| Friction stir welding(FSW) is a kind of solid phase bonding technique. It has a lot of advantages, such as no wires, no holes, no flippers, little residual stress, little deforma tion and so on. Therefore, it is used widely in automotive industry, shipbuilding industry and aerospace industry. But when it comes to high melting point alloys such as carbonsteel, stainless steel and Ti alloy,the low heat input limits the further development of friction stir welding. This research studies the EFSW(electric friction stir welding) which combines friction heat with electric heat to conquer the problem that the limits of traditional friction stir welding of high melting point alloys.For EFSW technology, there is no research about the effect of current type on microstructure and mechanical properties of same material EFSW process and the effect of current size on microstructure and mechanical properties of dissimilar material EFSW process. This research applies 4 different current types: 1) DC, 2) pulse DC, 3) rectangular square AC, and 4) pulse rectangular square AC, to weld AZ31 B magnesium alloy by EFSW and discusses the effect of current type on EFSWed AZ31 B magnesium alloys based on macroscopic morphology of joints cross section, formation mechanism of weld, micro hardness, tensile property and fracture mechanism. AZ31 B magnesium alloy and ZK60 magnesium alloy were welded by EFSW with current size of 0A, 50 A, 100 A, 150 A. The effects of current size on the EFSW process of dissimilar material AZ31 B magnesium alloy and ZK60 magnesium alloy are evaluated from the aspects of joint combination performance, element diffusion, tensile property, and fracture morphology change. The results show that the width of weld joint increases when current type changes from DC to pulse DC or the peak current increases. The shape of weld cross section changes from funnel style to full bowel style. Both continuous dynamic recrystallization and discontinuous dynamic recrystallization happen in WNZ(Weld Nugget Zone). Geometric dynamic recrystallization happens in TMAZ(ThermoMechanically Affected Zone). Grain size in WNZ decreases with the increase of the peak current. The micro hardness of joint cross section shows a “W” shape. The weld zone turns soft. Electric current can help strengthen the hardness of soft zone. The tensile test shows that all the failures happen at the advancing side of TMAZ and the angle of fracture is 45°. Both adopting current and increasing peak current can improve the tensile strength of joints. There are many dimples in the fractures. Adopting current or increasing peak current can enlarge dimples. Adhesion appears during the EFSW process of combining AZ31 B magnesium alloy with ZK60 magnesium alloy. The cross section of the dissimilar material joint changes from a smooth overlapping type to a bend zigzag type with the increase of the current. And increasing the current also facilitates the diffusion of element Zn at the interface. Dimples of the dissimilar material joints cross section are elongated. Because of the adhesion problem, fracture type changes from ductile fracture to ductile brittle mixed fracture with the increase of the current.In addition, this paper also analyzes the mechanism of cavity forming and the effect of current type on cavity defect and discovers the effect of cavity defect on the joints mechanical properties. The results show that all cavities appear at the advancing side of joints and are near the bottom of the joints. The size of cavities decreases with the increase of the peak current. Fractures of joints all appear near the advancing side of TMAZ and HAZ(Heat-Affected Zone). The fracture direction is perpendicular to the left side of the cavity. Cavity defect decreases mechanical properties of joints sharply and weakens the improvement of joints' mechanical properties caused by the change of current size. But the change of joints' mechanical properties caused by changing current size isn't affected by the cavity defect. |
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