Microstructural evolution in the friction stir welded 6061 aluminum alloy (T6-temper condition) to copper

This paper concentrates on the temperature distribution and microstructural evolution of the friction stir welding of 6061 aluminum alloy (T6-temper condition) to copper. The mechanically mixed region in the joining of the dissimilar metals 6061 aluminum alloy and copper weld consists mainly of several intermetallic compounds such as CuAl2, CuAl, and Cu9Al4 together with small amounts of alpha-Al and the saturated solid solution of Al in Cu. Distributed at the bottom of the weld nugget are numerous deformed copper lamellae with a high solid-solubility of aluminum. An intercalated structure or vertex flow pattern consisting of CuAl4 and the saturated solid solution of Al in Cu is formed in the Cu-rich regions adjacent to the bottom of the weld by the mechanical integration of aluminum into copper. The measured peak temperature in the weld zone of the 6061 aluminum side reaches 580 degrees C, which is distinctly higher than the melting points of the Al-Cu eutectic or some of the hypo- and hyper-eutectic alloys. Higher peak temperatures are expected at the near interface regions between the weld metal and the stirred tool pin. The phases present in the welds can be explained from the Al-Cu equilibrium phase diagram with the assumption that a complete phase equilibrium is reached in the liquid state but not during solidification. The primary dendrites of alpha-Al, CuAl2, and CuAl, and the eutectic of alpha-Al/CuAl2 are formed in the weld nugget during solidification. Distinctly different micro-hardness levels from 136 to 760 HV0.2 are produced corresponding to various microstructural features in the weld nugget. (c) 2005 Elsevier B.V. All rights reserved.