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Compressive properties of cellular Mg foams fabricated by melt-foaming method
Materials Science and Engineering A 527 (2010) 5405–5409
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Compressive properties of cellular Mg foams fabricated by melt-foaming method Yang Dong-Hui a,∗ , Yang Shang-Run b , Wang Hui c , Ma Ai-Bin a , Jiang Jing-Hua a , Chen Jian-Qing a , Wang Ding-Lie a a
College of Mechanics and Materials, HoHai University, No. 1, Xikang Road, Nanjing 210098, Jiangsu, China School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China c State Key Laboratory for Advanced Metals and Materials, University of Science and Technology, Beijing 10083, China b
a r t i c l e
i n f o
Article history: Received 2 December 2009 Received in revised form 19 April 2010 Accepted 10 May 2010
Keywords: Cellular Mg foam Compression test Mechanical properties Energy absorption
a b s t r a c t By using CaCO3 powder as blowing agent and Ca particles as thickening agent, cellular pure Mg foams with homogenous pore structures were fabricated by melt-foaming method. Comparing with the Al/Al alloy foams, the cellular Mg foams possess superior comprehensive mechanical properties. The energy absorption characteristics and the effects of compression behavior on the energy absorption properties for the cellular Mg foams have been investigated and discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Extensive researches have been carried out to develop cellular metal foams with close pore structures because of their unique combination of physical, mechanical, thermal, electrical and acoustic properties [1]. In particular, cellular Al/Al alloy foams possess good strength, high specific strength with low density and are good at absorbing energy, which have been drawing broad attention in automobile and aerospace applications from the viewpoint of energy and environmental preservation [1–3]. But much less attention has been paid to develop cellular Mg/Mg alloy foams, which are expected to maintain that multifunctionality of the cellular structure with reduced weight. Up to now, although several methods have been considered to fabricate cellular Mg foams, such as powder metallurgical method [1], solid-state foaming method [4], high pressure casting method [5], vacuum foaming method [6] and GASAR method [7], the investigations have shown that these methods are relatively expensive and the fabrication procedures are somewhat complicated. Furthermore, studies on the mechanical properties of cellular Mg foams are still limited. Recently, our work [8] has reported that it is feasible to foam Mg melt by melt-foaming method, which is regarded as a cost-effective and easy controllable approach to prepare the cellular metals with large scale. In this
∗ Corresponding author. Tel.: +86 13813846261; fax: +86 83596895. E-mail addresses: yang [email protected], [email protected] (D.-H. Yang). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.05.017
paper, the cellular Mg foams with porosity of 53.0–71.1% were fabricated by melt-foaming method, their mechanical properties, the energy absorption characteristics and the effects of compression behavior on the energy absorption properties were investigated and discussed. 2. Experimental Commercial pure Mg ingot was used as the matrix material, Ca particles (purity >99.9 wt.%) was selected as the thickening agent and the as-received CaCO3 powder (purity >99.80 wt.%, 4 m) was chosen as the blowing agent. The melt-foaming method was applied to fabricate the cellular Mg foams and the specific preparation procedure has been elaborated in Ref. [8]. Briefly, a definite quantity of pure Mg (∼1 Kg) was melted in a crucible at a fixed temperature and then 2.0 wt.% Ca was introduced into the melted Mg to increase its viscosity. The thickened Mg melt was foamed by adding the blowing agent, CaCO3 powder (2.0 wt.%) at a proper foaming temperature (T). During the fabrication process, the stirring foaming time and holding foaming time were both set as 30 s. The pore structures of the final products were controlled by adjusting the foaming temperature in the range of 680–750 ◦ C, where the cellular Mg foam with relative low porosity (Pr) can be prepared at lower foaming temperature (e.g. T = 690 ◦ C, Pr ≈ 60.0%) and vice versa (e.g. T = 750◦ C, Pr ≈ 70.0%). As a result, cellular Mg foam samples with porosities of ∼53.0% to ∼72.0% and pore size around ∼2.0 mm were obtained. Generally, for a given porosity, the mechanical performance of a metallic foam is mainly determined by its cell wall strength.
