- Email: [email protected]
Compression temperature and binder ratio on a process for fabrication of open-celled porous Ti
Materials Research Bulletin 45 (2010) 355–358
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Compression temperature and binder ratio on a process for fabrication of open-celled porous Ti Il-Ho Kim, Wonsik Lee *, Se-Hyun Ko, Jin Man Jang Eco Materials & Processing Department, Korea Institute of Industrial Technology, Incheon 406-840, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 January 2009 Accepted 28 July 2009 Available online 16 December 2009
This study was performed to investigate the fabrication of open-celled porous Ti by space holder using spherical polymer balls, and to evaluate the effect of temperature and binder ratio in the compression process. In compression condition of 80 8C, the open cellular structure consisted of continuously connected Ti struts with pore fraction above 85% and with the pores distributed homogeneously with nominal diameters between 200 mm and 400 mm. The large stress of 700 MPa applied to the acryl balls, containing the Ti feedstock, were not crushed, because the uniaxal stress applied to the acryl balls was transferred into the pseudo-hydrostatic stress state. A compression temperature of 90 8C caused the agglomeration of Ti feedstock and penetration of Ti powders into the inner part of acryl balls by softening. The thinner struts were gradually developed with increase of binder ratio due to increase of deformation. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Metals A. Microporous materials D. Microstructure
In order to manufacture open-celled Ti foams, various technologies have been developed such as powder-space holder injection method, gel casting and slurry coating on open-celled polyurethane foam [1,2]. However, it seems that most of these techniques are not economical, or it is difficult to obtain homogeneous cell structures using them. Therefore, in this study, a new fabrication process of open-celled Ti foam, which uses a preform made of spherical polymer balls as a space holder, was developed and the effects of process temperature and binder ratio on foam shape were investigated.
perform was compressed under pressure of 700 MPa at temperatures of 70 8C, 80 8C and 90 8C, respectively. During injection process, the Ti feedstock was infiltrated into the paths and then, the composite body of feedstock and polymer preform was compressed by uniaxial stress. In order to remove the binder and the polymer preform, the samples were heated up to 600 8C from room temperature at heating rate of 1 8C/min in a furnace where high purity Ar gas flows. The brown parts were sintered for 4 h at 1300 8C in a vacuum furnace. Measuring microscope, OM and SEM observations were performed to investigate the sample shape and microstructure. The pore fraction was calculated simply by division of weight to measured volume.
2. Experiment
3. Results and discussion
For this study, the Ti feedstock was prepared of a mixture of Ti powders and binder of which major constituent is paraffin wax and the polymer balls were used with average diameter of 0.8 mm. Fig. 1 shows SEM image of the Ti powder(-#325) used in present study. The procedure manufacturing open cell Ti foam was as follows: firstly, the polymer balls were stacked and heated to 150 8C. Then, the balls were bonded by partial melting in contact points, resulting in formation of a preform for infiltration as shown in Fig. 2. Secondly, the feedstock was injected/infiltrated into the polymer preform of 12 mm diameter and then, Ti infiltrated
Fig. 3 shows OM images of the composite bodies compressed under pressure of 700 MPa at 70 8C, 80 8C and 90 8C, respectively. The shapes of the polymer balls and feedstock were varied with compression temperature. As shown in Fig. 3(a), in the sample compressed at 70 8C, the paths among the polymer balls are filled very well with Ti feedstock without deformation of the balls. However, it was observed that some of Ti feedstock surrounds the polymer balls due to break of the bonded areas between the balls, indicating that the bonding strength of the balls do not endure the compression stress at 70 8C. These morphologies resulted in closed cell-like structures after sintering. In case of the composite sample compressed at 80 8C, the bonded necks were somewhat grown during compression and the infiltrated Ti struts were thicker than
1. Introduction
* Corresponding author. Fax: +82 32 850 0430. E-mail address: [email protected] (W. Lee). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.12.002
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Fig. 1. SEM image of Ti powder used in present study.
