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Surface modification of TiNi-based shape memory alloys by dry electrical discharge machining
Journal of Materials Processing Technology 221 (2015) 279–284
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Surface modification of TiNi-based shape memory alloys by dry electrical discharge machining Tyau-Song Huang a , Shy-Feng Hsieh a , Sung-Long Chen b , Ming-Hong Lin b , Shih-Fu Ou a,∗ , Wei-Tse Chang a a b
Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan
a r t i c l e
i n f o
Article history: Received 3 September 2014 Received in revised form 2 February 2015 Accepted 11 February 2015 Available online 24 February 2015 Keywords: Shape memory alloys Dry electrical discharge machining Surface modification
a b s t r a c t This study attempts to decrease the martensite transformation temperature of Ti50 Ni50 shape memory alloy (SMA) for its use in biomedical applications by Cr addition. In addition, surface modification of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs using electrical discharge machining (EDM) is proposed. Nitrogen gas is used as a dielectric medium, and a pure titanium pipe is used as the tool electrode. The machining characteristics and surface properties of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs after EDM in nitrogen gas were investigated. Many electrical discharge craters and recast materials are observed on the EDMed surface of these SMAs. Material removal rate, electrode wear rate, and surface roughness increase with increasing pulse current and duration, and they share an inverse relationship with the thermal conductivity of these SMAs. After EDM, the SMAs continue to exhibit good shape recovery, and even the recast layers have high surface hardness. The recast layers, comprising TiN and CrN, with high hardness and good adhesion are expected to improve the SMAs’ wear and corrosion resistance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Shape memory alloys (SMAs) are used in many fields such as mechanical, aerospace, and military applications (Van Humbeeck, 1999). From among SMAs, Ti–Ni SMAs are widely applied in biomedical engineering because of their unique superelasticity (Otsuka and Ren, 2005), superior shape memory effect (Yoneyama et al., 1992), excellent corrosion resistance (Liu and Xiang, 1998), low elastic modulus (Wever et al., 1998), and non-ferromagnetic property (Duerig et al., 1999). However, from the viewpoint of longterm implantation, Ni release from Ti–Ni SMAs may cause allergy in the human body. Therefore, proper surface treatment of Ti–Ni implants is important to reduce Ni release and improve corrosion resistance. Hydroxyapatite (Lobo et al., 2011), zirconia (Qiu et al., 2010), titanium oxide (Qiu et al., 2011), titanium nitride (Jin et al., 2013), and apatite-collagen composite (Sun et al., 2011), are coated on Ti–Ni SMAs to improve the biocompatibility. Among these coating,
∗ Corresponding author. Tel.: +886 7 3814526x5415; fax: +886 7 3814526. E-mail addresses: [email protected] (T.-S. Huang), [email protected] (S.-F. Hsieh), [email protected] (S.-L. Chen), [email protected] (M.-H. Lin), [email protected] (S.-F. Ou), [email protected] (W.-T. Chang). http://dx.doi.org/10.1016/j.jmatprotec.2015.02.025 0924-0136/© 2015 Elsevier B.V. All rights reserved.
titanium nitride (TiN), possessing high hardness and good wear resistance, has been used for hip prostheses (Piscanec et al., 2004). Therefore, many surface treatment technologies, i.e., gas nitriding (Wu et al., 1999), filtered arcing ion plate technique (Jin et al., 2013), and powder immersion reaction assisted coating (Starosvetsky and Gotman, 2001), are proposed to coat TiN on Ti–Ni SMA surfaces. In order to develop better surface treatment technologies, the present study introduce a method to coat TiN on Ti–Ni SMAs by dry electrical discharge machining (EDM). Until now, many researchers have attempted to modify titanium and its alloys’ surface (Manjaiah et al., 2014) by EDM. However, to the best of our knowledge, EDM applied to Ti–Ni SMAs is hardly reported. In our previous study, TiO and TiC were formed on the surface when Ti–6Al–4V alloys were EDMed in kerosene and distilled water, respectively (Chen et al., 1999). In EDM, the work-piece is melted, vaporized, and then cooled rapidly by a dielectric fluid; this leads to the formation of a recast layer on the substrate surface. In order to improve the biocompatibility of Ti–Ni SMAs, this study aims to fabricate nitride films on Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs by dry EDM. Nitrogen gas was chosen as the dielectric fluid to improve material removal rate and keep the work-piece contamination-free. The microstructure, composition, hardness, and roughness of the EDMed surfaces were investigated.
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in Taiwan. Fig. 1 illustrates the configuration of EDM equipment. Highly pure nitrogen gas (99%) was jetted from pipe tool electrode under a pressure of 97.1 KPa as the dielectric medium. A cylindrical Ti pipe (99.7 wt.%) with outer and inner diameters of 6 mm and 5 mm, respectively, was used as the tool electrode. The rotational speed of the Ti pipe electrode was 120 rpm. In this study, EDM was used to fabricate fine-machined products. Hence pulse currents of 3, 5, and 7 A with pulse durations of 15, 30, 45, 60, and 75 s were selected. 2.3. Characteristics of Ti–Ni SMAs before and after EDM
Fig. 1. Schematic diagram of the dry EDM equipment.
