| Friction stir welding(FSW)is a new and efficient solid-phase bonding technology developed by The Welding Institute(TWI)in 1991 based on traditional friction welding.As the core component of FSW technology,the structure design of tool pin plays a decisive role in obtaining high quality welded joints.In this paper,2024 aluminum alloy with 4mm thickness is selected as the experimental material,and the conical thread tool pin is used for welding.A large number of experiments show that when the rotation speed is1000r/min and welding speed is 500mm/min,the inclination of spindle is 2.5° and preheating time is 5s,and the down pressure is 0.20 mm,the inner part of the weld is intact and the surface is beautiful.Under this parameter,the tensile strength of the joint reaches341 Mpa,about 81.2% of the base metal,and the elongation of the joint is 10.4%.On the basis of this test,three groups of different tool pin profiles were redesigned,which were conical thread(A),conical thread with side groove(B),conical thread with three platform(C).Under the above welding conditions,4 mm thick 2024 aluminum alloy was welded.The welding thermal cycle curve,microstructure,mechanical properties and fracture morphology of three groups of welded joints were compared and analyzed.The results show that the thermal cycle curves of the three groups are similar,and the heat input of the advancing side(AS)is slightly higher than that of the retreat side(RS).The welding heat input of tool pin A is the largest,and the peak temperature of AS and RS is 423 ? and408.6 ? respectively;the welding heat input of tool pin C is the smallest,and the peak temperature of AS and RS is 368.7 ? and 345.1 ? respectively.The welding heat input is directly proportional to the total friction area of the tool pin.The results indicate that all the tools can weld good joints,and the grain size of nugget zone formed by tool pin C is the smallest,about 7?13 ?m.The higher the welding peak temperature is,the larger the grain size is.The lowest hardness appeared at the junction of the thermal-mechanical affect zone and the heat affected zone,but the hardness of the weld formed by the tool pin C was 71.76 Hv higher than that of other tool pin,and the tensile strength of the joint formed by the tool pin C was 364.27 Mpa,which was 86.73% of the base metal.The tensile fracture morphology analysis shows that all the fracture modes of the joints are plastic fracture,so the performance of the joint formed by tool pin C is the best.The metal plastic flow behavior of the cross section,longitudinal section and horizontal plane of the weld formed by three groups of tool pin was analyzed.It was found that the number of metal in the AS of the weld was significantly more than that in the RS.The counter clockwise right-hand thread generates a downward driving force,which makes the metal flow to the lower layer and continue to rotate on the AS of the same horizontal plane.The amount of the lower layer metal increases continuously,forming the "onion ring" pattern.The thread on the surface of the tool pin is the main driving force for the downward flow of plastic metal,and the tool pin B has the strongest ability to drive the downward flow of metal.Under the extrusion of the metal on the AS of the nugget zone,the metal on the RS moves upward,around the cavity above the nugget zone,and converges with the metal on the AS to form a turbulent zone.The metal fluidity of the tool pin is C the best,the fish scale like lamellar structure is formed in the turbulent zone.The metal flow in weld longitudinal section is divided into four parts: shoulder influence zone,transition zone,vertical flow zone and extrusion flow zone.The results show that the plasticized metal in the shoulder influence zone moves forward along the welding direction with a certain inclined angle;due to the shear stress on the thread surface of the tool pin,the metal in the vertical flow zone bends backward and migrates downward;most of the extrusion zone flows from both sides to the upper part of the nugget zone,and finally meets in the disordered zone,and a small part flows horizontally away from the welding direction.At the same time,the microstructure and properties,temperature field and plastic metal flow behavior of 2024 aluminum alloy Bobbin tool friction stir welding(BT-FSW)joint were studied on the basis of FSW.The results show that the temperature field of BT-FSW is similar to that of FSW,but the peak temperature of RS is slightly higher than that of AS,and the temperature difference is about 20 ?.The grain size of the upper part of nugget zone is slightly smaller than that of the lower part.In the process of BT-FSW,the metal plasticity on the AS driven by the upper and lower shaft shoulders is transferred to the nugget zone,where it meets and mixes with the metal in the circumferential nugget zone driven by the tool pin to form strip structure.The movement of these two kinds of shaping metal constitutes the mechanism of BT-FSW joint. |
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