Surface Integrity Enhancement of Stainless Steel and Copper Alloy Golf Putter Faces via Ultrasonic Nanocrystal Surface Modification: Performance Improvement and Restoration and Their Implications for the Circular Economy

This study explores the application of ultrasonic nanocrystal surface modification (UNSM) technology to both new and aged golf putters to improve shot performance. Unlike drivers and irons, putters are used for short-distance shots, making directional precision a critical factor for putting performance. The scientific contribution of this work lies in isolating the micro-tribological mechanisms of nanostructured surfaces under low-energy impact via a robot-assisted testing framework, while the industrial contribution establishes a room-temperature surface remanufacturing paradigm for extending the lifecycle of sports equipment. Here we demonstrate that improving the surface integrity of the putter face using UNSM enhances putting straightness and distance consistency. Empirical testing using a semi-automated swing robot was conducted to evaluate both directional precision and successful hole-ins (based on a standard 108 mm hole cup). For new AISI 304 putters, UNSM treatment reduced the standard deviation (SD) of lateral displacement by 78.7% (from 49.97 to 10.64 mm), and directly boosted the hole-in success rate from 66.7% (20/30 successful trials) to 96.7% (29/30 trials). For the aged copper (Cu) alloy putter, UNSM successfully restored mechanical performance, yielding a 45.04% improvement in shot consistency (standard deviation decreased from 10.79 to 5.93 mm), reducing lateral deviation offset from −7.72 to −5.84 mm, and elevating the hole-in success rate from 84% (21/25 trials) to 96% (24/25 trials). Surface characterization analysis revealed that UNSM reduced surface roughness average (Ra) by 76.04% for AISI 304 and 67.98% for Cu alloy, while increasing surface hardness from 332 to 353 HV (AISI 304) and from 610 to 755 HV (Cu alloy). Life cycle assessment indicates that extending the service life of premium Cu alloy putter via room-temperature UNSM prevents primary metal production and forging, mitigating 1.5 to 6.5 tons of CO2 emissions per ton of remanufactured steel and heavy alloy components, thereby directly contributing to UN sustainable development goals (SDGs 12 and 13) and the circular economy.