2. Experimental procedure 2.1. Ti–Ni SMAs preparation The conventional tungsten arc-melting technique was employed to prepare the Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs. Ti (purity, 99.7 wt.%), Ni (purity, 99.9 wt.%), and Cr (purity, 99.8 wt.%) totaling to about 150 g were melted and remelted at least six times in argon atmosphere. In addition, pure Ti buttons were melted and used as getters. The mass loss during melting was negligible. The as-melted buttons were homogenized at 950 ◦ C for 72 h. The homogenized buttons were cut into several discs (ϕ 6 mm × 1 mm) using a low-speed diamond saw. The disk specimens were evacuated in quartz tubes and annealed at 900 ◦ C for 2 h, followed by quenching in water. Before EDM, the specimens were mechanically polished with SiC sandpaper (180, 240, 400, 600, and 800 grit). 2.2. EDM process The specimens were electrical discharge – machined using a diesinking EDM machine (model LS-250C) made by Lien-Sheng Co.
The martensitic transformation temperatures of SMAs were measured by differential scanning calorimetry (DSC, TA Q10) at a controlled heating/cooling rate of 10 ◦ C min−1 . The shape recovery effect was examined by conducting a bending test (Lin and Wu, 1992). The microstructures of the EDMed surfaces were examined with X-ray diffraction (XRD, Siemens D5000) at 2 scanning rate of 3 min−1 . The morphologies of EDMed surface were observed using scanning electron microscopy (SEM, JOEL 6330 TF). Cross-sectional specimens were prepared by polishing and subsequent etching using the following solution: HF (1 ml) + HNO3 (2 ml) + HCl (2 ml) + H2 O (100 ml). The etching periods for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 were 30 s and 90 s, respectively. A Talysurf profilometer was used to evaluate quantitatively the roughness of the EDMed surface, denoted as Ra. The cut-off was selected as 0.8 mm. For each piece, the average of the readings taken at seven points on the machining plane was considered the surface roughness value. The surface hardness was measured using a micro Vickers hardness tester (FUTURE.TECH FM.700) under a load of 20 g for 15 s. For each specimen, the average of at least five test readings was calculated. The recast layer-SMA adhesion was tested using a scratch tester (MFT-4000). 3. Results and discussion 3.1. Characteristics of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs Fig. 2(a) and (b) show the DSC curve of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 in both forward and reverse transformation, respectively. It is indicated that decreased martensite transformation temperatures (M* and A*) can be achieved by adding a small amount of Cr. Both SMAs exhibit almost 100% shape recovery at various bending strains, can be inferred from Table 1. Furthermore, Ti50 Ni49.5 Cr0.5 has higher hardness than Ti50 Ni50 SMA, which can be ascribed to Cr solid-solution hardening (Hsieh et al., 2012). This hardening phenomenon is appeared in Ti52 Ni47 Al1 alloy (Hsieh and Wu, 1999) as well.
Fig. 2. DSC curves of (a) Ti50Ni50 and (b) Ti50Ni49.5Cr0.5 SMA.
T.-S. Huang et al. / Journal of Materials Processing Technology 221 (2015) 279–284 Table 1 The measured shape recovery (RSME ), hardness, and critical load measured during scratch test of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs before and after EDM. Shape recovery, RSME (%) Bending strain, εs
2%
5%
8%
Ti50 Ni50 Ti50 Ni50 (EDM) Ti50 Ni49.5 Cr0.5 Ti50 Ni49.5 Cr0.5 (EDM)
100 95 100 94
100 87 100 84
95 67 94 65
Hardness (Hv)
Critical load (N) 5 A, 15 s
207 723 253 746
– 58.32 – 64.57
3.2. Surface morphology and composition analysis of TiNi-based SMAs after EDM Fig. 3(a) and (b), respectively, show SEM micrographs of the Ti50 Ni50 SMA surfaces after EDM under the following conditions: pulse current (IP ) = 3 A and pulse duration ( P ) = 15, 75 s. Many discharge craters, melting drops, and recast materials were observed in the EDMed surfaces. Fig. 3(c) and (d) show the EDMed surfaces of Ti50 Ni49.5 Cr0.5 SMA, which are similar to those of EDMed Ti50 Ni50 SMA. In Fig. 3(b) and (d), cracks can be seen on the surfaces of both EDMed SMAs, and cracking appears to have occurred more frequently on the EDMed surfaces subjected to longer pulse duration. Comparing these findings with results of EDM in distilled water (Hasc¸alık and C¸aydas¸, 2007) and kerosene (Tang and Du, 2014), there is no significant difference in surface characteristics of alloys EDMed in gas and liquid electrolytes. The cross-sectional SEM micrographs of the EDMed Ti50 Ni50 SMAs (Fig. 4(a) and (b)) show recast layers formed on SMA surfaces, and the recast layer thickness increases with increasing pulse duration. In addition, this phenomenon is found for EDMed Ti50 Ni49.5 Cr0.5 SMA, as shown in Fig. 4(c) and (d). The recast layer thickness depends mainly on the electrical discharge energy. Increasing the pulse duration
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results in an increase in the ratio of cation flow in the plasma channel (Dibitonto, 1989), thus increasing the amount of electrical discharge energy acting on the work-piece. Fig. 5 shows the XRD patterns of the surfaces of the EDMed SMAs. For the EDMed Ti50 Ni50 SMA, the XRD patterns show peaks from the recast layer (TiN) and the substrate (B19 and B2), while the recast layer of the EDMed Ti50 Ni49.5 Cr0.5 SMA consists of TiN and CrN. The recast layer was formed because the work-piece was melted and re-solidified in dielectric medium during EDM. The composition of the recast layer varied with work-piece material and electrolyte composition. In study of EDM Ti alloys in kerosene, kerosene is decomposed to carbon and therefore Ti alloys’ surfaces are carbonized (Hasc¸alık and C¸aydas¸, 2007). Similarly, when distilled water is used as EDM dielectric medium, water is decomposed into hydrogen and oxygen and hence Ti oxide is formed (Chen et al., 1999). In this study, electrical discharge induces nitrogen decomposition and enhances high energetic nitrogen to react with work-piece (Czerwiec and Bergmann, 1998). Moreover, work-piece is melted due to the localized high temperature, which results in increased nitridation of work-piece. The composition of biomedically used material surface is very important because it affects material’s biocompatibility. According to the results of cytotoxicity tests, TiN coatings on TiNi SMA (Jin et al., 2013) and stainless steel (Park et al., 2003) show good biocompatibility. CrN coatings on CoCr alloys show decreased cytotoxic effect on cells compared to commercially used CoCr alloys (Williams et al., 2003). 3.3. Machining characteristics and surface roughness of TiNi-based SMAs after EDM Fig. 6(a) and (b) shows the relationship between the material removal rate (MRR) and the pulse duration for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 , respectively. For both SMAs, the MRR increases with increasing pulse duration and current. In general, increasing the
Fig. 3. SEM micrographs of EDMed Ti50Ni50 SMA surfaces under (a) 3 A × 15 s and (b) 3 A × 75 s. SEM micrographs of EDMed Ti50Ni49.5Cr0.5 SMA surfaces under (c) 3 A × 15 s and (d) 3 A × 75 s.
