Processing of Metal Powders and
Processing of Ceramics and Glass
Group 10
Kevin Burns
Jared Adams
Chris Chaves
Drew Smith
James Colovos
March 6, 2006
Topics
Processing of Metal Powders
• Production of Metal Powders
• Compaction of Metal Powders
• Sintering
• Secondary and Finishing Operations
• Design Considerations
• Process Capabilities
• Economics of Powder Metallurgy
Processing of Ceramics, Glass, and Superconductors
• Shaping Ceramics
• Forming and Shaping of Glass
• Techniques for Strengthening and Annealing glass
• Design Considerations for Ceramics and Glasses
• Processing of Superconductors
Processing of Metal Powders
• Powder Metallurgy Process (P/M process)
– The process where metal powders are
compacted into desired and often complex
shapes and sintered to form a solid piece
• Process was first used five thousand years
ago by Egyptians to make iron tools
• Net-shape Forming
– The ability to produce parts to net dimensions
Parts and Components Made with
the P/M Process
• Balls for ballpoint pens
• Automotive components, makes
up 70% of P/M process (ex.
piston rings, connecting rods,
brake pads, gears, cams,
bushings)
• Tool steels, tungsten carbides
• Graphite brushes inserted with
copper for electric motors
• Magnetic materials
• Metal filters and oilimpregnated bearings with
controlled porosity
• Metal foams
• Surgical implants
• Other items used for
aerospace, nuclear and
industrial applications
• Advances in the P/M
process permit structural
parts of aircraft (ex.
landing gear components,
engine-mount supports,
engine disks, impellers,
engine frames
• The P/M process has
become competitive for
complex parts made of
high strength and hard
alloys with processes such
as casting, forging, and
machining)
• Common metals used in
the P/M process
– Iron
– Copper
– Aluminum
– Tin
– Nickel
– Titanium
– Refractory metals
Production of Metal Powders
• The powder metallurgy process
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Powder production
Blending
Compaction
Sintering
Finishing operations
Powder Production
• First step in P/M process
• Methods
– Atomization
– Reduction
– Electrolytic deposition
– Carbonyls
– Comminution
– Mechanical alloying
– Miscellaneous methods
Atomization
• Produces a liquid-metal stream by injecting molten metal
through a small orifice
• The stream is broken up by jets of inert gas or air
• The size and shape of the
particles from atomization
depend on the
temperature, flow rate,
size of nozzle, and the jet
characteristics
• When water is used it
creates a slurry metal
powder and leaves a liquid
at the bottom of the
atomization chamber
• The water cools the metal
faster for a higher
production rates
Centrifugal Atomization
• The process in which the molten-metal drops onto a
rapidly rotating disk or cup
• The centrifugal forces break up the molten-metal stream to
generate particles
• Another method is that a consumable electrode is rotating
rapidly in a helium filled chamber
Reduction of Metal Oxides
• A process that uses gases
as a reducing agent
– Hydrogen and carbon
monoxide
• Also known as the
removal of oxygen
• Very fine metallic oxides
are reduced to the metallic
state
• Spongy and porous
powders are produced
Electrolytic Deposition and Carbonyls
• Electrolytic Deposition
utilizes either aqueous
solutions or fused salts
• Makes the purest powders
that are available
• Metal carbonyls are
formed by letting iron
or nickel react with
carbon monoxide
• Reaction product is
decomposed to iron
and nickel
• Forms small, dense,
uniform spherical
particles
Mechanical Comminution
• Also known as pulverization
• Involves roll crushing, milling
in a ball mill, or grinding of
brittle or less ductile metals into
small particles
• Brittle materials have angular
shapes
• Ductile metals are flaky and
not particularly suitable for P/M
Mechanical Alloying
• Powders of two or more pure metals are
mixed in a ball mill
• Under the impact of the hard balls the
powders fracture and bond together by
diffusion, forming alloy powders
• The dispersed phase can result in
strengthening of the particles or can impart
special electrical or magnetic properties
Miscellaneous Methods
• Precipitation from a
chemical solution
• Production of fine metal
chips by machining
• Vapor condensation
Types of Powders
• Nanopowders
– Consist of mostly copper,
aluminum, iron, titanium
– Are pyrophoric (ignite
spontaneously)
