Research on fabricating gradient nanostructure and enhancing wear resistance in aluminum alloy via two-dimensional ultrasonic surface burnishing

The present study employs a two-dimensional ultrasonic surface burnishing process (2D-USBP) to enhance the surface properties of aluminum alloy. A multi-ball surface burnishing tool, operating under the synergistic effects of ultrasonic vibration and machining pressure, induces a high strain rate of plastic deformation on the aluminum alloy surface. The deformation-induced heat and convective heat transfer from the lubricant achieve a dynamic equilibrium around 90 °C, facilitating quasi-isothermal processing conditions. Microstructural characterization of the machined specimens uses electron backscatter diffraction and transmission electron microscopy. Additionally, a microhardness tester and micro-measurement in-situ mechanical testing system are employed to evaluate the mechanical properties of the surface layer before and after machining. Lubricated wear tests are performed to compare the wear resistance of the original and machined specimens. The test results indicate that the 2D-USBP generates a gradient nanostructure approximately 660 μm thick on the specimen surface, with dislocation density exhibiting a high-low-high distribution pattern along the depth. The surface hardness of the machined specimens reached approximately 120 HV, 1.6 times that of the original specimens, with the hardened layer extending to a depth of approximately 800 μm. The machined specimens demonstrated higher yield strength, fracture strength, and elongation under high strain rates. Wear test results revealed that the wear scar area of the machined specimens is only 50 % of the original specimens, significantly enhancing wear resistance.