Mold Design and Temperature Field Analysis on Electrical Current Sintering of Electrically...

8/2/2019 Mold Design and Temperature Field Analysis on Electrical Current Sintering of Electrically Conductive Powders

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Section 4 Others

MOLD DESIGNAND TEMPERATURE FIELD ANALYSIS ON

ELECTRIC CURRENT SINTERING OF ELECTRICALLY

CONDUCTIVE POWDERS

YANG Junyi, LI Yuanyuan, LI Xiaoqiang, Guo Liang

*School ofMechanical Engineering, South China University ofTechnology, Guangzhou 510640, China

Fax: (86) 20-87112948, E-mail: [email protected]

Keywords: Field activated sintering, temperature field, finite

element modelling, powder metallurgy.

Abstract

In order to probe and improve temperature field during

electric current sintering, high strength ceramic and graphite

were used respectively as female die during sintering.

Experiments and finite element method (FEM) were

conducted to characterize the electric current density,

resistance heating and temperature distribution in the

specimen/die/punch. Obviously higher heating rate, higher

final sintering temperature and higher gradient temperature

were found in specimens when high strength ceramics was

used as female die, compared with graphite female die under

the conditions of the same die dimension and sintering

parameters. A composite die was proposed to reduce and even

eliminate the differences as well as to improve heating rate ofthe specimen. The experimental results are in good agreement

with calculated values by FEM, which indicates that FEM

may be an effective analysis method to simulate temperature

field of electric current sintering.

1 Introduction

Electric current sintering (ECS) techniques, especially spark

plasma sintering (SPS), have shown some great merits such as

faster heating and cooling, shorter holding time, fewer

processing steps, lower sintering temperature, controlled

atmosphere and external press, elimination of the need for

sintering aids, and near net shape capability [1]. They have

been widely used in fabrication and investigation of magneticmaterials, Ceramics, bulk amorphous alloy, intermetallic

compounds, metal matrix composites , functional gradient

materials (FGM), ul tra-fine grained and nanostructured

materials, and so on, which have shown excellent

performance [2-8].

Currently, the main die material for ECS is graphite because

of its benefits such as high melting point, medium electric

resistance, low cost and so on. However, graphite has

disadvantages for sintering. Its low strength makes graphite

punch not bear a relatively high uniaxial-pressure, otherwise

the horizontal pressure generated by uniaxial-press can

destroy the female die or shorten its working-life. In addition,

sintering velocity is also lowered by low sintering pressure.

Another adverse factor of graphite is its high thermal

conductivity (about three t imes than that of steel) when

graphite is used in female die. Due to the high thermal

conductivity, its heat transfer is fast, which accelerates theheat loss of die surface. So, without appropriate insulated

measures, the temperature of graphite die is difficult to rise,

particularly for those electrical current sintering techniques in

which the maximal pulsed electric current is relatively low. It

can also be imagined that the heat transfer in powders, which

contact interior die surface, is also fast because of the high

thermal conductivity of graphite, this is one of the causes that

result in large temperature gradient in powders [9], in

particular for SPS which has fast heating rate. It is known that

the heat energy in powder specimen during ECS is transferred

from graphite punches and comes from its Joule heat [10,11].

Hence, the heating rate may be greater when using high

strength ceramic die, because of its lower thermal

conductivity and higher electrical resistance compared withgraphite material. This makes lower electric current sintering

possible.

In this paper, to further clarify the advantages of ceramics die,

both experiments and FEM were adopted to analyse the

temperature field during electric current sintering of

electrically conductive powders.

2 Experimental study

Experiments were carried out using an electric current

sintering equipment, which had a maximal uniaxial-pressure

of 1OOOKN. The power supply could provide a pulsed direct

electric current up to 3000 A through the rams into the die set

that contained specimen. Fig.l shows the Schematic diagramof heating system. A 5 mm thick celluloid plane was used to

separate graphite punch electrode from the system. Graphite

and alumina ceramic were adopted as female die materia ls

respectively. Since the thermal stress generated by the

temperature difference between interior and outer die surface

can destroy the die when it is beyond the ceramics thermal

shock resistance [11]. In this paper alumina ceramic die

surface was enveloped by iron sheath by thermal interference

fit. A thermocouple was placed 3mm away from the top of the

lower punch.

Experimental parameters: the base, duty ratio, frequency,

holding time and peak of pulsed direct current were 360 A, 50

%, 50Hz, 6minutes and 2400 A, respectively. Uniaxial



International Technology and Innovation Conference 2006

pressure was 30MPa and was supplied throughout sintering.

