Influence of Particles on Cell Structure of Al-Mg Alloys

Foamed by Melt Foaming Method

Soo-Han Parka, Hwan-Goo Seongb, Yeong Hwan Songc Chang-Hwan Seo, Zulkifli and Bo-Young Hurd

i-Cube Center, K-MEM R&D Cluster-GSNU, School of Nano and Advanced Materials Engineering,

Gyeongsang National University, 900, Gazwa-dong, Jinju, 660-701, Korea.

asuhan2121@nate.com,

buw98hgs@gnu.ac.kr,

cyhsong@nate.com,

dhurby@gsnu.ac.kr

Keywords: foamed Al-Mg alloys, melt foaming method, foam stability, MgAl2O4 particles

Abstract. Al-Mg alloy foams containing different Mg contents were synthesized via a conventional

melt foaming method. The surface structures of pores formed in resultant foamed alloys was

characterized by scanning electron microscopy and x-ray diffractometer. It was found that the pore

structures were deteriorated with increasing Mg contents, while the percent porosities did not vary

with increasing Mg contents, about 90% and 3~5 mm in pore size. The detailed microstructural

examination conducted on increasing Mg containing alloy foams revealed presence of various oxide

particles on the surface of individual cells, including MgAl2O4 particles in a form of fine spinel; its

small amount for Al-1wt%Mg alloy foam but relative high amount of Al-4wt%Mg one. This

suggested the negative effect of MgAl2O4 on the stable pore and thus cell structures in corresponding

alloy foams. The possible mechanism associated with MgAl2O4 formation was discussed in the

present study.

Introduction

Metallic foams are typically porous metals with high porosity. They have been attractive as

multifunctional engineering materials for increasing usage in various applications, including sound

and energy absorption devices [1,2]. Among various processes the melt foaming method is most

common for making metallic foams because of its cost-efficiency and ease of handling. In detail, Ca

and TiH2 are introduced in molten aluminum and stirred mechanically to produce uniform

distribution of pores in solidified foams. Although intensive researches have been conducted on pure

aluminum [3-7], little work has been done on the effect of addition of alloying elements on pore

structures in terms of pore sizes and their distribution, as well as foaming stability. The magnesium

added in the aluminum has been shown to increase in tensile strength, enhance machinability and

corrosion resistance. As alloying element, in particular, the magnesium is strongly reactive to form

oxides within the aluminum melt and thereby influence formation of the pores that is expected to

determine uniform sizes of pores during fabrication [8].

Therefore, in the present study, the metallic foams Al-Mg alloy were fabricated by conventional

melt foaming method. The microstructures of resultant metallic foams were then characterized by

using scanning electron microscopy and x-ray diffractometer, so as to investigate morphology of

oxides and foam stability of corresponding foams.

Experimental Procedures

Al-Mg alloy with different Mg contents were diluted by adding AM60 magnesium alloys into pure

molten aluminum. The diluted Al-Mg alloys with targeted compositions (1~4 wt% Mg) were melted

in the electric furnace up to 720oC for foaming. Thickening agent, 2wt% of Ca (<1mm) was added to

individual molten Al-Mg alloy at 720oC, which was followed by mechanical stirring at 500 rpm for 10

min. Then, 1.5wt% TiH2 powders, smaller than 45mm, was incorporated for pore formation. For

Materials Science Forum Vols. 544-545 (2007) pp 363-366Online available since 2007/May/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.544-545.363

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uniform distribution of TiH2, melt was mechanically stirred about 1000 rpm for 20 sec.

To evaluate the effect of Mg addition on pore structures, the porosities were estimated by using the

equation (1) shown below.

100)1(% ×−=

s

f

P

PP (1)

Where P is the porosity, Pf is the density of foamed Al-Mg alloy, Ps is the apparent density of

Al-Mg alloy. In this case, the density of alloy foams was measured by the Archimedes principle. Then

the specimens were sectioned and microscopically analyzed using digital image analyzer to

characterize the surface structures of sectioned foamed alloys. The distribution and chemical

composition of oxide particles existed on the cell surface were analyzed by various analytical

facilities, including scanning electron microscopy (SEM) and x-ray diffractometer.

