Study On The Effect Of Temperature Field On Welding Performance Of Thick Plate By Friction Stir Welding

With the rapid development of modern society,the demands for large equipment such as rail transit,automobile and aerospace,ship and so on are increasing.Therefore,the requirements for materials are stricter than before with the more extensive use of lightweight heavy plate.Friction stir welding(FSW),as a green welding method,is used to avoid defects for the traditional joining methods of aluminum alloy.The invention of FSW successfully promotes the development and application of aluminum alloy.In this paper,the 2A12 aluminum alloy with thickness of 30mm was connected by FSW.The microstructure of the joint was analyzed by scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),X-ray diffraction(XRD).The mechanical properties of the joint such as tensile strength,hardness and impact toughness were checked by layered method and the temperature fields of welding process were studied by thermocouple measurement and numerical simulation.According to the different microstructure,the joints are divided into seven parts:the upper part of the nugget zone(NZ).the bottom part of the nugget zone.the thermo-mechanic ally affected zone(TMAZ)of the advancing side,the thermo-mechanically affected zone of the retreating side,the heat affected zone(HAZ)of the advancing side,the heat affected zone of the retreating side and the base material.The grain size at the bottom of the nugget zone is smaller than the upper part;the heat affected zone is affected by the rotation of the pin,which forms the interlaced existence phenomenon between the elongated grain belt and the growing grain belt;the grain of the heat affected zone appears coarsening phenomenon owing to the thermal environment.XRD analysis of joints showed that there are lots of S phases(Al2CuMg)in the joints,and the second phase mainly occurs at grain boundaries with different quantities in different regions.The results of tensile test show that the tensile strength of the joint decreases gradually along the thickness direction;the tensile strength of the second layers is the largest and the tensile strength of the sixth layer is the smallest.And the fracture position lies in the border edge between thermo-mechanically affected zone and heat-affected zone.The hardness values of the cross section of the welded joint is "W" shape.The hardness values of the nugget area are the highest and the hardness values of the heat affected zone are the lowest.Due to the interlaced microstructure between the elongated grain belt and the heated grain belt,the hardness values fluctuate.At the same time,it is found that the hardness decreases gradually along the thickness direction.The impact toughness of the joint is greater than the base material.In impact test of the two layer of weld,the impact toughness of the lower part of NZ is the highest and the impact toughness of the upper part of TMAZ is the smallest.Along thickness direction,the impact toughness of the upper layer is less than the lower layer.The impact toughness of the TMAZ is smaller than that in the HAZ,which is biggest in the NZ,and its fracture mode is ductile fracture.The temperature field in the welding process is studied by two methods:the thermocouple real-time measurement and the ANSYS numerical simulation.It is found that the upper layer temperature is high;the lower layer temperature is low and the temperature gradient along the thickness direction is larger.The temperature of the stir pin is the highest,which gradually decreases around.and the temperature field present elliptical shape.The temperature of the advancing side is lower than the retreating side.The maximum temperature is 550 ? during the welding process,which is still lower than the melting point of 2A12 aluminum alloy.The shape and the quantity of the second phase are influenced by the temperature of each region during welding process.The mechanical properties of welded joints are further affected.According to the Hall-Petch formula and metal hardening principle,it is found that the change of mechanical properties of joints is not only related to the change of microstructure in different regions but also to the quantity of the second phase in different regions.