Assessment of microstructure and mechanical properties of friction stir welded AA2014-O and AA2014-T6 sheets

In this study, friction stir welding of AA2014-O and AA2014-T6 aluminum alloy was performed at various welding speeds to evaluate the influence of temper conditions of base metal (BM) on the properties of the welded joints. The results showed strong influence of BM temper conditions on the microstructural morphologies and mechanical behavior of the welded joints. In the 2014-O joints, different zones of weld joint were diffused into each other and there was no clear interface between them. In 2014-T6 joints, there was a distinct demarcation between the nugget zone (NZ), thermomechanically affected zone TMAZ, heat-affected zone (HAZ), and BM. The welded joints in 2014-O temper condition showed increase in hardness in the vicinity of weld center due to grain refinement whereas, in 2014-T6, softening occurred in the same region by the dissolution of strengthening precipitates. The mechanical properties of 2014-O joints were equivalent to the base metal showing a 100% weld efficiency with fracture located in the base metal, whereas 2014-T6 welds exhibited about 70% weld efficiency with fracture located at the NZ/TMAZ interface. All the samples in mechanical testing fractured at retreating side (RS) which exhibited heterogeneity in the mechanical properties of the welded joints. Scanning electron microscopy fractographic analysis revealed a ductile fracture mode comprising of dimples in both temper conditions. The size and shape of the dimples were strongly dependent on base metal temper condition.