Nd:YAG laser processing of thick NiTi wires to locally alter transformation properties towards achieving multiple memory shape memory alloys
Subject Areas : Mechanical EngineeringAmin Alipour 1 , Mahmoud Kadkhodaei 2 , Ehsan Foroozmehr 3
1 - Department of Mechanical Engineering,Isfahan University of Technology, Isfahan 8415683111, Iran
2 - Isfahan University of Technology
3 - Department of Mechanical Engineering,Isfahan University of Technology, Isfahan 8415683111, Iran
Keywords: Multiple Memory Shape Memory Alloy, Nd:YAG Laser, Post-processing, Thick NiTi Wire,
Abstract :
Every commercial NiTi (Nitinol) Shape Memory Alloy (SMA) has its own transformation temperatures, which may cause limitations in ever-growing demands for the application of these alloys in novel engineering design. Among various methods proposed to achieve multiple functional characteristics, laser processing offers effective solutions in locally controlling the transformation properties of NiTi parts. The current work describes the application of laser technique followed by post processing to locally alter transformation temperatures and impose phase transition for thick NiTi wires. To this end, various laser parameters are applied, and the influences of peak power and pulse width on the functional, microstructural and mechanical properties of laser processed samples are studied. A four-sided laser processing protocol is proposed to process almost the whole cross section of thick Nitinol wires. It is also shown that post-processing heat treatment is required to recover the shape memory properties of as-processed Nitinol specimen. The transformation temperatures of final processed Nitinol wire increase by about 50 °C compared to those of the unprocessed base material.
[1] Khan, M. I., Pequegnat, A., and Zhou, Y. N., Multiple Memory Shape Memory Alloys, Advanced Engineering Materials, Vol. 15, No. 5, 2013, pp. 386-393.
[2] Pequegnat, A., Panton, B., Zhou, Y. N., and Khan, M. I., Local Composition and Microstructure Control for Multiple Pseudoelastic Plateau and Hybrid Self-Biasing Shape Memory Alloys, Materials and Design, Vol. 92, 2016, pp. 802-813.
[3] Miura, F., Mogi, M., and Ohura, Y., Japanese NiTi Alloy Wire: Use of The Direct Electric Resistance Heat Treatment Method, The European Journal of Orthodontics, Vol. 10, No. 1, 1988, pp. 187-191.
[4] Mahmud, A. S., Liu, Y., and Nam, T. H., Design of Functionally Graded NiTi by Heat Treatment, Physica Scripta, T129, 2007, pp. 222.
[5] Sevilla, P., Martorell, F., Libenson, C., Planell, J. A., and Gil, F. J., Laser Welding of NiTi Orthodontic Archwires for Selective Force Application, Journal of Materials Science: Materials in Medicine, Vol. 19, 2008, pp. 525-529.
[6] Kang, S. W., Cho, G. B., Yang, S. Y., Liu, Y., Yang, H., Miyazaki, S., and Nam, T. H., Transformation Temperatures and Shape Memory Characteristics of a Ti–45Ni–5Cu (at%) Alloy Annealed by Joule Heating, Physica Scripta, T139, 2010, pp. 014068.
[7] Tang, C., Huang, W. M., Wang, C. C., and Purnawali, H., The Triple-Shape Memory Effect in NiTi Shape Memory Alloys, Smart Materials and Structures, Vol. 21, No. 8, 2012, pp. 085022.
[8] Shariat, B. S., Liu, Y., and Rio, G., Mathematical Modelling of Pseudoelastic Behaviour of Tapered NiTi Bars, Journal of Alloys and Compounds, Vol. 577, 2013, pp. 76-82.
[9] Shariat, B. S., Liu, Y., and Rio, G., Modelling and Experimental Investigation of Geometrically Graded NiTi Shape Memory Alloys, Smart Materials and Structures, Vol. 22, No. 2, 2013, pp. 025030.
[10] Shariat, B. S., Liu, Y., and Bakhtiari, S., Modelling and Experimental Investigation of Geometrically Graded Shape Memory Alloys with Parallel Design Configuration, Journal of Alloys and Compounds, Vol. 791, 2019, pp. 711-721.