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Fig. 4. Cross-sectional SEM micrographs of EDMed Ti50Ni50 SMA under (a) 3 A × 15 s and (b) 3 A × 75 s. Cross-sectional SEM micrographs of EDMed Ti50Ni49.5Cr0.5 SMA under (c) 3 A × 15 s and (d) 3 A × 75 s.
pulse duration can lead to sustained work-piece surface melting and, therefore, a higher MRR. Fig. 6(c) shows that the MRR of EDMed Ti50 Ni50 SMA is higher than that of Ti50 Ni49.5 Cr0.5 for pulse durations of 15–75 s. A previous study has reported that the MMR is inversely related to the product of melting temperature () and thermal conductivity (K˛) of work-piece (Lin et al., 2001). The × K˛ values of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMA are 111.8 and 310.1 W/cm ◦ C, respectively. According to Fig. 6(c), the MRRs of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 follow the abovementioned relationship. Fig. 7(a) and (b) shows that the electrode wear rate (EWR) increases with increasing pulse duration for both EDMed Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs. When the pulse duration is 15 s, the EWR is very low and no significant difference in the EWR between variant pulse current (5 A and 7 A). The low EWR is due to TiN formed on the surface of tool electrode (Ti) and the melting point of TiN (3200 ◦ C) is higher than Ti (1668 ◦ C). Theoretically, the tool
electrode has high melting temperature results in low electrode wear rate (Hasc¸alık and C¸aydas¸, 2007). In addition, the variation in EWR with pulse duration exhibits a tendency similar to that of MRR with pulse duration, as shown in Fig. 6. The EWR during the machining of Ti50 Ni49.5 Cr0.5 was lower than the EWR measured during the machining of Ti50 Ni50 under high discharge current (5 and 7 A) and long pulse duration (60 and 75 s). This is because Ti50 Ni49.5 Cr0.5 has a high × K˛ value, and it absorbs most of the thermal energy, thus reducing the heat transferred to the electrode. In addition, in this study, nitrogen gas flow from the hollow electrode helps prevent debris from adhering to the electrode. Fig. 8(a) and (b) show the change in the roughness of the EDMed surfaces with pulse duration at IP = 3, 5, 7 A for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMA, respectively. The roughness of the EDMed surface increases with increasing pulse current and pulse duration. Furthermore, the roughness is affected by the electrical discharge energy mode in EDM, and the relationship can be represented as Ra = C(Ip × p ) . The constants C and  in this empirical equation were obtained from Fig. 9. In this study, the surface roughness (Ra) of the EDMed surface for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMA follow the relationships Ra = 283.68(Ip × p )0.607 and Ra = 295.63(Ip × p )0.625 , respectively. The constants C and  are related to the electrical discharging energy mode and tool–work-piece material characteristics, for example, mechanical properties (Rebelo et al., 2000), material structure, and thermal properties (Jeswani, 1978).
3.4. Surface mechanical properties of TiNi-based SMAs after EDM
Fig. 5. XRD patterns of EDMed surfaces of Ti50Ni50 and Ti50Ni49.5Cr0.5 SMA.
According to Table 1, after EDM, the shape recovery ability of SMAs is reduced, and this reduction is attributed to increased surface hardness of the EDMed SMAs. The increased hardness is associated with TiN and CrN (Fig. 5) formed on the recast layer during EDM. Hsieh et al. (2009) concluded that after wire EDMed in distilled water, the surface hardness of TiNiZr alloy increase
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Fig. 6. Relationship between MMR and pulse duration for (a) Ti50Ni50 SMA, (b) Ti50Ni49.5Cr0.5 SMA, and (c) Ti50Ni50 and Ti50Ni49.5Cr0.5 SMAs EDMed at IP = 7 A.
