Hot pressing is a technique that combines powder compaction and sintering into a single step. Powder is placed in a graphite die and sintered under pressure and heat. This allows densification of materials that otherwise have poor sintering behavior. The process involves placing powder in a die and applying heat and pressure simultaneously. The temperature increases during compaction to allow sintering through mechanisms like plastic deformation and diffusion. Hot pressing produces fully dense materials in less time compared to conventional sintering. It finds applications in advanced ceramics and dispersion strengthened metals.

1. H o t p r e s s i n g
Presented by
AKHIL PS
Materials Technology
Roll No: 165551
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2. contents
• Introduction
• Why hp
• process
• Mechanism
• Die design and materials
• Manufacturers and Activities in india
• Applications
• Advantages and disadvantages
• Conclusion
• reference
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3. Introduction
• powder is uniaxially pressed between two
graphite punches in a graphite die and heated in
a defined atmosphere (vacuum or inert gases)
• Combines powder compaction and sintering into
single step
• graphite heating elements in HP surround the
pressing tool, transferring heat by
radiation/convection
• Suitable for materials with poor sintering
behaviour- ceramics and dispersion
strengthened metals
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4. Why hp
• We have to compress metal powders in a mold to form
a coherent mass or compact and sinter the compact to
form a metallic bond between the particles.
• The sintering requires considerable time, and has been
a matter of hours and, in some instances, of days.
• This makes high cost of operation and the process
becomes non-competitive
• 100% of pore removal is not possible in conventional
sintering
• the compact usually is sintered in an unconfined state
after withdrawal from the forming die, changes in
dimension and distortion are likely to occur.
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5. process
1. Powder or a cold compacted preform is placed
in the die mould
2. Mould is heated
1. Resistance heating
2. Induction heating
3. Powder in die cavity is pressurised
4. Temperature is increased during compacting
5. Dwell time- P&T are held constant
6. Mould is cooled slowly under pressure
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6. mechanism
• Different types of mechanism
– Plastic yielding: plastically deforming powder
particles> increases contact area
– Creep: high temperature deformation
– Lattice and boundary diffusion: increases with
temperature
– Viscous flow mechanism: applicable for glass or
other amorphous materials at elevated
temperature. Flow depends on viscosity
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7. 7
Fig. densification behaviour of alumina powder with and without hot
pressing

8. Die design
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Fig. schematic design of hot pressing unit

9. Die design
• Choice of die materials
– Strong enough at high temperature
– No plastic deformation at working temperature
– Should not react with powder
• The punches must resist plastic deformation
under the axial loads of pressing, while the die
body must constrain the lateral pressure of
pressing without yielding
• Metallic dies and punches should water cooled
• Temp upto 600° metal dies may be used- 135Mpa
• Above 600 graphite must be used: 15-20Mpa
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10. • To avoid reactivity between the tooling and
powder, improve ejection of the pressed material
from the die, die wall lubricants are frequently
used- Colloidal graphite suspension in water or oil
• Die washers, coatings, spacers, liners and sleeves
are used
– decrease the die compact interaction
– decrease contamination
• Because thermal expansion occurs during
heating, the dies and punches should be made of
the same material
• Vacuum is most common atmosphere
– Removing air from powder body- no air entrapment
– Decreases the chance of oxidation
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11. Die materials
• Graphite: stable up to 2500°, low CTE, ease of
machining, low cost, increases strength with temp
• Oxide ceramic: used in oxidizing atm, eg.Al2O3
• Carbides: limited applications, high cost and difficult to
machine
• Titanium boride: excellent high temp strength,
increases strength with temp, limited use because of
difficulty in machining
• tungsten and molybdenum: creep at high temp.
• If steel or superalloy dies and punches are used, they
must be kept at lower temperatures by water cooling
through internal passages
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12. Manufacturers and Suppliers
• Pratishna Engineers, Mumbai, Maharashtra -
400072, India
• Caple Industrial Solutions, Malad East,
Mumbai
• Jai Industries, Rakhial, Ahmedabad
Companies where hot press is available:
ARCI India, DMRL India
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13. applications
• high-performance ceramic components-boron
carbide, tungsten carbide, titanium diboride,
and sialon
– Tungsten carbide powders are hot pressed generally in
graphite molds induction heated to 1400 °C and
pressed at pressures up to 17 MPa.
– Hot pressing of beryllium powders prevents grain
coarsening, which is the major limitation in casting
this material. Beryllium is hot pressed at 1050 °C in
graphite tooling in a vacuum. Used in nuclear reactor
modulators and in missile and aerospace applications
• friction material industry- production of sintered
brake pads and clutches
• metal-diamond-composite
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14. advantages
• Lower pressure required- resistance of metal
particles to plastic deformation decreases
rapidly with inceasing temperature
• minimizing opportunity for distortion and
saving much time
• high density (i. e., little or no porosity)
• Less sensitive to powder characteristics-shape,
size, distribution(imp in cold pressing)
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15. disadvantages
• Expensive technique- die, integrated heating and
pressurizing system
• Control over sintering atmosphere is necessary –
vacuum is most preferred
• loose powder particles entering the die cavity
heat up rapidly, causing particles to bond
together or weld to the tool cavity and impede
further flow of powder into the die.
• the large free surface area of loose powder
increases the likelihood of oxidation of the
powder at the high temperatures of hot pressing
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16. Conclusion
• Hot pressing involves compression of loose
powder or a powder compact fitted tightly in the
die cavity
• Hot pressing generally is carded out at very low
deformation rates in hydraulic presses
• Suitable method for densifying materials with
poor sintering ability
• Application of pressure decreases the sintering
time and improves density
• Wide range of application in manufacturing of
difficult to process material
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17. reference
• Direct comparison between hot pressing and electric field-assisted
sintering of submicron alumina. Jochen Langera, , , Michael J.
offmannb, Olivier Guillona
• POWDER METALLURGY George D. Cremer, Pelham Manor, N. Y.,
assignor to Hardy Metallurgical Company, New York, N. Y., a
corporation of Delaware Application March 4, 1942, Serial No.
433,336, US 2355954 A
• Field-Assisted Sintering Technology/Spark Plasma Sintering:
Mechanisms, Materials, and Technology Developments†
Authors:Olivier Guillon, Jesus Gonzalez-Julian, Benjamin Dargatz,
Tobias Kessel,
• Challenges and Opportunities for Spark Plasma Sintering: A Key
Technology for a New Generation of Materials M. Suárez, A.
Fernández, J.L. Menéndez,
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