Rheological Properties of Molten Al-Cu alloys for Manufacturing Metallic Foam

Soo-han Park1,a, Yong-su Um1,b and Bo-young Hur1,c

1K-MEM R&D Cluster–GSNU, AMRC, Division of Advanced Materials Engineering,

Gyeongsang National University, Jinju 660-701, Korea.

asuhan2121@nate.com, bumys51@gshp.gsnu.ac.kr, churby@gsnu.ac.kr

Keywords: metallic foam, surface tension, viscosity, Al -Cu alloys

Abstract. The surface tension and the viscosity characteristics of molten metal are the most important

factors in casting process and metallic foam manufacturing especially. The surface tension (by the

modified ring method) and the viscosity (by the rotational method) of molten Al-Cu alloys have been

measured under high purity Ar gas atmosphere. The surface tension and the viscosity of Al-Cu alloys

were investigated in the temperature range of 660-800oC, and the effects of the additional elements

were investigated at the 660~680�. The result show that the surface tension and viscosity of these

alloys decrease with increasing temperature together. The viscosity of Al-Cu alloys near the melting

point is about 4.7 to 5.7 [mPa.s]. The effect of additional thickening elements has the tendency that is

the surface tension decreased and the viscosity increased. This anomalous behavior has the relation of

the preferential adsorption of high activity elements on the surface.

Introduction

Metallic foam is porous metal with pores in the metal matrix. It can be used as functional material

such as sound absorption, isolation and impact absorption. In fabrication of metallic foam with

chemical reaction, usually two methods have been used [1]. Many metallic foam produced by Cast

method (Directly Melt Foaming) have coarse and irregular cell structures. This is particularly true of

material created using melt-based routes, which are economically attractive. A primary current aim is to

fabricate of various metallic foams with more uniform structure and cell size. It is important to

understand the mechanisms and factors controlling. Rheological properties are very important factors in

casting process. Such as melting point, pouring temperature, density, viscosity and surface tension in

molten metals. For the control of the bubble in molten metal, such as life, death, shape and size of

bubble in molten metal, the physical properties of liquid metal which have great influence on

fabricating properties of metal foam must be given adequate attention [2]. Thus this paper investigated

the bubble behavior in the molten metal based in the most important two parameters: surface tension

and liquid viscosity.

These two factors are considered with two liquid mechanisms operating in foam. The first is

gravity-driven melt flow from the top to base of foam column. The second is capillarity-driven melt

flow from cell face to plateau borders. This leads to cell face thinning and often to cell face rupture [2,

3].

In this experiment, the modified drop weight method and the rotational method were used because of

simple measurement method. They are also directly applicable to the fabrication process. Viscosity and

surface tension of Al-Cu alloy, which is used as foaming materials, were investigated. Influences of

temperature and additional thickening elements (Ca) to viscosity and surface tension were also studied.

Of course in the actual foaming process the appearance of the rheological characteristics are very

much influenced from the existing of particles [4]. However the fundamental rheological data do not

lose the importance.

Solid State Phenomena Vols. 116-117 (2006) pp 656-660Online available since 2006/Oct/15 at www.scientific.net© (2006) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.116-117.656

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Experimental procedure

Surface tension was measured by the modified drop weight method, which applies the capillary

phenomenon to measuring the maximum force and contact angle when the ring is pulled out from the

melt surface [5,6]. Fig. 1 shows a schematic illustration of the experimental apparatus for the surface

tension measurement. Viscosity was measured by the rotational method can be calculated the viscosity

by measuring the torque that is a resistance force of the melt for the rotating rotor [5,7]. Fig. 2 shows a

schematic illustration of the experimental apparatus for the viscosity measurement. In this experiment,

graphite rotor and crucible were used.

In this experiment, the high purity argon gas sealing was used to prevent surface oxidation. The

flowing rate was set to 10(ℓ/min). The measured samples were Al-Cu (1~4 wt%) alloys. The

temperature range of measurement was set from 660oC to 800

oC. And the effects of the adding Ca were

investigated at the 660~680�.

