Are Modern “Cold” Femto Laser-Cut Components Truly HAZ-Free?

Contemporary laser cutting of Nitinol components is typically performed by either long-pulse continuous wave-modulated fiber lasers (“fiber”) or ultrashort pulse femtosecond lasers (“femto”). The primary advantage of the fiber laser is the faster speed at which parts can be cut, with the tradeoff of the creation of a molten recast layer and heat affected zone (HAZ). Conversely, femto laser cutting is relatively slower and purportedly produces no molten recast or HAZ. The present study challenges these generalizations by quantitatively comparing the microstructure of fiber versus femto laser-cut parts. Specifically, six different tubes were selected for challenge conditions (OD x wall): 1mm x 0.060mm, 1mm x 0.125mm, 1mm x 0.250mm, 5mm x 0.125mm, 5mm x 0.250mm, and 5mm x 0.500mm. The larger 5mm tubing, intended to simulate structural heart scale tubing, has ample inner lumen such that laser power could be quite large and beam expansion quite narrow without creating backwall damage on the opposite surface. The smaller 1mm tubing, intended to simulate neurovascular scale tubing, has minimal inner lumen such that power was quite low and larger beam expansion to prevent backwall damage. Variations in wall thickness were selected to assess the contribution of cut depth on the HAZ formation. Characterization of HAZ was performed in three ways: i) etch-tinted metallurgical cross-sectioning, ii) nano-indentation, and iii) surrogate stent radial expansion strain limit determination to challenge the maximum bending strain limit when surfaces are compromised by HAZ.