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.

A new technique of filling friction stir welding (FFSW) relying on a semiconsumable joining tool has been developed to repair the keyhole left at the end of friction stir welding (FSW) seam. The conventional nonconsumable tool of FSW was transformed, and a semiconsumable joining tool consisting of alloy steel shoulder and aluminium alloy joining bit was designed to create a solid state joint. Using the combined plastic deformation and flow of the consumable joining bit and the wall of the keyhole, the FFSW process is able to repair the keyhole with both metallurgical and mechanical bonding characteristics, and the FSW seam can be achieved without keyhole or other defects. The relative tensile strength and elongation of the FFSW joint are 84?3 and 98?9% of the base weld without defects respectively.

In the present work, 7?8 mm thick AA 2219 rolled plates were successfully joined without keyholes using semiconsumable tools by filling friction stir welding (FFSW). The shoulder further effect was performed to enhance mechanical stir, and mechanical properties were enhanced effectively. The results showed that the bonding interface of AA 2219 bit and keyhole is defect free. The softened region on the advanced side is the weakest part of FFSW joint rather than the bonding interface of the keyhole. The average ultimate tensile strength and elongation are 172 MPa and 11?2%, equal to 90 and 82% of the base weld without defects respectively. Excellent bonding interface and mechanical properties of FFSW joints have been exhibited.

Friction stir butt barrier welding, an improved method based on the friction stir butt welding process, in which a barrier was configured on the butt plate, was applied to the Cu-Al combination. The resultant Cu-Al joints presented good appearance when Al was adopted as the barrier. The pin shifting towards the Al side could improve the interior quality of the weld. However, the effect was limited when the pin offset increased to a certain value. As the tool rotation rate increased, the amount and size of the voids decreased first and then increased. There were two types of joint depending on the interfacial morphology: the smooth interface and the chaotic interface. When the pin offset was zero, the interface was occupied by large Cu fragments. As the pin offset increased, the joint interface transited first to the smooth type and then to the chaotic type. As the rotation rate increased, the laminate density around the interface decreased, and the sublayers in the laminates thinned.

Underwater friction stir welding (FSW) has been demonstrated to be an effective method to improve the mechanical properties of joints. To illuminate the characteristics of underwater FSW, the microstructural evolution and its effect on mechanical performance of an underwater joint were investigated in the present paper. The weld nugget zone (WNZ) is characterised by the homogeneity of refined grain structures in the direction of thickness. The precipitate deterioration is gradually strengthened from the heat affected zone (HAZ) to the WNZ. The dislocation movement featured by different types of grain boundaries suggests that continuous dynamic recrystallisation has occurred in the WNZ during FSW. The evolutions of strengthening precipitates and grain structures have significant influence on hardness distributions and the tensile fracture features of the underwater joint.