Variable direction shear deformation induced strengthening mechanism of Ti-6Al-4V alloy treated by a novel ultrasonic milling-burnishing process

A novel ultrasonic milling-burnishing process (UMBP) is proposed for surface strengthening of Ti-6Al-4V alloy. By controlling rotational and feed speeds, variable direction shear deformation was achieved on the strengthened surface. This work systematically investigated the effects of tool rotational speed and cooling lubrication conditions on strengthening forces, surface integrity, including surface roughness, microstructure evolution, and mechanical behaviors. Under minimum quantity lubrication (MQL) condition, surface roughness was reduced by 24.6 % compared to dry condition. Based on the electron backscatter diffraction (EBSD) test of the strengthened subsurface, an 8 μm thick grain refinement layer formed on the strengthened surface, containing 84.1 % of the grains with grain sizes <1.5 μm. It exhibited high geometrically necessary dislocation (GND) density (5 × 10¹⁵/m²–7 × 10¹⁵/m²), kernel average misorientation (KAM) angles (1°-3°), and grain orientation spread (GOS) values (6–9), confirming severe lattice distortion and plastic deformation. The microhardness increased by a maximum of 28.8 % (about 411.3 HV) within 120 μm depth from strengthened surface. Meanwhile, the nanoindentation hardness increased by a maximum of 36 % (to 6.9 GPa) at top surface. Based on the subsurface microstructure evolution and mechanical behaviors, a combined strengthened mechanism of grain boundary strengthening and dislocation strengthening was reported for UMBP treated Ti-6Al-4V alloy. UMBP provides an effective surface strengthening strategy for mechanical components with complex geometries.