The maximum force (Fmax) that is measured by the ring method can be recalculated to a surface

tension(σst) by equation (1).

⋅

⋅=

r

R

V

Rf

R

F,

cos4

3max

θπσ (1)

Where 4πR is wetted length, Fmax is the total maximum force, f is the Harkins Jordan factor 8) and θ is

the contact angle (θ = 90o).

Viscosity (ηvisco) can be calculated from the measured torque (T) on the rotation rotor.

)(

15

32

a

h

ab

rNr

Tvisco

+=π

η (2)

Where T is the measured torque and N is the revolutions per min of the rotor. r is the radius of rotor :

26mm, a is side gap : 4mm, b is bottom gap : 5mm and h is wetted height : 100mm.

In addition, to compare the foamability of pure Al and Al-Cu, foaming tests were carried out. The

testing conditions were taken as shown in table 1.

(a) (b)

Fig. 1. Schematic diagram of the apparatus used for the (a) surface tension and (b) viscosity

measurement.

Table 2. The condition of foaming test

Thickening Agent / Stirring Ca (1.5wt% addition) / 15minutes

Blowing Agent / Stirring TiH2 (1.5wt% addition) / 20 seconds

Thickening / blowing 680oC / 680

oC

Temperature Curing 10minutes at 600

oC

Solid State Phenomena Vols. 116-117 657

Results and discussion

To investigate the surface tension and viscosity of molten Al-Cu by modified drop weight method

and rotational method [5]. We carried out experiments with temperature and composition dependencies

of Al-Cu alloy. Fig. 2 shows the surface tension and viscosity measured value of Al-Cu alloys with the

change of temperature. As shown in Fig. 2(a), the surface tension of Al-1wt%Cu is about 798[mNm-1]

near the melting point, and with the increasing temperature, the surface tension was decreasing lineally

following the relationship of σ=798-0.425(T-Tm)[mNm-1]. The value of the Al-Cu (1~4 wt%) alloys

belong to the range of figures regardless of each amount. The observation on those Al-Cu alloys

decreased linearly with the increasing temperature.

Fig. 2(b) shows the variation of the viscosity of the Al-Cu alloys with the change of temperature. The

viscosity of Al-Cu near the melting point is about 5.2[mPa·s]. According to the Ref. [2], the theoretical

value and data of pure aluminum is 4.2[mPa·s], which are smaller than the value obtained in this study.

Fig 3, in the case of adding Cu into the Al, there were no obvious visible changes of surface tension.

The observation that the amount of Cu increases the surface tension indicates that Cu is not surface

active elements. The effects of adding copper were shown in Fig. 2 as well. In Fig. 4(a), the effects of

Ca on the surface tension were plotted.

When Ca is added (see in Fig. 4(a)), it showed a rapidly decreasing. This reason can be explained

well by Gibbs's adsorption equation [9-11], which relates the excess surface concentration with the

dependence of the surface tension on the thermodynamic activity of the solute in melt.

Surface tension value of optimal conditions for the metal foam manufacturing is about 600[mNm-1]

[12-16]. In the case of Al-Cu alloys, it is possible that the optimal conditions of the surface tension can

be obtained through controlling the adding 1.5wt% Ca and temperature.

600 700 800600

700

800

900

Surface tension , mNm

-1

Temperature , oC

Al-Cu (1~4wt%)(a)

Melting

point

600 700 800 900 10003

4

5

6

7

Viscosity , mPa . s

Temperature , oC

Al-Cu (1~4wt%)(b)

Fig. 2. Temperature dependencies s of (a) surface tension and (b) viscosity of Al-Cu alloys.

Fig. 3. Effect of added Cu on the surface tension of pure Al at 600-608oC

0 1 2 3 4 5500

600

700

800

900

1000

Copper

Pure Al : 832mNm-1

Alloying element , wt%

Surface tension , mNm

-1

658 Semi-Solid Processing of Alloys and Composites

Fig. 4(b) shows the variation of the viscosity of Al-Cu alloys with the change of composition. As

shown in Fig. 4(b), adding Ca into the Al-Cu melt can increase the viscosity of the melt. We can see

that the viscosity increases remarkably with 1.5wt%Ca addition.

