Effect Of Auxiliary Condition Of Heat Pipe On The Thermal Cycle,Microstructure And Property Of The Friction Stir Welding

As a new solid phase bonding technology,friction stir welding provides a new approach for light alloy bonding.Friction stir welding uses the heat of friction and the heat of plastic deformation as the welding heat source and realizes the welding connection.However,due to the three-dimensional asymmetric distribution of the temperature field of the weld,the material flow process is not uniform.This results in the defects such as the coarse grain of the heat-affected zone and the gap of the bottom,which reduce the homogeneity of the microstructure and mechanical properties of the welded joint.In this paper,the excellent heat transfer performance of heat pipe was used to cool the welding test plate,and heat pipe was applied outside or inside the shaft shoulder respectively to regulate the weld heat cycle curve.The effect of heat pipe application on the welding heat cycle and joint structure was compared,so as to improve the joint mechanical properties.The characteristic points of the joint were measured by thermocouple,and the change rule of thermal cycle curve was analyzed.The results show that under three conditions of conventional friction stir welding,heat pipe outside and inside the weld area,the phenomenon of ‘double peak' at the initial characteristic points of welding all exists.Along the direction of welding,the peak temperature of each characteristic point tends to increase.Along the direction of weld width,the peak temperature and the duration of high temperature both decrease.Along the normal direction,the peak temperature decreases,but the duration slightly increases.Compared with the conventional friction stir welding,peak temperature and duration of high temperature decrease by applying heat pipe outside the shaft shoulder.Along the welding direction,the increasing amplitude of peak temperature decreases.The heat transfer process gets more uniform.Applying heat pipe inside the weld,the cooling effect on the upper area of the weld is stronger.The temperature difference along the transverse direction perpendicular to the welding direction gets further reduced.But for middle of the weld,the cooling effect is lower than that of applying heat pipe outside the shaft shoulder.Through the calculation of heat absorption of the weld,it is further verified that the application of heat pipe could effectively reduce the overall heat absorption of the weld.Especially when the heat pipe was applied outside the shaft shoulder,the overall heat absorption of the weld decreased by about 4.18 k J greatly compared with the conventional conditions.The axial force of the stirring head was measured by a force sensor and the influence of the heat pipe application mode was compared.The results show that the axial force at each stage of friction stir welding presented similar changing rules under different welding conditions.The average axial force in the welding stability section(95s?199s)is 4114 N.The application of heat pipe can increase the axial force of the welding stable section and the heat output of the stirring head.When the heat pipe is applied inside the shaft shoulder,the average axial force is the largest as 4984 N.The macroscopic and microscopic structure of the joint were observed by optical microscope.The results show that compared with the conventional friction stir welding,when the heat pipe is applied on the outside of the shaft shoulder,the width of the upper area and the middle area in the welding nugget zone are individually reduced by 4.88 mm and 2.59 mm.The width of the bottom slightly increases by 0.12 mm.The grain size of the upper part in the welding nugget zone decreases by about 0.9 ?m,4.0 ?m and 12.7 ?m compared with the conventional conditions along the welding direction.The recrystallization degree in the heat affected zone and the thermal-mechanical affected zone drops significantly.When the heat pipe is applied inside the shaft shoulder,the width of the upper part of the welding nugget zone is further reduced by 5.99 mm,while the width of the middle part is reduced slightly by2.48 mm.The width of the bottom part in the welding nugget zone is close to that of the conventional condition.The grain size measured at the upper part in the welding nugget zone is further reduced to 19.4 ?m,22.1 ?m and 23.6 ?m respectively along the welding direction.While the grain size in the middle area is reduced less.The effect of the heat pipe inside the shaft shoulder on the thermal-mechanical affected zone and the microstructure of the heat affected area is small.The mechanical properties of the joint were tested by tensile test and microhardness test and the effect of the different heat pipe assisted conditions were compared.The results show that the heat pipe applied outside the shaft shoulder is helpful to improve the tensile strength of welded joint,and the biggest increase was about 10 Mpa.When the heat pipe is applied in the weld area,the tensile strength of the welded joint is close to the conventional conditions.When heat pipe is applied during the friction stir welding process,the joint elongation generally shows a downward trend.When heat pipe is applied inside the weld,the joint elongation decreases significantly.The hardness of each section of the welding cross section increase when heat pipe is applied.When heat pipe is applied outside the shoulder,the average hardness of the welding nugget zone is the highest,and increasing amplitude is about 7 HV.