Tribological behavior of hierarchical ridge-pit microtextures on LPBF Ti-6Al-4V surfaces fabricated by longitudinal-torsional ultrasonic assisted milling: Anisotropy and heat treatment responses

Laser powder bed fusion (LPBF) enables near-net-shape fabrication and integrated manufacturing of complex structures. However, the non-equilibrium microstructure and poor surface integrity inherent to LPBF Ti-6Al-4V alloys limit their tribological performance. In this study, longitudinal-torsional ultrasonic assisted milling (LTUAM) was employed to improve the surface finish of LPBF Ti-6Al-4V alloys and fabricate hierarchical ridge-pit microtextured surfaces, with conventional milling (CM) serving as controls. The effects of laser scanning strategy, heat treatment, and lubrication condition on tribological behavior and wear mechanism were systematically investigated. The results demonstrate that LTUAM converts irregular machining marks generated by CM into periodic hierarchical microtextures, leading to improved surface finish and tribological performance. Under dry sliding, the hierarchical microtextures exert synergistic effects by reducing the real contact area, trapping wear debris, regulating frictional heat and strengthening the surface layer, resulting in a 36.56%-48.15% improvement in wear resistance. The wear mechanism on microstructured surfaces is predominantly mild abrasive wear accompanied by plastic deformation, whereas surfaces machined by CM exhibit a mixture of severe abrasive wear and adhesive wear. Under oil lubrication, lubricant retention, debris accommodation, and local hydrodynamic pressure further enhance tribological performance, reducing the coefficient of friction and wear rate by 23.86%-27.56% and 57.50%-65.54%, respectively. The as-built alloy exhibits pronounced tribological anisotropy, with the XOY surface outperforming the XOZ surface. Solution & aging further improve the tribological performance through microstructural homogenization and precipitation strengthening. This work provides fundamental insights into the integrated fabrication of tribological functional surfaces for additive-manufactured Ti-6Al-4V alloys.