# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 4 Effect of USRP on Mechanical and Tribological Properties of Engineering Materials  
The SPD developed due to the synergistic influence of mechanical rolling and high-frequency static loads during USRP directly influences the degree of hardening to the surface [57]. Generally, both the longitudinal and torsional loads generate a stress field that allows for rapid dislocation multiplication [22]. Consequently, the tribological and mechanical properties of USRP'ed materials are greatly improved. To demonstrate this, [PERSON] et al. [53] are among many who have created a link between the mechanical hardness and tensile performance of USRP components. In this work, selective laser melted (SLM) 316L SS was subjected to a four-pass treatment of USRP using a frequency of 28 kHz, the amplitude of 5 \(\upmu\)m, a static load of 1200 N, the scanning velocity of 0.02 ms\({}^{-1}\), and a tip diameter of 14 mm. The variable of study in this work was the differing thicknesses of the SLM component. These thicknesses varied from 2 mm and 4 mm, which can be noted as USRP2 and USRP4. Although the authors have decided to use the nomenclature of ultrasonic severe surface rolling (USSR) in this work, it's important to point out that their process is fundamentally the same as USRP. As expected, TEM images indicated a high-density dislocation build-up within the near-equivaxed grains of the topmost surface of the specimen. After USRP treatment, the strength of the SLM component was noticeably increased because of the combination of cellular restructuring, nanoprecipitate generation, martensite-induced transformation, and increased dislocation density. In fact, for both specimens, the mechanical hardness was increased from 243.9 HV to an astounding 412.4 HV (USRP2) and 400.4 HV (USRP4). The yield strength (YS) and ultimate tensile strength (UTS) also showed a similar trend where they were increased to a maximum value of 740.9 MPa, and 865 MPa (for USRP2) compared to the base values of 508.2 MPa and 658.4 MPa. The impediment of dislocation movement from the high-density dislocations has also contributed to this increase of YS and UTS. However, it should be noted that despite this, the ductility was slightly decreased. Although still relatively ductile, this decrease can most likely be attributed to the worsened surface conditions from processing [70]. Despite this, the strength-ductility performance was vastly improved. Similarly, non-ferrous alloys have also demonstrated identical behaviors when subjected to USRP, as [PERSON] et al. [71] explained in their work. Furthermore, the authors experimented with USRP on Ti6 Al4V and studied their tensile behavior. The authors observed a UTS of 1046 MPa and YS of 857 MPa when they performed the uniaxial tension test. They observed that the UTS and YS values for the untreated specimen are 1018 MPa and 972.7 MPa; however, they also reported that there were no changes for the elongation and area reduction. [PERSON] et al. [72] performed USRP on heat-treated Ti6 Al4V and analyzed the tensile and fatigue properties. The authors reported an increase of 3.6% and 3% of UTS and YS, respectively. In addition to that, they reported a 43% increase in surface hardness. The authors summarized that USRP has limited influence on the tensile properties of heat  
Figure 9: OM images of (**a**) untreated specimen, (**b**) USRP’ed specimen, (**c**) deformation layer (**d**) transition zone. Reprinted with permission from ref. [68]. Copyright 2020 MDPI. treated materials. [PERSON] et al. [40] conducted USRP experiments on the AZ31B sheet. They used an output frequency of 30 kHz, whereas the linear velocity, amplitude, and ball radius were set to 2 m min\({}^{-1}\), 7 \(\upmu\)m, and 7 mm. The variable of investigation in this work pertained to the pressure applied to the sheet, varying from 0, 0.1. 0.15, 0.2, and 0.25 MPa. The pressure, in turn, was related to the amount of rolling at 0, 0.04, 0.05, 0.06, and 0.07 mm being carried out.