To achieve weight reduction, multi-materials based on steel are being considered. In particular, bonding is linked to various issues, and understanding the state of bonding is an important quality issue.

Here are some examples using iron-to-iron spot welding. During spot welding, different microstructural structures are formed around the weld. Since each microstructure has a characteristic of high or low hardness, it is possible to estimate the vulnerable area due to the application of force.

The following is an example of fracture surface observation by FSW performed at the same pressure and rotation time but at different rotation speeds. Comparing the two samples, it can be inferred that the difference in heat input due to rotational speed affects the bonding strength, since the sample with stronger bonding strength has more aluminum uniformly remaining on the fracture surface, indicating cohesive