STRUCTURE FOR PURE METALSFOR PURE METALS:: At the mould walls, metal cools rapidly. Produces

solidified skin or shell (thickness depends on composition, mould temperature, mould size and shape etc)

• These are of equiaxed structure.• Grains grow opposite to heat transfer through the

mould• These are columnar grains• Driving force of the heat transfer is reduced away from

the mould walls and blocking at the axis prevents further growth

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PURE METALSPURE METALS-

Have clearly defined melting/freezing point, solidifies at a constant temperature.

Eg: Al - 6600C, Fe - 15370C,

and W- 34100C.

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Size and distribution of the overall grain structure throughout a casting depends on rate & direction of heat flow

(Grain size influences strength, ductility, properties along different directions etc.)

CONVECTION- TEMPERATURE GRADIENTS DUE TO DIFFERNCES IN THE DENSITY OF MOLTEN METAL AT DIFFERENT

TEMPERATURES WITHIN THE FLUID - STRONGLY EFFECTS THE GRAIN SIZE.

Outer chill zones do not occur in the absence of convectionOuter chill zones do not occur in the absence of convection

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FOR ALLOYS:• Alloys solidify over a range of temperatures• Begins when temp. drops below liquidous,

completed when it reaches solidous.• Within this temperature range, mushy or pasty state

(Structure as in figure)• Inner zone can be extended throughout by adding a

catalyst.- sodium, bismuth, tellurium, Mg (or by eliminating thermal gradient, i.e. eliminating

convection. (Expts in space to see the effect of lack of gravity in eliminating convection)

(refresh dendritic growth- branches of tree, interlock, each dendrite develops uniform composition, etc)

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SOLIDIFICATION TIMESOLIDIFICATION TIME

During solidification, thin solidified skin begins to form at the cool mould walls.

Thickness increases with time.For flat mould walls thickness time (time doubled, thickness by 1.414)

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CHVORINOV’S RULE solidification time (t) is a function of volume of the casting and its surface area

t = C ( volume/ surface area )2

C is a constant [depends on mould material, metal properties including latent heat, temperature]

A large sphere solidifies and cools at a much slower rate than a small diameter sphere. (Eg- potatoes, one big and other small)

Volume cube of diameter of sphere,

surface area square of diameter

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Solidification time for various shapesSolidification time for various shapes::

Eg: Three pieces cast with the SAME volume, but different shapes. (i)Sphere, (ii)Cube, (iii)Cylinder with height = diameter. Which piece solidifies the fastest?

Solution: Solidification time = C (volume/surface area)2

Let volume = unity. As volume is same, t = C/ surface area2.

Cylinder: V = πr2h = 2 π r3; ie, r = (1/2 π) 1/3

A = 2 πr2 + 2πrh = 6 πr2 = 5.54.

Then, t cube = 0.028C ; t cylinder = 0.033C ; t sphere= 0.043C

Metal poured to cube shaped mould solidifies the fastest.

Sphere: V= 4/3 (π r3); i.e. r = (3/4 π)1/3

A= 4 π r2 = 4 π (3/4 π)1/3 = 4.84

Cube: V = a3; ie a = 1; A = 6 a2 = 6.

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SHRINKAGE AND POROSITYSHRINKAGE AND POROSITY

METALS SHRINK(CONTRACT) DURING SOLIDIFICATION

- CAUSES DIMENSIONAL CHANGES

LEADING TO CENTRE LINE SHRINKAGE, POROSITY, CRACKING TOO

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Time

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SHRINKAGE DUE TO:(1).CONTRACTION OF

MOLTEN METAL AS IT COOLS PRIOR TO SOLIDIFICATION

(2) CONTRACTION OF SOLIDIFYING METAL,

LATENT HEAT OF FUSION

(3) CONTRACTION OF SOLIDIFIED METAL

DURING DROP TO AMBIENT TEMP

OUT OF THESE, LARGEST SHRINKAGE DURING OUT OF THESE, LARGEST SHRINKAGE DURING

COOLING OF CASTINGCOOLING OF CASTING (ITEM 3) eg:pure metal

SOLIDIFICATION CONTRACTION FOR VARIOUS METALSSOLIDIFICATION CONTRACTION FOR VARIOUS METALS

METAL Volumetric Solidification Contraction Al 6.6 Grey cast Iron Expansion 2.5 Carbon Steel 2.5 to 3 Copper 4.9Magnesium 4.2 Zinc 6.5

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• POROSITY DUE TO SHRINKAGE OF GASES

AND METAL TOO. RELATED TO DUCTILITY AND SURFACE FINISH(DUCTILITY V/S POROSITY CURVES FOR

DIFFERENT METALS) - ELIMINATION BY VARIOUS MEANS(ADEQUATE SUPPLY OF LIQUID METAL, USE

OF CHILLS, NARROWING MUSHY ZONE- CASTING SUBJECTED TO ISOSTATIC PRESSING

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POROSITY BY GASESPOROSITY BY GASES

LIQUID METALS HAVE HIGH SOLUBILITY FOR GASESDISSOLVED GASES EXPELLED FROM SOLUTION DURING SOLIDIFICATION(Hydrogen, Nitrogen mainly)ACCUMULATE IN REGIONS OF EXISTING POROSITY ORCAUSE MICROPOROSITY IN CASTING- TO BE CONTROLLED

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Effect of microporosity on the ductility of quenched and tempered cast steel – Porosity affects the ‘pressure tightness’

of cast pressure vesselD

uct

ility

Porosity(%)

Elongation

Reduction of area

0 5 10 15

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FLOW OF MOLTEN METAL IN MOULDSFLOW OF MOLTEN METAL IN MOULDSImportant: pouring basin, mould cavity & riser

GATING SYSTEM Design -fluid flow, heat transfer, influence of temperature gradient,

FLUID FLOWWithout turbulence

or with minimized turbulence

HEAT FLOW INFLUENCED BY MANY FACTORS

FLUIDITY-A characteristic related to viscosity.