Fig. 7. Relationship between EWR and pulse duration for (a) Ti50Ni50 SMA, and (b) Ti50Ni49.5Cr0.5 SMA.
from 320 Hv to 980 Hv and it is attributed to the formation of TiO2 , TiNiO3 , ZrO2 and the deposition debris in the recast layer. Lin et al. (2001) proved that the hardness of EDMed TiNiCu alloy increased from the inner to surface machined surface. The hardening effect is because of the recast layer consisting of TiO2 , TiNiO3 and debris.
Moreover, a scratch test was performed to evaluate friction and adhesion between the recast layer and the substrate. The measured critical load in Table 1 indicates the normal force at which failure occurs. The recast layer of EDMed Ti50 Ni49.5 Cr0.5 can withstand a higher critical load than that of EDMed Ti50 Ni50 . Hence, it can be
Fig. 8. Change in roughness of EDMed surfaces with pulse duration for (a) Ti50Ni50 SMA and (b) Ti50Ni49.5Cr0.5 SMA.
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Fig. 9. Surface roughness versus product of IP and P for Ti50Ni50 and Ti50Ni49.5Cr0.5 SMA.
said that the EDMed Ti50 Ni49.5 Cr0.5 surface has better wear resistance because its recast layer is harder and has higher adhesion. In Fig. 4, the substrate–recast layer interface cannot be distinguished clearly, which suggests that the recast layer has good adhesion. This is because the recast layer solidifies slowly in gas, and the stress between the recast layer and substrate is well released. 4. Conclusions In this study, Cr was added to Ti50 Ni50 shape memory alloy (SMA) to decrease its martensite transformation temperature in order to make it suitable for biomedical applications. The machining characteristics and surface properties of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs after EDM in nitrogen gas were investigated. Material removal rate, electrode wear rate, and surface roughness of Ti50 Ni49.5 Cr0.5 SMA were lower than those of Ti50 Ni50 SMA because they share an inverse relationship with the thermal conductivity of these SMAs. After EDM, the SMAs exhibited improved surface hardness while retaining good shape recovery ability. The recast layer, comprising TiN and CrN, that was well adhered on the SMAs is expected to provide good wear and corrosion resistance, and can prevent the metal substrate from contacting body fluid when EDMed SMAs are implanted in human body. Acknowledgement The authors sincerely acknowledge the financial support of this research by the National Science Council (NSC), Republic of China, under Grants NSC 101-2221-E-151-009 and NSC 102-2221-E-151013. References Chen, S.L., Yan, B.H., Huang, F.Y., 1999. Influence of kerosene and distilled water as dielectrics on the electric discharge machining characteristics of Ti–6Al–4V. J. Mater. Process. Technol. 87, 107–111.
Czerwiec, T., Bergmann, M.H., 1998. Low-pressure, high-density plasma nitriding: mechanisms, technology and results. Surf. Coat. Technol. 108–109, 181–182. Dibitonto, D.D., 1989. Theoretical models of the electrical discharge machining process. I: Simple cathode erosion model. J. Appl. Phys. 66, 4095–4103. Duerig, T., Pelton, A., Stockel, D., 1999. An overview of nitinol medical applications. Mater. Sci. Eng. A 273, 149–160. Hasc¸alık, A., C¸aydas¸, U., 2007. Electrical discharge machining of titanium alloy (Ti–6Al–4V). Appl. Surf. Sci. 253, 9007–9016. Hsieh, S.F., Chen, S.L., Lin, H.C., Lin, M.H., Chiou, S.Y., 2009. The machining characteristics and shape recovery ability of Ti–Ni–X (X = Zr, Cr) ternary shape memory alloys using the wire electro-discharge machining. Int. J. Mach. Tool. Manuf. 49, 509–514. Hsieh, S.F., Chen, S.L., Lin, H.C., Lin, M.H., Huang, J.H., Lin, M.C., 2012. A study of TiNiCr ternary shape memory alloys. J. Alloy Compd. 464, 155–160. Hsieh, S.F., Wu, S.K., 1999. A study on a Ti52 Ni47 Al1 shape memory alloy. J. Mater. Sci. 34, 1659–1665. Van Humbeeck, J., 1999. Non-medical applications of shape memory alloys. Mater. Sci. Eng. A 273, 134–148. Jeswani, M.L., 1978. Roughness and wear characteristics of spark-eroded surfaces. Wear 51, 227–236. Jin, S., Zhang, Y., Wang, Q., Zhang, D., Zhang, S., 2013. Influence of TiN coating on the biocompatibility of medical NiTi alloy. Colloids Surf. B 101, 343–349. Lin, H.C., Lin, K.M., Cheng, I.C., 2001. The electro-discharge machining characteristics of TiNi shape memory alloys. J. Mater. Sci. 36, 399–404. Lin, H.C., Wu, S.K., 1992. Scripta Metall. Mater. 2, 59–62. Liu, Y., Xiang, H., 1998. Apparent modulus of elasticity of near-equiatomic NiTi. J. Alloys Compd. 270, 154–159. Lobo, A.O., Otubo, J., Matsushima, J.T., Corat, E.J., 2011. Rapid obtaining of nanohydroxyapatite bioactive films on NiTi shape memory alloy by electrodeposition process. J. Mater. Eng. Perform. 20, 793–797. Manjaiah, M., Narendranath, S., Basavarajappa, S., 2014. A review on machining of titanium based alloys using EDM and WEDM. Rev. Adv. Mater. Sci. 36, 89–111. Otsuka, K., Ren, X., 2005. Physical metallurgy of Ti–Ni-based shape memory alloys. Prog. Mater. Sci. 50, 511–678. Park, J., Kim, D.-J., Kim, Y.-K., Lee, K.-H., Lee, K.-H., Lee, H., Ahn, S., 2003. Improvement of the biocompatibility and mechanical properties of surgical tools with TiN coating by PACVD. Thin Solid Films 435, 102–107. Piscanec, S., Ciacchi, L.C., Vesselli, E., Comelli, G., Sbaizero, O., Meriani, S., De Vita, A., 2004. Bioactivity of TiN-coated titanium implants. Acta Mater. 