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D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409 Table 1 Physical and mechanical properties of Mg, Al and Al alloys [9]. Material
Density (g/cm3 )
Yielding strength (MPa)
Tensile strength (MPa)
Mg Al AlSi9Cu2Mg AlSi7Mg
1.74 2.71 2.71 2.68
21 30 – 125
90 85 205 165
As shown in Table 1, Mg possesses comparable mechanical properties with Al, so, in this study, in order to compare the mechanical properties between cellular Mg foams and Al based foams, the cellular Al foams and porous Al/Al alloy foams with open pore structures were also fabricated via meltfoaming method [10–12] and the melt-infiltration method [13], respectively. The starting materials used in this study were commercial pure Al, AlSi9Cu2Mg (Si: 8.0–10.0 wt.%, Cu: 1.3–1.8 wt.%, Mg:0.4–0.6 wt.%, Mn: 0.10–0.35 wt.%, Ti: 0.10–0.35 wt.%, Al: Bal.), AlSi7Mg (Si:6.5–7.5 wt.%, Mg: 0.25–0.45 wt.%, Al: Bal.) and their related physical and mechanical properties are all listed in Table 1. Using fabricated foams, the cylinder compression specimens were made by electro-discharging machine into the size of 20 mm × 30 mm. The porosity Pr, pore fraction of a specimen was calculated from its mass (M) and volume (V) using the following equation: Pr (%) =
V − (M/s ) × 100% V
pores in both cellular Mg foam and cellular Al foam are isolated (as shown in Fig. 2a and b) while the pores in porous AlSi7Mg alloy foam are interconnected and the open pores (the holes connecting neighboring pores) are indicated by the pink arrows in Fig. 2c. The compressive stress–strain curves of cellular Mg foams with porosities of 53.0–71.1% are shown in Fig. 3a. In all cases, the stress–strain curves of cellular Mg foams consist of three regions: an initial, approximately linear deformation region until the peak stress followed by a stress drop to a plateau region, where the flow stress continuously drops with multiple serrations, indicative of a series of brittle cell edge fracture events (as shown in Fig. 3b and c), and finally a densification region, in which the stress increases steeply due to the collapsed cells having been almost fully compacted together. For a metallic foam, the peak stress is defined as yielding ∗ is defined as the yielding strength, y∗ , and the specific strength, y,s ∗ strength, y , divided by its apparent density, , namely
(1)
where s is the density of the matrix. The quasi-static compression tests were carried out at room temperature on the Universal Testing Machine (Suns Experimental Equipments Instrument Co. Ltd., Shenzhen, China) with a constant cross head speed of 2 mm/min and the stress ()–strain (ε) curves were obtained and recorded using a computer. 3. Results and discussion Fig. 1 shows the section images of the cellular Mg foam sample, the cellular Al foam sample and the porous AlSi7Mg alloy foam, which demonstrates that all the foam samples have homogeneous pore structures and especially, both the cellular Mg foam (Fig. 1a) and cellular Al foams (Fig. 1b) have the analogous appearance. The corresponding stereo micrographs of cellular Mg/Al foams and porous Al alloy foam are shown in Fig. 2, which demonstrates that
∗ y,s =
y∗
=
y∗ (1 − Pr) · s
(2)
where Pr is the porosity of a foam and s is the density of the metal ∗ , of cellular matrix. A summary of all the strength values, y∗ and y,s Mg foams as well as those of Al/Al alloy foams prepared in this study is shown in Fig. 4. Fig. 4a indicates that the yielding strengths of cellular Mg foams as well as the other Al based foams possess the tendency of increasing with decreasing of porosity, where the y∗ of cellular Mg foam increases from 8.69 MPa for porosity of 71.1% to 27.11 MPa for porosity of 53.0%. Although the Al alloys possess better mechanical properties than Mg (shown in Table 1), the y∗ of the cellular Mg foam is comparable or even higher than those of Al based foams for comparing at a given porosity. And particularly, it is apparent from ∗ of the Fig. 4b that for a given porosity, the specific strength, y,s cellular Mg foam is higher than those of all the Al based foams. The highest specific strength of cellular Mg foam with porosity of 53.0%
Fig. 1. Section images of (a) cellular Mg foam, (b) cellular Al foam and (c) porous AlSi7Mg alloy foam. The porosities are (a) 61.0%, (b) 60.0% and (c) 63.0%, respectively; the mean pore sizes are (a) 1.3 mm, (b) 0.8 mm and (c) 2.0 mm, respectively.
