Technology of full density powder
materials and products
Professor Jakob Kübarsepp
Institute of Materials Engineering
Tallinn University of Technology
1.
2.
3.
4.
5.
6.
7.
Introduction: full density processing
Hot pressing
Hot isostatic pressing
Sinter HIP
Powder extrusion
Powder forging
Alternative hot consolidation processes
1. Introduction: full density processing
• Powder metallurgy (P/M) is
the most diverse
manufacturing approach
among various metalworking
technologies.
• There are three main
reasons (see Fig.) for using
powder metallurgy - the art
and science of producing
powders and of utilizing
powders for the production
of massive materials and
shaped products
Introduction: full density processing (cntd. 1)
Influence of
porosity (density) on
impact strength and
tensile strength of
P/M materials.
Introduction: full density processing (cntd. 2)
The effect of porosity on fatique strength
Introduction: full density processing (cntd. 3)
• Permissible porosity depends on application
field
• Higher loads call for higher density
• Conventional P/M technology (press- and -sinter
technology) in most cases doesn`t enable to
achieve full density.
• Hot consolidation processes (hot pressing, HIP,
extrusion etc) enable to produce full-density or
near full-density or near full-density powder
materials/products
Introduction: full density processing (cntd. 4)
High performance applications
require higher densities
The symbols are:
P/M = press and sinter
reP = press, sinter and repress
P/S+F = press, sinter and forge
CIP+S= cold isostatically press and sinter
HIP = hot isostatically press
HIP+F = hot isostatically press and forge
Three variables influencing P/M processing methods – component size, density and performance (as a percentage of wrought
Introduction: full density processing (cntd. 5)
• Challenge of P/M technology – production of net-shape
(with final dimensions) and full density
materials/products
• Utilizing full density processing the performance levels
can exceed (see Fig.) those associated with wrought
materials (if control is gained over defects, impurities,
microstructure and product homogeneity)
The density effect on the strength and ductility of a forged 4640 steel
Introduction: full density processing (cntd. 6)
Liquid phase sintering (LPS) is widely employed to obtain full density products
without application of simultaneus pressure during sintering.
Applications of LPS: W-Ni-Fe heavy alloys, WC-Co cemented carbides, TiC-Fe
cermets, Co5Sm-base magnets etc.
Infiltration – other technique for obtaining full density (see section 7.9).
Phase diagram of an ideal
system for liquid-phase sintering
Typical microstructure of a liquid-phase sintering
and infiltration system
Introduction: full density processing (cntd. 7)
• The traditional P/M cycle involves the sequential application of
pressure (to compact powder) and temperature (to sinter the
powder). Many full density processes involve simultaneous
heating and pressurization.
• The simultaneous heating and pressurization add cost and
complexity that are best justified by increased performance.
Introduction: full density processing (cntd. 8)
The four concerns discussing hot consolidation of
powders are:
1) Temperature: full density processing is normally
effective at T>0.5Ts (Ts – melting temp.)
2) Stress: only full density does the effective stress at the
interparticle contacts equal to applied stress (because
pores act as stress concentrators)
3) Strain: large shear strains disrupt surface films on the
particles, but contribute to tool wear.
4) Strain rate:
 high strain rate reduces ductility (fracture is more likely)
 low strain rate gives more plastic deformation with a
higher final density
Introduction: full density processing (cntd. 9)
Three mechanisms of pore
elimination:
1) Plastic yielding – occurs when
the effective stress exceeds the
material yield strength
2) Power law creep – occurs when
both the stress and temperature
are high and rate of densification
depends on the diffusion rate for
dislocation climb
Initial and intermediate stages of densification for
spherical powders
3) Lattice and grain boundary
diffusion – occurs when the
diffusivity is highly sensitive to
temperature and the temperature
has the most important influence
on the densification rate
Introduction: full density processing (cntd. 10)
Application fields of full density or near-full density powder
materials
•
•
•
•
•
•
Structural ceramics
High-temperature composites
Metal-bonded diamond tools
P/M high speed steel tools
Products from refractory metals
Products from Ni-base superalloys and
corrosion resistant alloys
• Products from low-alloy steels etc.
2. Hot pressing
2.1 Process
• Uniaxial hot pressing =
pressing on sintering
temperature
• Uniaxial pressing in
graphite die at
pressures up to 50
MPa and temperatures
up to 2200C in
controlled atmosphere
(Ar, N2, vacuum)
Hot pressing (cntd. 1)
Hot pressing is the simplest
hot consolition technique:
Temperature:
up to 2200oC
Time: 1…3 hours
Pressure:
up to 50 MPa
Die material:
graphite, refractory
metals, ceramics
Hot pressing (cntd. 2)
Mechanism of pore elimination
(compaction):
• particle rearrangement and plastic
yielding/plastic flow (at point contacts)
• grain boundary and lattice diffusion (as
densification progresses)
Hot pressing (cntd. 3)
Hot pressing can be used for consolidation of different
material combinations:
• single powders or powder mix
• alternating layers of powders (for production of
laminated composites)
• mix of powder and whiskers
• powders + thin sheets of metal/ceramics
• sheets of continous fibers with powders (plasma
sprayed on fibers)
• powder cloth + fiber mats etc.
