Abstract
This study explores a novel technique using a hemispherical tool tilted towards the retreating side for friction stir welding AA6061-T6. Mechanical properties assessment proved the competitiveness of the new method over friction stir welding using a conventional tool with backwards tilting. The ultimate tensile strength, yield strength and elongation at break reach efficiencies of 60, 40 and 85% compared to BM. The new tool shape and the tilt towards the retreating side accentuate asymmetry in the stirred zone. Grain morphology and dislocations vary within the stirred zone when comparing the center and the sides due to different degrees of dynamic recovery and dynamic recrystallization. Dislocation density is increased at the center due to the complex stirring along an extended circular trajectory. The artificial aging at 180 °C for 18 h positively affects the joint strength while reducing the elongation at the break due to an inhomogeneous response of the softening zone to the aging. The highest hardness recovery within the stirred zone occurs at the center due to the higher concentration of strengthening precipitates which nucleate around dislocations. The weakest point of the weld identified in the stirred zone at the advancing side after artificial aging modifies the fracture behavior of the sample from a ductile to a rather ductile–brittle rupture.
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