Thixotropic Behaviour of a Cu – Eutectic Sn-Pb Slurry
C.Merizalde1,a, J.M. Cabrera1,2,b and J.M. Prado1,2,c 1Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica. Universidad Politécnica de
Cataluña – ETSEIB – Av. Diagonal 647,08028, Barcelona, Spain.
2CTM Centro Tecnológico de Manresa – Av. Bases de Manresa 1, 08042, Manresa, Spain
[email protected], [email protected], [email protected]
Keywords: Compocasting, thixotropic behavior, semisolid state
Abstract. Semi-solid materials (SSM) in the thixotropic state behave like liquids, i.e they show low
or null shear resistance and, at the same time, they behave like solids as do not fall to pieces under
applied forces. At present, the potential advantages and industrial applications of these materials are
well recognized, in particular for the production of Al-alloy components for the aerospace and
automotive sectors. This work is focused on the evaluation and characterization of the thixotropic
behaviour of a metal mixture in the semi-solid state obtained by “Compocasting”. The mixture is
obtained by mixing spherical solid Cu particles with a liquid eutectic tin-lead alloy. Measurements
of the time-dependence of the viscosity of the mixture using an instrumented rheometer showed
that, after mechanical stirring, the slurry acquires thixotropic properties. The best conditions to
obtain such mixture are presented. Additionally, once the mixture is cooled down, the material is
reheated and then tested in a laboratory backward extrusion process. The behaviour of the material
is analysed on the basis of the microstructure obtained, and the process parameters considered.
Introduction
Since the 70´s a great amount of researchers have observed that materials with spherical (non-
dendritic) microstructures possess rheological properties in the semi-solid estate which are quite
different from those of dentritic alloys. Flemings [1] and Spencer [2] investigated the rheological
behaviour of these materials. They found that they exhibit a pseudoplastic behaviour and, under
certain experimental conditions, they present thixotropy. Since then a great amount of forming
methods (semisolid metal processing methods, SSM) have been developed taking advantage of
these materials properties [3,4]. The SSM have a series of advantages compared with traditional
forming methods (forging, extrusion, etc). For example: the heat content involved is only about 2/3
of that used in casting, the viscosity is controllable and, at least, 100 times greater and, finally, the
solidification shrinkage is very much reduced [5]. These semisolid materials can be obtained by
melting an specific alloy into the solid-liquid region [1,5,6] or by mixing metallic slurries with
particles in solid state or fully liquid metals with solid particles. This later procedure is known as
“compocasting” [7], and will be investigated in this work.
Experimental Procedure
Materials. To achieve semisolid behavior, the material must be formed by a solid phase surrounded
by a liquid phase. The material selected was formed by an 63%Sn - 37%Pb alloy (which acts as the
union agent or liquid phase), and a given percentage of copper particles (which act as the solid
phase). This alloy has been chosen due to its a low melting temperature (183ºC), which facilitates
the preparation of the material. Moreover the small difference of density between the solid and
liquid phases (ρ(Cu solid) = 8.96 gr/cm3 and ρ(liquid 63Sn37Pb) = 8.01 gr/cm3) reduces
considerably the possibility that particle segregation or flotation takes place. The mixture procedure,
known as compocasting, consists of vigorously stirring of the copper spherical particles within the
liquid tin-lead alloy. This mixture is made at 240ºC to guarantee that the Sn-Pb alloy is in liquid
state while the copper particles remain in solid state. The material is prepared either “in situ” on a
Materials Science Forum Vol. 560 (2007) pp. 17-22online at http://www.scientific.net© (2007) Trans Tech Publications, Switzerland
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 147.83.80.91-12/06/07,11:53:43)
rheometer (for rheologic tests) or in a basculant furnace coupled with a stirring system (to obtain
material for industrial tests)
Rheology Measurements. The rheology measurements were carried out in a rotational
rheometer RC20-CPS (speed and stress controlled) using a parallel-plates configuration, equipped
with a temperature-controlled bottom measuring plate, which allows to maintain and to know the
temperature throughout the test. The material to be analysed fills the gap between the bottom static
plate of the instrument and the rotating one. The main parameter that was taken into account to
make the rheology experiments was the proportion of solid phases (between 0% and 50%
copper).The size of particles was kept at an average diameter of 100 µm. The principal objective of
the rheologic study is to corroborate the pseudoplastic and thixotropic behaviour of the slurry and to
determine some relative thixotropic values in order to make a more direct comparison between the
different experimental conditions.
