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Fabrication of high porous NiTi shape memory alloy by metal injection molding
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
journal homepage: www.elsevier.com/locate/jmatprotec
Fabrication of high porous NiTi shape memory alloy by metal injection molding Hu Guoxin ∗ , Zhang Lixiang, Fan Yunliang, Li Yanhong School of Mechanical & Power Engineering, Shanghai Jiaotong University, 200240 Shanghai, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper has investigated the fabrication of porous NiTi shape memory alloy (SMA) by
Received 22 August 2007
metal injection molding (MIM). A two-step debinding process was used to extract the binder
Received in revised form
in a molded compact which was produced under an injection pressure of 400 bar, a feedstock
8 November 2007
temperature of 353 K and a mould temperature of 328 K. The porous NiTi SMA was fabricated
Accepted 14 December 2007
by self-propagating high-temperature synthesis (SHS). The results showed that the weight loss of binder reached 98.8% for the solvent debinding. The solvent debinding rate was divided into three stages with time. The microstructure of porous NiTi alloy from molded
Keywords:
compact and pressed compact was observed by scanning election microscope (SEM) and the
MIM
X-ray diffraction (XRD) method was used to analyze the phase of the porous NiTi alloy. It was
SMA
found that both the porosity and the pores size for the molded compact were much higher
Porous
than those for the pressed compact. The B2 (NiTi) and B19 (NiTi) phases were predominantly
XRD
present in the specimen. © 2007 Elsevier B.V. All rights reserved.
SEM
1.
Introduction
NiTi shape memory alloy (SMA) has attracted much interest for its potential use as a functional material in many engineering applications such as an active, adaptive or smart structure as well as certain biomedical applications (Lipscomb and Nokes, 1996; Hosoda et al., 1998). Recently, considerable attentions have been attracted on the porous NiTi SMA. The porous NiTi SMA was usually used for hard tissue implants because of its porous structure, good mechanical properties and biocompatibility (Saito et al., 1993; Itin et al., 1994). Self-propagating high-temperature synthesis (SHS) (Chu et al., 1997), pre-alloy powder metallurgy (Igharo and Wood, 1986), and element powder metallurgy (EPM) (Green et al., 1997) were the three popular methods to produce porous NiTi SMA. SHS was concerned with the ignition of a compressed powder mixture in an inert atmosphere, producing a self-sustaining chemical reaction which released sufficient heat (Munir and Anselmi-Tamburini,
∗
Corresponding author. Tel.: +86 21 34206569; fax: +86 21 34206569. E-mail address: [email protected] (H. Guoxin). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.044
1989; Moore and Feng, 1995). SHS has been used to fabricate several intermetallic compounds of nickel and titanium with time and energy savings (Munir and Anselmi-Tamburini, 1989; Yi and Moore, 1989a,b). In the recent years, a few papers have been published on the investigation of porous NiTi by SHS. Li et al. (2000) have successfully fabricated porous NiTi SMA with a banded structure of channels and about 54 vol.% porosity. The synthesized NiTi SMA had an open porous structure, and its pores and channels with various sizes and shapes were three-dimensionally interconnected. Chu et al. (2004) have also fabricated a porous NiTi SMA for hard tissue implants by SHS and found that it had an isotropic pore structure with a general porosity of 57.3 vol.% and an open porosity ratio of about 86%. However, the compressive strength of the porous NiTi SMA was only 100 MPa. How to produce products with high desired porosity and ideal pore characteristics needed extensive investigations. The MIM process was a combination of powder metallurgy
396
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
Fig. 3 – Morphology of molded compact without debinding. Fig. 1 – Effect of debinding temperature on the weight loss of binder in chloroform for various debinding times.
and plastic injection molding technologies. Many materials have been produced by MIM such as 316 L stainless steel (Loh et al., 2001), TiAl alloy (Gerling et al., 2006), etc. In the present study, MIM was used to fabricate the porous NiTi SMA. The effect of debinding and preheating temperature on the pore characteristics was systematically investigated.
2.
