The effects of shape-setting on transformation temperatures of pseudoelastic shape memory alloy springs

Phase transformation temperatures of the produced springs are measured by differential scanning calorimetry (DSC), and the inﬂuences of effective parameters including cold work, heat tre[r]

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Original Article

Farideh Jahanbazi Asla, Mahmoud Kadkhodaeia,*, Fathallah Karimzadehb

aDepartment of Mechanical Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran bDepartment of Materials Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran

a r t i c l e i n f o

Article history: Received 13 April 2019 Received in revised form 21 October 2019 Accepted 24 October 2019 Available online November 2019 Keywords:

Shape memory alloy SMA

Transformation temperature Heat treatment

Shape-setting

a b s t r a c t

Since shape memory alloy (SMA) wires can hardly ever be reliably employed under compressive load-ings, SMA springs are developed as axial actuators with the ability of withstanding both tension and compression In this paper, shape memory alloy helical springs are produced by shape-setting two types of wires: one with shape memory effect (SME) and the other with pseudoelasticity (PE) at the ambient temperature Phase transformation temperatures of the produced springs are measured by differential scanning calorimetry (DSC), and the influences of effective parameters including cold work, heat treatment temperature and duration, and cooling rate are investigated on transformation temperatures of the products The results show that phase transition temperatures of the fabricated springs can be tuned by performing cold work and by adjusting temperature and duration of the conducted heat treatment as well as the subsequent cooling rate It is found that transformation temperatures of the springs fabricated using the SME wire increase as the heat treatment temperature increases However, for samples manufactured using PE wire, transformation temperaturesfirst increase and then decrease with the increase in the heat treatment temperature An increase in the cooling rate leads to a decrease in the austenitefinal temperature (Af), and an increase in the extent of cold work leads to the increase in transformation temperatures especially Af

1 Introduction

Shape memory alloys (SMAs) are a class of smart materials which exhibit two extraordinary behaviors of shape memory effect (SME) and pseudoelasticity (PE) (or superelasticity) owing to thermoelastic martensitic phase transformation between the two phases of martensite and austenite The so-called shape memory

effect is the thermally-driven recovery of the initial conﬁguration of

an SMA when residual strains appear after an inelastic loading/ unloading cycle In PE, recovery occurs spontaneously upon unloading once a large deformation is induced during inelastic

loadings [1] Shape memory alloys with several shapes have been

widely employed in variousﬁelds, such as robotics, biomechanics

and microelectromechanical systems [2] In particular, beside SMA

wires, spring actuators are vastly used owing to their simplicity of

fabrication [3] compared to other shapes such as tubes Moreover,

in contrary to wires, helical springs can be subjected to both tension and compression

Many studies have been so far accomplished to investigate the behaviors of SMA springs both theatrically and experimentally In theoretical modeling of SMA springs, the main goal is to predict

force-displacement response of a helical spring Toi et al [4]

pre-sented aﬁnite element formulation for analysis of superelasticity in

SMA springs using linear Timoshenko beam elements Aguiar et al

[5] proposed a numerical method based on the operator split

technique in this regard Mirzaeifar et al [6,7] studied pure torsion

of SMA bars with circular cross sections to investigate the pseu-doelastic response of SMA helical springs under an axial force

Heidari et al [8] proposed an enhanced one-dimensional

consti-tutive model to describe the shear stressestrain response within

the coils of an SMA spring Their model was based on the von-Mises effective stress and strain, and it was further extended to take large

deformations into account [9]

To fabricate SMA elements with a desirable shape, speciﬁc

thermomechanical treatments called“shape-setting” are required

* Corresponding author

E-mail addresses:fj.eng88@yahoo.com(F Jahanbazi Asl),kadkhodaei@cc.iut.ac ir(M Kadkhodaei),karimzadeh_f@cc.iut.ac.ir(F Karimzadeh)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.10.005

