# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 4 Effect of USRP on Mechanical and Tribological Properties of Engineering Materials
Similar to the previous work, the microstructural morphology of the alloy was refined to nanometer size, with the minimum grain size being at 338.8 nm for a rolling displacement of 0.07 mm. The hardness was proportionally increased with the rolling distance as the plastic deformation increased the total number of dislocation walls and tangles. The highest hardness was at 128.3 HV, nearly 84% greater than the base material. The increase in hardness also translates to the tensile performance of the Mg sheet, because of which the YS increased by 24.08%. Aside from the grain-refinement and dislocation strain strengthening from USRP, the authors in this work have also related the compressive stresses to tensile performance. Particularly, the presence of these stresses can prevent the generation of early part failure via microcracks. On a similar note, the fatigue properties of USRP components have also demonstrated promising results due to the combination of surface hardening, grain refinement, and improved structural integrity, which can largely prevent fatigue-induced crack initiations [73]. A recent study from [PERSON] et al. [59] thoroughly explores the influence of USRP on the fatigue behavior of Ti6 Al4V and its relation to surface integrity. This work set the static load, ultrasonic vibration frequency, and ultrasonic vibration amplitude to 600 N, 20 kHz, and 10 \(\upmu\)m. These conditions were repeated on different sections of the alloyed bar, varying from 1, 3, 6, and 12 repetitions, also listed as R1, R3, R6, and R12. With respect to fatigue stress vs. cycles to failure, it was shown that each USRP sample had an improvement in fatigue performance, as shown in Figure 10. However, as the number of processing limits increased, its fatigue limits gradually decreased. The primary mechanisms that have enabled fatigue performance enhancement are primarily due to the work hardening, microstructural refinement, compressive stresses, and surface quality induced from USRP. In combination, the residual stresses for the high cycle fatigue (HCF) condition were shown to be lower than the low cycle fatigue (LCF) condition for all specimens. In the case of specimen R1, its superior fatigue performance can be closely linked to combination of surface quality and the amount of compressive stresses induced. Despite not containing refined grains, the superior fatigue life was explained by the ratio of refined to coarse grains. In the case of this work, coarse grains are speculated to help deflect any propagation of cracks through the grain boundaries, unlike more refined grains. This in turn can result to a reduction of crack growth rate as there is a greater likelihood of crack closure. Figure 10: The fatigue of life of USRP’ed Ti6 Al4V specimens under a stress value of 700 MPa. Reproduced with permission from [59]. Copyright 2019 Elsevier. Other Ti alloys have also exhibited similar outcomes, as recently shown with TB8 from [PERSON] et al. [32]. The authors also used a vibration frequency of 25 kHz, a lathe rotational speed of 75 r min\({}^{-1}\), and a feed rate of 0.12 mm rev\({}^{-1}\). These parameters were applied in 1, 5, and 15 passes to the base substrate, namely USRP-1, USRP-5, and USRP-15. Interestingly, USRP-1 demonstrated some slight grain growth along the surface, contradicting the expected grain refining process of USRP. These findings were correlated to a defect annihilation mechanism from the vibrational frequency of the tool piece combined with below-par thermal conductivity. As the number of passes increased, the gradient microstructure of the specimens was gradually refined with an increasing number of grain boundaries with disorientations under 15\({}^{\circ}\), which were characterized as low angle grain boundaries (LAGB). In relation to the fatigue strength, all samples showed a moderate increase in fatigue life with recorded values of 335 MPa (USRP-1), 338 MPa (USRP-5), and 335 MPa (USPR-15) compared to the base sample values of 295 MPa.