Fundamental Study On Low Axial Force Robotic Friction Stir Welding Processes

Robotic friction stir welding has the advantage of high flexibility,high automatic operation and lower costs,thus it has become an important trend in welding field.However,a conventional industrial robot has the disadvantage of high low bearing force and poor stability,which severely restricted the application and development of robotic friction stir welding.Reducing the axial force has contribute to improve stability of robotic friction stir welding,and it has an important significance in promoting development of this technology.To reduce the axial force of robotic friction stir welding,design tools with different shoulder geometries and shoulder diameters,by using friction stir welding process with zero title angle to joined 6061-T6 Al-alloy with thickness of 5mm.This paper analysed the effects of shoulder geometry and diameter on zero title angle friction stir welding process and by establishing relevant models to study the influential mechanism of shoulder geometry and diameter on the axial force.The results of this research are as follows:According to the axial force features of friction stir welding with zero title angle,the welding process can be divided into 5 stages: pin insert,shoulder plung,dwelling,stable welding and pin retracement.In stable welding stage,upon increasing rotation speed,the axial forces decreased,when increasing welding speed,the axial forces raised.In friction stir welding with zero title angle,comparing with concave-shoulder tool,the axial force is lower when use flat-shoulder tool to operate welding experiment.Well organized and defects free weld joint with good weld surface appearance can be obtained by use flat-shoulder tool,but when use concave-shoulder tool,the weld surface become roughness and there are hole-defects in the joint.When rotation speed is 1500 rpm and welding speed is 95 mm/min,the axial force of the two tools reach their minimum,but axial force of flat-shoulder tool is about 200 N lower than concave-shoulder tool.By using flat-shoulder tool to operate welding experiment in friction stir welding with zero title angle,the smaller the shoulder diameter,the lower the axial force and more stable the force.When rotation speed is 1500 rpm and welding speed is 95 mm/min,the axial force of three different shoulder diameter tools reach their minimum,while tool shoulder diameter is 9mm,the axial force reached 2300 N,comparing with 15 mm and 12 mm shoulder diameters,the force declined about 1500 N and 600 N respectively.Process window shape of 6061 Al-alloy friction stir welding process with zero title angle is like a inclined polygon,the process range of concave-shoulder tool with 15 mm shoulder diameter is the most narrow one.Flat-shoulder tool with the shoulder diameter decrease,the corresponding process window moves to the left and extends upwards.Through model analysis,there are three reasons can explain why the axial force of flat-shoulder tool is lower than concave-shoulder tool.Firstly,in friction stir welding with zero title angle the effective friction area of flat shoulder is larger,which can produce more friction heat than the concave one to makes the material softer and deformation resistance lower.Secondly,during welding process,the concave shoulder is acted upon by a component which perpendicular to the workpiece.Finally,flat shoulder has smaller surface than concave shoulder.As for the flat-shoulder tool,the smaller the shoulder diameter,the smaller the product of pressure act on shoulder small elementary area and total shoulder area,so when shoulder diameter increase the axial force increase.