Chapter 4 - Structure of Metals

8/12/2019 Chapter 4 - Structure of Metals

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Revision 1: March 2011

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.

2




AMORPHOUS STRUCTURE

• The basic particles are arranged in random

formation.

• Non-crystalline solids are said to beamorphous.

• Examples:

Glass, pitch (tar), plastics and moltenmetals.

3




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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http://localhost/var/www/apps/conversion/tmp/Temp/TYPES%20OF%20CRYSTAL%20LATTICE%20STRUCTURES.doc






Revision 1: March 2011 5

(Materials for the Engineering Technician – R.A. Higgins)

Metals with this

structure are:

(i) Iron ()(ii) Chromium

(iii) Tungsten

(iv) Molybdenum





Metals with this

structure are:

(i) Iron ( )

(ii) Copper

(iii) Aluminium

(iv) Lead

(v) Gold





Metals with this

structure are:

(i) Zinc

(ii) Cadmium

(iii) Magnesium

(iv) Beryllium




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


The temperature is

constant during cooling

(between points B and C)

because of the dissipationof latent heat.




CRYSTAL GROWTH - cont’d

(Engineering Materials – Volume 1, R.L.. Timings)




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

18

Purest

Metal

Bulk ofimpurities

settles at the

grain

boundaries

Crystal




MAJOR SEGREGATION

19

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)




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.




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)




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

http://www.youtube.com/watch?v=FfXTUX4xTHk

http://www.youtube.com/watch?v=BX8w-GUPz1w

http://www.youtube.com/watch?v=rgL2Jn5mk1A




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




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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• 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.





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





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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Video

http://www.youtube.com/watch?v=1AgDGLNE6Es

http://www.youtube.com/watch?v=1AgDGLNE6Es




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





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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• 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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• 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)







Investment CastingProcess




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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Chapter 4 - Structure of Metals

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