Experimental
The raw materials used are titanium and nickel powders supplied by Yezhong Metals Powder Company. The chemical composition of titanium powders (wt.%) is Ti 99.86, Fe 0.04, O 0.036, N 0.02, C 0.01 and H 0.03. The chemical composition of nickel powders (wt.%) is Ni 99.6, O 0.3, C 0.006, Fe 0.003, Co 0.002, Zn 0.001 and Mg 0.002. Sizeing by means of Laser Particle Sizer (OMEC LS-POP (III)) shows that Ni and Ti particles have an average size of 2 m and 3 m, respectively. The porous NiTi products are analyzed with scanning election microscope (SEM, S-2150, Hitachi High-Technologies
Fig. 2 – Debinding rate as a function of the debinding time for molded compacts at 288–328 K.
Fig. 4 – Morphology of molded compact for debinding time 2 h and debinding temperature 308 K.
Corp., Japan) and X-ray diffraction (XRD, D/max-2200/PC, Rigaku, Japan). The pore size distributions are testified by the mercury intrusion method (Model 9500, Micyomeyific, USA). A completed MIM process is divided into four steps as follows: (1) Mixing: The mixed powders of 50Ni–50Ti (at.%) are blended using ball milling for 12 h under a pure argon atmosphere. The feedstock is a mixture of Ni + Ti powders and binder which is made up of paraffin, polyethylene and stearic acid. The binder content is 20 wt.%. Powders and binder are mixed in a Z-blade mixer at a temperature of 373–383 K under a pure argon atmosphere. (2) Injection molding: Cylinder with a diameter of 20 mm is molded. An injection pressure of 400 bar, a feedstock temperature of 353 K and a mould temperature of 328 K turned out to be the adequate parameters providing homogeneous properties of the molded compacts. (3) Debinding: A two-step debinding process is necessary for the selected binder system: chloroform is used as the solvent for extracting the soluble binder from the molded compacts in a temperature range between 288 K and 328 K and a time range between 1 h to 8 h. Thermal debinding
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
397
is used to crack and vaporize the remaining binder about 673 K under a vacuum condition. (4) Self-propagating combustion: Self-propagating combustion is performed in a preheated molybdenum alloy resistance furnace. The preheating temperature is in a range of 573–773 K. The preheated molded compact is ignited at one end by a tungsten coil in the SHS equipment under an atmosphere pressure of 99.98% pure argon atmosphere. Once ignited, combustion self-propagate along the axis to the other end of the compact in a very short time, and then the porous NiTi SMA is synthesized.
3.
Results and discussion
The density of the molded compact is 2.94 g/cm3 . Fig. 1 shows the morphology of the molded compact without debinding. The white particles represent the titanium and nickel powders, while the ribbons represent the binder. The binder fills the void space of the compact completely. No pores are observed in the molded compact. Fig. 2 shows the effect of debinding temperature on the extraction of binder. A high efficiency is achieved due to a large solubility and diffusivity of binder in the chloroform at a high temperature. For the situation of the debinding temperature 328 K and the debinding time 8 h, the weight loss of soluble binder reaches 98.8%. The weight loss of soluble binder almost remains unchanged after 5 h of debinding because the dynamic equilibrium between the compacts and the chloroform is reached. Similar results are found in Yang et al. (2003). In addition, the debinding rate as shown in Fig. 3, determined from the slopes of the curves in Fig. 2, can be divided into three stages in the debinding process. The debinding rate decreases sharply at the initial stage, which is caused by the large differential concentration of soluble binder between the molded compacts and the solvent. The debinding rate at the following stage is much less than that at the initial stage, which is complex due to the intercoupling of debinding time, temperature and concentration of binders in the molded compacts. At the last stage, the debinding rate remains approximately linear because the weight loss pattern of soluble binder is transferred from diffusion to solution, which is found in Wang et al. (2006). Fig. 4 shows the morphology of the molded compact for a debinding time of 2 h and a debinding temperature of 308 K.
Fig. 5 – Morphology of molded compact after thermal debinding.
