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Fabrication and characterization of a novel bionic manipulator using a laser processed NiTi shape memory alloy
Optics and Laser Technology 122 (2020) 105876
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Full length article
Fabrication and characterization of a novel bionic manipulator using a laser processed NiTi shape memory alloy
T
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Zhi Zenga, J.P. Oliveirab, Sansan Aoc, , Wei Zhangc, Jiangmei Cuid, Shuo Yana, Bei Penga a
School of Mechanical and Electrical Engineering, University of Electronic Science and Technology of China, Sichuan 611731, China UNIDEMI, Department of Mechanical and Industrial Engineering, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516 Caparica, Portugal c School of Material Science and Engineering, Tianjin University, Tianjin 300072, China d School of Materials Engineering, Chengdu Technological University, Sichuan 610071, China b
H I GH L IG H T S
manipulator was fabricated by laser processing of a NiTi alloy. • AThebionic transformation behaviour depend on the laser processing parameters. • The phase behavior is based on alloy shape memory effect. • The electrical-thermo-mechanical • bionic manipulator can be helpful for patients with hand paralysis.
A R T I C LE I N FO
A B S T R A C T
Keywords: NiTi Shape memory effect Laser processing Bionic manipulator
The thermo-mechanical properties of NiTi shape memory alloys (SMAs) have sparked significant research efforts seeking to exploit their bionic capabilities. Currently, the performance capabilities of NiTi-based devices have been inhibited by the retention of only one thermo-mechanical response in the as-received material, namely the shape memory effect (SME) or superelasticity (SE), which mainly depend on the transformation temperatures of the base material. In this work, a novel monolithic bionic manipulator was developed using a NiTi SMA by laser processing, which included both the shape memory and superelastic effects in a single Ni-rich monolithic structure. The device actuation and bending were achieved by resistive heating, which activates the SME of different laser processed regions. Each laser processed region has unique phase transformation onset temperatures and thermo-mechanical recovery characteristics thus providing distinct actuation characteristics. Additionally, the functional fatigue of the part was determined for rehabilitation training of patients including the fingers pairing and grabbing modes.
1. Introduction
could be achieved by smart devices based on SMAs covered by rubber, which uses the change in electrical resistance brought by changes in the crystal structure following a phase change. The introduction of metallurgical processing into SMA-based devices provides greater flexibility in terms of design options, allowing direct electrical coupling to control systems without heavy and complex motors [5,6]. Daly et al. designed and manufactured a monolithic micro-gripper using resistance heating [7]. Oliveira et al. reported the existence of SME on laser welded NiTi joints, subjected to bending tests, and correlated this effect with microstructural analysis performed with X-ray diffraction (XRD) [8]. Kohl et al. developed an integrated antagonistic NiTi SMA microgripper by laser micro-machining [9]. With the advent of additive manufacturing complex shape NiTi parts can be
Shape memory alloys (SMAs) are becoming an indispensable part of advanced design and manufacturing systems in this age of miniaturization, light weighting and game changing design concepts [1,2]. Electrically driven structures or robots have a major disadvantage when used in autonomous systems: they rely on motors. Industry has already started to produce bionic fingers based on NiTi SMAs, which are very complex structures and still need to be externally controlled [3].The shape memory effect (SME) of these unique materials is of particular use for flexible robots which are forecasted to be in high demand in the coming years [4]. For example, for patients with hemiplegia (complete paralysis of part of the human body), hand and fingers rehabilitation
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Corresponding author. E-mail addresses: [email protected] (J.P. Oliveira), [email protected] (S. Ao).
https://doi.org/10.1016/j.optlastec.2019.105876 Received 9 March 2019; Received in revised form 10 September 2019; Accepted 27 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 122 (2020) 105876
Z. Zeng, et al.
(LP) zones and mounted in epoxy resin. They were ground with SiC abrasive paper and then polished with diamond paste for microstructural analysis by an optical microscope (BX-51P, Olympus, Japan). A mixed acid solution composed of 14 mL HNO3, 3 mL HF and 82 mL H2O was used for 25–30 s to reveal the microstructure. The NiTi high temperature shape was achieved by heat treatment on a customized fixture for 2 h at 500 °C. For the thermo-mechanical training, first, the laser processed specimen was reverted to the austenite state by heating the specimen to a temperature above the austenite transformation finish temperature (Af) by direct current (DC). Secondly, an external force was applied to bend the specimen, and the bending stress was set at 60% of specimen's maximum tensile strength. Thirdly, liquid nitrogen was released into a dedicated fixture for cooling the specimen to a low temperature below the martensite transformation finish temperature (Mf). Finally, the external force was released and then the whole training step was repeated. In total, 20 cycles of training cycles were carried out to achieve the optimal two-way shape memory effect (TWSME). Normally, there are several rehabilitating methods to help patients with hand disabilities. Among the finger training schemes, the fingers pairing and grabbing modes are the main solution for hand’s accurate action which require the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints bend as 60°, 90° and 45°, 60° respectively [13]. Electrical-thermo-mechanical (bending) testing was performed on a custom setup shown in Fig. 1b, which simulates the fingers rehabilitation training. The prefabricated bending angles were set as 30°, 60° and 90°, so that the laser joints can be used as a bionic manipulator, such as PIP or DIP. The electrical-thermo-mechanical testing was performed as depicted in Fig. 1b. The DC power supply (PXIe-4113, National Instruments, USA), and DAQ system were used to control the motion of the NiTi wires. The applied direct current was set from 0.5 to 2.5 A, and a varying potential of nearly 2 V was used to heat the laser processed regions during actuation. This potential is applied for 5 s so that the temperature stabilizes. At least three samples were tested to determine the recovery angle by Keyence CV-X470A high-resolution charge coupled device (CCD) from top, left and back views. This guarantees that it is possible to observe the complete deformation experienced by the bionic device. Functional fatigue testing was conducted based on the above electrical-thermo-mechanical (bending) tests. After thermo-mechanical training, the 150 A and 170 A processed joints were bent up to 45°, 60° and 90° by electrical heating, then cooled down below Mf by a fan cooler to achieve the near-straight shape to complete one full functional cycle. The cycling tests were applied using a controlled heating/cooling system. The recovery was also measured by a high-resolution CCD camera from top, left and back views. In the fatigue testing, a cross head speed of 0.04 mm/min was applied in a first loading cycle up to a 6% strain followed by an unloading cycle. The same procedure was repeated 50 times for both parent and laser processed specimens. After completion of 50 cycles the specimens were deformed until fracture at a cross head speed of 0.4 mm/min.
