Dissimilar laser welding of NiTi shape memory alloys
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Date
2021-12-20
Authors
Shamsolhodaei, Amirali
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Welding of NiTi shape memory alloys has a great importance since it has a wide application in the medical industry due to the NiTi unique biocompatibility, superelasticity and shape memory effect. Laser microwelding is one of the leading candidates in manufacturing methods for medical devices due to its low heat input, minimum heat affected zone and good reproducibility. Despite all these benefits, some challenges such as formation of hard and brittle intermetallic compounds and selective vaporization, inherent in this technique still exist. Therefore, it is important to study and develop methods and parameters to minimize detrimental effects, such as the mentioned issues.
To have a basic understanding of the effect of laser parameters, similar laser welding of NiTi has been applied and the results show that applying high power resulted in Ni evaporation and changing the transformation temperature, and hence, the formation of the martensite phase within the fusion zone. These outcomes in addition to unfavorable texture inside the fusion zone (FZ) are the primary reasons for deterioration of superelasticity for the high power sample.
Laser offsetting strategy in the dissimilar weld of NiTi to stainless steel and pure Cu was studied to control the intermetallic compound formation and flow characteristics inside the fusion zone. Adopting laser offsetting welding on NiTi/SS joint could increase the mechanical response of the welded samples by suppressing the brittle intermetallics and modifying the microstructure. However, in the case of NiTi/Cu although the positioning of the laser varied the degree of the microstructure homogeneity, the mixing pattern and the presence of NixTiyCuz intermetallic compounds, it did not have any significant effect on the mechanical properties of the welds.
In the case of the NiTi/PtIr dissimilar joint, it was shown that by selecting proper parameters the NiTiPt phase formed in the fusion zone, which has a strong bonding with both base materials, causing superelasticity preservation in NiTi/PtIr joint. The mentioned NiTiPt, contains the P-phase and Ti2(Ni, Pt)3 nano-precipitates, which are the main characteristics of this high temperature shape memory alloy. This resulted in fabrication of NiTiPt high temperature shape memory alloy wire with a significant superelasticity at 250°C. This work could open a door to obtaining a better understanding of laser microwelding of NiTi shape memory alloys and improving its weld quality and functional properties and additionally making use of the laser microwelding potential to fabricate high temperature shape memory alloys by alloying a third element into the NiTi.
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Keywords
NiTi Shape memory alloys, Welding and Joining, Dissimilar microwelding, Laser microalloying, Advanced characterization, Biomaterials