these at 70 8C, as can be seen in Fig. 3(b). These results indicate that the increase of temperature causes softening of the polymer balls and deformation by compressive stress. The neck growth due to the deformation concentrates the feedstock toward the centers of the paths and results in feedstock structures with high density and strength. Fig. 3(c) shows the image after compression at 90 8C. The
polymer balls were severely deformed due to too high temperature and the resultant composite structures were very inhomogeneous. Fig. 3(d) is the cross sectional image of the sample compressed in pressure of 700 MPa at 80 8C. Although some acryl balls are deformed slightly, overall shapes are retained soundly sustaining spheres. Considering the common properties of polymers, the stress of 700 MPa must be very large to them. However, as can be seen in the results at 80 8C of Fig. 3, the balls did not crush severely down. This means that the deformation of the soft polymer balls was suppressed sufficiently due to pseudo-hydrostatic stress state caused by the infiltrated Ti feedstock [3,4]. That is, although uniaxial stress is supplied to the specimens during compression process, the real stress which the balls experienced is the pseudohydrostatic stress. As a result, the sound spherical shapes of the balls are retained after compression. Fig. 4 is SEM images in the samples compressed at 80 8C and 90 8C, respectively, after debinding and sintering process at 1300 8C. In the compression condition of 80 8C, the sound open cellular structures were obtained with homogeneous Ti struts and pore fraction above 85% and the pores had diameters between 300 mm and 500 mm, as shown in Fig. 4(a). Fig. 4(b) shows the microstructure of Ti strut and it is can be seen that the Ti feedstock was sintered successfully.
Fig. 2. Measuring microscope images of the preform made by bonding of polymer balls. (a) Polymer balls before bonding and (b) polymer perform after bonding at 150 8C.
Fig. 3. OM images of the composite bodies of feedstock and polymer preform compressed in pressure of 700 MPa at different temperature. (a) 70 8C, (b) 80 8C, (c) 90 8C, and (d) cross sectional image of the sample compressed at 80 8C.
I.-H. Kim et al. / Materials Research Bulletin 45 (2010) 355–358
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Fig. 4. SEM images after acryl removal and sintering of the samples with different compression temperature. (a) Low magnification image of sample compressed at 80 8C, (b) magnified image of Ti frame in (a), (c) low magnification image of sample compressed at 90 8C, and (d) magnified image of Ti frame in (c).
However, Ti foam compressed at 90 8C, as shown in Fig. 4(c), shows is inhomogeneous structure with size of pores between 100 mm and 600 mm and strut thickness over 150 mm. Also, relatively more porous Ti microstructure was obtained comparing to that at 80 8C (Fig. 4(d)) and this seems to result from decrease of feedstock densification due to severe deformation of polymer balls, as mentioned in Fig. 3. Fig. 5 shows thickness change of the struts with different binder ratio. Thin struts are developed gradually with increase of binder ratio from 40% to 60%. This result shows that the change of binder amount can adjust the flowability and
deformation amount in the mixture of Ti powder and binder. Therefore, in order to get a critical thickness of Ti struts during compression, it is necessary to decide a suitable amount of binder. Fig. 6 shows a photograph of the sample compressed at 80 8C and sintered for 4 h at 1300 8C. Ti foam with 12 mm in diameter was fabricated with homogeneous pores and struts. Fig. 7 is the results of pore fraction in the samples fabricated with different compression temperature. The pore fractions were calculated to 85.63%, 86.70% and 78.03% in the compression temperatures of 70 8C, 80 8C and 90 8C,
Fig. 5. Change of strut thickness with different binder ratio. (a) 40%, (b) 50%, and (c) 60%.
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Fig. 6. Photograph of open-celled porous Ti compressed at 80 8C and sintered at 1300 8C.
Fig. 7. Comparison to pore fractions in samples fabricated with different compression temperature.
respectively, and the highest pore fraction was obtained at the compression temperature of 80 8C. This means that in order to manufacture the open-celled Ti foam by the process suggested in present study, it is very important to decide the suitable compression temperature.
homogeneous Ti struts and pore fraction above 85% and with the pore diameters between 300 mm and 500 mm. However, increase of compression temperature to 90 8C caused severe deformation of the polymer balls and the resultant composite structures were inhomogeneous.
