Experimental Study of Back Wall Dross and Surface Roughness in Fiber Laser Microcutting of 316L Miniature Tubes Page: 12

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Micromachines 2018, 9, 4

avoiding the formation of debris and a recast layer [35]. In a related study, Demir et al. presented
the influence of submerging AZ31 alloy in three different liquids (water, alcohol and oil) to get a
dross-free cutting. An alcohol-water solution showed the best results in terms of chemical dross
dissolution [36]. In the current study, the surface topography images (Figure 8) showed a reduction of
the dross formations (conditions (a) and (d)) with low levels of pulse energy, avoiding the melting
and thermal effects on surface. However, these surfaces were not free of dross, therefore the use of
the dragging techniques could be of interest in order to reduce the molten deposits. Also, back wall
damage can be related to the tube thicknesses (i.e., thicker material should require higher energy).
However this comparison is not conclusive due to the differences in tube processing.
In related studies, the potential of femtosecond laser machining has been demonstrated in the
case of extremely delicate materials (e.g., polymeric stents and biodegradable stents) [37] and sensitive
materials (shape memory alloys) due to the reduction in thermal effects during laser machining [38].
Biffi et al. performed a comparison between a CW, or long pulsed laser, and a femtosecond laser for
Nitinol with thicknesses of 100 m and 130 m. From their results, the CW laser showed a large amount
of material deposited in the form of drops, while the femtosecond laser samples appeared smother and
the kerf was more precise and regular [39]. However, currently coronary stents from metallic alloys
(e.g., stainless steel and CoCr) are industrially manufactured with lasers under longer pulse.
5. Conclusions and Future Work
This research is focused on assessing the influence of process parameters on average surface
roughness (Ra) and back wall dross during the fiber laser microcutting of miniature tubes,
with potential application in medical implants such as coronary stents. Fiber laser microcutting
experimentation was conducted on 1.8 mm diameter miniature tubes (material: AISI 316L stainless
steel) with two different dimensions of wall thickness and hardness conditions (type A: annealed
with 110 m wall thickness and type B: hard drawn with 160 pm wall thickness). Hard drawn tubes
showed better results in terms of back wall dross when compared to the miniature tubes under the
annealed condition.
A quantitative evaluation of the back-wall dross adherence phenomenon was carried out which,
to our best knowledge is the first of its kind. Through the appropriate combination of process
parameters (i.e., high level of pulse overlapping factor and pulse energy below 32 mJ) it is possible to
achieve less than 1 m in Ra at the edge of the laser-cut tube and less than 3.5% dross deposits at the
back wall of the miniature tube.
In addition, a high degree of interaction is present among the studied process parameters
(i.e., pulse frequency, pulse width, peak power, cutting speed and gas pressure), together with the
central point curvature. Therefore, further analysis is required in order to develop a more robust
statistical model which would reduce the number of process variables and exploring spot overlap and
pulse energy parameters to appropriately predict quality indicators in the fiber laser micro-cutting of
miniature tubes.
Acknowledgments: This research was possible with support from the grant FOMIX NL-2010-C30-145045 and
from Tecnologico de Monterrey, through the research group in Advanced Manufacturing.
Author Contributions: Erika Garcia-Lopez performed the experimental work and statistical analysis.
Alexis G. Medrano-Tellez and Juansethi R. Ibarra-Medina advised in fiber laser machine calibration and operation,
as well as design of experiments for this study. Hector R. Siller specified the metrology technology for this study.
Ciro A. Rodriguez validated the regression models and reviewed the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

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García-López, Erika; Medrano-Tellez, Alexis G.; Ibarra-Medina, Juansethi R.; Siller, Héctor R. & Rodríguez, Ciro A. Experimental Study of Back Wall Dross and Surface Roughness in Fiber Laser Microcutting of 316L Miniature Tubes, article, December 26, 2017; Basel, Switzerland. (https://digital.library.unt.edu/ark:/67531/metadc1062077/m1/12/ocr/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Engineering.

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