Formation Mechanism,Microstructure And Mechanical Properties Of The Joint Prepared By Probeless Friction Stir Spot Welding Of An Al-li Alloy

As an innovative solid-state joining technology,friction stir spot welding(FSSW)has the advantages of small deformation,uniform weld microstructure and environmentally friendly,which has a significant application in manufacturing.It shows great potential to replace conventional spot joining technologies,like resistance spot welding and riveting.In order to eliminate the keyhole defect inherent in the conventional FSSW,which affects the joint strength and reliability,probeless FSSW(P-FSSW)has been proposed.However,it still has some key problems unsolved,such as interface defects and poor welding stability.Thus,the purpose of this study is to investigate the effect of welding tool,the joint formation mechanism,as well as the mircrostructure and properties of the P-FSSWed 2198-T8 Al-Li alloy joints with the assistance of experiment and simulation.The main conclusions are as follows:Based on a 3D thermo-mechanical finite element model of the P-FSSW,the distributions of the temperature and plastic strain as well as the material flow were investigated to clarify the forming characteristic of P-FSSWed joints.The distributions of temperature and plastic strain are symmetrical with respect to the centerline,which is in agreement with the macrostructure.According to the flow field,it is found that the material flow is consistent with the rotation direction.The material with preferential flow is located at about 1/2-2/3 of the radius,which is the same as the temperature distribution due to large heat flux outside the shoulder.In addition,the material flow does not follow closely the shear direction of the tool movement.But to a limited extent,it flows inwards along the rotation direction.And the centrifugal effect of the shoulder indeed exists,which facilitates the expulsion of plasticized material.The material flow in the thickness direction gradually weakens,resulting in a spiral pattern with a certain degree of contraction.The upward flow of the materials in the bottom sheet distorts the interface and forms the hook defect.The welding process was optimized by using response surface methodology.Results indicate that the connection strength is significantly affected by dwell time and rotation speed,followed by plunge rate.A sufficiently long dwell time is essential for heat conduction,material flow and expansion of the stir zone to form a sound joint.From the response surface graphs,it can deduced that the maximum tensile/shear strength is 7.83k N corresponding to the dwell time of 7.2s,rotation speed of 950rpm and plunge rate of 30mm/min.It is in good agreement with the experimental result,of which the experimental value reaches 7.84±0.42k N and the error is within 6%.Then,the morphology of the P-FSSW joint was quantitatively characterized,and a phenomenological model was established.It is found that when the ratio of the shoulder diameter(D)to sheet thickness(d)exceeds 6,sound joints could be obtained by parameter optimization.The larger the ratio,the wider the process.Conversely,when the ratio is less than 6,the interface connection is weak to form a good joint due to the limited maximum size of the stir zone.For a typical P-FSSWed 2198-T8 Al-Li alloy joint,the microstructure evolution as well as its effect on the local properties was analyzed.Results show that fine recrystallized grains are observed in the stir zone(SZ)due to severe friction heat and plastic deformation but without obvious texture.The thermo-mechanically affected zone(TMAZ)is characterized by highly deformed grains,and a large number of subgrain boundaries due to dynamic recovery.The grains in heat affected zone(HAZ)show similar morphology to the base metal(BM),which appear to be flattened and elongated.In addition,the secondary phases also change significantly during welding.Almost all the original T₁phase,which is the main strengthening phase in the BM,dissolves during welding due to severe thermal cycle in the SZ,only a little??phase re-precipitates.And some intermetallic particles(Al-Cu-Fe)with rodlike shape in the BM are broken into smaller sizes due to intense deformation at high temperature.The microhardness profiles change from‘W'type in the upper sheet to‘U'type in the lower sheet,with the lowest value at the boundary of HAZ and TMAZ due to partial dissolution or coarsening of the T₁phase.The corrosion results show that there is no significant difference in the circuit potential of each region,but the pitting potential of SZ is slightly higher than those of BM and HAZ due to grain refinement and solution strengthening.Pitting is the dominant corrosion type in all regions,which takes place mostly in the vicinity of the intermetallic particles with the particles acting as cathodes.And the interface microstructure indicates that local metallurgical bonding is attributed to stress-induced atomic diffusion behavior.The roughening and oxide cleaning are benefical to promote the diffusion to form a good connection.Aiming at improving the weak connection of the P-FSSWed joint,the double-side P-FSSW(DP-FSSW)was proposed.Results show that the material flow and plastic strain at the interface are significantly enhanced,which improves the interface connection.It makes up for the difference in welding efficiency and joint strength between the P-FSSW and other welding processes by virtue of rapid forming and high process stability,and facilitates the application of probeless forming technology.