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A study on the machining characteristics of TiNi shape memory alloys
Journal of Materials Processing Technology 105 (2000) 327±332
A study on the machining characteristics of TiNi shape memory alloys H.C. Lin*, K.M. Lin, Y.C. Chen Department of Materials Science, Feng Chia University, Taichung, Taiwan 400, PR China Received 16 May 1999
Abstract The machining characteristics of TiNi shape memory alloys (SMAs) have been studied. The time needed to cut TiNi SMAs is found to decrease with increasing blade rotation speed and cutting load in the early period and then approach constant values at higher blade rotation speeds and cutting loads. These features are ascribed to the effects of strain hardening, fatigue hardening and high strain-rate hardening. The Buehler 4217 emery blade being always composed of new cutting edges of SiC and Al2O3 powders, exhibits a better cutting rate than a diamond blade to cut TiNi SMAs. A high-speed steel drill coated with TiN ®lm exhibits a better drilling ability than a high-speed steel drill for the TiNi SMAs because the TiN ®lm has high hardness and excellent wear resistance. further, a tungsten±carbide drill exhibits the best drilling ability. The drilling forces for TiNi SMAs can exceed 5000 N which are higher than those for many commercial alloys. This feature may arise from the high toughness and viscosity, and the unique pseudoelasticity of TiNi SMAs. Wavy tracks appearing on the drilled surface are ascribed to sliding wear between the blunt drill and the drilled surface of TiNi SMAs. These wavy tracks will increase the frictional force, promote the vibration of the drilling machine and cause damage of the twist drill. Plastic deformation occurs during drilling and hence the specimen's hardness near to the drilled holes can reach 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. At the same time Ti50Ni50 SMA exhibits better drilling characteristics than Ti49Ni51 SMA. # 2000 Elsevier Science B.V. All rights reserved. Keywords: TiNi shape memory alloys; Mechanical cutting; Drilling
1. Introduction TiNi alloys are an important class of shape memory alloys (SMAs). They exhibit not only the shape memory effect (SME) [1] but also unusual pseudoelasticity [2,3] and high damping capacities [4,5]. These properties along with their superior ductility, fatigue strength and corrosion resistance have resulted in many applications. The basic characteristics of TiNi SMAs involving transformational crystallography, shape memory phenomena and the effects of thermomechanical treatments have been investigated intensively [6±20]. However, impediments to their development are caused by dif®culties 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 [1] but the severe strain hardening, high toughness and viscosity, and the unique pseudoelastic behavior have caused the machining characteristics of TiNi SMAs to be quite complicated. To overcome this dif®culty some special techniques have been used to machine TiNi SMAs such as laser machining, electric-discharge and wire-cut machining. *
Corresponding author. Tel.: 4-451-7250 ext. 5301; fax: 4-451-0014. E-mail address: [email protected] (H.C. Lin).
However, even these special machining techniques have some technical limits. As an approach is made to a solution to these problems, an understanding of the conventional machining characteristics of TiNi SMAs becomes important. To the best of the author's knowledge no systematic investigation of the conventional machining characteristics of TiNi SMAs has yet been reported. In the present study the aim is to investigate the machining characteristics of TiNi SMAs involving mechanical cutting and drilling and to discuss their optimal machining parameters. 2. Experimental procedure The conventional tungsten arc-melting technique was employed to prepare the Ti50Ni50 and Ti49Ni51 shape memory alloys. Pure titanium (99.7 wt.%) and nickel (99.98 wt.%), totaling nearly 120 g were melted and remelted at least six times in an argon atmosphere. The mass loss during the melting was negligible. The as-melted buttons were homogenized at 10508C for 24 h and then quenched in water. Specimens for mechanical cutting and drilling were carefully prepared from these buttons. These specimens were vacuum-sealed in quartz tubes, annealed at 8008C for 2 h and then quenched in water.