Results and Discussion

Fig. 1 shows the macrostructures of foamed Al-Mg alloys with different Mg contents. The pore sizes

for foamed Al-1wt%Mg alloy were finer than those for Al-4wt%wtMg alloy foam, and their

distributions were much more uniform. It is apparent that increasing Mg contents degenerate the pore

structures in foamed alloys. Further, no significant melt drainage was observed for all the foams as

can be illustrated from the bottom of individual foams. This is likely to formation of oxides and small

sized spinels. In order to confirm this, the detailed microstructural examination was conducted and

discussed later.

The porosity level was measured for all the foam specimens. It seemed that the percent porosities

did not vary with increasing Mg contents. They were almost close to about 90% and 3~5 mm in pore

size, except for Al-1wt%Mg alloy foam.

Fig. 2 exhibits surface structures of the cells observed in foamed Al-Mg alloys, showing a

difference in particle distribution and size. Several particles were existed and distributed on the cell

surfaces for all foamed alloys. These particles were identified to either MgAl2O4 or Al2O3 and

embedded on the oxide layer as confirmed in Fig. 3 (a) and (b). The formation of particles identified

was shown to be related to interaction among particles during melt stirring.

Fig. 4 shows a representative micrograph of the cell surface in foamed Al-1%Mg alloy, exhibiting a

clustering of MgAl2O4 particles and breakage of the cell surfaces. This stereological feature was

indeed observed on the cell surfaces for all foamed Al-Mg alloys. Nonetheless, such an inclination

was relatively less severe in the foamed Al-1wt%Mg alloy than in the Al-4wt%Mg alloy. For this

reason, in the present study, the foamed Al-1wt%Mg alloy was shown to have better cell structures

than other Al-Mg alloys.

Fig. 1. Representative macrostructures of foamed Al-Mg alloys with different Mg contents

at constant foaming temperature 720°C. a)Al-1wt%Mg, b) Al-2wt%Mg, c)Al-3wt%Mg,

d) Al-4wt%Mg.

a) b) c)

d)

364 Eco-Materials Processing and Design VIII

Fig. 2. SEM morphology of foamed Al-Mg alloys with 2wt% calcium.

(a)1 wt% Mg, (b) 2 wt% Mg, (c) 3 wt% Mg, (d) 4 wt% Mg

(a) X-ray diffractions pattern (b) element mapping

Fig. 3. (a) X-ray diffractions pattern and (b) element mapping of Al-Mg alloy foam surface.

The possible mechanism how the localized AlMg2O4 particles deteriorate the cell formation might

be associated with the following. As the small portion of magnesium element adds to the molten

aluminum, the MgO particles are easily formed over the period of foam fabrication. In this view, the

preformed MgO particles are likely to prevent other types of particles from forming in the aluminum

melt, leading to decrease in the foam stability. Alternatively, the MgAl2O4 particles through the

reactions between MgO and Al2O3 particle will be able to increase the viscosity of the aluminum melt.

Fig. 4. SEM image of Al-1wt%Mg alloy foam showed spinel oxides.

Materials Science Forum Vols. 544-545 365

In this case, however, the MgAl2O4 particles in form of fine clusters can interplay with oxide layers

covering the cell surfaces, probably causing the breakage of the gaseous pores and their coalescence

due to repulsive forces.

According to the previous work, the evaporating magnesium within the Al-Mg melt has generated

burrs and damaged the oxide films [9]. This can be related to the possible mechanism proposed by the

present authors. When viewed in the present study, the minimized formation of MgO particles is

expected to be needed for stable cell structures in making metallic foamed Al-Mg alloy.

Conclusions

The foamed Al-Mg alloys with different Mg contents were synthesized via the melt foaming method.

The preliminary results can be summarized as follows:

1. The pore structures in Al-Mg alloy foams were degenerated with increasing Mg contents. It was

apparent that increasing Mg contents damages the pore structures over the period of fabrication.