D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409
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Fig. 2. Stereo micrographs of (a) cellular Mg foam, (b) cellular Al foam and (c) porous AlSi7Mg alloy foam. The pink arrows in Fig. 2(c) indicate the open pores (the holes connecting neighboring pores) in the porous AlSi7Mg alloy foam.
is 33.2 MPa/(g cm−3 ), which is almost twice the value of the other Al based foams. Furthermore, in comparison to the other types of reinforced Al foam, i.e. Al-stainless-steel/low-carbon-steel (Al-SS/LCS) composite foams (their y∗ values are in the range of 70–90.4 MPa for the porosity range of 57–58%) [14] and ceramic microballoon reinforced Al matrix composites (microballoon MMCs) (their y∗ values are in the range of 146.9–56.5 MPa for the porosity range of 50–70%) [15], their yielding strengths are about three to six times the y∗ values of the present cellular Mg foams with porosities of
53.0% and 57.9% (y∗ = 27.11 MPa and 22.44 MPa, respectively) as shown in Fig. 5a. The higher strengths of these composites foams are likely attributed to the high volume fraction of SS/LCS hollow sphere (∼59 vol.%, 3.7 mm in diameter and 200 m in wall thickness) or ceramic microballoon (50–70 vol.%, 45 or 270 m mean diameter) distributed in the Al matrix. However, the apparent densities () of the Al-SS/LCS composite foams and the microballoon MMCs are around 2.40 × 103 kg/m3 [14] and 4.2 × 103 kg/m3 [15], respectively. So, according to Eq. (2), as shown in Fig. 5b, the
Fig. 3. Compression behavior of cellular Mg foams. (a) Compression stress–strain curve of cellular Mg foams with porosities of 53.0–71.1% and the small inserted figure shows the typical stress–strain curves for Al based foams prepared in this study. (b–e) Compression features with strain of (0.33, 0.50) for the cellular Mg foam (Pr = 59.4%) and the cellular Al foam (Pr = 58.0%), respectively.
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D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409
Fig. 4. Relationships of (a) yielding strength and porosity and (b) specific strength and porosity, for samples of cellular Mg foams with porosities of 53.0–71.1% and the Al based foams.
highest specific strengths of the Al-SS/LCS composite foam with porosity of around 58% and the microballoon MMCs with porosity around 50% are 37.7 MPa/(g cm−3 ) and 35.0 MPa/(g cm−3 ), respectively, which become comparable with the specific strengths of cellular Mg foam with porosity of 53.0% (33.2 MPa/(g cm−3 )) and 57.9% (30.6 MPa/(g cm−3 )). The energy absorption capacity (W) of the cellular Mg foams is calculated by integrating the stress–strain curves shown in Fig. 3a and so, the W values of the cellular Mg foams at the densification strains (Wd ) can be calculated according to the following equation:
Wd =
εd
(ε) · dε
(3)
0
where εd is the densification strain. The εd values for all the cellular Mg foams in this study are around 0.6. The relationships between
Fig. 6. Relationships of (a) energy absorption capacity, Wd , and porosity and (b) specific energy absorption capacity, Wd,m , and porosity, for samples of cellular Mg foams with porosity of 53.0–71.1%, the Al based foams and reinforced Al foams.
