Ultrafine grained structure and improved mechanical properties of low temperature friction stir spot welded 6061-T6 Al alloys

In this study, 1mm thick 6061-T6 aluminum alloy plates were subjected to friction stir spot welding (FSSW) at a very low rotation speed of 30, 40 and 50rpm, aiming to prevent the hardness reduction in the heat affected zone (HAZ) of this precipitate hardening Al joints. Two kinds of rotating tools, namely, one with a 1mm long probe and the other one with a flat shoulder without a probe, were used and the corresponding force applied to the rotating tools was ranged from 4 to 8tons. Under this special welding condition, the highest welding temperature was measured lower than 200°C. As a result, no hardness reduction of the HAZ region could be detected and an ultrafine grain structure with a high density of dislocations was formed in the stir zone. The average grain size obtained in the stir zone was less than 500nm for all the welds, which was much smaller than that in conventional FSSW processed 6061-T6 Al joints. In addition, no hook-like bonding ligaments were formed in the joints due to the very different material flows and the shear tensile tests showed that the spot welds prepared by the low speed FSSW can reach a maximum shear load of about 5400N, much higher than those prepared by the conventional FSSW technique. •Friction stir spot welding of Al alloy was successfully done at low rotation speed.•The highest welding temperature was lower than 200°C.•The average grain size of less than 500nm was formed in the stir zone.•No hardness reduction of the HAZ region could be detected.•Shear tensile tests of the welds revealed a maximum shear load of about 5400N.