TEST OF FLUIDITY - USING A SPIRAL MOULD.Fluidity Index

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TEST FOR FLUIDITY

USING A SPIRAL MOULD.

FLUIDITY INDEX IS THE LENGTH OF THE SOLIDIFIED METAL IN THE SPIRAL PASSAGE. GREATER THE LENGTH, GREATER THE FLUIDITY INDEX.

PATTERNPATTERN

• Model of a casting constructed such that it forms an impression in moulding sand

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PATTERNPATTERN• 1st step- Prepare model (pattern)

Differs from the casting

Differences Pattern Allowances.

• To compensate for metal shrinkage, • Provide sufficient metal for machining• Easiness in moulding

• As Shrinkage allowance, Draft allowance, Finishing allowance, Distortion or camber allowance,

Shaking or rapping allowance

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MATERIALMATERIAL1. WOOD.2. METAL Al, CI, Brass, 3. For special casting processes, Polystyrene which leaves mould as gas when heated also used. Types- many

Simple-Identical patterns; Complex, intricate- with number of pieces. Single or loose piece; Split; gated; Match Plate; Sweep; Segmental; Skeleton(frame, ribbed), skell; Boxed Up; Odd shaped etc. Sketches--

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MaterialMaterial1. WOOD.(+) Cheap, easily available, light, easiness in surfacing, preserving (by shellac coating), workable, ease in joining, fabrication

(-) Moisture effects, wear by sand abrasion, warp during forming, not for rough use. Must be properly dried/ seasoned, free from knots, straight grainedEgs. Burma teak, pine wood, mahogany, Sal, Deodar, Shisham, Walnut, Apple tree

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2. METAL:

For durability, strength

Egs: Al alloys, Brass, Mg alloys, Steel, cast Iron for mass production

(first, wooden pattern is made, then cast in the metal)

Type of material depends on shape, size, number of castings required, method of moulding etc.

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TYPES OF PATTERNS

1. SINGLE PIECE PATTERN.

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2. SPLIT PATTERN (TWO PIECE )

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2. a, THREE PIECE SPLIT PATTERN

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3. LOOSE PIECE PATTERNN

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4. COPE AND DRAG PATTERN

• COPE AND DRAG PARTS OF THE PATTERN MOUNTED ON SEPARATE PLATES.

• COPE HALF AND DRAG HALF MADE BY WORKING ON DIFFERENT MOULDING MACHINES.

• THIS REDUCES THE SEPARATE COPE AND DRAG PLATE PREPARATION.

• GENERALLY FOR HIGH SPEED MECHANISED MOULDING.

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5. MATCH PLATE PATTERN – Pattern generally of metal and plate making

parting line metal/wood.

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6. FOLLOW BOARD PATTERN. For thin sections.

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THIN PATTERN

7. GATED PATTERN - Gating system is a part of the pattern.

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8. SWEEP PATTERN – For large size castings in small numbers. Template of

wood attached to a sweep used.

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9. SEGMENTAL PATTERN – For rings, wheel rims, large size gears.

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10. SKELETON PATTERN.- Stickle board used to scrape the excess sand. Eg. Oil pipes, water pipes, pipe bends, boxes, valve bodies etc.

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Stickle board

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11. SHELL PATTERN

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12. BUILT UP PATTERN – Also called lagged up patterns- For barrels, pipes,

columns etc

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13. LEFT AND RIGHT PATTERN – For parts to be made in pairs. Eg: legs of sewing machine, wood working lathe, garden benches, J hangers for shafts, brackets for luggage racks etc.

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• Type of pattern depends on:

• Shape and size of casting,

• number of castings required,

• method of moulding employed,

• easiness or difficulties of the moulding operations,

• other factors peculiar to the casting.

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MetalPattern

Oversize Factor (each direction)

Finish Allowance (smaller number for larger sizes)

Min Wallmm

Aluminum 1.08 - 1.12 0.5 to 1.0 % 4.75

Copper alloys 1.05 - 1.06 0.5 to 1.0 % 2.3

Gray Cast Iron 1.10 0.4 to 1.6 % 3.0

Nickel alloys 1.05 0.5 to 1.0 % N/A

Steel 1.05 - 1.10 0.5 to 2 % 5

Magnesium alloys

1.07 - 1.10 0.5 to 1.0 % 4.0

Malleable Irons

1.06 - 1.19 0.6 to 1.6 % 3.0

Pattern, Finish Allowance, and Wall Thickness

CHARACTERISTICS OF PATTERN MATERIALS

CHARACTERISTIC RATING

WOOD AL STEEL PLASTIC CAST IRON

MACHINABILITY E G F G G WEAR RESISTANCE P G E F ESTRENGTH E G E G GWEIGHT E G P G PREPAIRABILITY E P G F GRESISTANCE TO:

• CORROSION (by water) E E P E P• SWELLING P E E E E

E- Excellent; G- Good; F-fair, P- Poor

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Functions of pattern

• Moulding the Gating system;

• Establishing a parting Line,

• Making Cores,

• Minimising casting Defects,

• Providing Economy in moulding

• Others, as needed