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Fabrication of cylindrical uni-directional porous metal with explosive compaction
Materials Letters 137 (2014) 323–327
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of cylindrical uni-directional porous metal with explosive compaction K. Hokamoto a,n, M. Vesenjak b, Z. Ren b a b
Institute of Pulsed Power Science, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2014 Accepted 6 September 2014 Available online 16 September 2014
A new method to fabricate uni-directional porous (UniPore) metal employing the explosive compaction process of cylindrical pipe assembly is proposed. The proposed fabrication method enables production of long rods with uniform longitudinal pores along the whole rod length, exhibiting typical porous material behavior in the transverse direction while retaining solid material behavior in the longitudinal direction under compressive mechanical loading. Micro structural analysis of the fabricated samples demonstrates strong interface bonding between pipe walls. Many practical applications of UniPore porous metal are to be expected due to its unique properties and some preliminary investigations show very high efficiency of such structure for heat exchangers or heat sink applications. & 2014 Elsevier B.V. All rights reserved.
Keywords: High-energy-rate forming Explosive compaction Explosive welding Porous material Uni-directional porous metal
1. Introduction Porous materials [1,2] have superior properties such as low density (light-weight structures), effective damping, high grade of deformation, high energy absorption capability, and high thermal [3] and acoustic isolation properties. Until recently only one type of porous structure with uni-directional pores, named “Gasar” or “Lotus” porous metal, has been successfully fabricated by process of unidirectional solidification in pressurized gas atmospheres, developed by Shapovalov [4] and Nakajima [5–7]. However, the length of unidirectional pores achieved with this fabrication process is limited to several millimeters only. The recent investigation resulted in development of a new method for fabrication of porous metal with longer and uniform unidirectional pores, named “UniPore”, using explosive compaction of cylindrical metal pipe assembly. The experiments were performed by explosive compression of a bigger outer pipe completely filled with smaller inner pipes, which in turn are filled with a paraffin filler to prevent their complete compaction. The structure of the UniPore porous metal made by this method exhibits uniform porous cross-section in the longitudinal direction with length of pores limited only by the size of specimens. A similar type of structure was proposed by Koh et al. [8] which was made by extrusion of two metal components to make the
n
Corresponding author. E-mail address: [email protected] (K. Hokamoto).
http://dx.doi.org/10.1016/j.matlet.2014.09.039 0167-577X/& 2014 Elsevier B.V. All rights reserved.
composite followed by leaching of one component but the forming process limits the thickness of each component due to necking during stretching [9,10]. It prevents fabrication of longer sized specimens with uni-directional pores. In contrast, with use of an explosive compaction technique it is possible to fabricate longer sized specimens on the order of several meters. However, the possible size conventionally achieved by the explosive welding technique has to be considered [11]. The other advantages of the proposed fabrication process is the possibility of minimizing the change in thickness of the pipe walls because the material is accelerated only in the radial direction towards the outer pipe center.
2. Experimental Fig. 1 shows a schematic illustration of the fabrication assembly consisting of the outer copper (JIS-C1220) pipe (30 mm in diameter, 1.5 mm in wall thickness and 210 mm in length) filled with the inner copper (JIS-C1220) pipes (3 mm in diameter and 200 mm in length x 63 pipes). Low, middle and high porosity samples were made by changing the thickness of the inner pipes from 0.5, 0.3 to 0.2 mm, respectively. All the pipes are commercially available in Japan. Each inner pipe was entirely filled with paraffin, and the gaps 5 mm in height on top and bottom part of the outer pipe were filled with epoxy resin. The primary explosive used was PAXEX produced by Kayaku Japan Co., Ltd. (detonation velocity: approximately 2.4 km/s, density: approximately 530 kg/m3)
324
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
which was provided as powder. 750 g of PAVEX explosive was weighted and filled in the gap between the outer pipe and PVC pipe as shown in Fig. 1. The primary explosive was ignited by an electric detonator with a small amount of SEP explosive (10 g) as booster. The PAVEX explosive is ANFO based and it is normally used for explosive welding.