Three different types of experiments
were performed on the rheometer. 1)
“Steady state” tests were carried out to
measure the apparent viscosity at
constant shear rate (300 s-1) and
temperature (240ºC) in order to
determine the influence of stirring time
on the rheologic behaviour, 2)
“Pseudoplasticity” tests were performed
at variable shear rate in order to observe
the shear rate dependence of the
apparent viscosity. 3) “Thixotropic” tests
were done in order to obtain the
conditions that maximize thixotropy.
Thixotropy can be defined as a decrease
in viscosity under applied stress,
followed by gradual recovery when the
stress is removed. This effect is both time and thermo-mechanical history dependent [6, 8, 9]. To
determine a relative value of thixotropy, the technique developed by Green and Weltmann [10] was
used in the present work. This technique consists of measuring a hysteresis loop on a shear stress-
shear rate plot. Once the steady state has been reached, the shear rate is increased continuously from
zero up to 600 s-1 and the change in induced shear stress is measured (up curve). At some specified
upper shear rate limit, the shear rate is maintained constant and then reversed to zero (down curve)
(Fig. 1). If the material is thixotropic, the up- and down- curves will not coincide. The thixotropy
will be higher as the area between the curves becomes bigger.
Back-Extrusion Tests. With the purpose of studying the behavior of the material under
industrial applications, a series of back-extrusion test were carried out. The experimental apparatus
consisted of a cylindrical die-plunger assembly driven by an Instron 4507 universal testing machine
operating in compression mode. The plunger velocity was 50 mm min-1 (0.8 mm s
-1). The
temperature of the system (plunger, cylindrical mould and re-heated material) was considered to be
homogeneous after soaking for 15 minutes at the test temperature. Two extrusion temperatures were
evaluated: 210 ºC and 240º C. The material was initially obtained in an external furnace and cooled
down. Then it was cut into small pieces and placed within the cylindrical mould. Values of force
and displacement were recorded during the tests and then used to correlate them with the macro and
microscopic characteristics of the extruded material.
0 50 100 150 200 250 3000
100
200
300
400
500
600
Shear rate (1/s)
time (s)
Figure 1. Procedure employed to generate the
hysteresis loop to derive thixotropiy values
loops, varying the shear rate.
Advanced Structural Materials III18
Results and Discussion
Rheology Tests - Steady State Tests. Plots of
apparent viscosity as a function of shear time
are presented in Fig 2. The experiments were
performed at a constant shear rate of 300 s-1 and
at constant temperature of 240ºC for solid
fractions fs = 0.35, 0.38, 0.40, 0.43, 0.45 and
0.50. It was assumed that fs remained constant
during the tests. The viscosity decreases as a
result of continued dissociation and breakdown
of agglomerated particles until an “apparent
steady state” is reached after 80 s. These results
are somewhat different to those reported in the
literature, where the time required to reach the
steady state is at least 10 minutes [11, 12] and can take up to several hours [5, 15]. In the present
experiments particles were added with the required spherical morphology (size and shape). In
contrast, results reported in the literature were obtained in samples where most of the stirring time
was spent in obtaining the optimal shape and size of particles. It is evident in Fig. 2 that the steady
state viscosity depends on the solid fraction. The curves exhibit two well defined regions. First,
after short times, the viscosity decreases very rapidly and then stabilizes at a constant value
depending on the solid phase fraction. At the beginning of the experiment, the apparent viscosity is
high because the slurry was kept static for some time and particles tend to agglomerate and even
combine by a local sintering process. Thus the slurry’s flow resistance increases. After some
stirring, the agglomeration of particles is destroyed by the shearing forces and the apparent viscosity
tend to decrease very rapidly with time, until reaching the apparent steady state. The same behavior
has been observed by other researchers [5,11-14].