0924-0136/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 5 6 - 7
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H.C. Lin et al. / Journal of Materials Processing Technology 105 (2000) 327±332
The mechanical cutting was carried out by using an Isomet 2000 Precision Saw made by Buehler Co. USA and equipped with an automatic cooling and saw-grinding system. The rotation speed and applied load of the cutting saws (Isocut diamond blade and Buehler 4217 emery blade) are controlled digitally. The lubricant used during cutting was nine parts water to one part Buehler Isocut Plus ¯uid. The mechanical drilling was carried out by using a radial drilling machine made by Leadwell Co. USA and equipped with a dynamometer to measure the drilling force. The rotational speed and feed rate of the twist drills are controlled digitally. Three kinds of twist drills, a high-speed steel drill (HSS), high-speed steel drill coated with TiN ®lm (HSS TiN) and a tungsten±carbide drill (TC) were used in this study. The surface morphologies of the machined specimens were observed by stereomicroscopy. Specimens for hardness tests were polished mechanically and measured in a micro-Vickers hardness tester with a 500 g load. For each specimen the average hardness value was obtained from at least ®ve test readings. 3. Results and discussion 3.1. The mechanical cutting of Ti50Ni50 and Ti49Ni51 SMAs To understand the machining characteristics of Ti50Ni50 and Ti49Ni51 SMAs some important mechanical properties of these alloys are presented in Table 1. From this table one can ®nd that the Ti50Ni50 and Ti49Ni51 SMAs exhibit B190 martensite and B2 parent phases, respectively. The Ti49Ni51 SMA has higher hardness and yield strength than Ti50Ni50 SMA. Also, both Ti50Ni50 and Ti49Ni51 SMAs exhibit excellent ductility. In this investigation 304 stainless steel (commercial available) is used as a comparative material. The experimental results show that the mechanical cutting rate of 304 stainless steel is twice as high as than that of Ti50Ni50 SMA [21], although their mechanical properties are similar as presented in Table 1. Hence, it is quite interesting and valuable to understand the machining characteristics of Ti50Ni50 and Ti49Ni51 SMAs. Fig. 1(a) and (b) show the time needed to cut the Ti50Ni50 and Ti49Ni51 specimens with a 5 mm 5 mm cross-sectional area, respectively using cutting saws of a diamond blade and a Buehler 4217 emery blade at various cutting Table 1 The important mechanical properties of Ti50Ni50, Ti49Ni51 SMAs and 304 stainless steel Mechanical properties at room temperature
Ti50Ni50
Ti49Ni51
304 Stainless steel
Structure (phase) Hardness (HV) Yield strength (MPa) Elongation (%)
B190 phase 200 120 25
B2 phase 275 250 22
FCC austenite 190 120 27
Fig. 1. The time needed to cut TiNi specimens of 5 mm 5 mm crosssectional area at various cutting loads and 3000 rpm blade rotational speed: (a) Ti50Ni50; (b) Ti49Ni51.
loads and 3000 rpm blade rotation and speed. In Fig. 1 the cutting time for both the Ti50Ni50 and Ti49Ni51 specimens is found to decrease with increasing cutting load in the early period and then approach constant values at higher cutting loads. This means that at constant blade rotational speed a higher cutting load can not effectively improve the cutting rate of TiNi SMAs. This phenomenon can be ascribed to the strain hardening and fatigue hardening effects [22] occurring at higher cutting loads as noted in Fig. 2, in which ®gure shows the stress±strain curves of TiNi SMA under the tensile (monotonic) test and cyclic (fatigue) test [22]. From Fig. 2, for strain 0:015 strain hardening occurs in the specimen of the monotonic test as shown in region A. However, both strain hardening and fatigue hardening are induced in the specimen of the fatigue test as shown in regions A and B, respectively. It is believed that the vibration of the cutting machine during the cutting process will cause fatigue in the TiNi SMAs because the vibration will induce a repeated cyclic load on the specimen. Therefore, both strain hardening and fatigue hardening concurrently cause a severe hardening effect and impair the cutting rate. This feature is especially true for higher cutting loads. Fig. 3(a) and (b) show the time needed to cut the Ti50Ni50 and Ti49Ni51 specimens, respectively at 300 g cutting load
H.C. Lin et al. / Journal of Materials Processing Technology 105 (2000) 327±332
Fig. 2. Stress±strain curves showing the phenomena of strain hardening and fatigue hardening appearing in TiNi alloy [22].
