# Superior Mechanical Behavior and Fretting Wear Resistance of 3D-Printed Inconel 625 Superalloy
## 3 Results and Discussion
### Tribological Tests  
Figure 6 shows the relationship between the friction behaviors of AM-Inconel 625 and the number of cycles at a contact load of 106 N. Figure 6a gives friction torques of the material tested after 2 \(\times\) 10\({}^{4}\) cycles. The friction torque increases rapidly over the first 5 \(\times\) 10\({}^{3}\) cycles and reaches a high value (\(\sim\)0.55). Then the friction torque maintains a relative steady state (\(\sim\)0.48). The friction behaviors of AM-Inconel 625 can be more clearly observed in the Figure 6b, in which the friction coefficient of the specimen is plotted against fretting cycles. The friction coefficient reaches a peak value of 0.62 at about 5 \(\times\) 10\({}^{3}\) cycles and decreases to a low value of \(\sim\)0.50 after 7 \(\times\) 10\({}^{3}\) cycles. The friction coefficient of AM-processed Inconel 625 is similar to that of the traditional Inconel 625 alloy reported in the previous work [8]. The typical friction torque versus angular displacement amplitude curves (\(T\)-\(\theta\) curves) at the fretting cycles of \(10^{0}\), \(10^{1}\), \(10^{2}\), \(10^{3}\), \(10^{4}\) and \(2\times 10^{4}\) are displayed in Figure 6c to evaluate the fretting running state. For the \(T\)-\(\theta\) curve with a parallelogram shape, the segments \(\overline{\text{AB}}\) and \(\overline{\text{CD}}\) correspond to the sliding of the two contact surfaces and the tilt segments \(\overline{\text{BC}}\) and \(\overline{\text{DA}}\) correspond to the static friction force. All the \(T\)-\(\theta\) curves show the shape of quasi-parallelograms, undoubtedly indicating the gross slip between the contact interfaces during the fretting wear process. For an individual loop, the tangential force is almost constant and without tips at the end, indicating that adhesion wear is mild between the contact surfaces. To explore the wear mechanism of AM-processed Inconel 625 alloy, optical images of the fretting wear tracks from tests at 40-106 N loads are shown in Figure 7. Figure 7a represents the measurements obtained from the test of AM-processed Inconel 625 at a contact load of 40 N, showing the material transfer from the counterpart. The optical image of the fretting wear track after test at 62 and 84 N are shown in Figure 7b,c, respectively. Similarly, material transfer can be observed on both wear tracks and apparent surface oxidation occurred at a contact load of 84 N. It has been confirmed that fretting wear decreased substantially for most friction couples (metals) as the relative humidity increases from zero to fifty percent [39]. In this study, with humidity about 60%, moisture promotes the formation of soft iron hydrates such as abrasive Fe\({}_{3}\)O\({}_{4}\), magnetite and a magnetic oxide of iron. Figure 7d shows the wear track obtained after test at a contact load of 106 N. Severe oxidation on the surface could be observed. All the tests at different contact loads last for \(2\times 10^{4}\) cycles without experiencing a significant amount of surface damage, indicating superior wear-resistance of AM-processed Inconel 625. Fretting wear is quantified according to a change in wear volume of the fretting wear track, as shown in Figure 8a. All the wear volume tests were carried out at the same condition for three times, yielding standard error bars. The 3D optical microscope is used to measure the wear volume and depth after fretting tests. For testing at 40-86 N, a slight increase in wear volume proves the mild wear in the wear surfaces. Since increment of wear volume is relative pronounced at the highest load of 106 N, the wear still remains slight. The critical load, at which the transition from mild to severe wear occurs, is primarily a material property [40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55]. The propagation of cracks which can result in the significant wear becomes more difficult as the fracture toughness of the material increases.