Viscosity value of optimal conditions for the metal foam manufacturing is about 10~14[mPa·s]. In

the case of Al-Cu alloys, it is possible that the optimal conditions of the viscosity can be obtained

through controlling the adding 1.5wt% Ca and stirring time.

The results of foaming test are shown Fig. 5. Al-Cu foam is produced by adding 1.5wt% Ca to

increase the melt viscosity and blowing agent (TiH2) to generate gas at 680oC.

0 1 2 3 4400

500

600

700

800

900

Al-1wt% Cu

Al-2wt% Cu

Al-3wt% Cu

Al-4wt% Cu

Alloying Calcuim , wt%

Surface tension , mNm

-1

(a)

0 5 10 15 20

0

4

8

12

16

20

Rotation speed 700rpm

temperature 660oC

Stirring time , min

Viscosity , mPa . s

Al-1wt% Cu with 1.5wt% Ca

Al-2wt% Cu with 1.5wt% Ca

Al-3wt% Cu with 1.5wt% Ca

Al-4wt% Cu with 1.5wt% Ca

(b)

Fig. 4. Effect of added Ca on the (a) surface tension and (b) viscosity of Al-Cu alloys at 660-680�.

The foam is produced by blowing H2 gas and taken out after up to 10min isothermal holding time at

600�. Fig. 5 shows photographs of the foamed pure Al and Al-Cu of a vertical section. Compared with

foamed pure Al, by this condition have a little big pore structures. The specimen heights increased

about 8 times for Al and Al-Cu alloys that original column height. The macroscopic porosity including

skins of the foamed columns was evaluated by Archimedes’ principle to be about 0.9. The foamed pure

Al has fine cell and shows uniform cell structure. Though a few larger pores of 3~4 mm exist at the

center of foam, the remaining small pores of less than 3 mm are dispersed uniformly around them. This

is because of the addition of Ca and particles in melt possesses, lower surface tension and higher

viscosity. The Al-Cu alloy foams have coarse and shows big pore and more irregular cell structures than

foamed pure Al.

(a) (b)

Fig. 5. Photographs of foamed (a) pure Al and (b) Al-Cu column.

Conclusion

The surface tension of the Al-Cu alloys followed the relationship σ=798-0.425(T-Tm)[mNm-1], and

with additional 1.5wt%Ca, the surface tension was decreased rapidly to the value of about

σ1.5Ca=570~590 [mNm-1] at the 660~680�. The viscosity of molten Al-Cu (1~4 wt%) alloys was η =

4.7~5.7 [mPa·s] in the temperature range, and with the additional Ca and stirring for 15min, the

Solid State Phenomena Vols. 116-117 659

viscosity was increased to η1.5Ca=10~13 [mPa·s] at the 660~680�, when adding 1.5wts% of Ca. The

Al-Cu alloy foams have coarse and shows big pore and more irregular cell structures than foamed pure

Al.

Acknowledgement

This work was supported by grant No. RTI04-01-03 from the Regional Technology Innovation Program

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

Reference

[1] H. Fusheng and Z. Zhengang: J. Mater. Sci. Vol. 34 (1999), pp. 291-299

[2] D. Weaire: Cellular Metals and Metal Foaming Technology (Verlag MIT Publishing 2001), pp.

63-68

[3] M. Meier, D. Hille and G. Wallot: Cellular Metals: Manufacture, Properties, Applications (Verlag

MIT Publication 2003), pp. 65-70

[4] J. Banhart: Journal of Metals Vol. 52 (2000), pp. 22-27

[5] T. Iida and P.I.L. Guthrie: The Physical Properties of Liquid Metals (Clarendon press. Oxford 1988),

pp. 109-198

[6] J.P. Anson, R.A.L. Drew and J.E. Gruzleski: Met. & Mater., Trans. B Vol. 30 (1998), pp.

