**3.2 Surface integrity**

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

**mm/min**

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:

20 30 2000 40 10 2.38 WC 25, 800

**Force, N Feed-rate, μm**

**Ball diameter, mm**

**Ball material** **Temperature, °C**

**Figure 2** shows the top surface LSM images of the samples. In general, SLM

unmelted powders and also the presence of gas-induced pores on surface as shown in **Figure 2(a)**. The size of unmelted powders was found to be in the range of 30–40 μm, while the size of gas-induced pores was approximately 12 μm. The surface of the UNSM-25C sample is shown in **Figure 2(b)**. Obviously, the UNSM was able to impinge against those unmelted powders and to expel the pores from the surface, which was flattened at the end with no unmelted powders, pores and even cracks. In order to investigate the amalgamation of UNSM with thermal energy, the sample was treated by UNSM at 800°C. **Figure 2(c)** shows the surface of the UNSM-800C sample. The UNSM leaves the trace in the roughness of the surface

creating cracks along the pathway of the WC tip. Doubtless, the unmelted powders and pores were not observed following the UNSM at 800°C, which led to the formation of some UNSM-induced isolated cracks noticed in **Figure 2(c)**. The initiated cracks are the indication of over-peening leading to an excessive strain hardening, where the amalgamated impact of UNSM and thermal energy resulted in surface degradation, and consequently imposing practical limitation in terms of surface quality rather than strength. An appearance of UNSM-induced isolated cracks can be explained in three stages: (1) the stage of strain hardening that consists of an intensive increase in surface roughness; (2) the stage of saturation, where a plastic shearing takes place leading to a reduction in dislocation density and cracks initiated; (3) the stage of surface damage, the integrity of surface is destroyed leading to an appearance of cracks, where the surface roughness increases. Actually, overpeening of surface peening technologies may deteriorate the surface integrity leading to a decrease in fatigue cycles or strength [28]. Over-peeing may cause inversion of stress that can reduce the compressive residual stress induced by UNSM. In this regard, it is always required to optimize the impact of surface peening technologies

PD resulting in refining grains and

fabricated samples demonstrated very rough surface due to the partially

that distorts by elasto-plastic deformation and S<sup>2</sup>

**118**

JEOL, JED2300, Japan).

*UNSM treatment parameters.*

**Table 2.**

**3.1 Surface topography**

**3. Results and discussion**

**Frequency, kHz Amplitude, μm Speed,** 

Surface roughness measurement direction (MD) and UNSM treatment direction (TD) for each samples are shown in **Figure 2**. The surface roughness was measured in perpendicular direction to the UNSM TD. As mentioned in the previous subsection, a rough surface of AM fabricated samples is still considered as one of the main issues. One can be seen that the actual surface contained irregularities in the form of peaks and valleys. **Figure 3(a)** shows the comparison in surface roughness *(R*a*)* profiles of the samples. The as-SLM sample had a roughness of about 9.541 μm, while it was drastically reduced up to 0.892 μm after UNSM at 25°C as shown in **Figure 3(a)**. The roughness of the UNSM-800C sample was 1.237 μm, which is about 7–8 times smoother than that of the as-SLM sample, but it is still rougher than UNSM-25C sample. A bit rougher surface of the UNSM-800C sample than that of the UNSM-25C sample is associated with plastic deformation during the heating that distorted the geometrically pattern resulting in increased the height of irregularities. In general, during the UNSM, a plastic deformation of the top and subsurface layers took place. Expelled pores and disposed peaks and valleys led to the reduction in surface roughness [25]. As those peaks and valleys between irregularities are notches that weaken the surface cause stress concentrations. Feed-rate pathways induced by UNSM and the presence of peaks and valleys are responsible

**Figure 3.** *Surface roughness (a) and hardness (b) of the as-SLM, UNSM-25C and UNSM-800C samples.*

for the relatively rough surface of the UNSM-800C sample compared to that of the UNSM-25C sample. The level of plastic deformation increased and lessened flow eliminating with increasing temperature. Smoothed surface by UNSM at 25°C is beneficial to improving main structural properties such as tribology, corrosion and fatigue. Commonly, a surface quality of AM fabricated materials is very rough upon completion of AM [29]. This means that surface is required to be machined or finished. In this study, it is worth mentioning that the surface with no any additional machining or milling was treated by UNSM.
