Study On Microstructure And Properties Of Friction Stir Welded Joints Of ZG61 Magnesium Alloys With RE

At present,magnesium alloy is the lighter metal material in engineering application.It could meet the modern industrial requirement of saving energy and reducing consumption,and widely used in aerospace,automobile manufacturing,transportation,3C and other field.Friction stir welding(FSW)as a solid connection technology,applied to the welding of magnesium could avoid cracks,pores,deformation and other defects generated by conventional welding methods,and obtain good formation welding joints.Extruded rare earth magnesium alloys ZG61 were investigated,ZG61 magnesium alloys and dissimilar magnesium alloys between extruded ZG61 and cast AZ91D were carried out with different parameters.The weld forming,joints microstructure,mechanical properties and corrosion behavior were researched and analyzed,the influence on the microstructure and properties of the joints with the rotation speed and the traverse speed were discussed.When the rotation speed were 1000r/min and 1200r/min,traverse speed were 80mm/min and 120mm/min,weld with good formation on the surface,and without gas holes or tunnel-type defects inside were obtained with all those four parameters.When the traverse speed was 40mm/min,the joints with poor quality on the surface and tunnel inside,cause the heat input were improper.The crown zone near the shoulder has a very thin layer(about 0.1-0.13mm)of fine structure.The refine and equiaxed grains at weld nugget are small and uniform,between the weld nugget zone and the heat affected zone of the advanced side has a clear boundary.Besides,the onion ring structure is not observed in ZG61 FSW joints,the large grain belt and the small grain belt are parallel distribution in the weld nugget zone while the welding parameter is 1000r/min-120mm/min.The study of ZG61 FSW joints on corrosion performance shows that the corrosion of joints is mainly local corrosion in the initial stage of corrosion.The ZG61 joints get the best corrosion resistance while the welding parameter is 1000r/min-80mm/min.The results of microhardness of the joints shows that the transverse microhardness distribution of the joints is presented ' W ' in shape.By the combination of three mechanisms of fine grain strengthening,working hardening and dispersing strengthening,the microhardness reaches the maximum 82.8HV in the junction between weld nugget zone and the thermo mechanical affected zone of advanced side.The tensile test results shows that the cracks of defective ZG61 FSW joints locate in weld and the fracture mode is brittle rupture,while the cracks of defect-free joints locate in the region of junction between weld nugget zone and the thermo mechanical affected zone of advanced side,and the fracture mode is brittle and toughness.The tensile strength increase first and then decrease with the increasing of traverse speed,the highest tensile strength is 260MPa and reaching 88%of the base metal while the welding parameter is 1000r/min-80mm/min.As for the FSW joints of ZG61/AZ91D,two kinds of metals are dynamically recrystallized to form fine uniform and equiaxed grains.Onion ring structure could be observed in the joints while the traverse speed was 120mm/min.Plastic flow of metal during friction stir welding is divided into three processes.Because of the influence of the shoulder,the metal in the central area of the crown zone flows downwards and the lower part of the metal is pressed upward along both sides.The metal in retreated side is driven by the stir pin then flows clockwise along the advanced side and terminates in the crown zone.The metal in advanced side is driven by stir pin and under the influence of suction and extrution then flows back to the retreated side and terminates in the 1/2 height of the joint.The corrosion behavior of ZG61 magnesium,AZ91D magnesium and ZG61/AZ91D FSW joints are studied by immersion test and electrochemical test,the results show that the corrosion of ZG61 magnesium can be divided into three stage:pitting corrosion,filiform corrosion and stress corrosion.The mainly corrosion types of AZ91D magnesium are pitting corrosion and exfoliation corrosion.Comparing the corrosion resistance of the three groups of the samples,it is concluded that the corrosion resistance of AZ91D magnesium alloy is the best,the ZG61/AZ91D FSW joint is the second,and the corrosion resistance of ZG61 magnesium alloy is the worst.The tensile test results of ZG61/AZ91D FSW joints at different traverse speed shows that the cracks of all the joints in different parameters locate in the heat affected zone of AZ91D side,and the fracture mode is brittle and toughness.The tensile strength decrease with the increasing of traverse speed,maximum tensile strength of the ZG61/AZ91D joints is 153MPa while the traverse speed is 80mm/min,reaching 78%of the AZ91D base metal.