Wd and porosity for the cellular Mg foams as well as those of the Al based foams prepared in this study and Al-SS/LCS composite foams reported in Ref. [14] are shown in Fig. 6a, which demonstrates that the Wd of cellular Mg foams as well as those of Al based foams possess the tendency of increasing with decreasing of porosity, where the Wd of the cellular Mg foam increases from 4.24 MJ/m3 (for porosity of 71.1%) to 12.72 MJ/m3 (for porosity of 53.0%). Fig. 6a indicates that the Wd of Al-SS/LCS composite foams with porosity around 58% (Wd = 44.0–24.4 MJ/cm3 ) [14] are higher than that of the cellular Mg foam with porosity of 57.9% (9.99 MJ/m3 ) by a factor of about 2.4–4.4. However, when normalizing the Wd by the apparent density () of a foam (resulting in specific energy absorption capacity, Wd,m , where Wd,m = Wd /), this advantage disappears as shown in Fig. 6b, where the Wd,m of the cellular Mg foam with porosity of 57.9% (13.6 J/g) is comparable with those of Al-SS/LCS
Fig. 5. Relationships of (a) yielding strength and porosity and (b) specific strength and porosity, for the cellular Mg foams with porosities of 53.0–71.1%, Al based foams prepared in this study and the reinforced Al foams (i.e. Al-SS/LCS composite foams [14] and ceramic microballoon reinforced Al matrix composites [15]).
D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409
composite foams (10.2–18.3 J/g) and the ratio is located in the range of 0.75–1.35. Fig. 6 also shows that for a given porosity, the Wd of cellular Al foam is higher than that of the cellular Mg foam by a factor of about 2, which can be attributed to the difference of their compression behavior. Comparing Fig. 3b and c with Fig. 3d and e indicates that during the compression process of the plateau region, the cell walls of the cellular Mg foam keep cracking and collapsing (indicated by the red arrow in Fig. 3b and c), which leads to its plateau stress dropping with multiple serrations until the densification strain while there is no such phenomenon happened to the cellular Al foam (as shown in Fig. 3d and e), which leads to the corresponding plateau stress keeping increasing. According to Fig. 3a and Eq. (3), the energy absorption capacity is directly determined by the area covered by the stress–strain curves, so the Wd of cellular Mg foams is significantly reduced, leading to the values of Wd and Wd,m being lower than the corresponding values of the cellular Al foams. 4. Conclusions In summary, for the cellular Mg foams prepared by meltfoaming method with porosity of 71.1–53.0%, their yielding strengths increase from 8.69 MPa to 27.11 MPa and their energy absorption capacities increase from 4.21 MJ/m3 to 12.72 MJ/m3 . Comparing with Al based foams, the cellular Mg foams exhibit excellent strength with reduced weight and particularly possess superior specific strength, where the highest specific strength reaches 33.2 MPa/(g cm−3 ). Although the energy absorption properties of cellular Mg foams are not better than those of cellular Al foams, their specific energy absorption capacities still possess comparable values with those of reinforced Al foams. All these conclusions imply that the cellular Mg foam should have promising future in the practical engineering applications.
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Acknowledgments This work is supported by Research Fund for Doctoral Program of Higher Education of China (grant no. 200802941010), Scientific Foundation for returned Scholar, Ministry of Education of China (grant no. 2009512712), the Fundamental Research Funds for the Central Universities (grant no. 2009B15514, 2009B16114), the Natural Science Foundation of HoHai University (grant no. 2008428001) and Scientific Research Startup Fund of HoHai University (grant no.2084140801109). References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
J. Banhart, Prog. Mater. Sci. 46 (2002) 559. J. Banhart, Int. J. Vehicle Des. 37 (2005) 114. W. Zheng, Y.N. Sun, Spacecraft Eng. 10 (3) (2001) 24. C.E. Wen, M.R. Barnett, J. Vella, Y. Yamada, in: H. Nakajima, N. Kanetake (Eds.), Porous Metals and Metal Foaming Technology, The Japan Institute of Metal, Japan, 2005, pp. 165–168. C. Körner, M. Hirschmann, V. Brautigam, R.F. Singer, Adv. Eng. Mater. 6 (2004) 385. K. Renger, H. Kaufmann, Adv. Eng. Mater. 3 (2005) 117. Z.K. Xie, M. Tane, S.K. Hyun, Y. Okuda, H. Nakajima, Mater. Sci. Eng. A 417 (2006) 129. D.H. Yang, B.Y. Hur, S.R. Yang, J. Alloys Compd. 461 (2008) 221. B.Y. Huang, C.G. Li, L.K. Shi, G.Z. Qiu, T.Y. Zuo, Handbook of Nonferrous Metal Materials, first ed., Chemical Industry Press, Beijing, 2009. D.H. Yang, D.P. He, S.R. Yang, Sci. China B 47 (6) (2004) 512. D.H. Yang, B.Y. Hur, Mater. Lett. 60 (2006) 3635. D.H. Yang, B.Y. Hur, D.P. He, S.R. Yang, Mater. Sci. Eng. A 445–446 (2007) 415. D.H. Yang, D.P. He, Sci. China B 44 (4) (2001) 411. L.J. Vendra, A. Rabiei, Mater. Sci. Eng. A465 (2007) 59. R.A. Palmer, K. Gao, T.M. Doan, L. Green, G. Cavallaro, Mater. Sci. Eng. A 464 (2007) 85.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Compressive properties of cellular Mg foams fabricated by melt-foaming method Yang Dong-Hui a,∗ , Yang Shang-Run b , Wang Hui c , Ma Ai-Bin a , Jiang Jing-Hua a , Chen Jian-Qing a , Wang Ding-Lie a a
