Microstructure And Mechanical Properties Of Bobbin Tool Friction Stir Welding Of 6005A-T6 Aluminum Alloy

Bobbin tool friction stir welding is an important technology innovation for friction stir welding.Because of its particular construction of self-support bottom shoulder,the bobbin tool friction stir welding not only solve the problem of incomplete root penetration in friction stir welding,but also improve the welding operation for narrow cavity and any other airtight structure.On the one hand,the bobbin tool friction stir welding observably decrease the difficulty during welding installation;on the other hand,the welding method improve the production efficiency.The bobbin tool friction stir welding possess a wide application prospect in shipping,aircraft industry,railway vehicles and any other industrial manufacture market,and many scientific workers have been paying close attention on it for a long time.The 6005A-T6 aluminum alloy which were widely adopted in the railway vehicles' fabrication were welded by the bobbin tool friction stir welding in different shoulder diameters and welding speeds.The influence of shoulder sizes and welding speeds on shaping properties,microstructure,mechanical properties and electrochemical properties of BT-FSW joints have been carefully analyzed.The results show that under the appropriate welding speeds and shoulder diameters,the surface of the welding line is very good.The macro morphology of the BT-FSW joints look like a dumbbell.The upper surface and the lower surface of the joints are more widely,however the middle surface is a little narrow.Because of the different microstructure,the BT-FSW joint can be divided into four areas,the specific areas are base metal,heat affected zone,the thermo-mechanically affected zone and the weld nugget zone.Under the thermo-mechanical coupling effects of the bobbin tool,the weld nugget zone went through a process of dynamic recrystallization and the equiaxed grain structure was obtained.The grain which on the heat affected zone of the advancing side experienced a very severe process of bending deformation,and the boundary between the heat affected zone and the thermo-mechanically affected zone on the advancing side is more clear than the retreating side.During the process of bobbin tool friction stir welding,the aluminum oxide which was not cleaned absolutely on the groove's surface of base metal came into the welding line and developed into the “S line”.The “S line” is regarded as a defect of weak connection,taking an important influence about tensile strength and fatigue strength.When shoulder diameter was 14 mm,rotational speed was 800r/min,welding speed was 600mm/min,the mechanical properties of BT-FSW joints were best.The average tensile strength of the BT-FSW joints which were 78.6% of the base metal was 221.8MPa,and the breaking elongation achieved to 5.2%.The micro-hardness from base metal to weld nugget zone show a distribution of high-low-high-high,the shape looked like “W”.The minimum micro-hardness value located in the junction of heat affected zone and the thermo-mechanically affected zone on the advancing side.The corrosion resistance on the weld nugget zone is superior to the base metal.Under the same welding process parameters,when the shoulder diameter is smaller,the corrosion resistance is better.The fatigue strength condition of the BT-FSW joints achieve to 95 MPa when the frequency of the cyclic stress was 10~7.