The Effect of Assist Gas Type on Nitinol Microsecond Laser Cut Edges: A Study on the Use of Oxygen, Argon, Nitrogen, Helium, and Compressed Air

Abstract

Historically, the published literature for laser cutting atomically balanced nickel-titanium alloy tubular devices assumes an inert argon assist gas is required for the laser cutting process, resulting in slower cutting rates. Herein, a novel application of an exothermic reactive oxygen assist gas during Nitinol laser micromachining enabled a 38.1 mm/s cut rate, a 4.5-times improvement from other studies, with the realization of improved cut quality: 2-times less dross, 2-times lower surface roughness, and a minimal heat-affected zone. Of the tested assist gases (oxygen, argon, nitrogen, helium, and compressed air), oxygen was found to provide the best cut quality, achieving dross-free cuts. Additionally, oxygen was shown to produce a relatively low arithmetic mean average surface roughness of 0.48 μm, when compared to argon at 0.85 μm. A decrease in surface roughness was found to be associated with an increase in cut rate. These findings suggest that assist gas melt flow dynamics has a higher contributing factor than laser pulse energy parameters. In-situ thermographic monitoring of melt flow during processing demonstrated a clear difference in the melt flow pattern between a reactive oxygen assist gas and inert argon assist gas. Furthermore, with the culmination of nanoindentation analysis and microstructural characterization, it was concluded that long-pulse laser machining can produce cuts with negligible microstructural grain recrystallization of the bulk material. This study quantitatively demonstrates that there are potential benefits observed during laser cutting Nitinol with a reactive oxygen assist gas when compared to previous studies that employ an inert argon assist gas.