obtained provided that appropriate heat treatment is applied to stabilize the mechanical and functional properties of the as-built parts [10,11]. However, the alteration of the physical and functional properties after laser processing are often followed by a deterioration on the mechanical and functional performance of these smart materials. As a results of the solidification conditions, namely the temperature gradient and cooling rate, columnar crystals are preferentially distributed in the fusion zone instead of equiaxed grains. In addition, the thermal cycle experienced by the material can lead to the development of non-uniform residual stresses, which would deteriorate the mechanical and functional properties of laser processed joints. Similar research results were shown in references [4,5,7]. Recently, most of rehabilitation robotics or systems rely on several motors to achieve more flexibility. These structures can be is too heavy and large for patients with hemiplegia. In order to improve the adaptability of rehabilitation training systems, a bionic manipulator based on NiTi shape memory alloy laser welds capable of self-moving and positioning is presented. This novel structure can benefit from weight reduction while allowing a higher degrees of freedom (DOF) based on distinct shape memory effect exhibited by multiple laser processed joints. Furthermore, accurately knowing the electrical-thermo-mechanical properties is essential to support the design and fabrication of these NiTi smart devices. Hence, these properties are studied and discussed in detail.
2. Experimental procedure 0.4 mm diameter near equiatomic NiTi (50.8% at. Ni) wires from Jiangsu Fasten Co., ltd. were used in this investigation. Laser processing was performed by a pulsed Nd:YAG laser welder (Model 400E, Chutian, China) that produces a laser beam with a wavelength of 1064 nm. The spot size was set at 600 µm. A special clamping system was built for gas protection to avoid oxidation. Argon was introduced at a flow rate of 18 l/min, opened 20 s before processing started and closed 15 s after laser processing was finished. It is known the laser current and pulse width can drastically modify the amount of Ni evaporation during laser processing. With lower heat input, preferential Ni evaporation is restricted, while higher heat input can deteriorate the mechanical properties of the alloy due to the formation of intermetallic compounds (IMCs) and micro-cracks [12]. For that reason, previous optimization of the laser processing parameters was performed. The laser current and pulse width were set at 150A, 170A and 3.1 ms, 3.5 ms, respectively, to achieve different shape memory properties and recovery, as depicted in Fig. 1a. Differential scanning calorimetry (DSC) was performed using a Thermal Analysis system (Q2000, TA Instrument, U.S.A) that was equipped with a refrigerated cooling system based on a modified ASTM F2004-05 standard. The modification consisted on the use of a temperature range between −75 °C and 120 °C and a heating/cooling rate of 5 °C min−1. The specimens for metallography were cut from the laser processed
Fig. 1. Laser processed NiTi bionic manipulator protocol (a. NiTi bionic finger diagram by laser processing b. bionic bending tests as self-positioning). LT – Low temperature; HT – High temperature. 2
Optics and Laser Technology 122 (2020) 105876
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Fig. 2. Microstructures of laser processed zones on NiTi joints (a. 150A and 3.1 ms; b. 170A and 3.5 ms).
3. Results and discussion
Table.1 Transformation temperatures (°C) of the base materials and laser processed joints.
3.1. Microstructure of laser processed joints The microstructure of the laser processed region of the bionic joints are shown in Fig. 2. The dendritic grains in the fusion zone (FZ) can be easily distinguished from the equiaxed grain structure of the heat affect zone (HAZ) and the cold-drawn elongated grains of the base material (BM). Grain recrystallization occurred in the HAZ and no significant grain coarsening was observed. In the FZ, grains can be observed solidifying epitaxially from the parent grains at the HAZ-FZ interface. The large reduction in grain boundary area, dislocations and precipitates that limit transformation induced plasticity make these coarse-grained structures less stable than the ideal NiTi BM during mechanical testing [13,14]. Some micro-pores were detected near the FZ boundary of the laser processed region when the laser current and interaction time increases to 170A and 3.5 ms, respectively. This phenomenon is also referred to as “excessive flash” in other works [15], and may introduce undesired defects in the laser processed metal. It is noted that the higher energy input could be the most important effect on the formation of these micro-pores, which causes a decrease in the mechanical properties of the joints.
