Superelastic shape memory alloys are difficult to machine by thermal processes due to the facility for Ti oxidation and by mechanical processes due to their superelastic behavior. In this study, femtosecond lasers were tested to analyze the potential for machining NiTi since femtosecond lasers allow nonthermal processing of materials by ablation. The effect of processing parameters on machining depth was studied, and material removal rates were computed. Surfaces produced were analyzed under SEM which shows a resolidified thin layer with minimal heat affected zones. However, for high cutting speeds, that is, for short interaction times, this layer was not observed. A depletion of Ni was seen which may be beneficial in biomedical applications since Ni is known to produce human tissue reactions in biophysical environments.
NiTi has been finding increasing applications in industry due to their superior mechanical properties and functional behavior including biocompatibility. As a consequence, NiTi is being applied in parts and microparts as in stents, sensors, and actuators for biomedics and transport industries.
Microfabrication of these alloys includes material removal, as cutting and drilling, where mechanical processes are difficult to use due to strain hardening experienced by NiTi. High pressure abrasive water jet has also been used for rough cutting operations but is not applicable in high precision cutting since the jet does not guarantee precision and tolerances compatible with those required in microparts. Electric discharge machining (EDM) is feasible but with limited flexibility [
Lasers are much more adequate, and Nd/YAG lasers have been successfully used. However, they induce a heat affected zone where oxidation of Ti occurs with depletion of this element and enrichment in Ni [
Basically, material decomposition and removal are a consequence of an energy introduced that is above the solid bonding energy. When a very short laser pulse targets a surface, a solid ablation can occur in two stages: absorption by coupling via multiphoton excitation of electrons in the conduction band and energy distribution into the bulk material. Since this diffusion is minimal, a limited amount of liquid phase is formed, and thus material removal occurs mainly by evaporation [
This study aimed at assessing the effect of processing parameters on the machining depth by computing the material removal rate. The produced surfaces were observed under SEM, and a resolidified thin layer was observed with solidification cracking across the surfaces.
A Ti-sapphire laser system from Coherent Inc. was operated to generate polarized laser pulses with 35 fs duration at a repetition rate of 1 kHz and a peak power of 2.5 W. The laser was emitted at a wavelength of 800 nm and had a Gaussian beam distribution, and the travel speed was set at 0.01, 0.1 and 1 mm/s. Laser pulses were focused by a lens with 13 cm focal length, and a mechanical shutter was used to select the number of pulses. Figure
Experimental set up: laser head and fixing system.
Machined samples were observed using electron scanning microscopy in backscattered and secondary emitted radiation. Compositional changes resulting from processing were determined by EDS in a Jeol scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) from Oxford Instruments model INCAx-sight. Semiquantification microanalysis was made using ZAF correction procedure.
The cross-section of the cuts obtained was analyzed under SEM with EDS and shows that the lower speed led to an ablation A of the total thickness of the sample (1 mm) while irradiations at lower speeds left lower depth marks. Analyzing the geometry of the surface and with a free image analysis software, the volume of material removed was calculated and plotted as a function of the interaction time as shown in Figure
Variation of material removal with the logarithm of the interaction time.
This figure shows a linear relationship between the material removal (MR) and the logarithm of the interaction time (
The penetration depth has a logarithmic decay with the travel speed as depicted in Figure
Penetration depth as a function of cutting speed.
Observing the cut surfaces first under optical microscopy and later under SEM, sharp cuts were seen as shown in Figure
Macrograph of sample machined by femtosecond laser (
Etching the samples cross-sections, it was not possible to identify any grain boundary structure since this is difficult to reveal in hot worked NiTi; however, there was no noticeable difference near the cut surface, suggesting that the very short interaction time prevented diffusion to occur into the bulk material with grain growth or phase transformations, and thus heat affected zone is inexistent.
Figure
SEM macrographs of cuts with different travel speeds. (a)
Observing these samples, a few interesting features can be identified. On the surface of the noncomplete cut, cracked material is seen suggesting a material removal mechanism as described before; that is, the successive laser irradiations coupled with the material induce the formation of cumulative defects that break the material into very small fragments leading to disaggregation. A rippled structure is seen free of redeposits or solidified layers as in the sample cut at lower travel speed. In this case, the energy introduced was well above the threshold for evaporation, and the exceeding heat dissipated into the bulk material, melting a thin layer of the surface. This layer resolidified, and this process is followed by cracking.
Elemental semiquantitative analysis in the base material and in the resolidified zone was performed with EDS. The X-ray spectrums are shown in Figure
SEM view of a cut surface in NiTi with microanalysis. (a) EDS spectrum of bulk material; (b) EDS spectrum in the resolidified layer.
The solidified layer is oxidized, and both Ti and Ni are lost in this oxidation process, with a reduction in Ni.
From this study, it was seen that femtosecond lasers can cut NiTi with a high precision and a minimal heat affected zone, empirical equations are proposed to calculate the material removal and penetration in ablation of NiTi with femtosecond lasers within the experimental range used, for low cutting speeds, the surfaces show a resolidified thin layer with solidification cracks and oxidation. This was not observed for high cutting speeds when the interaction time decreased, A depletion of Ni was seen which is beneficial specially in biomedical applications; though biocompatibility tests were required to confirm this statement.