College of Mechanics and Materials, HoHai University, No. 1, Xikang Road, Nanjing 210098, Jiangsu, China School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China c State Key Laboratory for Advanced Metals and Materials, University of Science and Technology, Beijing 10083, China b
a r t i c l e
i n f o
Article history: Received 2 December 2009 Received in revised form 19 April 2010 Accepted 10 May 2010
Keywords: Cellular Mg foam Compression test Mechanical properties Energy absorption
a b s t r a c t By using CaCO3 powder as blowing agent and Ca particles as thickening agent, cellular pure Mg foams with homogenous pore structures were fabricated by melt-foaming method. Comparing with the Al/Al alloy foams, the cellular Mg foams possess superior comprehensive mechanical properties. The energy absorption characteristics and the effects of compression behavior on the energy absorption properties for the cellular Mg foams have been investigated and discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Extensive researches have been carried out to develop cellular metal foams with close pore structures because of their unique combination of physical, mechanical, thermal, electrical and acoustic properties [1]. In particular, cellular Al/Al alloy foams possess good strength, high specific strength with low density and are good at absorbing energy, which have been drawing broad attention in automobile and aerospace applications from the viewpoint of energy and environmental preservation [1–3]. But much less attention has been paid to develop cellular Mg/Mg alloy foams, which are expected to maintain that multifunctionality of the cellular structure with reduced weight. Up to now, although several methods have been considered to fabricate cellular Mg foams, such as powder metallurgical method [1], solid-state foaming method [4], high pressure casting method [5], vacuum foaming method [6] and GASAR method [7], the investigations have shown that these methods are relatively expensive and the fabrication procedures are somewhat complicated. Furthermore, studies on the mechanical properties of cellular Mg foams are still limited. Recently, our work [8] has reported that it is feasible to foam Mg melt by melt-foaming method, which is regarded as a cost-effective and easy controllable approach to prepare the cellular metals with large scale. In this
∗ Corresponding author. Tel.: +86 13813846261; fax: +86 83596895. E-mail addresses: yang [email protected], [email protected] (D.-H. Yang). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.05.017
paper, the cellular Mg foams with porosity of 53.0–71.1% were fabricated by melt-foaming method, their mechanical properties, the energy absorption characteristics and the effects of compression behavior on the energy absorption properties were investigated and discussed. 2. Experimental Commercial pure Mg ingot was used as the matrix material, Ca particles (purity >99.9 wt.%) was selected as the thickening agent and the as-received CaCO3 powder (purity >99.80 wt.%, 4 m) was chosen as the blowing agent. The melt-foaming method was applied to fabricate the cellular Mg foams and the specific preparation procedure has been elaborated in Ref. [8]. Briefly, a definite quantity of pure Mg (∼1 Kg) was melted in a crucible at a fixed temperature and then 2.0 wt.% Ca was introduced into the melted Mg to increase its viscosity. The thickened Mg melt was foamed by adding the blowing agent, CaCO3 powder (2.0 wt.%) at a proper foaming temperature (T). During the fabrication process, the stirring foaming time and holding foaming time were both set as 30 s. The pore structures of the final products were controlled by adjusting the foaming temperature in the range of 680–750 ◦ C, where the cellular Mg foam with relative low porosity (Pr) can be prepared at lower foaming temperature (e.g. T = 690 ◦ C, Pr ≈ 60.0%) and vice versa (e.g. T = 750◦ C, Pr ≈ 70.0%). As a result, cellular Mg foam samples with porosities of ∼53.0% to ∼72.0% and pore size around ∼2.0 mm were obtained. Generally, for a given porosity, the mechanical performance of a metallic foam is mainly determined by its cell wall strength.
