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CASTINGControlled solidification of liquid metal into the shape required
1. Casting techniques
Ingot casting
Continuous casting
Sand casting
Investment casting
Die castingCasting direct to final shape
Post-cast forging, rolling etc
2. Casting alloys
3. Cast microstructure, defects and properties
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sand
casting
investment
casting
green
sand
shell
moulding
chemically
bonded
sand
lost
wax
loast
foam
plaster
moulding
die
casting
low P gravity high P ingot concast
intermediate
processing
Casting
processes
Sources
www.efunda.com
www.castmetalsfederation.com
www.key-to-metals.com
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How do we select manufacturing processes?
Material class Materials to which the process can be applied
characterised by melting point and hardness
Size Minimum and maximum overall size
measured by volume or weight
Shape Aspect ratio, web thickness-to-depth ratiosurface to volume ratio
Complexity Information content, symmetry etc
Tolerance Dimensional accuracy or precision
Roughness Surface finish
Surface detail Smallest radius of curvature at corner
Min batch size Minimum number of components to be madeProduction rate Time to produce one component, cycle time
Cost Cost per component
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Casting processes
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Ingot casting
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Continuous casting Molten steel enters a water-cooled
copper mould from the tundish.
Steel at the mould surface is rapidly
cooled and solidified to form a thin solid
shell.
Advantages over ingot casting:
Improved qualityless segregation and fewerinclusions
Less wasteingot top and bottom discarded dueto defects and inclusions
Higher productivity & efficiencycan feed straight into rolling mill.
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Sand castingForming a mould with the help of a pattern pressed into a sand
mixture and then removed, after which molten liquid metal is
poured into the cavity in the mould.
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Sand casting
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Sand Casting (contd)
Green Sand
Lost foam [Replicast]
Sands may be chemically
bonded around pattern
[See shell moulding process]
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Investment Casting(the lost-waxprocess)
a precision casting process to fabricate near-net-shaped metal
parts from almost any alloy
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Aluminium
alloy
Titanium
alloy
Ti / steel
bio-compatible alloys
Airbus track landing flap
Aluminium alloy
600mm x 500mm x 250mm
Liquid food-filling machine
part (Titanium alloy)
400mm x 120mm x 60mm
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Sacrificial pattern: made from special casting waxes
normally injection moulded
can be hand made for small numbers
many parts attached to single tree
Injection mould tool: tool steel, < 1,000,000, < 6 months lead time
Stage 1: Pattern making
Wax
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Stage 2: Tool making
based on alumino-
silicates, alumina,
and silica
Dip tree in
ceramic slurry
Dip tree in
refractory sand
Repeat dip / dry
5 10 mm shell
Melt/ vapourise
pattern
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Stage 3: Metal casting
Pre-heat shell Pour alloy
& solidify
Remove shell Finish machining
OVERALL IC PROCESS:
Sacrificial pattern - complex shapes
small features with good surface finish
Large casting size range up to 100 kg
Economic for small & large batch sizes
Most metal alloys with melting point < 2500 C:
Steels, Aluminium alloys, Titanium alloys, Precious metals
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e.g. Turbine blade (Ni superalloy) investment casting
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e.g. Turbine blade (Ni superalloy) investment casting
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Vacuum casting - stops oxidation
e.g. Turbine blade (Ni superalloy) investment casting
wax pattern tree
Shell-coated
with refractory
investment
wax pattern
ceramic or vitreous silica can be dissolved out of casting
Cores
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e.g. Turbine blade (Ni superalloy) investment casting
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DIE CASTINGfor mass production of non-ferrous (Zn, Al, Mg) components
Four types of die-casting:
Gravity Low pressure
High pressure: Hot chamber
Cold chamber
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Zinc die cast parts
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Aluminium die cast parts
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Manual, much like
sand casting, but with
coated (~1mm of
ceramic) steel or
graphite die
All alloys castable
(even steel - with
graphite or coated die)
Gravity die casting
Applications: car/truck pistons, gears, cylinder heads, pipe fittings
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Common for Al alloys
(molten metal not
exposed to air)
Compressed-gas
pressure forces
molten metal upwards
through refractory
pouring tube into die
Low-pressure die casting
Applications:Automotive wheels and cylinder heads, gearbox and
clutch covers, transmission and differential housings, electric motor
stators, transformer covers and heat sinks.
