电脉冲辅助车削 TC4 钛合金表面完整性 及其疲劳性能

Titanium alloys have excellent physical and mechanical properties and are widely used in many industries; however, their poor machinability not only reduces the machining efficiency but also the surface integrity, thus significantly decreasing the fatigue life. Based on the electroplastic effect and sliding electric-contact principles, electric pulse-assisted cutting can effectively reduce the workpiece strength and improve the tribological behavior between tool-chip/workpiece interfaces during cutting, thus significantly improving the machinability of metal materials to enhance the surface integrity of the workpiece. However, research on the fatigue performance of titanium alloys based on electric pulse-assisted cutting is insufficient. Considering the high-fatigue requirement of machined titanium-alloy components, this study performed electric pulse-assisted cutting on titanium alloys to investigate their surface integrity after they were subjected to electric pulse-assisted turning. A surface roughness tester, Vickers microhardness tester, scanning electron microscope, and electron backscattered diffraction system were used to characterize the workpiece surface roughness, surface morphology, surface work hardening, surface microstructure, and surface thickness of the plastic-deformation layer. Workpiece surface defects, including bonding, tearing, and tool marks, were significantly eliminated; the Vickers hardness reduced with increasing pulse-current parameters; and severe plastic deformation occurred on the workpiece surface along with significant grain refinement. Compared with the results of conventional dry turning, the surface roughness and work hardening of TC4 titanium alloy subjected to electric pulse-assisted turning decreased by 35.7% and 4.7%, respectively; the thickness of the strengthening layer increased by 2.14 times; and more α phases were changed to the β phase in the workpiece strengthening layer. The fatigue performance of the TC4 titanium alloy subjected to electric pulse-assisted turning was investigated using a fatigue-testing machine and scanning electron microscope. The fatigue life of the workpiece machined via electric pulse-assisted turning reached 528.18 × 10⁴ cycles, whereas that achieved via conventional turning was only 35.35 × 10⁴ cycles, thus signifying an improvement by 14.9 times. The fracture morphology of the fatigue specimen was observed using a scanning electron microscope. Fatigue cracks primarily originated from the machined workpiece surface owing to surface defects and stress concentration, whereas they were transferred from the surface to the subsurface in the machined workpiece, which resulted from the low surface roughness, smooth surface morphology, small surface work hardening, and large thickness of the plastic-deformation layer. In particular, the electroplastic effect accelerated dynamic recrystallization and reduced the dislocation density, thereby better coordinating the strain of the plastic-deformation layer on the machined-workpiece surface. Numerous fatigue striations were observed microscopically in the crack-propagation region. The fatigue striations were primarily formed by repeated sharpening and passivation of the crack tip under cyclic compressive and tensile stresses, and the direction of the fatigue striations was perpendicular to the crack-propagation direction. Each fatigue striation was equivalent to one cycle of load or strain, and the fatigue-striation width was closely related to factor such as the workpiece surface roughness, work-hardening rate, and microstructure. The fine grains, small dislocation density, and large plastic-deformation depth might have reduced the fatigue-striation width, thus hindering fatigue-crack propagation. The fatigue-striation width resulting from electric pulse-assisted turning was less than that yielded by the conventional turning. The fatigue-striation width at 50 ℃-0.4 A/mm²-500 Hz was 0.194 μm, which was 78.7% lower than that yielded by the conventional turning.