LAUSR

In this study, friction stir butt welding was applied to weld an aluminum–zinc alloy produced by semi-solid method (rheocasting). For this aim, the effect of tool rotational and traverse speeds on the microstructure was investigated. The microstructures of the processed materials were studied using optical microscopy, scanning electron microscopy equipped with energy dispersive spectroscopy, and X-ray diffraction. The Vickers hardness test was used to evaluate the hardness of the joints. The results showed that the friction stir welding using optimum parameters improved the structure of the rheocast alloy. The porosities were eliminated due to severe plastic deformation induced to the material, which caused filling the voids. Friction stir welding lead to replacement of the semi-solid structure by finer equiaxed grains, which are formed by continuous dynamic recrystallization mechanism. High rotational speeds and low traverse speeds (high heat inputs) caused formation of larger grains, due to grain growth at higher temperatures. On the other hand, at high heat inputs the eutectic phases were more scattered throughout the microstructure, and super saturated solid solution was formed. Because of this behavior, the relationship between the hardness and grain size of the joints did not follow the classical Hall–Petch equation.