3. Results and discussion After the explosive compaction, the sample was successfully recovered as shown in Fig. 2. The wall of the inner pipes was welded well without separated space in the cross-sectional area. The paraffin was easily removed by melting just by heating the specimens up to 100 1C. The porosity measurement were equals to 0.276, 0.482 and 0.527 for low, middle and high porosity samples, respectively. Fig. 3 shows the microstructure of the three porous samples (a– c) after etching and a trace of Vickers penetrators under 10 gf (9.8 mN) load for the high porosity sample (d). The welding interface can be clearly observed. The center position, where the three inner pipes collided, shows a molten spot which suggests the collision of metal jets caused by intensive deformation. Clean surfaces contributed to the welding of the pipes. The metal jet is significant to achieve tight bonding between the materials, consequently assuring high bonding strength [11]. The measured Vickers hardness in the molten zone showed a relatively low value ranging between 44–61 HV (hardness of annealed copper is approximately 60 HV). Therefore, there is a chance of formation of shrinkage cavities or other defects in a limited region. The other areas showed higher hardness ranging between 130–150 HV, which is higher than the average value of 120 HV for the as-received pipes. The area close to the molten zone showed quite high hardness due to an intense high-rate deformation inducing ultrafine grained structure. The longitudinal cross-section shows a typical wavy bonding interface as normally found in an explosively welded interface. Also, regions of planar interface are observed because it is slightly difficult to satisfy the condition for wave formation [11] due to the change in the inclination angle inside the gap of the inner pipes. No fracturing was observed in any of the samples in the welded inner pipes. Only a small number of neckings in the inner pipe wall (about 20% decrease in thickness) were observed for the high porosity sample. This means that pores are perfectly isolated between each other and this fact might be important for application of this material for some special purposes. To estimate the order of velocity at collision the terminal velocity of a flying plate can be evaluated by the Gurney equation [12] V P max ¼
Fig. 1. Experimental assembly.
" pffiffiffiffiffiffi 2E
3 5ðm=CÞ þ 2ðm=CÞ2 ððR þr 0 Þ=r 0 Þ þ ð2r 0 =r 0 Þ
Fig. 2. Appearance of samples recovered.
#1=2 ð1Þ
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
325
Fig. 3. Microstructure of cross-section of samples, low (a), middle (b), high porosity (c) and trace of Vickers penetrator (d).
where C is mass of explosive and m is mass of outer pipe per unit cross-section. R is outer radius of explosive and r0 is inner radius of outer pipe. The Gurney energy is estimated to be 1.18 MJ/kg [13] with
the terminal velocity of 1220 m/s. This is the maximum estimated value but the real one should be smaller due to the collisions of inner pipes. Besides, it is possible to estimate the acceleration by the
326
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
Fig. 4. Force–displacement curves for transverse (left) and longitudinal (right) compressive loading direction.
detonation gas pressure P (0.25 ρV c 2 ; ρ is density of explosive and Vc is detonation velocity) at 0.763 GPa [14]. The deformation velocity shall be analyzed more accurately by numerical simulation in future. Since the high pressure is maintained for a while due to the relatively thick explosive, it is possible to calculate the acceleration of the outer pipe with the equation of motion. The calculated velocity at a distance of 1 mm from the outer pipe was 337 m/s and at 1.5 mm (radius of inner pipe) it was 412 m/s. The lower welding velocity is known to be 200 m/s [11], which is high enough to induce the fluidization that has been achieved even close to the outer pipe. In case of using the cylindrical geometry, a mach stem is sometimes induced along with the center axis and destroys the region due to the convergence of high pressure [14]. In the present experiments, the use of low detonation velocity explosive PAVEX did not cause such destructive phenomena and the collision velocity appeared to be quite uniform all through the cross-sectional area. It is known that the pressure or the velocity might decrease when the energy is intensively dissipated by deformation toward the center which results in the un-compacted area for the case of explosive compaction [14]. The longer pressurizing time achieved by using a thick explosive, provides the energy for long enough to compact the sample which allows the fabrication of uniformly compacted sample. The fabrication of aluminum or titanium rods is considered to be much easier as far as the dimensions of the sample are not significantly different. The reason for this is the lower material density which contributes to higher acceleration and consequently to higher collision velocity that makes the welding easier. The three fabricated porous samples and an as-received outer pipe were sliced at a longitudinal distance of 27 mm for further compressive testing. The samples were compressed perpendicularly and longitudinally, with respect to to the pore direction using the servo-hydraulic testing machine according to the standard DIN 50 125 with the cross-head velocity of 0.1 mm/s [15]. The results are presented in Fig. 4. A force plateau region typical for compressed porous structures can be observed. By increasing the porosity, the plateau force decreases and densification strain increases. In contrast, the specimens compressed in the longitudinal direction exhibit much higher load carrying capability with typical solid material characteristics. A highly anisotropic mechanical behavior of the material is confirmed. Further
investigations on the mechanical properties of UniPore structure including the rate effect are being conducted.
