Multi-field Coupling Analysis Of Simultaneous Interaction Between Ultrasound And Thermo-mechanical Process In Friction Stir Welding Of Aluminum Alloy

Friction stir welding(FSW),as an environmental friendly solid state welding process,has been widely used in joining of light alloys.However,the FSW process requires large axial force and spindle torque to generate enough heat,which causes higher welding load and lower welding speed as well as tool wear.To solve the above problems,our group developed a process variant,i.e.ultrasonic-assisted friction stir welding(UaFSW),which applied ultrasonic vibration to the tool radially along the welding direction.The UaFSW process can reduce the welding load significantly,improved the weld forming quality and increased the welding efficiency.However,the previous research mainly focused on the experimental study of UaFSW process.The ultrasonic action mechanism in UaFSW process is not clear yet.Therefore,it is of great significance to combine numerical modeling with experimental verification to reveal the interaction mechanism between ultrasonic energy,heat generation and plastic deformation in UaFSW process,which will provide theoretical basis for the practical application of UaFSW process.Based on the ultrasonic exerting features of UaFSW,the ultrasonic transmission process of "ultrasonic horn-tool-workpiece" is described with the computational ultrasound method.The ultrasonic distribution and transmission in UaFSW are analyzed.The results show that the ultrasonic direction at the trailing side(TS)and leading side(LS)changes once in a period;the peak ultrasonic pressure is located at the pin root.With the increase of the distance from the tool axis,the ultrasonic field gradually attenuates.G-Z constitutive equation and S-T constitutive equation are modified by acoustic stress work,and the two constitutive equations are combined to determine the flow stress and strain in UaFSW.The modified constitutive equations and calculation method comprehensively consider various influencing factors such as strain,strain rate and temperature.Through numerical simulation of flow stress,strain/strain rate,plastic material flow and ultrasonic field in shear layer,the mechanism of acoustic softening effect and promoting plastic flow is explained.The results show that the modified constitutive equation can accurately predict the transverse cross-section boundary of the welds.Acoustic stress work compensates the thermal activation energy of plastic deformation,which can effectively reduce the flow stress,increase the strain and strain rate in the shear layer.Based on the acoustic stress work and thermal activation mechanism,the slip coefficient at the tool/workpiece contact interface is modifie.The friction/viscous velocity boundary model and force boundary model with ultrasonic effects are established.The ultrasonic effects on the material flow state and heat generation at the cantact interface are revealed.The results show that,although ultrasonic vibration has little effect on the total heat generation,it will change the interfacial slip state and the heat generation ratio,reduce the friction heat and increase the viscous heat generation.In view of the complicated interfacial stress state in UaFSW,Norton friction model is modified by acoustic stress work.The relative motion of material and tool is taken into account as the influencing factor for calculating friction shear stress.The welding load and stress state are compared and analyzed in FSW and UaFSW processes.The results show that the ultrasonic enhancing material flow can result in the antifriction effect,which would reduce the welding load.Considering the ultrasonic direction and intensity factors,the Coulomb friction model is modified to consider the ultrasonic antifriction mechanism.The results show that the antifriction mechanism in UaFSW process is determined by ultrasonic intensity and vibration direction.The directional relationship between ultrasonic vibration and material flow determines the morphological characteristics and antifriction range.And the ultrasonic intensity determines the antifriction strength.The friction coefficient is obviously reduced on the whole contact interface,especially at the pin root.On both LS(Leading Side)and TS(Trailing Side)sides,the region with minimum friction coefficient is in a double butterfly shape.The obvious reduction of friction coefficient will directly determine the reduction of friction shear stress.