Experimental Study of Back Wall Dross and Surface Roughness in Fiber Laser Microcutting of 316L Miniature Tubes Page: 11
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White zones in Figures 4 and 5 denote the areas where surface roughness (less than 1 m) and back
wall dross (less than 3.5%) are decreased, representing the best configuration of process parameters for
both responses. These zones were defined as the value below 70% of the maximum result in surface
roughness and 50% of the maximum result in back wall dross for both types of tubes. Wandera et al.
reported that the best quality in fiber laser cutting of stainless steel is found using the lowest level
of cutting speed . The results shown in Figure 6a,b are consistent with Wandera et al. Also,
Mahrle et al. described the achievable cutting speed according to the inclination cutting front in the
laser fusion process, which depends on the sheet thickness, absorptivity and the energy required to
melt a volume per unit of time . Their results indicate that a thicker sheet requires control of the
cut front inclination (sheet thickness up to 2.5 mm). Further studies should clarify this phenomena
in thinner wall thickness, such as those used for coronary stents. In a different study, according to
Thawari et al. , the periodic striations on laser cutting affect cut quality attributes like surface
roughness and kerf width. As the pulse overlap factor decreases, the kerf width tends to decrease and
the surface roughness tends to increase. Similarly, in this study, a low value of pulse overlap factor is
correlated with higher surface roughness (see condition e in Figure 5).
Tubes used for surgical applications are manufactured by methods described in ASTM F225 .
The specific manufacturing methods can be classified as cold drawn and annealing processes.
The materials used in this research were certified materials intended for the manufacturing of coronary
stents. It is well known that the fabrication of coronary stents includes several steps such as laser cutting,
electropolishing, passivation and annealing process . The Food and Drug Administration (FDA)
recommends the tracking of the stress history of the coronary stents, including the manufacturing
(fabrication, annealing, electropolishing, heat setting, etc.), crimping, expansion/deployment, stent
recoil and physiologic loading conditions .
In this work, laser cutting was studied with miniature tubes manufactured by annealing and hard
drawing processes. The annealing process is an effective method for softening the stent material ,
which provides high ductility even at high strain amplitudes, compared with the hard drawing process
characterized by a low ductility and high strength. Poncin et al. explained that although a fully
annealed condition is desirable, a cold worked tube is preferred in order to reduce the risk of handling
damage when the tube is cut. Certainly, after laser cutting and electropolishing, an annealing step
is recommended to release the residual stresses during laser cutting and to control the mechanical
properties and microstructure .
According to our results, as reported in Figure 7, the manufacturing process (i.e., annealing vs.
hard drawn) can affect some quality related parameters. Lavvafi H. et al., described the effects of
laser treatment on AISI 316L wires fabricated by the hard drawing and annealed processes .
Their results indicated that surface roughness on annealed tubes is directly related to the laser
power level. In addition, laser processing shows an effect on the bulk mechanical properties of
hard drawn wire (e.g., reduction in ultimate tensile strength (UTS) and micro hardness). From Figure 7,
our surface roughness results are dependent on five process parameters. Therefore further studies
should be performed in regards to the influence of the laser energy on the surface finish and mechanical
performance of the stent. Also, it was observed that the amount of back wall dross tends to increase
in the case of the annealed tubes. This implies that the annealed miniature tubes will require more
work post processing in order to eliminate this residue. For the harder condition derived from hard
drawing, there is also the risk of cracking due to temperature gradients. In this study, there was no
evidence of cracking.
In particular, surface roughness and back wall dross have implications on the coronary stent
quality. Muhammad et al. explored an underwater technique to drag the dross particles inside AISI
316L tubes, reducing the effects of back wall damage and cracks caused by the heat dissipation .
The hydrodynamics of water over the workpiece surface in laser cutting was explained as a thin
and thermally disturbed water layer which prevented the redeposition of the melted material, thus
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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/m1/11/: accessed March 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Engineering.