ON SINTERING OF W-Cu COMPOSITE ALLOYS

AT0100384U.U. Gomes et al. RM 24 177

15™ International Plansee Seminar, Eds. G. Kneringer, P. Rödhammerand H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

ON SINTERING OF W-Cu COMPOSITE ALLOYS

U. Umbelino Gomes, F.A. da Costa*, A.G.P. da Silva

Depto. de Física, Universidade Fed. R. G. do Norte, 59072-970, Natal, Brazil

*Depto. De Materials, IPEN, Cid. Universitaria, Säo Paulo, Brazil

Summary:

The sintering behavior of W-Cu composites is influenced by the coppercontent, the sintering temperature, the tungsten particle size and mainly thedispersion of the copper and tungsten phases. High sintered densities areobtained if the copper phase is well dispersed and fine tungsten is used.Powders consisting of composite particles, obtained by long time milling,exhibit high sinterability, and high final densities can be reached even forshort sintering times and low copper content. Alloys of different compositions,made with different tungsten powders and milled for different times, areprepared. The sintering behavior is characterized by the evolution of thestructure and dilatometric measurements. The sintering kinetics is describedand the influence of the dispersion of the powder is analyzed.

Keywords:

Composite, W-Cu, sintering, high energy milling

1. Introduction

W-Cu is a metal-metal matrix composite material. Its main properties are highelectric and thermal conductivity, high resistance to electric-arc-corrosion, lowcoefficient of thermal dilation and high density. These properties are easilyadjustable by varying the alloy composition. Its main applications are inelectric contacts for heavy-duty circuit breakers, welding electrodes and heatsinks for high power micro-electronic devices.Conventionally, W-Cu alloys are produced by infiltration of a porous, pre-sintered tungsten body by molten copper. Nevertheless, the structure ofpieces produced by infiltration can have residual porosity and heterogeneous

178 RM24 U.U. Gomes et al.15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

phase distribution, such as copper pools. Additionally, not all compositionscan be produced by infiltration. There is an upper limit for the copper contentbeyond which the capillary force is not strong enough to cause copperinfiltration. Another drawback of the infiltration technique is the impossibility ofproducing complex shapes, as required for use as heat sinks in electronics.The traditional route of powder metallurgy (mixing-pressing-sintering) is analternative technique for producing W-Cu alloys. Apparently, it would solveproblems such as structural heterogeneity and shape limitation. Theproduction scale could also be increased.In the liquid phase sintering of a structure with a given volume of liquid, onlysystems having high wettability (low contact angle) between liquid and solidphases or having some solubility of the solid phase in the liquid can besintered to densities near the theoretical value (almost pore-free structure).The hardmetal based on WC-Co is an example of a system with suchproperties. The W-Cu system, on the other hand, has low wettability of liquidcopper on tungsten and tungsten does not dissolve in liquid copper.Consequently, the W-Cu alloy structures produced by sintering have highporosity. The use of sintering as a method for producing the W-Cu alloysdepends on solutions to increase the sinterability of the system.The use of fine tungsten powders, milling instead of just mixing, and highersintering temperatures can increase the sintered density (1), but not enoughto produce the necessary pore-free structure. A fully dense structure can beobtained if sintering activators such as Co, Ni or Fe are used, but thepresence of segregated impurities deplete the properties of the material (2-4).There are a number of methods capable of preparing W-Cu powders that canbe sintered near full density (5-8). These methods are chemically ormechanically based. The powders produced by these methods have acommon characteristic: high dispersion and fine copper and tungsten phases.Sintering of W-Cu composites has been described as the solid state sinteringof the tungsten particles before and after the melting of copper, leading to theformation of a rigid skeleton. The rearrangement of the tungsten particles ascopper melts also contributes to the densification, but this contributiondepends on the composition of the alloy (1,3,9). Nevertheless, Ji-Chun Kim etal. describe sintering of a composite W-Cu powder prepared by mechanicalalloying as a double rearrangement process. In such a powder, each particleconsists of tungsten nanograins embedded in a copper matrix. Firstly, in solidphase, the composite particles sinter together as common particles (firstrearrangement). Then, as liquid forms, the tungsten grains rearrange,producing an homogeneous structure (second rearrangement).

