Tensile properties of a friction stir welded magnesium alloy: Effect of pin tool thread orientation and weld pitch

Microstructures, tensile properties and strain hardening behavior of a friction stir welded (FSWed) AZ31B-H24 magnesium alloy were studied at varying welding speeds, rotational rates and pin tool thread orientations. After friction stir welding (FSW) both yield strength (YS) and ultimate tensile strength (UTS) were observed to be lower but strain hardening exponent became much higher due to the presence of recrystallized grains in the stirred zone (SZ) and thermomechanically affected zone (TMAZ). The left-hand thread pin tool rotating clockwise generated good FSWed joints and mechanical properties due to the downward material flow close to the pin surface, while the right-hand thread pin tool turning clockwise caused an upward material flow and resulted in inferior joints. The YS and UTS increased and strain hardening exponent decreased with increasing welding speed. The YS as a function of grain sizes obeyed the Hall-Petch relationship well, and it also increased with decreasing rotational rate. Both YS and UTS were observed to increase linearly with increasing weld pitch (a ratio of welding speed to rotational rate). A significantly higher YS of ∼170 MPa was achieved at a high weld pitch of 1.2 mm/rev, in comparison with that (∼110 MPa) using a weld pitch ranging from 0.0039 to 0.24 mm/rev. All the FSWed AZ31B-H24 joints failed in-between the SZ and TMAZ. Dimple-like ductile fracture characteristics appeared in the base metal, while some cleavage-like flat facets together with dimples and river marking were observed in the FSWed samples.