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D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409 Table 1 Physical and mechanical properties of Mg, Al and Al alloys [9]. Material
Density (g/cm3 )
Yielding strength (MPa)
Tensile strength (MPa)
Mg Al AlSi9Cu2Mg AlSi7Mg
1.74 2.71 2.71 2.68
21 30 – 125
90 85 205 165
As shown in Table 1, Mg possesses comparable mechanical properties with Al, so, in this study, in order to compare the mechanical properties between cellular Mg foams and Al based foams, the cellular Al foams and porous Al/Al alloy foams with open pore structures were also fabricated via meltfoaming method [10–12] and the melt-infiltration method [13], respectively. The starting materials used in this study were commercial pure Al, AlSi9Cu2Mg (Si: 8.0–10.0 wt.%, Cu: 1.3–1.8 wt.%, Mg:0.4–0.6 wt.%, Mn: 0.10–0.35 wt.%, Ti: 0.10–0.35 wt.%, Al: Bal.), AlSi7Mg (Si:6.5–7.5 wt.%, Mg: 0.25–0.45 wt.%, Al: Bal.) and their related physical and mechanical properties are all listed in Table 1. Using fabricated foams, the cylinder compression specimens were made by electro-discharging machine into the size of 20 mm × 30 mm. The porosity Pr, pore fraction of a specimen was calculated from its mass (M) and volume (V) using the following equation: Pr (%) =
V − (M/s ) × 100% V
pores in both cellular Mg foam and cellular Al foam are isolated (as shown in Fig. 2a and b) while the pores in porous AlSi7Mg alloy foam are interconnected and the open pores (the holes connecting neighboring pores) are indicated by the pink arrows in Fig. 2c. The compressive stress–strain curves of cellular Mg foams with porosities of 53.0–71.1% are shown in Fig. 3a. In all cases, the stress–strain curves of cellular Mg foams consist of three regions: an initial, approximately linear deformation region until the peak stress followed by a stress drop to a plateau region, where the flow stress continuously drops with multiple serrations, indicative of a series of brittle cell edge fracture events (as shown in Fig. 3b and c), and finally a densification region, in which the stress increases steeply due to the collapsed cells having been almost fully compacted together. For a metallic foam, the peak stress is defined as yielding ∗ is defined as the yielding strength, y∗ , and the specific strength, y,s ∗ strength, y , divided by its apparent density, , namely
(1)
where s is the density of the matrix. The quasi-static compression tests were carried out at room temperature on the Universal Testing Machine (Suns Experimental Equipments Instrument Co. Ltd., Shenzhen, China) with a constant cross head speed of 2 mm/min and the stress ()–strain (ε) curves were obtained and recorded using a computer. 3. Results and discussion Fig. 1 shows the section images of the cellular Mg foam sample, the cellular Al foam sample and the porous AlSi7Mg alloy foam, which demonstrates that all the foam samples have homogeneous pore structures and especially, both the cellular Mg foam (Fig. 1a) and cellular Al foams (Fig. 1b) have the analogous appearance. The corresponding stereo micrographs of cellular Mg/Al foams and porous Al alloy foam are shown in Fig. 2, which demonstrates that
∗ y,s =
y∗
=
y∗ (1 − Pr) · s
(2)
where Pr is the porosity of a foam and s is the density of the metal ∗ , of cellular matrix. A summary of all the strength values, y∗ and y,s Mg foams as well as those of Al/Al alloy foams prepared in this study is shown in Fig. 4. Fig. 4a indicates that the yielding strengths of cellular Mg foams as well as the other Al based foams possess the tendency of increasing with decreasing of porosity, where the y∗ of cellular Mg foam increases from 8.69 MPa for porosity of 71.1% to 27.11 MPa for porosity of 53.0%. Although the Al alloys possess better mechanical properties than Mg (shown in Table 1), the y∗ of the cellular Mg foam is comparable or even higher than those of Al based foams for comparing at a given porosity. And particularly, it is apparent from ∗ of the Fig. 4b that for a given porosity, the specific strength, y,s cellular Mg foam is higher than those of all the Al based foams. The highest specific strength of cellular Mg foam with porosity of 53.0%