4. Summary The presented method using the explosive compaction process allows for fabrication of long rods with uniform pores in the longitudinal direction separated by fine walls (UniPore structure). The authors believe that it is also possible to fabricate materials with curved and/or crossed pores by just modifying the explosive compaction assembly. Preliminary investigations show very high efficiency of UniPore structure when used as heat exchangers or heat sinks, while even more practical applications are being expected due to its unique geometrical and physical properties.
Acknowledgments The authors are grateful for the support of the Amada Foundation AF-2013008, the Japan Society for Promotion of Science and the Slovenian Research Agency Grant no. BI-JP/11-13-004. References [1] Banhart J. Manufacture, characterisation and application of cellular metals and metal foams. Prog Mater Sci 2001;46:559–632. [2] Ashby MF, Evans A, Fleck NA, Gibson LJ, Hutchinson JW, Wadley HNG. Metal foams: a design guide. Burlington, Massachusetts: Elsevier Science; 2000. [3] Fiedler T, White N, Dahari M, Hooman K. On the electrical and thermal contact resistance of metal foam. Int J Heat Mass Transfer 2014;72:565–71. [4] Shapovalov VI. Porous metals. MRS Bull 1994;19:24–9. [5] Kashihara M, Hyun SK, Yonetani H, Kobi T, Nakajima H. Fabrication of lotustype porous carbon steel by unidirectional solidification in nitrogen atmosphere. Scr Mater 2006;54:509–12. [6] Nakajima H. Fabrication, properties and application of porous metals with directional pores. Prog Mater Sci 2007;52:1091–173. [7] Park JS, Hyun SK, Suzuki S, Nakajima H. Effect of transference velocity and hydrogen pressure on porosity and pore morphology of lotus-type porous copper fabricated by a continuous casting technique. Acta Mater 2007;55: 5646–54. [8] Koh H, Utsunomiya H, Miyamoto J, Sakai T. Fabrication of porous copper by cold extrusion and leaching. J Japan Inst Metals 2007;71-9:708–11.
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
[9] Osakada K, Limb M, Mellor PB. Hydrostatic extrusion of composite rods with hard core. Int J Mech Sci 1973;15:291–307. [10] Semiatin SL, Piehler HR. Formability of sandwich sheet materials in plane strain compression and rolling. Metall. Trans A 1979;10A:97–107. [11] Crossland B. Explosive welding of metals and its application. Oxford: Clarendon Press; 1982. [12] Meyers MA, Wang SL. An improved method for shock consolidation of powders. Acta Metall 1988;36–4:925–36.
