# Microstructure of Joint between Stranded Wire and Substrate Weld by Ultrasonic Welding
## 3 Results
### Microstructure at the Interface Between the Wire and Substrate  
In this section, the detailed microstructure of the interface was investigated in order to elucidate the welding mechanism.  
Figure 7 shows the typical interface microstructure between the wire and the substrate. Grey and white regions which correspond to the Ag and Au are observed between the Cu and the Ni. Ag is the coating layer of the Cu wire and Au is the outermost layer on the Cu substrate. The morphology of the Ag and Au layers were rough and the thickness of each layer varied depending on location suggesting that the plastic flow between Ag and Au occurred during the ultrasonic welding. Occasionally small pores were observed at the interface between the Ag and Au indicating the original surface position. No other defects were observed at the interface.  
Figure 5: Inverse pole figure (IPF) map taken from the cross-section of the wire cut perpendicular to the wire direction.  
Figure 6: Cross-section of the welded wire cut parallel to the wire direction.  
Figure 8 shows the IPF map and EBSD grain boundary map at the interface. Figure 8a shows the SEM image of the interface taken from a similar region as the EBSD analysis in Figure 7. In the IPF map in Figure 8b, elongated small Ag grains were observed in the Ag layer adjacent to the large Cu grains. Along the interface, grains extended from 1 to 2 \(\upmu\)m. In contrast, the thickness of the grains was about 200 nm in average and perpendicular to the interface. Figure 8c shows the EBSD grain boundary map. In this map, blue lines indicate that the grain boundary misorientation is over 15\({}^{\circ}\), and the grain boundary is a high angle grain boundary. Green lines indicate that the rotation angle ranges from 15\({}^{\circ}\) to 5\({}^{\circ}\). Red lines indicate that the rotation angle is less than 5\({}^{\circ}\). It was found that the Ag layer mostly consisted of high angle grain boundaries, and the Cu grains contained many small angle grain boundaries.  
Figure 8: **(a)** SEM image at the interface between the wire and the substrate **(b)** IPF map and **(c)** EBSD grain boundary map taken at the interface.  
Figure 7: Enlarged SEM image of the interface. The arrow denotes a small pore at the interface.  
To confirm the composition of each layer observed between the wire and the substrate, the composition mapping was taken using EDS installed in STEM. Figure 9 shows a typical STEM image around the interface and EDS mapping taken from the same region of the STEM image. The Ni and Au layers are present on the Cu plate as the substrate. On this substrate, there is Ag which is the coating layer on the Cu wire. It was revealed that after the ultrasonic welding, these layers remained intact and the contrast difference in each layer corresponded to the composition variation.  
Figure 10: Bright field image of the interface. The arrow indicates a twin boundary.  
Figure 9: STEM image and EDS mapping of each element around the interface.  
Figure 10 shows the bright field image at the welded interface between the wire and the substrate. In the Ag layer, twin boundaries were observed parallel to the interface, which was thought to be caused by the plastic deformation during the ultrasonic welding. No clear interface between the Au and Ag was observed at this region. Chemical composition mapping was taken at a similar region of the interface. Figure 11 shows the STEM image and the composition maps of the Ag grain adjacent to the welded interface. In the Ag grain, Au exists without a clear boundary, and the density of the Au increases near the substrate. An Ag/Au solid solution is present at this region.