The microstructure and tensile behavior of stir zones in the friction stir welded brass plates containing disordered secondary β phase at different heat inputs were investigated. At high heat inputs, β was transformed completely to α, and hence the final microstructure was single-phase. At lower heat inputs, fine β particles remained on α grain boundaries, causing a reduction in grain size from 3.78 to 1.08 μm. Despite the smaller grain sizes of the stir zones, they revealed lower elongation compared to that of the base metal, which was due to their higher dislocation densities with tangle structures. The stir zones had higher tensile strength than the base metal due to both grain boundaries and dislocation density strengthening mechanisms. The low heat input stir zone showed a synergic increase in the tensile strength and elongation compared to the higher heat input stir zone due to its smaller grains and presence of β particles on the grain boundaries.
