| Composite structure composed of aluminum and copper will contribute to weight and cost reduction and making full use of their advantages. The connection structure has been widely used in many areas due to their excellent heat and electric conductivity, corrosion resistance and formability, etc. However, due to the wide difference in their physical and chemical properties, the dissimilar combination of copper and aluminum is generally more difficult and hence limit their wide application. Various welding methods, including fusion welding, braze welding and pressure welding, have been applied to join Al-Cu dissimilar materials but many problems occurred. Thus we need to find out the better welding process for the connection of aluminum and copper.In this study, aluminum and copper were jointed by friction stir welding to solve those problems as mentioned above, so that the high-quality joints can be achieved. Firstly, the welding process parameters have been optimized, and on this basis not only did the macro and microstructure be studied, but also the phase composition, interfacial and mechanical properties were studied as well. At the same time, the relationship between interfacial structure and mechanical properties was analyzed from interface corrosion perspective. Beyond those, the flow behavior of the plastic deformation metal and the generation of defects in the joints were fully discussed.The results showed that the stir tool with complex shoulder morphology and threaded pin would attribute to the stir pin adhesion problem. The stir tool with an inward shoulder and none-threaded pin is proved to be the optimum selection to weld aluminum and copper. The pin offset technique and material arrangement exert a significant influence on the quality of the dissimilar joint. When T2 copper was located at advancing side and the tool pin was inserted towards the aluminum side about 0.8～1mm with suitable welding parameters, sound Al\Cu joints can be obtained. The lamellar structure formed at the of Cu\WN transition zone is conductive to the metallurgical bonding of aluminum and copper. Besides, a thin, uniform and continuous IMC layer can enhance the comprehensive performance of the joints further. However, the interfacial corrosion morphology revealed the weak bonding mechanism in the inner of the joints, which is the main reason why the crack in tended to initiate and propagate at the bonding interface in the tensile test. The tensile strength of Al\Cu joints was relatively lower than that of T2 cooper with a brittle or ductile-brittle mixed fracture mode. The grain refinement and high dislocation density caused by severe plastic deformation in Al\Cu dissimilar FSW are the prime reasons which cause the microhardness higher than that of base metals. Moreover, the average hardness at the upper side is relatively higher than that of the middle and bottom side. Beyond that, the Al4Cu9 and Al2Cu formation lead to a high fluctuation in hardness values.During FSW, the plastic flow is generated along the horizontal and the materials thickness direction at the same time, which attributed to the formation of an obvious boundary at WN\Cu interface. When the intercalation mixed structure is formed, the ratio of the element content of aluminum and copper is close to 1:1, which presents a good mixing condition. When the weld heat input is below 20 or the rotation and welding speed are higher than 1250rpm and 60mm\min, respectively, it causes obvious deterioration in the joints formation and mechanical properties. Furthermore, When T2 copper is placed at retreating side, the tunnel type defects and weak connection phenomenon are apt to initiate in the joints. Beyond that, the problems such as holes, cracks and poor mixing occur without a stir pin offsetting technique. |
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