4. Conclusions References The fabrication of open-celled Ti foam, by injection/infiltration of Ti feedstock and compression of composite body of polymer preform and the infiltrated Ti feedstock, were obtained. The foam structures depend on compression temperatures. In compression condition of 80 8C, the open cellular structure was obtained with
[1] [2] [3] [4]
G. Ryan, A. Pandit, D.P. Apatsidis, Biomaterials 27 (2006) 2651. J. Banhart, Prog. Mater. Sci. 46 (2001) 559. R.M. German, Sintering Theory and Practice, John Wiley & Sons, Inc., 1996. E. Klar, Powder metallurgy, vol.7: Metals Handbook, 9th ed., ASM International, Materials Park, OH, 1984.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Compression temperature and binder ratio on a process for fabrication of open-celled porous Ti Il-Ho Kim, Wonsik Lee *, Se-Hyun Ko, Jin Man Jang Eco Materials & Processing Department, Korea Institute of Industrial Technology, Incheon 406-840, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 January 2009 Accepted 28 July 2009 Available online 16 December 2009
This study was performed to investigate the fabrication of open-celled porous Ti by space holder using spherical polymer balls, and to evaluate the effect of temperature and binder ratio in the compression process. In compression condition of 80 8C, the open cellular structure consisted of continuously connected Ti struts with pore fraction above 85% and with the pores distributed homogeneously with nominal diameters between 200 mm and 400 mm. The large stress of 700 MPa applied to the acryl balls, containing the Ti feedstock, were not crushed, because the uniaxal stress applied to the acryl balls was transferred into the pseudo-hydrostatic stress state. A compression temperature of 90 8C caused the agglomeration of Ti feedstock and penetration of Ti powders into the inner part of acryl balls by softening. The thinner struts were gradually developed with increase of binder ratio due to increase of deformation. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Metals A. Microporous materials D. Microstructure
In order to manufacture open-celled Ti foams, various technologies have been developed such as powder-space holder injection method, gel casting and slurry coating on open-celled polyurethane foam [1,2]. However, it seems that most of these techniques are not economical, or it is difficult to obtain homogeneous cell structures using them. Therefore, in this study, a new fabrication process of open-celled Ti foam, which uses a preform made of spherical polymer balls as a space holder, was developed and the effects of process temperature and binder ratio on foam shape were investigated.
perform was compressed under pressure of 700 MPa at temperatures of 70 8C, 80 8C and 90 8C, respectively. During injection process, the Ti feedstock was infiltrated into the paths and then, the composite body of feedstock and polymer preform was compressed by uniaxial stress. In order to remove the binder and the polymer preform, the samples were heated up to 600 8C from room temperature at heating rate of 1 8C/min in a furnace where high purity Ar gas flows. The brown parts were sintered for 4 h at 1300 8C in a vacuum furnace. Measuring microscope, OM and SEM observations were performed to investigate the sample shape and microstructure. The pore fraction was calculated simply by division of weight to measured volume.
2. Experiment
3. Results and discussion
For this study, the Ti feedstock was prepared of a mixture of Ti powders and binder of which major constituent is paraffin wax and the polymer balls were used with average diameter of 0.8 mm. Fig. 1 shows SEM image of the Ti powder(-#325) used in present study. The procedure manufacturing open cell Ti foam was as follows: firstly, the polymer balls were stacked and heated to 150 8C. Then, the balls were bonded by partial melting in contact points, resulting in formation of a preform for infiltration as shown in Fig. 2. Secondly, the feedstock was injected/infiltrated into the polymer preform of 12 mm diameter and then, Ti infiltrated
Fig. 3 shows OM images of the composite bodies compressed under pressure of 700 MPa at 70 8C, 80 8C and 90 8C, respectively. The shapes of the polymer balls and feedstock were varied with compression temperature. As shown in Fig. 3(a), in the sample compressed at 70 8C, the paths among the polymer balls are filled very well with Ti feedstock without deformation of the balls. However, it was observed that some of Ti feedstock surrounds the polymer balls due to break of the bonded areas between the balls, indicating that the bonding strength of the balls do not endure the compression stress at 70 8C. These morphologies resulted in closed cell-like structures after sintering. In case of the composite sample compressed at 80 8C, the bonded necks were somewhat grown during compression and the infiltrated Ti struts were thicker than
1. Introduction
* Corresponding author. Fax: +82 32 850 0430. E-mail address: [email protected] (W. Lee). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.12.002
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Fig. 1. SEM image of Ti powder used in present study.
