Numerical Simulation And Experimental Investigation Of Al-to-Mg FSLW/UVeFSLW Process

Nowadays,there are urgent demands for lightweight structures in manufacturing sector of transportation vehicles.Al-to-Mg dissimilar alloys composite structures are widely applied in industry because of their low density and high strength.Thin-sheet Al-to-Mg composite parts are generally made by lap welding,and friction stir lap welding(FSLW)is the important forming process for such lap-joint parts.However,the influences of lap configurations(Al as the top sheet or Mg as the top sheet)and pin lengths on the microstructures&tensile shear strength of the lap joints are still unclear.In addition,the process mechanism in ultrasonic vibration enhanced friction stir lap welding(UVeFSLW)of Al-to-Mg dissimilar alloys needs to be further researched.In this work,numerical simulation is combined with experimental investigation to analyze the heat generation,heat transfer and material flow behavior in Al-to-Mg FSLW and UVeFSLW.It can provide basic data for the process optimization,thus,there are great theoretical significance and engineering application potential.Two lap configurations(Al/Mg,Mg/Al)and three levels of tool pin length(3.0 mm,3.8 mm and 5.2 mm)were adopted to carry out the Al-to-Mg FSLW experiments with sheet thickness of 3+3 mm.For Al/Mg configuration,when the pin length was increased from 3.0 mm to 3.8 mm,the interfacial temperature was increased,and the intermetallic compounds(IMCs)at Al-Mg interface became thicker.But when the pin length was increased from 3.8 mm to 5.2 mm,the interfacial temperature was decreased,and the IMCs at Al-Mg interface became thinner.For Mg/Al configuration,when the pin length was increased,the interfacial temperature was increased,and the IMCs at Al-Mg interface became thicker.For the two lap configurations,the tensile shear strength was decreased when the pin length was increased.For Al/Mg configuration,the temperature was higher,the material flow ability was better,the mixed region was larger,and the IMCs at Al-Mg interface was thicker than those in Mg/Al configuration.The thicker IMCs in Al/Mg configuration resulted in the lower tensile shear strength.The experiments alone are not enough to fully understand the heat generation,heat transfer and material flow behaviours during FSLW process.Numerical simulation is a powerful tool in revealing the process mechanism in FSLW.In practice,the tool was tilted backwards by a slight angle relative to the welding direction whether in FSW or FSLW.The shoulder is only contacted with the top sheet material in the Al-to-Mg FSLW process,which is like the similar metal FSW process.In order to learn about the influences of the tool tilt angle more intuitively,the Al FSW was taken as an example for numerical analysis.Then the treatment method was applied to the Al-to-Mg FSLW process.The fluid mechanics analysis of the materials at shoulder/workpiece contact interface was conducted,and the expressions to determine incomplete contact region and the deflection were further derived.Correspondingly,the non-uniform axial stress,slip rate,friction coefficient were modified in complete and incomplete contact regions.The influences of tool tilt angle on heat generation and material flow behavior in Al FSW process were quantitatively analyzed.A computational fluid dynamics(CFD)model of Al-to-Mg FSLW process was established by using the volume of fluid(VOF)method.The tool tilt angle,local liquation at lap interface and tool adhesive layer were considered.The differences between the predicted results with and without consideration of the local liquation and tool adhesive layer were compared,and the accuracy of the model with considering these two factors was verified.Numerical analysis results showed that the interfacial heat flux and viscous dissipation heat generation in Al/Mg configuration were higher,which resulted in higher temperature.The material flow was also stronger in Al/Mg configuration.The mixed region in Al/Mg configuration extended from the pin bottom to the position close to the top surface on the advancing side(AS),while that in Mg/Al configuration extended from the pin bottom to the lap interface.Then the effects of tool pin lengths were studied in Mg/Al configuration.When the pin length was increased,the interfacial heat flux and viscous dissipation heat generation got rising,the peak temperature was higher,the high-temperature region around the tool was obviously expanded.Therefore,tool could drive more materials to flow,and the mixed region expanded.The Al-to-Mg UVeFSLW experiments were conducted.The exerted ultrasonic vibration enhanced the materials flow ability,decreased the interfacial temperature,increased the effective sheet thickness(EST)and the effective lap width(ELW),and reduced the thickness of IMCs at Al-Mg interface.The ultrasonic effect made the tensile shear strength of UVeFSLW joints higher than that of FSLW joints.For Mg/Al configuration,the fracture mode was changed from brittle fracture mode in FSLW to mixed fracture mode of brittle and ductile in UVeFSLW.The dislocation strain energy in both grain interior and grain boundary were considered to modify the constitutive equation.The function related to temperature and strain rate was selected to calculate the grain size in different regions.The ultrasonic reflection and refraction at the lap interface,heat generation due to sonotrode friction and acoustic absorption,and the antifriction effect were considered in the model.Al-to-Mg UVeFSLW multi-physical model was developed,including ultrasonic field,temperature,plastic deformation and dissimilar materials intermixing.The influences of ultrasonic vibration were quantitatively analyzed in Mg/Al configuration.Ultrasonic vibration reduced the flow stress,the interfacial friction coefficient,and the heat generation,which resulted in lower temperature.The ultrasonic vibration enhanced the material flow ability in the horizontal direction,and decreased the material flow ability in the vertical direction.The materials distribution in the vertical direction near the lap interface(Z=3 mm)were more uniform.The numerical analysis results explained the reason why the UV decreased IMCs thickness,increased the EST and ELW,improved the tensile shear strength in the experimental results.