Depending on the material the lay-up sequence will
vary significantly.
Hot pressing (cntd. 4)
2.2 Applications
1.
2.
3.
4.
5.
6.
Large parts/products from WC-Co hardmetals
Products from Be alloys in nuclear reactors, missile
and aerospace applications
Diamond-metal composite cutting tools (e.g
diamond+Co)
Micro Infiltrated Macro Laminated Composites –
MIMLC
Fiber-reinforced materials
Sputtering targets for deposition of thin films etc.
Hot pressing (cntd. 5)
Ceramic materials are characterized by extremely low fracture
toughness. Efforts to improve the toughness include:
• Incorporation of particulates
• Incorporation of whiskers or fibers
• Cermet (ceramic/metal composite) technology in which the tough
metallic component absorbs energy
• Inclusion of a phase which undergoes tranformation due to the
stress field associated with a crack (“transformation toughening”).
Transformation leads to volume expansion producing compressive
stress on the crack tip
• Utilization of the concept of compositional gradient – MIMLC-type
materials
Hot pressing (cntd. 6)
Improved toughness of ceramic materials and
composites can be attributed to:
• crack branching and crack deflection
• crack bridging (e.g in cermets where metal matrix
ligaments effectively bridge the macroscopic crack
plane)
• fiber of whiscers matrix decohesion and pullout
(e.g in fiber/whiscers reinforced ceramic matrix
composites)
• plastic deformation = one of the best mathods of
improving fracture toughess of a material
(e.g in MIMLC-type materials)
Hot pressing (cntd. 7)
Conceptual MIMLC structure
Soft, ductile, low modulus
high toughness phase
Hard, brittle, high
modulus
low toughness phase
Schematic view of a conceptual MIMLC structure
Hot pressing (cntd. 8)
MIMLC – gradient material:
• On a macro scale is laminated with alternating
layers of hard and brittle ceramic-like material
and ductile and tough metal-like layer
• On a micro scale the ceramic-like material is
inter-penetrated with the ductile and tough metal
like material.
Hot pressing (cntd. 9)
Improved toughness of MIMLC: can be attributed to:
• Increased resistance (higher than that of
ceramic-metal composites) to fracture due to
energy absorbtion via plastic deformation
• Crack bridging in plastic zone of a composite
Hot pressing (cntd. 10)
Example of MIMLC:W-Ni-Fe grandient material
Hot pressing (cntd. 11)
MIMLC: Al2O3-Ni grandient material
One of the prime uses
is material against:
 ballistic penetration
 hyper-velocity impacts
Hot pressing (cntd. 12)
Hot pressing of Al2O3-Ni gradient material
MIMLC composite
lay up assembly
(for production of Al2O3-Ni
gradient material)
Hot pressing (cntd. 13)
Schematic processing sequence for producing the fiber reinforced
glass and glass-ceramic matrix composite by hot pressing
1000oC, 300 bar
Hot pressing (cntd. 14)
A typical stress-strain curve for fiber reinforced glass matrix composite
Hot pressing (cntd. 15)
Hot pressing of composites by utilizing
in-situ displacement reaction synthesis:
Examples of in-situ synthesized
(high-temperature) composites:
1) Mo2C + 5Si = 2MoSi2+SiC
(MoSi2 – based composite reinforced by SiC
particles)
2) 4.5NiAl + 3NiO = 2.5Ni3Al + Al2O3, or
12NiAl + 3NiO = 7.5NiAl + 2.5Ni3Al + Al2O3
(Al2O3 reinforced Ni-Al composite)
3. Hot isostatic pressing (HIP)
3.1 Process
• HIP = high-temperature
pressing (hot
consolidation) of
encapsulated powders or
sintered high-density
(density >92% of
theoretical) compacts.
• Pressure: 100...300 MPa
• Time: 2...6 hours
• HIP-low strain rate
process because stress
rise in slow
Hot isostatic pressing (HIP) (cntd. 1)
Preparative steps of HIP:
1.
•
•
•
2.
