Mats 214_l2 Casting 2011 Overheads(1)

8/3/2019 Mats 214_l2 Casting 2011 Overheads(1)

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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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Vacuum casting - stops oxidation


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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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:

Documents

Mats 214_l2 Casting 2011 Overheads(1)