Materials
As
Af
Ms
Mf
Base material Laser processed joints (150 A) Laser processed joints (170 A)
−3.6 44.1 52.4
11.6 91.7 97.5
−1.5 54.9 62.1
7.2 1.9 17.7
structure. The coarse grain, low dislocation density and lack of Ni-rich precipitates would lead to single stage transformation in the laser processed region [12]. Onset temperatures for both Ms and Af of the laser processed joint obtained with 150 A and 3.5 ms were in the range of 55 °C and 92 °C, respectively, with a common transformational hysteresis varying between 30 and 35 °C. Increasing laser current was seen to impact the transformation temperatures, which increased the Ms and Af temperatures. The transformation temperatures of NiTi SMAs are known to be affected by both thermo-mechanical processing and change of the Ti/Ni ratio during laser welding [16], processing [5] and additive manufacturing [17], both of which can be experienced during laser processing due to the applied thermal cycles and local vaporization, respectively. The B2 to B19′ ratio was also found to vary with the laser energy density and the annealing temperature [18]. The higher welding current, results in more Ni vaporization, further increasing the transformation temperatures of the material. Compared to previously published research, the current results reveal substantial changes on the local transformation temperatures induced by pulsed Nd:YAG laser processing. In particular, the stable room temperature phase was converted from austenite to martensite. Other works on laser processing of NiTi SMAs have shown that the high cooling rates associated with laser based process may be of importance to the stabilization of austenite in the processed material [19]. During the laser/material interaction the high temperature material experienced a resetting of the effects of
3.2. Phase transformation characteristics The phase transformation curves of the BM wires and the laser processed specimens are shown in Fig. 3 with the respective onset temperatures summarized in Table 1. For the base metal, both austenite finish (Af) and martensite start (Ms) temperatures were below room temperature at 11.6 °C and 7.2 °C, respectively. This indicates that the BM phase at room temperature is fully austenitic. DSC analysis of the laser processed joints suggests a modification of the original crystal
Fig. 3. Phase transformation of base materials and laser processed joints (a. Heating b. Cooling). 3
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recovery behavior when the applied current changed, which enables to separate the three stages based on the shape memory effect. The first stage presents a small recovery at low currents. This small recovery can then be compensated by increasing the applied current, so that the material reaches the austenitic domain, thus nearly fully recovering the imposed deformation. This electrical-thermo-mechanical behavior is related to the shape memory effect of the laser processed regions. Once the laser processed NiTi material is in the bent state and the stress has been released, heating to a temperature above As will cause the detwinned martensite to transform into austenite., which will induce a modification of the crystal structure which translates into the full recovery of the original trained shape at the macroscopic scale once the temperature surpasses the Af temperature [21]. The deformed crystal structure remembers its original orientation because of the lattice correspondence [22]. When the temperature is returned to below Mf the austenite structure fully changes back to martensite. Laser processing has been also seen to create local tensile residual stresses in the laser processed regions (fusion and heat affected zones) of the joints [23,24], so the applied stress required for martensitic transformation in the laser processed regions will depend on the residual stress state of each region. In this investigation, for the laser processed bionic joints made with 150 A-3.5 ms, the martensite phase was maintained when the temperature was below 44 °C (since the As temperature of this region is 44 °C) and no shape change occurred. However, when the temperature was increased by increasing the applied current, the shape memory effect was triggered. Since the amount of detwinned martensite in NiTi determines the magnitude of shape memory recovery [25], the martensite transformation pushed the bionic joints to recover more than 90% of the applied deformation due to the detwinned martensite in AsAf temperature range, corresponding to the current range from 1.1A to 2.2A, then the recovery stopped even when the temperature was above Af. Comparing to the low heat input laser processed samples, the bionic joints with 170 A-3.1 ms would recover at higher applied currents (1.4 A), and complete recovery is achieved for an applied current of 2.3 A. Meanwhile, the larger grain size and existence of micropores in the fusion zone of the laser processed regions may further hamper and deteriorate the complete shape memory recovery even when the material is above Af. The laser processed joints presented a residual plastic deformation after the electrical-thermo-mechanical tests. The degradation of the shape memory effect in the laser processed NiTi, is due to the quick buildup of dislocations that occurred in the coarse grained, low dislocation density heat affected and fusion zones [26,27]. This reveals that the martensite plasticity will confine the occurrence of the reverse transformation from the induced martensite phase to the austenite phase. The applied load puts great importance on the functional recovery of the laser processed joint samples. The accumulated irrecoverable strain increases with the applied bending angle. For the maximum applied load of 90°, the laser processed joints had the maximum residual deformation (about 6°) when compared to the lower primary bending load. Martensite plasticity can restrain the martensite transformation, which results in a stabilized response during the bendingrecovery testing at high temperature (above Af) and then severely degrades the superelasticity of the bionic joints. The plastic deformation of the laser processed joints produced with 150 A-3.5 ms is lower than those with 170 A-3.1 ms. It can be concluded that the micro-pores and coarser grain structure of the joint (Fig. 2), resulting from a higher laser current, deteriorates the shape memory recovery. The cold-work and heat treatment schedules used to make modern NiTi base materials makes them more resistant to the plastic deformation that occurs during the actuation of NiTi SMAs [28]. The combination of small grain size, high dislocation density and precipitates that are typical of these microstructures [29] is not found in the laser processed regions owing to the microstructural-induced changes by the laser/material interaction. As a result of these
Fig. 4. Electrical-temperature-mechanical relationship of different laser processed joints (a. Mechanical- electrical current relationship of 150 A joint b. Mechanical- electrical current relationship of 170 A joint c. Temperatureelectrical current relationship). ETR = electrical-thermo-recovery.
previous thermomechanical processing and returned the material to a near fully annealed state. Higher degree of base metal annealing occurred at higher laser current, which resulted in more well-defined transformation peaks [20]. These local modifications in the laser processed zones can affect the functional properties of NiTi SMAs and need to be considered in applications such as the bionic manipulator.
