"Thermal-fluid-structure" Coupling Numerical Analysis And Void Defect Prediction In Friction Stir Welding Process

Friction stir welding(FSW),as a solid-state joining process,is capable to achieve a joint with high mechanical properties and low welding distortion,and is widely used for joining of lightweight alloys such as aluminum and magnesium alloys.However,a large number of studies have shown that insufficient plastic material flow maybe occur in friction stir welding if improperly welding parameters are employed,which may lead to the formation of void defects in the joint and seriously affects the mechanical properties of the joint.In addition,premature failure of the FSW tool maybe also occur due to excessive force under those conditions.Those limit the critical welding speed and welding efficiency of friction stir welding to a low level.Thus,it is of great significance to study the formation mechanism of void defect in FSW joint and reliably predict the FSW tool life for optimizing the welding parameters,controlling the joint formation,improving the welding efficiency and reducing the production cost.According to the interaction between the FSW tool and the workpiece,a non-uniform distribution model of the pressure at tool-workpiece interfaces was proposed.A computational fluid dynamic(CFD)model of FSW process was established by employing the shear stress boundary condition at tool-workpiece contact interfaces.The void defects of the joints were observed by tracing discrete particles.The prediction results and experimental results of void defects were compared between the shear stress boundary CFD model and three classical velocity boundary CFD models(i.e.,slip rate is constant,velocity-dependent and temperaturedependent).It was found that the proposed shear stress boundary CFD model can reasonably predict the forming characteristics of FSW joints under different process parameters(whether a void is formed in the weld or not;and the size,morphology and location of void,etc.).Then,the shear stress boundary CFD model was verified by the shapes of thermal affected zone and the welding thermal cycles at typical positions.Finally,the numerical analysis of friction stir welding process with different welding speed and rotational speed was carried out,and a critical characteristic parameter for predicting the formation of void defects in the joint were proposed.In order to understand the formation mechanism of void defects in friction stir welding process,the simulation results at the welding speed of 20 mm/min(i.e.,the condition which does not form a void defect in the joint)and 180 mm/min(i.e.,the condition which forms a void defect in the join)were systematically analyzed at a constant rotational speed of 800 rpm.The thermal processes and plastic material flow behaviors during friction stir welding of these two conditions were quantitatively compared and analyzed.The results show that the plastic material flows in horizontal direction and can full fill the cavity behind the tool at the welding speed of 20 mm/min,which result in an excellent joint without void defect.However,as the welding speed is increased to 180 mm/min,the frictional stress behind the tool decreases significantly which leads to a decrease of the fluidity of the plastic material.As a result,the plastic material at the bottom of the weld is stagnated after bypassing the tool from the retreating side,and void defect is formed in the middle and lower part of the weld at the advancing side.In addition,the width of the void defect increases while its height decreases with an increase of the tool pin tip diameter at the same tool pin root diameter and welding parameters.Based on the CFD model,a thermal-fluid-structure coupling numerical model of friction stir welding process was established.The temperature and pressure results of CFD model were imported into the structure model by coupling the fluid-structure interfaces between the workpiece and FSW tool.The influences of welding parameters on the temperature,stress and strain distribution of the tool were quantitatively analyzed,and the bearing capacity and fatigue life of the tool were also investigated.The results show that the temperature of the tool decreases when its distance to the tool-workpiece contact interfaces increases.The maximum equivalent stress of the tool decreases with an increase of tool rotational speed,while it is increased with an increase of welding speed.The equivalent stress distribution on the tool is uniform at a low welding speed.While there is obvious stress concentration at the root of the pin at a high welding speed.The front of the tool is subjected to tensile stress while its rear is subjected to compressive stress,which will lead to the fracture of the pin at the root.The service life of the tool is decreased with an increase of welding speed while it is increased with an increase of rotation speed.The average normal stress of the tool varies periodically with its period is consistent with the rotation period of the tool.