U.U. Gomes et al. RM24 17915'n International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

This paper examines the sintering process of W-Cu powders prepared bydifferent methods and investigates the influence of the powder preparationmethod, the tungsten particle size and the composition on the sinteringkinetics. A sintering model is proposed to explain the different sinteringmechanisms mentioned in the literature.

2. Materials and Method:

Tungsten powders (Wolfram B.H. - Austria) of different mean particle sizes(0.54|im and 2.03u,m) and a copper powder were used in this work. Thecopper powder was produced by atomization and the particle size is in therange 4-30(xm.The copper and tungsten powders were wet milled with cyclohexane in aplanetary mill for different periods. Both the vessel and the milling sphereswere made of hardmetal to minimize contamination. After milling, the powderwas dried and granulated. The compaction was done in a 6mm diameter,single action die with a pressure of 220MPa. Table 1 shows the compositionsand the milling conditions of the prepared alloy powders. After drying, thepowders were observed under the electron microscope.

Table 1: Compositions and milling conditions used to prepare the alloys.Alloy

W19C2W40C2W75C2W47C2W47C6

W47C92W19C25fW47C56fW40C2-f

Copper ContentVol.%

194075

47

194740

Milling TimeHour

2

69225562

Tungsten Powder

2.03|j,m

0.54|j.m

The samples were sintered in a dilatometer and/or in a resistive type furnace.The sintering atmosphere used was flowing hydrogen and the heating rate forall experiments was 10°C/minute. Table 2 shows the sintering conditions inthe dilatometer and the resistive type furnace, the green and sintereddensities and the densification of each alloy.


Table 2: Sintering conditions, green and sintered densities and densification.The samples were sintered for 1 hour

Alloy

Pure WPure CuW19C2W40C2

W40C2-f

W75C2

W47C2W47C6

W47C92W47C92W19C56fW47C25f

Sintering Temperature°C

Dilatometer

1400990

1200

1070

——

120613001200—

ResistiveFurnace

——

925, 1000,1085, 1160

700, 800,925, 1000,

1060,1085, 1160

10761076———

1170(5min)

unless stated.RelativeGreenDensity

%6586677261

77

838265656468

RelativeSinteredDensity

%5589707880

85

848498989695

Densification(SD-GD)

%

-1033619

8

12

33333227

Pure tungsten and copper were sintered as reference. The sinteringconditions and densities are also shown in Table 2. The green and sintereddensities were measured by the Archimedes method and by the ratiomass/volume, where the volume is determined by measuring the dimensionof the piece with a micrometer. Both methods give comparable results, unlessthe sample is deformed or has pores open to the outer surface. In the firstcase the Archimedes method is used. In the green samples and in case ofsamples sintered at low temperatures, the ratio mass/volume is used. Thesintered samples were also cut, ground, polished, etched and observed underthe light microscope.The dilatometer measures the variations of the linear dimension of thesamples during sintering as a function of the temperature and time. Theresults are presented in terms of the sintering parameter and the sinteringrate, defined according to equations (1) and (2) respectively:

U.U. Gomes et al. RM 24 18115* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

AL _dAL

At ~ dt

;ri00 (D

(2)

where Lo and Lf are the initial and the final length of the sample and and Ls isthe length at a given moment during sintering. Positive values for equations(1) and (2) means swelling of the sample.

3. Results:

Figure 1 shows the curves of the sintering rate for pure copper and tungsten.Figure 2 shows the sintering rate of alloys W19C2, W40C2 and W75C2. Thevertical lines mark the moments at which copper melted and the isothermaltemperature was reached.

-5

Isotherm

Isotherm

50 100 150

Time (minute)

200

Figure 1: Sintering rate of pure tungsten and copper versus the sintering time.


6,0-

4,5-

3,0-

0,0

• I -4,5-

.£ -6,0 :

-7,5

-9,0

: ; Copper

: ."• Melting

• W40C2 '"-.

• W19C2 '-. •

'. W75C2 \ :

: Isotherm ••

i I'-ll i/iV

Isotherm :

r^* -.

0 40 80 120 160

Time (minute)

200

Figure 2: Sintering rate of alloys W19C2, W40C2 and W75C2 versus thesintering time.

