**4. Conclusions**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

crack propagation [37, 38].

Ti-6Al-4V alloy fabricated by SLM with no subsequent process. Furthermore, it is of interest to note up that the fatigue strength of the UNSM-800C sample may be detrimental due to the presence of cracks on surface (see **Figure 2(c)**) induced by UNSM at HT of 800°C because of the presence of continuous stress leading to a

**Figure 8** shows the SEM images along with EDX results and oxidation distribu-

tion of the samples. It can be realized from SEM image in **Figure 8(a)** that the adhesive wear mechanism was found to be a dominant for the as-SLM sample as it is softer than that of the counterface ball, while a combination of abrasive and adhesive wear mechanisms was dominant for the UNSM-25C sample as shown in **Figure 8(b)**. An increase in temperature of UNSM resulted in changing wear mode as shown in **Figure 8(c)**, where the abrasive wear mechanisms took place for the UNSM-800C sample. Apart from those wear mechanisms, an oxidative wear mechanism came up in all the samples with different oxidation levels as shown in **Figure 8**. For instance, an oxide content over the wear track of the as-SLM sample was about 9.68%, while it was about 9.89% and 11.62% for the UNSM-25C and

*SEM images along with EDX results of the as-SLM (a), UNSM-25C (b) and UNSM-800C (c) samples.*

**124**

**Figure 8.**

In this study, the influence of UNSM on the surface, tensile and tribological properties of Ti-6Al-4V alloy fabricated by SLM was evaluated. The as-SLM sample had a roughness of about 9.541 μm, which was drastically reduced up to 0.892 and 3.058 μm after UNSM at 25 and 800°C. The average surface hardness of the as-SLM sample was approximately 396.4 HV, which increased up to 455.7 and 877.6 HV for the UNSM-25C and UNSM-800C samples, corresponding to a 13.1% and 221.3% increase, respectively. The surface residual stress of both the UNSM-25C and UNSM-800C samples was transferred into compressive residual stress. The as-SLM sample demonstrated lower YS and UTS than UNSM-25C and UNSM-800C samples, but its elongation was shorter than that of the UNSM-25C sample and longer than that of the UNSM-800C sample. YS and UTS of the UNSM-25C sample was lower and higher than that of the UNSM-800C sample, while the elongation was also longer than that of the UNSM-800C sample. Friction coefficient of the as-SLM sample was reduced by the application of UNSM at 25 °C by about 25.8%, and it further reduced by about 305% increasing the UNSM temperature up to 800 °C. The wear rate of the as-SLM sample was reduced by about 41% and 246% compared to those of the UNSM-25C and UNSM-800C samples, respectively. As a main conclusion, a UNSM at RT and HT may be applied to Ti-6Al-4V alloy fabricated by SLM with the intention of enhancing tensile and tribological properties of various components in aerospace and biomedical applications. Indeed, a further investigation is required to improve the properties and performance of Ti-6Al-4V alloy fabricated by SLM to the wrought level due to the replacement possibility.