327
[13] Buczkowski D, Zygmunt B. Detonation properties of mixtures of ammonium nitrate based fertilizers and fuels. Central Euro J Energetic Mater 2011;8–2: 99–106. [14] Pruemmr RA, Bahat TB, Siva Kumar K, Hokamoto K. Explosive compaction of powders & composites. Enfield. New Hampshire: Scientific Publisher; 2006. [15] Vesenjak M, Krstulović-Opara L, Ren Z. Characterization of irregular open-cell cellular structure with silicone pore filler. Polym Test 2013;32:1538–44.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of cylindrical uni-directional porous metal with explosive compaction K. Hokamoto a,n, M. Vesenjak b, Z. Ren b a b
Institute of Pulsed Power Science, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2014 Accepted 6 September 2014 Available online 16 September 2014
A new method to fabricate uni-directional porous (UniPore) metal employing the explosive compaction process of cylindrical pipe assembly is proposed. The proposed fabrication method enables production of long rods with uniform longitudinal pores along the whole rod length, exhibiting typical porous material behavior in the transverse direction while retaining solid material behavior in the longitudinal direction under compressive mechanical loading. Micro structural analysis of the fabricated samples demonstrates strong interface bonding between pipe walls. Many practical applications of UniPore porous metal are to be expected due to its unique properties and some preliminary investigations show very high efficiency of such structure for heat exchangers or heat sink applications. & 2014 Elsevier B.V. All rights reserved.
Keywords: High-energy-rate forming Explosive compaction Explosive welding Porous material Uni-directional porous metal
1. Introduction Porous materials [1,2] have superior properties such as low density (light-weight structures), effective damping, high grade of deformation, high energy absorption capability, and high thermal [3] and acoustic isolation properties. Until recently only one type of porous structure with uni-directional pores, named “Gasar” or “Lotus” porous metal, has been successfully fabricated by process of unidirectional solidification in pressurized gas atmospheres, developed by Shapovalov [4] and Nakajima [5–7]. However, the length of unidirectional pores achieved with this fabrication process is limited to several millimeters only. The recent investigation resulted in development of a new method for fabrication of porous metal with longer and uniform unidirectional pores, named “UniPore”, using explosive compaction of cylindrical metal pipe assembly. The experiments were performed by explosive compression of a bigger outer pipe completely filled with smaller inner pipes, which in turn are filled with a paraffin filler to prevent their complete compaction. The structure of the UniPore porous metal made by this method exhibits uniform porous cross-section in the longitudinal direction with length of pores limited only by the size of specimens. A similar type of structure was proposed by Koh et al. [8] which was made by extrusion of two metal components to make the
n
Corresponding author. E-mail address: [email protected] (K. Hokamoto).
http://dx.doi.org/10.1016/j.matlet.2014.09.039 0167-577X/& 2014 Elsevier B.V. All rights reserved.
composite followed by leaching of one component but the forming process limits the thickness of each component due to necking during stretching [9,10]. It prevents fabrication of longer sized specimens with uni-directional pores. In contrast, with use of an explosive compaction technique it is possible to fabricate longer sized specimens on the order of several meters. However, the possible size conventionally achieved by the explosive welding technique has to be considered [11]. The other advantages of the proposed fabrication process is the possibility of minimizing the change in thickness of the pipe walls because the material is accelerated only in the radial direction towards the outer pipe center.
2. Experimental Fig. 1 shows a schematic illustration of the fabrication assembly consisting of the outer copper (JIS-C1220) pipe (30 mm in diameter, 1.5 mm in wall thickness and 210 mm in length) filled with the inner copper (JIS-C1220) pipes (3 mm in diameter and 200 mm in length x 63 pipes). Low, middle and high porosity samples were made by changing the thickness of the inner pipes from 0.5, 0.3 to 0.2 mm, respectively. All the pipes are commercially available in Japan. Each inner pipe was entirely filled with paraffin, and the gaps 5 mm in height on top and bottom part of the outer pipe were filled with epoxy resin. The primary explosive used was PAXEX produced by Kayaku Japan Co., Ltd. (detonation velocity: approximately 2.4 km/s, density: approximately 530 kg/m3)
324
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
which was provided as powder. 750 g of PAVEX explosive was weighted and filled in the gap between the outer pipe and PVC pipe as shown in Fig. 1. The primary explosive was ignited by an electric detonator with a small amount of SEP explosive (10 g) as booster. The PAVEX explosive is ANFO based and it is normally used for explosive welding.
