Laser welding of shape memory NiTi wires for biomedical applications

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Laser welding of shape memory NiTi wires for biomedical applications

 

Author: Chan, Chi Wai
Title: Laser welding of shape memory NiTi wires for biomedical applications
Degree: Ph.D.
Year: 2013
Subject: Laser welding.
Shape memory alloys.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xx, 285 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2615881
URI: http://theses.lib.polyu.edu.hk/handle/200/6983
Abstract: The critical problem for the fabrication of NiTi micro-components by conventional joining methods is the remarkable degradation in the mechanical and functional properties at the welded regions (weld zone (WZ) and heat-affected zone (HAZ)) due to the high heat input and significant amount of thermal induced defects. Laser welding has been one of the promising techniques in the clinical and medical device industry by virtue of its capability to deliver the advantages of high precision with small and localized heat input. However, the laser-welded NiTi parts still have the problem that needs to be addressed in conjunction with the removal of thermally induced defects. Post-weld heat-treatment (PWHT) is always suggested after laser welding because it can relieve the thermal induced defects in the welded regions. It is interesting to investigate whether this positive effect on the mechanical and functional properties originated from the precipitation of secondary particles could bring to the laser-welded NiTi. On the other hand, long-term stability of the NiTi implants with respect to corrosion is a pre-requisite for in vivo applications, and such implants are often working under continuous loading and unloading conditions in the human body. However, studies on the corrosion and stress corrosion cracking (SCC) behaviours of NiTi after laser welding are still limited, though the welded regions are often recognized as zones which are prone to different kinds of corrosion. It is expected that the balance between the corrosion resistances, mechanical and functional properties could be optimized by proper control of PWHT. Moreover, systematic studies on the in vitro cell responses of NiTi after laser welding are lacking from the literature. It is of interest to study whether laser welding affects the biocompatibility of NiTi.
In this research, PWHT between 300 °C to 450 °C was applied on the laser-welded NiTi. The microstructure and the phases present in the welded NiTi wires were studied by scanning-electron microscopy (SEM), transmission-electron microscopy (TEM) and x-ray diffractometry (XRD). The phase transformation temperatures and the cyclic stress-strain behaviours were studied using differential scanning calorimetry (DSC) and cyclic tensile tests. The hardness was studied by the Vickers method and nano-indentation. The corrosion behaviours in Hanks' solution at 37.5 °C were studied by the potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS), while the passive film composition was analysed by the X-ray photoelectron spectroscopy (XPS). The susceptibility to SCC in Hanks' solution at 37.5 °C was studied by the slow strain-rate test (SSRT) at open-circuit potential (OCP) and at different applied anodic potentials. The biocompatibility was studied by examining the in vitro Mesenchymal stem cells (MSCs) responses. A constitutive mathematical model for the super-elastic behaviours of the laser-welded NiTi wires under cyclic tension was also developed. The results in this research show that the mechanical and functional properties as well as the corrosion resistances in Hanks' solution of the laser-welded NiTi could be improved by PWHT at 350 °C because of the precipitation of fine and coherent Ni₄Ti₃ particles. Although the welded NiTi is susceptible to the SCC in Hanks' solution, the good biocompatibility of the welded NiTi has been firstly demonstrated in this research, as evidenced by the success of cell adhesion and cell spreading of MSCs onto the different regions (WZ, HAZ, and base metal (BM)). The simulation of the constitutive model for the laser-welded NiTi on a range of tensile cyclic deformation is consistent with the results of a series of experiments. Thorough understanding of the biocompatibility and SCC behaviours, as well as successful application of PWHT on the laser-welded NiTi with proper improvements in the corrosion resistances, mechanical and functional properties contribute to the reliable and safe applications of NiTi implants.

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