Defocus-controlled dual-pulse femtosecond laser shock peening of titanium: Strengthening performance and coloration mechanisms

Femtosecond laser shock peening (fs-LSP) stands out as a precision surface enhancement technique, offering superior advantages over conventional nanosecond laser shock peening (ns-LSP) in microscale processing and thermal damage mitigation. However, traditional fs-LSP treatment of titanium encounters a core trade-off: insufficient shockwave intensity at low energies versus excessive surface roughness (Ra >1.2 μm) at high energies. Here, we propose an innovative dual-pulse coupled defocusing strategy to address the power limitations in conventional fs-LSP processing. Defocusing effectively curbs excessive heat flux, minimizing ablative damage on titanium surfaces. Experiments show optimal performance at defocus distance of 0.8-1.0 mm, yielding surface roughness below Ra 0.8 μm and microhardness enhancement over 210 HV, comparable to conventional LSP outcome. This work systematically study the micro-morphology, microstructure, shockwave dynamics, and residual stress of the hardened layer. The fs-LSP hardening mechanism is mainly oxidation-driven, with an affected depth of ∼50 μm featuring a distinct martensitic transformation at the interface. Notably, the process introduce unique coloration, which closely relate with surface oxidation behavior and microstructure but minimally with laser-induced periodic surface structures. The coloration patterns mirror hardening efficacy, with green-toned surfaces exhibiting superior surface quality and hardness. Our defocusing approach enables fs-LSP to achieve both superior surface quality/hardness and anti-counterfeiting functionality, establishing a new way for titanium surface engineering.