EDM surface characteristics and shape recovery ability of Ti35.5Ni48.5Zr16 and Ni60Al24.5Fe15.5 ternary shape memory alloys

EDM surface characteristics and shape recovery ability of Ti

35.5

Ni

48.5

Zr

16

and Ni

60

Al

24.5

Fe

15.5

ternary shape memory alloys

S.F. Hsieh

a

_{, Albert W.J. Hsue}

a

_{, S.L. Chen}

b,

⇑

_{, M.H. Lin}

b

_{, K.L. Ou}

c

_{, P.L. Mao}

a

a

Dept. of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan

b

Dept. of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan

c

Graduate Institute of Biomedical Materials and Engineering, Taipei Medical University, Taipei 110, Taiwan

a r t i c l e

i n f o

Article history:

Received 4 September 2012

Received in revised form 1 March 2013 Accepted 12 March 2013

Available online 30 March 2013 Keywords:

Electro-discharge machining (EDM) Roughness

TiNiZr shape memory alloys

a b s t r a c t

Electro-discharge craters and recast materials are observed in the electro-discharge machining (EDM) surface of Ti35.5Ni48.5Zr16and Ni60Al24.5Fe15.5alloys. The recast layer is composed of TiO, NiO, ZrO2,

Fe2O3, Al2O3.The hardening effect near the outer surface for EDMed TiNiZr alloy originates from the

sur-face oxides of the recast layer. The thickness of the recast layer increases upon growing pulse energy. The EDMed Ti35.5Ni48.5Zr16 alloy still exhibits a good shape recovery, but the shape recovery is slightly

reduced due to the depression of the recast layer. The material removal rate (MRR) of Ti35.5Ni48.5Zr16alloy

is less than that of Ni60Al24.5Fe15.5alloy because of the larger h  kavalue. A high discharge energy should

have a larger and deeper craters, and a rougher surface. The surface roughness of the EDMed TiNiZr and NiAlFe SMAs is found to obey the empirical equation of Ra = C(IP

s

P)b. Having a larger h  Kavalue,

Ti35.5Ni48.5Zr16 alloy has a lower Ra value than that of Ni60Al24.5Fe15.5 alloy after electro-discharge

machining.

Ó 2013 Published by Elsevier B.V.

1. Introduction

TiNi alloys are known as the most important shape memory

alloys (SMAs), with good shape memory effect (SME) and

pesudo-elasticity (PE). The unique SME and PE characteristics of SMAs have

attracted interest from industries due to their engineering and

bio-medical applications. TiNi alloys have been widely used as

materi-als for medical implants because of their superior corrosion

resistance and excellent biocompatibility. The biocompatibility of

the implant devices, however, relies on a corrosion-resistant

tita-nium oxide surface layer to eliminate the toxic and allergic effects

of nickel

[1]

. Ti-based oxide layers can be formed by several surface

techniques, such as laser and electron-beam irradiation,

electro-chemical treatment, heat treatment and others

[1–3]

. The

proper-ties of TiNi binary alloys can be also affected by various thermal–

mechanical treatments

[4–7]

. Furthermore, the addition of a third

element to replace Ni and/or Ti has a substantial effect on phase

transformation behavior of these SMAs. The starting temperature

of martensitic transformation, Ms, of TiNi alloys decreases

follow-ing the substitution of Ni with V, Cr, Mn, Fe or Co

[8–10]

, but

in-creases remarkably following the substitution of Ni with Au, Pd

or Pt in amounts not less than 15–20 at.%

[11–13]

. However, these

alloys cannot be used at temperatures above 100 °C. For this

rea-son, other ternary TiNiX SMAs must be investigated. Among them,

TiNiZr and TiNiHf alloys are considered as the most prospective

candidates due to their low cost

[14–17]

.

The high ductility, severe work hardening and the unique

pseudoelastic behavior in TiNi SMAs cause their machining to be

quite complicated

[18,19]

, and it is difficult to machine TiNi alloys

using traditional techniques such as mechanical drilling, cutting

and shaping. To overcome machining difficulty in the

manufactur-ing process, some special techniques, such as electro-discharge

machining (EDM) and laser machining, may exhibit an excellent

ability for machining the TiNi alloys. However, to the best of our

knowledge, few investigations of these special techniques in

machining the TiNi SMAs have been reported

[20,21]

. Recently,

EDM, which has provided a useful technique for machining

im-plants and modifying their surfaces

[22,23]

, was employed for

forming nanoporous titanium oxide as well as bioactive titanium

on pure Ti metal

[24]

. To extend the applications of ternary TiNiZr

SMAs, some machining technologies for the production of

compli-cated shapes with high accuracy should be urgently developed. In

this study, we aim to investigate the machining characteristics of

TiNiZr alloys involving EDM by using de-ionized water as a

dielec-tric medium. The microstructure, phase composition and surface

roughness of EDMed surfaces are also discussed. In addition, the

Ni

60

Al

24.5

Fe

15.5

SMA is used as a comparative material.

