3.1 Metallurgy of additive manufactured titanium alloy
To examine and compare the phase existence in the additive manufactured titanium alloy with the base titanium alloy, both the materials were x-ray diffracted and the results were plotted as shown in Fig. 2. The diffraction obtained for the base material match with the reference pattern 98-008-0571 exhibits the alpha titanium with (hkl) indices of (002), (011), (012), and (013). The diffraction obtained for the additive processed material match with the reference pattern 98-009-2053 exhibits the titanium with (hkl) indices of (010), (002), (011), (012), and (013).The observed peak intensities of the additive manufactured sample is found to increase compared to the base material and this is due to the modification of material structure as observed in Fig. 3. Fig. 3 shows the microstructure of base titanium alloy and additive manufactured titanium alloy. The base titanium alloy exhibits the equiaxed alpha and beta structure and the additive titanium alloy exhibits the needle shaped acicular a phase(elongated structure) with the transformation of beta due to rapid cooling of material [5].
3.2 Wear performance
The additive manufactured titanium alloy exposed to dry sliding wear exhibited the wear rate as shown in Fig. 4. The exhibited wear rate of titanium alloy under 1 ms-1 sliding velocity at varied loads are depicted in Fig. 4a, the wear rate of additive manufactured titanium alloy shows a lower wear rate at lower load of 9.81 N and it exhibits a higher wear rate at medium load of 19.62 N. When the load was at higher level i.e., 29.43 N, it exhibits an intermediate wear rate between the lower and higher condition. The wear rate is found to be in steady state after 1500 m sliding for the higher and medium loads, whereas at lower load it is achieved after 2500 m sliding distance. This is due to the higher frictional force acting on the sliding surface at higher loads results in the reduction of asperity to asperity contact. Similar trend of wear rate was perceived in the additive manufactured titanium under 2 ms-1 sliding velocity. But the reduction of wear is clearly visible after the 1500 m sliding distance at medium and higher load, which is resulted from the oxidation mechanism, at increased sliding velocity and load. Whereas at lower load, there is no such declining trend, this shows there is no enough amount of frictional force and temperature to induce the oxidation formation. Though the wear rate of titanium alloy at 3 ms-1seems to be similar like 1 ms-1and 2 ms-1, the wear rate at lower and medium load is found to be increased at the initial run and it is declined up to the sliding distance of 1000 m. The possible effect for this reduction are the surface oxidation of the contact asperities at the initial run period due to higher contact stress. Beyond 1000 m sliding distance, it showed an increasing trend of wear rate and it is due to the removal of oxidized asperities from the pin surface. Almost in all the velocities, higher load exhibited higher wear rate and drop in load results in reduction in wear rate. Initially the wear rate seems to be lower at the lower sliding velocity and it found to be increasing with the increase in sliding velocity to 2 ms-1.Further, the increase in sliding velocity decreased the wear rate, and this is attributed due to the mechanism of oxidation. Similar trend of reduction in wear rate with an increased sliding velocity and increased wear rate with increased load is reported by Straffelini and Molinari[25]. The comparison of wear results with the base material under different working conditions are presented in Table 2, which shows that the additive manufacture sample has better wear resistance compared with the base alloy.
The additive manufactured titanium alloy exposed to dry sliding wear exhibited the coefficient of friction as shown in Fig. 5. The exhibited coefficient of friction of the titanium alloy under 1 ms-1 sliding velocity at varied loads are shown in Fig. 5a.It shows that higher coefficient of friction at higher load and it got reduced with the reduction in load. Also, there is a slight fluctuation in the coefficient of friction and this is due to the variation in the surface topography during sliding. Similar trend was observed under sliding velocity of 3 ms-1, whereas for sliding velocity of 2 ms-1, the coefficient of friction is higher for medium load for 19.62 N load. The possible reason for this is at sliding velocity 3 ms-1, oxidation occurs, that leads to the reduction in coefficient of friction as the oxide layer acts as lubricant. Also, the coefficient of friction is found to be reduced with the increase in sliding velocity and this effect is due to the formation of oxide layer.
3.3 Wear mechanism
The dry sliding of titanium alloy pin results in the destruction of sliding surface, which helps to reveal the involved wear mechanism. After dry sliding, the pin which exhibits the best wear rate and worst wear rate was analysed under scanning electron microscope as shown in Fig. 6. The surface topography of wear pin corresponds to the worst wear (i.e., load of 29.43 N with sliding velocity of 2 ms-1) in Fig. 6a, it shows the presence of crater, scratches, and spalling that evident the delamination, and abrasion mechanism. The observation of fragment and laminates confirms the mechanism of delamination and abrasion from Fig. 7a. The topography of wear pin surface corresponds to the best wear (i.e., load of 9.81 N with sliding velocity of 3 ms-1) is shown in Fig. 6b, with spalling, ridges, shallow crater, ploughs, and scratches. The ridges on observed surface morphology raised due to the deformation of deformed layers due to adhesion and the ploughs and scratches evident abrasion mechanism. The observation of ribbon flakes, laminates, and fine debris confirms the mechanism of delamination, abrasion and oxidation from Fig. 7b. The mechanism of oxidation is confirmed with the presence of oxygen content in the worn pin corresponds to the best wear condition.