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Effect of solidification velocity on pore morphology of lotus-type porous copper fabricated by unidirectional solidification
Materials Letters 57 (2003) 3149 – 3154 www.elsevier.com/locate/matlet
Effect of solidification velocity on pore morphology of lotus-type porous copper fabricated by unidirectional solidification S.K. Hyun *, H. Nakajima The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Received 21 November 2002; accepted 25 December 2002
Abstract The effect of solidification velocity on pore size and porosity of lotus-type porous copper fabricated by unidirectional solidification in the pressurized hydrogen and argon has been investigated. The pore diameter and the pore number density are significantly affected by the solidification velocity, although the porosity is not varied much. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous copper; Unidirectional solidification; Solidification velocity; Porosity; Hydrogen
1. Introduction Recently, a new type of porous metals whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification method at a pressurized hydrogen or nitrogen gas with argon gas [1– 15]. These porous metals should be distinguished from the conventional porous metals whose pores are almost isotropic and randomly distributed such as foamed metals, cellular and sintered metals. Hereafter, we designate these porous metals with elongated pores as lotus-type porous metals, because they look like lotus roots. The tensile and compressive properties of the lotus-type porous copper whose pores are aligned in one direction are superior to that of conventional
* Corresponding author. Tel.: +81-6-6879-8435; fax: +81-66879-8439. E-mail address: [email protected] (S.K. Hyun).
porous metals [12,16 –18], which are much affected by the pore morphology of the porous metals. Therefore, the control of the pore morphology is important. The pore size and the porosity of the lotus-type porous metals can be controlled by the gas pressure during melting and solidification, the solidification velocity, the temperature during melting and so on. Thus, various kinds of porous metals can be fabricated by controlling such experimental conditions. Recently, Yamamura et al. [13,14] and Nakajima et al. [12] reported that the porosity and the pore size in the lotus-type porous copper were significantly affected by the partial pressures of hydrogen and argon and the temperature during melting. However, the effect of solidification velocity on the porosity and the pore morphology of the lotus-type porous metals has not been studied yet. From this point of view, the present work was undertaken to investigate the effect of solidification velocity on the porosity and the pore morphology of the lotus-type
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00012-0
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porous copper, which was fabricated by controlling the two different solidification velocities at a given pressure of hydrogen and argon.
2. Experimental procedure The lotus-type porous copper were fabricated by the vacuum-assisted and pressurized casting apparatus consisting of a graphite crucible (110 mm in outside diameter, 90 mm in inside diameter, and 170 mm in length) with a hole (20 mm in diameter) on the bottom of the crucible, a stopper stick for preventing the melt flow through the hole, an induction heating coil and a mould (70 mm in diameter and 100 mm in length) with water-cooled copper plate.
High purity copper (99.99%) was melted in the crucible by radio-frequency heating under high-pressure mixture gas of hydrogen and argon. The purities of gases used for these experiments were 99.999%. The temperature of the molten copper in the crucible was monitored by a W –5Re/W – 26Re thermocouple and was set to be 1473 K. In order to make hydrogen dissolve in the molten copper the pressurized condition was maintained for 1.8 ks. Thus, hydrogen was dissolved uniformly into the molten copper to the equilibrium concentration according to the Sieverts’ law in the hydrogen atmosphere. The partial pressures of hydrogen and argon during melting and solidification were set 0.4 and 0.7 MPa, respectively. Then, the molten copper was poured into the mould whose bottom plate was cooled down with
Fig. 1. Schematic of measurement for solidification velocity.
S.K. Hyun, H. Nakajima / Materials Letters 57 (2003) 3149–3154
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The samples were solidified at different solidification velocities through the condition whether a ceramic sheet was inserted between a carbon plate and a copper plate of chiller as shown in Fig. 1. Rtype thermocouples were inserted in the mould and coupled with a computer-controlled data acquisition system for recording the cooling curves. The solidified ingots were sectioned vertically by using a sparkerosion wire-cutting machine (Model A320D, Sodick). These sections were observed under an optical microscope. The pore diameter was measured by using an image analysis system. The porosity was evaluated from a measurement of its weight and volume for each configuration. Fig. 2. Comparison of solidification velocity.
3. Results and discussion water circulated through a chiller. The lateral side of the mould was made of alumina-coated stainless steel tube, which was suitable for heat-insulating material in order to be solidified in one direction from the bottom to the top. During the solidification, hydrogen in the melt was rejected at the solid– liquid interface due to the solubility gap of hydrogen between in the liquid and in the solid and forms cylindrical pores that were aligned parallel to the solidification direction.
