Fracture of friction stir welded butt joints structure of aluminum-magnesium alloy

The results of aluminum-magnesium alloy friction stir welded joints fracture investigations are put forward in the article. Through the example of welded joints of plates with different thickness it is illustrated that macro- and microstructure of the welds have both common features of strained metal and peculiars caused by the impact of welding temperature-time criteria. It is demonstrated that the significant difference between weld microstructure and original metal was formation of the gradient fine-grained microstructure without signs of texture. It is revealed that as a result of heating and cooling temperature-time terms variation during welding of thick alloyed plates in the weld zone along with dynamic cell formation partial strained structure recrystallization can occur. It is illustrated that in case of recrystallization the joint fracture was along the boundary of the areas with different structures. A comparative analysis of the fracture topography of the broken specimen joints that were obtained from the alloy plates with different thickness is carried out. This analysis revealed that both cases showed similar fracture pattern. A crack originated in the root and run to a distance that is approximately equal to middle of the specimen gauge along to transition boundary of the base metal structure to gradient one. "Pest" degradation goes further. Nature of the alloy structuring mechanism in the weld zone stimulated with heating and intensive plastic deformation is discussed.