The sintering temperature was measured every 30 seconds via

the thermocouple. And the whole sintering was carried out in

the air.CelluloidPlane

/ Upper PunchElectrode

Lower PunchEletrode

Fig.l: schematic diagram of heating system

of electric current sintering equipment.

3 Finite element simulation

The heat transfer during electric current sintering can be

considered as three dimensional transient heat transfer in a

long cylinder which had heat source. Due to its symmetry, it

can be simplified to two dimensional axisymmetric transient

heat transfer, and the heat energy was offered by Joule heat

generated by electric current through it.

The transient heat transfer equation was described by the

energy balance equation [12]:

!.-(re r) .!.!..(n 8r)+!..(A 8 r ) + ~ (I )at rOr Or Oz Oz

Where p, p ' t , T and A represents the density, specific

heat, time, temperature, thermal conductivity, respectively.

#fis the heat produced by an internal heat source in unit time

and volume.

The current distribution is expressed by Kirchhoffs law

8(rir ) z =r az

Where irand iz represent the current density in the radial and

axial directions, respectively. The specific power #fis related

to the current density by Ohm's law

#f= irPr izpz

Where Pr and Pzare the electrical resistivity in radial and

axial directions, respectively.

In order to simplify problem and save solved time, we

assumed that temperature and the electric potential were

continuous at the interface, that was to say, the thermal and

electric contact resistance can be ignored. Thermal

conductivity and electrical resistivity were functions of

temperature only. Due to powder volume change derived

from densification process, which was an extremely

complicated process, we assumed there was no volume

change but only density change. So, density change of

powders can be considered as volume change. Fig.2 shows

FE model, the thermoelectrical properties of materials were

given in table.1 [8,9,13]. The boundary conditions for

temperature and electric field were given as follows.

Initial and boundary conditions for temperature field: (a)

T(to) =293 K; (b) At the interface between upper (lower)

punch and celluloid plane, - 0; (c) At the

outer surfaces of all parts,

- n· £a s4amb -

4) •

Where n is the normal vector; h denotes nature convection

heat transfer coefficient, being 50 W/m K; ~ n and Tamb

represent the external and ambient temperature,

~ n =Tamb =293K; is emissivity, being 0.85; as is the

Stefan-Boltzmann constant, being 5.669 xIO-8W/m2K4•

Initial and boundary conditions for electric field: (a)

(t0) (b) At the interface between upper punch and

celluloid plane, - = where is current density,

J is inward current density; (c) At the interface between

lower punch and celluloid plane, V=O; (d) At the outer

surfaces of all components, n · =The model of temperature field during electric current

sintering was built by Electro-Thermal Interaction module in

FEM analysis software COMSOL Multiphysics. The virtual

modeling was set up by Drawing functional module in two

dimensional axisymmetric coordinates. Sintering variables,

material properties and boundary conditions were set in

Expressions, Subdomain Settings and Boundary Settings

functional modules, respectively. And the meshing (Fig.2)

was executed by mesh menu.

Fig.2: mesh partition.

4 Results and discussion

4.1 Electric current distribution

A series of simulations were performed. Fist the results

without thermal contact resistance at the interface were

presented. Current density distribution played a very

important role during electric current sintering since it

directly effected the distributions of heating rate and

temperature. Fig. 3 shows the magnitude of electric current in

the electrode/graphite plate/specimen/die/punch. The results

showed that the direction of the current in punches was

vertical for both alumina ceramic die and graphite die.

However, for alumina ceramic die, the electric current density



Section 4 Others

was higher in the punches and exhibited a maximum at the

outside r i m ~ fewer electric current went through the alumina

ceramic die and iron sheath. For graphite d i e ~ the area with a

current flow in the horizontal direction was at the top of the

punch and at the contact of the punch with the top surface of

the d i e ~ electric current density was approximate except

outside rim of graphite punches which exhibited a m a x i m u m ~the specimen and the die were essentially in a parallel electric

network. The current path depended primarily on the ratio of

resistance of punch and d i e ~ i . e . ~ the electric conductivity of

the die and punch and the conductive cross section areas of

each component. Because the graphite punch was equally

conductive as the d i e ~ the majority of the current was diverted

to that which had a larger cross section area. As a w h o l e ~electric current density in punch for graphite die was less than

that of alumina ceramic die under the same conditions of die

dimension and sintering parameters. As shown in F i g . 3 ~ it was

also seen that the total valueof

current density in punch foralumina ceramic die was larger than that of graphite die.

4.2 Joule heat distribution

Fig. 4 depicts the Joule heat generation in punch/sample/die.

