# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 5 Applications and Recent Advances of USRP
The authors reported that after UNSM, a hardened layer of 125 \(\upmu\)m was formed, surface microhardness increased by 34.2%, surface roughness was reduced from 12.1 \(\upmu\)m to 9 \(\upmu\)m, reduced subsurface porosity, corrosion resistance, and wear resistance were improved. The enhancement in corrosion resistance and wear resistance were attributed to the formation of a hardened layer due to USRP. [PERSON] et al. [110] conducted UNSM experiments on CoCrFeMnNi high entropy alloy and studied the effect on mechanical properties and microstructural features. The SPD, due to UNSM, led to the formation of the GNS layer with higher dislocation densities, deformation twins, and shear bands. Furthermore, the presence of high dislocation densities and deformation twins in the SPD layer improved the tensile properties of the USRP'ed specimen. USRP can provide remarkable hardening of the surface due to SPD; however, in the subsequent stages of the USRP, the surface hardened layer prevents further improvement. On the other hand, this surface hardened layer hinders further plastic deformation. This is attributed to the presence of immobile dislocation and enhanced deformation resistance [115]. To improve the deformation ability, manipulate the microstructure, and remarkably enhance surface quality, scholars have used electropulsing techniques with USRP. The conventional method of USRP, integrated with electropulsing, led to the development of a novel electropulsing assisted ultrasonic surface rolling process (EP-USRP) [116; 117]. Figure 18 represents the schematic illustration of the EP-USRP. In this process, a high electropulsing current is applied to the specimen for a short period coupled with USRP. During electropulsing, the flow stress of the material drops, and the plasticity of the specimen increases. In addition, electropulsing enhances the mobility of dislocations due to thermal and athermal effects [118; 119]. The main aim of EP-USRP is to optimize the mechanical properties and microstructural features of the specimen. The electropulsing during EP-USRP causes a reduction in deformation resistance of the specimen, which improves ductility and surface quality. This is attributed to the significant reduction in work hardening and defects by promoting phase transformation, recrystallization, and crack healing [120; 121; 122]. [PERSON] et al. [123] studied the effect of EP-USRP on AISI 304 SS substrate material. The authors reported that the surface undergoes extensive refinement, and the indications of deformation were observed up to the depth of 200 \(\upmu\)m. The deformation was mainly due to the twinning activity. They attributed relatively low stacking fault energy (SFE) of the SS (16 mJ/m\({}^{2}\)) as the cause of twinning dominant deformation. [PERSON] et al. [124] performed EP-USRP on pure Ti and demonstrated significant improvement in mechanical properties compared to USRP. The authors observed lower surface roughness with reduced cracks on the specimen surface. The untreated specimen has a surface roughness of 0.867 \(\upmu\)m, reduced to 0.045 \(\upmu\)m during USRP and further reduced to 0.033 \(\upmu\)m. After EP-USRP, authors observed smooth morphology with reduced defects on the specimen surface. In addition, the authors reported an improvement in the surface microhardness by 32.1% during EP-USRP compared to 19.5% during USRP. The enhancement in surface hardness is attributed to the thermal and athermal effect due to electropulsing, which accelerates the dislocation mobility and diffusion of atoms. Table 2 corresponds to the recent advancements of the USRP technique, which summarizes the UMSM and EP-USRP. [PERSON] et al. [125] studied the mechanical properties and microstructural features of EP-USRP on Ti6 Al4V. The authors reported a significant enhancement in surface microhardness, wear resistance, deformation layer depth, strengthened RCS in the deformation layer, and superior healing of the surface cracks.