Reverse-Feed Ultrasonic Burnishing for Interlaced Micro-Texture and Hydrophobic Control of 316 Stainless Steel Pipes

Using the ultrasonic burnishing process to fabricate micro-textures is one of the effective methods to improve the hydrophobic properties of workpiece surfaces. In this study, three ultrasonic burnishing strategies—single-pass ultrasonic burnishing process (SUBP), two-pass ultrasonic burnishing process with reverse feed direction (TUBP-RF), and two-pass ultrasonic burnishing process with forward feed direction (TUBP-FF)—were employed to fabricate micro-textures on 316 stainless steel pipes. The effects of burnishing strategy and feed rate on surface morphology and hydrophobic performance were investigated. TUBP-RF introduces reverse feed in the second pass, generating tangential forces in the opposite direction that induce secondary plastic flow and material accumulation at texture intersections. The results show that surface hydrophobicity first increased and then decreased with increasing feed rate, reaching its maximum at 0.7 mm/r. TUBP-RF achieved the highest contact angle of 108°, representing increases of 18.4% and 12.1% compared with SUBP and TUBP-FF, respectively. Among the three strategies, TUBP-RF produces interlaced micro-textures with larger peak height Rp, medium peak spacing RSm, and reduced effective solid contact area, facilitating air entrapment beneath water droplets and promoting a Cassie–Baxter wetting state. Furthermore, under the optimal parameters of the TUBP-RF process, the machined surface improved droplet sliding speed, reduced the sliding time by 61.7% compared with the original surface. The TUBP-RF strategy effectively enhances surface hydrophobic properties by constructing interlaced micro-textures, offering new insights for optimizing the ultrasonic burnishing process.