52, 1237–1245. Qiu, D., Wang, A., Yin, Y., 2010. Characterization and corrosion behavior of hydroxyapatite/zirconia composite coating on NiTi fabricated by electrochemical deposition. Appl. Surf. Sci. 257, 1774–1778. Qiu, D., Yang, L., Yin, Y., Wang, A., 2011. Preparation and characterization of hydroxyapatite/titania composite coating on NiTi alloy by electrochemical deposition. Surf. Coat. Technol. 205, 3280–3284. Rebelo, J.C., Dias, A.M., Mesquita, R., Vassalo, P., Santos, M., 2000. An experimental study on electro-discharge machining and polishing of high strength copper–beryllium alloy. J. Mater. Process. Technol. 103, 389–397. Starosvetsky, D., Gotman, I., 2001. Corrosion behavior of titanium nitride coated Ni–Ti shape memory surgical alloy. Biomaterials 22, 1853–1859. Sun, T., Lee, W.C., Wang, M., 2011. A comparative study of apatite coating and apatite/collagen composite coating fabricated on NiTi shape memory alloy through electrochemical deposition. Mater. Lett. 65, 2575–2577. Tang, L., Du, Y.T., 2014. Experimental study on green electrical discharge machining in tap water of Ti–6Al–4V and parameters optimization. Int. J. Adv. Manuf. Technol. 70, 469–475. Wever, D.J., Veldhuizen, A.G., Vries, J., de Busscher, H.J., Uges, D.R., Van Horn, J.R., 1998. Electrochemical and surface characterization of a nickel–titanium alloy. Biomaterials 19, 761–769. Williams, S., Tipper, J.L., Ingham, E., Stone, M.H., Fisher, J., 2003. In vitro analysis of the wear, wear debris and biological activity of surface-engineered coatings for use in metal-on-metal total hip replacements. Proc Inst. Mech. Eng. H 217, 155–163. Wu, S.K., Lin, H.C., Lee, C.Y., 1999. Gas nitriding of an equiatomic TiNi shape memory alloy-II. Hardness, wear and shape memory ability. Surf. Coat. Technol. 113, 13–16. Yoneyama, T., Doi, H., Hamanaka, H., Okamoto, Y., Mogi, M., Miura, F., 1992. Superelasticity and thermal behavior of Ni–Ti alloy orthodontic arch wires. Dent. Mater. J. 11, 1–10.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Surface modification of TiNi-based shape memory alloys by dry electrical discharge machining Tyau-Song Huang a , Shy-Feng Hsieh a , Sung-Long Chen b , Ming-Hong Lin b , Shih-Fu Ou a,∗ , Wei-Tse Chang a a b
Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan
a r t i c l e
i n f o
Article history: Received 3 September 2014 Received in revised form 2 February 2015 Accepted 11 February 2015 Available online 24 February 2015 Keywords: Shape memory alloys Dry electrical discharge machining Surface modification
a b s t r a c t This study attempts to decrease the martensite transformation temperature of Ti50 Ni50 shape memory alloy (SMA) for its use in biomedical applications by Cr addition. In addition, surface modification of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs using electrical discharge machining (EDM) is proposed. Nitrogen gas is used as a dielectric medium, and a pure titanium pipe is used as the tool electrode. The machining characteristics and surface properties of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs after EDM in nitrogen gas were investigated. Many electrical discharge craters and recast materials are observed on the EDMed surface of these SMAs. Material removal rate, electrode wear rate, and surface roughness increase with increasing pulse current and duration, and they share an inverse relationship with the thermal conductivity of these SMAs. After EDM, the SMAs continue to exhibit good shape recovery, and even the recast layers have high surface hardness. The recast layers, comprising TiN and CrN, with high hardness and good adhesion are expected to improve the SMAs’ wear and corrosion resistance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Shape memory alloys (SMAs) are used in many fields such as mechanical, aerospace, and military applications (Van Humbeeck, 1999). From among SMAs, Ti–Ni SMAs are widely applied in biomedical engineering because of their unique superelasticity (Otsuka and Ren, 2005), superior shape memory effect (Yoneyama et al., 1992), excellent corrosion resistance (Liu and Xiang, 1998), low elastic modulus (Wever et al., 1998), and non-ferromagnetic property (Duerig et al., 1999). However, from the viewpoint of longterm implantation, Ni release from Ti–Ni SMAs may cause allergy in the human body. Therefore, proper surface treatment of Ti–Ni implants is important to reduce Ni release and improve corrosion resistance. Hydroxyapatite (Lobo et al., 2011), zirconia (Qiu et al., 2010), titanium oxide (Qiu et al., 2011), titanium nitride (Jin et al., 2013), and apatite-collagen composite (Sun et al., 2011), are coated on Ti–Ni SMAs to improve the biocompatibility. Among these coating,
∗ Corresponding author. Tel.: +886 7 3814526x5415; fax: +886 7 3814526. E-mail addresses: [email protected] (T.-S. Huang), [email protected] (S.-F. Hsieh), [email protected] (S.-L. Chen), [email protected] (M.-H. Lin), [email protected] (S.-F. Ou), [email protected] (W.-T. Chang). http://dx.doi.org/10.1016/j.jmatprotec.2015.02.025 0924-0136/© 2015 Elsevier B.V. All rights reserved.
