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Effect of Zn addition on mechanical properties of brass hollow spheres
Materials Science and Engineering A 483–484 (2008) 254–257
Effect of Zn addition on mechanical properties of brass hollow spheres Ja-Myeong Koo a , Hideki Araki b , Seung-Boo Jung a,∗ a
Department of Advanced Materials Science & Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, Republic of Korea b Department of Materials Science and Engineering, Faculty of Engineering Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 6 June 2005; received in revised form 5 January 2006; accepted 21 January 2006
Abstract Polymer spheres coated with a slurry, consisting of copper(I) oxide + Zn (0–30 wt.%) powders, water and polyacrilamide (PA), were sintered at temperatures ranging from 800 to 930 ◦ C under a hydrogen atmosphere to fabricate high-porous brass hollow spheres. The metallurgical and mechanical properties of the spheres were investigated with different Zn contents and sintering temperatures. Increasing sintering temperature accelerated the Zn evaporation and sphere densification. Increasing Zn content and decreasing sintering temperature increased the porosity of the spheres, but degraded their mechanical properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper–zinc alloy (brass); Powder metallurgy; Hollow sphere; Porous metal; Sintering; Zn evaporation; Compression test
1. Introduction Development of light materials is one of the most critical issues in modern industry, particularly in transport industries where saving energy is paramount. Porous materials are increasingly considered to hold promise as light-weight structural materials and functional materials with superior specific strength, sound damping, specific surface area, heat emission efficiency and filtering property [1,2]. Many researchers have already reported fabrication techniques, characteristics and applications of various porous metals. In particular, the development of low-cost processing techniques has increased the interest [1–4]. Aluminum (Al) foams have been tested and used in the automobile, aerospace and railway industries. Copper (Cu) foams have higher thermal/electrical conductivity, melting point and better mechanical properties than Al foams. However, the fabrication of Cu foams has been limited due to the high density of Cu [1]. In two previous studies, Cu hollow spheres with different sphere diameters and shell thicknesses were fabricated successfully using the sintering process with copper oxide powder under a hydrogen atmosphere [3,4]. In the present work,
∗
Corresponding author. Tel.: +82 31 290 7383; fax: +82 31 290 7371. E-mail address: [email protected] (S.-B. Jung).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.183
the effect of Zn addition on the microstructural evolutions and mechanical properties of brass hollow spheres were investigated to study the feasibility of high-porous brass hollow spheres. Zn has a much higher vapor pressure than Cu at the sintering temperature, and therefore evaporates easily during sintering [5]. To increase and/or control the sphere porosity, Zn powder was added to the slurry containing Cu2 O powder. 2. Experimental procedure A 15 mm-diameter-polymer sphere was used as the core of the hollow sphere. The solvent and binder were water and polyacrilamide (PA), respectively. A slurry containing 3 mdiameter-copper (I) oxide powder, 30 m-diameter-Zn powder, water, and PA was prepared using a stirring machine. The polymer spheres were dip-coated with the slurry and dried for the shell thickness of 1.8 mm. The samples were sintered at temperatures ranging from 800 to 930 ◦ C for 1 h under a hydrogen atmosphere. The polymer ball was burned out and eliminated by evaporation during sintering. After sintering, the samples were mounted in cold epoxy and mechanically polished for the microstructural examination using scanning electron microscopy (SEM). The densities of the samples were measured by an electronic scale utilizing the Archimedes’ principle. The chemical compositions of the samples were analyzed using an electron-probe micro-analyzer
J.-M. Koo et al. / Materials Science and Engineering A 483–484 (2008) 254–257
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Fig. 1. Cross-sectional SEM images of the brass hollow spheres sintered with Cu2 O powder (a) and Cu2 O + 30 wt.% Zn powders (b and c) at 930 ◦ C (a and b) and 800 ◦ C (c). The gray and dark regions, marked as A and B, indicate the brass phase and pore, respectively.
