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

The effects of shape-setting on transformation temperatures of

pseudoelastic shape memory alloy springs

a Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran

b Department 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 1 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

open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

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

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

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

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

SMA springs using linear Timoshenko beam elements Aguiar et al

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

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

* 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.

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

Journal of Science: Advanced Materials and Devices 4 (2019) 568e576

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[10] Shape-setting mainly includes annealing but may be

accom-panied by cold work and quenching on the products Although

investigate the detailed mechanisms of the various stages in

shape-setting, macroscopic studies have gained a great attention in order

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

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

the R-phase appends to austenite and martensite, several

manipulated by adjusting the heat treatment parameters including

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

micro-structure, martensitic transformation, shape memory effect and

concluded that, in the course of elevating the annealing

tempera-ture, the transformation temperatures increased for annealing

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

phase transformation and mechanical properties of Ni-rich NiTi

furnace-cooled sample has the highest phase transformation

superelasticity, the shape memory effect and the hardness of NiTi

alloy They found that the shape memory recovery is diminished by

augmenting the extent of applied cold work, the products would

proposed a procedure to fabricate an SMA spring and to

charac-terize its microstructural evolution during the production Costanza

and presented a technique to design a linear actuator made of

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,

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

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

illustrate results of differential scanning calorimetry (DSC), and

Table 1shows the transformation temperatures of these wires To

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

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

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

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

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,

were used to determine the transformation temperatures of the

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:

1 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

F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices 4 (2019) 568e576 569

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Fig 1 Transformation temperatures for the SME wire.

Fig 2 Transformation temperatures for the PE wire.

Table 1

Transformation temperatures of the utilized wires.

Initial wire A f ( C) A s ( C) R s ( C) R f ( C) M s ( C) M f ( C)

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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

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

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

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

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

applicable temperature for achieving a pseudoelastic spring

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

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

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

Since pseudoelastic springs are planned to be manufactured in

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

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

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 3 Constrained SMA wire on a screw.

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minimum temperature of 750C, Asand Afare higher than the

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

It is observed that the transformation temperatures strongly depend

three trends for the variations in the transformation temperatures:

1 Transformation temperatures are enhanced by elevating the

Fig 5 Transformation temperatures for sample S-13.

Fig 6 Transformation temperatures for sample S-24.

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3 Transformation temperatures except Rsand Rfare nearly

con-stant with the increase in the heat treatment temperature

heat treatment temperature, transformation temperatures

remain fairly constant This is why heat treatment at

3.1.2 Effect of cooling rate

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

increases by increasing the cooling rate This trend coincides with

furnace-cooled specimen has the highest phase transformation hysteresis

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

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

Fig 7 A fabricated pseudoelastic helical spring.

Table 2

Transformation temperatures of Rhombohedral phase during heating

cycle.

Specimen Code R s ( C) R f ( C)

Fig 8 Effect of the heat treatment temperature on the transformation temperatures

for the spring made from SME wire.

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

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

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

10 min followed by quenching in the water-ice bath Specimen S-13

was manufactured with the use of a 3.5% cold-worked SME wire

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

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

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,

the increase in the heat treatment temperature The increase in

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

Depending on the desired type of products with either the

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

(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

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

Fig 11 Effect of heat treatment duration on the transformation temperatures of

springs made from PE wire.

Fig 12 Schematic illustration of the effect of various parameters on transformation

temperatures for springs sample made of PE wire.

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Appendix 1 Summary of Heat Transfer Conditions

Appendix 2 Transformation temperatures of selected

specimens

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