[11] Shariat, B. S., Bakhtiari, S., Yang, H., ansd Liu, Y., Computational and Experimental Analyses of Martensitic Transformation Propagation in Shape Memory Alloys, Journal of Alloys and Compounds, Vol. 806, 2019, pp. 1522-1528.
[12] Shariat, B. S., Bakhtiari, S., Yang, H., and Liu, Y., Controlled Initiation and Propagation of Stress-Induced Martensitic Transformation in Functionally Graded NiTi, Journal of Alloys and Compounds, Vol. 851, 2021, pp. 156103.
[13] Meng, Q., Wu, Z., Bakhtiari, R., Shariat, B. S., Yang, H., Liu, Y., and Nam, T. H., A Unique “Fishtail-Like” Four-Way Shape Memory Effect of Compositionally Graded NiTi, Scripta Materialia, Vol. 127, 2017, pp. 84-87.
[14] Khan, M. I., Pulsed Nd: YAG Laser Processing of Nitinol, Ph.D. Dissertation, Mechanical and Mechatronics Engineering Dept., University of Waterloo, Waterloo, Ontario, Canada, 2011.
[15] Daly, M., Pequegnat, A., Zhou, Y. N., and Khan, M. I., Fabrication of a Novel Laser-Processed NiTi Shape Memory Microgripper with Enhanced Thermomechanical Functionality, Journal of Intelligent Material Systems and Structures, Vol. 24, No. 8, 2013, pp. 984-990.
[16] Panton, B., Zhou, Y. N., and Khan, M. I., A Stabilized, High Stress Self-Biasing Shape Memory Alloy Actuator, Smart Materials and Structures, Vol. 25, No. 9, 2016, pp. 095027.
[17] Zeng, Z., Oliveira, J. P., Ao, S., Zhang, W., Cui, J., Yan, S., and Peng, B., Fabrication and Characterization of a Novel Bionic Manipulator Using a Laser Processed NiTi Shape Memory Alloy, Optics and Laser Technology, Vol. 122, 2020, pp. 105876.
[18] Ge, F., Peng, B., Ke, W., Teshome, F. B., Du, X., and Zeng, Z., Design and Fabrication of Bionic Finger Driven by NiTi Shape Memory Alloys, Proceedings of the Eighth Asia International Symposium on Mechatronics, Springer Nature Singapore, Singapore, 2022, pp. 616-627.
[19] Khan, M. I., Zhou, Y., Effects of Local Phase Conversion on The Tensile Loading of Pulsed Nd: YAG Laser Processed Nitinol, Materials Science and Engineering: A, Vol. 527, No. 23, 2010, pp. 6235-6238.
[20] Michael, A., Pequegnat, A., Wang, J., Zhou, Y. N., and Khan, M. I., Corrosion Performance of Medical Grade NiTi After Laser Processing, Surface and Coatings Technology, Vol. 324, 2017, pp. 478-485.
[21] Mehrpouya, M., Gisario, A., Brotzu, A., and Natali, S., Laser Welding of NiTi Shape Memory Sheets Using a Diode Laser, Optics and Laser Technology, Vol. 108, 2018, pp. 142-149.
[22] Michael, A., Laser Processing and Modelling of Multiple Memory Shape Memory Alloys, Ph.D. Dissertation, Mechanical and Mechatronics Engineering Dept., University of Waterloo, Waterloo, Ontario, Canada, 2018.
[23] Panton, B., Laser Processing, Thermomechanical Processing, and Thermomechanical Fatigue of NiTi Shape Memory Alloys, Ph.D. Dissertation, Mechanical and Mechatronics Engineering Dept., University of Waterloo, Waterloo, Ontario, Canada, 2016.
[24] Pequegnat, A., Novel Laser Based NiTi Shape Memory Alloy Processing Protocol for Medical Device Applications, Ph.D. Dissertation, Mechanical and Mechatronics Engineering Dept., University of Waterloo, Waterloo, Ontario, Canada, 2014.
[25] Shamsolhodaei, A., Zhou, Y. N., and Michael, A., Enhancement of Mechanical and Functional Properties of Welded NiTi by Controlling Nickel Vapourisation, Science and Technology of Welding and Joining, Vol. 24, No. 8, 2019, pp. 706-712.