The ribbons disappear and only a small amount of binder adheres on the Ni and Ti particles, meaning that the most of the soluble binder (polyethylene glycol, PEG) has been extracted. Similar results were also obtained by Krauss et al. (2007), noted that the weight loss of soluble binder is about 71.4% at this situation. After the solvent debinding, the compact is heated to 673 K with a heating rate of 120–300 K/h to extract the remaining binder under an atmosphere pressure of 99.98% pure argon. The hold time is about 6 h. Fig. 5 shows the morphology of the molded compact after thermal debinding. Pores with different sizes are formed in the molded compact and the binder is almost debound completely. The macrograph of the porous NiTi SMA from molded compact is shown in Fig. 6(a) for a preheating temperature of 773 K and Fig. 6(b) for a preheating temperature of 573 K, respectively. Fig. 7(a) shows the macrograph of the porous NiTi SMA from pressed compact for a preheating temperature of 773 K, while Fig. 7(b) shows the macrograph of the porous NiTi SMA from pressed compact for a preheating temperature of 573 K. It can be seen from Fig. 6 that there are numerous pores with various sizes and shapes. The pores are almost
Fig. 6 – SEM macrograph of porous NiTi SMA from molded compact (a) preheating temperature 773 K and (b) preheating temperature 573 K.
398
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
Fig. 7 – SEM macrograph of porous NiTi SMA from pressed compact (a) preheating temperature 773 K and (b) preheating temperature 573 K.
exothermic heat is used to melt the product if the preheating temperature is higher (Merzhanov, 1994). Comparing Fig. 6 with Fig. 7, we can see that both the porosity and the pores size for the molded compact are much higher than those for the pressed compact because pores with different sizes are formed during the process of the solvent and thermal debinding as shown in Figs. 4 and 5.
Fig. 8 – Column map of pore size distribution of the porous NiTi (a) preheating temperature 773 K and (b) preheating temperature 573 K.
three-dimensionally interconnected, which forms an open porous structure. The porosity is about 75 vol.% and the density is about 1.56 g/cm3 , which is measured by the Archimedes principle. It seems that the molten phenomenon occurs during the process of the SHS. The molten fraction increases with an increase in the preheating temperature because more
Fig. 9 – XRD pattern of the porous NiTi SMA by MIM (a) preheating temperature 773 K and (b) preheating temperature 573 K, () B2 (NiTi); () B19 (NiTi); () (NiTi2 ).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
Fig. 8 shows the column maps of the pore size distribution of the porous NiTi from the molded compact for preheating temperatures of 773 K and 573 K, respectively. The macropores (about 1000 m in diameter) is predominant, which would be produced from the pores formed in the debinding process. There are still some micropores (about 200 m or less in diameter) in the porous NiTi SMA as shown in Fig. 6, which is formed in a similar way to the pressed compact. The preheating temperature seems to have an important effect on the pore size distribution of porous NiTi product. As shown in Fig. 8(a) and (b), the pores size of the specimen for a preheating temperature of 573 K is smaller and more homogenous than that for a preheating temperature of 773 K because the molten fraction increases with an increase in the preheating temperature. The larger molten fraction would lead to the smaller size for the micropores and larger size for the macropores. Fig. 9 shows the XRD pattern of the synthesized porous NiTi SMA from the molded compact for preheating temperatures of 773 K and 573 K, respectively. It can be found that the SHS process results in the formation of several intermetallic compounds such as NiTi and NiTi2 and no reflections corresponding to either of the reactants are observed, whether the preheating temperature is 573 K or 773 K. The amount of the B2 phase increases with an increase in the preheating temperature. The B2 (NiTi) and B19 (NiTi) phases, which are the desired products, are predominantly present.
4.
Conclusions
This paper has investigated the fabrication of NiTi SMA by MIM. Homogeneous compacts can be obtained at an injection pressure of 400 bar, a feedstock temperature of 353 K and a mould temperature of 328 K. A high efficiency is achieved due to a large solubility and diffusivity of binder in the chloroform at a high temperature. The maximum weight loss of soluble binder is 98.8%. The debinding rate is divided into three stages with time. The porosity of the porous NiTi alloy from molded compact is about 75 vol.%. The pores are almost threedimensionally interconnected, which forms an open porous structure. Both the porosity and the pores size for the molded compact are much higher than those for the pressed compact. The SHS process results in the formation of several intermetallic compounds such as NiTi and NiTi2 , and no reflections corresponding to either of the reactants are observed. The B2 (NiTi) and B19 (NiTi) phases are predominantly present in the porous NiTi specimen.
