Transcript PROCESSING OF POWDER METALS,CERAMICS,GLASS AND SUPERCONDUCTORS

Processing of Powder Metals,
Ceramics, Glass & Superconductors
Powder Metals
• Commonly used metals in P/M
– Iron,Tin, Copper, Aluminum, and Nickel
• It is a completive process with forging and
machining
• Parts can weigh as much as little as 2.5Kg or up
to 50Kg
Powder Metallurgy
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c
Fig: (a)Examples of typical parts made by
powder-metallurgy processes. (b) Upper
trip lever for a commercial irrigation
sprinkler, made by P/M. This part is
made of unleaded brass alloy; it replaces
a die-cast part, with a 60% savings. (c)
Main-bearing powder metal caps for 3.8
and 3.1 liter General Motors automotive
engines.
Production of Metal Powders
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Powder production
Blending
Compaction
Sintering
Finishing operations
Particle Size, Distribution, and shape
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Particle size is measured by
screening
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In addition to screen analysis one
can use:
– Sedimentation – measuring the
rate that particles settle in a
fluid
– Microscopic analysis – using a
scanning electron microscope
– Light scattering
– Optical – particles blocking a
beam of light that is sensed by a
photocell
– Suspending particles in a liquid
& detecting particle size and
distribution
Fig: Particle shapes in metal powders,and the
processes by which they are produced.Iron
powders are produced by many of these
processes
Powder Particles
Fig : (a) Scanning electron-microscopy photograph of iron-powder particles made by atomization.
(b) Nickel-based superalloy powder particles made by the rotating electrode process.
Methods of Powder Production
Fig : Methods of metal-powder
production by atomization;(a) melt
atomization; (b) atomization with a
rotating consumable electrode
Fig: Methods of mechanical
communication, to obtain fine particles: (a)
roll crushing, (b) ball mill, & (c) hammer
milling
Blending Powders
• Blending powders is the second
step in the P/M process
• Powders made by different
processes have different sizes
and shapes and must be well
mixed
• Powders of different metals can
be mixed together
• Lubricants can be mixed with
the
powders to improve their flow
characteristics
Fig: Some common equipment geometries
for mixing or blending powders. (a)
cylindrical, (b) rotating cube, (c) double
cone, and (d) twin shell.
Compaction of Metal Powders
• Blended powders are pressed
together
• The powder must flow easily into
the die
• Size distribution is an important fact
– They should not be all the same
size
– Should be a mixture of large and
small
particles
• The higher the density the higher
the strength
Fig: Compaction of metal powder to form a
bushing.The pressed powder part is called green
compact. (b) Typical tool and die set for
compacting a spur gear
Equipment
• Uses 100-300 ton press
• Selection of the press
depends on the part and
the configuration of the
part
Fig: A7.3 MN (825 ton) mechanical press for
compacting metal powder.
Isostatic Pressing
• Cold isostatic Pressing (CIP)
– Metal powder is placed in a
flexible
rubber mold
– Pressurized hydrostatically
– Uses pressures up to 150
KSI
– Typical application is
automotive
cylinder liners
Fig: Schematic diagram, of cold isostatic, as
applied to forming a tube.The powder is
enclosed in a flexible container around a solid
core rod.Pressure is applied iso-statically to
the assembly inside a high-pressure chamber.
Isostatic Pressing
• Hot Isostatic pressing
– Container is made of highmelting-point sheet metal
– Uses a inert gas as the
pressurizing medium
– Common conditions for
HIP are 15KSI at 2000F
– Mainly used for super
alloy casting
Fig: Schematic illustration of hot isostatic
pressing.The pressure and temperature
variation vs.time are shown in the diagram
Punch and Die Materials
• Depends on the abrasiveness of the powder
metal
• Tungsten-carbide dies are used
• Punches are generally made of the similar
materials
• Dimensions are watched very close
Metal Injection Molding
• MIM uses very fine metal powders blended with
a polymer
• The molded greens are then placed in a furnace
to burn off the plastics
• Advantages of injection molding
– Produces complex shapes
– Mechanical properties are nearly equal to
those of wrought products
Other Shaping Processes
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Rolling – powder is fed though the roll gap and is used to make coins
and sheet metal
An example of powder rolling
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Extrusion – has improved properties and parts my be forged in a closed
die to get final shape
Pressureless compaction – gravity filled die and used to make porous
parts
Ceramic molds – molds are made by made by investment casting and
the powder is compressed by hot isostatic pressing
Spray deposition – shape-generation process
Sintering
• Sintering - Green compacts are heated in a furnace to a temperature
below melting point
• Improves the strength of the material
• Proper furnace control is important for optimum properties
Fig: Schematic illustration of two
mechanism for sintering metal
powders: (a) solid-state material
transport; (b) liquid-phase
material transport.R= particle
radius, r=neck radius, and
p=neck profile radius
Sintering
• Particles start forming a bond by diffusion
• Vapor-phase transport – heated very close to melting
temperature allows metal atoms to release to the vapor phase
Mechanical Properties
Secondary & Finishing Operations
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To improve the properties of
sintered P/M products several
additional operations may be
used:
– Coining and sizing –
compaction operations
– Impact forging – cold or hot
forging may be used
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Parts may be impregnated with
a fluid to reduce the porosity
Fig: Examples of P/M parts,showing poor designs and
good ones.Notes that sharp radii and re entry
corners should be avoided and that threads and
transverse holes have to be produced separately by
additional machining operations.
Secondary & Finishing Operations
• Infiltration – metal infiltrates the pores of a
sintered part to produce a stronger part and
produces a pore free part
• Other finishing operations
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Heat treating
Machining
Grinding
Plating
Design Considerations for P/M
• Design principles to consider
– Shape of the compact must be simple and
uniform
– Provision must be made for the ejection of the
part
– Wide tolerances should be used when ever
possible
Process Capabilities
• It is a technique for making parts from high
melting point refractory metals
• High production rates
• Good dimensional control
• Wide range of compositions for obtaining special
mechanical and physical properties
Process Capabilities
• Limitations
– High cost
– Tooling cost for short production runs
– Limitations on part size and shape
– Mechanical properties of the part
• Strength
• Ductility
Economics of Powder Metallurgy
• Competitive with casting and forging
• High initial cost
• Economical for quantities over 10,000 pieces
• Reduces or eliminates scraps
Shaping Ceramics
Processing ceramics
– Crushing or grinding the raw materials in to very fine particles
– Mixing with additives
– Shaping, drying , and firing the material
SLIP CASTING
Processing steps involved in making ceramic parts