Abstract
Here the aim of this research is annealing the surface of NiTi wire for shape memory alloy, super-elastic wire by solid state laser beam. The laser surface treatment was carried out on the NiTi wire locally with fast, selective, surface heat treatment that enables precisely tune the localized material properties without any precipitation. Both as drawn (hard) and straight annealing NiTi wire were considered for laser annealing with input power 3 W, with precisely focusing the laser beam height 14.3 % of the Z-axis with a spot size of 1 mm. However, straight annealing wire is more interest due to its low temperature shape setting behavior and used by companies for stent materials. The variable parameter such as speed of the laser scanning and tensile stress on the NiTi wire were optimized to observe the effect of laser response on the sample. Superelastic, straight annealed NiTi wires (d: 0.10 mm) were held prestrained at the end of the superelastic plateau (ε: 5 ∼6.5 %) above the superelastic region by a tensile machine ( Mitter: miniature testing rig) at room temperature (RT). Simultaneously, the hardness of the wires along the cross-section was performed by nano-indentation (NI) method. The hardness of the NiTi wire corresponds to phase changes were correlated with NI test. The laser induced NiTi wire shows better fatigue performance with improved 6500 cycles.
Highlights
NiTi based shape memory alloys are used in a wide variety of engineering applications [1] due to their unique thermomechanical functional properties
The hardness measurement was performed on laser treated NiTi wire mounted on the epoxy resin with capillary fixed at the end point by nano indentation method
Surface treatment of as drawn sample from hard to super elastic behavior with laser modification The hard samples of NiTi wire were fixed at Metter tensile machine with 220 MPa applied stress at a fixed position
Summary
NiTi based shape memory alloys are used in a wide variety of engineering applications [1] due to their unique thermomechanical functional properties. According to theoretical prediction the solid state conversion in NiTi alloy martensite (monoclinic, ) daughter phase stable at low temperature derived from austenite ( parent phase (cubic) stable at high temperature towards could deliver millions of mechanical or thermomechanical cycles as the function of strain or stress [3]. The main reason for the obstacles is the surface oxide layer formation during conventional heating method [4] of NiTi alloy for the phase transformation during heat treatment in a furnace at the range of 400 to 500°C for 30 minutes. To improve the heating process, the conventional method shifts towards laser technology that improves the temperature profile avoids the oxide layer formation on the surface of NiTi wire.
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