Ultrasonic-assisted chemical milling: surface roughness characterization and material removal enhancement of Al 6061

Chemical milling (CHM) is widely used for thinning and pocketing components, especially in aerospace applications, but its low material removal rate (MRR) requires high etchant consumption and often leads to poor surface integrity. This study introduces a novel method, direct ultrasonic-assisted chemical milling (UACM), as an efficient and sustainable alternative to CHM. A full-factorial design examined three ultrasonic powers (50, 150, 250 W) across acidic and alkaline etchants at three concentration levels. MRR and roughness parameters (Ra, Rz, Sa) were experimentally quantified and modeled using ANOVA-based regression, while multi-region SEM/EDS elucidated oxide disruption, pit formation, and surface evolution on Al 6061. In 10 % HCl at 250 W, UACM increased MRR by 591 % through cavitation-enhanced agitation and rapid renewal of reactive species, enabling≈85.5 % reduction in acid use. Moreover, at matched MRR, UACM yielded 15-30 % lower Sa than CHM, producing uniform microporosity beneficial for bonding and coating applications. The best acidic condition was 15 % HCl at 250 W, where MRR increased from 0.70 to 1.30 mm/m/s and produced a clean, crack-free, homogeneous surface with optimal integrity. The optimal alkaline condition was 50 % NaOH at 250 W, where MRR increased from 0.08 to 0.23 mm/m/s (≈65.2 % time reduction), and the disruption of hydrogen-bubble shielding generated a uniform and stable morphology. Key experimental challenges included maintaining thermal stability under high acoustic power, controlling bubble adhesion in alkaline media, and achieving a reproducible initial surface. Overall, UACM substantially enhances removal efficiency, improves surface integrity, and reduces chemical consumption, offering a practical pathway toward high-precision and greener manufacturing.