Encapsulation:
container production
leak testing
filling
Degassing (evacuation of absorbed
gases and moisture) and crimping of
the can
Hot isostatic pressing (HIP) (cntd. 2)
Typical HIP cycles:
I HIP in unit with internal heating zone
II HIP of externally heated encapsulated part
Hot isostatic pressing (HIP) (cntd. 3)
A cross section view of the HIP vessel
Hot isostatic pressing (HIP) (cntd. 4)
Recent developments of HIP equipment:
1. Increased Cooling HIP systems (providing
productivity and decrease in cost)
2. Ultra High Pressure and Temperature Systems
• pressures up to 1000 MPa
• Temperatures up to 3000C
3. Duplex systems for HIP, WIP, CIP (hot
isostatic pressing, warm isostatic pressing,
cold isostatic pressing)
Hot isostatic pressing (HIP) (cntd. 5)
Advantages of HIP:
1. Materials/products of higher performance
2. Cost savings because of
 lower lifecycle cost (of higher performance
materials)
 the HIP near-net shape process
 lower unit costs of large parts and
production volumes of small-weight parts
Hot isostatic pressing (HIP) (cntd. 6)
Disadvantages of HIP:
• little shear on the particle surfaces (because of
consolidation by hydrostatic stress). Therefore:
1) it is necessary (in some applications) to subject
HIP-compact to post-consolidation deformation
or
2) the microstructural defects can be minimized by
use of powders atomized with rapid
solidification rate and clean handling in inert
gas
Hot isostatic pressing (HIP) (cntd. 7)
3.2 Mechanism of consolidation
• Full density is
achieved more
easily under the
conditions of higher
packing density
• Packing density of
commercial
spherical powders
vary from
0.62...0.72
• Mostly pre-alloyed
powders are used
Hot isostatic pressing (HIP) (cntd. 8)
Two models of the HIP process
1. Microscopic model: calculates density as a
function of the process variables: time,
pressure, temperature, initial packing density,
material properties. Model predicts density.
2. Macroscopic model: shape change under
pressure is analyzed utilizing plastic theory of
porous metals. Macroscopic models are tools
to predict the size and shape change
(important in production of net-shape parts).
Hot isostatic pressing (HIP) (cntd. 9)
Microscopic model: three stages of consolidation process
0) stage of
packing density
1) stage of
connected
porosity
2) stage of closed
porosity
Hot isostatic pressing (HIP) (cntd. 10)
Microscopic model:
• HIP maps (density vs
temperature maps) at
constant pressure and
initial packing density
• Two mechanisms of
HIP densification:
(1) Plastic yielding
(2) Power-law creep (the
dominant mechanism
of densification)
Hot isostatic pressing (HIP) (cntd. 11)
Macroscopic model:
A comparison of the
predicted shape change for
axisymmetric product to the
actual shape change after
HIP
Hot isostatic pressing (HIP) (cntd. 12)
3.3 Application fields:
1. P/M tool steels:
– high speed steels
– cold work tool steels (0.8…3.9% C)
– wear and corrosion resistant steels (1.7…3.7% C)
– HIP/clad products (see section 7.10)
2. Ni-base superalloys
3. Corrosion resistant Ni- and Fe-based alloys
4. Titanium and titanium aluminide
5. Metal matrix composites – MMC
6. Hardmetals and cermets etc.
Hot isostatic pressing (HIP) (cntd. 13)
Characterization of P/M tool steels:
• Microstructure is characterized by fine and
evenly distributed carbides which improve
toughness, wear resistance, grindability,
workability
• High-carbon (up to 3.9%) and high-carbidevolume (up to 45 vol%) steels
• The largest tonnage application at about 12 000
tons worldwide
Hot isostatic pressing (HIP) (cntd. 14)
All of the P/M tool and high speed steels have:
• fine and evenly distributed carbides
• improved toughness and wear resistance
• improved technological properties such as hot
workability, machineability (in annealed condition), more
uniform heat treatment response in large section,
predictable size change after heat treatment
Primary carbides in heat treated P/M T15 (left) and conventional T15 (right) high-speed steels
Hot isostatic pressing (HIP) (cntd. 15)
HIP fabrication of metal matrix composites Ti/SiC
4. Sinter HIP
• Disadvantages of HIP:
1) Encapsulation prior HIP-ing is as
expensive and time consuming step
2) Canning in most cases precludes the
fabrication of compex shapes
• Sinter HIP techniques (Sinter+HIP,
Sinter/HIP or Sinter-HIP etc.) enable to
produce complex and net shape full
density products
Sinter HIP (cntd. 1)
4.1 Sinter + HIP
Both Sinter + HIP
and Sinter/HIP =
containerless
pressureassisted
consolidation
technique of
sintered parts
(with porosity
below 8%)
Sinter HIP (cntd. 2)
Different technological schemes of Sinter+HIP:
1.
2.
3.
4.