3.3. Electrical-thermo-mechanical tests In order to obtain separate electrical-thermo-mechanical characterization of the bionic joints based on laser processed NiTi SMAs regions, bending-recovery tests were conducted on different joints including 170 A-3.1 ms and 150 A-3.5 ms, which reproduce the finger movement in this investigation. Fig. 4 depicts the recovery for three sets of maximum applied loads, which correspond to 30°, 60° and 90° as bending angle. The bionic laser processed joints exhibited a similar 4
Optics and Laser Technology 122 (2020) 105876
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Fig. 5. The bionic index-finger moving based on laser processed NiTi SMAs joints (a finger-grabbing b. finger-pairing).
microstructural differences, the plastic strain of the laser processed bionic joints was higher than the base material after the ETR (electricalthermo-recovery) used tests in this investigation. Using DSC and thermo-mechanical recovery results as a guideline, a resistance-heating protocol was designed to sequentially activate each of the two processed bionic joints for imitating the finger move and training. Fig. 5 provides photographs of two moving positions achieved by heating the 150 A and 170 A laser processed bionic joints with predetermined applied currents, which simulates the fingers pair and grab trainings. Normally, for fingers pairing, when the current was set as 1.6 A, the index-finger PIP joints (170A) would bend to 57.2° and the DIP joints with 150 A and 3.5 ms could recover about 43.3° as anticipated for the high temperature shape during thermal-mechanical training. Another training mode of critical important for patient is hand grabbing, where the PIP joints changed to 85.6° (near 90°) and MIP could transform to 56.8° under 2.2 A of applied current. The recorded recovery temperatures did not explicitly match with the DSC data because of external loads imposed on the device from fixture and also from the heat sinking effects of the training process [7,30]. According to the trends reported by Kakeshita et al., sudden increases in the bulk resistance of solution-treated NiTi are indicative of a martensite to austenite phase change [31]. While it was expected that the NiTi material would show some increase in resistivity due to heating, the increase in the magnitude of resistance in the bionic manipulator could not be explained by joule heating alone.
Fig. 6. Functional fatigue of laser processed joints.
significant dislocation activity during electrical-thermal-recovery cycling, which decreases fatigue resistance [32,34,35]. 4. Conclusions This is the first investigation to quantify the electrical-thermal-recovery properties of laser processed/welded NiTi shape memory alloys, to be applied as bionic manipulators for finger rehabilitate training. The following was observed:
3.4. Functional fatigue of bionic joints Normally, if the dissipation energy per cycle decreases to a level lower than 4% of that for the first cycle, then ‘functional fatigue’ occurs in welded structures [32]. In this investigation, the manipulator training positions would be accurate enough for high rehabilitation effect, and the functional fatigue was controlled so that no more than 4% deviation of the first recovery angle, according to clinical requirements, would occur. It is concluded that the prefabricated load puts great effect on the functional fatigue life, where the recovery deteriorated as the bending angle increases, as depicted in Fig. 6. For electrical-thermal-recovery tests, when the applied bending is of 90°, the accurate recovery, that is, without functional fatigue, would be no more than 30 times when applied to the 170A and 3.1 ms joints, and if the pre-bending angle was of 30°, it could to train the material for more than 70 times. These results are corroborated by other references, where the higher pre-load would lead to lower fatigue life on NiTi laser welds [33]. The coarser dendritic structure in the laser processed region, presence of columnar crystals, dendritic and pores enabled
1. Several dendritic grains developed in the fusion zone, and the high energy input would result in micro-pores. Meanwhile, the different phase transformation characteristics was achieved by different laser processing parameters, which is the key effect on an electrically driven bionic manipulator training. 2. It can be concluded that the electrical-thermo-mechanical appearance is related to the shape memory effect of the laser processed regions by different applied currents, and the higher pre-load, higher plastic deformation and lower functional fatigue life was observed. 3. A laser processed NiTi bionic manipulator prototype was successfully achieved, simulating the fingers pair and grab training scheduled using optimized laser process parameters of 150 A/3.1 ms and 170 A/3.5 ms.. It is noted that the designed smart device could be helpful for completing the finger rehabilitation training without motors only by using a resistance-heating protocol.
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Further research focusing on improving the functional fatigue of these smart devices is currently in progress.
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Fabrication and characterization of a novel bionic manipulator using a laser processed NiTi shape memory alloy
T
⁎
Zhi Zenga, J.P. Oliveirab, Sansan Aoc, , Wei Zhangc, Jiangmei Cuid, Shuo Yana, Bei Penga a
School of Mechanical and Electrical Engineering, University of Electronic Science and Technology of China, Sichuan 611731, China UNIDEMI, Department of Mechanical and Industrial Engineering, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516 Caparica, Portugal c School of Material Science and Engineering, Tianjin University, Tianjin 300072, China d School of Materials Engineering, Chengdu Technological University, Sichuan 610071, China b
H I GH L IG H T S
manipulator was fabricated by laser processing of a NiTi alloy. • AThebionic transformation behaviour depend on the laser processing parameters. • The phase behavior is based on alloy shape memory effect. • The electrical-thermo-mechanical • bionic manipulator can be helpful for patients with hand paralysis.
A R T I C LE I N FO
A B S T R A C T
Keywords: NiTi Shape memory effect Laser processing Bionic manipulator
The thermo-mechanical properties of NiTi shape memory alloys (SMAs) have sparked significant research efforts seeking to exploit their bionic capabilities. Currently, the performance capabilities of NiTi-based devices have been inhibited by the retention of only one thermo-mechanical response in the as-received material, namely the shape memory effect (SME) or superelasticity (SE), which mainly depend on the transformation temperatures of the base material. In this work, a novel monolithic bionic manipulator was developed using a NiTi SMA by laser processing, which included both the shape memory and superelastic effects in a single Ni-rich monolithic structure. The device actuation and bending were achieved by resistive heating, which activates the SME of different laser processed regions. Each laser processed region has unique phase transformation onset temperatures and thermo-mechanical recovery characteristics thus providing distinct actuation characteristics. Additionally, the functional fatigue of the part was determined for rehabilitation training of patients including the fingers pairing and grabbing modes.
