Effects of welding speed on the microstructure and hardness in friction stir welding joints of 6005A-T6 aluminum alloy

Abstract The 6005A-T6 aluminum alloy was friction stir welded at different welding speeds. The peak temperature, microstructure and mechanical properties were examined for these joints. A special attention was devoted to the relationship between the precipitates evolution within different zones and the local hardness. In the nugget zone (NZ) experiencing the highest peak temperature, the β ″ precipitates dissolved into α-Al matrix during welding, and the hardness of NZ depended on the level of natural aging (NA) at different welding speeds. The thermo-mechanically affected zone (TMAZ) is characterized by elongated grains with a high density of dislocations. The welding speed had not a significant effect on hardness in this zone. The heat-affected zone (HAZ) contains the transformation of β ″– β ′, the precipitation of Q ′ and the coarsening of precipitates. The HAZ close to the joint center line exhibited the minimum hardness due to the coarsening of β ′ and Q ′ precipitates while the HAZ far from it having a high hardness level was mainly related to coherent β ″ precipitates. The HAZ hardness and joint strength have an increased tendency with increasing the welding speed. It can be explained by increasing the density of Q ′ or β ″ precipitates.