Rong-Shi Chen

To address poor room-temperature bending formability of AZ31B magnesium alloy sheets, this study proposes an ultrasonic rolling (UR) + static recrystallization annealing (A) process for surface microstructure tailoring. 1/2-pass UR with varying annealing temperatures successfully fabricated a fine gradient microstructure (surface grain size < 2 μm) on the sheet surface. Specifically, UR-1-pass + 180 °C/1 h annealing formed a surface gradient structure: fine-grained layer (~220 μm thick, grain size < 5 μm), transition layer, and inner coarse-grained substrate. Bending performance tests show that the specimen treated with UR-1-pass + 180 °C annealing achieves a maximum bending angle of over 95°, representing an approximately 15.7% improvement compared to the as-prepared state (82.7°), with a smooth and flat surface after bending. Microstructural analysis indicates that this fine gradient microstructure significantly promotes the activation of multiple slip systems (especially pyramidal slip) during bending, suppresses the formation of surface twins and deformation bands, and shifts the strain concentration zone and crack initiation site from the surface to an inner depth of ~ 400–500 μm. This study elucidates micro-mechanistically that the surface fine gradient microstructure effectively delays crack initiation/propagation via surface slip activation and inward crack initiation shift, providing a new pathway to enhance magnesium alloy sheets’ room-temperature bending formability.