A Study on Al-Mg Alloy Foams by Melt Foaming Method

Younghwan Song, Soo Han Park, Sang Youl Kim, Changhwan Seo and

Boyoung Hur

The Research Center for Aircraft Parts Technology(RCAPT), School of Nano and Advanced Materials Engineering, Gyeongsang National University, 900, Gazwa-dong, Jinju, 660-701, Korea

Corresponding author: hurby@gsnu.ac.kr

Keywords: Al-Mg alloy foam, metal foam, foaming, melt foaming method

Abstract.

Al-Mg alloy foams were synthesized via conventional melt foaming method. Ca and TiH2 were

introduced into molten Al-Mg alloys with different magnesium contents. The macrostructures of

resultant alloy foams were analyzed and correlated with compressive properties estimated by

compression test. It is shown that the pore structures observed in alloy foams degraded with

increasing Mg contents. This tendency was shown to be consistent with compressive

characteristics of corresponding alloy foams. In detail, plateau strength was high for Al-1wt%Mg

alloy foams, exhibiting a gradual decrease in plateau strength with increasing magnesium content.

Introduction

Metallic foams are 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 fabrication techniques, the melt foaming method is most

common because of its cost-effective one and ease of handling. In details, Ca and TiH2 are introduced

in molten aluminum and stirred mechanically to produce uniform distribution of pores in solidified

foams. Although intensive researches has been performed on pure aluminum[3-8], 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 mechanical properties. Therefore, in the present study, the

macrostructures and compressive characteristics of Al-Mg alloy foams were evaluated to examine the

effect of Mg contents.

Experimental Procedures

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

molten aluminum. The chemical compositions used as starting materials are shown in Table 1. 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 uniform

distribution of TiH2, melt was mechanically stirred about 1000rpm for 20 sec. The home-made

apparatus used for fabricating Al-Mg alloy foams are shown in Fig.1.

Table 1 Chemical composition of pure aluminum and AM 60: (wt %)

Comp. Al Zn Mn Cu Si Fe Ni Mg Other

Pure Al 0.06 0.10 0.14

AM 60 6.0 0.22 0.6 0.01 0.1 0.005 0.002 <0.3

�

Solid State Phenomena Vols. 124-126 (2007) pp 1841-1844Online available since 2007/Jun/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.124-126.1841

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Fig.1. Schematic drawing of experimental

apparatus���

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 Al-Mg alloy metal foam, Ps is the apparent density of

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

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

characterize the surface structures of sectioned alloy foams. Compression tests, using Instron 8872,

were carried out on pieces of alloy foams cut as 3303030 mm×× at a strain rate of 20mm/min at room

temperature to evaluate the effect of magnesium addition to strength of solidified alloy foams.

Results and Discussion

Fig. 2 shows the macrostructures of Al-Mg alloy foams with different Mg contents. The pore sizes

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

distributions were much more uniform. It is apparent that increasing Mg contents (degenerate)

deteriorate the pore structures in solidified foams. Further, no significant melt drainage was observed

for all the foams as can be illustrated from the bottom of individual foams. This is possibly due to a

decrease in viscosity of melt surface resulting from formation of magnesium oxides because

magnesium oxides may agglomerate locally with other oxides such as aluminum and calcium oxides.

The percentage porosity was measured for all the foam specimens. Fig.3 shows percent porosity of

individual foam specimens as a function of Mg contents. It seems that the percent porosities do not

vary with increasing Mg contents. They were almost close to about 85%, except for Al-1wt%Mg alloy

foam.

a) b) c) d)

1842 Advances in Nanomaterials and Processing

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Fig.2. Representative macrostructures of solidified Al-Mg alloy foams 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.

Fig.4 shows the compressive stress-strain curves of Al-Mg alloy foams, exhibiting a typical

deformation with three different areas. It seems that the areas are consisted of a linear elastic, plateau,

and densification area which has been proposed by Ashby and Gibson[12]. When viewed on

individual curves, the plateau strength for Al-1wt%Mg alloy foam was about 7 Mpa at 5% strain but

decreases with increasing Mg content. On the other hand, the plateau area increases up to over 50%

strain with increasing Mg content. The observed decrease in the plateau strength (with increasing Mg

contents) is likely to non-uniform pore structures presented in corresponding Al-Mg foams. These

results are much consistent with macrostructures of al-Mg alloy foams as indicated in Fig.2.

1 2 3 460

65

70

75

80

85

90

95

100

Porosity [ %

]

Mg Contents [ wt% ]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

5

10

15

20

25

Stress [ MPa ]

Strain

Al-1Mg

Al-2Mg

Al-3Mg

Al-4Mg

Pure Al

Fig.3. Porosity of foamed Al-Mg alloy������������Fig.4. Compressive stress-strain curves of foamed

Al-Mg alloy

Conclusions

Al-Mg alloy foams with different Mg contents were fabricated via melt foaming method. The

experimental results can be summarized as follows:

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

is possibly due to a decrease in viscosity of melt surface resulting from formation of magnesium

oxides because magnesium oxides may agglomerate locally with other oxides such as

aluminum and calcium oxides.

2. No significant change in percent porosities were wholly found at Al-Mg alloy foams,

irrespective of Mg contents.

3. The plateau strength was high for Al-1wt%Mg alloy foams. It tended to decrease with

increasing Mg content due primarily to non-uniform pore structures.

Solid State Phenomena Vols. 124-126 1843

Acknowledgement

This work was supported by grant No. RTI-04-01-03 from the Regional Technology Innovation

Program of the Ministry of Commerce, Industry and Energy(MOCIE).

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Advances in Nanomaterials and Processing 10.4028/www.scientific.net/SSP.124-126 A Study on Al-Mg Alloy Foams by Melt Foaming Method 10.4028/www.scientific.net/SSP.124-126.1841

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