MICRO WAVE SINTERING OF REFRACTORY METALS –W,Mo,Re

Presentation by

Suresh Beera

12ETMM11

M.Tech-I,2nd Sem.,

Materials Engineering

SEST, UoH.

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CONTENTS

Introduction

Microwave Sintering

Microwave Vs. Conventional Heating

Microwave Sintering Devices

Microwave Sintering Of Refractory Metals

Consolidation Of Tungsten

Consolidation Of Molybdenum

Consolidation Of Rhenium

Summary

References

INTRODUCTION

REFRACTORY METALS:

Refractory metal can withstand at high temperature, pressure and

they are well known for their high mechanical properties

SINTERING PROCESS:

Sintering is a heating process that causes particle to bond together,

resulting in significant strengthening and improved properties

MICRO WAVE :

The microwave part of the electromagnetic spectrum corresponds to

frequencies between 300 MHz and 300 GHz. Wavelength of 1CM –

100micron However, most research and industrial activities involve

microwaves only at 2.45 GHz and 915 MHz frequencies

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MICROWAVE SINTERING APPLICATIONS

Microwave energy has been in use for a variety of

applications for over 50 years.

Some of the early applications include communication,

navigation , wood processing , medical therapy and

drying of food items.

In the past two decades, the remarkable success of

domestic microwave ovens has revolutionised home

cooking

The most recent development in microwave

applications is in sintering of metal powders,

This technology can be used to sinter various powder

metal components, and has produced useful products

ranging from small cylinders, rods, gears and automotive

components in 30-90 min.

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Which Metals have been Microwave Sintered?

Many commercial powder-metal components of various alloy compositions,

including iron and steel, copper, aluminum, nickel, molybdenum, cobalt,

tungsten, rhenium, tungsten carbide, tin, and their alloys have been sintered

using microwaves, producing essentially fully dense bodies. Figure 1 illustrates

some of the metallurgical parts processed using microwave technology. The

biggest commercial steel component that has been fully sintered in our system so

far is an automotive gear of 10 cm in diameter and about 2.5 cm in height.

Figure 1. Metallic parts produced by microwave sintering such as gears cylinders, rods and discs.

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Microwave vs. Conventional HeatingThe use of microwave energy for materials processing has major potential, and

real advantages over conventional heating. These include:

Time and energy savings

Rapid heating rates

Considerably reduced processing time and temperature

Fine microstructures and hence improved mechanical properties and better

product performance

Lower environmental impact

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Figure 2. Schematic of a microwave sintering furnace.

The sintering chamber consists of ceramic insulation housing (batch system) or an alumina

tube insulated with ceramic insulation from outside, figure 2. The primary function of the

insulation is to preserve the heat generated in the work piece. The temperatures are

monitored by optical pyrometers, IR sensors and/or sheathed thermocouples placed close

to the surface of the sample. The system is equipped with appropriate equipment to

provide the desired sintering atmosphere, such as H2, N2, Ar, etc, and is capable of

achieving temperatures up to 1600°C. The technology can be easily commercialized by

scaling up the existing microwave system.

.

Microwave Sintering Devices

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MICROWAVE SINTERING OF REFRACTORY METAL

Refractory metals and alloys are well known for their high

mechanical properties which make them useful for wide range of

high temperature applications.

Conventional P/M processing is a viable sintering technique for

these refractory metals. One of the constraints in conventional

sintering is long residence time which results in undesirable micro

structural coarsening conditions.

These refractory metals and alloys (W, Mo, Re, W-Cu, W-Ni-

Cu and W-Ni-Fe) have been successfully consolidated using

microwave sintering.

. Most refractory metals used for various applications are tungsten

with fusion point of 3420°C, molybdenum of 2620°C and rhenium of

3180°C.

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Sintered tungsten is an excellent material for many applications

such as lightings, heating, aerospace, electronic, and military Uses,

due to its high melting point, high-density of 19.3 g/cm3, high hardness

of 9.75GPa, moderate elastic modulus of 407 GPa, low coefficient of

thermal expansion, good thermal conductivity

Rhenium metal is only second to tungsten, among the metallic elements, in

melting point. Its density of 21.0 g/cm3 is higher than that of tungsten.

Annealed material has exhibited tensile strengths of about 120KPa. with 25%

ductility at room temperature, and it is somewhat harder. Other properties,

such as its corrosion resistance and electrical properties make it promising for

incandescent lamp filaments and electrical contacts.