(EPMA). X-ray diffraction (XRD) was employed to analyze the reduction of Cu2 O to Cu and the solid-solution of Zn in the reduced Cu matrix after sintering. The Vickers hardness was measured on transverse sections of the hollow spheres with a testing time of 10 s and a load of 50 g mass. The compression strength of one hollow sphere was evaluated at a displacement rate of 1 mm/min. After testing, the fracture samples were observed using an optical microscope. 3. Results and discussion Fig. 1 shows the cross-sectional micrographs of brass hollow spheres sintered with different Zn contents and sintering temperatures. The Cu hollow sphere sintered with Cu2 O powder at 930 ◦ C showed a dense typical Cu structure, as shown in Fig. 1a. However, the addition of Zn powder increased the porosity of the sphere, as shown in Fig. 1b. As Zn has extremely high vapor pressure at sintering temperatures, it was likely that the added Zn evaporated easily to form the pores in the spheres, rather than remaining as solid-solute in the reduced Cu matrix [5]. The sphere porosity increased with decreasing sintering temperature, as shown in Fig. 1c, because of the decreased diffusivities of Cu and Zn atoms [6]. Volume shrinkage with many inhomogeneous micro-pores was also observed in the spheres. Fig. 2shows the chemical compositions (wt.%) of the hollow spheres sintered with different Zn contents and sintering temperatures. The Zn concentration in the spheres decreased
Fig. 2. EPMA analysis results of the brass hollow spheres after sintering.
Fig. 3. XRD analysis results of the brass hollow spheres sintered with Cu–30 wt.% Zn powders at 800 ◦ C.
with increasing sintering temperature and decreasing the amount of Zn powder contained in the slurry, due to the Zn evaporation. In particular, increasing sintering temperature accelerated the Zn evaporation. The Zn concentration in the spheres sintered with Cu2 O + 30 wt.% Zn powders decreased from 24.5 wt.% to 8.0 wt.% as the sintering temperature increased from 800 ◦ C to 930 ◦ C, because the Zn vapor pressure increases linearly with the exponential function of temperature [5]. The Zn concentrations of the spheres sintered with 20 wt.% and less Zn powder at 800 ◦ C were higher than the Zn contents contained in the slurry. In this study, the Zn content
Fig. 4. Density of the brass hollow spheres after sintering.
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Fig. 5. Vickers hardness of the brass hollow spheres after sintering.
Fig. 7. Compression strength of the brass hollow spheres after sintering.
was the mass ratio of Zn powder to Zn and Cu2 O powders before sintering, whereas the Zn concentration was that of Zn to Zn and Cu after sintering. It was suggested that the oxygen in the Cu2 O was reduced and evaporated during sintering, thereby increasing the Zn concentration in the spheres. Fig. 3 shows the XRD analysis results of the hollow sphere sintered with Cu2 O + 30 wt.% Zn powders at 800 ◦ C. The Cu2 O and Zn peaks disappeared after sintering, irrespective of Zn content and sintering temperature, indicating that the Cu2 O and Zn were completely transformed to Cu with solid-solute Zn. The Cu peaks of the spheres, sintered with Cu2 O + 30 wt.% Zn powders at 800 ◦ C, shifted leftwards compared to the reference Cu value. Based on the Cu–Zn binary alloy phase diagram, Zn has a large solubility in the Cu matrix [7]. Therefore, it is reasonable that Cu and Zn were completely alloyed after sintering. Fig. 4 shows the density of the hollow spheres sintered with different Zn contents and sintering temperatures. The polymer core burned out and a large pore remained at the center of the sphere during sintering. Therefore, the density was below 4.0 g/cm3 in spite of the densification of the metal shell. Increasing Zn content and decreasing sintering temperature decreased the sphere density. In particular, the sphere density increased with increasing sintering temperature, in spite of Zn evaporation. Therefore, it was suggested that the densification of the spheres by the diffusion of the Cu and Zn atoms was more apparent with
increasing sintering temperature than the pore formation by the Zn evaporation. Fig. 5 shows the Vickers hardness of the hollow spheres with increasing Zn content and sintering temperature. According to the previous studies, a 5–30 wt.% addition of Zn in pure Cu leaded to a 15–60% increase in the hardness of the bulk sample, and the grain size of Cu hollow sphere increased with increasing sintering temperature [4,8]. However, our results were not in agreement with this tendency. The hardness was mainly determined by the density rather than the solid-solution hardening by the Zn addition and the Hall–Petch effect by the grain growth. Therefore, decreasing Zn content and increasing sintering temperature increased the hardness of the samples. Fig. 6 shows the macro-images of the compression-tested hollow spheres after sintering with different Zn contents and sintering temperatures. The pure Cu spheres sintered at 930 ◦ C were deformed completely without any crack during the test [4]. However, the tendency toward brittle fracture was intensified with increasing Zn content and decreasing sintering temperature. It was likely that the pores inside the spheres acted as the initial sites and propagation paths of the cracks during the test. Fig. 7 shows the compression force of the hollow spheres sintered with different Zn contents and sintering temperatures. According to the previous study, the shell thickness of the Cu hollow spheres increased with the sphere contraction as the sintering temperature increased [4]. Therefore, the compression property of the spheres decreased with increasing Zn content and decreas-
Fig. 6. Optical microscope images of the fractured brass hollow spheres after sintering with Cu2 O + 30 wt.% Zn powders at 800 ◦ C (a) and 930 ◦ C (b).
