**2.2 Application of UNSM technology**

A UNSM is a cold-forging process that uses a tungsten carbide (WC) tip with a diameter of 2.38 mm to strike the sample surface at 20 kHz, which results in elastoplastic and surface severe plastic deformation (S2 PD), and heating, whereas forming a nanostructured layer at RT and HT. Due to a small radius of the tip, the contact area with a sample is relatively small causing high contact pressure up to 30 GPa. Advantages of UNSM over other surface peening technologies for particular AM materials are that it smooths out the surface, which is usually rough after AM and also increases the strength simultaneously. Moreover, it somehow shrinkages pores due to the compressive strike. The samples were treated by UNSM using the following variables listed in **Table 2** at 25 and 800°C. The combination of UNSM and LHT was described in our previous study [27]. The main variables are important, while force being the most important because it's magnitude determines the intensity of strain hardening. The force is directly proportional to the surface hardness, the grain size, strain-hardened layer and the compressive residual stress. The roughness is inversely proportional to the force, while it is directly proportional to the feed-rate.


**Table 1.** *SLM parameters.*

**Figure 1.** *SEM image of a single powder showing its shape and diameter.*


#### **Table 2.**

*UNSM treatment parameters.*

Roughness data were obtained measured by non-contact laser scanning microscope (LSM: VK-X100 Series, Keyence, Japan). Hardness data were collected by hardness tester (MVK-E3, Mitutoyo, Japan) at a load of 300 gf. X-ray diffraction (XRD) was performed with a Cu Kα radiation (k = 1.54056 Å), a tube current of 40 mA and a voltage of 30 kV over the range of 30–90 with a scanning rate of 100/min by Bruker D8 Advance X-ray diffractometer. Compressive residual stress induced after UNSM was measured by portable device (μ-360 s, Pulstec, Japan), which is a nondestructive method. Tensile-induced fracture and wear mechanisms were investigated by SEM (JEOL, JSM-6010LA, Japan) and chemistry reacted at the contact interface was characterized by energy-dispersive X-ray spectroscopy (EDX: JEOL, JED2300, Japan).