[26] Shamsolhodaei, A., Razmpoosh, M. H., Maletta, C., Magarò, P., and Zhou, Y. N., A Comprehensive Insight into The Superelasticity Measurement of Laser Welded NiTi Shape Memory Alloys, Materials Letters, Vol. 287, 2021, pp. 129310.
[27] Vâlsan, D. D., Bolocan, V. M., Novac, A., Chilnicean, G. A., and Crăciunescu, C. M., Analysis of Pulsed Laser Spot Effects on NiTi Wires, Solid State Phenomena, Vol. 332, 2022, pp. 59-66.
[28] Ge, F., Zeng, Z., Peng, B., Teshome, F. B., and Chen, L., Study on The Effect of Double-Sided Laser Welding of NiTi Shape Memory Alloys Wire, The International Journal of Advanced Manufacturing Technology, Vol. 120, No. 11-12, 2022, pp. 8201-8209.
[29] Amrollahipour, R., Kadkhodaei, M., Influence of Strain Rate on Stress–Strain Response of Ni–Mn–Ga Ferromagnetic Shape Memory Alloy Single Crystals, Iranian Journal of Science and Technology, Transactions of Mechanical Engineering, Vol. 41, 2017, pp. 265-268.
[30] Jamalimehr, A., Ravanbakhsh, H., Kadkhodaei, M., and Kamrani, M., Investigation of Dog-Bone Geometry for Simple Tensile Test of Pseudoelastic Shape Memory Alloys, Iranian Journal of Science and Technology, Transactions of Mechanical Engineering, Vol. 40, 2016, pp. 337-345.
[31] Brinson, L. C., One-Dimensional Constitutive Behavior of Shape Memory Alloys: Thermomechanical Derivation with Non-Constant Material Functions and Redefined Martensite Internal Variable, Journal of Intelligent Material Systems and Structures, Vol. 4, No. 2, 1993, pp. 229-242.
[32] He, X., Deb Roy, T., and Fuerschbach, P. W., Composition Change of Stainless Steel During Microjoining with Short Laser Pulse, Journal of Applied Physics, Vol. 96, No. 8, 2004, pp. 4547-4555.
[33] Jandaghi, M., Parvin, P., Torkamany, M. J., and Sabbaghzadeh, J., Alloying Element Losses in Pulsed Nd: YAG Laser Welding of Stainless Steel 316, Journal of Physics D: Applied Physics, Vol. 41, No. 23, 2008, pp. 235503.
[34] Sadiq, H., Wong, M. B., Al-Mahaidi, R., and Zhao, X. L., The Effects of Heat Treatment on The Recovery Stresses of Shape Memory Alloys, Smart Materials and Structures, Vol. 19, No. 3, 2010, pp. 035021.
Int. J. Advanced Design and Manufacturing Technology, 2024, Vol. 17, No. 1, pp. 39-49
DOI: 10.30486/ADMT.1403.873806 ISSN: 2252-0406 https:// sanad.iau.ir/journal/admt
Nd:YAG Laser Processing of Thick NiTi Wires to Locally Alter Transformation Properties Towards Achieving Multiple Memory Shape Memory Alloys
Amin Alipour, Mahmoud Kadkhodaei*, Ehsan Foroozmehr
Department of Mechanical Engineering,
Isfahan University of Technology, Isfahan 8415683111, Iran
E-mail: alipoor68@gmail.com, kadkhodaei@iut.ac.ir, eforoozmehr@iut.ac.ir
*Corresponding author
Received: 11 November 2023, Revised: 12 January 2024, Accepted: 14 February 2024
Keywords: Multiple Memory Shape Memory Alloy, Nd:YAG Laser, Post-Processing
Biographical notes: Amin Alipour received his PhD in Mechanical Engineering from Isfahan University of Technology, Isfahan, Iran, in 2022. Mahmoud Kadkhodaei is a Professor of Mechanical Engineering at the Isfahan University of Technology. He received his PhD in Mechanical Engineering from Isfahan University of Technology in 2007. His current research focuses on Smart Materials, Additive Manufacturing, Biomechanics, and Metal Forming. Ehsan Foroozmehr received his PhD in Mechanical Engineering from Southern Methodist University, Dallas, Texas, in 2009. He is currently an Associate Professor at the Department of Mechanical Engineering, Isfahan University of Technology. His current research interest includes Laser Material Processing, Additive Manufacturing and Fiber Optic Sensors.