It is considered in the present work that each solidification velocity is constant since the beginning time of solidification linearly increases with increasing distance from bottom of mould as shown in Fig. 2, in which the thermal conductivity of copper can be sufficiently high. The samples are solidified with solidification velocities of 1.185 and 0.697 mm s 1 through different condition whether ceramic sheet was
Fig. 3. Microstructure of lotus-type porous copper in perpendicular section to solidification direction.
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inserted between the graphite plate and the copper plate of chiller or not. The optical micrographs of the cross-section of the lotus-type porous copper are shown in Fig. 3. It was apparent that the pore morphology is significantly affected by the solidification velocity. The average pore diameter is plotted against the distance from the bottom of the mould as shown in Fig. 4. The pore diameter in the porous copper fabricated with solidification velocity of 0.697 mm s 1 is twice larger that that with 1.185 mm s 1. The pores are considered to nucleate heterogeneously [13]. The oxide layer may remain at the solid – liquid interface, which becomes a nucleation site of the pore. During the unidirectional solidification, only 50% of the hydrogen amount in the melt contributes to the pore formation according to the evaluation of the porosity in the porous copper; half of the hydrogen may escape from the melt to the atmosphere during and/or solidification [13]. The pores grow by the insoluble hydrogen rejected in the solid –liquid interface and hydrogen diffusion from the solid copper to the pore. It can be understood that the amount of hydrogen diffusing from liquid to the pores increases with decreasing solidification velocity, and then the pores formed with the velocity of 0.697 mm s 1 become larger than those with 1.185 mm s 1. Fig. 5 shows the distribution of pores in the cross-sectional area at 25 mm from the bottom of the mould in the porous copper fabricated with different velocity. It is apparent in Fig. 5 that the number density of pores at 1.185 mm s 1 is higher than that
Fig. 4. Average pore diameter plotted against distance from bottom of mould.
Fig. 5. Distribution of pores in the cross-sectional area at 25 mm from bottom of mould.
at 0.697 mm s 1. As the solidification velocity increases, the pore size decreases but the number density of pores increases. Increasing the solidifica-
Fig. 6. Porosity plotted against distance from bottom of mould.
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the present work is much lower than the atmospheric pressure of 1.1 MPa. From this point of view, the porosity increases a little with the solidification velocity. A further investigation is required to understand the effect of solidification velocity on the pore morphology quantitatively.
4. Conclusions
Fig. 7. Capillary pressure for copper melt.
tion velocity causes an increase in the hydrogen supersaturation of the solid– liquid interface and then the driving force to nucleate pores increases. If the entire amount of hydrogen dissolved in the melt is diffused into the pores, the pore size can decrease with increasing number density of pores. Consequently, it can be understood that the solidification velocity can affect the average pore diameter of lotus-type porous copper. On the other hand, the porosity as shown in Fig. 6 is not varied with the solidification velocity so much as like pore diameter. The porosity at 0.697 mm s 1 is approximately 4% higher than that at 1.185 mm s 1. Neglecting the hydrostatic pressure of the melt above the pore, the pressure P in the pore is given by P ¼ PH2
2r ; þ PAr þ r
In the present work, the lotus-type porous copper having long aligned pores was fabricated unidirectional solidification of the melt in a mixture gas of hydrogen and argon. An increase of solidification velocity from 0.697 to 1.185 mm s 1 leads to a decrease of pore diameter as well as porosity and an increase of pore number density in the lotus-type porous copper. The pore diameter and the pore number density are significantly affected by the solidification velocity, although the porosity is not varied much.
Acknowledgements The present work was supported by Grant-in-Aid for Scientific Research A (No. 09355025) and for University and Society Collaboration (No. 11792022) of the Ministry of Education, Culture, Sports, Science and Technology.