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High-pressure die casting
Al, Zn and Mg alloys
Molten metal injected
into tool using plunger
Good for thin walls and
fine detail replication
Fast cycle time
Applications: clutch and gearbox housings; motor frames
and cases, switchgear housings; general applications:
pulleys, rotating parts, record player parts, etc.
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Die Casting
repeated use of a permanent (usually steel) die
rapid cooling and solidification (due to high thermal conduction of steel
die (c.f. sand/investment) from just above Tmelt
--> small equi-axed grains and a high production rate.
(steel, graphite.)
(coat with graphite, silicone)
The Die
must have high temp stability and good thermal conductivity
must have low adhesion between casting and die
component & die must be designed to release casting quickly
complex design, long lead times, high cost (1000 - 100000)
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COMPARISON OF CASTING PROCESSESSand casting
Investment casting
Die casting
Choice of casting process dependant on many factors, including
metal alloy fluidity, and melt temp (determines mould material)
cast component tolerances/detail/shape/size
control required overmicrostrucure (via heat-flow etc)
Casting to final shape preferred to forming of ingot when:
a large complicated shape required
quality and strength of component NOT very important(internal defects and cast microstructure generally give poor
mechanical properties c.f. forming)
ductility of alloy is too low to allow hot or cold working
it is cheaper !!
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Comparison of Casting Processes
SAND DIE INVESTMENT
Metals Any Non-Fe Any
Sizes (kg) 10-2 -105 10-3 -1 10-2 -1
Minimum dimension
Surface finish
CAPITAL COSTS Low High Average
LABOUR COSTS
Lead-time Days Months Days
Production rates Low Very high Average
Minimum quantity
5 mm 1 mm 1 mm
Poor Good Good
High Low High
1 ~10000 10 -1000
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Defects in castings
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Defects in castings
Casting defects:
shrinkage
porosity
inclusions
cracking and tearing
Effect on properties:
Reduced pressure
tightness
Reduced strength
Poor fatigue
properties
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Contraction on solidification is ~7% for Al and ~3% for Fe. (However, Bi and Si
both expand on solidification!)
Coefficient of linear thermal expansion: ~23x10-6 oC-1 for solid Al
~11x10-6 oC-1 for solid Fe
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Shrinkage compensation
All metals shrink on
solidification
Must compensate by
scaling partdimensions
Metal / Alloy
Volumetric
solidification
shrinkage
(%)
Al 6.6
Zn 6.5
Mg 4.2
Al-12Si 3.8
Steel 2.5 3.0Shrinkage Factor (%)
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Gas porosity in castings
e.g. as Al solidifies, only ~5% of the hydrogen dissolved in the liquid is retained in
solution in the solid under equilibrium conditions. The other 95% will be rejected
and can form gas pores.
Even if it doesnt precipitate out, it can cause embrittlement (e.g. H in steel)
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Grain structure in castings
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Segregation in cast alloys
Variation in the freezing
point of alloys leads to
separation of alloys
during cooling.
Leads to variation in
structure and properties
Can be reduced by
prolonged hightemperature heat
treatment (soaking).
