Investigation On Joint Formation Mechanism And Mechanical Properties Of Bobbin Tool Friction Stir Welding Of Al-Li Alloys

Bobbin tool friction stir welding(BT-FSW)is a newly developed solid-state joining technology,has a significant application in aircraft tank and fuselage manufacturing.However,BT-FSW is still a very young welding technology,especially in welding of high strength aluminum lithium alloys.Moreover,the BT-FSW of thin sheet still has many problems unsolved,such as the harsh requirement in welding controlling and poor welding stability.Thus,in this study the designing of welding equipment and its controlling method,welding process,weld formation mechanism are investigated to promote the application of BT-FSW on thin aluminum lithium alloy sheets.A brief introduction to this work and its main achievements obtained are as follow.Based on the characteristics of BT-FSW of thin sheet,the mechanism behind the welding instability is studied and an adjustable gap size BT-FSW welding system is reasonably developed.To minimize the risk of welding tool failure,the new developed welding tool shoulders and probes are installed separately and uninstall integrally.An unbalanced force loading resulted by the “COLD” contact at the beginning of welding can easily lead to tool failure,thus the tool is designed and a low welding speed is proposed for the welding process.At last,a displacement controlling system assisted by force sensors is proposed for BT-FSW of thin sheets,thus the welding stability is significantly improved.For the typical BT-FSW joints of AA2198-T851 ally,the effect of heat input on the microstructure and mechanical properties are characterized,as well as the local and global mechanical properties of joints.Results show that BT-FSW joints have symmetric hour glass shape;the stirred zone(SZ)has a relatively low fraction of high angle grain boundary(~36%),and high density dislocation and sub-grain boundaries inside the recrystallized grains.The shoulder affected zone is much bigger than that of the probe(<0.5mm),the joint consist of main simple C {001}<110> shear texture and little A texture,and a discontinuous zone is found in the mid thickness zone.With the dissolve and/or coarsening of precipitates,the weld zone has a sharp hardness drop.As the rotational speed increases,the material flow is significantly improved in the shoulder affected zone,while a small impact happens on the probe affected zone,and the hardness profile along the mid thickness line changes from U-shaped to W-shaped.During tensile testing,the heat affected zone(HAZ)and SZ are prone to strain localization while the base material(BM)has hardly plastic deformation,which leads to a poor global elongation of the Al-Li alloy joint.The fracture locations of joints are closely related to the welding heat input and material flow,the best joint is broken along the border of thermal-mechanically affected zone(TMAZ)and HAZ.A 3D coupled Eulerian-Lagrangian thermomechanical finite element model is,for the first time,developed for BT-FSW to investigate the temperature field,strain field and material flow behavior to understand the joint formation mechanism.Simulations show that the temperature and the effective plastic strain gradient on the advancing side(AS)are higher than those of retreating side(RS),which agree well with the experiment.A weak material flow area is found behind the welding tool on AS during welding,the joint and defect formation process is visualized in this model.Finally,a symmetrical mechanism of weld formation is proposed as: on one hand,an original cavity forms behind the tool;On the other hand,the layered plasticized material is driven to refill the cavity periodically.Once the periodical layers have the same distance and period,some big closed voids will remain in the joint,which is well validated by both simulation and experiment.Based on an asymmetric material flow assumption,the dual-rotational BT-FSW(DBT-FSW)process is proposed to improve the welding stability and quality.The evolutions of the welding torque,weld formation,microstructure and mechanical properties of joints are investigated with different welding parameters.Compared to BT-FSW,an unbalanced force between upper and lower shoulders is observed in DBT-FSW with an evident torque difference of 22.9%~80.6%,and the DBT-FSW joints show asymmetric weld zone with waved structure in TMAZ on AS,indicating that DBT-FSW significantly improves the material flow.Defect free joints can be stably obtained in a wider welding parameters.The DBT-FSW joints have a typical simple shear texture,but the microstructures,microhardness and tensile properties of joints show significant difference related to the welding rotational speed and welding speed.The optimal tensile strength of 404 MPa(85% of the BM)is much higher than that of the BT-FSW joint(80% of the BM),and the tensile elongation reaches to 8.4%.Moreover,the fracture of all joints locate in the border between TMAZ and HAZ.