titanium nitride (TiN), possessing high hardness and good wear resistance, has been used for hip prostheses (Piscanec et al., 2004). Therefore, many surface treatment technologies, i.e., gas nitriding (Wu et al., 1999), filtered arcing ion plate technique (Jin et al., 2013), and powder immersion reaction assisted coating (Starosvetsky and Gotman, 2001), are proposed to coat TiN on Ti–Ni SMA surfaces. In order to develop better surface treatment technologies, the present study introduce a method to coat TiN on Ti–Ni SMAs by dry electrical discharge machining (EDM). Until now, many researchers have attempted to modify titanium and its alloys’ surface (Manjaiah et al., 2014) by EDM. However, to the best of our knowledge, EDM applied to Ti–Ni SMAs is hardly reported. In our previous study, TiO and TiC were formed on the surface when Ti–6Al–4V alloys were EDMed in kerosene and distilled water, respectively (Chen et al., 1999). In EDM, the work-piece is melted, vaporized, and then cooled rapidly by a dielectric fluid; this leads to the formation of a recast layer on the substrate surface. In order to improve the biocompatibility of Ti–Ni SMAs, this study aims to fabricate nitride films on Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs by dry EDM. Nitrogen gas was chosen as the dielectric fluid to improve material removal rate and keep the work-piece contamination-free. The microstructure, composition, hardness, and roughness of the EDMed surfaces were investigated.
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in Taiwan. Fig. 1 illustrates the configuration of EDM equipment. Highly pure nitrogen gas (99%) was jetted from pipe tool electrode under a pressure of 97.1 KPa as the dielectric medium. A cylindrical Ti pipe (99.7 wt.%) with outer and inner diameters of 6 mm and 5 mm, respectively, was used as the tool electrode. The rotational speed of the Ti pipe electrode was 120 rpm. In this study, EDM was used to fabricate fine-machined products. Hence pulse currents of 3, 5, and 7 A with pulse durations of 15, 30, 45, 60, and 75 s were selected. 2.3. Characteristics of Ti–Ni SMAs before and after EDM
Fig. 1. Schematic diagram of the dry EDM equipment.
2. Experimental procedure 2.1. Ti–Ni SMAs preparation The conventional tungsten arc-melting technique was employed to prepare the Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs. Ti (purity, 99.7 wt.%), Ni (purity, 99.9 wt.%), and Cr (purity, 99.8 wt.%) totaling to about 150 g were melted and remelted at least six times in argon atmosphere. In addition, pure Ti buttons were melted and used as getters. The mass loss during melting was negligible. The as-melted buttons were homogenized at 950 ◦ C for 72 h. The homogenized buttons were cut into several discs (ϕ 6 mm × 1 mm) using a low-speed diamond saw. The disk specimens were evacuated in quartz tubes and annealed at 900 ◦ C for 2 h, followed by quenching in water. Before EDM, the specimens were mechanically polished with SiC sandpaper (180, 240, 400, 600, and 800 grit). 2.2. EDM process The specimens were electrical discharge – machined using a diesinking EDM machine (model LS-250C) made by Lien-Sheng Co.
The martensitic transformation temperatures of SMAs were measured by differential scanning calorimetry (DSC, TA Q10) at a controlled heating/cooling rate of 10 ◦ C min−1 . The shape recovery effect was examined by conducting a bending test (Lin and Wu, 1992). The microstructures of the EDMed surfaces were examined with X-ray diffraction (XRD, Siemens D5000) at 2 scanning rate of 3 min−1 . The morphologies of EDMed surface were observed using scanning electron microscopy (SEM, JOEL 6330 TF). Cross-sectional specimens were prepared by polishing and subsequent etching using the following solution: HF (1 ml) + HNO3 (2 ml) + HCl (2 ml) + H2 O (100 ml). The etching periods for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 were 30 s and 90 s, respectively. A Talysurf profilometer was used to evaluate quantitatively the roughness of the EDMed surface, denoted as Ra. The cut-off was selected as 0.8 mm. For each piece, the average of the readings taken at seven points on the machining plane was considered the surface roughness value. The surface hardness was measured using a micro Vickers hardness tester (FUTURE.TECH FM.700) under a load of 20 g for 15 s. For each specimen, the average of at least five test readings was calculated. The recast layer-SMA adhesion was tested using a scratch tester (MFT-4000). 3. Results and discussion 3.1. Characteristics of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs Fig. 2(a) and (b) show the DSC curve of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 in both forward and reverse transformation, respectively. It is indicated that decreased martensite transformation temperatures (M* and A*) can be achieved by adding a small amount of Cr. Both SMAs exhibit almost 100% shape recovery at various bending strains, can be inferred from Table 1. Furthermore, Ti50 Ni49.5 Cr0.5 has higher hardness than Ti50 Ni50 SMA, which can be ascribed to Cr solid-solution hardening (Hsieh et al., 2012). This hardening phenomenon is appeared in Ti52 Ni47 Al1 alloy (Hsieh and Wu, 1999) as well.