1 Introduction
The unique responses of Shape Memory Alloys (SMAs), including Shape Memory Effect (SME) and Pseudoelasticity (PE), make them an ideal option to be widely applied in various high tech. industries. SME is the ability to eliminate the residual strains after the loading cycle upon heating whereas, based on the PE effect, a large amount of strains can be spontaneously recovered through a mechanical loading/unloading cycle. Due to better strength, corrosion resistance, ductility, stability of transformation temperatures, and biocompatibility, NiTi is the most well-known and applicable shape memory alloy [1]. The extraordinary features of SME and PE depend on the composition and processing history of Nitinol alloys [1-2]. Since these characteristics are uniform through commercial NiTi elements, they pose only one set of transformation properties leading to the so-called single memory SMAs. However, new engineering demands urge promoting functionality and enhancing design adjustability of applied components. One way to address this issue is embedding more transformation properties by locally controlling the microstructure and/or composition of NiTi-based SMAs. Consequently, various approaches have been so far proposed to locally tune the properties of monolithic Nitinol alloys.
As one of the first attempts, Miura et al. [3] used different Direct Electric Resistance Heat Treatment (DERHT) for each section of an orthodontic wire. Hence, each section experienced a distinct processing history and, consequently, achieved varying transformation properties. Gradient annealing of SMA wires using tube furnace [4], joining of dissimilar SMA wires [5], inducing gradient heat treatment over an SMA wire by Joule Heating [6], establishment of a triple-SME applying R-phase [7], applying geometric gradient [8-12], and compositionally grading NiTi plates via surface diffusion of Ni through the plate thickness [13] can be mentioned as the reported attempts to enhance the functional properties of NiTi SMAs. Khan [14] introduced a laser induced controlled vaporization technique to manage the transformation temperatures of SMAs. Employing the high power density of an Nd:YAG laser, they could impart variations in the alloy’s chemical composition resulting from the preferential vaporization of Nickel. Moreover, they studied the influences of laser parameters (pulse time, peak power, and the number of pulses per spot) on microstructure, transformation temperatures, and mechanical properties of laser-processed monolithic sheets [14]. This technique was utilized to produce various multiple memory NiTi-based devices [15-18]
Although NiTi based SMAs are vastly utilized in various smart structures in the form of wire or ribbon, major investigations on laser processing have been concentrated on sheets [1-2], [19-21]. There are limited works dedicated to study the processing of NiTi in the form of wires. As one of these attempts, Michael [22] applied pulsed Nd:YAG laser to process Nitinol wires. They focused on the superior vaporization of Ni in the course of laser processing and developed a model to predict changes in Ni concentration of the processed samples in terms of peak power and laser pulse duration. Moreover, Panton [23] studied the thermomechanical fatigue properties of NiTi laser welded joints. To this end, a NiTi wire containing a single laser-processed spot was investigated. They also explored the characterization of laser-processed NiTi wires. Based on their analysis results, a thermomechanical treatment was presented to enhance the properties of the products. Additionally, Pequegnat [24] investigated the use of pulsed Nd:YAG laser processing accompanying post-processing to adjust the local SME as well as PE properties of uniform NiTi wires. They utilized this technique to induce various memories in a desired section of a monolithic NiTi linear actuator. Deformation of the product was further characterized and compared with that of a single memory actuator. In another part of their work, microstructure and composition of the processed material were determined to uncover the influences of laser processing accompanied by cold working and heat treatment. Besides laser processing, there are several works regarding the effects of laser welding on the shape memory properties of Nitinol wires [25-28].
All the wires applied in the previous studies [22-24] were very thin with a diameter no bigger than 700 μm, commonly used for medical devices. However, for thicker NiTi wires, the energy required to process the wire as well as laser processing conditions would change. Although only one-sided laser processing was considered and studied in previous works, due to the small sizes of wires, more than one-side processing may be required to reach almost full penetration in rather thick wires. Therefore, distinct investigations are required to study the laser processing of such thick Nitinol wires.
