Influence of process parameters on surface integrity and topography of nitinol stents manufactured by fiber laser micro-cutting

Abstract

Nitinol alloys are commonly used for the manufacture of self-expanding stents. These materials have super-elasticity and shape memory properties, allowing better navigability and placement of the stent. Aortic valve implants have three manufactured parts to be considered: the support mesh (i.e., stent), a cover, and leaflets. This work explains the laser cutting procedure and the parameter variation for the stent manufacture, as well as the postprocessing techniques of electropolish and shape setting. A preliminary study was conducted to analyze the influence of the process parameters (spot overlap and pulse energy) on the dross, surface roughness, kerf width, and chemical composition. These outcomes have an effect on the stent surface integrity, which is related to microcracks and residual stresses, which negatively affect the stent performance. Our results indicated that dross surface percentage decreased with lower pulse energy (30.82 mJ) and higher spot overlap (83.31%), suggesting a relationship between these parameters. Results showed a reduction in the kerf width using lower pulse energy levels (i.e., 30.82 mJ, 31.49 mJ). Ra and Rz's surface roughness parameters were reduced with the highest pulse energy (32.16 mJ). According to the results, no consistent relationship was found between the spot overlap and the responses of the chemical composition and kerf width. Laser-cut samples presented a recast layer showing an increase in the Oxygen concentration reduction of the Nickel and Titanium, as well as an increase in the microhardness of the affected zone. It was found that these indicators are more prominent in certain zones where the cutting direction did not allow a proper ejection of the molten material, suggesting a relationship between the cutting direction and surface integrity for the proposed setup. The electropolish process achieved proper results to reduce Ra from 2.19 μm to 1.13 μm and from 1.3 μm to 0.91 μm, however, for samples with lower initial roughness it was not possible to reduce the response. For the shape setting treatment, best parameters (550 °C and 12 min) achieved an expansion of the stent with a small recoil of 0.8 mm.