3. Results and discussion After the explosive compaction, the sample was successfully recovered as shown in Fig. 2. The wall of the inner pipes was welded well without separated space in the cross-sectional area. The paraffin was easily removed by melting just by heating the specimens up to 100 1C. The porosity measurement were equals to 0.276, 0.482 and 0.527 for low, middle and high porosity samples, respectively. Fig. 3 shows the microstructure of the three porous samples (a– c) after etching and a trace of Vickers penetrators under 10 gf (9.8 mN) load for the high porosity sample (d). The welding interface can be clearly observed. The center position, where the three inner pipes collided, shows a molten spot which suggests the collision of metal jets caused by intensive deformation. Clean surfaces contributed to the welding of the pipes. The metal jet is significant to achieve tight bonding between the materials, consequently assuring high bonding strength [11]. The measured Vickers hardness in the molten zone showed a relatively low value ranging between 44–61 HV (hardness of annealed copper is approximately 60 HV). Therefore, there is a chance of formation of shrinkage cavities or other defects in a limited region. The other areas showed higher hardness ranging between 130–150 HV, which is higher than the average value of 120 HV for the as-received pipes. The area close to the molten zone showed quite high hardness due to an intense high-rate deformation inducing ultrafine grained structure. The longitudinal cross-section shows a typical wavy bonding interface as normally found in an explosively welded interface. Also, regions of planar interface are observed because it is slightly difficult to satisfy the condition for wave formation [11] due to the change in the inclination angle inside the gap of the inner pipes. No fracturing was observed in any of the samples in the welded inner pipes. Only a small number of neckings in the inner pipe wall (about 20% decrease in thickness) were observed for the high porosity sample. This means that pores are perfectly isolated between each other and this fact might be important for application of this material for some special purposes. To estimate the order of velocity at collision the terminal velocity of a flying plate can be evaluated by the Gurney equation [12] V P max ¼
Fig. 1. Experimental assembly.
" pffiffiffiffiffiffi 2E
3 5ðm=CÞ þ 2ðm=CÞ2 ððR þr 0 Þ=r 0 Þ þ ð2r 0 =r 0 Þ
Fig. 2. Appearance of samples recovered.
#1=2 ð1Þ
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
325
Fig. 3. Microstructure of cross-section of samples, low (a), middle (b), high porosity (c) and trace of Vickers penetrator (d).
where C is mass of explosive and m is mass of outer pipe per unit cross-section. R is outer radius of explosive and r0 is inner radius of outer pipe. The Gurney energy is estimated to be 1.18 MJ/kg [13] with
the terminal velocity of 1220 m/s. This is the maximum estimated value but the real one should be smaller due to the collisions of inner pipes. Besides, it is possible to estimate the acceleration by the
326
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
Fig. 4. Force–displacement curves for transverse (left) and longitudinal (right) compressive loading direction.
detonation gas pressure P (0.25 ρV c 2 ; ρ is density of explosive and Vc is detonation velocity) at 0.763 GPa [14]. The deformation velocity shall be analyzed more accurately by numerical simulation in future. Since the high pressure is maintained for a while due to the relatively thick explosive, it is possible to calculate the acceleration of the outer pipe with the equation of motion. The calculated velocity at a distance of 1 mm from the outer pipe was 337 m/s and at 1.5 mm (radius of inner pipe) it was 412 m/s. The lower welding velocity is known to be 200 m/s [11], which is high enough to induce the fluidization that has been achieved even close to the outer pipe. In case of using the cylindrical geometry, a mach stem is sometimes induced along with the center axis and destroys the region due to the convergence of high pressure [14]. In the present experiments, the use of low detonation velocity explosive PAVEX did not cause such destructive phenomena and the collision velocity appeared to be quite uniform all through the cross-sectional area. It is known that the pressure or the velocity might decrease when the energy is intensively dissipated by deformation toward the center which results in the un-compacted area for the case of explosive compaction [14]. The longer pressurizing time achieved by using a thick explosive, provides the energy for long enough to compact the sample which allows the fabrication of uniformly compacted sample. The fabrication of aluminum or titanium rods is considered to be much easier as far as the dimensions of the sample are not significantly different. The reason for this is the lower material density which contributes to higher acceleration and consequently to higher collision velocity that makes the welding easier. The three fabricated porous samples and an as-received outer pipe were sliced at a longitudinal distance of 27 mm for further compressive testing. The samples were compressed perpendicularly and longitudinally, with respect to to the pore direction using the servo-hydraulic testing machine according to the standard DIN 50 125 with the cross-head velocity of 0.1 mm/s [15]. The results are presented in Fig. 4. A force plateau region typical for compressed porous structures can be observed. By increasing the porosity, the plateau force decreases and densification strain increases. In contrast, the specimens compressed in the longitudinal direction exhibit much higher load carrying capability with typical solid material characteristics. A highly anisotropic mechanical behavior of the material is confirmed. Further
investigations on the mechanical properties of UniPore structure including the rate effect are being conducted.
