Thickness Prediction Model Of Intermetallic Compounds In FSW/UVeFSW Welds Of Al/Mg Dissimilar Alloys

With the rapid development of society,the demands for energy conservation and emission reduction have increased.For transportation industry,lightweight structure is becoming increasingly important.As lightweight alloys,aluminum alloys and magnesium alloys are widely used in aerospace,high-speed train and automobile industries.However,Al and Mg differ greatly in crystal structure and physical properties,which is a great technical challenge to achieve the high-quality joining.As a solid welding technique,friction stir welding(FSW)has great advantages for Al/Mg dissimilar alloys welding.The FSW has been improved and the ultrasonic vibration enhanced friction stir welding(UVeFSW)process has been developed by our research group.Previous experiments have proved that ultrasonic vibration can reduce the thickness of intermetallic compounds(IMCs)in Al/Mg dissimilar alloys UVeFSW process.However,the formation mechanism of IMCs and the influence mechanism of ultrasonic vibration on IMCs remain unclear.In this study,the prediction model of IMCs thickness in the Al/Mg dissimilar alloys FSW/UVeFSW was established,and the formation mechanism and evolution law of IMCs were explored by combining numerical simulation with experimental test.This will provide a basis for optimizing welding parameters and regulating the performance of Al/Mg dissimilar joints,which has important theoretical significance and engineering application value.The process tests of Al/Mg dissimilar alloys FSW and UVeFSW were carried out.The weld transverse cross-section and horizontal cross-section samples were characterized by SEM,EDS and XRD,respectively.It was found that the structure of IMCs in the UVeFSW weld was changed from double layer to single layer,almost no Al12Mg17 layer was found,and the thickness of IMCs was reduced by 2-3 μm.The welding temperature was measured,and it was found that the temperature on the tool/workpiece contact interface was about 620 K,lower than the Al/Mg eutectic temperature.The dislocation density on both sides of the binding interface was obtained by W-H method.It was found that the dislocation density of Al side decreased from 0.3235×1020 m-2 to 0.2403×1020 m-2 at UVeFSW.The dislocation density of Mg side decreased from 1.1203×1020 m-2 to 0.9432×1020 m-2 at UVeFSW.The macro thermo-mechanical model of Al/Mg dissimilar alloys FSW/UVeFSW based on Deform-3D was established.Based on the mechanism of solid atomic diffusion,the micro model of IMCs at the weld bonding interface of Al/Mg dissimilar alloys FSW/UVeFSW was established.The temperature and strain rate of inspected point were extracted from the macro thermo-mechanical model as input parameters of the micro model.For UVeFSW process,considering the effect of acoustic plastic effect and ultrasonic energy on atomic diffusion,the formula of dislocation density and atomic diffusion activation energy in micro model were modified.Applying the prediction model of IMCs thickness,the IMCs thickness in FSW and UVeFSW welds was calculated.The diffusion coefficients at characteristic time instants was quantitatively analyzed with the variation of temperature,strain rate and dislocation density.The calculation results show that the welding temperature of the UVeFSW weld center was about 20 K lower than that of FSW weld center.The dislocation density of Al side decreased from 0.3559×1020 m-2 to 0.0771 ×1020 m-2 at UVeFSW.The dislocation density of Mg side decreased from 0.9759×1020 m-2 to 0.1967×1020 m-2 at UVeFSW.Al3Mg2 was the main IMCs,but Al12Mg17 still existed.The predicted values of IMCs thickness in FSW and UVeFSW welds were 3.9 μm and 1.8 μm,respectively.The simulation results of welding temperature,dislocation density and IMCs thickness were in good agreement with the measured results.On the weld transverse cross-section,the IMCs thickness in the top part(TOP)was 2.6 μm,and the IMCs thickness in the middle and bottom part(MID and BOT)were about 1.85 μm.On the weld horizontal cross-section,with the increase of the distance from the exit hole,the IMCs thickness first increased and then decreased,and finally stabilizd at the edge of the shoulder.These calculated results agree with the experimental characterization.The formation and evolution of the IMCs in the welding process were explored by combining the welding test of "sudden stop+sudden cooling" with the simulation of the IMCs thickness at different moments.The results showed that the growth and evolution of IMCs can be divided into three stages:formation of IMCs,growth of IMCs and decomposition of Al12Mg17.The formation mechanism of IMCs was deduced by thermodynamic calculation and effective heat of formation model.The temperature and dislocation density were decreased and the atomic diffusion degree was weakened by ultrasonic vibration.Therefore,the formation and growth of IMCs were inhibited,which results in the decrease of IMCs thickness.