Research On Process And Mechanism Of Bobbin Tool Friction Stir Welding For ZK60 Magnesium Alloy

Bobbin tool friction stir welding(BT-FSW)born out of traditional FSW.It not only retains the advantages of solid phase connection technology,but also simplifies the welding tool and overcomes the problem of root defect.In this thesis,the 8 mm thick 2219 aluminum alloy was bobbin tool friction stir welded using the specially designed tool.The influence of process parameters on microstructure characteristics and mechanical properties of joints were investigated in detail and the optimization of process parameters was obtained based on this,which laid a foundation for practical engineering application of high-strength magnesium alloy.The effects of different shoulders and pin on joint forming were analyzed.The optimum design of plane shoulder with threads and tapered platform with threaded stirring joint were used to conduct welding experiments.Finally,a well-formed and defect-free joint was obtained.At the same time,the welding temperature and motor torque are monitored in real time.The influence of process parameters on temperature and torque was established.It is found that with the increase of heat input(the increase of stirring head speed or the decrease of welding speed),the peak temperature will rise significantly,and the torque of spindle and X-axis motor will decrease in stable state.On the contrary,the peak temperature decreases and the torque of the motor rises.Due to the existence of two shoulders,the weld area of friction stir welding of ZK60 magnesium alloy dual-shaft shoulder is dumbbell-shaped.The region which is directly stirred and composed of equiaxed crystals is called WNZ.Located on both sides of WNZ,it is driven by the extrusion of WNZ plastic metal,and the region mainly composed of slender and strip grains is called the TMAZ.Areas not subjected to direct or indirect mechanical action are called HAZ.The grain size increases with the increase of heat input.ZK60 magnesium alloy is a typical precipitation-strengthened magnesium alloy.Under the intense thermal effect of BT-FSW process,the precipitated phase of the weld will dissolve or break to different degrees,leading to the destruction of the precipitation strengthening of the joint.In the micro-hardness test of the joint,the base material area has the highest hardness value,the welding core area is seriously damaged due to precipitation strengthening,and the hardness value is the lowest.The shape and size of the joint hardness softening area are affected by the welding heat input.In the tensile performance test,the optimal tensile strength of the joints without defects is 264 MPa,and the elongation is 9.6%,which is equivalent to 95% and 65% of the base metal respectively.The tensile fracture of the joints without defects is located at the junction of the welding core zone and the influence zone of thermal engine on the forward side,and the joints have a ductile fracture.When the heat input is too low or too high,because the joint internal or surface defects,the defect formation during tensile stress concentration,lead to crack on the germination and expand,and intensity is greatly reduced,the fracture position is located in the weld defect,the defect in tensile fracture surface SEM images have no obvious plastic deformation,lead to joint the overall SEM appears as ductile-brittle-ductile mixed mode.Through the BT-FSW process,the impact work of joint WNZ and HAZ increased slightly compared to the parent material,and the impact work of HAZ was the highest.Different process parameters have a great impact on WNZ impact work of joints,but have no significant impact on HAZ impact work.The main reason is that unreasonable process parameters lead to defects in WNZ,which will seriously affect its impact work.