MICROWAVE SINTERING OF REFRACTORY METALS

1. 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

3. 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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4. 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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5. 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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6. Microwave vs. Conventional Heating
The 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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7. 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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8. 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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9. 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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10. 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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11.  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.

12. 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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13. 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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14. 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

15. 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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16. 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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17. T
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