**3.5 Tribology**

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

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

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

**Figure 3(b)** shows the surface hardness measurement results of the samples. 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%, respectively. It is well documented in the literature that the increase in hardness is due to the combination of grain refinement by Hall-Petch expression and increased dislocation density, which

subsurface layers [30]. The deformation usually refined the grains, which hinder further deformation and gradually diminishes with the depth of the surface layer. Moreover, Zhang et al. reported that the UNSM-induced work-hardening by plastic strain may also play a major role in increasing the hardness [31]. Moreover, in particular for AM materials, the expelled pores after peening technologies may be

**Figure 4** shows XRD patterns of the as-SLM, UNSM-25C and UNSM-800C samples. The change in diffraction peaks and phase transformation were confirmed by the relative intensity and the formation of a new peak after UNSM at 800°C. It was found that the intensity of all alpha phase peaks reduced except for (002) phase after UNSM at both 25 and 800°C. The intensity of alpha (101) phase increased for UNSM-800C sample and reduced for UNSM-25C sample. For as-SLM and UNSM-25C samples, the microstructure exhibited a full α/α`-phase, where α phase resulted from decomposition of α` during the SLM. For UNSM-800C sample, a precipitation of beta (110) phase was detected leading to a microstructure consisting of α and β phases as the temperature was higher than that of Ms. (575°C) [33]. Further, a broadening in full width at half maximum (FWHM) of the α peaks took place after UNSM, which led to the increase in dislocation density [27]. It is also obvious that

PD took place in the top and

machining or milling was treated by UNSM.

are the results of elasto-plastic deformation and S<sup>2</sup>

contributed to the increase in hardness [32].

**3.4 Phase transformation**

**3.3 Surface hardening**

**Figure 3.**

**120**

**Figure 5** shows the friction coefficient as a function of sliding cycles of the samples. It can be seen that all the samples came into contact with bearing steel (SAE 52100) underwent a running-in and steady-state frictional behavior. As shown in **Figure 5**, the friction coefficient of the as-SLM sample was found to be approximately 0.36 at the beginning of the friction test and increased continuously up to 0.52 for about 2000 cycles, which is considered as a running-in period. Then the friction coefficient continued being stable with a friction coefficient of 0.58 till the end of the test. **Figure 5** also shows the friction coefficient of the UNSM-25C sample. It was found that the friction coefficient demonstrated a similar friction behavior to the as-SLM sample, but the UNSM was able to reduce the friction coefficient in both the running-in and steady-state periods, where the friction coefficient was approximately 0.38 and 0.43, respectively. Overall, the friction behavior of the as-SLM sample was very highly fluctuated, which is associated with the initial rough surface. The frictional behavior of the UNSM-25C sample was relatively lower fluctuated, where the reduced surface roughness after UNSM is responsible for it. In addition,


**Table 3.** *Calculated FWHM results based on XRD pattern.*

#### **Figure 5.**

*Friction coefficient of the as-SLM, UNSM-25C and UNSM-800C samples.*

as shown in **Figure 5**, the friction coefficient of the UNSM-800C sample was found to be approximately 0.19 at the beginning of the friction test and then continued to be stable for about 3600 cycles, which is considered as a running-in period. Then the friction coefficient gradually increased up to 0.51 and subsequently approached a stable friction coefficient till the end of the test. From the tribological tests, it was obvious that the as-SLM and UNSM-25C samples demonstrated a similar friction behavior, but the UNSM-800C sample extended the running-in period. Essentially, a lower friction coefficient was dominated by initial roughness of the samples, while an increase in hardness of the UNSM-800C sample, which came into first contact with the surface of counterface ball, had harder asperities that could increase the level of plastic deformation. A similar friction behavior was confirmed in the previous study on stainless steel 316 L that the friction coefficient was lower at the beginning of the test due to the initial surface roughness, where the asperities came into contact first and it deformed plastically with continuing reciprocating sliding [25]. Obtained friction coefficient results under dry conditions are in good consistency with the ultra-fined Ti-6Al-4V alloy fabricated by SLM [35].

