Burr formation and influence mechanism analyzing of ultrasonic vibration-assisted micro-milling for 2A21 aluminum U-shaped grooves

Micro-milling is widely applied in aerospace and precision equipment manufacturing due to its excellent material adaptability, capability to machine complex 3D geometries and superior surface quality. Introducing high-frequency ultrasonic vibration further improves machining performance, yet significant micro-burrs still form in U-shaped grooves, degrading surface quality and service performance and inducing tip discharge. This work investigates the burr formation mechanism in ultrasonic-assisted micro-milling of 2A21 aluminum alloy. Tool motion and the cutting region under one-dimensional ultrasonic vibration are analyzed, and material removal behavior is simulated via finite element modeling. A four-factor, three-level Box-Behnken Design experiment is conducted, and a burr thickness prediction model is established to evaluate process parameter effects. Results show that ultrasonic vibration reduces metal tearing and burr generation by modifying tool-workpiece contact modes, with an average prediction error of 10.38%. Within the feasible range, the optimal parameters (feed speed: 100 mm/min, pitch: 0.05 mm, frequency: 20 kHz, amplitude: 8 μm) yield the minimum burr thickness of 245.27 μm.