2. Powder Metallurgy
Lesson Objectives
In this chapter we shall discuss the following:
1. What is powder metallurgy (PM)
2. Need of PM
3. Advantages, Limitations & Application of
PM
4. Basic steps in PM
5. Design considerations in PM
6. Secondary & finishing operations
Learning Activities
1. Look up
Keywords
2. View Slides;
3. Read Notes,
4. Listen to
lecture
Keywords: Powder, Blending, Sintering, Particle size and shape,
Infiltration etc.
3. What Is Powder Metallurgy ?
OR
It may also be defined as “material
processing technique used to consolidate
particulate matter i.e. powders both metal
and/or non-metals.”
Powder metallurgy may defined
as, “the art and science of
producing metal powders and
utilizing them to make
serviceable objects.”
4. Why PM?
Because:
• PM parts can be mass produced to net shape or near
net shape.
• PM products have doctored properties.
• No need for subsequent machining
• PM process wastes very little material ~ 3%.
• PM parts can be made with a specified level of
porosity, to produce porous metal parts
− Examples: filters, oil-impregnated bearings and gears
5. Some More Reasons For PM …
• Certain metals that are difficult to fabricate by
other methods can be shaped by powder metallurgy
− Example: Tungsten filaments for incandescent lamp bulbs
• Certain alloy combinations and cermets made by PM
cannot be produced in other ways
• PM compares favorably to most casting processes in
dimensional control
• PM production methods can be automated for
economical production.
6. Parts Made by PM
Fig (a) Examples of typical
parts made by PM processes.
(b)
(c)
Fig (c) Main-bearing metal-powder caps for
3.8 and 3.1 liter General Motors automotive
engines.
Fig(b) Upper trip lever for a commercial
sprinkler made by PM.
This part replaces a die-cast part of unleaded
brass alloy; with a 60% savings.
(b
(a)
8. Advantages of PM
Cost Advantages:
1. Zero or minimal scrap.
2. High production rates
3. Avoids high machining cost
needed for holes, gear teeth,
key-ways etc.
4. Extremely good surface
finish
5. Very close tolerance
without a machining
operation;
6. Assembly of two or more
parts (by I/M) can be made
in one piece;
Properties Advantages of sintered
components:
1. Complex shapes can be produced
2. Wide composition / property variations are
possible
3. Physical properties are comparable with cast
materials and wrought materials.
4. Ability to retain lubricants reduces wear and
lengthens life of bearings;
5. Improved surface finish with close control of
mass, volume and density;
6. Components are malleable and can be bent
without cracking.
7. Hard tools like diamond impregnated are
made for cutting porcelain, glass & WC.
8. Reactive and non-reactive metals can be
processed.
9. Limitations of PM Process
Major limitations are as follows:
1. Principal limitations of the process are those imposed by the size and
shape of the part, the compacting pressure required and material used.
2. High initial investment in machinery and dies.
3. Economically viable for production ranges in excess of 10,000.
4. High material cost.
5. Inferior strength properties.
6. Limitations on part geometry due to limited flowability of powders.
7. Varying density of part may be a problem, for complex geometries.
8. Can not make undercuts and re-entrant angles.
9. Problems in storing and handling metal powders e.g. degradation over
time, fire hazards with certain metals.
10. Limited cross-sectional area and length of the component .
11. Copper-based materials which are hot-worked have not so far been made
by PM successfully.
10. Basic Steps In PM
Powder metallurgy is the process of blending fine powdered
materials, compacting the same into a desired shape or
form inside a mould followed by heating of the compacted
powder in a controlled atmosphere (sintering) to facilitate
the formation of bonding of the powder particles to form
the final part.
The four basic steps of PM include:
(1) powder manufacture,
(2) blending of powders,
(3) compacting of powders in a mould or die, and
(4) sintering.
11. Steps In Making PM
Fig 2 Outline of processes and operations involved in making powder-metallurgy parts.
12. Powder Blending
• A single powder may not have all the requisite
properties and hence, powders of different materials
are blended to form a final part with desired properties.
• Blending is carried out for several purposes as follows:
1. To imparts uniformity in the shapes of the powder
particles.
2. To facilitates mixing of different powder particles.
3. To impart wide ranging physical and mechanical
properties.
4. To improve the flow characteristics of the powder
particles reducing friction between particles and dies.
5. To enhance green strength of parts by adding binders.
13. Is Blending & mixing same?
• Blending: process of mixing powder of the
same chemical composition but different
sizes.
• Mixing: process of combining powders of
different chemistries.
14. Devices For Blending & Mixing
Blending and mixing are accomplished by mechanical means.
Some bowl geometries are shown below:
Rotating drum Rotating
double cone
Screw Mixture Blade Mixture
Since metal powders are abrasive,
mixers rely on the rotation or tumbling
of enclosed geometries as opposed to
using aggressive agitators.
