Abstract
The surface melting of a NiTi superelastic alloy using a high-power laser Yb:Fiber was investigated. The influence of this process on the microstructural and mechanical properties was also examined. The reference material was a 3 mm nitinol strip with a homogeneous austenitic B2 phase. For the laser surface melting process, input fluences were applied from 17.5 to 45 J/mm2. The morphology of the structure and the chemical composition of several regions were determined by optical microscopy, scanning electron microscopy, dispersive energy spectra, and X-ray diffraction techniques. The mechanical properties, such as modulus of elasticity and hardness, were determined using nanoindentation and microindentation techniques. The greatest surface finishing of the fusion zone was observed for the condition 35 J/mm2. Three well-defined regions (fusion zone (FZ), heat-affected zone (HAZ), base metal (BM)) could be observed and dimensions of grain size, width, and depth of the melted pool were directly affected by the laser fluence. The geometry of the molten pool could be controlled by the optimization of the laser parameters. High laser fluence caused preferential volatilization of nickel, dynamic precipitation of intermetallic phases, including Ti2Ni, Ni3Ti, and Ni4Ti3, as well as solubilization of TiC in the matrix, which led to grain refinement. Thus, high laser fluence is a suitable technique to enhance mechanical properties such as hardness and Young’s modulus.
Highlights
Characteristics of NiTi alloy include shape memory, superelasticity, low modulus of elasticity, high corrosion resistance, and biocompatibility [1]
In biomedicine, which is a big field for the NiTi alloys, shape memory alloys have been applied for orthodontic wires [16], self-expanding stents for cardiovascular applications [22], treatment of patella fracture by claw-like shape memory alloy [23], and more, due to their unique properties
The results indicated that the corrosion resistance of NiTi treated by laser was significantly increased, which was demonstrated by increased passive film-breaking potential and decreased corrosion current
Summary
Characteristics of NiTi alloy include shape memory, superelasticity, low modulus of elasticity, high corrosion resistance, and biocompatibility [1]. According to material requirements for the potential application, the surface of the NiTi alloy can be laser-treated, which can provide improvements, including corrosion resistance, cavitation erosion. The list of treatments with addition of materials include laser cladding [31,32], laser surface alloying [33], and laser welding processes both similar [34] and dissimilar [35,36]. GPa. A high-power laser was used by ZD Cui et al [28] to melt the surface of NiTi shape memory alloy. The present work investigates the effects of high laser fluence surface melting on the structural and mechanical properties of NiTi, which would be of significant interest to those working in the field of laser materials processing
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