Research On Corrosion Behavior Of Friction Stir Welded Joints Of AA5052 Aluminum Alloy/AZ31B Magnesium Alloy

With the development of science and technology,people are more concentrate on green development,energy conservation and environmental protection,thus green manufacturing is developing rapidly,and the process of automobile lightweight is developing faster and faster.As excellent materials in lightweight materials,aluminum alloys and magnesium alloys are widely used in the fields of rail transit,aerospace,and vehicle engineering.To accelerate the reengineering application of aluminum/magnesium alloy dissimilar materials,it is necessary to research its corrosion resistance besides the strength of the connection.In this paper,AA5052 aluminum alloy and AZ31B magnesium alloy with a thickness of 3 mm were used as experimental materials,and the alloys were butt welded using appropriate welding process parameters.Taking the obtained welded joint as the research object,the corrosion behavior of the welded joint under neutral salt spray environment was studied,and the corrosion law and mechanism were analyzed by combining the morphology of corrosion microstructure and electrochemical test.In order to study the corrosion behavior of welded joints in different environments,the response surface method was used to design an experimental program with 3 factors and 5 levels,and the corrosion behavior and corrosion characteristics of welded joints were explored under different Cl~-concentration,p H values and corrosion time.In addition,COMSOL Multiphysics finite element software were used to model and numerically simulate the complex system of macro-galvanic corrosion in welded joints in practical applications,and the differential behavior of galvanic corrosion in different areas of welded joints were analyzed.The main conclusions are as follows:(1)The corrosion resistance of AA5052 aluminum alloy/AZ31B magnesium alloy friction stir welded joints in a neutral salt spray environment is not as good as that of the two base metals,showing a high corrosion tendency.Different degrees of corrosion occur in different areas of the entire joint.The zone corrosion severe most is the thermo-mechanical affected zone,which is located at the junction of the stir zone and the magnesium alloy base metal.In addition,through the analysis of the corrosion micro-morphology,it is found that the micro-corrosion morphology of each area is quite different.The corrosion morphology of the stir zone is lamellar,the corrosion expansion direction is horizontal,but the corrosion morphology of the thermo-mechanical affected zone is shown as corrosion pits,and the corrosion expansion direction is longitudinal.Throughout the neutral salt spray test,the corrosion rate of welded joints showed a tendency to increase first and then decrease.After the magnesium on the surface of the stir zone dissolves,the insoluble aluminum and intermetallic compounds hinder the expansion of corrosion to the inside.(2)The central composite design of response surface method was used to design a test matrix with 3 factors and 5 levels.The empirical relationship between the corrosion rate of joints and the Cl~-concentration,p H value and corrosion time was established through regression analysis.The analysis of variance shows this empirical relationship is very significant.The corrosion products produced in the corrosion process hinder the further expansion of corrosion to the inside,resulting in the corrosion rate of the welded joint showing a decreasing trend as the corrosion time increases.Since the corrosion product is difficult to stably exist in an acidic environment but can stably exist in a neutral or alkaline environment,the corrosion rate of the welded joint in the acidic environment is the largest,and the corrosion rate in the alkaline environment is the smallest.(3)Through the modeling of the galvanic corrosion system composed of the magnesium alloy base material,the stir zone and aluminum alloy base metal in the welded joint,the significant macroscopic galvanic corrosion effect is found among the magnesium alloy base metal,the stir zone,and the aluminum alloy base metal.There are differences in electrochemical characteristics between the stir zone and the other two base metals,resulting in uneven current density on the entire welded joint surface.As the only anode,the stir zone is preferentially dissolved in the entire galvanic corrosion system.The maximum current density in the entire welded joint is at the junction of the stir zone and the two base metals.The current density at the junction of the stir zone and the aluminum alloy base metal is greater than the current density at the junction of the magnesium alloy base metal and the stir zone.The galvanic corrosion formed by the stir zone and the aluminum alloy base metal makes more contribution to the galvanic corrosion of the entire welded joint.