Fig. 2. DSC curves of (a) Ti50Ni50 and (b) Ti50Ni49.5Cr0.5 SMA.
T.-S. Huang et al. / Journal of Materials Processing Technology 221 (2015) 279–284 Table 1 The measured shape recovery (RSME ), hardness, and critical load measured during scratch test of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs before and after EDM. Shape recovery, RSME (%) Bending strain, εs
2%
5%
8%
Ti50 Ni50 Ti50 Ni50 (EDM) Ti50 Ni49.5 Cr0.5 Ti50 Ni49.5 Cr0.5 (EDM)
100 95 100 94
100 87 100 84
95 67 94 65
Hardness (Hv)
Critical load (N) 5 A, 15 s
207 723 253 746
– 58.32 – 64.57
3.2. Surface morphology and composition analysis of TiNi-based SMAs after EDM Fig. 3(a) and (b), respectively, show SEM micrographs of the Ti50 Ni50 SMA surfaces after EDM under the following conditions: pulse current (IP ) = 3 A and pulse duration ( P ) = 15, 75 s. Many discharge craters, melting drops, and recast materials were observed in the EDMed surfaces. Fig. 3(c) and (d) show the EDMed surfaces of Ti50 Ni49.5 Cr0.5 SMA, which are similar to those of EDMed Ti50 Ni50 SMA. In Fig. 3(b) and (d), cracks can be seen on the surfaces of both EDMed SMAs, and cracking appears to have occurred more frequently on the EDMed surfaces subjected to longer pulse duration. Comparing these findings with results of EDM in distilled water (Hasc¸alık and C¸aydas¸, 2007) and kerosene (Tang and Du, 2014), there is no significant difference in surface characteristics of alloys EDMed in gas and liquid electrolytes. The cross-sectional SEM micrographs of the EDMed Ti50 Ni50 SMAs (Fig. 4(a) and (b)) show recast layers formed on SMA surfaces, and the recast layer thickness increases with increasing pulse duration. In addition, this phenomenon is found for EDMed Ti50 Ni49.5 Cr0.5 SMA, as shown in Fig. 4(c) and (d). The recast layer thickness depends mainly on the electrical discharge energy. Increasing the pulse duration
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results in an increase in the ratio of cation flow in the plasma channel (Dibitonto, 1989), thus increasing the amount of electrical discharge energy acting on the work-piece. Fig. 5 shows the XRD patterns of the surfaces of the EDMed SMAs. For the EDMed Ti50 Ni50 SMA, the XRD patterns show peaks from the recast layer (TiN) and the substrate (B19 and B2), while the recast layer of the EDMed Ti50 Ni49.5 Cr0.5 SMA consists of TiN and CrN. The recast layer was formed because the work-piece was melted and re-solidified in dielectric medium during EDM. The composition of the recast layer varied with work-piece material and electrolyte composition. In study of EDM Ti alloys in kerosene, kerosene is decomposed to carbon and therefore Ti alloys’ surfaces are carbonized (Hasc¸alık and C¸aydas¸, 2007). Similarly, when distilled water is used as EDM dielectric medium, water is decomposed into hydrogen and oxygen and hence Ti oxide is formed (Chen et al., 1999). In this study, electrical discharge induces nitrogen decomposition and enhances high energetic nitrogen to react with work-piece (Czerwiec and Bergmann, 1998). Moreover, work-piece is melted due to the localized high temperature, which results in increased nitridation of work-piece. The composition of biomedically used material surface is very important because it affects material’s biocompatibility. According to the results of cytotoxicity tests, TiN coatings on TiNi SMA (Jin et al., 2013) and stainless steel (Park et al., 2003) show good biocompatibility. CrN coatings on CoCr alloys show decreased cytotoxic effect on cells compared to commercially used CoCr alloys (Williams et al., 2003). 3.3. Machining characteristics and surface roughness of TiNi-based SMAs after EDM Fig. 6(a) and (b) shows the relationship between the material removal rate (MRR) and the pulse duration for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 , respectively. For both SMAs, the MRR increases with increasing pulse duration and current. In general, increasing the
Fig. 3. SEM micrographs of EDMed Ti50Ni50 SMA surfaces under (a) 3 A × 15 s and (b) 3 A × 75 s. SEM micrographs of EDMed Ti50Ni49.5Cr0.5 SMA surfaces under (c) 3 A × 15 s and (d) 3 A × 75 s.
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Fig. 4. Cross-sectional SEM micrographs of EDMed Ti50Ni50 SMA under (a) 3 A × 15 s and (b) 3 A × 75 s. Cross-sectional SEM micrographs of EDMed Ti50Ni49.5Cr0.5 SMA under (c) 3 A × 15 s and (d) 3 A × 75 s.
