Mechanical Properties and Structure of Friction Stir Welds of Rolled Zr-Modified AA5083 Alloy

Microstructure and mechanical properties of friction stir welds of Zr-modified AA5083 aluminum sheets were studied. The sheets were produced by cold or hot rolling with a total reduction of 80%. In both rolled conditions, the average high angle boundary spacing was 17-18 mu m. The density of free dislocations was similar to 5.6 x 10(13) and similar to 3.5 x 10(14) m(-2) in hot rolled and cold rolled conditions, respectively. The volume fraction of incoherent Al6Mn dispersoids with an average diameter of similar to 25 nm was measured to be similar to 0.076%. Defect-free welds were produced by double-side friction stir welding (FSW). Friction stir welding led to the formation of fully recrystallized microstructures with the average grain size about 2.5 mu m and low dislocation density in the stir zone in both conditions. The average size and volume fraction of Al6Mn particles increased to similar to 25 nm and similar to 0.1%, respectively. The joint efficiency of the friction stir welds for ultimate tensile strength was found to be 74 and 94% in the cold-rolled and hot-rolled preprocessed material conditions. The relatively low weld strength was attributed to the elimination of dislocation substructure strengthening during FSW.