Pseudoplastic tests. The existence of a transient behavior related with formation of a transitory
structure associated to a breakdown/buildup of the microstructure is traditionally studied using
shear rate change tests. Two types of tests were performed in this work. One type consisted of an
instantaneous increase (jump) or decrease (drop) in shear rate. The change of shear rate is
associated with a microstructure “breakdown time” for the jump test and to a microstructure
“building time” for the drop test. These behaviors have been observed previously by many
researchers. Some of them have measured transient stages lasting a few minutes [8] and others have
observed transients of less than 1 s [15]. In this work the shear rate change jump tests were
0 40 80 120 160 2000
1
2
3
4
5
6
Apparent viscosity (Pas)
time (sec)
50% Cu
45% Cu
43% Cu
40% Cu
38% Cu
35% Cu
Figure 2. Effect of solid fraction and time
on the apparent viscosity at 240ºC and
constant shear rate (300 s-1)
0 20 40 60 80 1000
50
100
150
200
250
300 40% Cu
35% Cu
20% Cu
10% Cu
0% Cu
Apparent viscosity (Pas)
Shear rate (1/s)
Figure 4. Effect of a continuos shear rate
change (0-100 s-1) on the apparent viscosity at
240º C for selected solid fractions.
200 240 280 320 3600,00
0,15
0,30
0,45
0,60 40% Cu
38% Cu
35% Cu
Apparent viscosity (Pas)
time (s)Figure 3. Effect of a shear rate change (300
to 600 s-1) on the apparent viscosity at 240 ºC
for selected solid fractions.
Materials Science Forum Vol. 560 19
performed in order to observe the
breakdown time of the microstructure.
The change in the shear rate was made
from the 300 s-1 to 600 s
-1. Prior to the
shear rate change, a 200 s stirring at
300 s-1 was applied to achieve an initial
steady state at the lower shear rate
(Fig. 3).
As shown in Fig. 3, the transient
took place in about 40 s. This time
represents the characteristic time
required to achieve the new “steady
state” condition. The apparent
viscosity decreases as a result of the
instantaneous increase in shear rate.
Quack [5] proposed two characteristic
times to describe a shear rate jump.
When the shear rate is increased, the
slurry undergoes an initial rapid
breakdown/buildup of the micro-
structure followed by a more gradual
breakdown/buildup controlled by diffusion.
In the second type of test the shear
rate is continuously increased from 0
to 100 s-1 in 1000 s. In this case, there
is an initially rapid decrease in
viscosity due to the fast removal of
loosely attached outer particles from
the agglomerates. Then, the rate of
viscosity decrease becomes slower
until an apparent “steady state” is
reached (Fig 4). These results agree
well with other works in material that
exhibited pseudoplastic behavior [6,
11,13,15].
Thixotropy Tests. The procedure
followed to determine the thixotropy
of the material was similar to that used
by other investigators in semisolids
materials [9]. First, a “steady state”
condition is established by stirring the
slurry during a specific time. Then, the
hysteresis loops are generated, as
described in the Fig. 1. The curves obtained for each fraction of solid Cu particles are shown in Fig.
5. As can be seen, the area of the histeresys loop increases with increasing solid fraction reaching a
largest value for a 50% solid Cu particles. It is noteworthy that, as the solid fraction increases, the
upper curve of the loop is strongly shifted upwards. This observation indicates an increase in the
resistance of the material to be sheared during stirring. For solid fractions lower than 0.35, the area
of the hysteresis loop cannot be measured accurately due to overlapping of the upper and lower
curves. This observation indicates that thixotropy is too small to be measured and that under the
compocasting conditions used in the present work, thixotropy can only be produced in slurries
processed with solid fractions larger than 35%.
