Numerical Analysis Of Ultrasonic Vibration Enhanced Friction Stir Welding Of Dissimilar Al/Mg Alloys

With the development of economy and society,the demand for energy conservation and emission reduction has increased,and light materials such as aluminum and magnesium alloys are widely used in automobile,aerospace and high-speed train industries.The joining of aluminum alloy and magnesium alloy can make full use of their advantages and make up for each others shortcomings.However,due to the great differences in crystal structure and physical properties between the two materials,the high-quality joining of Al/Mg dissimilar alloys faces special challenges.The ultrasonic vibration enhanced friction stir welding(UVeFSW)process has been developed by our research group.Previous experiments have proved that UVeFSW has unique advantages in joining dissimilar Al/Mg alloys.However,there are complex coupling mechanisms of multi-fields in the Al/Mg dissimilar UVeFSW process,and the influence of ultrasonic vibration on the "heat generation/temperature profile-stress/strain-material flow-material mixing" behaviors is more complex.Therefore,it is of great significance to model the UVeFSW process of dissimilar Al/Mg alloys and carry out the coupled numerical analysis of multi-physical fields,so as to reveal the influence mechanism of ultrasonic vibration on the formation of dissimilar Al/Mg joints and realize the effective utilization of ultrasonic energy field in fricition stir welding of dissimilar Al/Mg alloys.The computational fluid dynamics(CFD)model for friction stir welding(FSW)process of dissimilar Al/Mg alloys was established,where Mg alloys was on advanced side and A1 alloys was on retreating side.The involved physical issues,such as material transfer/mixing,heat generatio/transfer and the plastic material flow were described and analyzed.To improve the predicted accuracy of material mixing and distribution,the local turbulence under the tool pin was considered.The non-linear relationship between material-properties and the volume fraction was established.The flow,mixing and distribution of dissimilar materials in the weld nugget zone were quantitatively analyzed.The acoustic-plastic constitutive equation was established by considering the influence of both ultrasonic softening and residual hardening on the flow stress at different temperatures and strain rates.Two cases were compared:(i)ultrasonic softening and residual hardening effects were considered;and(ii)only ultrasonic softening was considered.The numerical results of UVeFS Welding of aluminum alloy show that in the inner annular region near the pin side,the calculated flow stress in case(i)is higher than that in case(ii),while near the outer ring,the calculated flow stress is contrary to the previous situation.The range of plastic deformation and material flow in UVeFSW is wider than that in FSW,which is consistent with the experimental results.The developed acoustic-plastic constitutive equation improves the prediction accuracy of material flow.Based on the ultrasonic induced friction reduction(UiFR)theory,the UiFR effect on friction coefficient in different relative directions at the FSW tool-workpiece interface were quantitatively calculated and analyzed.The projection of friction coefficient distribution at the tool-workpiece contact interface is in a butterfly shape,and the UiFR effect is more obvious at the advancing side and retreating side than that in front of the FSW tool.Consiering the ultrasonic induced reduction of friction coefficient improved the prediction accuracy of heat generation and temperature distribution in the UVeFSW process of aluminum alloy.The UVeFSW process model of dissimilar Al/Mg alloys was developed through including the above-mentioned acoustic-plastic constitutive equation and ultrasonic induced friction reduction.The ultrasonic sound pressure and energy are stronger at the aluminum alloy side.In the stirred zone,there is the pattern distribution of ultrasonic sound pressure and energy.The heat generation at tool-workpiece contact interface and viscous dissipation were reduced after applying ultrasonic vibration.Due to the UiFR effect,the projection of friction coefficient and heat flux distributions at the tool-workpiece interface show a "deformed" butterfly shape.The calculated results show that ultrasonic vibration enhanced the material flow and promoted the mixing and distribution of dissimilar materials.The model was experimentally validated.Finally,the atomic diffusion theory was used to estimate the intermetallic compounds(IMCs)thickness at a checking point on the Al/Mg interface in FSW.The influence of temperature,strain rate and dislocation density on the diffusion coefficients was considered.The IMCs thickness at characteristic times in welding process and the effects of dislocation density and temperature on IMCs formation at different stages were analyzed quantitatively.The prediction results of IMCs thickness were in good agreement with the measured ones.