The aim of this investigation was to study material flow during dissimilar friction stir welding of AA 5083-H111 to deoxidised high phosphorus copper plates of 1 mm thickness. The welds were performed using different tool geometries and welding parameters. The positions of the copper and aluminium plates, relative to the advancing and retreating sides of the tool, were also changed. It was found that the tool geometry and relative position of the plates deeply influence the morphology of the aluminium and copper flow interaction zones, influencing the distribution of both materials in the weld and the formation of intermetallic compounds. The material accumulated under the tool during welding was found as another important aspect determining weld morphology.