Fig. 1. Section images of (a) cellular Mg foam, (b) cellular Al foam and (c) porous AlSi7Mg alloy foam. The porosities are (a) 61.0%, (b) 60.0% and (c) 63.0%, respectively; the mean pore sizes are (a) 1.3 mm, (b) 0.8 mm and (c) 2.0 mm, respectively.
D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409
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Fig. 2. Stereo micrographs of (a) cellular Mg foam, (b) cellular Al foam and (c) porous AlSi7Mg alloy foam. The pink arrows in Fig. 2(c) indicate the open pores (the holes connecting neighboring pores) in the porous AlSi7Mg alloy foam.
is 33.2 MPa/(g cm−3 ), which is almost twice the value of the other Al based foams. Furthermore, in comparison to the other types of reinforced Al foam, i.e. Al-stainless-steel/low-carbon-steel (Al-SS/LCS) composite foams (their y∗ values are in the range of 70–90.4 MPa for the porosity range of 57–58%) [14] and ceramic microballoon reinforced Al matrix composites (microballoon MMCs) (their y∗ values are in the range of 146.9–56.5 MPa for the porosity range of 50–70%) [15], their yielding strengths are about three to six times the y∗ values of the present cellular Mg foams with porosities of
53.0% and 57.9% (y∗ = 27.11 MPa and 22.44 MPa, respectively) as shown in Fig. 5a. The higher strengths of these composites foams are likely attributed to the high volume fraction of SS/LCS hollow sphere (∼59 vol.%, 3.7 mm in diameter and 200 m in wall thickness) or ceramic microballoon (50–70 vol.%, 45 or 270 m mean diameter) distributed in the Al matrix. However, the apparent densities () of the Al-SS/LCS composite foams and the microballoon MMCs are around 2.40 × 103 kg/m3 [14] and 4.2 × 103 kg/m3 [15], respectively. So, according to Eq. (2), as shown in Fig. 5b, the
Fig. 3. Compression behavior of cellular Mg foams. (a) Compression stress–strain curve of cellular Mg foams with porosities of 53.0–71.1% and the small inserted figure shows the typical stress–strain curves for Al based foams prepared in this study. (b–e) Compression features with strain of (0.33, 0.50) for the cellular Mg foam (Pr = 59.4%) and the cellular Al foam (Pr = 58.0%), respectively.
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D.-H. Yang et al. / Materials Science and Engineering A 527 (2010) 5405–5409
Fig. 4. Relationships of (a) yielding strength and porosity and (b) specific strength and porosity, for samples of cellular Mg foams with porosities of 53.0–71.1% and the Al based foams.
highest specific strengths of the Al-SS/LCS composite foam with porosity of around 58% and the microballoon MMCs with porosity around 50% are 37.7 MPa/(g cm−3 ) and 35.0 MPa/(g cm−3 ), respectively, which become comparable with the specific strengths of cellular Mg foam with porosity of 53.0% (33.2 MPa/(g cm−3 )) and 57.9% (30.6 MPa/(g cm−3 )). The energy absorption capacity (W) of the cellular Mg foams is calculated by integrating the stress–strain curves shown in Fig. 3a and so, the W values of the cellular Mg foams at the densification strains (Wd ) can be calculated according to the following equation:
Wd =
εd
(ε) · dε
(3)
0
where εd is the densification strain. The εd values for all the cellular Mg foams in this study are around 0.6. The relationships between
Fig. 6. Relationships of (a) energy absorption capacity, Wd , and porosity and (b) specific energy absorption capacity, Wd,m , and porosity, for samples of cellular Mg foams with porosity of 53.0–71.1%, the Al based foams and reinforced Al foams.
