Metal Matrix Composite (MMC)

MatrixThe matrix is the monolithic material into which theThe matrix is the monolithic material into which the

reinforcement is embedded, and is completely continuous.

Thi th t th i th th h th t i t i tThis means that there is a path through the matrix to any point

in the material, unlike two materials sandwiched together.

• In structural applications the matrix is usually a lighter metal

such as aluminum, magnesium, or titanium, and provides a

compliant support for the reinforcement.

• In high temperature applications cobalt and cobalt nickel• In high temperature applications, cobalt and cobalt-nickel

alloy matrices are common.

Metal Matrix Composite (MMC)ReinforcementThe reinforcement can be either continuous, or discontinuous.

Discontinuous MMCs can be isotropic, and can be worked with standard

metalworking techniques, such as extrusion, forging or rolling.

Continuous reinforcement uses monofilament wires or fibers such as carbon

fiber or silicon carbide. Because the fibers are embedded into the matrix in a

certain direction, the result is an anisotropic structure in which the alignment

of the material affects its strength.

Discontinuous reinforcement uses "whiskers", short fibers, or particles. The

most common reinforcing materials in this category are alumina and silicon

carbide.

Metal Matrix Composite (MMC) Fabrication

Schematic overview of the approaches employed in the fabrication of MMCs

Metal Matrix Composite (MMC) Technology

Liquid State Fabrication:Stir CastingInfiltration Gas Pressure Infiltration Squeeze Casting Infiltration Squeeze Casting Infiltration Pressure Die Infiltration

These processes are suitable for discontinuous reinforcement ( i.e. Whiskers and particles)

Stir CastingS i C i i li id h d f i i lStir Casting is a liquid state method of composite materials

fabrication, in which a dispersed phase (ceramic particles,short fibers) is mixed with a molten matrix metal by means of) ymechanical stirring.

• A concern is to ensure that good particle wetting occurs.• It is the simplest and the most cost effective method of liquid

state fabrication

•The method using stirringmetal composite materials insemi-solid state is calledsemi solid state is calledRheocasting.High viscosity of the semi-solid

t i t i l bl b ttmatrix material enables bettermixing of the dispersed phase.

Stir Casting Disadvantages

Stir casting difficulties can arise from the increase in viscosity

on adding particles to a melton adding particles to a melt.

Microstructural inhomogeneities can arise, notably particle

agglomeration and sedimentation in the melt.

Redistribution as a result of particle pushing by an advancing

solidification front can also be a problem. This is reduced when

solidification is rapid.p

Interfacial reaction: Stir casting usually involves prolonged

li id i t t hi h b t ti l i t f i lliquid-ceramic contact, which can cause substantial interfacial

reaction.

Stir Casting Disadvantages

Microstructures (Lloyd 1991) of Al-7Si/

20 vol.% SiCpp

a) Stir Casting (slow cooling)

b) Pressure die casting (rapid cooling).b) essu e d e c s g ( p d coo g).

At the slower cooling rate SiC particles

have been pushed into the interdendriticp

regions by the growing dendrites, causing

severe clustering.

Infiltration

Liquid metal is injected into the interstices of anassembly of short fibers or particles, usually called apreform.

• The preform is designed with a specific shape to formi l f fi i h d d i han integral part of a finished product in the as-cast

form.P f l f b i t d b di t ti• Preforms are commonly fabricated by sedimentationof short fibers from liquid suspension.

• The process can also be adapted for production of• The process can also be adapted for production ofparticulate MMCs. In order for the preform to retainits integrity and shape, it is often necessary for aits integrity and shape, it is often necessary for abinder to be used.

Infiltration

The motive force of an infiltration process may be either capillary

force of the dispersed phase (spontaneous infiltration) or anforce of the dispersed phase (spontaneous infiltration) or an

external pressure (gaseous, mechanical, electromagnetic,

t if l lt i ) li d t th li id t i hcentrifugal or ultrasonic) applied to the liquid matrix phase

(forced infiltration).

Pressure less infiltration is less common but it is possible.

This technique is very appropriate for metal matrix compared to This technique is very appropriate for metal matrix compared to

polymer or glass matrix.

