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CHAPTER 4
STRUCTURES OF MET LS
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CRYSTALLINE STRUCTURES
OF METALSCrystalline Structure
• The basic particles are arranged in a definitethree dimensional pattern of geometrical
form.• The fundamental grouping of this
geometrical form is known as UNIT CELL.
• Unit cells can be repeated many times to
form SPACE LATTICE structure of thecrystal.
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AMORPHOUS STRUCTURE
• The basic particles are arranged in random
formation.
• Non-crystalline solids are said to beamorphous.
• Examples:
Glass, pitch (tar), plastics and moltenmetals.
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LATTICE STRUCTURES OF METALS
• All metals have crystalline structure whenthey are in the solid state.
• Different lattice structures are:(i) body-centred cubic structure (b.c.c.)
(ii) face-centred structure (f.c.c.)
(iii) close-packed hexagonal structure
(c.p.h.) which has the closest packingof atoms.
• More examples – see crystal lattice structures .
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(Materials for the Engineering Technician – R.A. Higgins)
Metals with this
structure are:
(i) Iron ()(ii) Chromium
(iii) Tungsten
(iv) Molybdenum
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(Materials for the Engineering Technician – R.A. Higgins)
Metals with this
structure are:
(i) Iron ( )
(ii) Copper
(iii) Aluminium
(iv) Lead
(v) Gold
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(Materials for the Engineering Technician – R.A. Higgins)
Metals with this
structure are:
(i) Zinc
(ii) Cadmium
(iii) Magnesium
(iv) Beryllium
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Body-centred cubic
(bcc)
Face-centred cubic
(fcc)
Close-packed
hexagonal (cph)
Chromium (Cr)
Molybdenum (Mo)Tungsten (W)
Niobium (Nb)
Aluminium (Al)
Copper (Cu)Lead (Pb)
Nickel (Ni)
Zinc (Zn)
Cadmium (Cd)Magnesium (Mg)
Beryllium (Be)
Iron (Fe)
SUMMARY
Metals with different lattice structures
(depends on the temperature)
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POLYMORPHISM
• The ability of a solid element to exist in more thanone different crystalline form.
(The term otropy is also used to describe thisphenomenon)
• Examples: Iron (Fe)
- below 910oC: -iron (b.c.c.)
- between 910oC and 1400oC: -iron (f.c.c.)
- above 1400oC and below 1550oC-iron (b.c.c.)
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DENDRITIC SOL IDIFICATION (CRYSTAL GROWTH)
• Metals possess mobility when they are in
the liquid state (i.e. the atoms are free to
move about with respect to one another).
• When metals solidifies, the arrangement of
atoms changes from an amorphous state
to one which is orderly (crystalline state).• Pure metals solidify at a fixed temperature
and latent heat is given off .
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DENDRITIC SOLIDIFICATION
(CRYSTAL GROWTH) – cont’d
• When the temperature drops below its
freezing point, crystallisation begins.
Atoms will join the crystal seed as it growsto visible size called “dendrite”.
• Geometric irregularities occurred when the
metal crystals are not free to grow. This
explains why grain boundaries areirregular.
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DENDRITIC SOLIDIFICATION
(CRYSTAL GROWTH) – cont’d
• Crystals commence to grow at the same
time and eventually collide with each other.
Upon collision, growth in that direction is
restricted and their grain boundaries
become distorted.
• The process of solidification is shown in
the Figures (Crystal Growth - 1,2 & 3)
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SOL IDIFICATION Of PURE METAL
(Materials for the Engineering Technician – R.A. Higgins)
The temperature is
constant during cooling
(between points B and C)
because of the dissipationof latent heat.
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CRYSTAL GROWTH - cont’d
(Engineering Materials – Volume 1, R.L.. Timings)
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DEFECTS IN CAST METALS
Although there are many defects in casting,
we are focussing on only two defects in this
lecture:
• Segregation(i) minor segregation
(ii) major segregation
• Porosity(i) dendritic porosity (shrinkage porosity)
(ii) gas porosity
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SEGREGATION
• Two types:
(i) minor segregation
(ii) major segregation
• Minor segregation – dissolved impurities
are found in the grain boundaries.
• Major segregation – the impurities are
concentrated at the core of the casting.
