Effect of tool rotational speed on the microstructure and mechanical properties of bobbin tool friction stir welding of Al-Li alloy, (2015),
AbstractA third-generation Al-Li alloy AA2198 has been successfully welded by bobbin tool friction stir welding. The stirred zone, displaying an hourglass shape, consists of recrystallized Al grains and precipitates remaining in solution. Joint line remnants have been found in all etched welds. Symmetrical hardness profiles have been obtained in the thickness direction, which indicates that the joints have homogenous through-thickness mechanical properties. As the rotational speed increases, the grain size of the stirred zone increases, whereas the density of strengthening particles decreases; the joint line remnants become compressed remarkably in the shoulder-dominated zone while less changes occur in the probe-dominated zone; the softest region shrinks and shifts outward, the average hardness of the stirred zone increases and the hardness profile along the cross section of the joint changes from the U-shaped to W-shaped. The tensile strength of the joint initially increases with rotational speed and then decreases with the maximal strength efficiency reaching 80%; three fracture modes have been observed and cracking initiates at the joint line remnant propagating towards the heat-affected zone, and finally to the border between the thermal-mechanically affected zone and stirred zone.
The effects of rotational and welding speeds on the microstructure and mechanical properties of bobbin-tool friction-stir welded (BT-FSW) Mg AZ31 were investigated. The results indicated that the thermo-mechanically affected zone (TMAZ) consisted of equiaxed grains, which were inconsistent with the deformed, rotated and elongated grains found in the TMAZs of BT-FSWed Al alloys and friction-stir welded Al and Mg alloys. The average grain size increased as the ratio of the rotational speed to welding speed increased. Excellent welds with no degradation in hardness were produced using a low heat input. Tensile tests revealed that the ultimate tensile strengths gradually increased with increasing welding speed while keeping the rotational speed constant while the rotational and welding speeds had only slight influences on the yield stresses and fracture elongations.
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