Ultrashort pulse laser cutting of NiTi in liquids and with pulse bursts

Shape memory alloys, particularly nickel-titanium, are widely used in medical technology for stent fabrication. Laser cutting of NiTi stents requires minimizing heat input and surface contamination to preserve the superelastic material properties and reduce post-processing effort. Ultrashort pulsed laser processing is well suited for this purpose, although a heat-affected zone remains. This study investigates ultrashort pulse laser cutting of NiTi foils in a liquid environment using a custom flow chamber. Compared with processing under argon atmosphere, laser ablation in liquids significantly reduces melt splash and debris adhesion, yielding clean cut edges with laser-induced periodic surface structures as the dominant surface feature. Transmission electron microscopy reveals that the microstructural change is confined to the outermost 250 nm of the kerf flank. Productivity in liquid is limited, however, since neither the repetition rate nor the pulse energy can be raised arbitrarily without bubble interactions or nonlinear effects degrading the result. Pulse bursts were therefore investigated as a means of depositing more energy while keeping the peak intensity of each sub-pulse below the relevant thresholds. MHz burst trains introduce kerf irregularities due to bubble interactions, whereas GHz bursts produce periodic sawtooth-like structures on the kerf flanks with a spatial period of approximately 7 µm, oriented perpendicular to the cutting direction, clearly distinguishing them from classical laser-induced periodic surface structures. In burst-in-burst mode experiments with GHz sub-bursts nested in MHz packages, cutting depth was increased by 40 % compared to single-pulse operation at comparable cut edge quality.