0925-8388/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2013.03.111

⇑

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

Journal of Alloys and Compounds 571 (2013) 63–68

Contents lists available at

SciVerse ScienceDirect

Journal of Alloys and Compounds

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 a l c o m

2. Experimental procedure

Conventional tungsten arc-melting was employed to prepare the Ti35.5Ni48.5Zr16

alloys. Titanium (purity, 99.7 wt.%), nickel (purity, 99.9 wt.%) and zirconium (purity, 99.8 wt %), totaling about 180 g, were melted and remelted at least six times in an argon atmosphere. Pure titanium buttons were also melted and used as getters. The mass loss during melting was negligibly small. The as-melted buttons were homog-enized at 950 °C in a 7  106_{torr vacuum furnace for 72 h. The homogenized}

but-tons were cut into several plates with a low speed diamond saw. Specimens for the electro-discharge machining (size: 50  25  5 mm3

) were carefully cut and ground from these plates. These specimens were annealed at 900 °C for 2 h in a vacuum fur-nace and then quenched in water.

In this paper, the specimens were performed on a die-sinking EDM machine model type LS-250C, made by Lien-Sheng Co. in Taiwan. The operation parameters used in this study are presented inTable 1. The microstructures of electro-discharge machined (EDMed) surfaces were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and secondary electron image (SEI). The X-ray analyses of EDMed surfaces were performed at room temperature by a Siemens D5000 XRD using Cu K

a

radiation. The power was 30 kV  20 mA and the 2h scanning rate was 3° min1_{. The morphologies of EDMed surface were observed using a JOEL 6330 TF}

SEM with SEI facility. A Talysurf profilometer was used to evaluate quantitatively the roughness of the machined surface, presented by Ra. The cut-off was selected as 0.8 mm. For each piece, the average of readings taken at seven places on the machining plane was used as the surface roughness value. Before surface hardness testing, the EDMed specimens were mounted, ground and polished to smooth sur-face. All specimens were cleaned ultrasonically with alcohol and/or distilled water to remove polishing particles. The hardness measurements were made with a reg-ular microvickers pyramid diamond indenter at 5 g load and 15 s dwell time in a FUTRE-TECH FM-700 machine. For each specimen, the average hardness value was taken from at least five test readings. The shape-recovery measurement was examined by the bending test[25].

3. Results and discussion

3.1. Surface morphology and composition analysis of a TiNiZr SMA

after EDM process

During the EDM process, the primary parameters discharge

cur-rent I

P

and pulse duration

s

P

were used, both of which are settings

of the power supply. Surface topography of Ti

35.5

Ni

48.5

Zr

16

and

Ni

60

Al

24.5

Fe

15.5

ternary SMAs after EDM process is presented in

Fig. 1

Many discharge craters, melting drops and recast materials

are observed in the EDM surface. The XRD patterns of the EDMed

surface layer for the Ti

35.5

Ni

48.5

Zr

16

and Ni

60

Al

24.5

Fe

15.5

alloys are

shown in

Fig. 2

from which the XRD peaks of TiO, NiO, ZrO

2

,

Fe

2

O

3

, Al

2

O

3

and Ni

3

Al can be observed. From

Fig. 2

a, the formation

of TiO, NiO and ZrO

2

oxides is ascribed to the high activity of Ti, Ni

and Zr atoms.

Figs. 3

a–d show the cross-sectional SEI micrographs

near the EDMed surface layer for the Ti

35.5

Ni

48.5

Zr

16

alloy under the

conditions of I

P

= 7 A and

s

P

= 3, 12, 25, 50

l

s, respectively. The

sim-ilar cross-sectional SEI micrographs having not shown here can be

observed for another discharge current versus pulse durations.

Carefully examining

Figs. 3

a–d, the thickness of the recast layer

in-creases with growing pulse duration, and these results can be

ex-plained as follows. During the EDM process, the

electrode-discharge plasma channel is composed of electron and ion flows.

Electron flow is dominant in the plasma channel during the initial

stage, and hence the cathode (workpiece) has lower pulse energy.

Meanwhile, the ratio of positive ions flow in the plasma channel

in-creases with growing pulse duration

[26]

and the workpiece has

greater discharge energy, so the thickness of the recast layer grows

during the early pulse duration. Thereafter, long pulse duration

will have relatively high accumulated discharge energy. This

makes more material be melted and re-solidified, as well as

dis-solving and depositing more de-ionized water dielectric medium

on the EDMed surface. Therefore, the recast layer becomes thicker.

Fig. 4

reveals the cross-sectional hardness at various distances

form the EDMed surface of the Ti

35.5

Ni

48.5

Zr

16

and Ni

60

Al

24.5

Fe

15.5

alloys under the conditions of I

P

= 15 A and

s

P

= 50

l

s. It indicates

that the specimen’s hardness near the outer surface can reach

Table 1

The machining parameters of EDM in this study.

discharge current (A) 1, 3, 5, 7, 15 Pulse duration (

l

s) 1, 3, 5, 12, 25, 50 Pause duration (

l

s) 1, 3, 5, 12, 25, 50

Gap voltage (V) 30

Electrode Ti (+); work-piece ()

Dielectric De-ionized water

melting drops

discharge craters

re-cast materials

Fig. 1. The SEM micrographs of the EDMed surface for: (a) Ti35.5Ni48.5Zr16alloy and

(b) Ni60Al24.5Fe15.5alloy.

Fig. 2. The XRD patterns of the EDMed surface layer for: (a) Ti35.5Ni48.5Zr16alloy

and (b) Ni60Al24.5Fe15.5alloy.

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