Figure 3 shows the sintering rate of alloys W47C92, W19C56-f and W40C2-f.

1

0

— -1cE -2

1 -4

1 -6

W40C2-f•W19C56-f-W47C92

CoppedMelting

Isotherm

40 80 120

Time (minute)

160 200

Figure 3: Sintering rate of alloys W40C2-f, W19C56-f and W47C92 versus thesintering time.

U.U. Gomes et al. RM24 18315'" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Figures 4 and 5 show the structures of alloys W40C2 and W40C2-frespectively. Both samples sintered at 925°C for 1 hour.

Figure 4: Alloy W40C2. The copperparticles are surrounded by a ring oftungsten particles.

Figure 5: Alloy W40C2-f. Elongatedstructures are seen. They are copperparticles deformed during milling.

Figure 6 shows the structure of alloy W40C2-f sintered at 1160°C for 1 hour.

Figure 6: Alloy W40C2-f. The poresare elongated. They were created bymelting of the copper plates.

Figures 7 and 8 show the powders of the alloy with 47v/o copper, made withthe finer tungsten powder milled for 25 and 92 hours respectively.


MLFigure 7: The powder of alloyW47C25-f. The plates are deformedCu particles with pricked tungsten.

Figure 8: The powder of alloyW47C92-f. The plates are fragmentedwith continued milling.

4. Discussion:

Two simultaneous effects must be taken into account in the analysis of thesintering rate curves: the thermal dilation and the shrinkage due to sintering.At low temperatures, the thermal dilation is more significant. Sinteringbecomes increasingly more important from a certain temperature thatdepends on the material under sintering. As seen in figure 1, sintering beginsin copper around 730°C and in tungsten around 1100°C. From thesetemperatures, the sintering rate continuously increases (negatively), reachinga maximum when the isothermal temperature is attained. The same behavioris repeated for alloys W40C2, W19C2 and W75C2, shown in Figure 2.Apparently, the sintering mechanisms, in solid and liquid state, depends insome way on the "state of heating". When heating stops, the mechanisms aredeactivated and sintering slows down.The sudden change of the sintering rate of copper around 285°C is probablyrelated to the reduction of the copper oxide by hydrogen. As seen in Table 2,copper shrank only 3% due to the low sintering temperature, 990°C, and thehigh green density. Tungsten swelled even after 1400°C for 1 hour.The sintering rate curves and the densification of the alloys milled for 2 hours,shown in Figure 2 and Table 2 respectively, show that the higher the coppercontent the higher is the sintering rate. Therefore, the sintering of W-Cucomposites in solid and liquid state, in the temperature range investigated inthis study, is due to the copper phase.

U.U. Gomes et al. RM 24 18515lh International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

Alloys W40C2 and W19C2 have a similar shrinkage behavior. Their sinteringrates exhibit 2 negative peaks between the melting point and the isothermaltemperature. Alloy W75C2 has a high sintering rate in solid state sintering.The structure begins to shrink at 825°C. Its relative density after sintering at1200°C is 85%. The sintering rate decreases immediately as the isothermaltemperature, 1070°C, is reached.

The structure of a green W-Cu sample is formed by tungsten particlessurrounding the larger copper particles. The distance between the copperparticles is variable, but the number of tungsten particles per copper particleand the mean distance between the copper particles depend on thecomposition of the material. A fraction of the copper particles are in contactwith each other. These particles sinter together as the temperature rises. If anetwork of sintering copper particles is formed throughout the structure, thesample shrinks during solid state sintering. Very fragile necks linking thetungsten particles must grow, but they do not contribute to the densification ofthe structure. The contacts between copper and tungsten particles also donot contribute for densification. The copper particles sinter together with theneighbor tungsten particles and these form a ring around the copper particles,as shown in Figure 4. In the alloys with 19% and 40% of copper, such anetwork of copper particles is not formed, but in the alloy with 75% of copperthe network exists and its structure shrank in solid state sintering.When copper melts, the liquid infiltrates the surrounding region, in the finespace between the tungsten particles. A large pore is created in place of thecopper particle. This pore is closed only if there is an excess of liquid in thestructure. During the infiltration of the liquid, the tungsten particles arerearranged due to the capillary forces. This rearrangement is the maincontribution to the densification. It corresponds to the rapid increase in thesintering rate just before the melting point, indicating that the copper phase isalready "soft" enough to infiltrate the structure. As the temperature rises, theliquid spreads further and the rearrangement continues, but as the isotherm isreached, the liquid stops infiltrating and the sintering rate decreases.The sintering rate of alloy W40C2-f, made with the finer tungsten powder, isvery different from that of alloy W40C2, with the same composition andmilling time, but with the coarser powder, as shown in Figure 3. The sinteringrate is clearly higher, mainly after the copper melting. The relative sintereddensity is comparable with that of alloy W40C2, but the densification is muchhigher, in part due to the low green relative density. It is expected that thedensification by the particle rearrangement is more intensive if a finertungsten powder is used because the pores between the tungsten particlesare smaller. The capillary forces are higher for finer pores. However, it was