J.-M. Koo et al. / Materials Science and Engineering A 483–484 (2008) 254–257
ing sintering temperature, because of decreasing their density and shell thickness. 4. Conclusion The metallurgical and mechanical properties of brass hollow spheres, sintered with Cu2 O + Zn (0–30 wt.%) powders at temperatures ranging from 800 ◦ C to 930 ◦ C for 1 h under a hydrogen atmosphere, were investigated. Increasing sintering temperature accelerated the Zn evaporation during sintering, due to higher vapor pressure of Zn, but increased the sphere density due to high diffusivity of Cu and Zn atoms. Based on the XRD analysis results, Cu2 O and Zn were alloyed completely to form the brass hollow spheres during sintering. Increasing Zn content and decreasing sintering temperature increased the porosity of the spheres, but degraded the mechanical properties of the spheres.
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Acknowledgement This work was supported by grant no. RTI04-03-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE). References [1] E. Zhang, B. Wang, Int. J. Mech. Sci. 47 (2005) 744–756. [2] S.K. Hyun, K. Murakami, H. Nakajuma, Mater. Sci. Eng. A 299 (2001) 241–248. [3] S.B. Jung, S.K. Kim, Y.J. Kim, Mater. Trans. 43 (2002) 55–57. [4] J.M. Koo, S.B. Jung, Mater. Sci. Forum 510–511 (2006) 730–733. [5] D.R. Gaskell, Introduction to the Thermodynamics of Materials, 3rd ed., Taylor & Francis, Washington, D.C., 1995. [6] R.M. German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, Princeton, NJ, 1994. [7] Binary Alloy Phase Diagram, 2nd ed., ASM International, OH, 1990. [8] ASM Handbook, vol.2, 10th ed., ASM International, OH, 1990.
Effect of Zn addition on mechanical properties of brass hollow spheres Ja-Myeong Koo a , Hideki Araki b , Seung-Boo Jung a,∗ a
Department of Advanced Materials Science & Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, Republic of Korea b Department of Materials Science and Engineering, Faculty of Engineering Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 6 June 2005; received in revised form 5 January 2006; accepted 21 January 2006
Abstract Polymer spheres coated with a slurry, consisting of copper(I) oxide + Zn (0–30 wt.%) powders, water and polyacrilamide (PA), were sintered at temperatures ranging from 800 to 930 ◦ C under a hydrogen atmosphere to fabricate high-porous brass hollow spheres. The metallurgical and mechanical properties of the spheres were investigated with different Zn contents and sintering temperatures. Increasing sintering temperature accelerated the Zn evaporation and sphere densification. Increasing Zn content and decreasing sintering temperature increased the porosity of the spheres, but degraded their mechanical properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper–zinc alloy (brass); Powder metallurgy; Hollow sphere; Porous metal; Sintering; Zn evaporation; Compression test
1. Introduction Development of light materials is one of the most critical issues in modern industry, particularly in transport industries where saving energy is paramount. Porous materials are increasingly considered to hold promise as light-weight structural materials and functional materials with superior specific strength, sound damping, specific surface area, heat emission efficiency and filtering property [1,2]. Many researchers have already reported fabrication techniques, characteristics and applications of various porous metals. In particular, the development of low-cost processing techniques has increased the interest [1–4]. Aluminum (Al) foams have been tested and used in the automobile, aerospace and railway industries. Copper (Cu) foams have higher thermal/electrical conductivity, melting point and better mechanical properties than Al foams. However, the fabrication of Cu foams has been limited due to the high density of Cu [1]. In two previous studies, Cu hollow spheres with different sphere diameters and shell thicknesses were fabricated successfully using the sintering process with copper oxide powder under a hydrogen atmosphere [3,4]. In the present work,
∗
Corresponding author. Tel.: +82 31 290 7383; fax: +82 31 290 7371. E-mail address: [email protected] (S.-B. Jung).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.01.183
