Low-temperature superplasticity of nugget zone of friction stir welded Al-Mg alloy joint

Commercial 5083Al rolled plates, 2.8 mm thick, were subjected to friction stir welding (FSW) with the aim of achieving low-temperature superplasticity (LTSP) in the nugget zone (NZ). Fine-grained microstructure with average grain sizes of 1.6 and 1.8 µm was obtained in the upper and lower parts of the NZ, respectively. The NZ was subjected to superplastic investigation at 250 and 300 °C. It was indicated that the upper and lower parts of the NZ exhibited similar LTSP values of 550-570% at 300 °C, much higher than those reported previously (< 300%) in friction stir processed 5083Al. This excellent LTSP was attributed to the extremely fine-grained microstructure and predominant high angle grain boundaries (> 84%). Grain boundary sliding was determined to be the dominant deformation mechanism, with grain boundary diffusion as the rate-controlling step.