Nanocrystallization and enhanced surface mechanical properties of commercial pure titanium by electropulsing-assisted ultrasonic surface rolling

In this study, gradient nanocrystallization and enhanced surface mechanical properties in commercial pure titanium, which were induced by an electropulsing-assisted ultrasonic surface rolling process (EP-USRP), were systematically investigated. The results indicated that EP-USRP at an optimum frequency of 500Hz is advantageous over a conventional ultrasonic surface rolling process (USRP) in achieving excellent surface mechanical properties, including a lower friction coefficient and less wear loss (maximum depth of wear scar decreased to two-thirds of that of USRP). Such enhancements may be attributed to a higher surface maximum hardness (308 HV, increased by 46.7% compared to the turning sample), deeper severe plastic deformation layer (480μm, ~0.5 times greater than that of USRP), smaller surface roughness (Ra 0.026μm), and higher compressive residual stresses. There is a balance between electropulsing-induced ductility and ultrasonic impact-induced work hardening. At the optimum conditions, the mobility of immobile dislocations is remarkably improved owing to the thermal and athermal effects of electropulsing as well as the ultrasonic vibration energy from USRP, leading to higher strains and dislocation densities; these induce further dynamic recrystallization in the sub-grains until a new balance is reached.