Suppression of root flaw in friction stir welded 6061-T6 aluminum alloy using double spiral tool

Friction Stir Welding (FSW) has been attracting attention for its usefulness such as relatively easy joining compared to conventional arc welding, and its application to industries is being expanded, for example thick aluminum plates for infra-structures. FSW is known to yield defects called root flaws depending on welding conditions. In order to suppress the defects, it is necessary to optimally control the plastic flow and to stir the materials to be welded cooperatively. The present study tried to make a sound joint of 6061-T6 aluminum alloy sheets with 3mm in thickness using a newly designed tool called “Double Spiral Tool” which can increase the volume of the stir zone around the tool. Two types of tools were used: a normal type with M4 to M3 screws (called single spiral tool) and a “double spiral” type with the same pitch and twice amount of lead length. The welding tests revealed that the root flaw was completely disappeared with the double spiral tool under the welding condition of 900rpm-50mm/min, while it still remained in the joint produced by the single spiral tool. Thus, the present study successfully proved that the double spiral tool is effective to suppress the root flaw in FSW joints of 6061 aluminum alloy.