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[10] Shape-setting mainly includes annealing but may be accom-panied by cold work and quenching on the products Although

several microstructural analyses have been carried out [11e16] to

investigate the detailed mechanisms of the various stages in shape-setting, macroscopic studies have gained a great attention in order

to directly observe the inﬂuences of each stage on the phase

tran-sition and other features of an SMA specimen after shape-setting

Wang et al [17] investigated the effect of annealing on the

trans-formation characteristics of TiNi shape memory alloys by differ-ential scanning calorimetry (DSC) Their results showed that the R-phase transformation appeared at low annealing temperatures The R-phase disappeared and austenite directly transformed to

martensite as the annealing temperature exceeded 550C When

the R-phase appends to austenite and martensite, several

ther-momechanical behaviors of an SMA may be affected [18e21,22,23]

Yeung et al [24] found that phase transition temperatures can be

manipulated by adjusting the heat treatment parameters including

duration, temperature and cooling rate Liu et al [25] showed that

the austeniteﬁnal temperature (Af) mostly increases after ageing

Eggeler et al [26] realized that, in general, the transformation

temperatures increase as the aging time increases Sadiq et al [27]

investigated the effect of annealing temperature on the trans-formation temperatures They found that, after annealing the NiTi alloy above the recrystallization temperature, the R-phase disap-pears so that a direct transformation from austenite to martensite takes place The transformation temperatures increase as the annealing temperature increases; however, the R-phase transforms

at lower temperatures Wang et al [28] investigated the

micro-structure, martensitic transformation, shape memory effect and

superelastic property of the Ti49.6Ni45.1Cu5Cr0.3 alloy They

concluded that, in the course of elevating the annealing tempera-ture, the transformation temperatures increased for annealing

temperatures in the range of 623 Ke873 K Then, they decreased for

the temperature range of 873 Ke1023 K and did not considerably

vary after 1023 K until 1273 K Heidari et al [8] fabricated SMA

helical springs by shape-setting the NiTi wires and evaluated the thermomechanical characteristics of their products They found that transformation temperatures of the fabricated springs increase as the annealing temperature increases; however, the start and ﬁnal temperatures of the phase transition decrease so that the R-phase eventually disappears at the annealing temperature of

600C

Motemani et al [29] studied the effect of cooling rate on the

phase transformation and mechanical properties of Ni-rich NiTi

shape memory alloy They realized that the austeniteﬁnal

tem-perature Af increases as the cooling rate decreases while the

martensite ﬁnal temperature Mf is reduced Consequently, a

furnace-cooled sample has the highest phase transformation

hys-teresis (AfeMf) compared to samples with lower cooling rates

Mitwally and Farag [30] studied the effect of cold work on the

superelasticity, the shape memory effect and the hardness of NiTi alloy They found that the shape memory recovery is diminished by

cold rolling as well as bending Lin and Wu [31] observed that, by

augmenting the extent of applied cold work, the products would

show lower residual strains upon unloading Grossmann et al [32]

proposed a procedure to fabricate an SMA spring and to charac-terize its microstructural evolution during the production Costanza

et al [33] investigated the shape memory effect in NieTi springs

and presented a technique to design a linear actuator made of

SMAs Follador et al [3] fabricated an SMA spring actuator They

further described the mechanical characteristics of the SMA spring by a simple linear-elastic model whose parameters depend on the crystalline characteristics

In most of the available works on fabrication of SMA springs, the products are martensitic at the ambient temperature Moreover,

the inﬂuences of all shape-setting parameters on transformation

temperatures of the fabricated springs are not thoroughly investi-gated Since pseudoelastic SMA springs at the ambient tempera-tures are vastly required, in this paper, the main purpose is to fabricate austenitic springs using shape memory alloy wires Two types of NiTi (Ti-55.87 at % Ni) wires (one of which is martensitic at ambient temperature and the other is austenitic) are utilized The wires are wound and locked on a screw and are then heat treated at

temperatures within the range of 300e1000C for 5e1260 min.

