Abstract
As a solution for reducing the load, which FSW machines suffer during welding, is to apply multi-layer welding, in which the parts to be welding with open grooves are filled by building-up metals step-by-step. In the present study, multi-pass FSW of 6061 aluminum alloy plates, which have rectangular opened grooves, is supplied from the front of the tool through the groove. The effect of the pass sequence on the weld microstructure and the mechanical properties are investigated. The opened grooves are successfully filled by the build-up metal in all welding conditions employed in the present study. The interior of the stir zone formed by multi-pass FSW contains continuously arrayed oxide particles, which is so-called the lazy S. The shape of the lazy S is similar to that of the groove. The larger offset of the weld interface to the retreating side from the probe center forms the more obvious lazy S. The pass sequence affects the peak temperature and the grade of aging at the heat affected zone, which dominates the mechanical properties of the joints. Multi-pass FSW joints performe almost the same tensile strength as ordinary FSW joints welded under the same condition. However, the tensile strength of multi-pass FSW joints depend on the pass sequence. The pass sequence and the welding speed determine the hardness distribution by ruling the state of aging.
