Experimental Study On Material Flow During Friction Stir Welding Of Al/Mg Dissimilar Alloys

Al/Mg composite components can take into account the performance advantages of the two alloys,further achieve lighter weight,and have broad application prospects in industries such as aerospace and automobile manufacturing.Due to the large differences in crystal structure and physical and chemical properties,the high-quality connection of Al/Mg dissimilar alloys faces huge challenges.Although friction stir welding(FSW)has obvious advantages when welding dissimilar materials,there are still many problems that need to be solved when it is used for welding of Al/Mg dissimilar alloys.In this project,the ultrasonic vibration enhanced friction stir welding(UVeFSW)process developed in the early stage was used to carry out the FSW and UVeFSW butt welding of 4 mm thick 6061-T6 aluminum alloy and AZ31B magnesium alloy.The material flow,the thickness of IMCs on the interface,the mechanical locking,the area of the weld nugget zone,and the fracture characteristics of the joints are compared and analyzed.The results show that under the adopted process conditions,when the tool is offset by 0.5 mm to the advancing side of the magnesium alloy,the welding forming and tensile properties are the best.In FSW,simply changing the rotation speed or welding speed,the tensile strength of joints shows a trend of first increasing and then decreasing.If the rotation speed and the welding speed are not properly matched,it is unfavorable to the welding forming and material flow.The material flow inside the joints obtained by comparing whether the pin is threaded is analyzed,and it is found that the thread can effectively promote the material flow and mixing in the weld nugget zone,thereby reducing or avoiding welding defects.The results of UVeFSW show that ultrasonic vibration can reduce the thickness of IMCs,enhance the mechanical locking of the interface.At the same time,it promotes the flow and mixing of materials at different parts of the joints,effectively expands the area of the weld nugget zone,and reduces or even eliminates welding defects.Under the conditions of typical parameters,the strength of UVeFSW joints is higher than that of FSW;when the heat input is low,the effect of ultrasonic vibration on tensile strength is more obvious.Under the experimental conditions,the maximum strength of FSW and UVeFSW joints is 156.49 MPa and 174.20 MPa,respectively.Compared with the FSW joints,the fracture positions of the UVeFSW joints are transferred from the Al/Mg interface to the HAZ-TMAZ interface on the magnesium alloy side during tension.At the same time,the crack propagation path will be more tortuous,and the plastic fracture area in the fracture will also be enlarged.The characteristics of plastic fracture become more obvious.Using 1060 aluminum foil as the marker material,it was respectively embedded in the Mg on the advancing side and Al on the receding side.After welding,the width of the aluminum foil at different positions was measured on the horizontal cross-section metallographic picture,and the principle of mass conservation and plane strain was applied to obtain the flow velocity,strain and strain rate of the marker material around the tool in the FSW and UVeFSW.The results show that ultrasonic vibration increases the flow velocity,strain and strain rate of the marker material near the tool.The copper powder was preset to different positions on the butt surface.After welding,according to the three-dimensional reconstruction results of CT detection,a three-dimensional model of the material flow near the tool during FSW and UVeFSW was constructed.The results show that the ultrasonic vibration increases the volume of the plastically deformed material near the tool,and promotes the material flow in the vertical direction at the same time.The flow behavior of the marker material around the tool is strongest when welding the same aluminum alloy,poor when welding the same magnesium alloy,and worst when welding Al/Mg dissimilar alloys.