Sinter + HIP in different cycles
Sinter + HIP in the same cycle eliminating extra
sintering and cooling step
CHIP = CIP/Sinter + HIP
PIM/Sinter + HIP
where:
CIP – cold isostatic pressing
PIM – powder injection molding
Sinter HIP (cntd. 3)
Sinter + HIP of Ti-alloys
Sinter HIP (cntd. 4)
The main advantages of Sinter+HIP technology are:
• possibility to produce complex and near net shape
products
• possibility to achieve full density
The most widely used process for consolidation of complex
in shape products are :
1.
2.
CHIP = CIP + Sinter + HIP
PIM/Sinter + HIP = PIM + Sinter + HIP
Sinter HIP (cntd. 5)
• The first stage of
PIM/Sinter + HIP
process, where:
PIM – powder injection
molding
Sinter HIP (cntd. 6)
4.2 Sinter/HIP or Sinter-HIP
1.
Sinter / HIP = pressure
assisted
sintering/overpressure
sintering
2. The main difference
between”Sinter+HIP in
the same cycle” and
Sinter/HIP:
– use of substainally
lower (almost an
order of magnitude)
gas pressure
– the requierments of
the equipment are
less demanding
Sinter HIP (cntd. 7)
Advantages of Sinter/HIP (Sinter-HIP) as against
conventional HIP:
1) Elimination of both containerization and
separate sintering cycle
2) Pressure (5-15 MPa) is generally an order of
magnitude lower
3) The requirements of the equipment are less
demanding and therefore the cost of lowpressure equipment is lower
Sinter HIP (cntd. 8)
Sinter/HIP process is preferable for production of low-cobalt alloys of
medium grain size (2m)
Sinter HIP (cntd. 9)
Sinter + HIP process is preferable for low-cobalt alloys of fine grain size (<1m)
Sinter HIP (cntd. 10)
Industrial Sinter/HIP cycle
Sinter HIP (cntd. 11)
Applications of Sinter/HIP:
1. Hardmetals (cemented carbides)
2. Composites (MoSi2+Al2O3)
3. P/M tool steels
4. P/M stainless steels
5. Si3N4 based structural ceramics
6. Complex shapes utilizing
CIP+Sinter/HIP and PIM+Sinter/HIP
Sinter HIP (cntd. 12)
4.3 Pulsed Sinter/HIP
Time-temperaturepressure profile of the
Pulsed Sinter/HIP:
1. Thermal Spike HIP
2. Pressure Spike HIP
5. Powder extrusion
Powder extrusion mechanics
Powder extrusion: high-temperature consolidation at high
strain rates and high stresses.
Three main groups of
processes:
1 – loose powder
extrusion
2 – extrusion of presintered powder
compacts
3 – extrusion of
encapsulated powders
Powder extrusion (cntd. 1)
• Conventional processes of extrusion are:
- extrusion of pre-compacted and pre-sintered compacts
(e.g Fe-based materials – high speed steels)
- extrusion of encapsulated powders (refractory and
reactive powders)
• Specific feature of extrusion: powder partides are
subjected to high shear and deformation helping braking
up oxide skin and promting nascent particle-particle
contact (the major adventage over HIP)
Powder extrusion (cntd. 2)
Extrusion pressure (for wrought
and particulate material):
P  K ln R
where
K – extrusion constant for the
material (combines flow
stress, friction, temperature,
material properties etc into
one parameter) decreases
with increasing temperature
R
Si
Sf
- extrusion ratio
(reduction ratio)
must exceed 10
for adequate
densification
Powder extrusion (cntd. 3)
Special processes of powder extrusion:
 duplex extrusion (co-extrusion)
 multi-temperature extrusion
 extrusion of composites in semi-solid
(mushy) state
 extrusion with changing direction of
powder flow
 extrusion using low extrusion ratios
Powder extrusion (cntd. 4)
Special process of extrusion – duplex extrusion (co-extrusion) is a
process which results on one material forming a layer on another over
the entire length of the final product (e.g tube on rod).
Special process of extrusion – multi-temperature extrusion for good
proportional co-reduction.
Examples are:
 co-extrusion of Cr (1300C) and steel (1050C)
 co-extrusion of UO2 (2000 C) and steel (700C)
Multi-temperature
co-extrusion
of chromium and steel
Powder extrusion (cntd. 5)
Special process of powder extrusion:
extrusion of composites in the semi-solid
state (mushy state extrusion).
Pecularity: liquid phase fraction 10…30 vol %
decrases extrusion pressure (flow stress)
markedly.
Example: metal matrix composites (MMC-s)
Powder extrusion (cntd. 6)
Special process of powder extrusion: extrusion with
changing direction of powder flow
1 – conventional extrusion
2 – single shear extrusion
3 – double shear extrusion
Powder extrusion (cntd. 7)
• Extrusion with changing direction of powder flow aims to
eliminate the problem of in homogeneous shear in the
conventional extrusion
• The total shear deformation in the double shear
extrusion process can be expressed (see Fig.)