A mixer
15. Compaction
• Compaction: Blended powers are pressed in dies under
high pressure to pressurize & bond the particles to form a
cohesion among powder particles to impart. required shape.
• The work part after compaction is called a green compact
or simply a green, (green means not yet fully processed.)
The compaction exercise imparts the following effects.
1. Reduces voids and enhance density of consolidated
powder.
2.Improves green strength of powder particles.
3.Facilitates plastic deformation of the powder particles
to conform to the final desired shape of the part.
4.Enhances the contact area among the powder particles
and facilitates the subsequent sintering process.
16. Guidelines For Compaction
General guidelines for metal powder compaction are:
1. Powder must fill die orifice completely.
2. A constant volume or constant weight may be used.
3. Use vibration filling to create denser packing to avoid
bridging and high porosity defects.
4. Apply pressure along more than one axis to minimize
defects.
5. Filling, Pressing and Ejection may be done automatically.
6. To facilitate compaction add additives to powder i.e.
– Lubricants: to reduce the particles-die friction
– Binders: to achieve enough strength before sintering
– Deflocculants: to improve the flow characteristics
during feeding
17. Compaction:
Process & Variables
Compaction process is shown below: Main variables are:
(a) Method of compaction
(b) Compaction pressure,
time and temperature
(c) Rate of compaction
(d) Compacting atmosphere
(e) Lubricants and other
additives of mix, and
(f) Die design
(g) Die materials
(h) Punch
(i) Carbide inserts
(j) Tolerances, clearances
and finishes
Further during compaction
tooling materials, clearances
and tolerances require
expertise.
18. Mechanism of
Compaction
• Consolidation generally occurs in three stages
(a) rearrangement of particles.
(b) particles contacting by plastic deformation.
(c) mechanical locking and cold welding of particles due to
surface shear strains.
• It is, therefore, easier to cold compact irregular particles than
spherical powder particles.
• During compaction green
density increases rapidly with
compaction pressure.
• Compaction pressure
determines mechanical
properties of parts
19. Methods of Compaction
1. With application of pressure
a) Unidirectional pressing
(single action or double
action pressing)
b) Isostatic pressing
c) Rocking die compaction
d) Powder rolling
e) Powder extrusion
f) Powder swaging
g) Powder forging
h) Powder Injection Molding
2. Without applying pressure
a) Slip mixing/ slip casting
b) Vibrational compaction
Single action Double action
20. Tool For Compaction (Presses)
• The basic types of compacting presses are:
1. Mechanical (single punch or rotary type) presses.
2. Hydraulic presses.
3. Hybrid-type presses (mechanical presses may make use of
auxiliary pneumatic or hydraulic devices).
• Minimum requirements for any powder metal press:
1. Adequate total pressure capability
2. Part ejection capability.
3. Controlled length and speed of
compression and ejection strokes.
4. Adjustable die fill arrangements.
5. Synchronized timing of press strokes.
6. Material feed and part removal systems.
A 7.3-MN
(825-ton
21. Compacting Presses:
Parts & Attachments
The presses systems used are;
(a) Single action press system consisting of:
• a die to form the outer contour of the part;
• an upper punch to form the top surface of the part;
• a lower punch to form the bottom surface of the part;
• if required, core rods to form any through holes (for class I parts).
(b) Double action opposed ram system consists of
• a die, upper punch, lower punch and core rods (for class I and class
II parts).
(c) Double action floating die system consists of
• moving upper punch, stationary lower punch, moving die table and
core rods (for class I – IV parts).
22. Density as a Function of Pressure and
Effects of Density on Other Properties
Figure (b) Effect of density on
tensile strength, elongation, and
electrical conductivity of copper
powder.
Fig: (a) Density of copper- and
iron-powder compacts as a
function of compacting pressure.
Density greatly influences mechanical & physical properties of PM parts.
23. Density Variation in
Compacting Metal Powders
Fig: Density variation in compacting metal powders in various dies:
(a) and (c) single-action press; (b) and (d) double-action press.
Note in (d) the greater uniformity of density from pressing with two
punches with separate movements when compared with (c).
(e) Pressure contours in compacted copper powder in a single-action press
25. Sintering
• Sintering bonds individual metallic particles, thereby
increases strength and hardness of final part.
• Compressed metal powder is heated in a controlled-
atmosphere furnace to a temperature (70% and 90% of Tm)
below its melting point, but high enough to cause diffusion
thereby bonding of neighboring particles.
• Powder performs are heated in a controlled, inert or
reducing atmosphere or in vacuum prevent oxidation.
• The primary driving force for sintering is not the fusion of
material, but formation and growth of bonds between
particles due to reduced of surface energy.
• Part shrinkage occurs during sintering due to pore size
reduction.
• Density increases due to filling up incipient holes and increasing
area of contact among powder particles in compact perform.
