Study On The Microstructure And Mechanical Properties Of Bobbin Tool Friction Stir Welding Of 6082-T6 Aluminum Alloy

Bobbin tool friction stir welding is developed on the basis the conventional friction stir welding.It not only retains the advantages of solid-state joining,but also solves problems of inefficiency,high demand for fixtures,welding keyhole in conventional friction stir welding.So it has a high value of research and application.In this paper,3mm and 5mm thick 6082-T6 aluminum alloy was welded by bobbin tool friction stir welding.The temperature field of 5mm thick plate was analyzed and the effect of surface forming was research.The microstructure of 3mm and 5mm thick welded joints were analyzed.The microstructures of the joints were investigated.The influence of rotational speeds,welding speeds,length of the pin and the type of the pin on the mechanical properties were analyzed.The fatigue properties of welding parts under two welding parameters were compared and analyzed.The results show that the temperature of the upper surface is lower than that of the lower surface,and the temperature of the RS is higher than that of the AS.With the increase of the rotational speed and the decrease of the welding speed,the peak temperature of each measuring point is increased.The results of the flow field test show that there is a confluent area of three-way material at the interface of WNZ and TMAZ,which is prone to tunnel defects.The experimental results showed that joint of 3mm and 5mm was similar to "dumbbell" shape.The joint was composed of four areas:nugget zone,thermo-mechanically affected zone,heat affected zone,base material.The microhardness test showed that the hardness of welded joints were "W"shape,the lowest hardness occured in HAZ near TMAZ of RS.tensile test shows when the rotation speed is too low and the welding speed is too high,the welding heat input is too low to produce the tunnel defects in joints,resulting in poor tensile properties.For the 5mm welded joint,when the length of the pin is 4.8mm.the rotation speed is 600rpm and the welding speed is 500mm/min,the tensile strength and elongation of the joint reach the maximum value of 246MPa and 9.71%respectively,which are 73.8%And 70.4%of base metal respectively.Fracture analysis showed that the joints were ductile fracture.For the 3mm welded joint,when the pin type is three-plane plus and double screw,the rotation speed is 600rpm and the welding speed is 500mm/min,the tensile strength and elongation of the joint reach the maximum value of 237MPa and 9.32%respectively,which are 73.8%And 70.4%of base metal respectively.Fracture analysis showed that the joints were ductile-brittle fracture and brittle fracture.3mm and 5mm of the positive bend and back bend strength is almost equal,indicating that can avoid the root defects in the conventional friction stir welding joints.The law of variation is consistent with the change of tensile strength.Under the same stress load,the fatigue performance of aluminum alloy base metal is better than that of welding parts.When the welding parameter is 600rpm/500mm/min,the fatigue limit is 154.6MPa,which is 91.91%of the fatigue limit of the base metal.When the welding parameters is 700rpm/500mm/min,the fatigue limit is 151.8MPa,which is 90.35%of the fatigue limit of the base metal.The location of the fatigue fracture is usually located in the heat-affected zone of the RS or AS.The crack source can clearly observed.Steady state expansion zone has a typical fatigue lines.Instantaneous fracture zone is ductile fracture.