1. Introduction
could be achieved by smart devices based on SMAs covered by rubber, which uses the change in electrical resistance brought by changes in the crystal structure following a phase change. The introduction of metallurgical processing into SMA-based devices provides greater flexibility in terms of design options, allowing direct electrical coupling to control systems without heavy and complex motors [5,6]. Daly et al. designed and manufactured a monolithic micro-gripper using resistance heating [7]. Oliveira et al. reported the existence of SME on laser welded NiTi joints, subjected to bending tests, and correlated this effect with microstructural analysis performed with X-ray diffraction (XRD) [8]. Kohl et al. developed an integrated antagonistic NiTi SMA microgripper by laser micro-machining [9]. With the advent of additive manufacturing complex shape NiTi parts can be
Shape memory alloys (SMAs) are becoming an indispensable part of advanced design and manufacturing systems in this age of miniaturization, light weighting and game changing design concepts [1,2]. Electrically driven structures or robots have a major disadvantage when used in autonomous systems: they rely on motors. Industry has already started to produce bionic fingers based on NiTi SMAs, which are very complex structures and still need to be externally controlled [3].The shape memory effect (SME) of these unique materials is of particular use for flexible robots which are forecasted to be in high demand in the coming years [4]. For example, for patients with hemiplegia (complete paralysis of part of the human body), hand and fingers rehabilitation
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Corresponding author. E-mail addresses: [email protected] (J.P. Oliveira), [email protected] (S. Ao).
https://doi.org/10.1016/j.optlastec.2019.105876 Received 9 March 2019; Received in revised form 10 September 2019; Accepted 27 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 122 (2020) 105876
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(LP) zones and mounted in epoxy resin. They were ground with SiC abrasive paper and then polished with diamond paste for microstructural analysis by an optical microscope (BX-51P, Olympus, Japan). A mixed acid solution composed of 14 mL HNO3, 3 mL HF and 82 mL H2O was used for 25–30 s to reveal the microstructure. The NiTi high temperature shape was achieved by heat treatment on a customized fixture for 2 h at 500 °C. For the thermo-mechanical training, first, the laser processed specimen was reverted to the austenite state by heating the specimen to a temperature above the austenite transformation finish temperature (Af) by direct current (DC). Secondly, an external force was applied to bend the specimen, and the bending stress was set at 60% of specimen's maximum tensile strength. Thirdly, liquid nitrogen was released into a dedicated fixture for cooling the specimen to a low temperature below the martensite transformation finish temperature (Mf). Finally, the external force was released and then the whole training step was repeated. In total, 20 cycles of training cycles were carried out to achieve the optimal two-way shape memory effect (TWSME). Normally, there are several rehabilitating methods to help patients with hand disabilities. Among the finger training schemes, the fingers pairing and grabbing modes are the main solution for hand’s accurate action which require the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints bend as 60°, 90° and 45°, 60° respectively [13]. Electrical-thermo-mechanical (bending) testing was performed on a custom setup shown in Fig. 1b, which simulates the fingers rehabilitation training. The prefabricated bending angles were set as 30°, 60° and 90°, so that the laser joints can be used as a bionic manipulator, such as PIP or DIP. The electrical-thermo-mechanical testing was performed as depicted in Fig. 1b. The DC power supply (PXIe-4113, National Instruments, USA), and DAQ system were used to control the motion of the NiTi wires. The applied direct current was set from 0.5 to 2.5 A, and a varying potential of nearly 2 V was used to heat the laser processed regions during actuation. This potential is applied for 5 s so that the temperature stabilizes. At least three samples were tested to determine the recovery angle by Keyence CV-X470A high-resolution charge coupled device (CCD) from top, left and back views. This guarantees that it is possible to observe the complete deformation experienced by the bionic device. Functional fatigue testing was conducted based on the above electrical-thermo-mechanical (bending) tests. After thermo-mechanical training, the 150 A and 170 A processed joints were bent up to 45°, 60° and 90° by electrical heating, then cooled down below Mf by a fan cooler to achieve the near-straight shape to complete one full functional cycle. The cycling tests were applied using a controlled heating/cooling system. The recovery was also measured by a high-resolution CCD camera from top, left and back views. In the fatigue testing, a cross head speed of 0.04 mm/min was applied in a first loading cycle up to a 6% strain followed by an unloading cycle. The same procedure was repeated 50 times for both parent and laser processed specimens. After completion of 50 cycles the specimens were deformed until fracture at a cross head speed of 0.4 mm/min.
obtained provided that appropriate heat treatment is applied to stabilize the mechanical and functional properties of the as-built parts [10,11]. However, the alteration of the physical and functional properties after laser processing are often followed by a deterioration on the mechanical and functional performance of these smart materials. As a results of the solidification conditions, namely the temperature gradient and cooling rate, columnar crystals are preferentially distributed in the fusion zone instead of equiaxed grains. In addition, the thermal cycle experienced by the material can lead to the development of non-uniform residual stresses, which would deteriorate the mechanical and functional properties of laser processed joints. Similar research results were shown in references [4,5,7]. Recently, most of rehabilitation robotics or systems rely on several motors to achieve more flexibility. These structures can be is too heavy and large for patients with hemiplegia. In order to improve the adaptability of rehabilitation training systems, a bionic manipulator based on NiTi shape memory alloy laser welds capable of self-moving and positioning is presented. This novel structure can benefit from weight reduction while allowing a higher degrees of freedom (DOF) based on distinct shape memory effect exhibited by multiple laser processed joints. Furthermore, accurately knowing the electrical-thermo-mechanical properties is essential to support the design and fabrication of these NiTi smart devices. Hence, these properties are studied and discussed in detail.