– Contaminated when exposed to
air
– The particle size is reduced
and becomes porous free when
subjected to large plastic
deformation by compression
and shear stress
– Posses enhanced properties
• Microencapsulated powders
– Coated completely with a
binder
– The binder acts as an insulator
for electrical applications
preventing electricity from
flowing between particles
– Compacted by warm pressing
– The binder is still in place
when used
Particle Size, Shape, and
Distribution
• Particle size is measured by a process called
screening
• Screening is the passing of metal powder through
screens of various mesh sizes
• The main process of screening is Screen Analysis
• Screen analysis uses a vertical stack of screens
with mesh size becoming finer as the powder
flows down through screens
Other Screening Methods
• Sedimentation
– Involves measuring the rate at
which particles settle in a fluid
• Microscopic Analysis
– Includes the use of
transmission and scanning
electron microscopy
• Optical
– Particles block a beam of light
and then sensed by a photocell
• Light Scattering
– A laser that illuminates a
sample consisting of particles
suspended in a liquid medium
– The particles cause the light to
be scattered, and a detector
then digitizes and computes the
particle-size distribution
• Suspending Particles
– Particles suspended in a liquid
and then detected by electrical
sensors
Particle Shape and Shape Factor
• Major influence on
processing characteristics
• Usually described by
aspect ratio and shape
factor
• Aspect ratio is the ratio of
the largest dimension to
the smallest dimension
• Ratio ranges from unity
(spherical) to 10 (flakelike, needle-like
• Shape factor (SF) is also
called the shape index
• Is a measure of the ratio of
the surface area to its
volume
• The volume is normalized
by a spherical particle of
equivalent volume
• The shape factor for a
flake is higher than it is
for a sphere
Size Distribution and Other
Properties
• Size distribution is important because it affects the processing
characteristics of the powder
• Flow properties, compressibility and density are other properties that
have an affect on metal powders behavior in processing them
• Flow
– When metal powders are being filled into dies
• Compressibility
– When metal powders are being compressed
• Density
– Theoretical density, apparent density, and the density when the
powder is shaken or tapped in the die cavity
Blending Metal Powders
• Blending (mixing) is the next step in P/M process
• Must be carried out under controlled conditions to
avoid contamination or deterioration
• Deterioration is caused my excessive mixing and
causes the shape to be altered or the particles
harden causing the compaction process to be
difficult
• Is done for several significant reasons
Reasons for Blending
• To impart special physical and mechanical properties and
characteristics
• Proper mixing is essential to ensure the uniformity of mechanical
properties throughout the part
• Even one metal can have powder vary in size and shape
• The ideal mix is one in which all of the particles of each material are
distributed uniformly
• Lubricants can be mixed with the powders to improve flow of metal
powder into dies, reduce friction between metal particles, and improve
the die life
• Binders are used to develop sufficient green strength
• Other additives can be used to facilitate sintering
Hazards
• Metal powders are explosive because of the high surface
area-to-volume ratio (mostly aluminum, magnesium,
titanium, zirconium, and thorium
• Most be blended, stored, handled with great care
• Precautions
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Grounding equipment
Preventing sparks
Avoiding friction as a source of heat
Avoiding dust clouds
Avoiding open flames
Avoiding chemical reactions
Compaction of Metal Powders
• The third step in the P/M process which the
blended powders are pressed into various shapes
in dies
• The purpose of compaction is to obtain the
required shape, density, and particle-to-particle
contact and to make the part sufficiently strong for
further processing
• Green compact is known as pressed powder and is
very fragile and can be crumbled like chalk
• The density of a green
compact depends on the
pressure applied
• Important factor in
density is the size
distribution of the
particles
• If all particles are the
same size then there will
always be porosity (ex.