This was shown to be due to formation of MgAl2O4 particles in a form of fine clusters.

2. The MgAl2O4 particles could cause the breakage of the gaseous pores and their coalescence due to

repulsive forces. It was shown that the possible interplay between MgAl2O4 particles and oxide

layer could result in uncontrolled pore structures in solidified foamed alloy, particularly, at high

Mg containing alloy foam.

Acknowledgement

The research was supported by the Program for the Training of Graduate Students in Regional

Innovation which was conducted by the Ministry of Commerce Industry and Energy of the Korean

Government.

References

[1] J. Banhart: Manufacture, Progress in Materials Science 46 (2001), p. 559.

[2] J. Baumeister, J. Banhart, M. Weber: Materials & Design Vol. 18, No. 46(1997), p. 217.

[3] T.Miyoshi, M. Itoh, S. Akiyam and A. Kitahara: Adv. Eng. Mat. 2(2000), p.179.

[4] A.A. Gokhale, S.N. Sahu, W.R. Kulkami, B. Sudhakar, and N.R. Rao: Porous metals and metal

foaming Technology (2005), p. 95.

[5] F. Han, Z. Zhu and J. Gao: Met. Trans. 29A (1998), p. 2497.

[6] C.C. Yang and H. Nakae: J. Mat. Processing Tech. 141(2003), p. 202.

[7] J. Banhart: Porous metals and metal foaming Technology (2005), p. 75.

[8] S.Y. Kim, Y.S.Um and B.Y. Hur: Materials Science Forum 510-511 (2006), p. 902.

[9] Z. Lijun, W. Jinbo, Q. Jiting and N. Qiu: Proceedings of the International Conference on Interface

in Metal-Ceramics Composites, Anaheim, California, Feb. 1990 (The Materials Society,

Warrendale, Pennsylvania, 1990, p. 213)

[10] N. Babcsan, D. Leutlmeier, H.P. Degischer and J. Banhart: Adv. Eng. Mat. 6 (2004), p. 421.

[11] N. Babcsan, D. Leutlmeier and H.P. Degischer: Mat.-wiss. u. Werkstofftech. 34 (2003), p. 1.

[12] N. Babcsan, F. Garcia-Moreno, J. Banhart: Porous metals and metal foaming Technology (2005),

p. 261.

[13] M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson and H.N.G. Wadley: Metal

foams: A Design Guide (Butterworth-Heinemann UK 2000)

[14] J. Gubicza, N.Q. Chinh , Z. Horita , T.G. Langdon: Materials Science and Engineering A387-389

(2004), p. 55.

366 Eco-Materials Processing and Design VIII

Eco-Materials Processing and Design VIII 10.4028/www.scientific.net/MSF.544-545 Influence of Particles on Cell Structure of Al-Mg Alloys Foamed by Melt Foaming Method 10.4028/www.scientific.net/MSF.544-545.363

DOI References

[1] J. Banhart: Manufacture, Progress in Materials Science 46 (2001), p. 559.

doi:10.1016/S0079-6425(00)00002-5 [8] S.Y. Kim, Y.S.Um and B.Y. Hur: Materials Science Forum 510-511 (2006), p. 902.

doi:10.4028/www.scientific.net/MSF.510-511.902 [12] N. Babcsan, F. Garcia-Moreno, J. Banhart: Porous metals and metal foaming Technology (2005), . 261.

doi:10.1016/j.colsurfa.2004.12.030 [14] J. Gubicza, N.Q. Chinh , Z. Horita , T.G. Langdon: Materials Science and Engineering A387-389 2004),

p. 55.

doi:10.1016/j.msea.2004.03.076 [12] N. Babcsan, F. Garcia-Moreno, J. Banhart: Porous metals and metal foaming Technology (2005), p .

261.

doi:10.1016/j.colsurfa.2004.12.030 [14] J. Gubicza, N.Q. Chinh , Z. Horita , T.G. Langdon: Materials Science and Engineering A387-389 (2004),

p. 55. [ [ [ [ 1 2 3 4 ] ] ] ]

doi:10.1016/j.msea.2004.03.076