InfiltrationG P I filt tiGas Pressure Infiltration is a forced infiltration method of liquid phase

fabrication of MMCs, using a pressurized gas for applying pressure on themolten metal and forcing it to penetrate into a heated preform.

Gas Pressure Infiltration method is used for manufacturing large composite parts. It results in low damage of the fibers. .

SInfiltration

Squeeze Casting Infiltration

SSqueeze Casting Infiltrationis a forced infiltration methodof liquid phase fabrication ofMetal Matrix Composites,using a movable mold part(ram) for applying pressure

th lt t lon the molten metal

The method is used for manufacturing simple small parts (automotiveengine pistons from aluminum alloy reinforced by alumina short fibers).

Pressure Die Infiltration

Pressure Die Infiltration is a forced infiltration method of liquid phase fabrication of Metal

Matrix Composites, using a Die casting technology, when a preformed dispersed phase

(particles, fibers) is placed into a die (mold) which is then filled with a molten metal

Advantages of Infiltration

Low damage of the reinforcement.

Gas infiltration is more suitable than squeeze casting Gas infiltration is more suitable than squeeze casting

for near net shape product.

Reduce subsequent machining to produce near net

shape composites, which is very difficult in some

composites systems.

Plastic deformation processes can used to reducePlastic deformation processes can used to reduce

porosity and improve the mechanical properties.

Solid State fabrication

Solid state fabrication of MMCs is the process, in which MMCs are

formed as a result of bonding matrix metal and reinforced phase due toformed as a result of bonding matrix metal and reinforced phase due to

mutual diffusion occurring between them in solid states at elevated

temperature and under pressure.p p

Low temperature of solid state fabrication process, as compared to liquid

state fabrication of MMCs, depresses undesirable reactions on the

boundary between the matrix and reinforcing phases.

Metal Matrix Composites may be deformed also by sintering operation

followed by rolling, forging, pressing, drawing or extrusion. The

deformation operation may be either cold or hot.

Solid State fabrication

Diffusion Bonding (continuous reinforcement)

Powder Metallurgy (discontinuous reinforcement)

Hot Pressing

Hot Isostatic Pressing

Hot Powder Extrusion

Diffusion BondingDiffusion Bonding is a solid state fabrication method, in which a matrix in form

of foils and a dispersed phase in form of long fibers are stacked in a particular

order and then pressed at elevated temperature.

The finished laminate composite material has a multilayer structure.

Diffusion Bonding is used for fabrication of simple shape parts (plates, tubes).

Hot Pressing (cont. reinforcement)

Hot pressing is a solid state fabrication method, in which a matrix in form of foils

and a reinforcement in form of long fibers. The reinforcement and the matrix

are consolidated with heat and pressure as shown below.

Flowchart of B-Al composite f b i tifabrication

Hot Pressing (cont. reinforcement)

The reinforcement is usually Boron, SiC or metal wire. They held

together with a binder, or woven with metal wire or ribbon.g ,

The matrix is normally in foil form with thickness in the range of

50 150 t k d i ti l d d th d t50-150 μm are stacked in a particular order and then pressed at

elevated temperature.

Typical the fiber spacing are in order of 4-10 per millimeter.

Temperature is raised first to offgas the binder under lightp g g

pressure and vacuum inside the chamber.

Aft th bi d i d th t t d After the binder is removed , the temperature and pressure are

raised to produced metal flow around the fibers.

Hot Pressing (cont. reinforcement)

After the diffusion bonding

occurs between the foils

and the fibers the pressure

is released and the chamberis released and the chamber

is allowed to cool down to

room temperatureroom temperature .

Cross section of filaments and foils

Metal Matrix Composite (MMC) Fabrication

Schematic overview of the approaches employed in the fabrication of MMCs

Powder MetallurgyP/M is a process, in which a powder of a matrix metal is mixed

with a powder of dispersed phase in form of particles or short

fibers for subsequent compacting and sintering in solid state .