2. Experimental procedure 0.4 mm diameter near equiatomic NiTi (50.8% at. Ni) wires from Jiangsu Fasten Co., ltd. were used in this investigation. Laser processing was performed by a pulsed Nd:YAG laser welder (Model 400E, Chutian, China) that produces a laser beam with a wavelength of 1064 nm. The spot size was set at 600 µm. A special clamping system was built for gas protection to avoid oxidation. Argon was introduced at a flow rate of 18 l/min, opened 20 s before processing started and closed 15 s after laser processing was finished. It is known the laser current and pulse width can drastically modify the amount of Ni evaporation during laser processing. With lower heat input, preferential Ni evaporation is restricted, while higher heat input can deteriorate the mechanical properties of the alloy due to the formation of intermetallic compounds (IMCs) and micro-cracks [12]. For that reason, previous optimization of the laser processing parameters was performed. The laser current and pulse width were set at 150A, 170A and 3.1 ms, 3.5 ms, respectively, to achieve different shape memory properties and recovery, as depicted in Fig. 1a. Differential scanning calorimetry (DSC) was performed using a Thermal Analysis system (Q2000, TA Instrument, U.S.A) that was equipped with a refrigerated cooling system based on a modified ASTM F2004-05 standard. The modification consisted on the use of a temperature range between −75 °C and 120 °C and a heating/cooling rate of 5 °C min−1. The specimens for metallography were cut from the laser processed
Fig. 1. Laser processed NiTi bionic manipulator protocol (a. NiTi bionic finger diagram by laser processing b. bionic bending tests as self-positioning). LT – Low temperature; HT – High temperature. 2
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Fig. 2. Microstructures of laser processed zones on NiTi joints (a. 150A and 3.1 ms; b. 170A and 3.5 ms).
3. Results and discussion
Table.1 Transformation temperatures (°C) of the base materials and laser processed joints.
3.1. Microstructure of laser processed joints The microstructure of the laser processed region of the bionic joints are shown in Fig. 2. The dendritic grains in the fusion zone (FZ) can be easily distinguished from the equiaxed grain structure of the heat affect zone (HAZ) and the cold-drawn elongated grains of the base material (BM). Grain recrystallization occurred in the HAZ and no significant grain coarsening was observed. In the FZ, grains can be observed solidifying epitaxially from the parent grains at the HAZ-FZ interface. The large reduction in grain boundary area, dislocations and precipitates that limit transformation induced plasticity make these coarse-grained structures less stable than the ideal NiTi BM during mechanical testing [13,14]. Some micro-pores were detected near the FZ boundary of the laser processed region when the laser current and interaction time increases to 170A and 3.5 ms, respectively. This phenomenon is also referred to as “excessive flash” in other works [15], and may introduce undesired defects in the laser processed metal. It is noted that the higher energy input could be the most important effect on the formation of these micro-pores, which causes a decrease in the mechanical properties of the joints.
Materials
As
Af
Ms
Mf
Base material Laser processed joints (150 A) Laser processed joints (170 A)
−3.6 44.1 52.4
11.6 91.7 97.5
−1.5 54.9 62.1
7.2 1.9 17.7
structure. The coarse grain, low dislocation density and lack of Ni-rich precipitates would lead to single stage transformation in the laser processed region [12]. Onset temperatures for both Ms and Af of the laser processed joint obtained with 150 A and 3.5 ms were in the range of 55 °C and 92 °C, respectively, with a common transformational hysteresis varying between 30 and 35 °C. Increasing laser current was seen to impact the transformation temperatures, which increased the Ms and Af temperatures. The transformation temperatures of NiTi SMAs are known to be affected by both thermo-mechanical processing and change of the Ti/Ni ratio during laser welding [16], processing [5] and additive manufacturing [17], both of which can be experienced during laser processing due to the applied thermal cycles and local vaporization, respectively. The B2 to B19′ ratio was also found to vary with the laser energy density and the annealing temperature [18]. The higher welding current, results in more Ni vaporization, further increasing the transformation temperatures of the material. Compared to previously published research, the current results reveal substantial changes on the local transformation temperatures induced by pulsed Nd:YAG laser processing. In particular, the stable room temperature phase was converted from austenite to martensite. Other works on laser processing of NiTi SMAs have shown that the high cooling rates associated with laser based process may be of importance to the stabilization of austenite in the processed material [19]. During the laser/material interaction the high temperature material experienced a resetting of the effects of
3.2. Phase transformation characteristics The phase transformation curves of the BM wires and the laser processed specimens are shown in Fig. 3 with the respective onset temperatures summarized in Table 1. For the base metal, both austenite finish (Af) and martensite start (Ms) temperatures were below room temperature at 11.6 °C and 7.2 °C, respectively. This indicates that the BM phase at room temperature is fully austenitic. DSC analysis of the laser processed joints suggests a modification of the original crystal
Fig. 3. Phase transformation of base materials and laser processed joints (a. Heating b. Cooling). 3