1999-2006

[7] Y. Shiraishi: J. Kor. Inst. Met & Mater. Vol. 25(11) (1987), pp. 824-834

[8] W.D. Harkins and H.F. Jordan: J. of Am. Chem. Soc. Vol. 52 (1930), pp. 1751-1760

[9] J.W. Gibbs: Thermodynamics (The Collected Works of J. W. Gibbs, vol. I, Yale Univer. Press, New

Haven, CT 1948)

[10] P. Kozakevitch: J. Met. (1969), pp. 57-62

[11] D. Skupien and D.R. Gaskell: Met & Mater. Trans. B Vol. 31 (2000), pp. 921-927

[12] S.H. Park and B.Y. Hur: Mater. Sci. Forum Vol. 486-487 (2005), pp. 464-467

[13] B.Y. Hur, H.J. Ahn, D.C. Choi and S.Y. Kim: Limat (2001), pp. 671-674

[14] B.Y. Hur, S.H. Cho and K.B. Kim: Proceedings of Fall Conference. Vol. 1 (2001), pp. 246-249

[15] B.Y. Hur, H.J. Ahn, D.C. Choi, S.H. Cho, K.D. Park, Y.J. Kim and S.H. Jun: Proceedings of the

Symposium on Solidification Process of Metals (2000), pp. 87-90

[16] S. Y. Kim, B. Y. Hur, C. K. Kwon, D. K. Ahn, S, H. Park and A. Hiroshi: Proceedings of the 65th

World Foundry Congress (2002), pp. 499-508

[17] H. Nakanishi, K. Nakazato, S. Asaba. K. Abe, S. Maeda and K. Terashima: J. Crystal growth Vol.

191 (1998), pp. 711-718

[18] G.H. Geiger and D.R. Poirier: Transport Phenomena in Metallurgy (4th ed., Daehan Printing &

Publishing Co., Ltd. 1994)

[19] S.K. Kim, T.H. Hong, S.H. Cho and Y.J. of Kim: J. Kor. Foundrymen's Soc. Vol. 18(5) (1998), pp.

419-425

[20] O.N. Yoon, H. Choi and D.H. Kim: J. Kor. Foundrymen's Soc. Vol. 19(4) (1999), pp. 324-329

[21] S.K. Kim and Y.J. Kim: Met. & Mater Int. Vol. 6(4) (2000), pp. 359-364

[22] K.D. Woo, H.S, Na, S.W. Kim, T. Sato and A. Kamio: Met. & Mater. Int. Vol. 7(6) (2001), pp.

613-619

[23] K.S. Lee , W.O. Yang , Y.G. Park and K W. Yi: Met. & Mater Int. Vol. 6(5) (2000), pp. 461-466

[24] S.H. Jun: Ph.D. Thesis, Gyeongsang Nat. Univ., Kor. (2002), pp. 10-16

[25] S.H. Park, B.Y. Hur, S.Y. Kim, D.K. Ahn and D.I. Ha: Proceedings of the 65th World Foundry

Congress (2002), pp. 515-524

[26] C.K. Kwon, B.Y. Hur, S. Y. Kim, D. K. Ahn and S. H. Park: Proceedings of the 65th World

Foundry Congress (2002) pp. 537-544

[27] S. Y. Kim, B. Y. Hur, C. K. Kwon, D. K. Ahn and S, H. Park: J. Kor. Inst. Met. & Mater. Vol.

40(8) (2002), pp. 905-910

660 Semi-Solid Processing of Alloys and Composites

Semi-Solid Processing of Alloys and Composites 10.4028/www.scientific.net/SSP.116-117 Rheological Properties of Molten Al-Cu Alloys for Manufacturing Metallic Foam 10.4028/www.scientific.net/SSP.116-117.656

DOI References

[8] W.D. Harkins and H.F. Jordan: J. of Am. Chem. Soc. Vol. 52 (1930), pp. 1751-1760

doi:10.1021/ja01368a004