UNIT 6 C
4 marks
question
1. Explain the
various steps involved in
powder metallurgy process.
2. What do you understand
by powder metallurgy?
What are the
main stages of powder
metallurgy process?
A. Refer
Question 1 Answers
3. State briefly
the process of
making a powder
metallurgy product having
improved properties.
A.Refer
Question 1 Answers
4. Enlist the products
made from powder
metallurgy. Explain all four
steps of power metallurgy.
A. Products
made from powder metallurgy
• Gears
• Cams
•
Connecting rods
• Crank shafts
• Bushings
& Bearings
• Piston rings
• Light
bulb filaments
• Cutting
tools
Remaining
Answer Refer Question 1
6. With the help
of neat sketch explain
cold isostatic pressing.
A. Photos
7. With the help
of neat sketch explain
hot isostatic pressing.
A. 
9. Explain the
mechanism of sintering.
A.
Machanism of sintering
• Sintering
mechanisms are complex and
depend on the composition of metal particles as
well as processing parameters.
• As temperature
increases two adjacent particles begin to form a bond
by diffusion (solid-state bonding).
• If
two adjacent particles
are of different metals, alloying
can take place at the interface of two particles.
• One of the
particles may have a lower melting point than
the other. In that case,
one particle may melt and
surround the particle that
has not melted (liquid-phase sintering).
13. Describe the atomization
process of making
powder in detail with neat sketch.
A. 
14. Give advantages, limitation &
applications of powder metallurgy.
A. Advantages of P/M
for Structural Components:
These may
be classified into
two main headings;
(a)
Cost advantages, and (b) Advantages due to
particular properties of
sintered components.
Cost
Advantages: (i) Zero or minimal
scrap;
(ii)
Avoiding high machining cost
in mass production
as irregularly shaped holes, flats, counter bores,
involute gear teeth,
key-ways can be molded into the
components;
(iii) Extremely good surface
finish at very low
additional cost after sizing and coining;
(iv)
very close tolerance without
a machining operation;
(v) Assembly of two or
more parts (by I/M) can
be made in one
piece;
(vi) Separate
parts can be combined before sintering.
(vii)
High production rates
Advantages due
to the
particular properties of sintered
components.
(i) By achieving up
to 95% density, the
mechanical and physical
properties are comparable
with cast materials and in
certain cases with wrought materials. In certain cases 99.9 %
dense structure can be
obtained (liquid phase sintering);
(ii) Platting is
also possible directly
at 90% density and above
and after impregnation
of the pores at lower densities.
(iii) Damping out
vibrations and noise property with controlled residual
porosity;
(iv) Ability to retain lubricants such
as lead, graphite and oil
giving less wear and longer
life to bearings;
(v) Achieving a close control
of porosity to
give a specified balance between strength and lubrication properties (a superiority over wrought
materials);
(i) Improved
surface finish with
close control of mass, volume
and density;
(ii) Components
are malleable and
can be bent without
cracking.
P/M makes possible
the production of hard
tools like diamond impregnated
tools for cutting porcelain, glass and
tungsten carbides.
Reactive and
non-reactive metals (both
having high m.p &low
m.p) can be
processed.
Limitations of P/M Process
(i) The
principal limitations of
the process are those imposed by
the size and shape
of the part, the compacting pressure
required and the material used.
(ii) The process is capital
intensive and initial high
costs mean that the production
ranges in excess of 10,000 are
necessary for economic viability (cost of dies
is very high).
(iii) The
configuration of the
component should be such that it can
be easily formed
and ejected from a die,
undercuts and re-entrant angles can not
be molded (when using conventional
pressing and sintering) and have
to be machined subsequently.
(iv) The capacity
and stroke of the
compacting press and the compacting pressure
required limit the cross-sectional area
and length of the
component.
(v) Spheres cannot
be molded and
hence a central cylindrical portion
is required.
(vi) Sintering
of low melting point
powders like lead, zinc,
tin etc., offer
serious difficulties.
Applications:
Motor Cycle
Parts
Vehicles
Engine Parts
Industrial Machines
Parts
For Electric Motors
15. Enlist methods of manufacturing metal powder. Discuss
carbonyl in detail.
A.
Methods of Powder
Production
•
Atomization
• Reduction
•
Electrolytic deposition
• Carbonyl
process
•
Comminution processes
•
Mechanical Alloying
Carbonyl
process
• React high purity Fe or Ni with CO to form
gaseous carbonyls
• The
reaction products are then decomposed to iron and nickel.
• Small, dense, uniformly spherical powders of
high purity
Carbonyl
metallurgy is useful as a low temperature metal coating technique that may well
find many applications in the future.
Iron
carbonyl is stable as iron pentacarbonyl, where five carbon monoxide molecules
are pendantly bonded to the iron atom, while nickel carbonyl is stable as
nickel tetracarbonyl, which has four carbon monoxide molecules pendantly bonded
to the nickel atom. Both can be formed by the exposure of the powdered metal to
carbon monoxide gas at temperatures of around 75 degrees Celsius. Both the
metal carbonyls decompose near 175 °C, resulting in a vapor plated
metallic coating. The thickness of thevapor plated deposit can be increased to
desired thicknesses by controlling the amount of metal carbonyl used and the
duration of the plating process.
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