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recovery behavior when the applied current changed, which enables to separate the three stages based on the shape memory effect. The first stage presents a small recovery at low currents. This small recovery can then be compensated by increasing the applied current, so that the material reaches the austenitic domain, thus nearly fully recovering the imposed deformation. This electrical-thermo-mechanical behavior is related to the shape memory effect of the laser processed regions. Once the laser processed NiTi material is in the bent state and the stress has been released, heating to a temperature above As will cause the detwinned martensite to transform into austenite., which will induce a modification of the crystal structure which translates into the full recovery of the original trained shape at the macroscopic scale once the temperature surpasses the Af temperature [21]. The deformed crystal structure remembers its original orientation because of the lattice correspondence [22]. When the temperature is returned to below Mf the austenite structure fully changes back to martensite. Laser processing has been also seen to create local tensile residual stresses in the laser processed regions (fusion and heat affected zones) of the joints [23,24], so the applied stress required for martensitic transformation in the laser processed regions will depend on the residual stress state of each region. In this investigation, for the laser processed bionic joints made with 150 A-3.5 ms, the martensite phase was maintained when the temperature was below 44 °C (since the As temperature of this region is 44 °C) and no shape change occurred. However, when the temperature was increased by increasing the applied current, the shape memory effect was triggered. Since the amount of detwinned martensite in NiTi determines the magnitude of shape memory recovery [25], the martensite transformation pushed the bionic joints to recover more than 90% of the applied deformation due to the detwinned martensite in AsAf temperature range, corresponding to the current range from 1.1A to 2.2A, then the recovery stopped even when the temperature was above Af. Comparing to the low heat input laser processed samples, the bionic joints with 170 A-3.1 ms would recover at higher applied currents (1.4 A), and complete recovery is achieved for an applied current of 2.3 A. Meanwhile, the larger grain size and existence of micropores in the fusion zone of the laser processed regions may further hamper and deteriorate the complete shape memory recovery even when the material is above Af. The laser processed joints presented a residual plastic deformation after the electrical-thermo-mechanical tests. The degradation of the shape memory effect in the laser processed NiTi, is due to the quick buildup of dislocations that occurred in the coarse grained, low dislocation density heat affected and fusion zones [26,27]. This reveals that the martensite plasticity will confine the occurrence of the reverse transformation from the induced martensite phase to the austenite phase. The applied load puts great importance on the functional recovery of the laser processed joint samples. The accumulated irrecoverable strain increases with the applied bending angle. For the maximum applied load of 90°, the laser processed joints had the maximum residual deformation (about 6°) when compared to the lower primary bending load. Martensite plasticity can restrain the martensite transformation, which results in a stabilized response during the bendingrecovery testing at high temperature (above Af) and then severely degrades the superelasticity of the bionic joints. The plastic deformation of the laser processed joints produced with 150 A-3.5 ms is lower than those with 170 A-3.1 ms. It can be concluded that the micro-pores and coarser grain structure of the joint (Fig. 2), resulting from a higher laser current, deteriorates the shape memory recovery. The cold-work and heat treatment schedules used to make modern NiTi base materials makes them more resistant to the plastic deformation that occurs during the actuation of NiTi SMAs [28]. The combination of small grain size, high dislocation density and precipitates that are typical of these microstructures [29] is not found in the laser processed regions owing to the microstructural-induced changes by the laser/material interaction. As a result of these
Fig. 4. Electrical-temperature-mechanical relationship of different laser processed joints (a. Mechanical- electrical current relationship of 150 A joint b. Mechanical- electrical current relationship of 170 A joint c. Temperatureelectrical current relationship). ETR = electrical-thermo-recovery.
previous thermomechanical processing and returned the material to a near fully annealed state. Higher degree of base metal annealing occurred at higher laser current, which resulted in more well-defined transformation peaks [20]. These local modifications in the laser processed zones can affect the functional properties of NiTi SMAs and need to be considered in applications such as the bionic manipulator.
3.3. Electrical-thermo-mechanical tests In order to obtain separate electrical-thermo-mechanical characterization of the bionic joints based on laser processed NiTi SMAs regions, bending-recovery tests were conducted on different joints including 170 A-3.1 ms and 150 A-3.5 ms, which reproduce the finger movement in this investigation. Fig. 4 depicts the recovery for three sets of maximum applied loads, which correspond to 30°, 60° and 90° as bending angle. The bionic laser processed joints exhibited a similar 4
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Fig. 5. The bionic index-finger moving based on laser processed NiTi SMAs joints (a finger-grabbing b. finger-pairing).
microstructural differences, the plastic strain of the laser processed bionic joints was higher than the base material after the ETR (electricalthermo-recovery) used tests in this investigation. Using DSC and thermo-mechanical recovery results as a guideline, a resistance-heating protocol was designed to sequentially activate each of the two processed bionic joints for imitating the finger move and training. Fig. 5 provides photographs of two moving positions achieved by heating the 150 A and 170 A laser processed bionic joints with predetermined applied currents, which simulates the fingers pair and grab trainings. Normally, for fingers pairing, when the current was set as 1.6 A, the index-finger PIP joints (170A) would bend to 57.2° and the DIP joints with 150 A and 3.5 ms could recover about 43.3° as anticipated for the high temperature shape during thermal-mechanical training. Another training mode of critical important for patient is hand grabbing, where the PIP joints changed to 85.6° (near 90°) and MIP could transform to 56.8° under 2.2 A of applied current. The recorded recovery temperatures did not explicitly match with the DSC data because of external loads imposed on the device from fixture and also from the heat sinking effects of the training process [7,30]. According to the trends reported by Kakeshita et al., sudden increases in the bulk resistance of solution-treated NiTi are indicative of a martensite to austenite phase change [31]. While it was expected that the NiTi material would show some increase in resistivity due to heating, the increase in the magnitude of resistance in the bionic manipulator could not be explained by joule heating alone.