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Hot Isostatic Pressing (HIPing)
Leads to improvedproperties of castings
Gas at high pressure
consolidates metal. Steps in the HIP
process: Place component in chamber
Evacuate
Re-fill with inert gas atpressure
Heat to soften metal
e.g. http://hipna.bodycote.com/index.asp?sid=markets&mn=frames.asp%3Fsid%3Dmarkets%26content%3D/main.asp
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MOULDDESIGN ISSUES
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Split Plane / Draft Angle
Select plane along which tool is
split opened to remove part
Re-entrant features require
sliding tool parts expensive Find simplest geometry
Apply draft angle to vertical faces
enable part removal
Draft angle
typically < 3
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Feeding system
Gate: point of entry intocavity
Runner: flow path fromsprue to gate or betweengates
Sprue: channel metalpoured into tapered toensure constant volumeflow rate
GATE
RUNNER
SPRUE
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Mould filling
Turbulent flow willcause air bubbles toform in the melt
These lead to GASPOROSITY in the solid
Aluminium die casting
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Mould filling
Often melt has solid
impurities (e.g. Al2O3 formed
on surface of molten Al)
Turbulent flow distributes
these through melt
inclusions in casting
Turbulence can even
damage sand tools leading
to sand inclusions
Steel sand casting
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Solution to turbulence defects
Design part geometry and tool and feed system toensure steady filling
However there will always be some solid impuritiesin the melt and
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Incomplete castings
Our tool currently gated bothsides at one end
Melt begins to cool as soon as
enters tool
M
ight solidify before it fillscavity completely
Solution: arrange feed system
to optimise fill rate
Gate
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Shrinkage Cavities
Solidification
shrinkage will lead to
cavities if casting is
not fed as it cools
Solution: use a gate
riser- a reservoir of
molten alloy that feedsto negate shrinkage
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Shrinkage porosity
Different areas of the casting cool and solidify at different
rates
Solidification shrinkage of some regions can be restricted
by adjacent regions of more solid metal leads to hot
tearing
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Shrinkage porosity
Some geometriesmore prone to this
than others
Solution: re-design
part
Solution: controlcooling rate
POROSITY
CHILLS
WATER
COOLING
SAND
TOOLS
METAL
TOOLS
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Reducing Gas Porosity / Solid
Inclusions Reduce turbulence
through tool design
Identify region of
casting that solidifies
last
Introduce a top riser
to trap bubbles /inclusions
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SPRUE GATE
RISER
TOP
RISER
GATE
RUNNER
Mould filling simulation
CAST
COMPONENT
Mould Design Issues: summary
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Mould Design Issues: summary
Poor Good
Runners and gating.
Feeding of a casting..
(a) no feeder-head
(b) feeder-head
eliminates cavity
(c) chills eliminate porosity
(d) tapering eliminates
porosity
Casting design problems..
(a) wall thickness variations, (b) corner
hot-spot, (c) cross-rib hot-spot, (d) hot-
tearing
(a) (b) (c) (d)
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CASTING ALLOYS
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Common Casting Alloys
Always SAND
-CAST:y Cast irons 1200OC
white, greyand nodular
y Steel 1500OC
Usually SAND-CAST:
y Copper alloys 1000OC
e.g. brass (20-30%Zn)..
USUALLY INVESTMENT CAST:
y Nickel superalloys - 1400OC
turbine blades.
DIE-CAST into permanent steel moulds:
y Al alloys 600OC
e.g. Al (3-4%Cu,3-12%Si)
y Mg alloys 600OC
e.g. Mg (10%Al)
y Zn alloys 400OC
e.g. Zn (2%Al, 1%Cu)
ALLOY % usage in casting approx T melt
84%
7%
2%
4%
1%
2%
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Cast iron
Grey iron (Fe + 2-4%C + 1-3%Si)
Si allows the C to form into graphiteflakes
Cheap, hard, stiff, weakLow Tmelt (c.f. steel) & good fluidity
Easy to machine
Vibrational damping
Little contraction on solidification
Spheroidal graphite (SG) iron
Modify with small addition ofMg
Improved strength, ductility and
toughness
Properties:
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Aluminium-silicon
LM6 casting alloy (Al-12% Si)
Low melting point
Narrow melting range
Little contraction on solidn Low ductility
Modify with 0.02% sodium to
refine microstructure
Properties: