Numerical Analysis Of Thermal Processes And Plastic Material Flow In Ultrasonic Vibration Enhanced Friction Stir Welding

Being a solid state welding technology, friction stir welding (FSW) requires sufficient heat generation and material flow to produce defect-free joints with good properties. Thus, very large axial force and tool torque are necessary to soften high strength and high hardness materials, such as high strength aluminum alloy, steels and titanium alloy. This poses a risk of rapid tool wear and limits the welding speed and thus the scope of FSW for widespread industries applications. The ultrasonic vibration energy can decrease the deformation resistance of materials, it has great potential to assist the plastic deformation, improve the material flow and lower the welding load in FSW. On this basis, the ultrasonic vibration enhanced friction stir welding (UVeFSW) was developed. In UVeFSW, the ultrasonic vibration energy is transmitted directly into the localized area of the workpiece near and ahead of the FSW tool. Preliminary experimental studies have shown that this novel FSW can obvious lower the welding loads and improve the weld quality. However, the process mechanism of UVeFSW is still unrevealed. A complete understanding of the underlying mechanisms of UVeFSW requires mathematical modeling and simulation of the process.In order to understand the underlying mechanisms of UVeFSW, a transient model is firstly developed to quantitatively analyze the heat generation, heat transfer, and material flow during the four stages of the FSW process. The computational fluid dynamic commercial code Fluent and its user defined function are used to implement the transient model. The plunge of the FSW tool was implemented by using the dynamic grid adaptive technology based on the layering algorithm in the commercial code Fluent.Then, to analyze the effects of ultrasonic vibration on the thermal processes and plastic material flow in FSW, a phenomenological model of UVeFSW is developed. A percent ultrasonic softening term is employed to quantify the degree of ultrasonic softening in the phenomenological model of UVeFSW. The phenomenon of ultrasonic softening in UVeFSW is quantitatively analyzed. The numerical simulation results of heat generation, temperature distribution and plastic material flow field are compared under the conditions with and without ultrasonic vibration. It is shown that the ultrasonic softening plays a dominant role in UVeFSW, while the thermal effect of ultrasonic vibration is insignificant. Superimposing ultrasonic vibration in FSW can produce an enhanced plastic material flow.A modified material constitutive equation characterizing the acoustic softening effect on aluminum alloys is proposed based on the thermal activation of dislocation under the influence of ultrasonic energy. The dependence of acoustic softening effect on temperature and strain rate is quantitatively analyzed. It reveals that the ultrasonic vibration energy density reduces the activation energy for deformation, resulting in a decrease of the flow stress during hot plastic deformation with superimposed ultrasonic vibration.The application of the proposed constitutive equation in modeling the heat transfer and material flow in UVeFSW shows that it can describe the effect of acoustic softening in friction stir welding of Al 6061-T6 plates. Through lowering the flow stress and viscosity of the plastic material near the tool, the ultrasonic vibration leads to an increase of material flow velocity and strain rate, and an enlargement of the flow region and deformation region in UVeFSW process.Subsequently, the modified constitutive equation is implemented into a CFD model of UVeFSW coupled with computational ultrasonic field to quantitatively analyze the effect of ultrasonic vibration on thermo-physical phenomena in UVeFSW. Evolution of sound pressure and distribution of ultrasonic energy density during the UVeFSW are calculated by computational ultrasonic field. The effect of ultrasonic vibration on the total heat generation, heat transfer, material flow patterns and deformation regions during the welding process are elucidated in detail. It is shown that the coupling of high ultrasonic energy near and in front of the FSW tool with the plastic deformed material pre-softened the material which resulting in a lower flow stress and viscosity. It reveals that superimposing ultrasonic vibration on FSW increases the material flow velocity and strain rate; enlarges the flow region and deformation region.The UVeFSW and conventional FSW experiments are conducted to validate the model. The calculated thermal cycles at typical locations, the welding torque and the boundaries of thermo-mechanically affected zone match with those obtained from experiments.