and various blade rotational speeds. In Fig. 3 the cutting time for both Ti50Ni50 and Ti49Ni51 specimens is found to decrease slightly with increasing blade rotational speed in the early period and then approach constant values at higher blade rotational speeds. In general, less time is required to cut commercial alloys (such as carbon steels or aluminium alloys) using a higher blade cutting speed. The distinctive phenomenon shown in Fig. 3, namely no obvious improvement of cutting rate by increasing the blade rotational speed
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may be ascribed to the high strain-rate hardening of TiNi SMAs [23,24]. Namely, the specimen's hardness prior to encountering the cutting edge will increase with increasing rotational speed of cutting blade. This strain-rate hardening will reduce the cutting rate, although a higher blade rotational speed can have a higher cutting frequency with TiNi specimens. This means that a higher blade rotational speed, for example 3000 rpm in this study cannot effectively increase the mechanical cutting rate of TiNi SMAs due to the high strain-rate hardening effect. Carefully examining the cutting time needed for TiNi SMAs in Figs. 1 and 3 one can easily see that the Buehler 4217 emery blade exhibits a better cutting rate than the diamond blade. This feature can be explained as below. Because of their high ductility some cut fragments of TiNi SMAs will adhere on the diamond blade during cutting [25]. These adhering fragments will impede further cutting. The greater the continuous cutting area, the more the number of adhered fragments that exist and hence the lower the cutting rate of the diamond blade. However, the Buehler 4217 emery blade being composed of the SiC and Al2O3 powders always maintains new cutting edges of SiC and Al2O3 powders due to the wear damage of the emery blade. These new cutting edges of Buehler 4217 emery blade will exhibit an excellent cutting ability and reduce the cutting time needed for TiNi SMAs. Further, from Figs. 1 and 3 it can also be seen that the cutting time needed for Ti50Ni50 and Ti49Ni51 SMAs are similar. This indicates that the Ti50Ni50 and Ti49Ni51 SMAs have the same cutting characteristics, although they exhibit the B190 martensite and B2 parent phases at room temperature, respectively. 3.2. The mechanical drilling of Ti50Ni50 and Ti49Ni51 SMAs
Fig. 3. The time needed to cut TiNi specimens of a 5 mm 5 mm crosssectional area at 300 g cutting load and various blade rotational speeds: (a) Ti50Ni50; (b) Ti49Ni51.
Table 2 presents the maximum depths of Ti50Ni50 and Ti49Ni51 SMAs that can be drilled by using three kinds of twist drills for nine settings of the drilling parameters. In Table 2 one can ®nd that the optimal drilling parameters for maximum drill depth are 163 rpm rotational speed and 0.07 mm/rev of feed rate, for the drill machine used in this study. One can also ®nd in Table 2 that the HSS TiN drill exhibits a better drilling ability than the HSS drill. Fig. 4(a) and (b) show the drilling forces of Ti50Ni50 and Ti49Ni51 SMAs, respectively when using HSS and HSS TiN twist drills. In Fig. 4 the increasing rates of drilling force, namely the upward slopes of the curve of drilling force versus drilling time of the HSS TiN drill are smaller than those of the HSS drill for both Ti50Ni50 and Ti49Ni51 SMAs. These features are ascribed to the high hardness and excellent wear resistance of the TiN ®lm coated to the HSS drill. The TiN ®lm can reduce both drilling force and wear damage, and hence improve the drilling ability of the HSS TiN drill for TiNi SMAs. In Fig. 4 the drilling forces for both Ti50Ni50 and Ti49Ni51 SMAs can exceed 5000 N which are higher than those for many commercial alloys. For example, the drilling force for S45C carbon steel is about 980 N. This
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Table 2 The maximum depths of Ti50Ni50 and Ti49Ni51 SMAs that can be drilled using three kinds of twist drills with nine settings of the drilling parameters Rotation speed (rpm)
88 88 88 163 163 163 305 305 305
Feeding rate (mm/rev)
0.07 0.13 0.22 0.07 0.13 0.22 0.07 0.13 0.22
Ti50Ni50 (mm)
Ti49Ni51 (mm)
HSS
HSSTiN
TC
HSS
HSSTiN
TC
4.476 2.600 2.332 11.799 10.891 3.066 10.658 8.635 3.146
4.052 2.668 1.851 22.943 12.151 3.131 12.224 9.414 8.367
± ± ± 39.596 ± ± ± ± ±
2.077 1.974 1.822 8.698 4.570 3.822 4.484 4.717 2.900
2.075 1.838 1.878 12.265 3.313 2.613 2.594 2.567 2.961
± ± ± 28.168 ± ± ± ± ±
feature may arise from their high toughness and viscosity, and unique pseudoelasticity. In Table 2 it can also be seen that the tungsten±carbide twist drill exhibits the best drilling ability as it has the largest drilling depth. The high hardness and strength of the tungsten±carbide drill are responsible for its excellent ability in the drilling of TiNi SMAs. However, the disadvantage of the tungsten±carbide drill are its brittle property and high cost. In Table 2 the maximum drilling depths for Ti50Ni50 SMA are found to be larger than those
for Ti49Ni51 SMA for all three kinds of twist drills. Not only the higher hardness but also the pseudoelasticity of Ti49Ni51 SMA is responsible for this phenomenon. The unique pseudoelasticity of Ti49Ni51 SMA will accommodate the applied strain near to the tip of twist drill and hence retard the progression of drilling. Fig. 5 shows the drilled surface morphology of TiNi SMAs when the twist drill has been retarded, numerous wavy tracks appearing on the drilled surface. These wavy tracks can be ascribed to the action of adhesive junction, abrasive deformation and wear chips, and the mechanical locking of surface pits during the sliding wear between the blunt drill and the drilled surface of the TiNi SMAs [24]. These wavy tracks will increase the frictional force, promote the vibration of the drilling machine and cause damage of the twist drill. Fig. 6 shows the specimen's hardness near to the drilled holes of TiNi SMAs. In this ®gure the specimen's hardness reaching 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. This feature could arise from the strain hardening effect because a large amount of plastic deformation occurs during drilling. The thickness of the hardened layer can reach 600 mm for TiNi SMAs. It is worthy to mention that, being a ductile material the
Fig. 4. The drilling forces of TiNi SMAs using the HSS and HSS TiN twist drills: (a) Ti50Ni50; (b) Ti49Ni51.