Ttotal = V2/V1 + V3/V2 = S1/S2 + S2/S3
where S1/S2 and S2/S3 represent the extrusion ratio for
zone 1 and zone 2, respectively
Schematic of the deformation in double
shear extrusion
Powder extrusion (cntd. 8)
Special process of powder extrusion:
extrusion using low extrusion ratios
Powder extrusion (cntd. 9)
Applications:
1.
2.
3.
4.
5.
6.
Intermetallic compounds (Ti3Al, Ni3Al etc.)
Cu-based alloys
Ni-based alloys
W-based alloys
P/M high speed steels
Oxide dispersion hardened (strengthened)
alloys etc.
6. Powder forging
The role of final density
of pure iron on the
mechanical properties
(tensile strength,
elongation, impact
resistance).
Powder forging offers a
process for producing
parts with very high
densities.
Powder forging (cntd. 1)
Different hot powder forging techniques:
1. Powder forging
(powder upsetting)
2. Powder repressing
3. Superplastic
forming of powders
The powder forging is very similar to conventional hot forging.
The major difference – preform is particulate material.
Powder forging (cntd. 2)
Differentiation between two schemes:
Powder forging:
 high strains
 better structure, good
mechanical properties
 high die wear
Powder repressing:
 low strains
 more defects, not
optimal mechanical
properties
 low die wear, more
complex shapes
Powder forging (cntd. 3)
• The main mechanism of pore elimination (compaction) – plastic yielding
(plastic flow)
• There take place changes during powder forging in:
 density (porosity)
 rate of work hardening (increasing with the porosity (see Fig.))
 Poisson`s ratio (0.30.5)
Compressive deformation of
porous iron with various initial
porosities
Powder forging (cntd. 4)
• The pore collapse in forging is significantly different from
that encountered under the hydrostatic conditions (HIP)
• The difference in pore collapse contributes to more shear
and bonding in powder forging (as against HIP)
Pore collapse in powder hot forging and hot isostatic pressing (HIP)
Powder forging (cntd. 5)
Height strains >50% ensure:
 pore elimination
 good interparticle bonding (due to disintegration of oxide films during
shear)
The preform and forged height to diameter ratio (H/D) showing the region of high
densification without fracture
Powder forging (cntd. 6)
• All mechanical characteristics except ductility (hardness, yield strength,
dynamic properties) of powder forged materials are often superior to
wrought products
• Inclusions seriosusly degrade properties in spite of full density (see Fig.)
The effect of oxigen inclusions on the impact toughness of full-density 4340
steel processed by powder forging
Powder forging (cntd. 7)
Features of powder forging:
a) powder forging in trap
die (to net shape) –
tight control of preform
mass is important
a)
b)
b) conventional forging
in an impression die
with flash – large
variations of the mass
of preform is permitted
Powder forging (cntd. 8)
Stages of powder forging process:
(a) powder fill
(b) pressing
(c) ejection of preform
(d) preheating (in
controlled
atmosphere)
(e) powder hot forging (in
controlled
atmosphere)
Powder forging (cntd. 9)
Superplastic forming:
Superplasticity – ability of particular
material under certain conditions to
undergo extraordinary high tensile
elongations. This phenomenon occurs in:
 metals/alloys
 intermetallic compounds
 glass ceramics
 ceramics
Powder forging (cntd. 10)
Powder superplastic forging (superplastic forming)
One of preconditions of superplastic forming –
structural superplasticity (relies on
microstructure)
Requirements of structural superplasticity:
1. Ultra fine (stable during the process) grain
size (metals < 10m, ceramics < 1 m)
2. Deformation temperature T > 0,5 TS
3. Low strain rates (102...10-6); high strain rate
sensitivity
Powder forging (cntd. 11)
Superplastic forming:
Important factor for superplastic
forming – strain rate sensitivity (=
resistance to “necking”):
Flow stress:
=Km
where
m – strain rate sensitivity exponent
(in superplastic condition m>0.3)
Strain rate sensitivity exponent m vs
temperature for the superplastic
forming of ultra high carbon steel
(UHC)
 – strain rate
K – material and structure constant
Powder forging (cntd. 12)
Changes on grain structure before and after superplastic
forming and conventional powder forging:
Dominating mechanisms of
deformation of metals and
intermetallic compounds:
• grain boundary sliding
• diffusion
• intergranular
dislocation slip
Powder forging (cntd. 13)
Difference between a conventional and isothermal die forging (nearly
identical to superplastic forming)
Powder forging (cntd. 14)
Another precondition of superplastic forming – transformation
superplasticity (internal stress superplasticity)
Requirements: for superplastic forming (of ceramics):
1. Ultra fine (sub-micron) grain size (<1m)
2. Stress-induced transformation toughening – internal
stresses generation by phase transformations
Example: Yttria-stabilized tetragonal zirconia (ZrO2+5,2
wt%Y2O3), elongation to failure up to 800% at 1550C
NB! The finer grain size makes superplastic ceramics prone to
concurrent grain growth
Powder forging (cntd. 15)
Advantages of powder forging over conventional
forging:
1) near-net shape capability
2) full-density products/materials
3) high degree of material utilization and energy
savings
4) enhanced mechanical, particularly dynamic
properties (fatique strength, fracture
toughness)
Powder forging (cntd. 16)
Applications of powder forging:
•
Components for automotive transmission (Fe-based
alloys)
Applications of powder superplastic forming:
1. Ultra high carbon steels
(UHC –steels)
2. Mechanically alloyed materials
( e.g TiAl – Ti3Al intermetallic compounds)
3. Superplastic ceramics
(e.g ZrO2-Y2O3, Al2O3+ZrO2-Y2O3) etc.