References ð1Þ
where PH2 and PAr are the pressures of hydrogen and argon in mixture gas atmosphere, respectively, r is the surface tension of the melt, which is 1.285 N m 1 for copper melt [19], and r is the radius of the pore. 2r/r is the capillary pressure which acts as resistance to the pore formation during the solidification. As the solidification velocity increases, the pore diameter decreases, and then the capillary pressure increases. Consequently, the volume fraction of pores (porosity) can decrease with increasing this capillary pressure related with the pressure in pores. As shown in Fig. 7, the calculated capillary pressure (0.05 –0.1 MPa) in
[1] O. Knacke, H. Probst, J. Wernekinck, Z. Met.kd. 70 (1979) 1. [2] I. Svensson, H.S. Fredriksson, Proc. Int. Conf. organized by the Applied Metallurgy and Metals Tech. Group of TMS, U. of Warwick, 1980, p. 376. [3] M. Imabayashi, M. Ichimura, Y. Kanno, Trans. JIM 24 (1983) 93. [4] L.V. Boiko, V.I. Shapovalov, E.A. Chernykh, Metallurgiya 346 (1991) 78. [5] H. Nakajima, Mater. Integr. 12 (1999) 37. [6] H. Nakajima, Prod. Tech. 51 (1999) 60. [7] H. Nakajima, Boundary 15 (1999) 9. [8] H. Nakajima, Funct. Mater. 20 (2000) 27. [9] H. Nakajima, J. High Temp. Mater. 26 (2000) 95. [10] H. Nakajima, S.K. Hyun, K. Murakami, J. Jpn. Copper Brass Res. Assoc. 39 (2000) 207. [11] H. Nakajima, Bull. Iron Steel Inst. Jpn. 6 (2001) 701.
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[12] H. Nakajima, S.K. Hyun, K. Ohashi, K. Ota, K. Murakami, Colloids Surf., A 179 (2001) 209. [13] S. Yamamura, Y. Shiota, K. Murakami, H. Nakajima, Mater. Sci. Eng., A 318 (2001) 137. [14] S. Yamamura, K. Murakami, Y. Shiota, H. Nakajima, Cellular Metals and Metal Foaming Technology, Verlag MIT, Bremen, Germany, 2001, p. 425. [15] S.K. Hyun, H. Nakajima, Mater. Trans. 43 (2002) 526. [16] S.K. Hyun, Y. Shiota, K. Murakami, H. Nakajima, in: M.
Koiwa, K. Otsuka, T. Miyazaki (Eds.), Proc. Int. Conf. on Solid – Solid Phase Transformations ’99, Japan Inst. Metals, Japan, 1999, p. 341. [17] S.K. Hyun, K. Murakami, H. Nakajima, Mater. Sci. Eng., A 299 (2001) 241. [18] S.K. Hyun, H. Nakajima, Mater. Sci. Eng., A 340 (2003) 258. [19] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, Butterworth-Heinemann, New York, USA, 1992, p. 12.13.
Effect of solidification velocity on pore morphology of lotus-type porous copper fabricated by unidirectional solidification S.K. Hyun *, H. Nakajima The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Received 21 November 2002; accepted 25 December 2002
Abstract The effect of solidification velocity on pore size and porosity of lotus-type porous copper fabricated by unidirectional solidification in the pressurized hydrogen and argon has been investigated. The pore diameter and the pore number density are significantly affected by the solidification velocity, although the porosity is not varied much. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous copper; Unidirectional solidification; Solidification velocity; Porosity; Hydrogen
1. Introduction Recently, a new type of porous metals whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification method at a pressurized hydrogen or nitrogen gas with argon gas [1– 15]. These porous metals should be distinguished from the conventional porous metals whose pores are almost isotropic and randomly distributed such as foamed metals, cellular and sintered metals. Hereafter, we designate these porous metals with elongated pores as lotus-type porous metals, because they look like lotus roots. The tensile and compressive properties of the lotus-type porous copper whose pores are aligned in one direction are superior to that of conventional
* Corresponding author. Tel.: +81-6-6879-8435; fax: +81-66879-8439. E-mail address: [email protected] (S.K. Hyun).
porous metals [12,16 –18], which are much affected by the pore morphology of the porous metals. Therefore, the control of the pore morphology is important. The pore size and the porosity of the lotus-type porous metals can be controlled by the gas pressure during melting and solidification, the solidification velocity, the temperature during melting and so on. Thus, various kinds of porous metals can be fabricated by controlling such experimental conditions. Recently, Yamamura et al. [13,14] and Nakajima et al. [12] reported that the porosity and the pore size in the lotus-type porous copper were significantly affected by the partial pressures of hydrogen and argon and the temperature during melting. However, the effect of solidification velocity on the porosity and the pore morphology of the lotus-type porous metals has not been studied yet. From this point of view, the present work was undertaken to investigate the effect of solidification velocity on the porosity and the pore morphology of the lotus-type
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00012-0
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S.K. Hyun, H. Nakajima / Materials Letters 57 (2003) 3149–3154
porous copper, which was fabricated by controlling the two different solidification velocities at a given pressure of hydrogen and argon.