Fig. 5. A stereomicroscopic picture of the drilled surface of TiNi SMAs showing the wavy tracks appearing on the drilled surface.
H.C. Lin et al. / Journal of Materials Processing Technology 105 (2000) 327±332
Fig. 6. The specimen's hardness at various distances from the edge of the drilled hole for TiNi SMAs.
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fragments of TiNi SMAs on the diamond blade will depress its cutting ability. 2. The HSS TiN twist drill exhibits a better drilling ability than the HSS twist drill for the TiNi SMAs because the TiN ®lm has high hardness and excellent wear resistance. Further, the tungsten±carbide twist drill exhibits the best drilling ability. The optimal drilling parameters are 163 rpm rotational speed and 0.07 mm/rev feed rate for the drill machine used in this study. 3. The drilling forces for TiNi SMAs can exceed 5000 N which are higher than those for many commercial alloys. This feature may arise from the high toughness and viscosity, and the unique pseudoelasticity of TiNi SMAs. Wavy tracks appearing on the drilled surface are ascribed to sliding wear between the blunt drill and the drilled surface of TiNi SMAs. These wavy tracks will increase the frictional force, promote the vibration of drilling machine and cause damage of the twist drill. 4. Plastic deformation occurs during drilling and hence the specimen's hardness near to the drilled holes can reach 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. The Ti50Ni50 SMA exhibits a better drilling characteristics than Ti49Ni51 SMA.
Acknowledgements
Fig. 7. A stereomicroscopic picture of drilled chips of TiNi SMAs.
drilled chips of TiNi SMAs are continuous as shown in Fig. 7. The continuous chips exhibit a yellow color because the temperature of the twist drill can exceed 3008C and some oxidation of drilled chips will occur during the drilling process. 4. Conclusions The machining characteristics of TiNi SMAs have been studied. The important conclusions are as follows. 1. The time needed to cut TiNi SMAs are found to decrease with increasing blade rotational speed and cutting load in the early period and then approach constant values at higher blade rotational speeds and cutting loads. These features are ascribed to the effects of strain hardening, fatigue hardening and high strainrate hardening. The Buehler 4217 emery blade always maintains new cutting edges of SiC and Al2O3 powders due to the wear damage of the emery blade and, hence exhibits a better cutting rate. However, the adhering
The authors are pleased to acknowledge the ®nancial support of this research by the National Science Council (NSC), Republic of China, under the Grant NSC86-2216E035-015.
References [1] S. Miyazaki, K. Otsuka, Y. Suzuki, Scripta Metall. 15 (1981) 287. [2] S. Miyazaki, Y. Ohmi, K. Otsuka, Y. Suzuki, Icomat-82, J. Phys. 43 (1982) C4±255. [3] S. Miyazaki, T. Imai, Y. Igo, K. Otsuka, Metall. Trans. A 17 (1986) 115. [4] H.C. Lin, S.K. Wu, M.T. Yeh, Metall. Trans. A 24 (1993) 2189. [5] H.C. Lin, S.K. Wu, Y.C. Chang, Metall. Trans. A 26 (1993) 851. [6] T. Tadaki, Y. Nakada, K. Shimizu, Trans. Jpn. Inst. Met. 28 (1987) 883. [7] S. Miyazaki, Y. Igo, K. Otsuka, Acta Metall. 34 (1986) 2045. [8] S.K. Wu, H.C. Lin, T.S. Chou, Acta Metall. 38 (1990) 95. [9] M. Nishida, T. Honma, Scripta Metall. 18 (1984) 1293. [10] M. Nishida, C.M. Wayman, T. Honma, Scripta Metall. 18 (1984) 1389. [11] S.K. Wu, H.C. Lin, Scripta Metall. Mater. 25 (1991) 1295. [12] Y. Okamota, H. Hamanaka, F. Miura, H. Tamura, H. Horikawa, Scripta Metall. 22 (1988) 517. [13] T. Todoroki, H. Tamura, Trans. Jpn. Inst. Met. 28 (1987) 83. [14] H.C. Lin, S.K. Wu, T.S. Chou, H.P. Kao, Acta Metall. Mater. 39 (1991) 2069. [15] H.C. Lin, S.K. Wu, Acta Metall. Mater. 42 (1994) 1623. [16] E.K. Eckelmeyer, Scripta Metall. 10 (1976) 667.
