Effect of Pin Geometry Features on Thermal Processes and Material Flow in Friction Stir Welding

Author:Sun Zhen

Supervisor:wu chuan song


Degree Year:2019





Tool is a critical component in friction stir welding(FSW)equipment,which directly determines the heat generation and material flow in the process,and ultimately influences microstructure,macrostructure of the weld and mechanical properties of the joint.Improving welding efficiency and welding quality by optimizing the geometry of welding tool is currently a strategy with the least modification to the original process,the lowest cost and remarkable effect.However,the design and manufacturing of the pin geometric shape still depends on trial and error method,and it is necessary to explore the mechanism of the influence of the pin profile on heat generation,heat transfer and material flow in the welding process.In this study,a three-dimensional computational fluid dynamics(CFD)model is established for two basic geometric features(thread and flat)which constitute different kinds of pin profiles on the basis of experiments.Nine pins with typical geometric features are selected to study the influence of pin profile on the material flow and heat generation in FSW quantitatively.Friction stir welding of AA6061-T6 is carried out using three kinds of pins:UP(unthreaded tapered pin),TP(tapered pin with right-hand thread)and LTP(tapered pin with left-hand thread).Tensile test results show that TP can effectively improve the tensile strength and elongation of the welding joints,especially under welding parameters with lower heat input.The marker material(copper)around the exit hole is frozen by the emergency stop of the tool,and then the distribution of the marker material can be reconstructed in three dimensions by means of CT technology.The results show that thread can effectively improve the uniformity of the distribution of marker material,increase the distribution width of the marker material on the horizontal planes,and play a very important role in avoiding or reducing welding defects.A force-driven velocity boundary condition is proposed based on the research foundation of computational solid mechanics model and computational fluid dynamics model in FSW.The screw characteristic of pin thread is retained,and the effect of the thread on workpiece is equivalent to pin side surface.At the same time,pin side surface is divided into thread area and arc area,and different velocity boundary conditions and thermal boundary conditions are applied respectively.Finally,the CFD model of FSW for TP is established.The effect of thread parameters(pitch,direction and starts)on the material flow and heat generation in FSW are analyzed quantitatively.The numerical simulation results show that thread can effectively increase the material flow velocity in the middle and lower regions of the nugget zone,improve the material flow in vertical direction,promote the mixing of the material in the thickness direction of the weld,and weaken or avoid the sudden change of velocity at advancing side(AS).When the tool pin length is certain,the peak value of material flow velocity increases with increase in thread pitch,however,the time maintaining high velocity becomes shorter,and the mixing degree of the material in vertical direction is weakened due to the reduction of the area of thread region.The material flow in vertical direction near pin side is just the opposite when the thread direction changes.The multiple threaded pin can effectively improve the material flow in the middle and lower regions of the nugget zone.With the increase in thread starts,the heat generation at the contact interface and the viscous dissipation heat generation in the shear layer all increase because of the increase in contact area and more intense plastic material flow.The driving effect of flat feature at pin side on the workpiece material is analyzed A comprehensive CFD model of FSW is established,which can cover the pin with two basic geometric features(thread and flat).Simulation results show that the volume of plastically deformed material can be significantly increased due to the flat feature on pin side,especially in the lower region of the nugget zone.The main function of the thread is to promote the material flow in the vertical direction of the weld.Finally,the numerical model established in this study are verified by the experimental results through thermal cycle,thermo-mechanically affected zone(TMAZ)boundary,and the distribution of marker material in the weld.