Acknowledgement The authors gratefully acknowledge financial support by the National Science Foundation of China (No. 50646024).
399
references
Chu, C.L., Li, B., Wang, S.D., Zhang, S.G., Yang, X.X., Yin, Z.D., 1997. Preparation of TiNi shape memory alloy by SHS. Trans. Nonferr. Met. Soc. China 7, 84–87. Chu, C.L., Chung, C.Y., Lin, P.H., Wang, S.D., 2004. Fabrication of porous NiTi shape memory alloy for hard tissue implants by combustion synthesis. Mater. Sci. Eng. A366, 114–119. Gerling, R., Aust, E., Limberg, W., Pfuff, M., Schimansky, F.P., 2006. Metal injection moulding of gamma titanium aluminide alloy powder. Mater. Sci. Eng. A 423, 262–268. Green, S.M., Grant, D.M., Kelly, N.R., 1997. Powder metallurgical processing of Ni–Ti shape memory alloy. Powder Metall. 40, 43–47. Hosoda, H., Hunada, S., Inoue, K., Fukui, T., Mishina, Y., Suzuki, T., 1998. Martensite transformation temperatures and mechanical properties of ternary NiTi alloys with offstoichiometric compositions. Intermetallics 6, 291–301. Igharo, M., Wood, J.V., 1986. Consolidation of rapidly solidified Ti–Ni intermetallics. Powder Metall. 29, 37–41. Itin, V.I., Gjunter, V.E., Shabalovskaya, S.A., Sachdeva, R.L.C., 1994. Mechanical properties and shape memory of porous nitinol. Mater. Charact. 32, 179–187. Krauss, V.A., Oliveira, A.A.M., Klein, A.N., Al-Qureshi, H.A., Fredel, M.C., 2007. A model for PEG removal from alumina injection moulded parts by solvent debinding. J. Mater. Process. Technol. 182, 268–273. Li, B.Y., Rong, L.J., Li, Y.Y., Gjunter, V.E., 2000. A recent development in producing porous Ni + Ti shape memory alloys. Intermetallics 8, 881–884. Lipscomb, I.P., Nokes, L.D.M., 1996. The Applications of Shape Memory Alloys in Medicine. Mechanical Engineering Publications, Suffolk, UK. Loh, N.H., Tor, S.B., Khor, K.A., 2001. Production of metal matrix composite part by powder injection molding. J. Mater. Process. Technol. 108, 398–407. Merzhanov, A.G., 1994. Solid flames: discoveries, concepts, and horizons of cognition. Comb. Sci. Technol. 98, 307–336. Moore, J.J., Feng, H.J., 1995. Combustion synthesis of advanced materials. Part I. Reaction parameters. Prog. Mater. Sci. 39, 243–273. Munir, Z.A., Anselmi-Tamburini, U., 1989. Self-propagating exothermic reaction. Mater. Sci. Rep. 3, 277–358. Saito, S., Wachi, T., Hanada, S., 1993. A new fabrication process of TiNi shape memory wire. Mater. Sci. Eng. A161, 91–96. Wang, R.F., Wu, Y.X., Zou, X., Tang, C.A., 2006. Debinding and sintering processes for injection molded pure titanium. Powder Metall. Technol. 24, 83–88. Yang, W.W., Yang, K.Y., Wang, M.C., Hon, M.H., 2003. Solvent debinding mechanism for alumina injection molded compacts with water-soluble binders. Ceram. Int. 29, 745–756. Yi, H.C., Moore, J.J., 1989a. Combustion synthesis of TiNi intermetallic compounds. Part 1 Determination of heart of fusion of TiNi and heat capacity of liquid TiNi. J. Mater. Sci. 24, 3449–3455. Yi, H.C., Moore, J.J., 1989b. Combustion synthesis of TiNi intermetallic compounds. Part 2. Effect of TiO2 formation. J. Mater. Sci. 24, 3456–3462.