The wear track dimensions of the samples were measured by 3D LSM as shown in **Figure 6**, which allowed to calculate the wear resistance based on the wear track width and depth dimensions. It can be seen that a significant difference was observed, where the wear track dimensions of the UNSM-800C sample was found to be the shallowest wear track compared to those of the as-SLM and UNSM-25C samples. The maximum peak-to-valley roughness height *(R*max*)* of the wear track was about 226.3, 131.6 and 87.8 μm for the as-SLM, UNSM-25C and UNSM-800C samples, while no remarkable distinct in wear track width was observed due to the same contact pressure, respectively. As shown in the inset of **Figure 6**, the wear rate of the as-SLM sample was reduced from 3.57 × 10−8 to 1.48 × 10−8 and 8.70 × 10−9 mm3 /N × m, corresponding to a ~ 41% and ~ 246% enhancement in wear resistance compared to those of the UNSM-25C and UNSM-800C samples, respectively. UNSM eliminated the effect of stress concentration in the inside of wear track, where it's depth did not exceed the thickness of strain-hardened layer containing refined nano-grains and compressive residual stress. Hence, the application of UNSM to the as-SLM sample at 25°C enhanced the wear resistance substantially due to the increase in hardness. Furthermore, a temperature increase of UNSM supplementary enhanced wear resistance. The reduction in surface roughness after UNSM led to the lower friction coefficient, while the increase in surface hardness was responsible for the higher wear resistance compared to that of the as-SLM sample. In addition, an induced compressive residual stress by UNSM

**123**

**Figure 7.**

*UNSM-800C (c) samples.*

**Figure 6.**

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

hindered the wear process [36]. As tribology considered as a system of two interacting surfaces, a wear scar of the counterface ball that came into contact with the as-SLM, UNSM-25C and UNSM-800C samples is shown in **Figure 7**. No significant difference in wear scar was observed, but the wear scar of the counterface ball that came into contact with the UNSM-800C sample was relatively smaller than those of the as-SLM and UNSM-25C samples. Beyond the wear scar of the counterface ball that came into contact with the as-SLM and UNSM-25C samples, accumulated debris were attached, while no any debris was found for the UNSM-800C sample. Finally, UNSM ensures improved surface integrity parameters and endurance of

*3D LSM images of the counterface ball that came into contact with the as-SLM (a), UNSM-25C (b) and* 

*3D LSM images of the as-SLM (a), UNSM-25C (b) and UNSM-800C (c) samples.*

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

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

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

as shown in **Figure 5**, the friction coefficient of the UNSM-800C sample was found to be approximately 0.19 at the beginning of the friction test and then continued to be stable for about 3600 cycles, which is considered as a running-in period. Then the friction coefficient gradually increased up to 0.51 and subsequently approached a stable friction coefficient till the end of the test. From the tribological tests, it was obvious that the as-SLM and UNSM-25C samples demonstrated a similar friction behavior, but the UNSM-800C sample extended the running-in period. Essentially, a lower friction coefficient was dominated by initial roughness of the samples, while an increase in hardness of the UNSM-800C sample, which came into first contact with the surface of counterface ball, had harder asperities that could increase the level of plastic deformation. A similar friction behavior was confirmed in the previous study on stainless steel 316 L that the friction coefficient was lower at the beginning of the test due to the initial surface roughness, where the asperities came into contact first and it deformed plastically with continuing reciprocating sliding [25]. Obtained friction coefficient results under dry conditions are in good consistency

The wear track dimensions of the samples were measured by 3D LSM as shown

/N × m, corresponding to a ~ 41% and ~ 246% enhancement in

in **Figure 6**, which allowed to calculate the wear resistance based on the wear track width and depth dimensions. It can be seen that a significant difference was observed, where the wear track dimensions of the UNSM-800C sample was found to be the shallowest wear track compared to those of the as-SLM and UNSM-25C samples. The maximum peak-to-valley roughness height *(R*max*)* of the wear track was about 226.3, 131.6 and 87.8 μm for the as-SLM, UNSM-25C and UNSM-800C samples, while no remarkable distinct in wear track width was observed due to the same contact pressure, respectively. As shown in the inset of **Figure 6**, the wear rate of the as-SLM sample was reduced from 3.57 × 10−8 to 1.48 × 10−8 and

wear resistance compared to those of the UNSM-25C and UNSM-800C samples, respectively. UNSM eliminated the effect of stress concentration in the inside of wear track, where it's depth did not exceed the thickness of strain-hardened layer containing refined nano-grains and compressive residual stress. Hence, the application of UNSM to the as-SLM sample at 25°C enhanced the wear resistance substantially due to the increase in hardness. Furthermore, a temperature increase of UNSM supplementary enhanced wear resistance. The reduction in surface roughness after UNSM led to the lower friction coefficient, while the increase in surface hardness was responsible for the higher wear resistance compared to that of the as-SLM sample. In addition, an induced compressive residual stress by UNSM

with the ultra-fined Ti-6Al-4V alloy fabricated by SLM [35].