4. Summary The presented method using the explosive compaction process allows for fabrication of long rods with uniform pores in the longitudinal direction separated by fine walls (UniPore structure). The authors believe that it is also possible to fabricate materials with curved and/or crossed pores by just modifying the explosive compaction assembly. Preliminary investigations show very high efficiency of UniPore structure when used as heat exchangers or heat sinks, while even more practical applications are being expected due to its unique geometrical and physical properties.
Acknowledgments The authors are grateful for the support of the Amada Foundation AF-2013008, the Japan Society for Promotion of Science and the Slovenian Research Agency Grant no. BI-JP/11-13-004. References [1] Banhart J. Manufacture, characterisation and application of cellular metals and metal foams. Prog Mater Sci 2001;46:559–632. [2] Ashby MF, Evans A, Fleck NA, Gibson LJ, Hutchinson JW, Wadley HNG. Metal foams: a design guide. Burlington, Massachusetts: Elsevier Science; 2000. [3] Fiedler T, White N, Dahari M, Hooman K. On the electrical and thermal contact resistance of metal foam. Int J Heat Mass Transfer 2014;72:565–71. [4] Shapovalov VI. Porous metals. MRS Bull 1994;19:24–9. [5] Kashihara M, Hyun SK, Yonetani H, Kobi T, Nakajima H. Fabrication of lotustype porous carbon steel by unidirectional solidification in nitrogen atmosphere. Scr Mater 2006;54:509–12. [6] Nakajima H. Fabrication, properties and application of porous metals with directional pores. Prog Mater Sci 2007;52:1091–173. [7] Park JS, Hyun SK, Suzuki S, Nakajima H. Effect of transference velocity and hydrogen pressure on porosity and pore morphology of lotus-type porous copper fabricated by a continuous casting technique. Acta Mater 2007;55: 5646–54. [8] Koh H, Utsunomiya H, Miyamoto J, Sakai T. Fabrication of porous copper by cold extrusion and leaching. J Japan Inst Metals 2007;71-9:708–11.
K. Hokamoto et al. / Materials Letters 137 (2014) 323–327
[9] Osakada K, Limb M, Mellor PB. Hydrostatic extrusion of composite rods with hard core. Int J Mech Sci 1973;15:291–307. [10] Semiatin SL, Piehler HR. Formability of sandwich sheet materials in plane strain compression and rolling. Metall. Trans A 1979;10A:97–107. [11] Crossland B. Explosive welding of metals and its application. Oxford: Clarendon Press; 1982. [12] Meyers MA, Wang SL. An improved method for shock consolidation of powders. Acta Metall 1988;36–4:925–36.
327
[13] Buczkowski D, Zygmunt B. Detonation properties of mixtures of ammonium nitrate based fertilizers and fuels. Central Euro J Energetic Mater 2011;8–2: 99–106. [14] Pruemmr RA, Bahat TB, Siva Kumar K, Hokamoto K. Explosive compaction of powders & composites. Enfield. New Hampshire: Scientific Publisher; 2006. [15] Vesenjak M, Krstulović-Opara L, Ren Z. Characterization of irregular open-cell cellular structure with silicone pore filler. Polym Test 2013;32:1538–44.