Powder forging (cntd. 17)
Powder forged automotive transmission components:
Powder forged
connecting rods
Powder forged internal ring gear
7. Alternative hot consolidation processes
7.1 Combustion synthesis (self-propagating high
temperature synthesis; SHS; SPS; reactive/ reaction
synthesis)
Combustion synthesis (SHS, SPS) = formation of
chemical compounds (usually refractory compounds)
involving self-propagating exothermic reactions between
the reactants.
Preconditions:
• exothermic reaction
• proper granulometry of powders
• external source of heat for initiation
Combustion synthesis (cntd 1)
Combustion synthesis reaction is as follows:
aiX + bjYj = Z + Q
where:
X – V, Mo, W, Hf, Ti etc metals
Y – C, B, N, Si, O, S – non-metals or metals (production of
intermetallic compounds)
Z – chemical compound
Q – exothermic release of energy
Variations of combustion synthesis:
 Propagation of combustion wave (SHS)
 Simultaneus combustion (SC)
Combustion synthesis (cntd 2)
Three different reaction methods:
- solid phase system
- solid-liquid system
- solid-gas system
Advantages of combustion synthesis (in comparison with
conventional P/M technology)
1) Exothermic process
2) Short processing time
3) A wide variety of different materials can be produced
4) Production of chemical compounds as well as
composites
5) High purity of the end product
Combustion synthesis (cntd 3)
Examples of reactions of combustion synthesis:
Ti + C = TiC
(all solid state)
Ni + 3Al = Ni3Al
(disappearing liquid phase)
2Al + N2 = 2AlN
(solid-gas system)
3SiO2 + 6C + 2N2 = Si3N4 + 6CO2
(gas-phase present in both reactant and product)
Combustion synthesis (cntd 4)
Exothermic combustion synthesis can be take place during
sintering of multi-component powder materials = reactive
sintering.
Reactive sintering – sintering process where an exothermic
reaction is initiated in a mixture of dissimilar (elemental)
powders. The reaction produces a compound and the
heat from the reaction is used to simultaneously sinter
the product.
7.2 Pressure-assisted combustion synthesis
• Pressure assisted combustion synthesis is used in
conditions where conventional process results in porosity
of product.
• The porosity could be due to: porosity of reaction
mixture, changes in molar volume, volatile impurities etc.
Pressure can be applied by various means:
1) Uniaxial pressure
2) Isostatic pressure
3) Pseudo-isostatic pressure
4) Shock wave
5) Centrifugal force
Pressure-assisted combustion synthesis (cntd. 1)
Different schemes of pressure-assisted combustion synthesis:
1)
2)
3)
4)
Reactive hot pressing (RHP)
Reactive hot isostatic pressing (RHIP)
Pseudo-HIP (P-HIP)
High pressure-assisted combustion synthesis
(HPCS)
Pressure-assisted combustion synthesis (cntd. 2)
Reactive hot pressing (RHP) (1):
Uniaxial hot pressing
and simultaneus
combustion
synthesis
Pressure-assisted combustion synthesis (cntd. 3)
Reactive hot pressing (RHP) (2):
Time-temperaturepressure profile for a
typical reactive hot
pressing operation:
Pressure: up to 50 MPa
Time:  600 sek
( 10 minutes)
Pressure-assisted combustion synthesis (cntd. 4)
Reactive hot pressing (RHP) (3):
Applications:
Ceramics (TiB2, TiC etc),
intermetallics (NiAl,
Ni3Al etc) and their
based composites
The effect of pressure on the residual
porosity for pressure-assisted combustion
synthesis of Ni3A and NiAl compositions
Pressure-assisted combustion synthesis (cntd. 5)
Reactive hot isostatic pressing (RHIP):
Time-temperaturepressure profile for a
typical reactive hot
isostatic pressing
operation
Pressure: 100…200 MPa
Time: 2…4 hours
Application: intermetallic
compounds (NiAl, FeAl,
Ni3Al, Al3Ta etc) and their
based composites
(Al3Ta – Al2O3 fibers,
NiAl – Al2O3 fibers,
NiAl – TiB2 etc.)
