Twenty One

POWDER METALLURGY
INTRODUCTION
• Earliest use of iron powder dates back to 3000 BC.

Egyptians used it for making tools
• Modern era of P/M began when W lamp filaments were
developed by Edison
• Components can be made from pure metals, alloys, or
mixture of metallic and non-metallic powders
• Commonly used materials are iron, copper, aluminium,
nickel, titanium, brass, bronze, steels and refractory
metals
• Used widely for manufacturing gears, cams, bushings,
cutting tools, piston rings, connecting rods, impellers etc.
PROCESS
• Powder production
• Blending
• Compaction
• Sintering
• Finishing Operations
POWDER PRODUCTION
Powder size: 0.1 to 1000 μm

Atomization
• Produce a liquid-metal
stream by injecting
molten metal through a
small orifice
• Stream is broken by jets
of inert gas, air, or water
• The size of the particle
formed depends on the
temperature of the metal,
metal flowrate through
the orifice, nozzle size
and jet characteristics
Variation:
• A consumable electrode is
rotated rapidly in a helium-
filled chamber. The
centrifugal force breaks up
the molten tip of the
electrode into metal
particles.
Fe powders made by atomization Ni-based superalloy made by
the rotating electrode process
Reduction
• Reduce metal oxides with H2/CO
• Powders are spongy and porous and they have uniformly
sized spherical or angular shapes
Electrolytic deposition
• Metal powder deposits at the cathode from aqueous
solution
• Powders are among the purest available
Carbonyls
• React high purity Fe or Ni with CO to form gaseous
carbonyls
• Carbonyl decomposes to Fe and Ni
• Small, dense, uniformly spherical powders of high purity
Comminution
• Crushing
• Milling in a ball mill
• Powder produced
– Brittle: Angular
– Ductile: flaky and not particularly suitable for P/M
operations
Mechanical Alloying
• Powders of two or more metals are mixed in a ball mill
• Under the impact of hard balls, powders fracture and join
together by diffusion
(a) Roll crusher, (b) Ball mill
BLENDING
• To make a homogeneous mass with uniform distribution
of particle size and composition
– Powders made by different processes have different
sizes and shapes
– Mixing powders of different metals/materials
– Add lubricants (<5%), such as graphite and stearic
acid, to improve the flow characteristics and
compressibility of mixtures
• Combining is generally carried out in
– Air or inert gases to avoid oxidation
– Liquids for better mixing, elimination of dusts and reduced
explosion hazards
• Hazards
– Metal powders, because of high surface area to volume ratio are
explosive, particularly Al, Mg, Ti, Zr, Th
Some common equipment geometries used for blending powders
(a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell
COMPACTION
• Press powder into the desired shape and size in dies

using a hydraulic or mechanical press
• Pressed powder is known as “green compact”
• Stages of metal powder compaction:
• Increased compaction pressure
– Provides better packing of particles and leads
to ↓ porosity
– ↑ localized deformation allowing new contacts
to be formed between particles
• At higher pressures, the green density approaches
density of the bulk metal
• Pressed density greater than 90% of the bulk density is
difficult to obtain
• Compaction pressure used depends on desired density
• Smaller particles provide greater strength mainly due to
reduction in porosity
• Size distribution of particles is very important. For same
size particles minimum porosity of 24% will always be
there
– Box filled with tennis balls will always have open space between
balls
– Introduction of finer particles will fill voids and result in↑ density
• Because of friction between (i) the metal particles and (ii)
between the punches and the die, the density within the
compact may vary considerably
• Density variation can be minimized by proper punch and
die design
(a) and (c) Single action press; (b) and (d) Double action press
(e) Pressure contours in compacted copper powder in single action press
Compaction pressure of some metal powders
Metal Powder Pressure (MPa)
Al 75-275
Al2O3 100-150
Brass 400-700
Carbon 140-170
Fe 400-800
W 75-150
WC 150-400
(a) Compaction of metal powder to form bushing
(b) Typical tool and die set for compacting spur gear
A 825 ton mechanical press for compacting metal powder
Cold Isostatic Pressing
• Metal powder placed
in a flexible rubber
mold
• Assembly pressurized
hydrostatically by
water (400 – 1000
MPa)
• Typical: Automotive
cylinder liners →
• FFT: Advantages?
SINTERING
• Green compact obtained after compaction is brittle and
low in strength
• Green compacts are heated in a controlled-atmosphere
furnace to allow packed metal powders to bond together
Carried out in three stages:
• First stage: Temperature is slowly increased so that all

volatile materials in the green compact that would
interfere with good bonding is removed
– Rapid heating in this stage may entrap gases and
produce high internal pressure which may fracture
the compact
Second stage: High temperature stage
• Promotes solid-state
bonding by diffusion.
• Diffusion is time-
temperature sensitive.
Needs sufficient time
•Promotes vapour-phase
transport
•Because material
heated very close to
MP, metal atoms will
be released in the
vapour phase from the
particles
•Vapour phase
resolidifies at the
interface
• Third stage: Sintered product is cooled in a controlled
atmosphere
– Prevents oxidation and thermal shock
Gases commonly used for sintering:

• H2, N2, inert gases or vacuum
Liquid Phase Sintering
• During sintering a liquid phase, from the lower MP

component, may exist
• Alloying may take place at the particle-particle interface
• Molten component may surround the particle that has
not melted
• High compact density can be quickly attained
• Important variables:
– Nature of alloy, molten component/particle wetting,
capillary action of the liquid
HOT ISOSTATIC PRESSING (HIP)
Steps in HIP
• Simultaneous compaction + sintering
• Container: High MP sheet metal
• Container subjected to elevated
temperature and a very high vacuum to
remove air and moisture from the powder
• Pressurizing medium: Inert gas
• Operating conditions
– 100 MPa at 1100 C
• Produces compacts with almost 100%
density
• Good metallurgical bonding between
particles and good mechanical strength
• Uses
– Superalloy components for aerospace
industries
– Final densification step for WC cutting tools
and P/M tool steels
READING ASSIGNMENT
• Kalpakjian
– Advantages and disadvantages of isostatic pressing
– P/M gears for a garden tractor
– Production of WC tools
Slip-Casting
(i) Slip is first poured into an absorbent mould

(ii) a layer of clay forms as the mould surface absorbs water
(iii)when the shell is of suitable thickness excess slip is poured away
(iv)the resultant casting
• Slip: Suspension of colloidal (small particles that
do not settle) in an immiscible liquid (generally
water)
• Slip is poured in a porous mold made of plaster
of paris. Air entrapment can be a major problem
• After mold has absorbed some water, it is
inverted and the remaining suspension poured
out.
• The top of the part is then trimmed, the mold
opened, and the part removed
• Application: Large and complex parts such as
plumbing ware, art objects and dinnerware
END

Twenty One

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POWDER METALLURGY

• Earliest use of iron powder dates back to 3000 BC.

Powder size: 0.1 to 1000 μm

• Press powder into the desired shape and size in dies

Metal Powder Pressure (MPa)

• First stage: Temperature is slowly increased so that all

Gases commonly used for sintering:

• During sintering a liquid phase, from the lower MP

(i) Slip is first poured into an absorbent mould

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