Abstract <p>In-situ high-speed (HS) imaging sheds light on the hydrodynamic behavior of the assist gas and melt film interaction during microsecond laser cutting of nickel–titanium alloy (Nitinol). A comprehensive comparison of multiple test conditions isolates reactive oxygen and inert argon assist gas cutting, unveiling notable characteristics of the thermal–mechanical–material relationship. The HS data shows that cutting with oxygen promotes the melt ejection velocity and has continual linear ejections of molten material. Whereas an argon cutting process results in an ~ 85% decrease in melt ejection velocity and elevates conditions conducive to the adhesion of resolidified material by decreasing the resultant Weber’s number of the melt flow by ~ 95% based on theoretical calculations. HS observations uncover periodic combustion cycles that take place during oxygen cutting and frequent vapor plume expansions during argon cutting. The conclusions and observations made with the HS data are paired with advanced analysis of surface morphology and topography, consisting of scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and energy dispersive X-ray spectroscopy (EDS). The SEM and CLSM maps reveal distinctive striation imprints post-cutting. EDS mapping uncovers the presence of surface oxidation, which aids in supporting the observations of local combustion events. Microstructural analysis of the specimens indicates inadequate melt flow dynamics, with inert gas cutting can promote a 2-times increase in the depth of the heat-affected zone (HAZ). Altogether, the results definitively demonstrate the importance of proper melt flow dynamics when laser cutting Nitinol.</p> Graphical Abstract <p></p>