Pressure-assisted combustion synthesis (cntd. 6)
Pseudo-HIP (P-HIP) (1):
Pseudo-HIP is similar to
conventional HIP, but
pressure is applied through
a granular particulate-based
medium instead of gas
Pressure-assisted combustion synthesis (cntd. 7)
Pseudo-HIP (P-HIP) (2):
Technological pecularities:
• Ceramical particulate-based medium (e.g SiO2) as the
pressure-transmitting medium
• Pressure: 100…700 MPa
• Time: 0,5 hours
Application: ceramics and ceramic-based composites
(TiAl-TiB2 etc).
NB! Pseudo-HIP is not isostatic pressing
7.3 Ceramic consolidation process
(CERACON-process)
• The CERACONprocess is similar to
pseudo-HIP, but the
pressure is not
isostatic (and
results in unequal
deformation of the
preform)
• The CERACONprocess = pseudoisostatic forming
process
Ceramic consolidation process (cntd.)
Technological pecularities:
• Preheating of both preform and ceramic granules
• Pressure: up to 1500 MPa
• Time: 30…60 sek
• Unequality (0.5%) of deformation of the preform
Application: near-net shape components from:
- metallic alloys (Ti, Ta, Al)
- intermetallic compounds (TiAl, NiAl etc)
- high-temperatures superconductors etc
7.4 Ceramic mold process
• Alternatives to produce full density net shapes:
Step 1: production of complex net shapes by:
1) loose powder packing on a complex shaped mold
2) cold isostatic pressing (CIP)
3) powder injection molding (PIM)
Step 2: Densification to full density by utilizing HIP (or
pseudo-HIP)
•
The ceramic mold process is a variation of conventional
HIP intended to drastically reduce machining
Ceramic mold process (cntd. 1)
Shematic of the
ceramic mold
process: production
of ceramic mold is
similar to that in
investment casting
Ceramic mold process (cntd. 2)
The ceramic mold process is
also suitable for producing:
1) Deep cavities (see Fig.)
2) Dual-property components
by bonding different
materials together in one
cycle
Ceramic mold process (cntd. 3)
Technological pecularities:
• an oversized ceramic mold to compensate for the
shrinkage during HIP
• application of pseudo-isostatic pressure through ceramic
particulate medium
Applications:

net-shape / near-net shape products from metallic
alloys (e.g Ti-alloys)
7.5 Rapid omnidirectional compaction
(fluid die process)
Technological pecularities:
• similar to hot isostatic
pressing where pressure
transmitting medium is a
“fluid die” (omnidirectional
nature of applied pressure)
• higher productivity in
comparison with
conventional HIP cycle
(rapid HIP)
Rapid omnidirectional compaction (cntd. 1)
Technological pecularities (cntd):
• pressure much higher than in
HIP
• Time 1…15 sek
• Temperature: lower than in HIP
Applications: WC-Co hardmetals,
intermetallic compounds etc.
Powder filled fluid die (made from low
carbon steels, Cu+10 Ni alloy, etc)
Rapid omnidirectional compaction (cntd. 2)
Drawback of utilizing metallic
“fluid” die – extensive machining
of fluid die
Solution: use of cast composite
fluid die (see Fig.) enabling
mechanical stripping of
consolidated part and
eliminating expensive canning
sequence
ROC process using a cast
composite fluid die
7.6 Plasma activated sintering (PAS)
(field activated sintering technique (FAST),
spark-plasma sintering)
Plasma activated
sintering = pressureassisted sintering
activated by electrical
discharges between
powder partides
Plasma activated sintering (PAS) (cntd. 1)
Time-temperature-pressure
cycle for the PAS-process:
1) application of pressure
2) initiation of pulsed
discharges resulting in
formation of pulse plasma
between particles (to
activate powder surface)
3) resistance heating (for
densification)
4) lowering of pressure and
temperature
Plasma activated sintering (PAS) (cntd. 2)
Technological parameters:
• Pressure: 10-65 MPa
• Total time: 10 minutes
• Pulse on-state duration: 30…60 sek
Application:
• ceramic materials
• metallic materials
• ceramic-metallic composites (hardmetals,
cermets) etc.
7.7 Consolidation by atmospheric pressure (CAP)
The CAP-technique
utilizes atmospheric
pressure to provide
pressure during the
course of sintering.
