*3.3. Mechanical Properties*

### 3.3.1. Hardness

Figure 8 shows the hardness distribution across the interfaces of dissimilar Ti-6Al-4V/Nitinol welds. It is observed that there was an increase in hardness on the Ti-6Al-4V side due to stain hardening that occurred during the friction welding process. However, no significant increase in hardness is observed on the Nitinol side, which confirms the previous

observation that no there was no significant deformation on the Nitinol side. Similar results were observed in previous studies involving friction welding of commercially pure Ti to 304L stainless steel [28,32]. Dissimilar friction welds prepared from Ti to 304L stainless steel have shown an increase in hardness on the Ti side due to the stain hardening effect when compared with the stainless steel side. However, a decrease in hardness on the stainless steel side was attributed to a limited amount of deformation or the lack of a strain hardening effect [28]. The Ti-6Al-4V and Nitinol base materials had average hardnesses of 367 ± 18 HV and 260 ± 16 HV, respectively. The highest hardness observed at the Ti-6Al-4V/Nitinol interface may be attributed to the formation of intermetallics of Ti and Ni, such as Ti2Ni, a result conformed by XRD analysis.

**Figure 8.** The hardness distribution across the interface of dissimilar Ti-6Al-4V/Nitinol welds.
