**2.1 Sample preparation**

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

reported that SLM fabricated components of nickel-based superalloys and titaniumbased alloys are known to have several issues, such as cracking due to thermal stress by rapid heating and quenching [5, 6], poor surface finish and voids inside the material [6, 7], tensile residual stress, which causes deformation, cracks, and worsening of fatigue strength [4, 8], and columnar grain structures, which cause anisotropy in mechanical properties [9, 10]. To cope with these issues, various post-processing techniques have been applied such as surface milling for removing rough surface layers and possible cracks [8, 11], heat treatment for increasing toughness by phase transformation [12–15], laser polishing for decreasing surface roughness [16, 17], etc. Laser polishing reduced the roughness of laser AM TC4 and TC11 by about 75% and at the same time enhanced the surface micro-hardness of TC4 and TC11 by about 32% and 42%, respectively [18]. The other post-processing method that usually performed is hot isostatic pressing (HIP), which is capable of reducing the porosity and tensile residual stress of SLM fabricated Ti-6Al-4V alloy [19]. Post-heat treatments and HIP can solve some of those issues of SLM fabricated Ti-6Al-4V alloy, but the strength is significantly reduced. While laser polishing can increase the micro-hardness, but cannot solve the porosity-related issues and improve ductility. For solving the aforementioned issues of metal AM, the application of surface treatment or modification technologies has been proposed, e.g., shot peening (SP), laser shock peening (LSP), and ultrasonic nanocrystal surface modification (UNSM). The application of SP showed a remarkable improvement in fatigue behavior of AlSi10Mg alloys fabricated by AM in the high cycle fatigue region [20]. A previous study reported that LSP can induce a deep level of compressive residual stress, which significantly improves the fatigue life of 316 L stainless steel fabricated by AM [21]. It was also reported that LSP provided more fatigue life improvement than SP, which is largely attributed to the depth of compressive

UNSM is one of the mechanical surface modifications that utilizes an ultrasonic vibration energy to improve mechanical properties, tribological behavior, corrosion resistance and fatigue strength of various materials including AM fabricated materials. It was found earlier reported that the fatigue strength of SUS 304 shaft was improved by approximately 80% and the surface hardness is enhanced by both the grain refinement and the martensitic transformation after treatment by UNSM [22]. Furthermore, UNSM induced enhancement of surface hardness, compressive residual stress and grain refinement that resulted in improvement of fretting wear and frictional properties of commercially pure Ti and Ti-6Al-4V alloy [23]. The application of UNSM to AM fabricated materials has also been investigated. For example, Zhang et al. reported that electrically-assisted UNSM reduced the porosity and surface roughness, and enhanced the surface hardness of 3D printed Ti-6Al-4V alloy [24]. In our previous study on SLM fabricated 316 L stainless steel, it was reported that UNSM improved the mechanical properties, tribological behavior and corrosion resistance [25]. In general, UNSM reduces the surface roughness, increases the surface hardness, refines grain size and induces high compressive residual stress with the depth of hardened layer in the range of ~0.1 to ~0.3 mm [23, 25]. UNSM temperature-dependent surface hardness and phase transformation of wrought Ti-6Al-4V alloy were reported earlier [26], but the influence of UNSM temperature-dependent mechanical and tribological properties of AM fabricated Ti-6Al-4V alloy was not investigated yet. Therefore, in this study, the synergy effect of UNSM and local heat treatment (LHT) on the improvement of tensile and tribological properties of SLM fabricated Ti-6Al-4V alloy is investigated. The improvement in tensile and tribological properties after UNSM at different temperatures was compared with the results of the as-printed

**116**

Ti-6Al-4V alloy.

residual stress.

In this study, the sample made of Ti-6Al-4V alloy were fabricated by SLM (EOS M290, Germany) under the parameters listed in **Table 1**. **Figure 1** shows that the powder was spherical with a diameter of about 30–40 μm. Detailed information of SLM can be found in our previous study [25]. The hardness and yield strength of the as-printed samples were about 380 HV and 820 MPa, respectively. As for chemical composition, the content of Al and V was about 5.8 and 4.2 in wt.%, while the rest was Ti, respectively. The sample fabricated by SLM was used in as-printed state, which is hereinafter referred to as as-SLM, while the UNSM-treated samples at 25°C and 800°C are hereinafter referred to as UNSM-25 Degree-C and UNSM-800 Degree-C, respectively.