box filled tennis balls
will always have space in
between them)
• The higher the
density, the higher
the strength and
elastic modulus
• The higher the
density, the higher
the amount of solid
metal in the same
volume and then the
higher the strength
Equipment
• The pressure required for
pressing metal powders
ranges from 70 MPa (10
ksi) to 800 MPa (120 ksi)
• The compacting pressure
required depends on the
characteristics and shape
of the particles, on the
method of blending, and
on the lubricant
Presses
• Press capacities ate on the
order of 200 to 300 tons
• Most projects require less
than 100 tons
• Small tonnage, crank- or
eccentric-type mechanical
presses are used
• For higher capacities,
toggle or knucklejoint
presses are employed
• Hydraulic presses can
have capacities up to
5,000 tons and are used
for large parts
• The type of press selected
depends on part size and
its configuration, density
requirements, and
production rate
Isostatic Pressing
Cold Isostatic Pressing
• Mold made of elastomer (neoprene rubber, urethane,
polyvinyl chloride)
• Commonly pressurized at 400 MPa, up to 1000 MPa
• Ex. Automotive cylinder liners
Hot Isostatic Pressing (HIP)
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High-melting-point sheet metal
High temp inert gas or vitreous fluid
Pressures as high as 100 MPa
Temperatures of 1200˚C (2200˚F)
Used for making high-quality parts
• Ex. valve lifter
Hot Isostatic Pressing (HIP)
• Advantages:
• Disadvantages:
• 100% density
• Good metallurgical
bonding of the particles
• Good mechanical
properties
• Compacts of uniform
grain structure and density
• Wider dimensional
tolerances
• Higher equipment cost
and production time
• Small production
quantities
Powder-injection molding (PIM)
• Metals melting above 1000˚C (1830˚F)
(carbon, stainless steels, copper, bronze, titanium)
• Ex. Watches, parts for guns, door hinges surgical knives
• Advantages
• Complex shapes
• Dimensional tolerances good
• High production rates
Disadvantage: high cost and limited availability of fine metal powders
PIM Process
Spray Deposition
• Shape-generation process
• Used to produce seamless
tubing and piping
• Produces 99% solid metal
density
• Osprey Process
Other Compacting and shaping processes
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•
•
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Powder rolling (roll compaction)
Extrusion
Pressureless compaction
Ceramic molds
Punch and Die materials
• Depends on abrasiveness of the powder metal and the
number of parts being produced
• Air- or oil-hardened tool steels
• Hardness range from 60 to 64 HRC
• Tungsten-carbide dies used for more severe applications
• Control of die and punch dimensions
• Die and punch surfaces lapped and polished
Sintering
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Green compacts
Temperature within 70-90% of melting point
Sintering time from 10 minutes to 8 hours
Furnace atmosphere (hydrogen, burned ammonia, partially
combusted hydrocarbon gases, nitrogen)
Diffusion mechanism
Vapor-phase transport
Liquid-phase sintering
Spark sintering
Sintering metal powders, sintering products,
sintering furnace
Secondary and finishing operations
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Coining and sizing
Impact forging
Machining
Grinding
Plating
Heat treating
Impregnating
Infiltration
17.6 Design Considerations
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Keep the shape simple (Avoid thin sections,
variations in thickness, and high length-to-diameter
ratios)
P/M parts should be made with the widest
acceptable tolerances
Parts should not be less than 1.5 mm thick
Letters can be pressed if oriented perpendicular to
the direction of the pressing and can be raised or
recessed
A radius cannot be pressed into an edge of a part
because it would require the punch to be feathered
to a zero thickness
17.6 (Cont.)
• Notches and grooves can
be made if they are
perpendicular to pressing
• Dimensional tolerances of
sintered P/M parts are
usually on the order of +.05 to .1 mm
To the right: Examples of P/M parts
showing poor and good designs.
17.7 Process Capabilities of P/M
Capabilities
• It is a technique for making parts from high-meltingpoint metals
• High production on relatively complex parts with less
labor
• P/M reduces scrap and waste, while eliminating
machining and finishing
• Wide range of compositions makes it possible to
obtain special mechanical and physical properties
(stiffness, vibration damping, hardness, density,
toughness, and magnetic properties)
17.7 (Cont.)
Limitations of P/M
• High cost of metal powder
• High cost for tooling and equipment for small
production runs
• Limitations on part size and shape
• Mechanical properties such as strength and
ductility are lower than by forging.
17.8 Economics of P/M
• P/M can produce parts neat net-shape, eliminating
secondary manufacturing and assembly operations.
• Because of initial costs of punches, dies, and
equipment; production of quantities of over 10,000
pieces are economical.
• Tooling costs for HIP and powder injection molding
are higher than powder processing (because its nearnet-shape manufacturing method, the cost of finishing
operations in P/M are low compared to casting and
forging.
18.2 Shaping of Ceramics
1- First, the raw materials must be ground or crushed down into fine
particles.
2- Next, the particles must be mixed with additives, which include:
binder- to hold particles together
lubricant- to reduce friction and aid in removing from mold
wetting agent- to improve mixing process (commonly water)
plasticizer- to improve ease of forming mixture
agents- control of foaming and sintering
deflocculent- to create uniform mixture by applying like
charges to all particles, causing them to repel
each other
3- Finally, the material must be shaped, dried, and fired.
Crushing
(a.k.a. comminution or milling)
Crushing is typically done in a ball mill, in either wet or dry conditions.
