Thixotropic Behaviour of a Cu – Eutectic Sn-Pb Slurry

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)

http://www.scientific.net

http://www.ttp.net

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


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


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





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.


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


Thixotropic Behaviour of a Cu – Eutectic Sn-Pb Slurry

Documents

Transcript of Thixotropic Behaviour of a Cu – Eutectic Sn-Pb Slurry