Good appearance and defect free friction stir welded joints of aluminium alloy to copper were obtained in a narrow range of welding conditions. The weld nugget (WN) consisted mainly of Cu(Al) solid solution, Al and several intermetallic compounds (IMCs), including CuAl 2 , CuAl, Cu 9 Al 4 and Cu 3 Al although the maximum peak temperature measured in the weld was 422uC. Intercalation was a typical structure distributed widely in the WN and consisted of many parallel alternate lamellae. At the WN/Cu interface, the intercalations were characterised by flat or bended feature and composed of CuAlzCu 9 Al 4 , AlzCuAl 2 zCu(Al) and Cu(Al) laminae. The formation mechanisms for these intercalation structures and IMCs were attributed to the intense stirring action of the tool pin and the short circuit diffusion caused by extreme deformation respectively. The fracture surface exhibited some IMCs, including Cu 9 Al 4 and CuAl 2 , which should be responsible for the limited tensile strength. The hardness profiles were consistent with the variation in the structures within the WN, and lower tool rotation rate resulted in higher peak hardness.

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Alclad 7B04-T74 aluminum alloy was friction stir lap welded at different welding parameters. The microstructural characteristics and mechanical properties of the joints were investigated. After welding, the initial Alclad at the faying interface upward migrated and penetrated into the stir zone from the retreating side. This reduced the postweld thickness of the upper sheet and formed a potential crack propagation path in the stir zone during tensile shear testing. During welding, the stirred Alclad was lifted by the probedriven material flow and pressed down by the shoulder-driven material flow. The higher tool rotation speed or the lower welding speed made the redistributed Alclad in the stir zone more disperse. Higher fracture strength of the joint was obtained when the retreating side of upper sheet was loaded. A mathematical relationship between the fracture strength and the welding parameters was developed by applying the BoxBehnken experimental design, and the optimized fracture strength of 749 N/mm was obtained.

A thermoplastic ABS and an aluminium alloy 6061-T6 were joined using friction lap welding (FLW). The joint characteristics were evaluated to investigate the effects of 6061-T6 surface and the welding parameters on the joint properties. ABS and 6061-T6 were joined via an interfacial MgO layer. The voids generated by thermal decomposition would affect the strength of the joint. Owing to the small temperature range between the melting point and the thermal decomposition point of ABS, controlling the welding temperature is the key to the success of the 6061-T6/ABS FLW process. The relationship between the 6061-T6 surface after welding and the adhesion properties of ABS and 6061-T6 under different processing parameters was investigated.

Temperature and material flow behavior during friction spot welding of Alclad 7B04-T74 aluminum alloy were studied by both numerical simulation and welding experiment. The Alclad 7B04-T74 aluminum alloy sequentially experienced solid solution treatment at 465°C, low temperature artificial aging at 120°C, and high temperature artificial aging at 180°C. During welding, the material which flowed into the sleeve cavity suffered from higher temperature, and the peak temperature in the stir zone was higher than the incipient melting temperature of the base material. Accordingly, the eutectic films along the grain boundaries can be observed in the stir zone after welding. The peak temperatures in the thermo-mechanically affected zone and the heat affected zone were lower than the solution temperature and higher than the artificial aging temperature of the base material. In the sleeve retreating stage of the welding process, the material in the sleeve cavity flowed downward out of the sleeve cavity, and then it flowed laterally and upward to fill the gap left by the retreating sleeve. Such a material flow path resulted in the "U-shaped" morphology of the bonding ligament, the upward curving of the hook, and the upward distortion of the grains in the thermomechanically affected zone.

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