For alumina ceramic die, the amount of Joule heat caused by

the punches was obviously higher than those by specimen and

die. Since the most of the Joule heat energy formed in the

p u n c h e s ~ the specimen was indirectly heated by thermal

conduction from the punches. However, for graphite d i e ~ the

values of the Joule heat in p u n c h e s ~ specimen and die were

approximate except for in the part of punches unenveloped by

the die, and the specimen was heated mainly by thermal

conduction from the punches and die. As shown in F i g . 3 ~ the

amount of the Joule heat in punches was larger when alumina

ceramic was used as die material replacing graphite.

From Fig.3 and F i g . 4 ~ the components were found having a

higher current density and correspondingly higher Joule

heating.

4.3 Temperature distribution

The temperature distribution in punch/specimen/die assembly

was characterized by the interplay of Joule heat generation

and heat transfer. Heat was mostly removed from the

punch/die/sample assembly via radiation and convection to

the ambient. As shown in Fig.5, for respectively alumina

ceramic die and graphite die, the part of punches close to the

specimen experienced the highest t e m p e r a t u r e ~ the specimen

experienced the medium temperature and the minimum

temperature was the one placed on the out die surface. is

implicit that Ti powder specimen is indirectly heated by

thermal conduction from the punches. For alumina ceramic

die, the highest temperature was about 900 °C the mean

temperature of specimen was above 850°C ;But for graphite

d i e ~ the maximum of temperature was about 500°C ~ t h e mean

temperature of specimen was less than 500°C .From Fig.5, we

also can see, temperature gradients were evidently in

punch/specimen/die for alumina ceramic d i e ~ compared with

graphite die.

Fig.6 shows temperature distribution in sample c e n t e r ~interior and outer die surface, outer iron sheath surface in

radial direction with time. For respectively alumina ceramic

die and graphite d i e ~ heating rate in the sample center and

interior die surface was very fast in the initial stage of

sintering. With prolonging of sintering t i m e ~ heating rate

reduced s l o w l y ~ but still remained a high value. As shown in

F i g . 6 ~ for alumina ceramics d i e ~ heating rate was obviously

higher than that of graphite d i e ~ but larger temperature

difference (about II 2°C) between center and outer sample

surface was found. This larger temperature difference may be

influence the consistency of the structure and performance of

the s a m p l e ~ in particular in the intermediate and final stage of

sintering.

From above results of temperature distribution, we knew that

alumina ceramic with lower electrical conductivity and lower

thermal conductivity may obviously improve heating rate andfinal sintering temperature in s p e c i m e n ~ but bring larger radial

temperature gradient in the sample. The reason may b e based

on above analysis of electrical current and Joule heat, the

punches experienced larger current density and Joule heat

generation for alumina c e r a m i c ~ resulting in faster heating

rate in specimen compared with graphite die; On the other

h a n d ~ heat loss of outer specimen was not supplied enough by

alumina ceramics die because of its fewer Joule heat

generation, this may result in relatively larger temperature

difference between center and surface of sample. When using

graphite d i e ~ due to existing Joule heat in graphite die

enveloping s p e c i m e n ~ the heat loss outer specimen was less

and resulting in lower temperature gradient in sample.

In this way, if combining the merits of graphite and c e r a m i c s ~we can design composite die which can not only improve

heating rate but also decrease temperature gradient under the

conditions of the same die dimension and sintering

p a r a m e t e r s ~ compared with graphite die and ceramics die.

Fig.7 shows temperature distribution of the specimen with

time using a composite die of graphite and ShN4 ceramic,

temperature difference between center and outer surface of

the sample was obviously lower (about 3 0 ° c ) ~ and the heating

rate was higher, compared with single ceramic die and

graphite die.

Fig.8 depicts the contrast between measured value and

calculated value by FEM. The finite element simulation with

no thermal contact resistance was found to be in reasonably

good agreement with experimental value in the range ofallowable error. Experimental observation and model results

showed that an almost linear increase of inside temperatures

on surface in the intermediate and final stage of sintering. In

the initial stage of s i n t e r i n g ~ the heating rates by experiment

and simulation were very fast due to large Joule heating

generated by large resistance. The measure value was higher

than calculation, the reason may be that: (I ) Overestimation

of air convection heat transfer coefficient between heating

system surface and ambient;(2) the effect of thermal contact

resistance on heat transfer had no not been considered.