the effect of Zn addition on the microstructural evolutions and mechanical properties of brass hollow spheres were investigated to study the feasibility of high-porous brass hollow spheres. Zn has a much higher vapor pressure than Cu at the sintering temperature, and therefore evaporates easily during sintering [5]. To increase and/or control the sphere porosity, Zn powder was added to the slurry containing Cu2 O powder. 2. Experimental procedure A 15 mm-diameter-polymer sphere was used as the core of the hollow sphere. The solvent and binder were water and polyacrilamide (PA), respectively. A slurry containing 3 mdiameter-copper (I) oxide powder, 30 m-diameter-Zn powder, water, and PA was prepared using a stirring machine. The polymer spheres were dip-coated with the slurry and dried for the shell thickness of 1.8 mm. The samples were sintered at temperatures ranging from 800 to 930 ◦ C for 1 h under a hydrogen atmosphere. The polymer ball was burned out and eliminated by evaporation during sintering. After sintering, the samples were mounted in cold epoxy and mechanically polished for the microstructural examination using scanning electron microscopy (SEM). The densities of the samples were measured by an electronic scale utilizing the Archimedes’ principle. The chemical compositions of the samples were analyzed using an electron-probe micro-analyzer
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Fig. 1. Cross-sectional SEM images of the brass hollow spheres sintered with Cu2 O powder (a) and Cu2 O + 30 wt.% Zn powders (b and c) at 930 ◦ C (a and b) and 800 ◦ C (c). The gray and dark regions, marked as A and B, indicate the brass phase and pore, respectively.
(EPMA). X-ray diffraction (XRD) was employed to analyze the reduction of Cu2 O to Cu and the solid-solution of Zn in the reduced Cu matrix after sintering. The Vickers hardness was measured on transverse sections of the hollow spheres with a testing time of 10 s and a load of 50 g mass. The compression strength of one hollow sphere was evaluated at a displacement rate of 1 mm/min. After testing, the fracture samples were observed using an optical microscope. 3. Results and discussion Fig. 1 shows the cross-sectional micrographs of brass hollow spheres sintered with different Zn contents and sintering temperatures. The Cu hollow sphere sintered with Cu2 O powder at 930 ◦ C showed a dense typical Cu structure, as shown in Fig. 1a. However, the addition of Zn powder increased the porosity of the sphere, as shown in Fig. 1b. As Zn has extremely high vapor pressure at sintering temperatures, it was likely that the added Zn evaporated easily to form the pores in the spheres, rather than remaining as solid-solute in the reduced Cu matrix [5]. The sphere porosity increased with decreasing sintering temperature, as shown in Fig. 1c, because of the decreased diffusivities of Cu and Zn atoms [6]. Volume shrinkage with many inhomogeneous micro-pores was also observed in the spheres. Fig. 2shows the chemical compositions (wt.%) of the hollow spheres sintered with different Zn contents and sintering temperatures. The Zn concentration in the spheres decreased
Fig. 2. EPMA analysis results of the brass hollow spheres after sintering.
Fig. 3. XRD analysis results of the brass hollow spheres sintered with Cu–30 wt.% Zn powders at 800 ◦ C.
with increasing sintering temperature and decreasing the amount of Zn powder contained in the slurry, due to the Zn evaporation. In particular, increasing sintering temperature accelerated the Zn evaporation. The Zn concentration in the spheres sintered with Cu2 O + 30 wt.% Zn powders decreased from 24.5 wt.% to 8.0 wt.% as the sintering temperature increased from 800 ◦ C to 930 ◦ C, because the Zn vapor pressure increases linearly with the exponential function of temperature [5]. The Zn concentrations of the spheres sintered with 20 wt.% and less Zn powder at 800 ◦ C were higher than the Zn contents contained in the slurry. In this study, the Zn content
Fig. 4. Density of the brass hollow spheres after sintering.
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Fig. 5. Vickers hardness of the brass hollow spheres after sintering.