2. Experimental procedure The lotus-type porous copper were fabricated by the vacuum-assisted and pressurized casting apparatus consisting of a graphite crucible (110 mm in outside diameter, 90 mm in inside diameter, and 170 mm in length) with a hole (20 mm in diameter) on the bottom of the crucible, a stopper stick for preventing the melt flow through the hole, an induction heating coil and a mould (70 mm in diameter and 100 mm in length) with water-cooled copper plate.
High purity copper (99.99%) was melted in the crucible by radio-frequency heating under high-pressure mixture gas of hydrogen and argon. The purities of gases used for these experiments were 99.999%. The temperature of the molten copper in the crucible was monitored by a W –5Re/W – 26Re thermocouple and was set to be 1473 K. In order to make hydrogen dissolve in the molten copper the pressurized condition was maintained for 1.8 ks. Thus, hydrogen was dissolved uniformly into the molten copper to the equilibrium concentration according to the Sieverts’ law in the hydrogen atmosphere. The partial pressures of hydrogen and argon during melting and solidification were set 0.4 and 0.7 MPa, respectively. Then, the molten copper was poured into the mould whose bottom plate was cooled down with
Fig. 1. Schematic of measurement for solidification velocity.
S.K. Hyun, H. Nakajima / Materials Letters 57 (2003) 3149–3154
3151
The samples were solidified at different solidification velocities through the condition whether a ceramic sheet was inserted between a carbon plate and a copper plate of chiller as shown in Fig. 1. Rtype thermocouples were inserted in the mould and coupled with a computer-controlled data acquisition system for recording the cooling curves. The solidified ingots were sectioned vertically by using a sparkerosion wire-cutting machine (Model A320D, Sodick). These sections were observed under an optical microscope. The pore diameter was measured by using an image analysis system. The porosity was evaluated from a measurement of its weight and volume for each configuration. Fig. 2. Comparison of solidification velocity.
3. Results and discussion water circulated through a chiller. The lateral side of the mould was made of alumina-coated stainless steel tube, which was suitable for heat-insulating material in order to be solidified in one direction from the bottom to the top. During the solidification, hydrogen in the melt was rejected at the solid– liquid interface due to the solubility gap of hydrogen between in the liquid and in the solid and forms cylindrical pores that were aligned parallel to the solidification direction.
It is considered in the present work that each solidification velocity is constant since the beginning time of solidification linearly increases with increasing distance from bottom of mould as shown in Fig. 2, in which the thermal conductivity of copper can be sufficiently high. The samples are solidified with solidification velocities of 1.185 and 0.697 mm s 1 through different condition whether ceramic sheet was
Fig. 3. Microstructure of lotus-type porous copper in perpendicular section to solidification direction.
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inserted between the graphite plate and the copper plate of chiller or not. The optical micrographs of the cross-section of the lotus-type porous copper are shown in Fig. 3. It was apparent that the pore morphology is significantly affected by the solidification velocity. The average pore diameter is plotted against the distance from the bottom of the mould as shown in Fig. 4. The pore diameter in the porous copper fabricated with solidification velocity of 0.697 mm s 1 is twice larger that that with 1.185 mm s 1. The pores are considered to nucleate heterogeneously [13]. The oxide layer may remain at the solid – liquid interface, which becomes a nucleation site of the pore. During the unidirectional solidification, only 50% of the hydrogen amount in the melt contributes to the pore formation according to the evaluation of the porosity in the porous copper; half of the hydrogen may escape from the melt to the atmosphere during and/or solidification [13]. The pores grow by the insoluble hydrogen rejected in the solid –liquid interface and hydrogen diffusion from the solid copper to the pore. It can be understood that the amount of hydrogen diffusing from liquid to the pores increases with decreasing solidification velocity, and then the pores formed with the velocity of 0.697 mm s 1 become larger than those with 1.185 mm s 1. Fig. 5 shows the distribution of pores in the cross-sectional area at 25 mm from the bottom of the mould in the porous copper fabricated with different velocity. It is apparent in Fig. 5 that the number density of pores at 1.185 mm s 1 is higher than that
Fig. 4. Average pore diameter plotted against distance from bottom of mould.
Fig. 5. Distribution of pores in the cross-sectional area at 25 mm from bottom of mould.
at 0.697 mm s 1. As the solidification velocity increases, the pore size decreases but the number density of pores increases. Increasing the solidifica-
Fig. 6. Porosity plotted against distance from bottom of mould.
