The machining characteristics and shape recovery ability of Ti–Ni–X(X = Zr, Cr) ternary shape memory alloys using the wire electro-discharge machining

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The machining characteristics and shape recovery ability of Ti–Ni–X (X ¼ Zr,

Cr) ternary shape memory alloys using the wire electro-discharge machining

S.F. Hsieh

a

, S.L. Chen

b,

, H.C. Lin

c

, M.H. Lin

b

, S.Y. Chiou

a

Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan 807, Republic of China b_{Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan 807, Republic of China} c

Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China

a r t i c l e

i n f o

Article history:

Received 5 September 2008 Received in revised form 3 December 2008

Accepted 23 December 2008 Available online 20 January 2009 Keywords:

WEDM process Roughness

TiNi shape memory alloys

a b s t r a c t

The wire electro-discharge machining (WEDM) characteristics of TiNiX ternary shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the maximum feeding rate without breakage of wire electrode of Ti35.5Ni49.5Zr15and Ti50Ni49.5Cr0.5alloys in the WEDM process exhibits a reverse relationship with the product of the alloy’s melting temperature and thermal conductivity. The surface roughness (Ra) of the machined TiNiX alloys increases with growing pulse duration. Having a larger

l

YKTvalue, Ti35.5Ni49.5Zr15alloy has a lesser Ra value and feeding rate of wire electrode than those of Ti50Ni49.5Cr0.5alloy after WEDM. Electro-discharge craters and recast materials are observed on the wire electro-discharge machined (WEDMed) surface of TiNiX alloys. The thickness of the recast layer varies with pulse duration. The hardening effect near the outer surface for WEDMed TiNiX alloys arises from the recast layer. The WEDMed TiNiX alloys still exhibit a good shape recovery, but a slight degradation of shape recovery occurs due to the depression of the recast layer.

1. Introduction

TiNi alloys are known as the most important shape memory

alloys (SMAs) because of their many applications based on shape

memory effect (SME) and pseudoelasticity (PE)

[1–4]

. To extend

their speciﬁc uses in various application ﬁelds, some TiNiX ternary

alloys still need to be developed and studied. Adding a third

element to replace Ti and/or Ni in TiNi alloys has a substantial

effect on their phase transformation behaviors. The temperature

of the start of the martensite transformation (Ms) increases

remarkably following the substitution of Ni with Au, Pd and Pt in

amounts not less than 15–20 at%

[5–7]

, but decreases

mono-tonously following the substitution of Ni with Cr, V, Fe, Mn and Co

elements

[8–11]

. On the other hand, adding Cr or Nb in a TiNi alloy

can widen the transformation temperature range

[12,13]

. Wide

thermal hysteresis is desirable for coupling and sealing

applica-tions. However, the applications of these alloys are limited at

temperatures lower than 100 1C. For this reason, ternary

high-temperature TiNiX SMAs must be investigated. Among them,

TiNiZr and TiNiHf alloys are considered as the most prospective

candidates due to their low costs

[14–18]

.

The impediments to TiNi SMA development are caused by

difﬁculties in the manufacturing process. It is well known that

TiNi alloys can be deformed with high ductility, but the high

degree of strain hardening and the unique shape memory

properties have caused the machinability of TiNi SMAs to be

quite complicated

[19,20]

. Therefore, it is difﬁcult to machine

TiNi alloys using the traditional techniques, namely mechanical

cutting, drilling and shaping. To overcome machining difﬁculty in

the manufacturing process, some special techniques, such as the

wire electro-discharge machining (WEDM) and laser machining,

may permit excellent machining ability in the TiNi alloys.

However, to the best of our knowledge, few investigations

of these special techniques in machining the TiNi SMAs have

been reported

[21]

. WEDM has been acquiring wide acceptance

for the machining of various conductive materials used in real

applications such as metals, ceramics, silicon and metal matrix

composites

[22–26]

. It is a numerically controlled, modiﬁed

electro-discharge technique where the workpiece geometry is

generated by a NC-controlled travelling wire. Therefore, WEDM

provides an effective technique for machining intricate and

complex shapes in conductive materials. In the present study,

we aim to explore the machinability of TiNiZr and TiNiCr ternary

SMAs involving WEDM. The microstructure, composition,

hard-ness

and

roughness

of

wire

electro-discharge

machined

(WEDMed) surfaces are also discussed. Meanwhile, the Ni

60

Al

25.5

Fe

14.5

high-temperature SMA is used as a comparative material.

2. Experimental procedure

The conventional tungsten arc-melting technique was

em-ployed to prepare Ti

35.5

Ni

49.5

Zr

15

and Ti

50

Ni

49.5

Cr

0.5

alloys.

Titanium (purity, 99.7 wt%), nickel (purity, 99.9 wt%), zirconium

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International Journal of Machine Tools & Manufacture

Corresponding author. Tel.: +886 7 381 4526x5342; fax: +886 7 383 1373. E-mail address:[email protected] (S.L. Chen).

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mode. It increases monotonically with increasing pulse duration.

Besides, the maximum feeding rate is found to have a reverse

relationship with the product of the material’s melting

tempera-ture and thermal conductivity. Longer pulse duration causes

rougher WEDMed surface in TiNiX alloys. Ti

50

Ni

49.5

Cr

0.5

alloy,

having a small

l

Y

K

T

value, exhibits a rougher machined surface

than Ti

35.5

Ni

49.5

Zr

15

alloy. The thickness of the recast layer for the

WEDMed TiNiX alloys decreases with growing pulse duration.

The specimen’s hardness near the outer surface can reach 875 and

807 Hv for WEDMed Ti

35.5

Ni

49.5

Zr

15

and Ti

50

Ni

49.5

Cr

0.5

alloys,

respectively. This hardening effect arises from the formation of

the oxides ZrO

2

, TiO

2

, TiNiO

3

, Cr

2

O

3

and the deposition particles

of the consumed brass wire electrode in the recast layer. The

WEDMed TiNiX alloys still exhibit a nearly perfect shape recovery

at a normal bending strain, but a slightly reduced shape recovery

at a higher bending strain due to their constrained effect on the

TiNiX matrix by the recast layer.

Acknowledgement

The authors sincerely acknowledge the ﬁnancial support

of this research by the National Science Council (NSC), Republic

of China, under Grants NSC 95-2221-E-151-013 and NSC

97-2221-E-151-024.

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ARTICLE IN PRESS

Fig. 7. Cross-sectional SEM micrographs of the WEDMed Ti35.5Ni49.5Zr15alloy with a pulse duration of (a) 1 and (b) 5

m

s.

Fig. 8. Specimen’s hardness at various distances from the WEDMed surfaces of TiNiX ternary SMAs.

Table 4

Measured shape recovery near the WEDMed surface of Ti35.5Ni49.5Zr15 and Ti50Ni49.5Cr0.5alloys.

Alloy Shape recovery (%)

e

¼3%

e

¼5%

e

¼8%

Ti35.5Ni49.5Zr15(as-annealed) 100 100 88

Ti35.5Ni49.5Zr15(WEDMed) 100 99 85

Ti50Ni49.5Cr0.5(as-annealed) 100 100 90

Ti50Ni49.5Cr0.5(WEDMed) 100 99 86

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