Wet milling is preferred because it strengthens particle bonds and
limits dust.
For correct sizing, the crushed particles are passed through a
sieve.
Mixing
Particles are then mixed with one of the additives listed and described on the
previous slide.
Casting
Slip Casting (Drain Casting)-The crushed particles are first mixed with
water, then are poured into a mold.
Pouring must be done properly to avoid air
pockets.
When some of the water has been absorbed, the
remainder of the mixture is poured out of
the top of the mold.
The top of the part can then be trimmed.
Advantages- inexpensive components
Disadvantages- limited control of dimensions & low production rate
Doctor-Blade Process- Used to produce ceramic sheets thinner than 1.5mm.
Ceramic mixture is forced under a blade to create a film, which is then dried in a
drying chamber (usually attached to the same machine).
Plastic Forming
Primary method of plastic forming is extrusion.
Extrusion- Ceramic particles mixed into a solution with 20-30% water.
Then mixture is pushed through a small die opening by a “screwtype piece of equipment.”
Advantages- low cost, high production
Disadvantages- wall thickness limited
Pressing
Dry Pressing- High pressure applied to ceramic particles with a moisture
content below 4%, causing compaction.
Require dies made of hardened steel and highly resistant to
wear, making them very expensive.
Friction causes large variation in density throughout mixture.
Wet Pressing- Part formed in mold under high pressure from a press.
Moisture content in mixture is typically 10-15%.
High Production, but high cost and limited dimensional control.
Isostatic Pressing- Used primarily to attain a uniform density in a part.
Accomplished by the application of inert gases before pressing.
Pressing (continued)
Jiggering- Similar to process of making clay pottery.
Ceramic particles are mixed with water, and then formed while
spinning.
Only for axisymetric parts, and little dimensional control.
Injection Molding- Used mostly in high cost operations where precision is
absolutely necessary.
Ceramic particles are mixed with a binder, which is then burned out.
Sections are usually less than 15mm thick, because anything thicker
tends to have internal cracks and voids.
Hot Pressing (Pressure Sintering)- Pressure and heat are applied at the same
time.
Combination reduces porosity of the part, which increases its
overall strength and density.
Drying and Firing
Variations in moisture content and thickness cause parts to crack while drying.
Moisture loss while drying can result in a size decrease of 15-20%.
“Green state” describes the state a part is in after it has been dried and before it is
fired because its softness makes it especially easy to machine.
Firing results in less (but still existent) shrinkage than drying.
Strength and hardness of ceramics come from firing due to a bond formed between
the oxide particles and reduced porosity.
Nanophase Ceramics- Fired at lower temperatures than regular ceramics.
Easier to fabricate due to the lower required temperature.
Finishing Operations
-Grinding
-Lapping and honing
-Ultrasonic machining
-Drilling
-Electrical-discharge machining
-Laser-beam machining
-Abrasive water-jet cutting
-Tumbling
Glazing- Applying a glaze or enamel to the ceramic before firing improves
both the final appearance and strength.
Forming and Shaping of Glass
Flat Sheet and Plate Glass
Float Method- Molten glass is floated over
a “bath” of molten tin before it is solidified
in a separate chamber.
No additional finishing is necessary.
Drawing Process- Molten glass is
squeezed through two rolls, then
moves on o two smaller rolls.
Rolling Process- Similar to drawing
process, but patterns are commonly
imprinted from the rolls onto the glass,
leaving a rough finish.
Molten Glass
Glass Tubing and Rods
Tubing- Molten glass is wrapped around a mandrel and taken out by two rolls.
Air is blown through the mandrel to prevent the tube from collapsing
into itself.
Some machines manufacture 2000 fluorescent light bulbs per minute
using this method.
Rods-
Rods are made in exactly the same way,
but without the air blown through
the mandrel.
This allows the glass to collapse and
become solid.
Discrete glass products
Processes used to make discrete glass objects
•
Blowing
•
Pressing
•
Centrifugal casting
•
Sagging
Blowing
• Blowing process: Blown air
expands a hollow gob of
heated glass against the inner
walls of a mold.
• A parting agent (such as oil
or emulsion) is usually used
to prevent the glass from
sticking to the mold.
Blowing
• Blow and blow
process: After
blowing a
second blowing
operation can be
used for
finalizing
product shape.
Blowing
• Applications:
Hollow and thinwalled glass items
(bottles, vases, and
flasks)
• Surface finish:
Acceptable for
most applications
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Pros and cons of blowing
• Pros: Very economical for
high-rate production.
