Materials Science and Engineering A 445–446 (2007) 486–492
Electrical discharge machining of TiNiCr and
TiNiZr ternary shape memory alloys
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, Taiwan 807, Republic of China bDepartment of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan 807, Republic of China
cDepartment of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China Received 10 July 2006; accepted 22 September 2006
Abstract
This study investigates the influence of the machining characteristics on TiNiX ternary shape memory alloys (SMAs) using electro-discharge machining (EDM). Experimental results show that the material removal rates (MRRs) of Ti50Ni49.5Cr0.5and Ti35.5Ni49.5Zr15alloys in the EDM
process exhibit a reverse relationship to the product of the alloy’s melting temperature and thermal conductivity. The surface roughness (Ra) of the
EDMed TiNiX alloys is found to obey the empirical equation of Ra=β(IP× τP)␣. Having a less T× KTvalue, Ti50Ni49.5Cr0.5alloy has a larger Ravalue than that of Ti35.5Ni49.5Zr15alloy after electro-discharge machining. Besides, a lower discharge current IPand a shorter pulse duration τPshould be selected to have a precise EDM machining of TiNiX SMAs. The hardening effect near the outer surface for EDMed TiNiX alloys 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. The EDMed TiNiX alloys still exhibit a nearly perfect shape recovery at a normal bending strain, but slight degradation of shape recovery occurs at a higher bending strain due to the constrained effect on the TiNiX matrix by the recast layer.
© 2006 Elsevier B.V. All rights reserved.
Keywords: EDM; Roughness; Ti–Ni shape memory alloys
1. Introduction
Although TiNi alloys are the most widely used shape memory alloys (SMAs), to extend their specific needs in various applica-tion fields, some TiNiX ternary alloys still need to be developed and studied. The addition of a third element to replace Ni and/or Ti in TiNi alloys has a substantial effect on their phase transfor-mation behaviors. The Ms temperature decreases monotonously following the substitution of Ni with Cr, V, Fe, Mn and Co elements [1–4], but increases remarkably following the sub-stitution of Ni with Au, Pd and Pt in amounts not less than 15–20 at.% [5–7]. On the other hand, the addition of Cr in a TiNi alloy can widen the transformation temperature range[8]. Wide thermal hysteresis is desirable for coupling and sealing applications. However, the applications of these alloys are lim-ited to use at temperatures lower than 100◦C. For this reason, high-temperature SMAs need to be investigated. Among them,
∗Corresponding author. Tel.: +886 7 381 4526x5342; fax: +886 7 383 1373.
E-mail address:[email protected](S.L. Chen).
the most significant candidates are TiNiZr and TiNiHf alloys, where Zr and Hf are used to replace Ti in these alloys[9–13].
The roadblocks to TiNi SMAs development are caused by difficulties in the manufacturing process. It is well known that TiNi alloys can be tensile-deformed in a ductile manner to about 50% strain prior to fracture, but the severe strain hardening and the unique pseudoelastic behavior have caused the machin-ing characteristics of TiNi SMAs to be quite complicated[14]. To overcome this difficulty, some special techniques, such as the electro-discharge machining (EDM) and laser machining, may exhibit an excellent ability in machining the TiNi SMAs [15]. EDM is an electro-thermal process in which the material is removed by electro-discharges occurring between the work-piece and tool electrode immersed in a liquid dielectric medium. These electro-discharges melt and vaporize minute amounts of the work-piece, which are then swept away by the dielectric. Therefore, EDM is a versatile technique in machining the stub-born materials, which are difficult to machine by conventional techniques. To extend the applications of TiNiX ternary SMAs, some machining technologies for production of complicated shapes with high accuracy should be urgently developed. Hence, 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
S.L. Chen et al. / Materials Science and Engineering A 445–446 (2007) 486–492 491
Fig. 9. The specimen’s hardness at various distance from the EDMed surfaces of TiNiX ternary SMAs under the condition IP= 10 A andτP= 100s. the recast layer is getting thinner, as shown inFig. 8(c). As to, an over-long pulse duration will have relatively high accumulated electro-discharge energy. This makes more material be melted and re-solidified, as well as more kerosene dielectric medium be dissolved and deposited on the EDMed surface. If this molten material is not swept away from the surface by the dielectric, it will solidify during the cooling process and form a recast layer. Therefore, the thickness of the recast layer is increased again. Based on the above discussion, a lower discharge current IPand a shorter pulse durationτPshould be selected to have a precise EDM machining of TiNiX ternary SMAs, but this approach is more time consuming.