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[17] R. Wasilewski, in: J. Perkin (Ed.), Shape Memory Effects In Alloys, Plenum, New York, 1975, pp. 245±271. [18] C.M. Hwang, M. Meichle, M.B. Salamon, C.M. Wayman, Philos. Mag. A 47 (1983) 9. [19] S.K. Wu, C.M. Wayman, Metallography 20 (1987) 359. [20] K. Enami, T. Yoshida, S. Nenno, Proc. Icomat 86 (1987) 103. [21] H.C. Lin, K.M. Lin, unpublished research.
[22] P. Clayton, Wear 162±164 (1993) 202. [23] T.H. Courtney, Mechanical Behavior of Materials, McGraw-hill, New York, 1990, pp. 16±17. [24] H.C. Lin, H.M. Liao, J.L. He, K.C. Chen, K.M. Lin, Metall. and Mater. Trans. A 28 (1997) 1871. [25] S.K. Wu, H.C. Lin, C.C. Chen, Mater. Lett., 1999, in press.
A study on the machining characteristics of TiNi shape memory alloys H.C. Lin*, K.M. Lin, Y.C. Chen Department of Materials Science, Feng Chia University, Taichung, Taiwan 400, PR China Received 16 May 1999
Abstract The machining characteristics of TiNi shape memory alloys (SMAs) have been studied. The time needed to cut TiNi SMAs is found to decrease with increasing blade rotation speed and cutting load in the early period and then approach constant values at higher blade rotation speeds and cutting loads. These features are ascribed to the effects of strain hardening, fatigue hardening and high strain-rate hardening. The Buehler 4217 emery blade being always composed of new cutting edges of SiC and Al2O3 powders, exhibits a better cutting rate than a diamond blade to cut TiNi SMAs. A high-speed steel drill coated with TiN ®lm exhibits a better drilling ability than a high-speed steel drill for the TiNi SMAs because the TiN ®lm has high hardness and excellent wear resistance. further, a tungsten±carbide drill exhibits the best drilling ability. The drilling forces for TiNi SMAs can exceed 5000 N which are higher than those for many commercial alloys. This feature may arise from the high toughness and viscosity, and the unique pseudoelasticity of TiNi SMAs. Wavy tracks appearing on the drilled surface are ascribed to sliding wear between the blunt drill and the drilled surface of TiNi SMAs. These wavy tracks will increase the frictional force, promote the vibration of the drilling machine and cause damage of the twist drill. Plastic deformation occurs during drilling and hence the specimen's hardness near to the drilled holes can reach 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. At the same time Ti50Ni50 SMA exhibits better drilling characteristics than Ti49Ni51 SMA. # 2000 Elsevier Science B.V. All rights reserved. Keywords: TiNi shape memory alloys; Mechanical cutting; Drilling
1. Introduction TiNi alloys are an important class of shape memory alloys (SMAs). They exhibit not only the shape memory effect (SME) [1] but also unusual pseudoelasticity [2,3] and high damping capacities [4,5]. These properties along with their superior ductility, fatigue strength and corrosion resistance have resulted in many applications. The basic characteristics of TiNi SMAs involving transformational crystallography, shape memory phenomena and the effects of thermomechanical treatments have been investigated intensively [6±20]. However, impediments to their development are caused by dif®culties 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 [1] but the severe strain hardening, high toughness and viscosity, and the unique pseudoelastic behavior have caused the machining characteristics of TiNi SMAs to be quite complicated. To overcome this dif®culty some special techniques have been used to machine TiNi SMAs such as laser machining, electric-discharge and wire-cut machining. *
Corresponding author. Tel.: 4-451-7250 ext. 5301; fax: 4-451-0014. E-mail address: [email protected] (H.C. Lin).