The current research is dedicated to the utilization of Nd:YAG laser processing together with post heat treatment to alter local transformation temperatures and induce phase conversion in thick NiTi wires. To this end, various laser parameters are examined on a relatively thick Nitinol wire, and the thermal, microstructural, and compositional properties of the processed samples are investigated via characterization analyses. The effects of pulse width and peak power are investigated in order to impart changes in the transformation properties of the monolithic NiTi wire and induce phase transition. A four-sided laser processing pattern is presented to fully penetrate into the whole cross-section of the wire. Then, annealing is applied to recover the shape memory property of the as-processed specimen. Finally, a laser-processed NiTi wire is successfully produced with new transformation temperatures so that the product shows SME at room temperature, unlike the as-received base material which exhibits PE properties.
2 Materials and Methods
A 1.35 mm diameter Nitinol wire manufactured by Small Parts; Inc. was employed in this study. The samples were treated by the provider to exhibit pseudoelasticity at room temperature. The nominal chemical composition of the purchased alloy was 49.3 at. % (44.2 wt.%) Ti and 50.7 at. % (55.8 wt.%) Ni. The wires were cut into 60 mm length samples, and only 35 mm of the central length of each sample was laser processed. To eliminate a dark oxide layer developed during the process, the wires were acid pickled in a solution containing HNO3, HF, and H2O with volume percentages of 20%, 7.5%, and 72.5%, respectively, for 40 seconds before the laser processing. Then, any probably remaining contaminants were removed by cleaning the wires with Ethanol and de-ionized water.
A 400 W pulsed Nd:YAG laser system with a wavelength of 1.06 μm, a nominal spot diameter of 1.3 mm, and a square profile was utilized. The laser power was measured by a Gentec-EO pronto-500 power meter. As illustrated in “Fig. 1”, the Nitinol wires were relied on a flat base plate, and two clamps held down their both ends to avoid any movement during laser processing.
Fig. 1 Laser setup.
A 3-axis CNC machine provided controlled movement of the NiTi wires while the laser processing was done. The laser spots were overlapped by 60% to optimize the surface finish in the course of removing a brittle terminal solidification region. The wire was surrounded by a plastic chamber according to “Fig. 2”. This chamber, filled with argon at the onset of laser processing, shielded the wire to minimize oxidation during the process. The flow rate of 10 lit/min for Argon shielding gas was found enough to avoid oxidation.
Fig. 2 Plastic chamber designed to avoid oxidation.
Differential Scanning Calorimetry (DSC) analysis at a controlled heating and cooling rate of 5°C/min was performed using a Mettler Toledo, DSC1 system equipped with a Refrigerated Cooling System (RCS). The chemical composition of the processed specimens was determined using EDAX Octane Elite system equipped with FEI Quanta 450 microscope. Santam universal testing machine model STM 50 equipped with a thermal chamber was employed to evaluate deformations of the as-received and the laser-processed wires. Tensile samples had a gauge length of 35 mm and were loaded at a strain rate of 1.5×10-3 1/s. The Effects of strain rate for a special kind of SMA [29] and gauge geometry in tensile test [30] were investigated previously. Tensile tests were carried out at the ambient temperature (25 °C). In order to investigate the microstructure of the processed specimens, metallographic samples were cross-sectioned, cold mounted in epoxy, and mechanically polished. Prior to mechanical polishing with colloidal alumina suspension, grinding with sequential sand paper of 400 to 1200-fine grit was performed. Finally, to reveal their microstructure, the samples were etched in a 3 mL HF, 14 mL HNO3, and 82 mL H2O solution for about 30 s.
3 Results and Discussion
To investigate the influence of laser processing on the transformation temperatures of the NiTi specimens, first, the unprocessed base Nitinol wire was needed to be characterized. Figure 3a shows the DSC results for the as-received material.
(a)
(b)
Fig. 3 DSC results for: (a): as-received material, and (b): HBM.