Molybdenum is a typical transition metal element having a high melting

point, high mechanical strength, and high modulus of Elasticity Most of the

applications for pure molybdenum metal and its alloys involve as electrodes

for electrically heated glass furnaces , nuclear energy applications, missile and

aircraft parts, thermocouple sheaths, flame and corrosion resistant coatings for

other metals.

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CONSOLIDATION OF TUNGSTEN

Usually the consolidation of W powder by conventional heating is difficult and

requires very high temperature (2200°C or more) in electrical resistance

sintering under hydrogen atmosphere.

The requirement of excessive high temperature and special technique

makes the process more expensive and imparts a restriction in the sizes and

shapes of the sintered products

sintering temperature is related to the powder size, when the size is in nano -

scale, the sintering temperature can be decreased up to several hundred

degrees.

It is long been know that the melting temperature of very fine particles

decreases with the size of the particles.

Therefore in addition to the faster sintering kinetics, the faster densification

in nano structured material could be attributed to the lower melting

temperature of nano particles.

10 to 12% higher sintered density in microwave sintering as compare to their

conventional

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Role of HFO2, Y2O3 as successful grain growth inhibitors.

They also observed that the introduction of a secondary oxide

(HfO2 and/or Y2O3) had a significant effect on the powder

morphology and in reducing the primary particle size of the as

synthesized tungsten powders. The particle size was reduced

from 350 nm to 80-100nm, and the crystallite size was reduced

from 48 nm to 25 nm with the addition of dopents

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Fig 3.describes typical thermal

profile used for their experiments in

both conventional as well as

microwave heating mode.

Fig. 2. SEM micrographs of (left) conventional and (right)microwave sintered W at

1600°C for 30 min in H2

atmosphere

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CONSOLIDATION OF MOLYBDENUM

Conventionally the sintering of molybdenum powder is

conducted using a resistance or induction sintering furnace in

an inert atmosphere (Ar) or in a reducing atmosphere (H2) .

High temperatures in the range of 2000°C are employed,

resulting in densities of 90–95% of theoretical, depending upon

the sintering time.

The sintering of molybdenum using vacuum furnaces and

obtained densities of 97 to 98.5% at a sintering temperature of

1750°C with times ranging from 10 to 40 h. This also results in

abnormal grain growth.

sintering of nano molybdenum powder to obtain submicron

grain size microstructure using microwave energy.

Samples with densities as high as 98% of theoretical density

(TD) were obtained with limited grain growth in 5 min of sintering

time in microwaves, compared to conventional process.

using microwave energy 99%TD could be obtained at 1400°C in

just 30 min. This conclusively shows that microwave sintering is

much faster than conventional sintering.

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CONSOLIDATION OF RHENIUM

Arc melting of rhenium in an inert atmosphere or vacuum is possible

but the metal produced tends to have coarse grain size and may have

segregation of rhenium oxides at the grain boundaries.

Rhenium powder is consolidated using pressure techniques to a

density of approximately 60% of the theoretical density. The pressed

compacts are then presintered in a hydrogen atmosphere to facilitate

handling before final sintering.

Relatively high sintered density in the order

of 95% of theoretical has been achieved in

microwave heating at 2000°C, 10 min soaking

time. Figure 4 shows a SEM micrograph of

as-pressed and microwave sintered rhenium

compact

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Fig. 4: SEM micrographs of Re

pallet in (left) as pressed and

(right) microwave sintered at

2000°C for 10 min

SUMMARY

Pure refractory metals such as, W, Re and Mo can be effectively

sintered using microwave energy to high densification.

Microwave sintering provides about 80% reduction in total processing

time. Microwave sintering leads to higher sintered densities (of as high as

98% of theoretical density).

Finer grain sizes and superior mechanical properties have been

achieved in microwave sintering irrespective of the material.

In case of W sintering addition of Y2O3 and HfO2 (grain growth

inhibitors) have been successfully used to restrict grain growth.

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REFERENCES

Journal of Microwave Power and Electromagnetic Energy, 44 (1), 2010,

pp. 28-44.A Publication of the International Microwave Power Institute

Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-

Ni-Cu and W-Ni-Fe Alloys Avijit Mondal1, Dinesh Agrawal2, Anish

Upadhyaya

Agrawal, D. (1999). “Microwave Sintering of Ceramics, Composites,

Metals, and Transparent Materials.” J Mater. Edu. vol.

19 (4, 5, 6), pp. 49-58.

Agrawal, D.K. (1998). “Microwave Processing of Ceramics: A Review.”

Current Opinion in Solid State & Mat Sci, vol. 3 (5), pp. 480-86.

Primary author: Prof. Dinesh Agrawal Source: Materials World, Vol. 7 no. 11 pp.

672-73 November 1999.

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