*Friction coefficient of the as-SLM, UNSM-25C and UNSM-800C samples.*

**122**

8.70 × 10−9 mm3

**Figure 5.**

hindered the wear process [36]. As tribology considered as a system of two interacting surfaces, a wear scar of the counterface ball that came into contact with the as-SLM, UNSM-25C and UNSM-800C samples is shown in **Figure 7**. No significant difference in wear scar was observed, but the wear scar of the counterface ball that came into contact with the UNSM-800C sample was relatively smaller than those of the as-SLM and UNSM-25C samples. Beyond the wear scar of the counterface ball that came into contact with the as-SLM and UNSM-25C samples, accumulated debris were attached, while no any debris was found for the UNSM-800C sample. Finally, UNSM ensures improved surface integrity parameters and endurance of

#### **Figure 7.**

*3D LSM images of the counterface ball that came into contact with the as-SLM (a), UNSM-25C (b) and UNSM-800C (c) samples.*

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 crack propagation [37, 38].

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

**Figure 8.** *SEM images along with EDX results of the as-SLM (a), UNSM-25C (b) and UNSM-800C (c) samples.*

**125**

**Acknowledgements**

**4. Conclusions**

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

UNSM-800C samples, respectively. It is clear from oxide distribution mapping (see **Figure 8**) that the level of oxidation of the as-SLM and UNSM-25C samples was nearly consistent, but a relatively high level of oxidation occurred for the UNSM-800C sample. Zhang et al. reported that at a high temperature, a hardness of oxide layer become remarkably higher due to the presence of much oxide [39]. Hence, it is reasonable to hypothesize that it may significantly increase the wear resistance by obtaining low friction coefficient [40]. Typically, there is an advantage to forming an oxide layer between two mating surface as it prevents direct metal-to-metal contact resulting in lower friction coefficient and higher wear resistance. Furthermore, a nearly same amount of Fe, which was transferred from the counterface ball can be seen from the chemical composition table as shown in the inset of **Figure 8**. The presence of Fe along with occurred oxide may react together and form a ferrosoferric (Fe3O4) layer, which provides an advantageous environment for achieving a better tribological behavior and performance [27]. Furthermore, as the surface roughness of the samples deteriorated during the dry tribological tests as shown in **Figure 8**, where the post-test surface roughness of the UNSM-25C and UNSM800C samples was lower than that of the as-SLM sample. Hence, it can be considered that the surface roughness after dry tribological tests is much important than post-surface treatment because the surface during the dry tribological tests comprises a number of grooves of different depth and sharpness – causing local stress concentrations and decreasing the wear resistance.

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.

This study was supported by the Industrial Technology Innovation Development

Project of the Ministry of Commerce, Industry and Energy, Rep. Korea (No.

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

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

UNSM-800C samples, respectively. It is clear from oxide distribution mapping (see **Figure 8**) that the level of oxidation of the as-SLM and UNSM-25C samples was nearly consistent, but a relatively high level of oxidation occurred for the UNSM-800C sample. Zhang et al. reported that at a high temperature, a hardness of oxide layer become remarkably higher due to the presence of much oxide [39]. Hence, it is reasonable to hypothesize that it may significantly increase the wear resistance by obtaining low friction coefficient [40]. Typically, there is an advantage to forming an oxide layer between two mating surface as it prevents direct metal-to-metal contact resulting in lower friction coefficient and higher wear resistance. Furthermore, a nearly same amount of Fe, which was transferred from the counterface ball can be seen from the chemical composition table as shown in the inset of **Figure 8**. The presence of Fe along with occurred oxide may react together and form a ferrosoferric (Fe3O4) layer, which provides an advantageous environment for achieving a better tribological behavior and performance [27]. Furthermore, as the surface roughness of the samples deteriorated during the dry tribological tests as shown in **Figure 8**, where the post-test surface roughness of the UNSM-25C and UNSM800C samples was lower than that of the as-SLM sample. Hence, it can be considered that the surface roughness after dry tribological tests is much important than post-surface treatment because the surface during the dry tribological tests comprises a number of grooves of different depth and sharpness – causing local stress concentrations and decreasing the wear resistance.
