The microstructures evolution mechanism of friction stir welded AZ31 magnesium alloy

The typical microstructure of the friction stirred weld is the formation of the weld nugget. AZ31 magnesium alloy was friction stir welded and the microstructures and the evolution mechanism of the weld were studied using optical microscopy (OM) and transmission electric spectroscopy (TEM) in this paper. The results show that a very thin layer microstructures of about 37 mu m similar to 47 mu m thickness with fine grains is formed under the shoulder. There exist a partly thermodynamic recrystallization, and superplastic grains appear in the thermo-mechanical affected zone. There is a coarse layer of the weld about 100 mu m similar to 130 mu m thickness near the bottom. The weld nugget is composed of fine, re-crystallization grains, and the grain size is different along the thickness direction. The microstructure evolution mechanism is analyzed and a simple model on weld nugget microstructure evolution is also presented in this paper. The mechanism and the factors on microstructures are theoretically analyzed. The heating and the mechanical stirring play the main role in the microstructure evolution.