The effects of various production parameters such as cold work, heat treatment temperature and duration, and the subsequent cooling rate on transformation temperatures of the fabricated springs are further investigated

2 Materials and methods

The present investigation is carried out on commercial SMA wires provided by Memry Co with the nominal composition of Ti-55.87% Ni and the diameter of 1.5 mm Two types of wires, one with shape memory effect (SME) and the other with pseudoelasticity

(PE) at the ambient temperature, were utilized Fig 1andFig

illustrate results of differential scanning calorimetry (DSC), and

Table 1shows the transformation temperatures of these wires To

fabricate springs using SME and PE wires, according toFig 3, each

wire wasﬁrst wound and locked on a screw Once the temperature

of the furnace has reached a desired number, the specimen was placed in the furnace for the required time interval Then, the specimen was cooled down To prevent oxidation of the specimens, the whole wound wire together with the fastening components were covered by a stainless steel foil Heat treatments were per-formed on the fabricated springs at different conditions so that

various combinations of the inﬂuential parameters were examined

For instance, the springs S-11, S-12 and S-13 were made using SME

wire by heat treating at 750C for 10 min; the samples 11 and

S-13 were quenched in a water-ice bath, but S-12 was quenched in a dry-ice bath Cold work was performed on S-13, but specimens S-11 and S-12 were fabricated with no cold work Sample S-14 indicates

the SME wire which was heat treated at 1000C for 10 and

ﬁnally was quenched in the water-ice bath For fabricating the

spring S-15, sample S-14 wasﬁrst wound and locked on the screw;

then, it was heat treated at 1000 C for 10 Eventually,

quenching in the water-ice bath was performed This procedure of heat treatment followed by quenching was repeated to fabricate spring S-16 from S-15 A summary of heat treatment temperatures,

duration, and cooling method is given in Appendix DSC tests

were used to determine the transformation temperatures of the

products DSC samples were cut into 30e50 mg in mass, and the

tests were performed at the heating/cooling rate of C/min

between50C andỵ150C.

To induce a thermal shock in order to reduce the transformation temperatures of the samples S-13 and S-16, heat treatment without using stainless steel foil was carried out in three stages as follows: Heat treatment for the initial wire at the required temperature

followed by quenching in the water-ice bath;

2 Wounding the heat-treated wire on the screw followed by quenching in the water-ice bath;

3 Heat treatment of the fabricated spring followed by quenching in the water-ice bath

Additionally, cold work was carried out on some of the samples in order to obtain desirable transformation temperatures To this end, cold rolling was used to reduce the thickness by the amounts

of 3.5% and 32.1% A wide range of temperatures from 300C to

1000 C was chosen for the shape-setting; however, in order to

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Fig Transformation temperatures for the SME wire

Fig Transformation temperatures for the PE wire

Table

Transformation temperatures of the utilized wires

Initial wire Af(C) As(C) Rs(C) Rf(C) Ms(C) Mf(C)

SME wire 64.00 43.63 50.00 28.00 8.00 26.00

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prevent oxidation of the samples, heat treatment at lower tem-peratures is more preferred

3 Results and discussion

In this section, the effects of temperature and duration of the heat treatments, cooling rate, and cold work on the transformation tem-peratures of the products are investigated Transformation

tempera-tures of the products are given inAppendix Since the utilized

instrument was limited to the minimum temperature of 50C,

determination of Mfand Msfor some specimens was not possible.Fig

shows one of these cases for the sample S-09 R-phase transition

oc-curs for most of the specimens (for instance, inFig 5where the DSC

curve of sample S-13 is shown) while it is not seen for some specimens

(for instance, in sample S-24 whose DSC curve is shown inFig 6) The

samples S-18, S-19, S-20, S-21, S-22 and 23 were pseudoelastic springs at the room temperature while the other ones showed shape memory

effect.Fig 7illustrates one of the fabricated pseudoelastic springs from

two different views As shown inAppendix 2, the austenitic

trans-formation temperatures of the specimens S-18, S-19, S-20 and S-22 are lower than the ambient temperature It should be noted that sample S-21 is similar to S-22 except the difference that sample S-S-21 was placed into a quartz capsule under vacuum to prevent from oxidation as much

as possible The specimen S-23 was heat treated toﬁnd the lowest

applicable temperature for achieving a pseudoelastic spring

Accord-ingly, 630C was the minimum temperature to optimize the process of

fabricating a pseudoelastic spring with the PE wire Samples S-21 and S-23 were mechanically shown to be pseudoelastic: they were examined under inelastic stretch followed by inelastic compression