Wd and porosity for the cellular Mg foams as well as those of the Al based foams prepared in this study and Al-SS/LCS composite foams reported in Ref. [14] are shown in Fig. 6a, which demonstrates that the Wd of cellular Mg foams as well as those of Al based foams possess the tendency of increasing with decreasing of porosity, where the Wd of the cellular Mg foam increases from 4.24 MJ/m3 (for porosity of 71.1%) to 12.72 MJ/m3 (for porosity of 53.0%). Fig. 6a indicates that the Wd of Al-SS/LCS composite foams with porosity around 58% (Wd = 44.0–24.4 MJ/cm3 ) [14] are higher than that of the cellular Mg foam with porosity of 57.9% (9.99 MJ/m3 ) by a factor of about 2.4–4.4. However, when normalizing the Wd by the apparent density () of a foam (resulting in specific energy absorption capacity, Wd,m , where Wd,m = Wd /), this advantage disappears as shown in Fig. 6b, where the Wd,m of the cellular Mg foam with porosity of 57.9% (13.6 J/g) is comparable with those of Al-SS/LCS
Fig. 5. Relationships of (a) yielding strength and porosity and (b) specific strength and porosity, for the cellular Mg foams with porosities of 53.0–71.1%, Al based foams prepared in this study and the reinforced Al foams (i.e. Al-SS/LCS composite foams [14] and ceramic microballoon reinforced Al matrix composites [15]).
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composite foams (10.2–18.3 J/g) and the ratio is located in the range of 0.75–1.35. Fig. 6 also shows that for a given porosity, the Wd of cellular Al foam is higher than that of the cellular Mg foam by a factor of about 2, which can be attributed to the difference of their compression behavior. Comparing Fig. 3b and c with Fig. 3d and e indicates that during the compression process of the plateau region, the cell walls of the cellular Mg foam keep cracking and collapsing (indicated by the red arrow in Fig. 3b and c), which leads to its plateau stress dropping with multiple serrations until the densification strain while there is no such phenomenon happened to the cellular Al foam (as shown in Fig. 3d and e), which leads to the corresponding plateau stress keeping increasing. According to Fig. 3a and Eq. (3), the energy absorption capacity is directly determined by the area covered by the stress–strain curves, so the Wd of cellular Mg foams is significantly reduced, leading to the values of Wd and Wd,m being lower than the corresponding values of the cellular Al foams. 4. Conclusions In summary, for the cellular Mg foams prepared by meltfoaming method with porosity of 71.1–53.0%, their yielding strengths increase from 8.69 MPa to 27.11 MPa and their energy absorption capacities increase from 4.21 MJ/m3 to 12.72 MJ/m3 . Comparing with Al based foams, the cellular Mg foams exhibit excellent strength with reduced weight and particularly possess superior specific strength, where the highest specific strength reaches 33.2 MPa/(g cm−3 ). Although the energy absorption properties of cellular Mg foams are not better than those of cellular Al foams, their specific energy absorption capacities still possess comparable values with those of reinforced Al foams. All these conclusions imply that the cellular Mg foam should have promising future in the practical engineering applications.
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Acknowledgments This work is supported by Research Fund for Doctoral Program of Higher Education of China (grant no. 200802941010), Scientific Foundation for returned Scholar, Ministry of Education of China (grant no. 2009512712), the Fundamental Research Funds for the Central Universities (grant no. 2009B15514, 2009B16114), the Natural Science Foundation of HoHai University (grant no. 2008428001) and Scientific Research Startup Fund of HoHai University (grant no.2084140801109). References [1] [2] [3] [4]
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