Material thermal conductivity (W/mK) electrical conductivity specific heat (J/kgK) Density3

)

3970

8960

1850

7770.25T/(249.4+T)+

790.15T/(249+T)+0.008T

355.3+0.IT/(5.5X 10-9+3.8x 10·IIT)

( 65181330.4+T)/

(-669628.8+8175.85T)

420.66+0.07T

Alumina

Graphite 65-0.0l7T 23789.87359+172.54179T 310.5+1.09T

-0.12989T

2

+4.2089x 10-5T3

-5.169x10-9y4

10-8

Copper

Electrode

[ron 11.36+0.0136T 446.5+0.162T 7850

Ti 38-9.6x 10-2T+1.7x 10-4T2

-1.3x I0-7T3+4.Ox 10·IIT4

1I(3.85x 10-7+4.75x 10-IOT) 331-0.67T+2.6x 10-4T2

-2.6x 10-8T3

4500-0.12T-1.1 x10-5T2

Table. 1: thermo-electrical properties ofmaterials [8,9,13].

In :lu :

CUi

0.6

(a) Alumina ceramic die (b) Graphite die

Fig.3: current density distribution.

In :

\t.i)$

lin:

lu :

1. 2

O.!

0.2

O.·fX

I.i!j;

(a) Alumina ceramic die

Fig.4: joule heat distribution.

(b) Graphite die




5 Conclusions

In this work we analyzed the temperature distribution in ECS

using experimental measurements and finite element

modeling. We found that

a) Under the same conditions of die dimension and sintering

parameters, higher heating rate, final sintering temperature

and temperature gradient exist in powder specimen when

ceramic is used as die material instead ofgraphite.

b) Although the radial temperature difference in specimen

during electric current sintering is inevitable, a composite die

of graphite and ceramic can reduce the difference and

improve the heating rate of samples.

c) The finite element modeling is a convenient and useful

way to analyze the temperature distributionof ECS.

AcknowledgementsThis work was supported by National Science Fund of China

for Distinguished Young Scholars (No. 50325516) and

Guangdong Research Project for Key Problems (No.

59872024).

References

[1] W. Chen, U. Anselmi-Tamburini, 1. E. Garay, 1. R.

Groza, Z. A. Munir, "Fundamental investigations on the

spark plasma sintering/synthesis process: Effect of dc

pulsing on reactivity", Materials Science and

Engineering A, 394, 132-138,2005.

[2] S. Paris, E. Gaffet, F. Bernard, Z.A. Munir, "Spark

plasma synthesis from mechanically activated powders:

a versatile route for producing dense nanostructured iron

aluminides", ScriptaMaterialia, 50,691-696, 2004.

[3] K. A. Khor, L. G. Yu, Andersen, G. Stephani, "Effect

of spark plasma sintering (SPS) on the microstructure

and mechanical properties of randomly packed hollow

sphere (RHS) cell wall", Materials Science and

Engineering A, 356, 130-135, 2003.

[4] Y.W. Gu, K.A. Khor, P. Cheang, "Bone-like apatite

layer formation on hydroxyapatite prepared by spark

plasma sintering (SPS)", Biomaterials, 25, 4 127-4 134,

2004.

[5] S. Cha, S. H. Hong, "Microstructures of binderless

tungsten carbides sintered by spark plasma sintering

process", Materials Science and Engineering A, 356,

381-389, 2003.

[6] L. L. Ye Z. G. Liu K. Raviprasad, M. X. Quan, M.

Umemoto, Z.Q. Hu, "Consolidation of amorphous

NiTi powders by spark p lasma sintering", Materials

Science andEngineeringA, 241, 290-293, 1998.

[7] Wang Yucheng, Fu Zhengyi, "Study of temperature field

in spark plasma sintering", Materials Science and

Engineering 90, 34-37, 2002.

[8] A. Z., 1. Z., M. K., J. R. G., "Temperature evolutionduring field activated sintering", Materials Science and

Engineering A, 379, 218-228, 2004.

[9] U. Anselmi-Tamburini, S. Gennari, 1. E. Garay, Z. A.

Munir, "Fundamental investigations on the spark plasma

sintering/synthesis process Modeling of current and

temperature distributions", Materials Science and

Engineering A, 394, 139-148, 2005.

[10] K. Vanmeensel, A. Laptev, 1. Hennicke, 1. Vleugels, O.

Van Der Biest, "Modelling of the temperature

distribution during field assisted sintering" Acta

Materialia, 53, 4 379-4 388, 2005.

[11] R. J. Li, Ceramics- metal composite materials (the 2nd

ed.). Beijing: Publishing company ofmetallurgy industry,

2004.[12] H. 1. Zhang, Heat Conduction. Beijing: Publishing

company ofhigher education, 1992.

[13] K. Matsugi, H. Kuramoto, T. Hatayama, O. Yanagisawa,

"Temperature distribution at steady state under constant

current discharge in spark sintering process of Ti and

Ah03 powders", Journal of Materials Processing

Technology,146,274-281,2004.

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