Powder Processing Flashcards
What is Powder Processing also known as?
Powder Metallurgy
Can be applied to a variety of materials including ceramics
Outline the basic concept of Powder Processing
COMPACTION- Powder is formed under pressure to make agreen part
SINTERING the green part is then heated (not melted) to allow the powder particles to stick together
What are the stages of the Powder Metallurgy Process?
– powder production
– blending
– compaction
– sintering
– finishing
What are the different powder production routes?
Atomisation
Chemical Reduction
Electrolysis
Outline Atomisation
Most common powder production route
Molten material is forced through a small orifice to produce a thin stream of fluid material
The stream is then hit by a jet of water or air (gas or water atomisation)
The stream disintegrates, and the droplets cool to form powder particles
– Particle size controlled by jet velocity
– Powders are often spherical in geometry
Chemical Reduction
Metallic compounds are reduced to metal powders
Use of reducing agents such as hydrogen or carbon monoxide
Frequently used in the production of iron powders
Electrolysis
Source of metal is anode in electrolytic cell or uses
aqueous solution
Metal powder of high purity deposited on cathode
Explain the geometric characteristics of metal powders
• Post production, powders are often screened for several characteristics:
Size (assessed via screening, microscopy or optical
methods)
Shape – defined by aspect ratio and shape factor
Size distribution – defined by frequency plot
Outline Powder Blending
Blending of powders is carried out either:
– to produce a mixture of two materials if an alloy
is to be produced
– to add lubricants or binders to improve the
powder processing properties
– to generate a uniform powder size distribution
throughout the batch
• Usually the powder production and blending will be
carried out by a specialist supplier, and the powder
processing company will buy powder off the shelf
Outline Compacting (diagram)
Compresses the powder within a die
The density of the green part depends on the pressure applied (which must be uniform)
The strength and modulus of the final product are
density dependent
Outline Sintering (diagrams)
The green part is heated in a furnace at a
temperature below the melting temperature of at
least one of the materials, but high enough to
allow the metal particles to bond together
Normally the atmosphere is tightly controlled to
ensure that there is no oxidation (vacuum, H2, N2,
Ar atmosphere)
Where only one material is present, bonding
between adjacent powder particles may occur by
diffusion (solid state sintering)
Liquid phase sintering normally occurs where two
metals are involved, one metal will melt and form a
“neck” between two solid powder particles
The sintering time will normally be less than an
hour for metals (can be much longer for ceramics)
Usually takes place at absolute temperature 70-
90% of the melting point
Some shrinkage and some residual porosity usually occur
Finishing
Three common finishing operations:
Coining or sizing to increase the density of the
component - improves accuracy or adds surface detail
Infiltration with a lower melting point metal to
remove porosity - Gives more uniform density and improved toughness
Impregnation with oil for lubrication - For self-lubricating bearings (automotive industry)
How do different powder properties effect the performance?
Particle size and size distribution - affects packing
and how well the powder “flows”
Particle shape - affects how well the powder flows,
the surface area available in sintering, and packing
Minimum wall thickness of compact about 2 mm
Fillets and draft angles necessary to make compact
removal easy
Large overhangs can cause problems in the green
stage when compact is weak
What are the Advantages of Powder Processing?
Good surface finish and close tolerances
Net shape or near net shape so little subsequent
machining and high material utilisation
Can be used with materials which can’t be easily
processed any other way e.g. ceramics
What are the disadvantages of Powder Processing?
Powder and processing equipment expensive
Tooling can be expensive (depends on powder
properties)
Porosity means inferior mechanical properties
