Journal of Alloys and Compounds 464 (2008) 446–451
Electrical discharge machining of a NiAlFe ternary shape memory alloy
S.L. Chen
a,∗, S.F. Hsieh
b, H.C. Lin
c, M.H. Lin
a, J.S. Huang
baDepartment of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC bDepartment of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC
cDepartment of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC Received 15 August 2007; received in revised form 30 September 2007; accepted 2 October 2007
Available online 7 October 2007
Abstract
The electro-discharge machining (EDM) characteristics of a NiAlFe ternary shape memory alloy (SMA) have been investigated in this study. Experimental results reveal that the material removal rates (MRRs) of Ni60Al24.5Fe15.5and Ti35.5Ni49.5Zr15alloys in the EDM process exhibit a reverse relationship to the product of the alloy’s melting temperature and thermal conductivity. In addition, a precise EDM machining of NiAlFe SMA can be obtained by setting the machine parameters at low pulse energy. The surface roughness (Ra) of the EDMed NiAlFe SMA is found to follow the empirical equation of Ra =λ(IP× τP)β. Having a less T× KTvalue, Ni60Al24.5Fe15.5alloy has a larger Ra value than that of Ti35.5Ni49.5Zr15
alloy after electro-discharge machining. The recast layer consists of the oxides Fe2O3, Al2O3, NiO and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The hardening effect near the outer surface for EDMed NiAlFe alloy originates from the recast layer. The thickness of the recast layer varies with the pulse duration and exhibits a minimum value at the maximal MRR.
© 2007 Elsevier B.V. All rights reserved.
Keywords: EDM; Roughness; NiAlFe shape memory alloys
1. Introduction
Among many shape memory alloys (SMAs), TiNi alloys are the most popular due to their superior properties in shape memory effect (SME) and superelasticity[1–4]. However, the applications of these alloys are limited to use at temperatures lower than 100◦C. For this reason, high-temperature SMAs need to be developed and studied. Nickel-rich NiAl alloys contain-ing about 63–65 at.% Ni have been reported as promiscontain-ing shape memory alloys. The quenched Nickel-rich NiAl alloys undergo a thermoelastic martensitic transformation from a B2 (-phase) structure to a metastable L10structure with either 3R or 7R stack-ing. The martensitic transformation starting temperature Ms of the-phase occurs over a wide temperature range up to approxi-mately 500◦C, depending on the nickel content[5,6]. However, only a small bending shape memory effect was found due to its limited ductility[7–9]. Adding Fe to NiAl binary SMAs is used for ductility and SME improvement[10–15]. Some authors used rapid solidification to produce a ductile NiAlFe alloy with the
∗Corresponding author. Tel.: +886 7 381 4526x5342; fax: +886 7 383 5015. E-mail address:[email protected](S.L. Chen).
quench-induced martensite structure that exhibited a reversible SME[13,16]. Ishida et al. also found that with proper heat treat-ments and quenching procedures, the cast material could be produced with room temperature ductility and shape recovery [15,17].
Ni–Al binary system has drawn much attention for many years owing to their importance in the development of high-temperature materials, i.e. it shows excellent oxidation resistance and has the advantageous of heat conductivity[7,18], as well as high SMAs[5–6]. Unfortunately, they are difficult for machine and hot working due to ordered structure. To overcome machining difficulty in the manufacturing process, some special techniques, such as the electrical discharge machining (EDM) and laser machining, may be useful in machining the hard, brittle and tough materials[19–21]. The die-sinking EDM process is a non-traditional, thermoelectric process in which the material is removed by electro-discharges occurring between the workpiece and tool electrode immersed in a liquid dielectric medium. The electrical discharge melts and vaporizes minute amounts of the workpiece, which are then ejected and swept away by the dielec-tric. Hence, EDM is an effective technique in machining the difficult-to-cut materials. To extend the applications of NiAlFe ternary SMAs, some machining technologies for the production 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
S.L. Chen et al. / Journal of Alloys and Compounds 464 (2008) 446–451 447
Table 1
The machining parameters of EDM in this study
Discharge current (A) 3, 6, 10, 19 Pulse duration (s) 3, 6, 12, 25, 50, 100 Pause duration (s) 3, 6, 12, 25, 50, 100
Gap voltage (V) 50
Electrode Cu (+); workpiece (−)
Dielectric Kerosene
of complicated shapes with high accuracy should be urgently developed. Therefore, the objective of this study is to investi-gate the machining characteristics of NiAlFe alloys involving EDM. The microstructure, composition, roughness and hard-ness of EDMed surfaces are discussed. The Ti35.5Ni49.5Zr15 high-temperature SMA is used as a comparative material.
2. Experimental procedure
The conventional tungsten arc-melting technique was employed to prepare the Ni60Al24.5Fe15.5ternary alloy. Nickel (purity, 99.9 wt.%), Aluminum (purity, 99.9 wt.%), and iron (purity, 99.95 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 negligi-bly small. The as-melted buttons were homogenized at 1200◦C for 72 h. The homogenized buttons were cut into several plates with a low speed diamond saw. Specimens for the electro-discharge machining (size: 55 mm× 20 mm × 5 mm) were carefully cut and ground from these plates. The specimens were then evacuated in quartz tubes and annealed at 1150◦C for 2 h, followed by a water-quench.