However, even these special machining techniques have some technical limits. As an approach is made to a solution to these problems, an understanding of the conventional machining characteristics of TiNi SMAs becomes important. To the best of the author's knowledge no systematic investigation of the conventional machining characteristics of TiNi SMAs has yet been reported. In the present study the aim is to investigate the machining characteristics of TiNi SMAs involving mechanical cutting and drilling and to discuss their optimal machining parameters. 2. Experimental procedure The conventional tungsten arc-melting technique was employed to prepare the Ti50Ni50 and Ti49Ni51 shape memory alloys. Pure titanium (99.7 wt.%) and nickel (99.98 wt.%), totaling nearly 120 g were melted and remelted at least six times in an argon atmosphere. The mass loss during the melting was negligible. The as-melted buttons were homogenized at 10508C for 24 h and then quenched in water. Specimens for mechanical cutting and drilling were carefully prepared from these buttons. These specimens were vacuum-sealed in quartz tubes, annealed at 8008C for 2 h and then quenched in water.
0924-0136/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 5 6 - 7
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The mechanical cutting was carried out by using an Isomet 2000 Precision Saw made by Buehler Co. USA and equipped with an automatic cooling and saw-grinding system. The rotation speed and applied load of the cutting saws (Isocut diamond blade and Buehler 4217 emery blade) are controlled digitally. The lubricant used during cutting was nine parts water to one part Buehler Isocut Plus ¯uid. The mechanical drilling was carried out by using a radial drilling machine made by Leadwell Co. USA and equipped with a dynamometer to measure the drilling force. The rotational speed and feed rate of the twist drills are controlled digitally. Three kinds of twist drills, a high-speed steel drill (HSS), high-speed steel drill coated with TiN ®lm (HSS TiN) and a tungsten±carbide drill (TC) were used in this study. The surface morphologies of the machined specimens were observed by stereomicroscopy. Specimens for hardness tests were polished mechanically and measured in a micro-Vickers hardness tester with a 500 g load. For each specimen the average hardness value was obtained from at least ®ve test readings. 3. Results and discussion 3.1. The mechanical cutting of Ti50Ni50 and Ti49Ni51 SMAs To understand the machining characteristics of Ti50Ni50 and Ti49Ni51 SMAs some important mechanical properties of these alloys are presented in Table 1. From this table one can ®nd that the Ti50Ni50 and Ti49Ni51 SMAs exhibit B190 martensite and B2 parent phases, respectively. The Ti49Ni51 SMA has higher hardness and yield strength than Ti50Ni50 SMA. Also, both Ti50Ni50 and Ti49Ni51 SMAs exhibit excellent ductility. In this investigation 304 stainless steel (commercial available) is used as a comparative material. The experimental results show that the mechanical cutting rate of 304 stainless steel is twice as high as than that of Ti50Ni50 SMA [21], although their mechanical properties are similar as presented in Table 1. Hence, it is quite interesting and valuable to understand the machining characteristics of Ti50Ni50 and Ti49Ni51 SMAs. Fig. 1(a) and (b) show the time needed to cut the Ti50Ni50 and Ti49Ni51 specimens with a 5 mm 5 mm cross-sectional area, respectively using cutting saws of a diamond blade and a Buehler 4217 emery blade at various cutting Table 1 The important mechanical properties of Ti50Ni50, Ti49Ni51 SMAs and 304 stainless steel Mechanical properties at room temperature
Ti50Ni50
Ti49Ni51
304 Stainless steel
Structure (phase) Hardness (HV) Yield strength (MPa) Elongation (%)
B190 phase 200 120 25
B2 phase 275 250 22
FCC austenite 190 120 27
Fig. 1. The time needed to cut TiNi specimens of 5 mm 5 mm crosssectional area at various cutting loads and 3000 rpm blade rotational speed: (a) Ti50Ni50; (b) Ti49Ni51.
loads and 3000 rpm blade rotation and speed. In Fig. 1 the cutting time for both the Ti50Ni50 and Ti49Ni51 specimens is found to decrease with increasing cutting load in the early period and then approach constant values at higher cutting loads. This means that at constant blade rotational speed a higher cutting load can not effectively improve the cutting rate of TiNi SMAs. This phenomenon can be ascribed to the strain hardening and fatigue hardening effects [22] occurring at higher cutting loads as noted in Fig. 2, in which ®gure shows the stress±strain curves of TiNi SMA under the tensile (monotonic) test and cyclic (fatigue) test [22]. From Fig. 2, for strain 0:015 strain hardening occurs in the specimen of the monotonic test as shown in region A. However, both strain hardening and fatigue hardening are induced in the specimen of the fatigue test as shown in regions A and B, respectively. It is believed that the vibration of the cutting machine during the cutting process will cause fatigue in the TiNi SMAs because the vibration will induce a repeated cyclic load on the specimen. Therefore, both strain hardening and fatigue hardening concurrently cause a severe hardening effect and impair the cutting rate. This feature is especially true for higher cutting loads. Fig. 3(a) and (b) show the time needed to cut the Ti50Ni50 and Ti49Ni51 specimens, respectively at 300 g cutting load
H.C. Lin et al. / Journal of Materials Processing Technology 105 (2000) 327±332
Fig. 2. Stress±strain curves showing the phenomena of strain hardening and fatigue hardening appearing in TiNi alloy [22].