3.3. The shape recovery ability near EDMed surfaces of TiNiX ternary alloys
Fig. 9 shows the cross-sectional hardness versus dis-tance form the EDMed surfaces of Ti50Ni49.5Cr0.5 and Ti35.5Ni49.5Zr15 alloys under the conditions of IP= 10 A and
τP= 100s. It indicates that the specimen’s hardness near the outer surface can reach 913 Hv for Ti50Ni49.5Cr0.5 alloy, but 1087 Hv for Ti35.5Ni49.5Zr15alloy. This hardening effect is due to the formation of the oxides Cr2O3, ZrO2, TiO2, TiNiO3, car-bides TiC and the deposition particles in the recast layer. Besides, the hardness of the matrix in TiNiX alloys is not affected by the EDM process.
Table 4depicts the measured shape recovery near the EDMed surface of the Ti50Ni49.5Cr0.5 and Ti35.5Ni49.5Zr15 alloys. The Table 4
The measured shape recovery near the EDMed surface of Ti50Ni49.5Cr0.5and Ti35.5Ni49.5Zr15alloys
Alloy Shape recovery (%)
ε = 3% ε = 5% ε = 8%
Ti50Ni49.5Cr0.5(as-annealed) 100 100 90
Ti50Ni49.5Cr0.5(EDMed) 100 99 83
Ti35.5Ni49.5Zr15(as-annealed) 100 100 88
Ti35.5Ni49.5Zr15(EDMed) 100 98 82
specimen’s thickness for SME test is 0.6 mm, which is much thicker than the recast layer (<100m). It shows inTable 4that the EDMed alloys exhibit almost perfect shape recovery at 3 and 5% bending strains, but a slightly reduced shape recovery at 8% bending strains, as compared with that of as-annealed TiNiX alloys. This feature indicates that the recast layer formed during the EDMed process has no obvious effect to depress the shape recovery of these alloys at normal bending strains. Thus, at higher bending strains, the shape recovery will be slightly reduced because the recast layer does not exhibit the shape mem-ory effect. Furthermore, their constrained effect of the recast layer on the matrix will also depress the shape recovery of the matrix on TiNiX alloys. Therefore, in the application of thin plates, the recast layer on the EDMed surface of TiNiX SMAs should be mechanically ground and/or machined by electro-chemical polishing before the SME treatment to improve their shape recovery characteristics.
4. Conclusion
The MRRs of Ti50Ni49.5Cr0.5 and Ti35.5Ni49.5Zr15 ternary SMAs in the EDM process significantly relate to the electro-discharge energy mode. It increases monotonically with growing discharge current, but appears a maximum value at an opti-mal pulse duration, sayingτP= 12s at IP= 10 A in this study. Besides, their MRRs are found to have a reverse relationship to the product of the material’s melting temperature and ther-mal conductivity. The roughness of EDMed surface increases with the discharge current and pulse duration, and follows the empirical equation Ra=β(IP× τP)α. The Ti50Ni49.5Cr0.5 alloy, having a less T× KTvalue, exhibits a rougher EDMed surface than that of Ti35.5Ni49.5Zr15 alloy. The thickness of the recast layer for the EDMed TiNiX alloys varies with the pulse dura-tion and exhibits a minimum value at the maximal MRR. The specimen’s hardness near the outer surface can reach 913 and 1087 Hv for EDMed Ti50Ni49.5Cr0.5and Ti35.5Ni49.5Zr15alloys, respectively. This hardening effect is due to the formation of the oxides Cr2O3, TiO2, TiNiO3, ZrO2, carbides TiC, and the deposition particles of the consumed Cu electrode and dissolved dielectric medium in the recast layer. The EDMed 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 the Grant NSC 94-2212-E-151-013.
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