Fig. 7. Compression strength of the brass hollow spheres after sintering.
was the mass ratio of Zn powder to Zn and Cu2 O powders before sintering, whereas the Zn concentration was that of Zn to Zn and Cu after sintering. It was suggested that the oxygen in the Cu2 O was reduced and evaporated during sintering, thereby increasing the Zn concentration in the spheres. Fig. 3 shows the XRD analysis results of the hollow sphere sintered with Cu2 O + 30 wt.% Zn powders at 800 ◦ C. The Cu2 O and Zn peaks disappeared after sintering, irrespective of Zn content and sintering temperature, indicating that the Cu2 O and Zn were completely transformed to Cu with solid-solute Zn. The Cu peaks of the spheres, sintered with Cu2 O + 30 wt.% Zn powders at 800 ◦ C, shifted leftwards compared to the reference Cu value. Based on the Cu–Zn binary alloy phase diagram, Zn has a large solubility in the Cu matrix [7]. Therefore, it is reasonable that Cu and Zn were completely alloyed after sintering. Fig. 4 shows the density of the hollow spheres sintered with different Zn contents and sintering temperatures. The polymer core burned out and a large pore remained at the center of the sphere during sintering. Therefore, the density was below 4.0 g/cm3 in spite of the densification of the metal shell. Increasing Zn content and decreasing sintering temperature decreased the sphere density. In particular, the sphere density increased with increasing sintering temperature, in spite of Zn evaporation. Therefore, it was suggested that the densification of the spheres by the diffusion of the Cu and Zn atoms was more apparent with
increasing sintering temperature than the pore formation by the Zn evaporation. Fig. 5 shows the Vickers hardness of the hollow spheres with increasing Zn content and sintering temperature. According to the previous studies, a 5–30 wt.% addition of Zn in pure Cu leaded to a 15–60% increase in the hardness of the bulk sample, and the grain size of Cu hollow sphere increased with increasing sintering temperature [4,8]. However, our results were not in agreement with this tendency. The hardness was mainly determined by the density rather than the solid-solution hardening by the Zn addition and the Hall–Petch effect by the grain growth. Therefore, decreasing Zn content and increasing sintering temperature increased the hardness of the samples. Fig. 6 shows the macro-images of the compression-tested hollow spheres after sintering with different Zn contents and sintering temperatures. The pure Cu spheres sintered at 930 ◦ C were deformed completely without any crack during the test [4]. However, the tendency toward brittle fracture was intensified with increasing Zn content and decreasing sintering temperature. It was likely that the pores inside the spheres acted as the initial sites and propagation paths of the cracks during the test. Fig. 7 shows the compression force of the hollow spheres sintered with different Zn contents and sintering temperatures. According to the previous study, the shell thickness of the Cu hollow spheres increased with the sphere contraction as the sintering temperature increased [4]. Therefore, the compression property of the spheres decreased with increasing Zn content and decreas-
Fig. 6. Optical microscope images of the fractured brass hollow spheres after sintering with Cu2 O + 30 wt.% Zn powders at 800 ◦ C (a) and 930 ◦ C (b).
J.-M. Koo et al. / Materials Science and Engineering A 483–484 (2008) 254–257
ing sintering temperature, because of decreasing their density and shell thickness. 4. Conclusion The metallurgical and mechanical properties of brass hollow spheres, sintered with Cu2 O + Zn (0–30 wt.%) powders at temperatures ranging from 800 ◦ C to 930 ◦ C for 1 h under a hydrogen atmosphere, were investigated. Increasing sintering temperature accelerated the Zn evaporation during sintering, due to higher vapor pressure of Zn, but increased the sphere density due to high diffusivity of Cu and Zn atoms. Based on the XRD analysis results, Cu2 O and Zn were alloyed completely to form the brass hollow spheres during sintering. Increasing Zn content and decreasing sintering temperature increased the porosity of the spheres, but degraded the mechanical properties of the spheres.
257
Acknowledgement This work was supported by grant no. RTI04-03-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE). References [1] E. Zhang, B. Wang, Int. J. Mech. Sci. 47 (2005) 744–756. [2] S.K. Hyun, K. Murakami, H. Nakajuma, Mater. Sci. Eng. A 299 (2001) 241–248. [3] S.B. Jung, S.K. Kim, Y.J. Kim, Mater. Trans. 43 (2002) 55–57. [4] J.M. Koo, S.B. Jung, Mater. Sci. Forum 510–511 (2006) 730–733. [5] D.R. Gaskell, Introduction to the Thermodynamics of Materials, 3rd ed., Taylor & Francis, Washington, D.C., 1995. [6] R.M. German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, Princeton, NJ, 1994. [7] Binary Alloy Phase Diagram, 2nd ed., ASM International, OH, 1990. [8] ASM Handbook, vol.2, 10th ed., ASM International, OH, 1990.