and various blade rotational speeds. In Fig. 3 the cutting time for both Ti50Ni50 and Ti49Ni51 specimens is found to decrease slightly with increasing blade rotational speed in the early period and then approach constant values at higher blade rotational speeds. In general, less time is required to cut commercial alloys (such as carbon steels or aluminium alloys) using a higher blade cutting speed. The distinctive phenomenon shown in Fig. 3, namely no obvious improvement of cutting rate by increasing the blade rotational speed
329
may be ascribed to the high strain-rate hardening of TiNi SMAs [23,24]. Namely, the specimen's hardness prior to encountering the cutting edge will increase with increasing rotational speed of cutting blade. This strain-rate hardening will reduce the cutting rate, although a higher blade rotational speed can have a higher cutting frequency with TiNi specimens. This means that a higher blade rotational speed, for example 3000 rpm in this study cannot effectively increase the mechanical cutting rate of TiNi SMAs due to the high strain-rate hardening effect. Carefully examining the cutting time needed for TiNi SMAs in Figs. 1 and 3 one can easily see that the Buehler 4217 emery blade exhibits a better cutting rate than the diamond blade. This feature can be explained as below. Because of their high ductility some cut fragments of TiNi SMAs will adhere on the diamond blade during cutting [25]. These adhering fragments will impede further cutting. The greater the continuous cutting area, the more the number of adhered fragments that exist and hence the lower the cutting rate of the diamond blade. However, the Buehler 4217 emery blade being composed of the SiC and Al2O3 powders always maintains new cutting edges of SiC and Al2O3 powders due to the wear damage of the emery blade. These new cutting edges of Buehler 4217 emery blade will exhibit an excellent cutting ability and reduce the cutting time needed for TiNi SMAs. Further, from Figs. 1 and 3 it can also be seen that the cutting time needed for Ti50Ni50 and Ti49Ni51 SMAs are similar. This indicates that the Ti50Ni50 and Ti49Ni51 SMAs have the same cutting characteristics, although they exhibit the B190 martensite and B2 parent phases at room temperature, respectively. 3.2. The mechanical drilling of Ti50Ni50 and Ti49Ni51 SMAs
Fig. 3. The time needed to cut TiNi specimens of a 5 mm 5 mm crosssectional area at 300 g cutting load and various blade rotational speeds: (a) Ti50Ni50; (b) Ti49Ni51.
Table 2 presents the maximum depths of Ti50Ni50 and Ti49Ni51 SMAs that can be drilled by using three kinds of twist drills for nine settings of the drilling parameters. In Table 2 one can ®nd that the optimal drilling parameters for maximum drill depth are 163 rpm rotational speed and 0.07 mm/rev of feed rate, for the drill machine used in this study. One can also ®nd in Table 2 that the HSS TiN drill exhibits a better drilling ability than the HSS drill. Fig. 4(a) and (b) show the drilling forces of Ti50Ni50 and Ti49Ni51 SMAs, respectively when using HSS and HSS TiN twist drills. In Fig. 4 the increasing rates of drilling force, namely the upward slopes of the curve of drilling force versus drilling time of the HSS TiN drill are smaller than those of the HSS drill for both Ti50Ni50 and Ti49Ni51 SMAs. These features are ascribed to the high hardness and excellent wear resistance of the TiN ®lm coated to the HSS drill. The TiN ®lm can reduce both drilling force and wear damage, and hence improve the drilling ability of the HSS TiN drill for TiNi SMAs. In Fig. 4 the drilling forces for both Ti50Ni50 and Ti49Ni51 SMAs can exceed 5000 N which are higher than those for many commercial alloys. For example, the drilling force for S45C carbon steel is about 980 N. This
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Table 2 The maximum depths of Ti50Ni50 and Ti49Ni51 SMAs that can be drilled using three kinds of twist drills with nine settings of the drilling parameters Rotation speed (rpm)
88 88 88 163 163 163 305 305 305
Feeding rate (mm/rev)
0.07 0.13 0.22 0.07 0.13 0.22 0.07 0.13 0.22
Ti50Ni50 (mm)
Ti49Ni51 (mm)
HSS
HSSTiN
TC
HSS
HSSTiN
TC
4.476 2.600 2.332 11.799 10.891 3.066 10.658 8.635 3.146
4.052 2.668 1.851 22.943 12.151 3.131 12.224 9.414 8.367
± ± ± 39.596 ± ± ± ± ±
2.077 1.974 1.822 8.698 4.570 3.822 4.484 4.717 2.900
2.075 1.838 1.878 12.265 3.313 2.613 2.594 2.567 2.961
± ± ± 28.168 ± ± ± ± ±