Table 1 Critical temperatures of HBM
| Transformation and Peak Temperatures, °C | ||||||
Sample |
|
|
|
|
|
| |
HBM | - 59.4 | - 11 | - 26.2 | 6.3 | - 35.5 | - 4.5 |
| Laser Parameters | ||
Sample | Pulse width, ms | Peak Power, kW | Frequency, 1/s |
LP 1 | 10 | 1 | 10 |
LP 2 | 10 | 1.2 | 10 |
To analyze the influence of laser processing with mentioned parameters on the transformation temperatures, a DSC test was conducted for LP 1 as well as LP 2 samples. The DSC results are illustrated in “Fig. 4”, and the corresponding critical temperatures are summarized in “Table 3”. By comparing the data of “Table 3” and “Table 1”, it can be inferred that the laser processing has not affected the critical temperatures, especially the transformation peak temperatures, of LP 1 and LP 2 compared to those of the reference HBM sample. It has been shown that the composition change is the primary mechanism responsible for the alteration of SMA transformation properties via laser processing [1]. The high energy density of the laser can induce vaporization in SMA elements. Due to the larger vapor pressure of Nickel compared to that of Titanium, more Nickel vaporizes during laser processing resulting in a composition change in the material. Hence, no change in the transformation temperatures of LP 1 and LP 2 can be due to insufficient power provided by these laser parameters which cannot raise the temperature highly enough for element vaporization and composition change.
(a)
(b)
Fig. 4 DSC results for: (a): LP 1, and (b): LP 2.
Table 3 Critical temperatures of LP 1 and LP 2
| Transformation and Peak Temperatures, °C | ||||||
Sample |
|
|
|
|
|
| |
LP 1 | - 59.5 | - 20 | - 26.7 | 14.4 | - 31 | - 4.4 | |
LP 2 | - 50.6 | - 12.2 | - 19 | 11.6 | - 30.6 | - 5.1 |
| Laser Parameters | ||
Sample | Pulse width, ms | Peak Power, kW | Frequency, 1/s |
LP 3 | 7 | 1.5 | 10 |
LP 4 | 8 | 1.5 | 10 |
Relative changes in material composition during laser processing mostly depend on a balance between the dilution of the molten pool and the vaporization flux of alloy elements [32-33]. Since these two issues are strongly associated with laser processing parameters [1], it is essential to study the effects of both pulse duration and peak power, as the key laser parameters, on the resultant transformation temperatures.
To investigate the effect of higher peak power, the amount of peak power was increased, and two new sets of processing conditions as LP 3 and LP 4 were defined according to “Table 4”.
In contrast, the amount of pulse width was decreased slightly to avoid any breakage of the wire while processing. The samples LP 3 and LP 4 were subjected to DSC analysis, and the results are shown in “Fig. 5”.
(a)
(b)
Fig. 5 DSC results for: (a): LP 3, and (b): LP 4.
The critical temperatures are extracted and reported in “Table 5”. As noted, the transformation temperatures of the newly processed wires do not considerably vary compared to the reference temperatures. In depth comparison of transformation temperatures showed that, while and temperatures were almost the same as the reference temperatures, showed a different approach for each of the two processed samples, and showed a slight increase for both cases. As mentioned above, this inconsistency is attributed to the method of deriving the transformation temperatures and depends on how the tangents to the DSC graphs are taken. However, the transformation peak temperatures for HBM, LP 3, and LP 4 are closer than the one observed for the transformation temperatures, especially much closer for values, which presents a difference of less than 2 °C. Consequently, all the transformation temperatures can be considered unchanged compared to the reference ones.
Table 5 Critical temperatures of LP 3 and LP 4
| Transformation and Peak Temperatures, °C | ||||||
Sample |
|
|
|
|
|
| |
LP 3 | - 57.5 | - 13 | - 33.7 | 14.6 | - 29.9 | - 4.7 | |
LP 4 | - 56 | - 11 | - 19.2 | 10.8 | - 33.1 | - 5.9 |
| Transformation and Peak Temperatures, °C | ||||||
Sample |
|
|
|
|
|
| |
LP 5 | - 61 | - 13.3 | - 20.1 | 12.8 | - 34.8 | - 4.5 |
| Transformation and Peak Temperatures, °C | ||||||
Sample |
|
|
|
|
|
| |
HLP 6 | - 3 | 30.5 | 20.9 | 67.4 | 13.2 | 45.5 |