The goal: production
of full dense
materials in an
inexpensive manner
(without HIP)
Consolidation by atmospheric pressure (CAP) (cntd. 1)
Advantages of CAP technique:
1) The ease of production of complex shape glass
molds
2) Assistance of pressure without use of
expensive high-pressure equipment (required
for hot extrusion or HIP)
3) No costly protective atmospheres are required
4) The ease of glass mold stripping
7.8 Thixomolding; (molding of thixotropic
alloys)
Thixotropic state/structure
can be achieved when any
material in the mushy state
is subjected to higher
shearing action (e.g by
stirring).
Result – formation of
nearly spheroidal
degenerate dendritic
partides and viscosity
drops markedly
Thixomolding (cntd. 1)
Al-Mg phase diagram
with a Thixomolding alloy
composition and
thixomolding temperature
range for Al-9% Mg alloy.
The goal:
production of near-net
shape or net-shape parts
in a single step
Thixomolding (cntd. 2)
Process characteristics (as against conventional die
casting):
1) Lower molding temperatures – material is in semi-solid
state
2) The temperature control at the exit end +/-2C
3) Elimination of most foundy operations (handling of
molten metal, use of fluxes, waste management etc.)
4) Production of complex components directly from
particulate material
5) Potential to produce dispersion strengthened/hardened
materials
Thixomolding (cntd. 3)
Application and
advantages:
Schematic of the injection molding
machine used for Thixomolding
•
Large scale production
of complex products
from low-temperature
alloys (Mg, Al)
•
Significant increase in
die life (as against die
casting)
•
Low porosity and
enhanced mechanical
properties of material
7.9 Infiltration
For wetting liquid melt, the capillary pressure:
P = 2cos()/d
where
d- size of pores
 - surface energy of the liquid
 - contact angle (=wetting angle formed at the intersection of
liquid, solid and vapor phases)
Infiltration sequence where
capillary forces pull a molten
metal into the open pores
Infiltration (cntd. 1)
• Infiltration – the process of filling the pores of compact
with open porosity with a lower melting temperature
metal/alloy or other material (glass)
• During infiltration a liquid metal may fill pores by
capillary-action (capillary infiltration) or by external force
(external pressure infiltration)
Infiltration (cntd. 2)
1. Capillary infiltration techniques (see also next Fig.)
a) Capillary-dip infiltration
b) Full-dip infiltration
c) Contact infiltritation
Applications:
electrical contacts, ferrous structural components for
automobiles; metal matrix composites (MMC) etc.
Infiltration (cntd. 3)
Infiltration (cntd. 4)
2. External pressure infiltration techniques:
a) gravity-feed infiltration
b) centrifugal pressure infiltration
c) vacuum infiltration
d) HIP-infiltration (HIP-impregnation) (see also next Fig.)
Applications:
mainly nonwetting combinations such as: graphite-Cu,
Al-SiC, Mg-SiC
Infiltration (cntd. 5)
HIP-infiltration – full
density consolidation in
conditions when capillary
forces are ineffective
because of poor wetting,
unsuitable pore size
distribution or high
viscosity of the liquid
7.10 Bonding by hot consolidation
For bonding/joining of different materials hot
consolidation techniques can be used, e.g.:
 Hot isostatic pressing (HIP-diffusion bonding)
 Reactive joining (SHS-joining)
Bonding by hot consolidation (cntd. 1)
1) HIP diffusion bonding provides bonding
between:
 two full-dense materials
 a full-dense and a porous materials
 two porous materials
HIP process provides concurrent densification
and bonding of different materials.
Bonding by hot consolidation (cntd. 2)
1) HIP diffusion bonding (for production of HIP/clad
products):
•
enables production of gradient-structured materials
NB!
large internal stresses maybe built up due to the
thermal expansion and elastic modulus mismatch.
The stresses increase with an increase in the thickness
of joined materials.
Bonding by hot consolidation (cntd. 3)
Advantages of HIP diffusion
bonding (compared with
conventional diffusion
bonding):
1) Isostatic prssure enables
bonding of shaped surfaces
2) Plastic deformation only at
microscopic scale enables
HIP diffusion bonding at
higher temperatures
HIP diffusion bonding of powder to solid
3) Powder and porous bodies
can be simultaneusly
densified to a substrate
Bonding by hot consolidation (cntd. 4)
2) Ractive joining (SHS-joining):
•
uses combustion synthesis to join different materials
•
is similar to exothermic welding or exothermic brazing
Advantages of SHS joining:
1) minimal heat induced damage to the substrate since
generated heat is localized
Bonding by hot consolidation (cntd. 5)
Advantages (cntd.)
2) Ability to form
functionally graded
material joints between
dissimilar substrates
thereby overcoming
matching problems.
Schematic set-up of two reactive joining processes
Thank you for attention!