journal homepage: www.elsevier.com/locate/jmatprotec
Fabrication of high porous NiTi shape memory alloy by metal injection molding Hu Guoxin ∗ , Zhang Lixiang, Fan Yunliang, Li Yanhong School of Mechanical & Power Engineering, Shanghai Jiaotong University, 200240 Shanghai, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper has investigated the fabrication of porous NiTi shape memory alloy (SMA) by
Received 22 August 2007
metal injection molding (MIM). A two-step debinding process was used to extract the binder
Received in revised form
in a molded compact which was produced under an injection pressure of 400 bar, a feedstock
8 November 2007
temperature of 353 K and a mould temperature of 328 K. The porous NiTi SMA was fabricated
Accepted 14 December 2007
by self-propagating high-temperature synthesis (SHS). The results showed that the weight loss of binder reached 98.8% for the solvent debinding. The solvent debinding rate was divided into three stages with time. The microstructure of porous NiTi alloy from molded
Keywords:
compact and pressed compact was observed by scanning election microscope (SEM) and the
MIM
X-ray diffraction (XRD) method was used to analyze the phase of the porous NiTi alloy. It was
SMA
found that both the porosity and the pores size for the molded compact were much higher
Porous
than those for the pressed compact. The B2 (NiTi) and B19 (NiTi) phases were predominantly
XRD
present in the specimen. © 2007 Elsevier B.V. All rights reserved.
SEM
1.
Introduction
NiTi shape memory alloy (SMA) has attracted much interest for its potential use as a functional material in many engineering applications such as an active, adaptive or smart structure as well as certain biomedical applications (Lipscomb and Nokes, 1996; Hosoda et al., 1998). Recently, considerable attentions have been attracted on the porous NiTi SMA. The porous NiTi SMA was usually used for hard tissue implants because of its porous structure, good mechanical properties and biocompatibility (Saito et al., 1993; Itin et al., 1994). Self-propagating high-temperature synthesis (SHS) (Chu et al., 1997), pre-alloy powder metallurgy (Igharo and Wood, 1986), and element powder metallurgy (EPM) (Green et al., 1997) were the three popular methods to produce porous NiTi SMA. SHS was concerned with the ignition of a compressed powder mixture in an inert atmosphere, producing a self-sustaining chemical reaction which released sufficient heat (Munir and Anselmi-Tamburini,
∗
Corresponding author. Tel.: +86 21 34206569; fax: +86 21 34206569. E-mail address: [email protected] (H. Guoxin). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.044
1989; Moore and Feng, 1995). SHS has been used to fabricate several intermetallic compounds of nickel and titanium with time and energy savings (Munir and Anselmi-Tamburini, 1989; Yi and Moore, 1989a,b). In the recent years, a few papers have been published on the investigation of porous NiTi by SHS. Li et al. (2000) have successfully fabricated porous NiTi SMA with a banded structure of channels and about 54 vol.% porosity. The synthesized NiTi SMA had an open porous structure, and its pores and channels with various sizes and shapes were three-dimensionally interconnected. Chu et al. (2004) have also fabricated a porous NiTi SMA for hard tissue implants by SHS and found that it had an isotropic pore structure with a general porosity of 57.3 vol.% and an open porosity ratio of about 86%. However, the compressive strength of the porous NiTi SMA was only 100 MPa. How to produce products with high desired porosity and ideal pore characteristics needed extensive investigations. The MIM process was a combination of powder metallurgy
396
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
Fig. 3 – Morphology of molded compact without debinding. Fig. 1 – Effect of debinding temperature on the weight loss of binder in chloroform for various debinding times.
and plastic injection molding technologies. Many materials have been produced by MIM such as 316 L stainless steel (Loh et al., 2001), TiAl alloy (Gerling et al., 2006), etc. In the present study, MIM was used to fabricate the porous NiTi SMA. The effect of debinding and preheating temperature on the pore characteristics was systematically investigated.
2.
Experimental
The raw materials used are titanium and nickel powders supplied by Yezhong Metals Powder Company. The chemical composition of titanium powders (wt.%) is Ti 99.86, Fe 0.04, O 0.036, N 0.02, C 0.01 and H 0.03. The chemical composition of nickel powders (wt.%) is Ni 99.6, O 0.3, C 0.006, Fe 0.003, Co 0.002, Zn 0.001 and Mg 0.002. Sizeing by means of Laser Particle Sizer (OMEC LS-POP (III)) shows that Ni and Ti particles have an average size of 2 m and 3 m, respectively. The porous NiTi products are analyzed with scanning election microscope (SEM, S-2150, Hitachi High-Technologies
Fig. 2 – Debinding rate as a function of the debinding time for molded compacts at 288–328 K.
