/steel joints were investigated.The microstructure of joints was researched,on this basis,the thickness and intermetallic compounds of interface layer were further analyzed.In addition,the corrosion behavior and mechanism of joint were studied through the corrosion test.The results showed that BC-MIG welding technology could obtain stable process and good welding appearance for joining marine aluminum/steel dissimilar metals.It was the key to obtain the high quality joint that taking reasonable welding parameters which include MIG current,bypass TIG current,welding speed and the space between tungsten and wire.The typical welding brazing morphologies of Al/Fe joint was divided into the fusion zone,weld zone,central interface zone and base metal.The metallurgical bonding between aluminum alloy and steel was a thin continuous reaction layer,which consisted of FeAl3,Fe₂Al₅Znx and other slight Fe-Al phases.As the thickness of the reaction layer consisted of intermetallic compounds has its effect primarily on the mechanical properties of welded joints,so the thickness is mainly depended on the reaction temperature and thickness of Zn coated steel in the BC-MIG process.The tensile strength of aluminum/steel BC-MIG welded joints changes differently under different welding parameters.Within a certain range of welding parameters,the tensile strength of joints increases gradually with the increase of main MIG welding current;joint tensile strength decreases firstly and then increases as bypass TIG current increases;as welding speed increases,the tensile strength of joints decreases after increases;as the space between tungsten and weld filler increases,the increase of joints tensile strength shows a changing trend,and when tungsten and weld filler are located in the center of lap joint,the joint shows a maximum tensile strength.Experiment shows that,the maximum tensile strength of the joint is 187.1MPa and joints fracture in the aluminum base metal regions,showing a typical ductile fracture.The corrosion rate of the weld zone was lower than the steel base metal and central interface later owning to silicon particle in the filler metal and the oxide film forming on the weld region played important role to protect the weld zone against corrosion.Higher charger transfer resistance was presented on EIS plots for central interface layer than base metal,which means the corrosion rate of central interface layer was lower than steel base metal.Due to the amount of intermetallic phase particle in per unit area of different region was different.It causes distrinct electrochemical corrosion reactions occurred in per unit area of different region during corrosion process,which leads different the corrosion rate in different region.BC-MIG welding technology overcomes some difficulties that caused by traditional welding methods,and improve the welding efficiency of aluminum/steel for shipbuilding.It is shown that the great economic and advanced technology value in future shipbuilding.In addition,the new welding method of BC-MIG provides new ideas for the development of shipbuilding.