Fig. 1. (a) Optical microscopy image of an electron beam weld of the Cu-Nb NLC panels (b) Higher magnification of the boxed region in (a) shows the edge of the weld melt zone where a dendritic Nb structure has formed within a Cu matrix [12].

Fig. 2. (a) FSW diagram showing the process parameters, workpiece material, tool, advancing and retreating sides. (b) A schematic of the transverse view of a FSW which contains the weld nugget, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and parent material (PM).

Fig. 3. Diagram showing the transverse view layout of the double pass weld.

Fig. 4. Tensile dogbone dimensions with panel transverse direction (TD) and rolling direction (RD) labeled. The FSW was made along the rolling direction of the ARB panels. All dimensions are in mm.

Fig. 5. Schematic of plan view of FSW zone in which material enters in slip lines around the FSW tool [after 17]. The material travels from right to left while the tool rotates in a counter clockwise direction.

Fig. 6. DIC εxx strain field maps showing large local strain on the AS for single (a) and double pass (b) FSWs. Corresponding scale-bar shows the strain with units of mm/mm.

Fig. 7. SEM backscatter image of PM microstructure showing the nano-lamellar structure. The lighter layers are Nb and the darker layers are Cu.

Fig. 8. (a) Location of SEM image in single pass FSW. (b) SEM backscatter image of the FSW nugget.

Fig. 9. (a) Location of SEM image in single pass FSW region. (b) SEM backscatter image of RS of the FSW nugget before removal of root side lack of penetration (LOP) region for the tensile test specimens.

Fig. 10. (a) Location of SEM image in single pass FSW region. (b) SEM backscatter image of the AS of the FSW nugget showing the most extreme changes in direction and layer refinement.

Fig. 11. (a) Location of SEM image in FSW region for the double pass FSW. (b) SEM image of transverse section of the double pass FSW showing loss of visible layered structure, in the SEM, at the AS interface between the nugget and the TMAZ region.

Fig. 12. (a) Location of SEM image in FSW region for the double pass. (b) FSW Hardness map across the double pass FSW at the AS nugget/TMAZ interface. Hardness values are shown in GPa.

Fig. 13. SEM backscattered image of waviness, swirls and vortices in the FSW region of a double pass weld in the Cu-Nb nano-lamellar layers.

Fig. 14. Profile of the FSW velocity that imparts a shear flow gradient parallel to the layers.