0 100 200 300 400 500 6000
200
400
600
800
1000
1200
Thixotropy (35% Cu)=13810 Pa/s
Thixotropy (38% Cu)=30278 Pa/s
Thixotropy (40% Cu)=84070 Pa/s
Thixotropy (45% Cu)=96110 Pa/s
Thixotropy (50% Cu)=326326 Pa/s
Shear stress (Pa)
Shear rate (1/s)
Figure 5. Effect of solid fraction on hysteresis loops.
A
B
C
D
100 µm
Figure 6. Microstructures of a sample with 0.35 solid
fraction after shearing at 100 s-1 during 25 min at
240º C, (A) Copper particle, (B) eutectic structure,
(C) intermetallic compounds, (D) lead island.
Advanced Structural Materials III20
Microstructure of the Material Tested in the Viscometer. A typical microstructure of the
compocast material after solidification was completed is shown in Fig 6. As can be seen, the
microstructure consists of spherical solid Cu particles in a non-dendritic eutectic Sn-Pb matrix. The
presence of intermetallic compounds (IMC) between the solids particles and the eutectic matrix is
also apparent in the microstructure. These IMC’s have been identified as Cu6Sn5 and Cu3Sn [16,
17]. It has been observed in this work that, after long periods of stirring (20 min) at 240º C,
material with high Cu content exhibit a large amount of IMC’s. These IMC layers generate drastic
changes in the microstructure and the rheological behavior of the material. Indeed, for very long
stirring times (1 hour or more) at high temperature, although the material is kept agitated, the
thixotropic properties are lost.
Backward Extrusion Tests - Load-Displacement Curves. Back extrusion tests were carried
out with the purpose of verifying that the material resistance stays below an acceptable range for
SSM. Additionally, some characteristic features in the curves are indicative of the material’s
behaviour during the processing. The load-displacement curves obtained during the back extrusion
of the present semisolid processed material at different solid fractions are represented in Fig. 7.
Different types of behaviors were observed (leading to different F-d curves) depending on the
parameters used for preparation of the material and the extrusion procedure. In general, the F-d
curves display three stages. First, a relatively slow increase of the force was observed followed by a
stabilization stage. After this, the force falls or tends to a relatively constant value. In the final stage,
a rapid increase in force takes place. At this point the test had to be stopped to avoid overloading of
the test apparatus. These results agree well with results reported by other authors for similar tests
[3,4,18]. In most cases, the curves that exhibit a stabilization stage (such curves d and c in Fig.7)
are those having the best macrostructural characteristics after the extrusion. Monotonically
increasing curves (such as a and b in Fig.7) seem to be associated with excess of liquid or solid
during the re-heating time.
Micro and Macrostructural Analysis of
the Back-Extruded Pieces. Figures 8a and b
illustrate the external and internal
macroscopic characteristics of an extruded
piece. Figures 8.c and d shows the
microstructure of the same piece of material.
As can be seen, there is no evidence of
fibering or flow localization during the
process. In general, the best extrusion results
were obtained for the larger Cu contents (45-
50% Cu). At low solid fractions, typical
defects observed in liquid state forming
processes (i.e. gas and shrinkage porosity, etc)
are also formed in semisolid processed
material. In addition, the large amount of
liquid present renders the material incapable
of maintaining its shape once the plunger was
retired. This result is in agreement with
previous measurements of thixotropy that indicated a minimum solid fraction of 35% for adequate
processing of semisolid slurries. As shown in Fig. 7, the F-d curves of material tested at 210ºC
displays lower resistance values than those tested at 240º C. This increase in resistance can be
explained by the increase in the amount of IMC’s formed at higher temperatures. The IMC’s layers
grow very rapidly, making a more rigid “skeleton” structure and increasing the overall solid
fraction. From the experimental results, it appears that longer stirring times favours the thixotropic
behavior because promote the development of a more homogeneous solid particle distribution,
promotes better cohesion between particles and matrix, and limits growth of IMC’s.
