Effect of laser shock peening direction on twinning behavior and bending formability in AZ31 Mg alloy

Mg alloys offer exceptional lightweight potential but suffer from intrinsically limited room-temperature formability because of their hexagonal close-packed crystal structure and strong basal texture. This study addresses this limitation through directionally controlled laser shock peening (LSP) to tailor twinning pathways without additional heat treatment, and the strength-ductility trade-off was successfully overcome by optimizing the impact direction. LSP was applied along the normal direction (ND) and rolling direction (RD) of homogenized AZ31 alloy sheets. The lattice stress state governed by the impact direction was found to be the key factor governing twin variant selection. Peening along the RD induced c -axis tensile stress, which predominantly activated extension twins in the interspot region, yielding a twin boundary fraction approximately six times higher than that produced by ND peening. This microstructural distinction led to markedly different bending performances. The RD-peened specimen achieved simultaneous increases of 14.93% in the flexural strength and 13.78% in flexural elongation, a synergy that overcomes the typical strength-ductility trade-off. Digital image correlation and crystallographic analyses identified the synergistic coupling of three mechanisms underpinning the enhanced formability: (i) efficient deformation partitioning between hardened zones and the soft matrix, (ii) additional strain accommodation through the detwinning of pre-introduced extension twins during bending, and (iii) local softening arising from an elevated Schmid factor for basal slip. These results demonstrate that controlling the LSP impact direction alone enables active microstructural design of Mg alloys, providing a practical and effective processing guideline for overcoming the fundamental limitations of room-temperature formability.