Microstructure And Properties Of Ultrasonic Friction Stir Welded 2219-T6 Aluminum Alloy And Mechanism Of Acoustoplastic Effect

Friction stir welding(FSW),as a solid-state joining technology,has been successfully applied in the aircraft and aerospace field due to its superiorities in joining high-strength aluminum alloys.Welding defects involving porosity and crack in traditional fusion technologies can be completely avoided in FSW.However,the properties of conventional FSW joints are very sensitive to the heat input.Higher heat input results in a serious joint softening,however,lower heat input is inevitable to introduce cavity,tunnel and kissing bond,leading to a reduction of the resultant mechanical properties.Ultrasonic friction stir welding(UFSW)is a promising technology to solve the foregoing problems in the conventional FSW by employing an extra ultrasonic field.Therefore,the microstructures and mechanical properties of the joints can be significantly improved.Considering the characterization of the temperature distribution and material flow patterns during FSW process as well as the damping of ultrasonic energy,the ultrasonic should be applied from the bottom surface of the workpieces where the defects are easily formed due to the lowest temperature and the most terrible material flow ability.However,the published literatures have never achieved this because of the existence of backing plate in the conventional FSW.An innovative UFSW with the ultrasonic applied from the bottom surface of the workpieces was proposed in the present study and the backing plate was replaced by a designed support column.During the welding process,the support column undertook the axial force to ensure the formation of UFSW joints.It should be highlighted that the support column can dramatically free the space under the workpieces,providing a promising future for the further development of hybrid FSW.The results of the macrostructure and microstructure reported that the volume of stir zone(SZ)in the UFSW was larger than that in the conventional FSW,indicating that the ultrasonic can activate more materials participating in the plastic deformation of 2219-T6 aluminum alloy.When the heat input was lower,there were S line and kissing bond in the conventional FSW joints.While when the ultrasonic was applied,the original butt interface can be completely eliminated and no defect was induced.During the dynamic recrystallization,the average grain size in the SZ of UFSW joint was finer than that in the SZ of conventional FSW joint because the strain rate of the plastic materials was increased by the ultrasonic.On the other hand,during the static recrystallization,the velocity of grain growth was significantly improved by the ultrasonic.Therefore,the resultant grain size was determined by the battle between the two processes.Furthermore,the precipitation strengthening can be significantly enhanced by the ultrasonic.In comparison,the density and size of the reprecipitated ?' phase in the SZ of UFSW joint were higher than those in the SZ without the effect of the ultrasonic.Furthermore,with the welding speed increasing,the density and size of ?' phase in the SZ of the conventional FSW joint were conspicuously decreased,however,a large number of reprecipitated precipitates can still be observed in the SZ of UFSW joint.Ultrasonic can improve the mechanical properties of the 2219-T6 aluminum alloy joints and enlarge the processing window of FSW.First,the tensile strength of the joint was improved by the UFSW.The maximum tensile strength was increased to 361 MPa with a corresponding joint efficiency of 82%.Second,the processing window was greatly enlarged and the joint efficiencies of most UFSW joints were more than 80% in the selected welding parameters.The microhardness distribution displayed a “W” shape and the average hardness values in UFSW joint were higher.Third,the elongations of joints were significantly improved by the ultrasonic.In detail,at high heat input conditions,the elongations of the UFSW joints were improved by 16%?36% compared with the conventional FSW joints,while at low heat input conditions,the elongations of the UFSW joints can be improved by 55%?71% compared with the conventional FSW joints.The positron annihilation spectroscopy(PAS)and molecular dynamics(MD)were employed to measure the vacancy concentration and calculate the vacancy formation energy,respectively.The results indicated that the ultrasonic introduced excess vacancies into the UFSW process by reducing the vacancy formation energy.When the temperature was 750 K,the vacancy concentration in the conventional FSW was 3×10-4,however,it was elevated to 5×10-5 in the UFSW,i.e.the vacancy concentration in the UFSW was improved by almost one order of the magnitude.When the vacancy concentration was increased,grain boundary sliding and migration were both facilitated in the UFSW due to the enhanced movement of grain boundary dislocations and grain boundary self-diffusion,leading to a larger growth rate of equiaxed grains in the SZ.Based on the analyses of precipitates,a vacancy-induced ?' precipitation mechanism was proposed.Considerable Cu-vacancy pairs as a result of the high-density vacancies induced by the ultrasonic provided copious nucleation sites for ?' phases.The vacancy migration and condensation onto the pre-existing phase interface can relieve the coherency strains,permitting a rapidly unrestricted growth of the precipitates.Finally,the precipitation strengthening was enhanced by the ultrasonic.The physical property of the materials was altered by the ultrasonic.The mechanism of the acoustoplastic effect was summarized as follows.A supersaturated vacancy concentration induced by the ultrasonic increased the width of dislocation core and thus resulted in a reduced Peierls stress,namely acoustic softening during the ultrasonic effect.The formation of appreciable helical dislocations arising from the sinking of vacancies on the screw dislocations reduced the mobility of dislocations,leading to the residual hardening after the ultrasonic effect.