Study On The Welding Parameters And Microstructure Properties Of Friction Stir T-welded Joints For 6061-T6 Aluminum Alloy

This paper focuses on the research friction stir welded(FSW) AA6061-T6 T-joints, a series of experiments were carried out. 4mm thick 6061 aluminum were welded by friction stir welding in different welding parameters. Influence of the slotted tool pin and ultrasonic on the T-joint was investigated. The influences of different parameters on the appearance of T-joints, welding defects, hardness profiles and tensile strength were discussed specifically. The results will provide important reference to study on FSW-T joints in the future.In the experiment, T-joints without Macro defect can be obtained. What is more, the welding deformation is small. However, unreasonable welding parameters were selected, macro defect appeared at a rotation speed of 1500 rpm and the traverse speed is above160mm/min. The effect of welding speed on tunnel defect was important. If other welding parameters were constant, when the welding speed up to 200mm/min tunnel defect was appeared. In a series of situations, bonding line defect could not be removed. Bonding line defect mainly appeared in NZ and THAZ, which are extruded downward to the stringer or sideward from the original joining surfaces to the advancing side of the skin under severe plastic deformation. Bonding line defect t is the main factor affecting the tensile properties along the stringer.The ultra tensile strength(UTS) of T-butt-lap joint is less then that of T-lap joint, the UTS along the stringer of the T-butt-lap joint is only 97.4MPa, which is less then 13% that of f T-lap joint. While the others welding parameters were certain and the radius the fillet was 1mm,the T-joints with good interface and less defects were gotten; bonding line, Z line and tunnel defects appeared in the T-joints when the radius was 3mm. The UTS along the stringer of the T-joints was the highest while the radius was 1mm, which value was 130 MPa. In the same parameters, the UTS along the skin obtained by slotted tool pin in FSW increases by 31%, while reducing by 10% along the stringer. The UTS along the stringer of the T-joint also increases by 20% by ultrasonic assisted FSW. Two methods aboved could not removed bonding line defect, however, which improved the size and distribution of the bonding line defect.During Vicker’s hardness test, it was shown that the variation of hardness in the skin presented W-shapes. The variation of hardness in the stringer presented “Up-Down-Up” trend respectively.The highest hardness is in NZ and the lowest hardness is in THAZ. The fracture morphology along the skin are different and which along the stringer were Brittle fracture. Fracture surface analysis showed that the fracture which located in the bonding line interface defects did not form metallurgical bonding.