feature may arise from their high toughness and viscosity, and unique pseudoelasticity. In Table 2 it can also be seen that the tungsten±carbide twist drill exhibits the best drilling ability as it has the largest drilling depth. The high hardness and strength of the tungsten±carbide drill are responsible for its excellent ability in the drilling of TiNi SMAs. However, the disadvantage of the tungsten±carbide drill are its brittle property and high cost. In Table 2 the maximum drilling depths for Ti50Ni50 SMA are found to be larger than those
for Ti49Ni51 SMA for all three kinds of twist drills. Not only the higher hardness but also the pseudoelasticity of Ti49Ni51 SMA is responsible for this phenomenon. The unique pseudoelasticity of Ti49Ni51 SMA will accommodate the applied strain near to the tip of twist drill and hence retard the progression of drilling. Fig. 5 shows the drilled surface morphology of TiNi SMAs when the twist drill has been retarded, numerous wavy tracks appearing on the drilled surface. These wavy tracks can be ascribed to the action of adhesive junction, abrasive deformation and wear chips, and the mechanical locking of surface pits during the sliding wear between the blunt drill and the drilled surface of the TiNi SMAs [24]. These wavy tracks will increase the frictional force, promote the vibration of the drilling machine and cause damage of the twist drill. Fig. 6 shows the specimen's hardness near to the drilled holes of TiNi SMAs. In this ®gure the specimen's hardness reaching 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. This feature could arise from the strain hardening effect because a large amount of plastic deformation occurs during drilling. The thickness of the hardened layer can reach 600 mm for TiNi SMAs. It is worthy to mention that, being a ductile material the
Fig. 4. The drilling forces of TiNi SMAs using the HSS and HSS TiN twist drills: (a) Ti50Ni50; (b) Ti49Ni51.
Fig. 5. A stereomicroscopic picture of the drilled surface of TiNi SMAs showing the wavy tracks appearing on the drilled surface.
H.C. Lin et al. / Journal of Materials Processing Technology 105 (2000) 327±332
Fig. 6. The specimen's hardness at various distances from the edge of the drilled hole for TiNi SMAs.
331
fragments of TiNi SMAs on the diamond blade will depress its cutting ability. 2. The HSS TiN twist drill exhibits a better drilling ability than the HSS twist drill for the TiNi SMAs because the TiN ®lm has high hardness and excellent wear resistance. Further, the tungsten±carbide twist drill exhibits the best drilling ability. The optimal drilling parameters are 163 rpm rotational speed and 0.07 mm/rev feed rate for the drill machine used in this study. 3. The drilling forces for TiNi SMAs can exceed 5000 N which are higher than those for many commercial alloys. This feature may arise from the high toughness and viscosity, and the unique pseudoelasticity of TiNi SMAs. Wavy tracks appearing on the drilled surface are ascribed to sliding wear between the blunt drill and the drilled surface of TiNi SMAs. These wavy tracks will increase the frictional force, promote the vibration of drilling machine and cause damage of the twist drill. 4. Plastic deformation occurs during drilling and hence the specimen's hardness near to the drilled holes can reach 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. The Ti50Ni50 SMA exhibits a better drilling characteristics than Ti49Ni51 SMA.
Acknowledgements
Fig. 7. A stereomicroscopic picture of drilled chips of TiNi SMAs.
drilled chips of TiNi SMAs are continuous as shown in Fig. 7. The continuous chips exhibit a yellow color because the temperature of the twist drill can exceed 3008C and some oxidation of drilled chips will occur during the drilling process. 4. Conclusions The machining characteristics of TiNi SMAs have been studied. The important conclusions are as follows. 1. The time needed to cut TiNi SMAs are found to decrease with increasing blade rotational speed and cutting load in the early period and then approach constant values at higher blade rotational speeds and cutting loads. These features are ascribed to the effects of strain hardening, fatigue hardening and high strainrate hardening. The Buehler 4217 emery blade always maintains new cutting edges of SiC and Al2O3 powders due to the wear damage of the emery blade and, hence exhibits a better cutting rate. However, the adhering
The authors are pleased to acknowledge the ®nancial support of this research by the National Science Council (NSC), Republic of China, under the Grant NSC86-2216E035-015.
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