and could return to their original conﬁguration after unloading The

samples S-24, S-25 and S-26 are 32.1% cold-worked SME wires As formerly indicated, in some specimens, the reverse trans-formation (during the heating cycle) occurs in two stages: from martensite to an intermediate R-phase and then to austenite Similarly, for some samples, the forward transformation (during the cooling cycle) occurs in two stages: from austenite (parent

phase) to R-phase and then to martensite Table 2shows

trans-formation temperatures of R-phase during the heating cycle for such specimens

Since pseudoelastic springs are planned to be manufactured in

the present work, Af of the products should be lower than the

ambient temperature Therefore, the effects of the various param-eters on the austenite transformation temperatures are investi-gated in the following subsections

3.1 Inﬂuence of heat treatment on the transformation temperatures

Heat treatment parameters including temperature, cooling rate, and the duration affect transformation temperatures of the

fabri-cated springs Here, theﬁndings of this work are presented

3.1.1 Effect of the heat treatment temperature

Fig 8shows the inﬂuence of the heat treatment temperature on the austenite transformation temperatures of the springs fabricated by using the SME wire It is seen that transformation temperatures increase with the increment in the heat treatment temperature Moreover, for the manufactured spring at the

Fig Constrained SMA wire on a screw

Fig Transformation temperatures for sample S-09

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minimum temperature of 750C, Asand Afare higher than the

ambient temperature The rapid growth of Afwith the increase in

the heat treatment temperature indicates that the fabrication of a pseudoelastic spring with an SME wire is impossible at these conditions

Variationsof the transformation temperatures for thesprings made

from PE wire with the heat treatment temperature are shown inFig

It is observed that the transformation temperatures strongly depend

on the heat treatment temperature According to thisﬁgure, there are

three trends for the variations in the transformation temperatures: Transformation temperatures are enhanced by elevating the

heat treatment temperature until 500C.

2 Transformation temperatures, except Ms, are reduced by

increasing the heat treatment temperature between 550C to

650C

Fig Transformation temperatures for sample S-13

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3 Transformation temperatures except Rsand Rfare nearly

con-stant with the increase in the heat treatment temperature

beyond 650C until 750C In fact, with further increasing the

heat treatment temperature, transformation temperatures remain fairly constant This is why heat treatment at

tempera-tures above 650C results in pseudoelastic products

3.1.2 Effect of cooling rate

According to the data provided inAppendix 2, the comparison

between the transformation temperatures of specimens S-19 and S-22 and between those of S-10 to S-12 indicates that trans-formation temperatures of the products mostly decrease when quenching in the water-ice bath is done instead of cooling in the furnace for both series of springs made using PE and SME wires

However, by comparing the samples S-11 and S-12, it is seen that Mf

increases by increasing the cooling rate This trend coincides with

theﬁndings of investigations carried out by Motemani et al [29] on

Ni-rich NieTi shape memory alloys In other words, the

furnace-cooled specimen has the highest phase transformation hysteresis

(AfeMf) compared to the other samples The increase in

trans-formation temperatures for the samples quenched in dry ice, compared to the samples quenched in water-ice, is due to the application of the steel foil to prevent oxidation of the parts When the sample is cooled in the water-ice bath, since the steel foil is not completely sealed, water may leak during the cooling time Consequently, cooling using dry ice is performed more gradually 3.1.3 Effect of the heat treatment duration

Variations of the austenite transformation temperatures with the duration of the heat treatment for springs made from the SME

wire are shown in Fig 10 As it is seen, Asand Afﬁrst increase

slightly with the increase in the heat treatment duration Then, by further prolonging the heat treatment, their increase is suppressed

Fig A fabricated pseudoelastic helical spring

Table

Transformation temperatures of Rhombohedral phase during heating cycle

Specimen Code Rs(C) Rf(C)