Cold compaction is a process that powder materials is compressedCold compaction is a process that powder materials is compressed

in a temperature region where high temperature deformation

mechanics like diffusional creep can be neglected Coldmechanics like diffusional creep can be neglected. Cold

compressing is the most important compaction method in powder

metallurgy. It starts from bulk powders containing very small

amounts of lubricant or binder additions.

Powder Metallurgy

Sintering is a method involving consolidation of powderparticles by heating the “green” compact part to a hightemperature below the melting point, when the materialof the separate particles diffuse to the neighboringpowder particlespowder particles.

In contrast to the liquid state fabrication of MMCs,sintering allows obtaining composites containing up tosintering allows obtaining composites containing up to50% reinforcement.

MixingMixing

Some common equipment geometries for mixing or blending powders:blending powders:

(a) cylindrical, (b) rotating cube, (c) double cone, and (d) twin shell.

CompactionPowder Metallurgy

p

(a) Compaction of metalpowder to form a bushing.The pressed powder part iscalled green compact.(b) Typical tool and die set(b) Typical tool and die setfor compacting a spur gear.

Si iPowder Metallurgy

Sintering Burn off stage High temperature stage (occurs at 0.7 to 0.9 melting

temperature) approximately 1120 °C

Bonding SOLID STATE DIFFUSION LIQUID STATE SINTERING

Cooling stageCooling stage

Powder Metallurgy Sintering can be considered to proceed in three stages During Sintering can be considered to proceed in three stages. During

the first, neck growth proceeds rapidly but powder particlesremain discrete.

During the second, most densification occurs, the structurerecrystallizes and particles diffuse into each other.

D i th thi d i l t d t d t b h i l d During the third, isolated pores tend to become spherical anddensification continues at a much lower rate.

The words Solid State in Solid State Sintering simply refer to theg p ystate the material is in when it bonds, solid meaning the materialwas not turned molten to bond together as alloys are formed.

Hot and Cold Isostatic PressingPowder Metallurgy

gSchematic diagram of cold isostatic pressing, asapplied to forming a tube. The powder is enclosed in aflexible container around a solid core rod. Pressure isapplied isostatically to the assembly inside a high-pressure chamber.

Schematic illustration of hot isostatic pressing. The pressure and temperature variation vs. time are shown in the diagram.

Hot Isostatic PressingHot Isostatic Pressing of MMCs – sintering under a pressureapplied from multiple directions through a liquid or gaseousmedium surrounding the compacted part and at elevatedmedium surrounding the compacted part and at elevatedtemperature.

Hot Pressing

Hot Pressing of MMCs – sinteringunder a unidirectional pressure appliedby a hot press.y p

Hot pressing lowers the pressuresrequired to reduce porosity and speedswelding and grain deformationprocesses.

Al i iAlso, it permits;♦ Better dimensional control of theproduct,♦ Lessened sensitivity to physical♦ Lessened sensitivity to physicalcharacteristics of starting materials.♦Allows powder to be driven tohigher densities than with coldhigher densities than with coldpressing, resulting in higher strength.

Hot Powder Extrusion

Hot Powder Extrusion Fabrication of Metal Matrix Composites – sinteringunder a pressure applied by an extruder at elevated temperature.

Optical microscope photographs of copper-matrix titanium diboride plateletcomposites fabricated by powder metallurgy. a 15 vol% TiB2, coated filler method;p y p gy , ;b 15 vol% TiB2, admixture method; c 60 vol% TiB2, coated filler method; d 60vol% TiB2, admixture method.

In-situ fabrication of Metal Matrix Composites

In situ fabrication of Metal Matrix Composite is a process in which dispersedIn situ fabrication of Metal Matrix Composite is a process, in which dispersed(reinforcing) phase is formed in the matrix as a result of precipitation from themelt during its cooling and Solidification.

In-situ fabrication of Metal Matrix Composites

Outstanding high temperature properties can be

achieved with directionally solidified eutectics,

natural composites consisting of high strength fibersnatural composites consisting of high strength fibers

in a ductile matrix. Eutectics, are thermodynamically

in equilibrium even at melting point.

This technique offer unique solution to the problemThis technique offer unique solution to the problem

of fiber/matrix incompatibility.