Fig. 4 – Morphology of molded compact for debinding time 2 h and debinding temperature 308 K.
Corp., Japan) and X-ray diffraction (XRD, D/max-2200/PC, Rigaku, Japan). The pore size distributions are testified by the mercury intrusion method (Model 9500, Micyomeyific, USA). A completed MIM process is divided into four steps as follows: (1) Mixing: The mixed powders of 50Ni–50Ti (at.%) are blended using ball milling for 12 h under a pure argon atmosphere. The feedstock is a mixture of Ni + Ti powders and binder which is made up of paraffin, polyethylene and stearic acid. The binder content is 20 wt.%. Powders and binder are mixed in a Z-blade mixer at a temperature of 373–383 K under a pure argon atmosphere. (2) Injection molding: Cylinder with a diameter of 20 mm is molded. An injection pressure of 400 bar, a feedstock temperature of 353 K and a mould temperature of 328 K turned out to be the adequate parameters providing homogeneous properties of the molded compacts. (3) Debinding: A two-step debinding process is necessary for the selected binder system: chloroform is used as the solvent for extracting the soluble binder from the molded compacts in a temperature range between 288 K and 328 K and a time range between 1 h to 8 h. Thermal debinding
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
397
is used to crack and vaporize the remaining binder about 673 K under a vacuum condition. (4) Self-propagating combustion: Self-propagating combustion is performed in a preheated molybdenum alloy resistance furnace. The preheating temperature is in a range of 573–773 K. The preheated molded compact is ignited at one end by a tungsten coil in the SHS equipment under an atmosphere pressure of 99.98% pure argon atmosphere. Once ignited, combustion self-propagate along the axis to the other end of the compact in a very short time, and then the porous NiTi SMA is synthesized.
3.
Results and discussion
The density of the molded compact is 2.94 g/cm3 . Fig. 1 shows the morphology of the molded compact without debinding. The white particles represent the titanium and nickel powders, while the ribbons represent the binder. The binder fills the void space of the compact completely. No pores are observed in the molded compact. Fig. 2 shows the effect of debinding temperature on the extraction of binder. A high efficiency is achieved due to a large solubility and diffusivity of binder in the chloroform at a high temperature. For the situation of the debinding temperature 328 K and the debinding time 8 h, the weight loss of soluble binder reaches 98.8%. The weight loss of soluble binder almost remains unchanged after 5 h of debinding because the dynamic equilibrium between the compacts and the chloroform is reached. Similar results are found in Yang et al. (2003). In addition, the debinding rate as shown in Fig. 3, determined from the slopes of the curves in Fig. 2, can be divided into three stages in the debinding process. The debinding rate decreases sharply at the initial stage, which is caused by the large differential concentration of soluble binder between the molded compacts and the solvent. The debinding rate at the following stage is much less than that at the initial stage, which is complex due to the intercoupling of debinding time, temperature and concentration of binders in the molded compacts. At the last stage, the debinding rate remains approximately linear because the weight loss pattern of soluble binder is transferred from diffusion to solution, which is found in Wang et al. (2006). Fig. 4 shows the morphology of the molded compact for a debinding time of 2 h and a debinding temperature of 308 K.
Fig. 5 – Morphology of molded compact after thermal debinding.