Example: Highly-automated
blowing machines can make around
2000 incandescent light bulbs per
minute.
• Cons: Difficult to control the
wall thickness of the product
Pressing
• Pressing process: A gob of molten glass is placed
into a mold and pressed by a plunger into a
confined shape.
• Molds may be one piece or split. Solidifying glass
acquires the shape of the mold-plunger cavity.
• Similar to closed-die forging.
Pressing
• One-piece molds cannot be used in pressing if the plunger
cannot be retracted.
• One-piece molds cannot be used for thin-walled items
• Split molds can accommodate thin-walled products
Pressing
• Pressing can produce higher dimensional
accuracy than blowing.
Press and blow process
• After a part is pressed, it is blown to further
expand the glass into the mold.
Centrifugal Casting (Spinning)
• Centrifugal casting
process: The centrifugal
force pushes the molten
glass against the wall.
• TV picture tubes and
missile nose cones can be
made with centrifugal
casting.
Sagging
• Sagging process: A sheet of glass is placed
over a mold and heated. The glass sags by
its own weight and takes the shape of the
mold.
• Typical applications include dishes,
sunglass lenses, mirrors for telescopes, and
lighting panels.
Glass ceramics manufacture
• Trade names: Pyroceram, Corningware
• Contain large proportions of several oxides.
• Manufacturing involves a combination of
methods used for ceramics and glasses.
• Shaped into discrete products (such as
dishes and baking pans) then heat treated.
• After heat treating glass is devitrified
(recrystallized).
Glass Fibers
• Continuous glass fibers are drawn through
multiple orifices (200 to 400 holes) in heated
platinum plates at speeds as high as 500 m/s
(1700ft/s).
• Fiber diameters as small as 2m (80in.)
• Coated with chemicals to protect fiber surface.
• Short fibers (chopped) are made as compressed air
or steam passes the fiber as it passes through the
orifice.
Glass Fibers - Glass wool
• Glass wool is short glass fibers.
• Glass wool is used for thermal and acoustic
insulation.
• Made by a centrifugal spraying process.
Molten glass is ejected (spun) from a
rotating head.
• Glass wool fiber diameter is typically 20 to
30 m (800 to 1200 in.)
Techniques for Strengthening and
Annealing Glass
• Glass can be strengthened by thermal
tempering, chemical tempering, and
laminate strengthening.
• Finishing operations can be used to impart
desired properties and surface
characteristics.
Thermal Tempering
• Surfaces of the hot glass are cooled rapidly by a
blast of air. The surfaces solidify and are forced to
contract as the bulk of the glass begins to cool.
• Surfaces develop residual compressive stresses.
• The interior develops tensile stresses.
• Compressive surface stresses improve the strength
of the glass.
Thermal Tempering
Chemical Tempering
• The glass is heated in a bath of molten
KNO3, K2SO4, or NaNO3, depending on the
type of glass.
• Ion exchanges take place and larger atoms
replace smaller atoms on the surface of the
glass.
• Residual compressive stresses develop on
the surface.
Laminated Glass
• Glass is strengthened
through a method called
laminate strengthening.
• Two pieces of flat glass have
a thin sheet of tough plastic
in between.
• When the glass is cracked,
its pieces are held together
by the plastic sheet.
Bulletproof Glass
• Bulletproof glass basically consists of glass
laminated with a polymer sheet.
• Thickness ranges from 7 to 75 mm (.3 to 3
in.) Thinner glass is for handguns and the
thicker glass is for rifles.
Finishing Operations
• Annealing removes residual stresses by heating
the glass to a certain temperature and then cooling
gradually.
• Annealing time ranges from a few seconds to 10
months.
• Glass products may be cut, drilled, ground, and
polished.
• Care should be exercised in all finishing
operations to ensure there is no surface damage.
Design Considerations for
Ceramics and Glasses
• Ceramic and glass products require careful
selection of composition, processing
methods, finishing operations, and methods
of assembly with other components.
References
• http:// www.designinsite.dk/gifs/pb1007.jpg
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www.cullenconsulting.com.au/ epsi/images/
www.scielo.br/.../ jbsmse/v26n1/a07fig03.gif
www.turkcadcam.net
www.esrf.fr/.../2002/ Materials/MAT3/fig081
www.mrf-furnaces.com/ images/4station.jpg
met.iisc.ernet.in/ ~govind/Spray-forming.jpg