S-18 7

S-19 7

S-20 10

S-24 55 66

S-25 53 68

S-26 52 66

Fig Effect of the heat treatment temperature on the transformation temperatures for the spring made from SME wire

Fig Effect of the heat treatment temperature on the transformation temperatures of samples manufactured using PE wires

Fig 10 Effect of the heat treatment duration on the transformation temperatures of sample springs made from SME wire

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so that Asis reduced a little.Fig 11shows the effect of the heat

treatment time on the transformation temperatures for springs made from the PE wire It is seen that the increase in duration of the

heat treatment leads to a very negligible rise in Asand Afcompared

to the observed increase for the springs made of the SME wire Moreover, the effect of the heat treatment duration on trans-formation temperatures is less than that of the other production parameters

3.2 Effect of cold work

Using a ﬂat rolling machine, the SME wire with the initial

diameter of 1.5 mm was cold-rolled to achieve a 0.75 mm-thick strip The cold-rolled specimen was then further wound and locked

on a screw Then, the prepared spring was heat treated at 750C for

10 followed by quenching in the water-ice bath Specimen S-13 was manufactured with the use of a 3.5% cold-worked SME wire

According toAppendix 2, comparison of transformation

tempera-tures for samples S-11 and S-13 indicates that the SME wire be-comes austenitic at the room temperature when the cold work percentage increases to 32.1% Moreover, transformation tempera-tures of 32.1% cold-worked wires S-24 to S-26 are generally higher than those of the as-received wire This is in agreement with the

ﬁndings reported by Mitwally and Farag [30]

According to theﬁndings of this work, the effects of the various

shape-setting stages on the transformation temperatures for the fabricated samples using PE wire are schematically illustrated in

Fig 12 By elevating the heat treatment temperature, the trans-formation temperatures increase and then decrease Consequently, in early stages of annealing when the temperature is not high enough, the possibility of achieving pseudoelastic parts may be lessen even with the use of initially austenitic wires In other words, if one needs to reduce the transformation temperatures after annealing, the set temperature is recommended to be chosen as high as possible A more pronounced cooling rate gives rise to the decrease in transformation temperatures; thus, severe cooling such

as quenching is beneﬁcial when pseudoelastic products are

desir-able Cold work on the specimens leads to the rise in transformation temperatures; therefore, such processes may be useful in achieving martensitic parts and are not recommended when austenitic products are desired

4 Conclusion

In this paper, pseudoelastic helical springs were manufactured by shape-setting two types of NiTi wires: one was martensitic at the ambient temperature and the other was austenitic The effects of the various stages such as cold work, heat treatment temperature and duration, and the subsequent cooling rate on the trans-formation temperatures of the products were investigated The main results obtained in this research can be summarized as fol-lows: Heat treatment duration has the less effect on the trans-formation temperatures than the other adjustments of the shape-setting process Transformation temperatures of the springs fabri-cated using the SME wire increase as the heat treatment temper-ature increases However, for samples manufactured using PE wire,

transformation temperaturesﬁrst increase and then decrease with

the increase in the heat treatment temperature The increase in

cooling rate results in a decrease in Afwhich is beneﬁcial to

fabri-cate PE springs at the ambient temperature Cold work leads to an

increase in transformation temperatures, especially Af

Depending on the desired type of products with either the

pseudoelasticity or the shape memory effect, the presented

ﬁnd-ings regarding the effects of the various shape-setting stages on transformation temperatures of an SMA spring can be considered

as guidelines for fabricating SMA springs with a predeﬁned phase

(austenite or martensite) at the ambient temperature Metallurgical investigations will improve the present understanding, and such studies are under way In addition to the procedures used in the current study, the various thermomechanical treatments cause a

well-developed dislocation sub-structure or nanocrystalline

structure and may be used for improving the properties of SMAs in future works

Declaration of interests

The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have

appeared to inﬂuence the work reported in this paper

Fig 11 Effect of heat treatment duration on the transformation temperatures of springs made from PE wire

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Appendix Summary of Heat Transfer Conditions

Appendix Transformation temperatures of selected specimens

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S-25 77.5 75 42.5 27.5

S-26 77.5 74 42 22.5

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