Kinematic boundaries govern load redistribution and surface quality in longitudinal-torsional ultrasonic-assisted helical milling of near-β TB6 titanium alloy

High-quality connection holes are critical to the assembly reliability of aerospace components made of near- β TB6 titanium alloy. However, the high strength, adhesion tendency and elastic recovery of TB6 promote high cutting loads, tool wear and surface–subsurface damage during hole making. Ultrasonic vibration can regulate tool–workpiece contact, but existing studies have not clarified how the intermittent cutting induced by ultrasonic vibration affects the machining process and load transfer, or how parameter-dependent changes in the tool–workpiece separation state influence surface quality. This study aims to identify the kinematic boundary of longitudinal-torsional ultrasonic-assisted helical milling (LTUHM) and determine how it governs hole-wall surface quality. Kinematic and material-removal models are combined with single-factor experiments comparing LTUHM with conventional helical milling (HM) in chip formation, cutting forces, hole-wall quality, subsurface microstructure and tool wear. LTUHM transforms continuous ribbon chips into short curled fragments, reduces cutting forces under conventional parameters and generates regular wave-like hole-wall microtextures. Under high-load conditions, it suppresses roughness deterioration and entry–exit diameter differences, shifts the near-surface microstructure toward a substructure-dominated state and reduces geometrically necessary dislocation (GND) density in the β matrix. Nevertheless, excessive axial feed eliminates the ultrasonic retreat-gap and causes axial-force reversal, whereas under low-load conditions, deterministic textures and micro-peening can produce higher roughness and more severe work hardening than HM. These findings demonstrate that LTUHM benefits exist within a finite kinematic window and provide a mechanistic basis for selecting parameters in low-damage TB6 hole making.