these at 70 8C, as can be seen in Fig. 3(b). These results indicate that the increase of temperature causes softening of the polymer balls and deformation by compressive stress. The neck growth due to the deformation concentrates the feedstock toward the centers of the paths and results in feedstock structures with high density and strength. Fig. 3(c) shows the image after compression at 90 8C. The
polymer balls were severely deformed due to too high temperature and the resultant composite structures were very inhomogeneous. Fig. 3(d) is the cross sectional image of the sample compressed in pressure of 700 MPa at 80 8C. Although some acryl balls are deformed slightly, overall shapes are retained soundly sustaining spheres. Considering the common properties of polymers, the stress of 700 MPa must be very large to them. However, as can be seen in the results at 80 8C of Fig. 3, the balls did not crush severely down. This means that the deformation of the soft polymer balls was suppressed sufficiently due to pseudo-hydrostatic stress state caused by the infiltrated Ti feedstock [3,4]. That is, although uniaxial stress is supplied to the specimens during compression process, the real stress which the balls experienced is the pseudohydrostatic stress. As a result, the sound spherical shapes of the balls are retained after compression. Fig. 4 is SEM images in the samples compressed at 80 8C and 90 8C, respectively, after debinding and sintering process at 1300 8C. In the compression condition of 80 8C, the sound open cellular structures were obtained with homogeneous Ti struts and pore fraction above 85% and the pores had diameters between 300 mm and 500 mm, as shown in Fig. 4(a). Fig. 4(b) shows the microstructure of Ti strut and it is can be seen that the Ti feedstock was sintered successfully.
Fig. 2. Measuring microscope images of the preform made by bonding of polymer balls. (a) Polymer balls before bonding and (b) polymer perform after bonding at 150 8C.
Fig. 3. OM images of the composite bodies of feedstock and polymer preform compressed in pressure of 700 MPa at different temperature. (a) 70 8C, (b) 80 8C, (c) 90 8C, and (d) cross sectional image of the sample compressed at 80 8C.
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Fig. 4. SEM images after acryl removal and sintering of the samples with different compression temperature. (a) Low magnification image of sample compressed at 80 8C, (b) magnified image of Ti frame in (a), (c) low magnification image of sample compressed at 90 8C, and (d) magnified image of Ti frame in (c).
However, Ti foam compressed at 90 8C, as shown in Fig. 4(c), shows is inhomogeneous structure with size of pores between 100 mm and 600 mm and strut thickness over 150 mm. Also, relatively more porous Ti microstructure was obtained comparing to that at 80 8C (Fig. 4(d)) and this seems to result from decrease of feedstock densification due to severe deformation of polymer balls, as mentioned in Fig. 3. Fig. 5 shows thickness change of the struts with different binder ratio. Thin struts are developed gradually with increase of binder ratio from 40% to 60%. This result shows that the change of binder amount can adjust the flowability and
deformation amount in the mixture of Ti powder and binder. Therefore, in order to get a critical thickness of Ti struts during compression, it is necessary to decide a suitable amount of binder. Fig. 6 shows a photograph of the sample compressed at 80 8C and sintered for 4 h at 1300 8C. Ti foam with 12 mm in diameter was fabricated with homogeneous pores and struts. Fig. 7 is the results of pore fraction in the samples fabricated with different compression temperature. The pore fractions were calculated to 85.63%, 86.70% and 78.03% in the compression temperatures of 70 8C, 80 8C and 90 8C,
Fig. 5. Change of strut thickness with different binder ratio. (a) 40%, (b) 50%, and (c) 60%.
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Fig. 6. Photograph of open-celled porous Ti compressed at 80 8C and sintered at 1300 8C.
Fig. 7. Comparison to pore fractions in samples fabricated with different compression temperature.
respectively, and the highest pore fraction was obtained at the compression temperature of 80 8C. This means that in order to manufacture the open-celled Ti foam by the process suggested in present study, it is very important to decide the suitable compression temperature.
homogeneous Ti struts and pore fraction above 85% and with the pore diameters between 300 mm and 500 mm. However, increase of compression temperature to 90 8C caused severe deformation of the polymer balls and the resultant composite structures were inhomogeneous.
4. Conclusions References The fabrication of open-celled Ti foam, by injection/infiltration of Ti feedstock and compression of composite body of polymer preform and the infiltrated Ti feedstock, were obtained. The foam structures depend on compression temperatures. In compression condition of 80 8C, the open cellular structure was obtained with
[1] [2] [3] [4]
G. Ryan, A. Pandit, D.P. Apatsidis, Biomaterials 27 (2006) 2651. J. Banhart, Prog. Mater. Sci. 46 (2001) 559. R.M. German, Sintering Theory and Practice, John Wiley & Sons, Inc., 1996. E. Klar, Powder metallurgy, vol.7: Metals Handbook, 9th ed., ASM International, Materials Park, OH, 1984.