Effect of ultrasonic surface rolling on the surface morphology and hardness of dual-phase LZ91 alloy

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Dual-phase Mg-Li alloys are promising for industrial applications due to their low density and superior plasticity, but their low hardness limits performance under high-stress conditions. This study employed ultrasonic surface rolling processing (USRP) to enhance the surface properties of the dual-phase LZ91 alloy. The effects of USRP on surface morphology, microstructure, and hardness were systematically investigated. Results showed that USRP generated a ∼400 μm gradient surface layer. TEM revealed grain refinement to ∼300 nm in both β-Li and α-Mg phases. Surface hardness increased from 52.7 HV to 82.6 HV (56.7% improvement) under identical loading. These findings demonstrate that USRP effectively strengthens the LZ91 alloy by refining grains and forming gradient structures.