26. Movements of Atoms During
Sintering
Fig: A three particle sketch of sintering, showing several
possible paths of atomic motion involved with particle bonding
(neck growth) and pore shrinkage (densification).
27. Mechanisms For Sintering
Metal Powders
Fig: Schematic illustration of
two mechanisms for sintering
metal powders: (a) solid-
state material transport;
and (b) vapor-phase material
transport.
Where R = particle radius, r =
neck radius, and p = neck-
profile radius.
Bonding among the powder particles takes places in three ways:
(1) melting of minor constituents in the powder particles,
(2)diffusion between the powder particles, and
(3)mechanical bonding.
28. Solid State Sintering
• Solid state sintering involves heating the powder below
the melting point to allow solid-state diffusion and
bonding the particles together.
• Particle bonding is initiated at contact point, which
then grow into necks, reducing pores between particles.
• Prolonged heating develops grain boundaries between
particle in place of necked regions.
29. Liquid Phase Sintering
Liquid phase sintering
usually involves mixing
an iron powder
With a liquid forming
powder ( Boride,
carbide, phosphide,
copper ,tin
And heating to a
temperature where the
liquid forms, spread
and contributes to
particle bonding and
densifications.
Fig: Liquid phase sintering
30. Factors In Sintering
• The nature and strength of the bond between
the particles depends on:
1. The mechanism of diffusion,
2. Plastic flow of the powder particles, and
3. Evaporation of volatile material from the
compacted preform.
• The three critical factors that control the sintering process are:
1) time,
2) temperature and
3) the furnace atmosphere
35. Finishing Operations
• A number of secondary and finishing operations can be
applied after sintering, some of them are:
1. Sizing: cold pressing to improve dimensional accuracy
2. Coining: cold pressing to press details into surface
3. Impregnation: oil fills the pores of the part
4. Infiltration: pores are filled with a molten metal
5. Heat treating, plating, painting
36. Impregnation and Infiltration
• Porosity is a unique and inherent characteristic of PM
technology.
• It can be exploited to create special products by
filling the available pore space with oils, polymers,
or metals
• Two categories:
1. Impregnation
2. Infiltration
37. Impregnation
• The term used when oil or other fluid is permeated
into the pores of a sintered PM part
• Common products are oil-impregnated bearings,
gears, and similar components.
• An alternative application is when parts are
impregnated with polymer resins that seep into the
pore spaces in liquid form and then solidify to create
a pressure tight part.
38. Infiltration
• An operation in which the pores of the PM part are
filled with a molten metal.
• The melting point of the filler metal must be below
that of the PM part.
• Involves heating the filler metal in contact with the
sintered component so capillary action draws the
filler into the pores
• The resulting structure is relatively nonporous, and
the infiltrated part has a more uniform density, as
well as improved toughness and strength.
39. General Classification of
Powder Metallurgy Parts
1) Class I parts with a diameter (or
thickness) up to 65 mm and single level
parts of any contour that can be pressed
with a force from one direction.
2) Class II parts are single level
components of any thickness and any
contour that must be pressed from two
directions.
3) Class III parts are two level components
of any thickness and contour that must
be pressed from two directions.
4) Class IV parts are multilevel components
of any thickness and contour that must
be pressed from two direction.
(a) Class I,(b) Class II
(c) Class III,(d) Class IV
40. Design Considerations for P/M
1. Shape of compact must be kept as simple and uniform as possible.
2. Provision must be made for ejection of the green compact without damaging
the compact.
3. P/M parts should be made with the widest acceptable tolerances to
maximize tool life.
4. Part walls should not be less than 1.5 mm thick;
5. Walls with length to thickness ratios above 8:1 are difficult to press.
6. Steps in parts can be produced if they are simple and their size doesn’t
exceed 15% of the overall part length.
7. Letters can be pressed if oriented perpendicular to pressing direction.
8. Raised letters are more susceptible to damage in the green stage and
prevent stacking.
9. Flanges or overhangs can be produced by a step in the die.
10. A true radius cannot be pressed; instead use a chamfer.
11. Dimensional tolerances are on the order of ±0.05 to 0.1 mm.
12. Tolerances improve significantly with additional operations such as sizing,
machining and grinding.
41. Poor & Good Designs of P/M Parts
Fig: Examples of P/M parts showing poor and good designs.
Note that sharp radii
and reentry corners
should be avoided
and that threads and
transverse holes have
to be produced
separately by
additional machining
operations.
42. Design Features for Use with
Unsupported Flanges or Grooves
Fig: (a) Design features for use with unsupported flanges.
(b) Design features for use with grooves.
43. Die Design for Powder-
Metal Compaction
Fig: Die geometry & design features for P/M compaction.
44. Further reading
• Fundamentals of powder metallurgy W. D. Jones
• Powder Metallurgy: Principles & Applications F. V. Lenel
• Fundamentals of P/M I. H. Khan
