Laser micro-welding of a NiTi shape memory alloy for biomedical applications

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Laser micro-welding of a NiTi shape memory alloy for biomedical applications

 

Author: Chan, Chi-wai
Title: Laser micro-welding of a NiTi shape memory alloy for biomedical applications
Degree: M.Phil.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Laser welding.
Shape memory alloys.
Department: Dept. of Industrial and Systems Engineering
Pages: xvi, 193 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2307159
URI: http://theses.lib.polyu.edu.hk/handle/200/4484
Abstract: The welding of thin foils of shape memory alloys for biomedical applications is a challenging issue due to the complex relationships between the heat transfer mechanisms involved in thin foil welding. These can affect the weld metallurgy, and affect the shape memory (SME) and the pseudoelasticity (PE) behavior of the foil. This project investigated how high quality laser welds in thin foil NiTi shape memory alloy could be obtained and how their SME, PE and corrosion properties in Hanks solution could be optimized by using post-weld-heat-treatment. The Taguchi experimental design approach, for optimizing the key process parameters for the micro-welding of thin shape memory NiTi foils using a 100W CW fibre laser, was applied. Two sets of L27 Taguchi experiments were conducted in order to analyze the effect of laser power, scanning velocity, focus position, gas flow rate and the type of shielding gas, on the bead dimensions of NiTi welds. A preliminary LI 6 Taguchi study was performed on SS316L foils for the purpose of understanding the interrelationships of laser parameters and various properties of commercial biomaterials. Among the process parameters, the interaction between power and scanning velocity produced the strongest effect on the aspect ratio of the NiTi welds. The power was found to be the predominant factor that drives the interaction with other factors to appreciably affect the aspect ratio. The change of shielding environment from argon to helium induced a considerable decrease in the aspect ratio. Exact penetration NiTi welds with satisfactory shape memory effect (SME), satisfactory pseudoelasticy (PE) and good anti-corrosion performance were successfully obtained. The NiTi welds obtained are mainly of a cellular-dendrite structure. A small amount of Ni3Ti was confirmed by x-ray diffraction (XRD) analysis in the NiTi welds. The onset of the transformation temperatures, As and Ms, were shifting to more negative values compared to those of the as-received NiTi in the differential scanning calorimeter (DSC) measurements. Ultimate tensile stress of the welds was comparable to the as-received NiTi, but a small reduction in the pseudoelastic properties was noted. A subsequent heat treatment process was recommended to improve both the shape memory and pseudoelastic properties of the NiTi, after laser welding. The effects of aging on the laser-welded NiTi alloy at a temperature from 573K to 1173K on the SME and PE as well as the corrosion properties were investigated. With the SME, the DSC results showed that the laser-welded NiTi aged at 573K showed coherent thermal transformation behaviours owing to the precipitation of Ni4Ti3 particles. As for the PE, the stress-strain measurements revealed that the laser-welded samples, after aging, still exhibited slightly inferior PE to that of the as-received samples. Both the laser-welded and the as-received NiTi aged at 573K showed the highest pitting potential in the potentiodynamic polarization curves. The aging temperature of 573K was proposed after laser welding on account of the highly coherent SME and PE and improved corrosion resistance. The results of the project can be applied to ensure a reliable weld is obtained for joining thin foils of NiTi in order to achieve optimal properties in SME, PE and corrosion resistance properties.

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