**3. Results and discussion**

#### **3.1 Surface topography**

**Figure 2** shows the top surface LSM images of the samples. In general, SLM fabricated samples demonstrated very rough surface due to the partially 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 that distorts by elasto-plastic deformation and S<sup>2</sup> PD resulting in refining grains and 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

**119**

*Tribology of Ti-6Al-4V Alloy Manufactured by Additive Manufacturing*

in order to avoid such crack initiation with the intention of preventing catastrophic failures of structures. There is still much improvements and optimizations in terms of microstructure- and ductility-based issued of AM materials to be done to be fully replaced with wrought alloys, but a progress in materials science and AM leading to overcome the faced challenge a couple of decades ago. Importantly, there is a way to get rid of from AM-based defects such as pore, unmelted and incompletely melted powders by HIP, which is highly-priced and time consuming that receiving a

*Top surface LSM images of the as-SLM (a), UNSM-25C (b) and UNSM-800C (c) samples.*

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

*DOI: http://dx.doi.org/10.5772/intechopen.93836*

cautious welcome from various industries.

**3.2 Surface integrity**

**Figure 2.**

*Tribology of Ti-6Al-4V Alloy Manufactured by Additive Manufacturing DOI: http://dx.doi.org/10.5772/intechopen.93836*

**Figure 2.** *Top surface LSM images of the as-SLM (a), UNSM-25C (b) and UNSM-800C (c) samples.*

in order to avoid such crack initiation with the intention of preventing catastrophic failures of structures. There is still much improvements and optimizations in terms of microstructure- and ductility-based issued of AM materials to be done to be fully replaced with wrought alloys, but a progress in materials science and AM leading to overcome the faced challenge a couple of decades ago. Importantly, there is a way to get rid of from AM-based defects such as pore, unmelted and incompletely melted powders by HIP, which is highly-priced and time consuming that receiving a cautious welcome from various industries.