The ribbons disappear and only a small amount of binder adheres on the Ni and Ti particles, meaning that the most of the soluble binder (polyethylene glycol, PEG) has been extracted. Similar results were also obtained by Krauss et al. (2007), noted that the weight loss of soluble binder is about 71.4% at this situation. After the solvent debinding, the compact is heated to 673 K with a heating rate of 120–300 K/h to extract the remaining binder under an atmosphere pressure of 99.98% pure argon. The hold time is about 6 h. Fig. 5 shows the morphology of the molded compact after thermal debinding. Pores with different sizes are formed in the molded compact and the binder is almost debound completely. The macrograph of the porous NiTi SMA from molded compact is shown in Fig. 6(a) for a preheating temperature of 773 K and Fig. 6(b) for a preheating temperature of 573 K, respectively. Fig. 7(a) shows the macrograph of the porous NiTi SMA from pressed compact for a preheating temperature of 773 K, while Fig. 7(b) shows the macrograph of the porous NiTi SMA from pressed compact for a preheating temperature of 573 K. It can be seen from Fig. 6 that there are numerous pores with various sizes and shapes. The pores are almost
Fig. 6 – SEM macrograph of porous NiTi SMA from molded compact (a) preheating temperature 773 K and (b) preheating temperature 573 K.
398
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
Fig. 7 – SEM macrograph of porous NiTi SMA from pressed compact (a) preheating temperature 773 K and (b) preheating temperature 573 K.
exothermic heat is used to melt the product if the preheating temperature is higher (Merzhanov, 1994). Comparing Fig. 6 with Fig. 7, we can see that both the porosity and the pores size for the molded compact are much higher than those for the pressed compact because pores with different sizes are formed during the process of the solvent and thermal debinding as shown in Figs. 4 and 5.
Fig. 8 – Column map of pore size distribution of the porous NiTi (a) preheating temperature 773 K and (b) preheating temperature 573 K.
three-dimensionally interconnected, which forms an open porous structure. The porosity is about 75 vol.% and the density is about 1.56 g/cm3 , which is measured by the Archimedes principle. It seems that the molten phenomenon occurs during the process of the SHS. The molten fraction increases with an increase in the preheating temperature because more
Fig. 9 – XRD pattern of the porous NiTi SMA by MIM (a) preheating temperature 773 K and (b) preheating temperature 573 K, () B2 (NiTi); () B19 (NiTi); () (NiTi2 ).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 6 ( 2 0 0 8 ) 395–399
Fig. 8 shows the column maps of the pore size distribution of the porous NiTi from the molded compact for preheating temperatures of 773 K and 573 K, respectively. The macropores (about 1000 m in diameter) is predominant, which would be produced from the pores formed in the debinding process. There are still some micropores (about 200 m or less in diameter) in the porous NiTi SMA as shown in Fig. 6, which is formed in a similar way to the pressed compact. The preheating temperature seems to have an important effect on the pore size distribution of porous NiTi product. As shown in Fig. 8(a) and (b), the pores size of the specimen for a preheating temperature of 573 K is smaller and more homogenous than that for a preheating temperature of 773 K because the molten fraction increases with an increase in the preheating temperature. The larger molten fraction would lead to the smaller size for the micropores and larger size for the macropores. Fig. 9 shows the XRD pattern of the synthesized porous NiTi SMA from the molded compact for preheating temperatures of 773 K and 573 K, respectively. It can be found that the SHS process results in the formation of several intermetallic compounds such as NiTi and NiTi2 and no reflections corresponding to either of the reactants are observed, whether the preheating temperature is 573 K or 773 K. The amount of the B2 phase increases with an increase in the preheating temperature. The B2 (NiTi) and B19 (NiTi) phases, which are the desired products, are predominantly present.
4.
Conclusions
This paper has investigated the fabrication of NiTi SMA by MIM. Homogeneous compacts can be obtained at an injection pressure of 400 bar, a feedstock temperature of 353 K and a mould temperature of 328 K. A high efficiency is achieved due to a large solubility and diffusivity of binder in the chloroform at a high temperature. The maximum weight loss of soluble binder is 98.8%. The debinding rate is divided into three stages with time. The porosity of the porous NiTi alloy from molded compact is about 75 vol.%. The pores are almost threedimensionally interconnected, which forms an open porous structure. Both the porosity and the pores size for the molded compact are much higher than those for the pressed compact. The SHS process results in the formation of several intermetallic compounds such as NiTi and NiTi2 , and no reflections corresponding to either of the reactants are observed. The B2 (NiTi) and B19 (NiTi) phases are predominantly present in the porous NiTi specimen.
Acknowledgement The authors gratefully acknowledge financial support by the National Science Foundation of China (No. 50646024).
399
references
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