Post on 17-Dec-2016
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
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 134.99.128.41, Universitaets- und Landesbibliothek Duesseldorf, Duesseldorf, Germany-20/11/13,05:08:45)
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).
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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
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