S.K. Hyun, H. Nakajima / Materials Letters 57 (2003) 3149–3154
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the present work is much lower than the atmospheric pressure of 1.1 MPa. From this point of view, the porosity increases a little with the solidification velocity. A further investigation is required to understand the effect of solidification velocity on the pore morphology quantitatively.
4. Conclusions
Fig. 7. Capillary pressure for copper melt.
tion velocity causes an increase in the hydrogen supersaturation of the solid– liquid interface and then the driving force to nucleate pores increases. If the entire amount of hydrogen dissolved in the melt is diffused into the pores, the pore size can decrease with increasing number density of pores. Consequently, it can be understood that the solidification velocity can affect the average pore diameter of lotus-type porous copper. On the other hand, the porosity as shown in Fig. 6 is not varied with the solidification velocity so much as like pore diameter. The porosity at 0.697 mm s 1 is approximately 4% higher than that at 1.185 mm s 1. Neglecting the hydrostatic pressure of the melt above the pore, the pressure P in the pore is given by P ¼ PH2
2r ; þ PAr þ r
In the present work, the lotus-type porous copper having long aligned pores was fabricated unidirectional solidification of the melt in a mixture gas of hydrogen and argon. An increase of solidification velocity from 0.697 to 1.185 mm s 1 leads to a decrease of pore diameter as well as porosity and an increase of pore number density in the lotus-type porous copper. The pore diameter and the pore number density are significantly affected by the solidification velocity, although the porosity is not varied much.
Acknowledgements The present work was supported by Grant-in-Aid for Scientific Research A (No. 09355025) and for University and Society Collaboration (No. 11792022) of the Ministry of Education, Culture, Sports, Science and Technology.
References ð1Þ
where PH2 and PAr are the pressures of hydrogen and argon in mixture gas atmosphere, respectively, r is the surface tension of the melt, which is 1.285 N m 1 for copper melt [19], and r is the radius of the pore. 2r/r is the capillary pressure which acts as resistance to the pore formation during the solidification. As the solidification velocity increases, the pore diameter decreases, and then the capillary pressure increases. Consequently, the volume fraction of pores (porosity) can decrease with increasing this capillary pressure related with the pressure in pores. As shown in Fig. 7, the calculated capillary pressure (0.05 –0.1 MPa) in
[1] O. Knacke, H. Probst, J. Wernekinck, Z. Met.kd. 70 (1979) 1. [2] I. Svensson, H.S. Fredriksson, Proc. Int. Conf. organized by the Applied Metallurgy and Metals Tech. Group of TMS, U. of Warwick, 1980, p. 376. [3] M. Imabayashi, M. Ichimura, Y. Kanno, Trans. JIM 24 (1983) 93. [4] L.V. Boiko, V.I. Shapovalov, E.A. Chernykh, Metallurgiya 346 (1991) 78. [5] H. Nakajima, Mater. Integr. 12 (1999) 37. [6] H. Nakajima, Prod. Tech. 51 (1999) 60. [7] H. Nakajima, Boundary 15 (1999) 9. [8] H. Nakajima, Funct. Mater. 20 (2000) 27. [9] H. Nakajima, J. High Temp. Mater. 26 (2000) 95. [10] H. Nakajima, S.K. Hyun, K. Murakami, J. Jpn. Copper Brass Res. Assoc. 39 (2000) 207. [11] H. Nakajima, Bull. Iron Steel Inst. Jpn. 6 (2001) 701.
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[12] H. Nakajima, S.K. Hyun, K. Ohashi, K. Ota, K. Murakami, Colloids Surf., A 179 (2001) 209. [13] S. Yamamura, Y. Shiota, K. Murakami, H. Nakajima, Mater. Sci. Eng., A 318 (2001) 137. [14] S. Yamamura, K. Murakami, Y. Shiota, H. Nakajima, Cellular Metals and Metal Foaming Technology, Verlag MIT, Bremen, Germany, 2001, p. 425. [15] S.K. Hyun, H. Nakajima, Mater. Trans. 43 (2002) 526. [16] S.K. Hyun, Y. Shiota, K. Murakami, H. Nakajima, in: M.
Koiwa, K. Otsuka, T. Miyazaki (Eds.), Proc. Int. Conf. on Solid – Solid Phase Transformations ’99, Japan Inst. Metals, Japan, 1999, p. 341. [17] S.K. Hyun, K. Murakami, H. Nakajima, Mater. Sci. Eng., A 299 (2001) 241. [18] S.K. Hyun, H. Nakajima, Mater. Sci. Eng., A 340 (2003) 258. [19] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, Butterworth-Heinemann, New York, USA, 1992, p. 12.13.