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

Based on experimental research, three types of friction stir welded (FSW) T-joints (A, B, C) for 3mm thick AA6061-T4 was performed.The influences of welding parameters on the microstructures, 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 paper, it was found that the microstructure features in the skin welds had the same patterns as that of butt joints through the metallographic observation in different zones of FSW-T joints. The TMAZ, which was formed by stir and friction of the tip of the tool, was in the stringer. The unique microstructures of T-joints were the two fillet zones where they were made of fine grains which were formed by forging and extrusion.There were obvious“kissing-bond”interface defects and“Z”line defects in the FSW-T joints. Regarding A-joint and B-joint, the“Z”lines tended to appear at the faying surfaces between the stringer and the skin when the rotating speed was from 1008rpm to 2256rpm. However, the typical characteristic of faying surfaces was“kissing-bond”interface for A-joint and B-joint when the rotating speed was from 400pm to 600rpm. In addition, the tunnel defects had a closed relationship with heat input, which were easily produced at the toe corner of stringer nugget along advancing side and at the bottom of nugget zone in all the three types of T-joints.Tunnel defects nearly disappeared for the three types of FSW-T joints which were welded by the tool with 20mm shoulder.During Vicker’s hardness test, it was shown that the variation of hardness in the skin and in the stringer presented W-shapes and N-shape, respectively. For FSW-T joints that were welded by tool with 20mm shoulder, the hardness of HAZ in the skin and TMAZ in the stringer was reduced 35.4% and 27.7% with respect to the hardness of base material. For given rotating speed, the softened zones became smaller as the increased welding speed. In turn, the softened zoned expended a little as the increased rotating speed. However, the hardness in the nugget zone increased along with the increased rotating speed. Therefore, the average hardness was over 15% compared to the hardness of base material with n=2256rpm.In the scope of this thesis, the mechanical tests indicated that A-joint had the best tensile strength corresponding to n/v=1541/142 r/mm. A-T and A-L could be up to 73.8% and 83.4% of the base material strength. In stringer direction, the smaller tunnel defects and zigzag lines had no pronounced effect on the tensile strength for A -joint. For B-joint, the best ratio of ultimate tensile strengths for T-joints and base materials in the skin direction and stringer direction could be 71.1% and 66.8% of the base material with n/v=1008/75 r/mm. The larger tunnel defects as well as clear and continuous“Z”lines would reduce the ultimate tensile strength of the T-joints and cause the joints to fracture at the joining interface. Regarding C-joint, n/v=500/50 was corresponding to welded joints without tune defects, the welded joint coefficients of C-T and C-L were 79.9% and 71.2%, respectively. Softened zones were the weakest zones for C-T and C-L.With varing rotating speed 400rpm~600rpm, the locations of fractuture of A-L, B-T, B-L, C-T and C-L were related to the“kissing-bond”interface defects, clear and continuous“Z”line defects and larger tunnel defects. Tensile fracture of A-L occurred in the softened zones at the rotating speeds of 1541rpm and 2256rpm.Fracture surface analysis showed that the fracture which located in the“kissing-bond”interface defects and clear and continuous“Z”line defects did not form metallurgical bonding. Dimples with varing size and shape were the main feature for the welded joints which failed in the lowest hardness zones.