Characteristics of Al Foam with High Porosity by Melt Foaming Method

Sang-Youl Kim1, Hwan-Goo Seong2 and Bo-Young Hur 1, a 1AMRC-ERI, School of Nano and Adv. Mat. Eng., Gyeongsang National University

2i-Cube Center, School of Nano and Adv. Mat. Eng., Gyeongsang National University

900 Kajoa-dong Jinju, Korea aCorresponding author:hurby@gnu.ac.kr

Keywords: Metal foam, particle, viscosity, pore size, Ostwald ripening.

Abstract. Al foams with high porosity were fabricated via melt foaming method. In this process, Ca and TiH2 are used as thickening and foaming agent. The macrostructure of aluminum foams exhibited homogeneous distribution of fine pore sizes at shorter holding time. From comparison of the actual pore sizes with ones calculated with modified Ostwald ripening, the measured pore sizes were well fitted to the calculated ones. These results suggest that hydrogen diffusion through pore walls from smaller pores to a larger one is the main process of pore evolution at least in the later period of holding. The detailed are discussed in the context.

Introduction

Metallic foams are very attractive engineering materials that combine solid and pores, exhibiting superior properties, such as sound absorption, impact absorption, lightweight. They have been manufactured by various processing techniques, including melt foaming method[1], continuous casting[2], powder metallurgy[2]. In the melt forming process that employs the addition of TiH2 in pure aluminum melt, pores generated leave individual hydrides and enter into the pure molten aluminum due partly to buoyancy forces and due partly to the thermodynamic instability of systems with enhanced liquid-gas interfaces[3]. In this view, achieving uniform distribution of controlled pore sizes in a foaming process can be influenced by the interaction among pores in the molten aluminum as the titanium hydrides releases the hydrogen gases via their thermal decompositions in contact with the molten aluminum. Many efforts have been done on the growth behavior of pores[4-5]. Song et al[8,9] reported the role of holding time on the pore size distributions in the molten aluminum, indicating that the pore size increased with holding time, but decreased with the melt viscosity. Nevertheless, there was no detailed mention on the interplay among pores. Thus, the present study discusses the dependence of holding time on the pore sizes and evaluate the pore sizes predicted via the modified Ostwald ripening theory[10] in comparison with ones measured in solidified aluminum foams.

Experimental procedures

Pure aluminum was used to high porosity metal foam by melt foaming method[11]. In detailed process, pure aluminum of 1 kg was melted in a high-frequency induction furnace. The molten aluminum was poured in home-made cylindrical steel mold for individual heats. Then, 2 wt% Ca chosen as a thickening agent were added to increase the melt viscosity, and the Ca thickened melt were subjected to a mechanical stirring at 600 rpm for 20 min. Then, the TiH2 were added to the Ca treated melt, followed by additional stirring at 1200 rpm for about 40 sec. Chemically treated melts were held at an identical melt temperature (700 °C) for various times, i.e. 60 to 1200 sec. For macrostructural observations, the resultant aluminum foams were sectioned at a direction parallel to the foaming direction. The pore sizes in the aluminum foams were measured and analyzed in comparison with the calculated pore sizes.

Solid State Phenomena Vols. 124-126 (2007) pp 1801-1804Online available since 2007/Jun/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.124-126.1801

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Results and Discussion

Fig.1 shows the macrostructures of melt-treated aluminum foams at different holding. The pore size measured at solidified foam after 120 sec holding time was much finer compared to ones measured at longer holding time; it was approximately 1.0 ~ 1.5 mm at 120 sec but 3.5 ~ 4.5 mm at 1200 sec. It is apparent that increasing pore sizes at increasing holding time are contributed to hydrogen diffusion taking places among different sizes of pores. This phenomenon can be described by the modified Ostwald ripening.

Fig. 1. Microstructures of aluminum foam with holding time for (a) 120 sec (b) 300 sec (c) 600 sec (d) 1200 sec Assuming that the growth of pores in molten aluminum can be considered the particle size in

conventional Ostwald ripening formulation, generalized Ostwald ripening formulation is given as shown in eq.(1).

RT

tr )1(983

ασ−

= ( )∑ ⎥⎦

⎤⎢⎣

⎡− DCCC NNNN

2θ (1)

Where r is average radius (m) of particles or pores, σ is surface tension of particles or pores (N/m), α is one third power of porosity ratio. R is gas constant (8.31J/mole), T is temperature (K), t is elapse time (s), θ

NC is molar fraction / molar volume of element N in pore (m-3), CN is molar fraction/molar volume of element N in matrix (m-3). DN is diffusion coefficient of element N in matrix (m2/s).

However, eq.(1) was developed mainly for solid precipitates, where the ratio of particle size between developing particles and average particles (hereafter k) was taken as 1.5[12-13]. In order to apply this equation for the case of gas pores, it is necessary to modify the original Ostwald ripening formulation as shown in eq.(2).

