LECTURE 2: POWDER METALLURGY PROCESS
Production of Metallic Powders
Production of Metallic Powders
Any metal can be made into powder form. The three principal
methods by which metallic powders are commercially produced are;
1. Atomization
2. Chemical
3. Electrolytic
In addition, mechanical methods are occasionally used to reduce
powder sizes such as ball milling technique.
Gas Atomization Method
In this method, a high velocity gas stream flows through the
expansion nozzle, siphoning molten metal and spraying it into
chamber
Atomization
• It is achieved by bringing the thin molten metal stream in
contact with the impact of high-energy jets of gas or
liquid.
• By varying the design and configurations of the jets,
pressure and volume of the atomising fluid, thickness of
the stream of metal etc., it is possible to control the
particle size distribution over a wide range.`
Bennett University
Water Atomization Method
In this method, a high
velocity water stream
flows through nozzles,
rapidly cooling and
solidifying molten metal
into collection chamber
Electrolysis
1. The desired metal is made as anode in an electrolytic cell,
such that it is dissolved by the electrolyte in the cell and then
transported and deposited on the cathode in a spongy or
powdery form.
2. The deposit is removed, washed, and dried to get the metal
powder. Copper is the primary metal produced by
electrolysis, but iron, chromium, and magnesium powders
are also produced using this process.
Bennett University
Iron Powders for PM
Iron powders produced
by water atomization
(photo courtesy of
T.F.Murphy and
Hoeganaes
Corporation)
Electrolysis Process
1. Copper powder => Solution containing copper sulphate and
sulphuric acid; crude copper as anode•
Reaction: at anode: Cu -> Cu++ e-
at cathode: Cu++ e-->Cu
2. Iron powder=> anode is low carbon steel; cathode is stainless
steel. The iron powder deposits are subsequently pulverized by
milling in hammer mill. The milled powders are annealed in
hydrogen atmosphere to make them soft
3. Mg powder=> electrodeposition from a purified magnesium
sulphate electrolyte using insoluble lead anodes and stainless steel
cathodes
Conventional Press and Sinter
Conventional PM part-making sequence consists of:
1. Blending and mixing of powders
2. Compaction - pressing into desired shape
3. Sintering - heating to temperature below melting point to
cause solid-state bonding of particles and strengthening of
part
4. In addition, secondary operations are sometimes
performed to improve dimensional accuracy of the part
Conventional PM Production Sequence
(1) Blending and mixing, (2) compacting, and (3) sintering
Powder Metallurgy Die
Blending and Mixing of Powders
1) For successful results in compaction and sintering, the starting
powders must be homogenized
a. Blending - powders of the same chemistry but possibly
different particle sizes are intermingled. Different particle
sizes are often blended to reduce porosity.
b. Mixing - powders of different chemistries are combined
Blending/ Mixing Machines
Compaction
It comprises of application of high pressure to the powders to
form them into the required shape
1. Conventional compaction method is pressing, in which
opposing punches squeeze the powders contained in a die
2. Work part after pressing is called a green compact, the word
green meaning not fully processed
3. The green strength of the part when pressed is okay for
handling but far less than after sintering
Conventional Pressing in PM
Pressing in PM: (1) filling
die cavity with powder by
automatic feeder; (2)
initial and (3) final
positions of upper and
lower punches during
pressing, (4) part ejection
450 kN (50-ton) hydraulic
press for conventional
pressing of PM parts (photo
courtesy of Dorst America,
Inc.).
Typical Compacting Pressures for Various Applications
Bennett University 173
Sintering
1. It refers to the heat treatment to bond the metallic particles,
thereby increasing strength and hardness of the green compact.
2. Usually carried out at 70% to 90% of the metal's melting point
(absolute scale)
3. The primary driving force for sintering is reduction of surface
energy. Part shrinkage occurs during sintering due to pore size
reduction
Typical Sintering Temperatures for Some Common Metals &
Materials
Bennett University 178
Sintering Sequence on a Microscopic Scale
(1) Particle bonding initiated at contact points; (2) contact points
grow into "necks"; (3) pores between particles are reduced in
size; (4) grain boundaries develop between particles in place of
necked regions
Sintering
1. It refers to the heat treatment to bond the metallic particles,
thereby increasing strength and hardness of the green compact.
2. Usually carried out at 70% to 90% of the metal's melting point
(absolute scale)
3. The primary driving force for sintering is reduction of surface
energy. Part shrinkage occurs during sintering due to pore size
reduction
The driving force for sintering is the reduction in the
total free energy of the particulate system, ΔG, which is
composed of free energy due to changes of volume
ΔGV , boundaries Δ Gb, and surfaces Δ Gs
Types of Sintering
1) Solid State Sintering
2) Liquid Phase Sintering
3) Pressure Assisted Sintering
Solid State Sintering
Development of ceramic
microstructure during
sintering: (a) Loose powder
particles; (b) initial stage;
(c) intermediate stage; and
(d) final stage.
From W. E. Lee and W. M. Rainforth, Ceramic Microstructures, p. 37., Kluwer Academic Publishers.
Initial Stage - SSS
Initial stage
• Rearrangement of
particles to increase
points of contact -
maximize the
coordination number
• Neck formation - bonding
at points with highest
surface energy
• Density - up to 75 %
theoretical density
Intermediate Stage - SSS
Intermediate stage
• Neck growth
• Volume shrinkage
• Grain boundaries formed at the
contacts
• Grain growth - lengthening of
grain boundaries
• Process ends when pore system
becomes discontinuous
• Density - up to 75-95%
theoretical density
Final Stage - SSS
Final stage
• Grain growth
• Pores isolated
• Grain boundary pores eliminated
• Inner porosity closes
• Density > 95 % theoretical density
Mechanisms of Solid-State Sintering
• The densifying mechanisms, grain boundary diffusion, lattice
diffusion from the grain boundary to the neck, and plastic flow
cause neck growth as well as densification (shrinkage). When the
non-densifying mechanisms dominate, coarsening leads to the
production of a porous particle, whereas a dense particle is
favored under conditions when the densifying mechanisms
dominate.
• Grain boundary diffusion and lattice diffusion are important
densification mechanisms in metals and ceramics.
• Plastic flow, by dislocation motion in response to the sintering
stress, plays essentially no role in the sintering of ceramics
because of the low dislocation density.
Lattice Diffusion
https://upload.wikimedia.org/wikipedia/commons/5/5f/Chemical_surface_diffusion_slow.gif
Sintering Cycle and Furnace
(a) Typical heat treatment
cycle in sintering; and
(b) schematic cross
section of a continuous
sintering furnace
