Abstract
This study investigated the effect of Ultrasonic Nanocrystalline Surface Modification (UNSM) on the microstructure, local mechanical properties, surface condition, and tribological behavior of a laser-welded butt joint of AISI 304 austenitic stainless steel. Laser welding was performed using ER308LSi filler wire at approximately 560 W (28% of 2000 W) and a frequency of 3000 Hz. UNSM treatment was subsequently applied using a Designmecha VM200A system under a static load of 10 N, a scanning speed of 1000 mm/min, a frequency of 20 kHz, and an amplitude of 20 μm. The joint was characterized by SEM/EDS, surface profilometry, instrumented indentation, and tribological testing. Laser welding produced a heterogeneous structure consisting of the base metal, heat-affected zone, and a cellular-dendritic fusion zone. After UNSM, the specimen surface exhibited severe plastic deformation and structural refinement. Surface roughness increased in all zones, while microhardness rose from 212 to 411 HV in the base metal, from 194 to 422 HV in the heat-affected zone, and from 244 to 496 HV in the fusion zone. The cross-sectional area of the wear track decreased from 63,241 to 29,648 μm2. These results demonstrate that UNSM effectively strengthens laser-welded AISI 304 joints and significantly improves their wear resistance.