Research On Formation Mechanism,Microstructure And Mechanical Properties Of Bobbin Tool Friction Stir Welded Joint Of ZK60 Alloy

Mg alloys are the lightest metallic engineering materials.Therefore,Mg alloys are attractive candidates for applications in many industries.Welding is an important fabricating technology of structural component,so that application of welding in Mg alloys conduces to extending their applications.Bobbin tool friction stir welding（BTFSW）is a variant of friction stir welding,which possesses the self-supporting characteristic and is an attractive technique for welding of Mg hollow extrusions.However,BTFSW of Mg alloys is troubled with poor formability,tunnel defect,tool fracturing and strong basal textures.Therefore,the present study investigated the tool design,formation mechanism,microstructure evolution and texture control in BTFSW of ZK60 alloy by the approach of experiments combined with simulation.The results from this investigation can offer some reference to further application of BTFSW to the joining of large hollow extrusions of Mg alloys in the engineering field.Lagrangian-Eulerian coupled model was developed for the BTFSW process using the ABAQUS software,the mesh distortion was overcomed,and the temperature distribution,plasticized metal flow as well as strain field were revealed.The prediction accurancy of the model was validated by experiments in the aspect of weld formation,temperature distribution as well as flow behavior.The formation mechanism of the defect was analyzed.It was revealed that the refilling of the plasticized metal behind the tool was a symmetrical process,where the upper and lower layers of plasticized metal driven by the shoulders had higher refilling rate,while the middle layer of plasticized metal driven by the probe had lower refilling rate.Therefore,a weak flow zone formed at the middle layer.When the upper and lower layers of plasticized metal converged at the middle,the gaps between the layered refilling metal might encounter since the two layers had the same refilling rate.Therefore,some voids were enclosed in the SZ.Effects of the probe geometry on the flow field were especially investigated,and the optimization mechanism of tool design was clarified.The refill of plasticized metal behind the tool exhibited an asymmetrical pattern when a tapered probe was employed,so that the upper and lower layers of plasticized metal had different refilling rate.This resulted in the gaps between the layered refilling metal from the upper and lower layers dispatched with each other when they converge,and the risk of enclosing voids was lowered.Besides,the swept volume of a tapered probe was lower as compared to a cylindrical probe,so that the stirred zone was smaller,and the refilling of the plasticized metal was more adequate,which was also beneficial to minisize the risk of defect formation.Effect of adding thread on probe on the microstructure evolution of the SZ was investigated.The thread induced vertical motion as well as accelerated the strain rate of the plasticized metal.The shearing layer around the threaded probe changed periodically as the probe rotates,which complicated the deformation pattern of the metal.The complicated deformation mode and accelerated strain rate induced by the threaded probe activated the non-basal slip systems including prismatic{10-10}<1-210>and pyramidal{10-11}<1-210>\{10-11}<-1-123>slips to accommodate the strain along with basal{0001}<1-210>slip,which introduced the P₁ and C₁ fiber texture component into the SZ and weaken the B fiber texture component.Meanwhile,as compared to the smooth probe,employing a threaded probe induced a higher nucleation rate so as to obtain finer grain size in the SZ.To the end of reducing the process forces engaged on the tool during BTFSW,forces senors were emplyed to investigate effect of tool geometry on the process parameters.The radical forces and torque on the tool were successfully reduced by optimizating the tool design,so that the risk of breaking of the stir porbe during BTFSW was substantically lowered.By this approach,sound BTFSW joints of 4 mm thick ZK60 alloy were obtained,and the stable welding speed was increased from 200mm/min to 600 mm/min.Based on the optimizated bobbin tool,defect-free joints were obtained in a wide range of parameters,and the weld formation,microstructure evolution and mechanical properties of the joints were investigated.Microstructure evolution during BTFSW of ZK60 alloy was complicated and involved several stages.In the thermo-mechanically affected zone（TMAZ）,grains were firstly deformed by the geometrized effects of strain.Abundant{10-12}<10-11>twins were activated in the deformed grain,and the grain orientation changed from<0001>//ND to<0001>//TD.Most of theβ₁’andβ₂’precipitates dissolved into the matrix,and a certain amount ofβprecipitates remained.In close proximity to the SZ,grain structure evolution was governed by the extensive continuous dynamic recrystallization.Owing to the high ratio of basal<a>and pyramidal<c+a>dislocations being participated in the dislocation rearrangement,the recrystallized grains possessed the preficial orientation of<11-20>//WD.However,the recrystallized nuclei continued to experience shear strain induced by the rotating stir probe,this caused their<0001>gradually to turn to WD.Therefore,a<11-20>～<0001>//WD texture formed.Within the SZ,geometrized as well as discontinuous dynamic recrystallization were also participated in the grain structure evolution,and grain boundary misorientation development was associated with the texture development.The peak temperature in the SZ reached 460℃,so that all theβ₁’andβ₂’precipitates dissolved and onlyβremained.Transverse tensile properties of the BTFSW joints reached 266 MPa,and the joint efficiency reached 0.992.The joints fractured at the TMAZ-side or the SZ/TMAZ interface region during tensile tests.The joints fracturing at the TMAZ-side possessed higher tensile properties.Fracturing mechanism was analyzed,the gains orientations in the TMAZ-side were prone to both{10-12}<10-11>twinning and{0001}<11-20>slip.The soft grain orientation along with decomposition of theβ₁’andβ₂’precipitates corporately resulted in the TMAZ-side to be firstly deformed under tensile loading.High density of dislocations in the TMAZ-side and the grain size mutation across the SZ/TMAZ interface made it hard to induce the adjacent region,i.e.,SZ-side deform.The above led to the necking and fracture in the TMAZ-side.On the other hand,strong textures were evolved in the SZ,and the different sub-regions within the SZ had different easiness for activation of basal slip and extension twinning under tensile loading.Specifically,the grains in the SZ-side had high schmid factor for extension twinning,while the grains in the adjacent region had high schmid factor for basal slip.The different deformation mechanism led to deformation incompatibility between the two adjacent regions,so that induced fracture along the SZ/TMAZ interface.Impact toughness at the SZ/TMAZ interface of the joints lay in the range of 5.9-7.0 J/cm².On the other hand,impact toughness at the SZ of the joints lay in the range of 4.8-5.8 J/cm².Bending angle of the joint reached 54.5°.The bending properties of the joint were comparable to those of the extruded plate.