The EDM specimens were performed on a die-sinking EDM machine model type 30-TP, made by Topedm 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␣ radiation. The power was 30 kV × 20 mA and the 2θ scanning rate was 3 min−1. 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 EDMed surface, pre-sented by Ra. The cut-off is selected as 0.8 mm. For each piece, the average of readings taken at seven places on machining plane was chosen as the surface roughness value. The surface hardness was measured in a microvickers hardness tester with a load of 25 g for 15 s. For each specimen, the average hardness value was taken from at least five test readings.
3. Results and discussion
3.1. The material removal rate and surface roughness of a NiAlFe alloy after EDM process
Table 2presents the transformation temperatures, hardness and crystal structures at room temperature for Ni60Al24.5Fe15.5 and Ti35.5Ni49.5Zr15 ternary SMAs. Some important metallur-gical properties of NiAlFe alloy inTable 2 aid to investigate
Fig. 1. The material removal rate vs. the pulse durationτPat IP= 10 A for the Ni60Al24.5Fe15.5and Ti35.5Ni49.5Zr15alloys.
the machining characteristics of this alloy in the EDM pro-cess. From this Table, one can find that Ni60Al24.5Fe15.5 and Ti35.5Ni49.5Zr15alloys exhibit the mixture ofmartensite and ␥-phase, and (Ti,Zr)2Ni,1-phase and B19phase, respectively. The material’s intrinsic properties and several machining param-eters, e.g. the electrode material, electrode polarity, discharge current IP and pulse duration τP, can significantly affect the EDM characteristics.
In this study, we hope to determine the effect of some of the most important parameters implicated in the EDM process on a NiAlFe ternary alloy including discharge current IPand pulse duration τP.Fig. 1 depicts the material removal rate (MRR) versus the pulse duration at IP= 10 A for Ni60Al24.5Fe15.5and Ti35.5Ni49.5Zr15 alloys. As can be seen fromFig. 1, the MRR of Ni60Al24.5Fe15.5alloy is larger than that of Ti35.5Ni49.5Zr15 alloy [22] at various pulse durations during the EDM pro-cess. This feature is associated with their melting temperature (T) and thermal conductivity (KT). Materials with higher melting temperature, leading to less melting and evaporation, and higher thermal conductivity, causing more heat transfer of discharge energy to the nearby matrix, will show a lower MRR in the EDM process. Hence, the product of the melt-ing temperature and thermal conductivity of materials can be used to evaluate the EDM characteristic in NiAlFe and TiNiZr ternary SMAs. The MRRs of TiNi-based SMAs have a reverse relationship to the product of the materials’ melting temper-ature and thermal conductivity [20,22]. Table 3 presents the product of the melting temperature and thermal conductiv-ity of Ni60Al24.5Fe15.5 and Ti35.5Ni49.5Zr15 alloys. A careful examination of Fig. 1 andTable 3 shows that the MRRs of Ni60Al24.5Fe15.5and Ti35.5Ni49.5Zr15 alloys are in accordance with the above-mentioned relationship.
Table 2
The crystal structures and some basic properties of Ni60Al24.5Fe15.5and Ti35.5Ni49.5Zr15ternary alloys
Alloy M* (◦C) A* (◦C) Hardness (Hv) Crystal structure
Ni60Al24.5Fe15.5 171 (B2→ ) 205 (→ B2) 260 +␥-phase
Ti35.5Ni49.5Zr15 176 217 320 B19+1-phase + (Ti,Zr)2Ni
S.L. Chen et al. / Journal of Alloys and Compounds 464 (2008) 446–451 451
a shorter pulse durationτP should be selected to have an excellent machined surface finish on a NiAlFe alloy. (2) The EDMed Ni60Al24.5Fe15.5 ternary SMAs demonstrate
increasing surface roughness as the discharge current and pulse duration increase. The roughness of EDMed surface increases with the discharge current and pulse duration, and follows the empirical equation Ra =λ(IP× τP)β. The Ni60Al24.5Fe15.5alloy, having a less T× KTvalue, exhibits a rougher EDMed surface than that of Ti35.5Ni49.5Zr15alloy. (3) The thickness of the recast layer increases in the early stage, drops to the minimum value at the maximal MRR, and then grows in the extended pulse duration again. The recast layer consists of the oxides Fe2O3, Al2O3, NiO and the deposi-tion particles of the consumed Cu electrode and dissolved dielectric medium in the recast layer.
(4) The specimen’s hardness near the outer surface can reach 527 Hv for EDMed Ni60Al24.5Fe15.5 alloy, but, the hard-ness of the matrix is not affected by the EDM process. This hardening effect is due to the formation of the oxides Fe2O3, Al2O3, NiO and the deposition particles of the consumed Cu electrode and dissolved dielectric medium in the recast layer.
Acknowledgements
The authors sincerely acknowledge the financial support of this research by the National Science Council (NSC), Republic of China, under the grant NSC 95-2221-E-151-013.
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