pulse duration can lead to sustained work-piece surface melting and, therefore, a higher MRR. Fig. 6(c) shows that the MRR of EDMed Ti50 Ni50 SMA is higher than that of Ti50 Ni49.5 Cr0.5 for pulse durations of 15–75 s. A previous study has reported that the MMR is inversely related to the product of melting temperature () and thermal conductivity (K˛) of work-piece (Lin et al., 2001). The × K˛ values of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMA are 111.8 and 310.1 W/cm ◦ C, respectively. According to Fig. 6(c), the MRRs of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 follow the abovementioned relationship. Fig. 7(a) and (b) shows that the electrode wear rate (EWR) increases with increasing pulse duration for both EDMed Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs. When the pulse duration is 15 s, the EWR is very low and no significant difference in the EWR between variant pulse current (5 A and 7 A). The low EWR is due to TiN formed on the surface of tool electrode (Ti) and the melting point of TiN (3200 ◦ C) is higher than Ti (1668 ◦ C). Theoretically, the tool
electrode has high melting temperature results in low electrode wear rate (Hasc¸alık and C¸aydas¸, 2007). In addition, the variation in EWR with pulse duration exhibits a tendency similar to that of MRR with pulse duration, as shown in Fig. 6. The EWR during the machining of Ti50 Ni49.5 Cr0.5 was lower than the EWR measured during the machining of Ti50 Ni50 under high discharge current (5 and 7 A) and long pulse duration (60 and 75 s). This is because Ti50 Ni49.5 Cr0.5 has a high × K˛ value, and it absorbs most of the thermal energy, thus reducing the heat transferred to the electrode. In addition, in this study, nitrogen gas flow from the hollow electrode helps prevent debris from adhering to the electrode. Fig. 8(a) and (b) show the change in the roughness of the EDMed surfaces with pulse duration at IP = 3, 5, 7 A for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMA, respectively. The roughness of the EDMed surface increases with increasing pulse current and pulse duration. Furthermore, the roughness is affected by the electrical discharge energy mode in EDM, and the relationship can be represented as Ra = C(Ip × p ) . The constants C and  in this empirical equation were obtained from Fig. 9. In this study, the surface roughness (Ra) of the EDMed surface for Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMA follow the relationships Ra = 283.68(Ip × p )0.607 and Ra = 295.63(Ip × p )0.625 , respectively. The constants C and  are related to the electrical discharging energy mode and tool–work-piece material characteristics, for example, mechanical properties (Rebelo et al., 2000), material structure, and thermal properties (Jeswani, 1978).
3.4. Surface mechanical properties of TiNi-based SMAs after EDM
Fig. 5. XRD patterns of EDMed surfaces of Ti50Ni50 and Ti50Ni49.5Cr0.5 SMA.
According to Table 1, after EDM, the shape recovery ability of SMAs is reduced, and this reduction is attributed to increased surface hardness of the EDMed SMAs. The increased hardness is associated with TiN and CrN (Fig. 5) formed on the recast layer during EDM. Hsieh et al. (2009) concluded that after wire EDMed in distilled water, the surface hardness of TiNiZr alloy increase
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Fig. 6. Relationship between MMR and pulse duration for (a) Ti50Ni50 SMA, (b) Ti50Ni49.5Cr0.5 SMA, and (c) Ti50Ni50 and Ti50Ni49.5Cr0.5 SMAs EDMed at IP = 7 A.
Fig. 7. Relationship between EWR and pulse duration for (a) Ti50Ni50 SMA, and (b) Ti50Ni49.5Cr0.5 SMA.
from 320 Hv to 980 Hv and it is attributed to the formation of TiO2 , TiNiO3 , ZrO2 and the deposition debris in the recast layer. Lin et al. (2001) proved that the hardness of EDMed TiNiCu alloy increased from the inner to surface machined surface. The hardening effect is because of the recast layer consisting of TiO2 , TiNiO3 and debris.
Moreover, a scratch test was performed to evaluate friction and adhesion between the recast layer and the substrate. The measured critical load in Table 1 indicates the normal force at which failure occurs. The recast layer of EDMed Ti50 Ni49.5 Cr0.5 can withstand a higher critical load than that of EDMed Ti50 Ni50 . Hence, it can be
Fig. 8. Change in roughness of EDMed surfaces with pulse duration for (a) Ti50Ni50 SMA and (b) Ti50Ni49.5Cr0.5 SMA.
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Fig. 9. Surface roughness versus product of IP and P for Ti50Ni50 and Ti50Ni49.5Cr0.5 SMA.
said that the EDMed Ti50 Ni49.5 Cr0.5 surface has better wear resistance because its recast layer is harder and has higher adhesion. In Fig. 4, the substrate–recast layer interface cannot be distinguished clearly, which suggests that the recast layer has good adhesion. This is because the recast layer solidifies slowly in gas, and the stress between the recast layer and substrate is well released. 4. Conclusions In this study, Cr was added to Ti50 Ni50 shape memory alloy (SMA) to decrease its martensite transformation temperature in order to make it suitable for biomedical applications. The machining characteristics and surface properties of Ti50 Ni50 and Ti50 Ni49.5 Cr0.5 SMAs after EDM in nitrogen gas were investigated. Material removal rate, electrode wear rate, and surface roughness of Ti50 Ni49.5 Cr0.5 SMA were lower than those of Ti50 Ni50 SMA because they share an inverse relationship with the thermal conductivity of these SMAs. After EDM, the SMAs exhibited improved surface hardness while retaining good shape recovery ability. The recast layer, comprising TiN and CrN, that was well adhered on the SMAs is expected to provide good wear and corrosion resistance, and can prevent the metal substrate from contacting body fluid when EDMed SMAs are implanted in human body. Acknowledgement The authors sincerely acknowledge the financial support of this research by the National Science Council (NSC), Republic of China, under Grants NSC 101-2221-E-151-009 and NSC 102-2221-E-151013. References Chen, S.L., Yan, B.H., Huang, F.Y., 1999. Influence of kerosene and distilled water as dielectrics on the electric discharge machining characteristics of Ti–6Al–4V. J. Mater. Process. Technol. 87, 107–111.
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