Fig. 6. Functional fatigue of laser processed joints.
significant dislocation activity during electrical-thermal-recovery cycling, which decreases fatigue resistance [32,34,35]. 4. Conclusions This is the first investigation to quantify the electrical-thermal-recovery properties of laser processed/welded NiTi shape memory alloys, to be applied as bionic manipulators for finger rehabilitate training. The following was observed:
3.4. Functional fatigue of bionic joints Normally, if the dissipation energy per cycle decreases to a level lower than 4% of that for the first cycle, then ‘functional fatigue’ occurs in welded structures [32]. In this investigation, the manipulator training positions would be accurate enough for high rehabilitation effect, and the functional fatigue was controlled so that no more than 4% deviation of the first recovery angle, according to clinical requirements, would occur. It is concluded that the prefabricated load puts great effect on the functional fatigue life, where the recovery deteriorated as the bending angle increases, as depicted in Fig. 6. For electrical-thermal-recovery tests, when the applied bending is of 90°, the accurate recovery, that is, without functional fatigue, would be no more than 30 times when applied to the 170A and 3.1 ms joints, and if the pre-bending angle was of 30°, it could to train the material for more than 70 times. These results are corroborated by other references, where the higher pre-load would lead to lower fatigue life on NiTi laser welds [33]. The coarser dendritic structure in the laser processed region, presence of columnar crystals, dendritic and pores enabled
1. Several dendritic grains developed in the fusion zone, and the high energy input would result in micro-pores. Meanwhile, the different phase transformation characteristics was achieved by different laser processing parameters, which is the key effect on an electrically driven bionic manipulator training. 2. It can be concluded that the electrical-thermo-mechanical appearance is related to the shape memory effect of the laser processed regions by different applied currents, and the higher pre-load, higher plastic deformation and lower functional fatigue life was observed. 3. A laser processed NiTi bionic manipulator prototype was successfully achieved, simulating the fingers pair and grab training scheduled using optimized laser process parameters of 150 A/3.1 ms and 170 A/3.5 ms.. It is noted that the designed smart device could be helpful for completing the finger rehabilitation training without motors only by using a resistance-heating protocol.
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Further research focusing on improving the functional fatigue of these smart devices is currently in progress.
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Acknowledgements Zhi Zeng acknowledge National Natural Science Foundation of China (Grant No. 51775091) and Science and Technology Project of Sichuan Province (No. 2018GZ0284 and 19ZDYF1064). Sansan Ao acknowledge the Natural Science Foundation of Tianjin City (No. 18JCQNJC04100), and National Natural Science Foundation of China and Civil Aviation Administration of China (No. U1933129), and JPO acknowledges Fundação para a Ciência e a Tecnologia (FCT-MCTES) for its financial support via the project UID/EMS/00667/2019 (UNIDEMI). References [1] J. Mohd Jani, M. Leary, A. Subic, M.A. Gibson, A review of shape memory alloy research, applications and opportunities, Mater. Des. 56 (2014) 1078–1113. [2] W. Zhang, S.S. Ao, J.P. Oliveira, Z. Zeng, Y.F. Huang, Z. Luo, Microstructural characterization and mechanical behavior of NiTi shape memory alloys ultrasonic joints using Cu interlayer, Materials 11 (10) (2018) 1830. [3] I.K. Hermano, F. Mark, P.B. Stephen, Rehabilitation robotics: pilot trial of a spatial extension for MIT-Manus, Neuro Eng. Rehabil. 1 (2004) 1–15. [4] Z. Zeng, M. Yang, J.P. Oliveira, D. Song, B. Peng, Laser welding of NiTi shape memory alloy wires and tubes for multi-functional design applications, Smart Mater. Struct. 25 (2016) 085001. [5] M.I. Khan, A. Pequegnat, Y.N. Zhou, Multiple memory shape memory alloys, Adv. Eng. Mater. 15 (2013) 386–393. [6] S.M. Seyyed Aghamiri, M. Nili Ahmadabadi, H. Shahmir, F. Naghdi, Sh. Raygan, Study of thermomechanical treatment on mechanical-induced phase transformation of NiTi and TiNiCu wires, J. Mech. Behav. Biomed. Mater. 21 (2013) 32–36. [7] M. Daly, A. Pequegnat, Y.N. Zhou, M.I. Khan, Fabrication of a novel laser-processed NiTi shape memory microgripper with enhanced thermomechanical functionality, J. Intell. Mater. Syst. Struct. 24 (8) (2012) 984–990. [8] J.P. Oliveira, F.M.B. Fernandes, N. Shell, R.M. Miranda, Shape memory effect of laser welded NiTi plates, Funct. Mater. Lett. 6 (2015) 1550069. [9] M. Kohl, E. Just, W. Pfleging, S. Miyazaki, SMA microgripper with integrated antagonism, Sens. Actuat., A 83 (1–3) (2000) 208–213. [10] N.S. Moghaddam, A. Jahadakbar, A. Amerinatanzi, M. Elahinia, Recent advances in laser-based additive manufacturing, Laser-based Additive Manufacturing of Metal Parts: Modeling, Optimization, and Control of Mechanical Properties, CRC Press, Boca Raton, USA, (2017). [11] H. Meier, C. Haberland, J. Frenzel, Structural and functional properties of NiTi shape memory alloys produced by selective laser melting, Innovative Developments in Design and Manufacturing: Advanced Research in Virtual and Rapid Prototyping, CRC Press, Boca Raton, FL, USA, 2011, pp. 291–296. [12] Z. Zeng, B. Panton, J.P. Oliveira, A. Han, Y.N. Zhou, Dissimilar laser welding of NiTi shape memory alloy and copper, Smart Mater. Struct. 24 (2015) 1–8. [13] K. Li, X.D. Zhang, A novel two-stage framework for musculoskeletal dynamic modeling: an application to multifingered hand movement, IEEE Trans. Biomed. Eng. 26 (7) (2009) 1949–1957. [14] B. Panton, J.P. Oliveira, Z. Zeng, Y.N. Zhou, M.I. Khan, Thermomechanical fatigue of post-weld heat treated NiTi shape memory alloy wires, Int. J. Fatigue 92 (1) (2016) 1-7. 9.
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