expected that the solid state sintering for the alloy with the finer tungstenpowder would be more difficult than that of the alloy with the coarser powderbecause the mean distance between copper particles increases, making theformation of a copper network more difficult. This behavior was not observedin our experiments.The reason for this is the formation of elongated particles during the millingprocess. Figure 5 shows the structure of alloy W40C2-f sintered at 925°C. Atthis temperature, the form of the particles after milling is still retained. Thecollisions during milling deform the copper particles and tungsten particlesenter at the surface and get distributed inside the copper particles. This kindof particle is not seen in the powder made with the coarser tungsten powder.For some reason, the use of finer powders makes the formation of suchparticles easier.The structure of alloy W40C2-f looks denser than that of alloy W40C2 whensintered before the melting point. This indicates that the densification in solidstate sintering is more intense. The elongated particles should connect toeach other more easily and the shrinkage of the structure is possible. Whencopper melts, the liquid infiltrates the surroundings, as in the alloy with thecoarser powder, and pores are formed. Figure 6 shows elongated, randomlyoriented pores.The powders milled for longer times (longer than 25 hours) have relativedensities higher than 95%. Their densification is also high, but the relativegreen densities are lower than those for the alloys of the same compositionmilled for 2 hours, as shown in Table 2. The curves of sintering rate are alsovery different from those of the briefly milled alloys, as seen in Figure 3. Thesamples begin to sinter at comparatively lower temperatures. Thedensification in solid state is pronounced and, in contrast to the briefly milledalloys, the sintering rate maximum is reached shortly after the copper melting.After this the sintering rate falls rapidly. This indicates that the structure hasalready a high density shortly after the melting temperature.The reason for the high sinterability exhibited by the powders milled for a longtime is in the particles. During milling, the collisions between the millingmedia, the wall of the vessel and the particles of tungsten and copper deformthe copper particles, so that their shape becomes plate-like. The tungstenparticles are broken and/or embedded in the copper plates. The successivecollisions further deform the copper plates in different directions and embedmore tungsten particles. These particles are called composite particles.The number of tungsten particles not embedded (free tungsten particles) incopper continuously decreases and the copper phase hardens by cold work.Figure 7 shows the powder of alloy W47C25-f after 25 hours milling. Large

U.U. Gomes et al. RM 24 18715" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

plates are seen together with much debris, which is also plate-like, formed bythe fragmentation of the larger plates. Very small free tungsten particles arealso seen. The fragmentation of these plates caused by the copper hardeningchanges completely the appearance of the powder. Figure 8 shows thepowder of the same composition, but after 92 hours of milling. The particlesare smaller, do not have the plate shape and free tungsten particles are notvisible.The composite particles are formed by very fine tungsten grains embedded incopper. The tungsten grains are placed in the bulk and also on the surface. Acompact of composite particles has, therefore, an excellent dispersion ofcopper (in each particle of the powder) and very fine tungsten. This is thereason for the high sinterability. During heating, at around 700°C, thetemperature at which pure copper starts to sinter, the copper phase presentin each particle begins to sinter. Necks of copper link the composite particlesin contact. In the sintering rate curves, alloys W47C92 and W19C56-f beginto sinter between 650°C and 700°C. The copper necessary to grow the neckscomes from the bulk of the composite particles. This causes a copperenrichment of the surface of the particles. The tungsten grains in the bulkapproach each other. Those tungsten grains in contact sinter together. Thustungsten grain growth by sintering and coalescence occurs.When the melting temperature is reached, the structure is in an advancedstate of densification. The liquid rapidly fills the remaining pores. Thetungsten grains, which were grouped in the bulk of the solid compositeparticles, are rearranged and the structure becomes more homogeneous.The final structure is dense, homogeneous and very fine.The high sinterability of powders consisting of composite particles is not verydependent on the sintering temperature, sintering time and composition. Thesintering of alloy W47C92 at 1206° and at 1300°C for 1 hour yielded thesame relative density, 98%. A relative density of 95% was obtained with alloyW47C25-f sintered at 1170°C for only 5 minutes. The most surprising result,however, is found for the alloy containing only 19v/o copper milled for 56hours. Its relative density was 95% after sintering at 1200°C for 60 minutes. Apowder of the same composition, milled for 2 hours and sintered at the sameconditions yielded a relative density of only 70%.

