Study On Microstructures And Mechanical Properties Of Bobbin Tool Friction Stir Welding Joints Of 6082-T6 Aluminum Alloy

6082-T6 aluminum alloy is widely used in the production of railway vehicles,so the realization of high-quality aluminum alloy joining has become increasingly important.However,when the aluminum alloy is welded with traditional fusion welding method,some problems such as joint softening,porosity,hot crack and so on will occur,which will seriously affect the joint strength.Bobbin tool friction stir welding(BT-FSW),as a new joining technology,can effectively avoid defects in the traditional fusion welding method.But at present,there is little research on BT-FSW at home and abroad.Therefore,the study on microstructures and mechanical properties of BT-FSW joints of aluminum alloy has important theoretical significance and practical value.The microstructural characteristics of BT-FSW joint of 6082-T6 aluminum alloy plate with 4mm in thickness and the effect of welding process parameters on the microstructures and mechanical properties of the joint were studied in this paper.The welding temperature field was simulated and analyzed by ABAQUS software.The results show that the BT-FSW joint can be divided into base metal(BM),nugget zone(NZ),thermal-mechanical affected zone(TMAZ)and heat affected zone(HAZ)according to the microstructural characteristics.The grain size in the BM is different and the phase composition is ?-Al phase and ? " phase.The microstructure in the BM is fine equiaxed grains formed after dynamic recrystallization and the phase composition is ?-Al phase and GP zones.The grain size in the TMAZ has the tendency of being elongated and the phase composition is still ?-Al phase and GP zones.The original ?" phase is transformed into ?' phase and Q' phase in the HAZ.The grain size of the HAZ is larger than that of the BM,in spite of the HAZ and the TMAZ smaller.The grain size of the HAZ is the smallest.The results show that the welding parameters(rotational speed and welding speed)have obvious influences on welding heat input,microstructures and mechanical properties.With the increase of the rotational speed,the heat generated by the tool increases and the welding heat input increases,which leads to the increase of the peak temperature in the welding heat cycle curve and the decrease of the cooling rate.With the increase of the rotational speed,the grain size of the NZ increases and the ratio of small angle grain boundaries decreases gradually.The microhardness distribution of the joint is W-shape.The hardness value in the BM is the highest and the NZ is the second.With the increase of the rotational speed,the hardness of the NZ increases gradually and the hardness of the HAZ decreases.The lowest point appears in the HAZ near to the TMAZ.The tensile strength and elongation of the joint increase with the increase of the rotational speed and reach the maximum when the rotational speed is 800r/min.That the rotational speed further increase results in the decrease of the tensile strength and elongation of the joint.The influence law of welding speed is opposite to the rotational speed.Increasing welding speed leads to the decrease of welding heat input,grain size and hardness in the NZ,and the increase of hardness in the HAZ.The tensile strength increases with the increase of welding speed.When the welding speed is 500mm/min,the tensile strength reaches the maximum value.But when the welding speed is further increased,the strength and elongation of the joint decrease.Therefore,under the condition of this test,the optimum welding parameters are the rotational speed of 800r/min and the welding speed of 500mm/min.The tensile strength and elongation of the joint are respectively266.9MPa and 7.8%,and the strength of the joint reaches 82.1% of the BM strength.The temperature field of BT-FSW joint of 6082-T6 aluminum alloy is simulated by ABAQUS software.The heat production process is analyzed and calculated.It is found that the heat production of BT-FSW is mainly composed of the tool and the plastic deformation.The heat produced by the tool is made up of the upper and lower shoulders and the side surface of the pin.Through the analysis above,the corresponding model is established and the heat input is set up.The simulated valuesare in agreement with the measured values and the simulation results have a certain guiding effect.