technology of full density powder materials and products 01.12.2010

Technology of full density powder materials and products

Professor Jakob Kübarsepp Institute of Materials Engineering

Tallinn University of Technology

1. Introduction: full density processing2. Hot pressing3. Hot isostatic pressing4. Sinter HIP5. Powder extrusion6. Powder forging7. 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


• 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


Three variables influencing P/M processing methods – component size, density and performance (as a percentage of wrought

High performance applications require higher densities

The symbols are:

P/M = press and sinterreP = press, sinter and repressP/S+F = press, sinter and forge

CIP+S= cold isostatically press and sinterHIP = hot isostatically pressHIP+F = hot isostatically press and forge


The density effect on the strength and ductility of a forged 4640 steel

• 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)


Phase diagram of an ideal system for liquid-phase sintering

Typical microstructure of a liquid-phase sintering and infiltration system

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).


• 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.


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


Initial and intermediate stages of densification forspherical powders

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

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 pressing2.1 Process

• Uniaxial hot pressing = pressing on sintering temperature

• Uniaxial pressing in graphite die at pressures up to 50 MPa and temperatures up to 2200C in controlled atmosphere (Ar, N2, vacuum)

Hot pressing (cntd. 1)

Temperature:

up to 2200oC

Time: 1…3 hours

Pressure:

up to 50 MPa

Die material:

graphite, refractory

metals, ceramics

Hot pressing is the simplest hot consolition technique:


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 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. Large parts/products from WC-Co hardmetals

2. Products from Be alloys in nuclear reactors, missile and aerospace applications

3. Diamond-metal composite cutting tools (e.g diamond+Co)

4. Micro Infiltrated Macro Laminated Composites – MIMLC

5. Fiber-reinforced materials

6. Sputtering targets for deposition of thin films etc.


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


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 modulushigh toughness phaseHard, brittle, high moduluslow 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 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. Encapsulation:

• container production

• leak testing

• filling

2. 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 3000C3. 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

• 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)

Microscopic model:

Hot isostatic pressing (HIP) (cntd. 10)

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 alloys4. Titanium and titanium aluminide 5. Metal matrix composites – MMC6. 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-carbide-volume (up to 45 vol%) steels

• The largest tonnage application at about 12 000 tons worldwide

Hot isostatic pressing (HIP) (cntd. 14)

Primary carbides in heat treated P/M T15 (left) and conventional T15 (right) high-speed steels

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

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 pressure-assisted consolidation technique of sintered parts (with porosity below 8%)

Sinter HIP (cntd. 2)Different technological schemes of Sinter+HIP:

1. Sinter + HIP in different cycles

2. Sinter + HIP in the same cycle eliminating extra sintering and cooling step

3. CHIP = CIP/Sinter + HIP

4. PIM/Sinter + HIP

where:

CIP – cold isostatic pressing

PIM – powder injection molding

Sinter HIP (cntd. 3)Sinter + HIP of Ti-alloys

The main advantages of Sinter+HIP technology are: • possibility to produce complex and near net shape products• possibility to achieve full densityThe most widely used process for consolidation of complex in shape products are :

Sinter HIP (cntd. 4)

1. CHIP = CIP + Sinter + HIP2. PIM/Sinter + HIP = PIM + Sinter + HIP

• 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


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 low-pressure equipment is lower

Sinter HIP (cntd. 8) Sinter/HIP process is preferable for production of low-cobalt alloys of

medium grain size (2m)

Sinter HIP (cntd. 9) Sinter + HIP process is preferable for low-cobalt alloys of fine grain size (<1m)

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 steels4. P/M stainless steels5. Si3N4 based structural ceramics6. Complex shapes utilizingCIP+Sinter/HIP and PIM+Sinter/HIP

Sinter HIP (cntd. 12) 4.3 Pulsed Sinter/HIP

Time-temperature-pressure profile of the Pulsed Sinter/HIP:

1. Thermal Spike HIP

2. Pressure Spike HIP

5. Powder extrusionPowder extrusion mechanics

Powder extrusion: high-temperature consolidation at high strain rates and high stresses.

Three main groups of processes:

1 – loose powder extrusion2 – extrusion of pre-sintered powder compacts3 – extrusion of encapsulated powders

• 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. 1)

Extrusion pressure (for wrought and particulate material):

RKP lnwhere

K – extrusion constant for the material (combines flow stress, friction, temperature, material properties etc into one parameter) decreases with increasing temperature

f

iS

SR - extrusion ratio (reduction ratio) must exceed 10 for adequate densification



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


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 (1300C) and steel (1050C)

co-extrusion of UO2 (2000 C) and steel (700C)

Multi-temperature co-extrusion of chromium and steel

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


• 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


Special process of powder extrusion:extrusion using low extrusion ratios

Powder extrusion (cntd. 9) Applications:

1. Intermetallic compounds (Ti3Al, Ni3Al etc.)

2. Cu-based alloys

3. Ni-based alloys

4. W-based alloys

5. P/M high speed steels

6. 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: Powder repressing:

low strains more defects, not

optimal mechanical properties

low die wear, more complex shapes

high strains better structure, good

mechanical properties high die wear

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 (0.30.5)

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)


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

b) conventional forging in an impression die with flash – large variations of the mass of preform is permitted

a)

b)

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 < 10m, 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:

=Km

where

m – strain rate sensitivity exponent (in superplastic condition m>0.3)

– strain rate

K – material and structure constant

Strain rate sensitivity exponent m vs temperature for the superplastic forming of ultra high carbon steel (UHC)

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 (<1m)

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 1550C

NB! The finer grain size makes superplastic ceramics prone to concurrent grain growth


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)


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 processes7.1 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)

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.

Combustion synthesis (cntd 4)

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) Reactive hot pressing (RHP)

2) Reactive hot isostatic pressing (RHIP)

3) Pseudo-HIP (P-HIP)

4) High pressure-assisted combustion synthesis (HPCS)

Pressure-assisted combustion synthesis (cntd. 2) Reactive hot pressing (RHP) (1):

Uniaxial hot pressing and simultaneus combustion synthesis


Time-temperature-pressure profile for a typical reactive hot pressing operation:

Pressure: up to 50 MPa

Time: 600 sek ( 10 minutes)


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-temperature-pressure 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 CERACON-process is similar to pseudo-HIP, but the pressure is not isostatic (and results in unequal deformation of the preform)

• The CERACON-process = pseudo-isostatic forming process

Ceramic consolidation process (cntd.)


• 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


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



• 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)


• 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)

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. 1)

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 = pressure-assisted 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


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 +/-2C3) 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


Application and advantages:

• 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

Schematic of the injection molding machine used for Thixomolding

7.9 InfiltrationFor wetting liquid melt, the capillary pressure:

P = 2cos()/dwhere

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)


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.


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

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.


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.


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

3) Powder and porous bodies can be simultaneusly densified to a substrate

HIP diffusion bonding of powder to solid


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


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!

technology of full density powder materials and products 01.12.2010

Documents

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