5. Conclusions:

Copper is the responsible for the sintering of W-Cu composites. Powdersmilled for short times and powders milled for long times sinter differently. For

188 RM24 U.U. Gomes et al.1Sm International Plansee Seminar, Eds. G. Kneringer, P. Rbdhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

the alloys briefly milled, in the solid state, densification occurs if a network ofcopper particles sintering together is formed. This condition is fulfilled only forcopper rich alloys. In the liquid state, the liquid copper spreads into the finepores between the tungsten particles and promotes a particle rearrangement.Again the contribution of the particle rearrangement to the densification isdependent on the copper content. Large pores are formed when copper meltsbecause the liquid infiltrates the surrounding pores. High sintered densitiesare obtained only for copper rich alloys.If finer tungsten powders are used, the densification in the solid and liquidstate sintering increases. The finer pores increase the capillary forcesresponsible for the particle rearrangement.When the powders are milled for longer times, composite particles are formedand these particles have high sinterability. The composite particles areformed by the continuous deformation and cold work of the copper particlesand the insertion of small tungsten particles into the copper. The milling timecontrols the shape and size of the composite particles and also the size andnumber of tungsten particles inserted into the copper phase.The powders formed by composite particles exhibit high sinterability and adifferent sintering behavior. In the solid state, the composite particles sintertogether by the growth of copper necks linking the particles. When the liquidis formed, it flows in the remaining pores. There is growth of the tungstengrains during the whole sintering process and a rearrangement of thesegrains when copper melts.

Acknowledgements

The authors are grateful to Dr. Simon and Eng. Hashemi at the TechnicalUniversity in Vienna for the metallographic preparation and imaging of thesamples, to Prof W.D. Schubert for the valuable discussion and to CAPES,Projeto Nordeste , RHAE and OAD for the partial financial support.

References:

01. J.L. Johnson, R.M. German: Met. Mat. Transactions B, vol. 27B, (1996)pg.900-909

02. J.L. Johnson, R.M. German: Int. J. Powder Metallurgy, vol.30, No.1,(1994)pg.91-102

U.U. Gomes et al. RM 24 189^

15™ International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1

03. A. Upadhyaya, J.L. Johnson, R.M. German: Proc. Third Int. Conf. OnTungsten and Refractory Metals, USA, (1995) pg.3-19

04. I.H. Moon, J.S. Lee: Powder Metallurgy, No.1, (1979) pg.5-7.

05. C.C. Yu, R. Kumar, T.S. Sudarshan: Proc. Second Int. Conf. On Tungstenand Refractory Metals, USA, (1994) pg.29-36

06. J.L. Sepulveda, L.A. Valenzuela: Metal Powder Report, June (1998) pg.24-27

07. J.C. Kim, S.S. Ryu, Y.D. Kim, I.H. Moon: Scripta Materialia, vol.39, No.6,(1998)pg.669-676

08. C.Qian, E. Wu, Z. Zou, Y. Zhang: Proc. 13th Int. PLANSEE Sem., Austria,vol.1, pg. 461-470(1993)

09. R.J. Nelson, D.R. Milner: Powder Metallurgy, vol.14, No.27, (1971) pg.39-63

Documents

ON SINTERING OF W-Cu COMPOSITE ALLOYS