Influence Of Bottom Shoulder Cooling On Joint Properties Of 6061-T6 Aluminum Alloy By BTFSW

Bobbin-tool Friction Stir Welding(BTFSW)as a variant of conventional Friction Stir Welding(FSW),using upper shoulder and lower shoulder force offset each other,effectively reduce the spindle load,requirement of rigidity in background,and can avoid the root defects of the joint.But in the BTFSW welding process,the upper shoulder has a big measure of area contact with the welding machine,good heat dissipation,but the volume of lower shoulder is small,poor heat dissipation conditions,resulting in the temperature difference between the upper and lower surface of the weld,further affecting the weld forming quality.How to reduce the temperature of the lower shoulder of the BTFSW tool,balance the upper and lower surface heat of the weldment and reduce the temperature difference between the upper and lower surfaces become an urgent problem of BTFSW.In this paper,we designed a new type of BTFSW tool and a matching cooling device for the shortcomings of the original BTFSW tool.The cooling device is installed at the bottom of the BTFSW tool,and connected to stirring pin with screw thread.In the welding,the cooling device and BTFSW tool rotation together,and it makes the cooling liquid flow through the cooling device and take a lot of welding heat away.The cooling device mainly cools the lower shoulder to balance the upper and lower surface temperature of the weld and taking away the excess welding heat.By studying the effect of the lower shoulder cooling on the joint properties of the 6061-T6 aluminum alloy by BTFSW,it is better to understand the effect of the cooling condition on the thermal cycling.At the same time,on the basis of studying the relationship between the phase change and the heat output and performance,it can effectively guide the design of welding parameters,optimize the performance of joints,and lay the theoretical and technological basis for the 6061-T6 aluminum alloy by BTFSW.The experimental results show that:in the rotating speed of 810 r/min and welding speed of 160 mm/min,welding joint will be beautiful and no obvious defects on the lower shoulder cooling.By studying the temperature field of the BTFSW with the cooling device,it was found that the use of the cooling device will reduce the temperature of the weld joint area and has a great cooling effect on the lower surface of joint.At the same time,the temperature difference between the upper and lower surfaces of the workpiece is reduced,which ensures that the upper and lower surfaces of the weld are evenly heated.The coolingdevice can reduce the duration of the workpiece in the high temperature range,and short high temperature duration inhibit the growth of grain,So the grain structure is refined and joint strength is improved.After the use of cooling device,the whole range of joint area narrowing,and the width of the connector is reduced by about 4mm,nugget grain size change small,arranged more uniform,the hardness generally improved,and the minimum hardness is increased from 55 HV to 60HV;Thermal Mechanically Affected Zone become narrow,strengthening precipitates increased,and grain growth is suppressed,result to the average hardness increases and the strength reduced;The heat affected zone is reduced and the hardness of the forward side is reduced because of too low forward side temperature.