Laser-assisted ultrasonic surface rolling of titanium alloys: Governing the softening-strengthening transition for fatigue enhancement

This study investigates the dual role of laser heating in laser-assisted ultrasonic surface rolling (LA-USRP) of titanium alloys, revealing a unique softening-strengthening transition governed by thermal-mechanical interactions. Through systematic experiments and microstructural characterization, we demonstrate that optimal laser parameters promote deeper plastic deformation by concurrently activating thermally-assisted dislocation motion and twinning behavior in a synergistic manner - a combined mechanism unachievable through conventional ultrasonic impact treatment. The resulting stabilized compressive residual stresses and refined surface microstructures lead to significant fatigue improvement. However, excessive heating induces competing effects: dislocation recovery causes material weakening while surface oxidation introduces brittle failure sites. The balance between these opposing effects is precisely regulated by a critical thermal processing window, whose proper maintenance enables maximum fatigue performance enhancement through optimal microstructure control. These findings establish fundamental guidelines for implementing laser-assisted surface engineering in high-performance titanium alloy applications.