RTt

kkr )1(

213

3

ασ

−⎟⎠⎞

⎜⎝⎛ −

= ( )∑ ⎥⎦

⎤⎢⎣

⎡− DCCC NNNN

2θ (2)

Here, since k is the ratio of radius between developing pores and average (shrinking) pores, rk × is the radius of developing pores (rd) as given in eq.(3).

RT

ktrd )1()1(23

ασ−

−= ( )∑ ⎥

⎦

⎤⎢⎣

⎡− DCCC NNNN

2θ (3)

Neglecting aluminum term because aluminum atoms can change their position easily by liquid flow, then, eq.(3) is simplified as eq.(4).

( )23

)1()1(2

HH

HHd

CCDC

RTtkr

−×

−−

=θα

σ (4)

Moreover equation (4) is rewritten as (5) by using Gay-Lussac’s law (PV=RT),

0

3

)1()1(2

Ptkrd α

σ−

−=

( )2HH

HHH

CC

DCC

−×

θ

θ≒ θα

σ

H

HH

CPDCk

0)1()1(2

−− t (5)

Surface tension of melt (σ), σ is 0.55 N/m[14], Molar fraction per molar volume of hydrogen in melt (CH), Solute hydrogen in melt, CH, should be equilibrium with hydrogen pressure. From the

a) b) c) d)

1802 Advances in Nanomaterials and Processing

binary Al-H phase diagram, hydrogen solubility in aluminum melt at 1atm (105 Pa) is given to be 0.0012 atomic%[15] and the specific density of Aluminum melt is 2.5gr/cm3[16]. Then, Molar fraction per molar volume of hydrogen in melt CH is calculated as 0.000012/2×√ (P/P0) / (26.98 / 2.5)×106 /m3, that is, 0.56 /m3 at P/P0=1. Here P0 is 105Pa (atmospheric pressure). Molar fraction per molar volume of hydrogen in pore(Cθ H), Hydrogen gas molar volume is 0.0224×T /273 (m3/mole). Therefore, Cθ H is 13.1 /m3 at T=933K and P=1 atm(105Pa). Since it is difficult to get diffusion coefficient of hydrogen in aluminum melt, the diffusion

coefficient of hydrogen in aluminum solid was (tentatively) used in this study. The diffusion coefficient of hydrogen in aluminum solid is reported about ~ 1.0×10-7 m2/s[17-18]. The binary Al-H phase diagram shows hydrogen solubility in melt is 10 times higher than in solid. Therefore, the diffusion coefficient estimated in the melt is also 10 times higher in solid, i.e. ~ 10-6 m2/s. Based upon a schematic model for foamed metal structure as shown in Fig. 2, the whole spaces

among pores can be filled entirely by globular shell melts with hollow inside. Each shell and pore radius are defined as rb and ra ( = r) keeping the similarity and α is defined as ra/rb. As porosity ratio is equal to (ra/rb )3, α is calculated one third power of porosity ratio. In case of 80% porosity, a, which is ra/rb, is calculated to be 0.93.

Fig.2. a) Model of precipitated particles and their territories b) model of closed-cell metal foam.

According to Song et al [9], particle size ratio between developing particles and average particles k is obviously greater than 1.5. Here we use k = 4. Thus, eq.(5) is calculated as eq.(6) under the condition above mentioned.

rd3 = 0.020 t mm3 (6)

The calculated pore sizes as a function of holding time are plotted with the actual pore sizes of aluminum foams as shown in Fig. 3. As indicated before, radii of actual pore were 1.0-1.5 mm at 120 second of holding time, 1.5-2.0 mm at 300 second, 2.5-3.5 mm at 600 second, 3.5-4.5 mm at 1200 second. These experimental data seemed to be in well agreement with ostwald ripening theory.

Fig. 3. Developing pore radius and holding time at foaming process.

Solid State Phenomena Vols. 124-126 1803

Conclusion

This work clearly shows that the evolution of foamed aluminum structure is controlled by hydrogen diffusion in the melt. The main conclusions are summarized as follows. 1. Ostwald ripening theory can successfully explain the hydrogen pore evolution in the latter half

holding period. This means that hydrogen diffusion through the pore walls from the smaller pores to the larger ones is the main process of pore evolution.

2. The extra highly pressurized pores release their pressure immediately if there is no restriction by surface tension. This means only ultra fine pores can stay longer in stirring melt as its shape.

3. Pore evolution in the initial half of holding period seems that the minority of large pore is developing while the majority of ultra fine pores are shrinking, just like a precipitation in super saturated solid solution. On this basis, pore development is calculated by modified Ostwald ripening model and .the results are well in agreement with the experimental data reported previously.

Acknowledgement

This work was supported (in part) by ERI-AMRC at Gyeongsang National University and Ministry of Environment as “The Eco-technopia 21 project” and 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

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Advances in Nanomaterials and Processing 10.4028/www.scientific.net/SSP.124-126 Characteristics of Al Foam with High Porosity by Melt Foaming Method 10.4028/www.scientific.net/SSP.124-126.1801

DOI References

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