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MINOR SEGREGATION
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Purest
Metal
Bulk ofimpurities
settles at the
grain
boundaries
Crystal
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MAJOR SEGREGATION
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During solidification,
the columnar crystals
formed will push theimpurities toward the
central region of the
casting. The central
region is the last to
solidify.
Chill crystals
Columnar
crystals
Large
equi-axed
crystals
(core)
Pipe (The last
area to solidify –
Metal tends to be
porous and
spongy
containing
blowholes and
impurities)
(Engineering Materials – Volume 1, R.L. Timings)
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GAS POROSITY
The voids are ofir regular shapes
and they occu r at
a lmo st any point
in the stru ctu re.
Voids
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DENDRITIC POROSITY
(SHRINKAGE CAVITIES)
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Shrinkage cavit ies
tend to fo l low theshape of the
dendr i te arms and
occu r at the crysta l
boundar ies.
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EFFECT OF COOLING RATE
ON GRAIN GROWTH
• Increase in the rate of cooling will favourthe formation of small grains.
• There will be zones of different types of
crystals formed in the casting: – Outer skin will have a layer of small hard
chilled crystals,
– Next to the outer skin where the rate of cooling
is less severe, heat flows outwards will favourthe formation of elongated columnar crystals,
– At the core of the ingot where solidification islast, large equi-axed crystals are formed.
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TYPES OF CRYSTALS FORMED DUE
TO THE RATE OF COOLING
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Effect of cooling
rate on grain
growth
Small
hard
“chill”
crys tals atthe
surface of
cast ing
due to
rapid
cool ing
Plane of
weakness
Large equi-axed crystals at core of
cast ing due to slow coo l ing
(Engineering Materials – Volume 1, R.L. Timings)
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METAL CASTING PROCESSES
• Sand Casting
• Die Casting
– Gravity casting – Pressure die casting
– Cold chamber die casting process
• Investment Casting
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Video
Video
Video
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SAND CASTING PROCESS
• Process for making a simple sand mould:
– The pattern of a simple gear blank is first laid ona moulding board along with the drag half (lowerhalf) of the moulding box
– Moulding sand is riddled over the woodenpattern and rammed sufficiently for the particles
to adhere to each other. When the drag hasbeen filled, the sand is levelled with the edge ofthe box.
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• In sand casting, a sand mould has to be preparedfirst. Molten metal is then poured into the mould to
cast out a component
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SAND CASTING – cont’d
– The assembly is then turned over.
– A layer of parting sand (dry clay-free material) is
now sprinkled on to the sand surface.
– The cope half (upper half) of the moulding box isthen placed in position along with the “runner”and “riser” pins. The riser and runner pins areheld steady by means of a small amount ofmoulding sand pressed around them.
• The runner in the finished mould is to allow the
molten metal to flow into the mould cavity.• The riser acts as a reservoir from which molten
metal can feed back into the casting as itsolidifies and shrinks.
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• Moulding sand in now used to fill the cope and it is
rammed around the pattern, runner and riser.
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SAND CASTING – cont’d
• The cope is then lifted off, the pattern is removed,
and the cope is replaced in position.
• The finished mould is then ready to receive its
charge of molten metal.
• For pattern of much complex shape, the mould
must be split into several sections and
consequently a multi-part box is used.
• Cores may be required to form holes and cavitiesin the casting.
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SAND CASTING – cont’d
Advantages:
(i) A large number of metals and alloys can be
sand-cast into intricate shapes. Suitable for
casting both ferrous and non-ferrous metals.
(ii) Small quantity of castings can be made
economically because outlay on simple
equipment and wooden patterns required is
low.
(iii) Largest casting sizes achievable.
(iv) High levels of sand reuse are achievable.
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Moulding
with asimple
pattern
(Materials for the Engineering Technician – R.A. Higgins)
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DIE CASTING
• In die casting, a permanent metal mould is
used.
• Molten metal is allowed to run in under the
action of gravity (gravity die casting) or forced
in under pressure (pressure die casting).
• Types of die casting processes:
– Gravity die casting
– Pressure die casting – Cold chamber die casting process
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DIE CASTING – cont’d
(GRAVITY DIE CASTING)
• The die is made of metal and may be part of a
multi-part design if the shape of the casting is
complex.
• Cores may be of sand or metal. – Metal cores of complex shape must be split to
allow for their removal from the finished
casting.