0 2 4 6 8 10 12 140
5000
10000
15000
20000
Force (N)
Displacement (mm)
a
b
c
d
a) ST=3min TT=240º C
b)ST=13 min TT=240º C
c)ST=3min TT=210º C
d)ST=13 min TT=210º C
Fig.7. Force vs. displacement. SSM with a
solid fraction of 0.5, prepared at 240 ºC. Key:
ST: Shearing time; TT: test temperature
Materials Science Forum Vol. 560 21
Conclusions
Compocast eutectic SnPb alloy mixed with
spherical Cu particles presents a non-
dendritic microstructure with equiaxed solid
particles. The particles are well insulated by
the continuous liquid matrix. This
microstructure results in rheologic behavior
similar to SSM studied by other authors. The
material exhibits considerable thixotropy for
solid fractions larger than 0.35. The degree
of thixotropicity increases with increasing
amounts of Cu in the mixture (fs), and
reaches a maximum level with 50% Cu. For
Cu contents greater than 50%, the material
loses its semisolid character and behaves as
a solid from a rheological point of view. The
best extrusion was obtained for the larger
percentage of solid (fs=0.45 – 0.5), the
lower extrusion temperature (210ºC) and for
the longest stirring times (13 minutes).
Acknowledgements
Authors are grateful for the fruitful discussion with Prof. H. Atkinson and want to thanks to Montse
Marsal by her support in the development of the microstructural study of the material.
References
[1] M.C. Flemings: Metall. Trans. Vol 22A (1991), p. 957-981.
[2] D.B. Spencer, R. Mehrabian and M.C. Flemings: Metall. Trans. Vol 3A. (1972), 1925- 1932
[3] P. Kapranos, T.Y. Liu, H.V. Atkinson and D.H Kirkwood: J. Mater. Process. Technol. Vol.
111 (2001), p. 31-36
[4] H.V. Atkinson: Progress in Materials Science. Vol. 50 (2005), p. 341-412.
[5] C.J. Quaak, M.G Horsten and W.H Kool: Mat. Sci. Eng. Vol. 183A (1994), p 247-256.
[6] P.A. Joly and R. Mehrabian: J. Mat. Sci. Vol. 11 (1976), p. 1393-1418.
[7] S. Abis and V. Pellegrini: Journal of the Less-Common Metals. Vol. 139 (1988), p. 203-210.
[8] T.Y. Liu, H.V Atkinson, P.J. Ward and D.H. Kirkwood: Metall.Trans. 34A (2003) 409-417.
[9] D.C.-H. Cheng: Int. J. Cosmetic Science. Vol. 9 (1987), p. 151-191.
[10] R. Weltmann: J. Appl. Phys. Vol 14 (1943), p. 343-350.
[11] T.Z. Kattamis and T.J Piccone: Mat. Sci. Eng. Vol. 131A (1991), p. 265-272.
[12] N.G. Henderson, A.R.A McLelland, H.V Atkinson, and D.H. Kirkwood: Proc. 2nd Int.
Conf. Semi-solid Proc. of alloys and Composites, Cambridge, USA (1994), p. 349-358.
[13] J. Koke and M. Modigell: J.of Non-Newtonian Fluid Mech. Vol. 112 (2003), p. 141-160.
[14] M. Mada and F. Ajersh: Mat. Sci. Eng. Vol. 212A (1996), p. 157-170.
[15] A.R.A. McLelland, N.G. Henderson, H.V. Atkinson, G.H. Kirkwood: Mat. Sci. Eng. A. Vol.
232 (1997), p. 110-118.
[16] D.Lin, G. Wang, T. Srivatsan, Al-Hajri and M. Petraroli: Materials Letters 53 (2002) 333.
[17] A. Sharif, Y.C. Chan: Mat.Sci.Eng. Vol. 106B (2004), p. 126-131.
[18] E.R. De Freitas, E. Ferracini and A. Ferrante: J. Mat. Proc. Tech. Vol. 146 (2004)241-249.
Figure 8. Sample with 0.5 solid fraction, prepared
at 240 º C. a) external aspect of the upper part of
the piece, b) view of the longitudinal section, c)
and d) micrograph of the longitudinal section
5 mm 100 µm
c
a b
d
Advanced Structural Materials III22
Top Related