• The die cavity may be filled by hand pouring or byautomatic feeding in a modern high-speed plant.
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DIE CASTING – cont’d
(PRESSURE DIE CASTING)
• Two types of pressure die casting. Cold chamber
die casting process and the Hot chamber die
casting process. The former is more widely used.
• Cold chamber die casting:
– Charge of molten metal is forced into the die by
means of a plunger.
– As soon as the casting is solid, the moving platen
is retracted.
– As the moving platen comes against the fixed
block, ejectors are activated and the casting is
pushed out of the mould. Cycle time is rapid.
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Cold Chamber
Die Casting
Process
(Materials for the Engineering Technician – R.A. Higgins)
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ADVANTAGES OF DIE CASTING
(COMPARED TO SAND CASTING)
• Internal structure is more uniform and the grainsare finer due to rapid cooling.
• Output rates are much higher.• Greater dimensional accuracy achievable
• Better surface finish.
• Process can be automated easily.
• Reduces or eliminates secondary machiningoperations.
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DISADVANTAGES OF DIE CASTING(COMPARED TO SAND CASTING)
• For alloys of high shrinkage coefficients, sand
casting is preferred.
• Die casting is confined mainly to zinc, aluminium-
or manganese-based alloy. These materials have
high fluidity in their molten state.
• Die casting is only economical if a large quantity
of casting is required (e.g. of the order of 5000)
and this is due to the high cost of the metal dies.
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INVESTMENT CASTING
• A permanent mould is used to make the wax
patterns.
– This master mould could be machined in steel or produced
by casting a low-melting-point alloy around a master
pattern.• To produce wax patterns, the two halves of the
mould are clamped together and molten wax is
injected. When the wax pattern has solidified, it is
removed from the mould.• The wax gate is suitably trimmed using a heated
hand-held tool.
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INVESTMENT CASTING – cont’d
• The wax pattern is then attached to a centralrunner. The assembled runner with its tree ofpatterns is then fixed to a flat-bottom plate by a blobof molten wax.
• A metal flask is placed over the assembly. The gapbetween the flask and the bottom plate is sealedwith wax.
• Investment material is then poured into the flask.
• The bottom plate together with the assembly isvibrated to expel trapped air or bubbles duringsolidification of the investment materials.
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INVESTMENT CASTING – cont’d
• For low-temperature casting, the investment
material is composed of fine silica sand and
Plaster of Paris
• The investment material is allowed to dry for someeight hours. The base plate is then detached and
the inverted flask is passed through an oven at
about 150oC.
• Most of the wax will melt and run out leaving a
mould cavity in the investment material.
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INVESTMENT CASTING – cont’d
• Before receiving its charge of molten metal, the
investment mould is pre-heated to a temperature
range between 700oC and 1000oC.
– The objective of pre-heating is to remove the lasttraces of wax and to complete the decomposition of
ethyl silicate bond to silica and to prevent chilling of the
cast metal so that it will flow into every corner of the
mould cavity.
• Investment casting process is also known as“Lost Wax” casting process.
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(Video)
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(Materials for the Engineering Technician – R.A. Higgins)
Investment CastingProcess
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ADVANTAGES OF
INVESTMENT CASTING
• Very high dimensional accuracy of the product can be
achieved and thereby reduce the need for secondary
machining operations.
• Complicated shape can be produced.
• Absence of disfiguring parting line on the component.
• Can be used for manufacturing small components from
metals and alloys. These small components cannot be
shaped by forging and machining operations.
– Examples: gas turbine and jet engine blades, millingcutters, precision gauges, forming and
swaging dies and spray nozzles.
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DISADVANTAGES OF
INVESTMENT CASTING
• High cost – investment material is very
expensive.
• The size/weight of a component is limitedto about 2 kg.
• Individual pattern is required for each
casting
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SUMMARY OF LECTURE
• In this lecture, the following topics were discussed: – Crystalline and amorphous structures
– BCC, FCC and CPH structures – polymorphism
– Dendritic solidification – crystal growth
– Defects in cast metals – segregation and porosity – The effect of cooling rate on the formation of different types of
crystals.
– Different types of metal casting processes – sand moulding, die
casting and investment casting.
– Advantages and disadvantages of investment casting.
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