design of castings - concordia...

Mech 423 #2 1

Casting-Comparisons

Mech 423 #2 2

Lecture 2

Introduction

Credits: 3.5 Session: Fall

MECH 423 Casting, Welding, Heat

Treating and NDT

Time: _ _ W _ F 14:45 - 16:00

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Solidification/Freezing

• Casting is a process where

molten material is allowed to

freeze and take the final shape

• Final product property that

depend of structural features

are formed during solidification

• Many defects gas porosity and shrinkage also happen this time

• These defects can be reduced by controlling the solidification

• Refinement of grain size is also possible by controlling solidification

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• Nucleation:- formation of stable particle of solid material within

the molten liquid.

• Growth:- growth of solid particles to convert remaining liquid to

solid.

• Nucleation – while material changes state, internal energy

reduces as at low temperature solid phase is stable than liquid

• New surfaces are created at the interface between solid and liquid

which requires energy

• There is balance between the energy levels


• Due to this balance in energy, nucleation occurs at

temperatures below the melting point


• The temperature difference

between the melting point

and the actual temperature

at which nucleation starts is

called super or

undercooling

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• Homogeneous nucleation takes place inside liquid metal

when atoms bond together to form large enough particle that

does not remelt (latent heat of fusion). Rare in industry.

• Heterogeneous nucleation takes place at foreign bodies e.g.,

mould walls, impurities etc. Most common type industrially


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• Each nuclei grows to form grain (crystal)

so in given volume, more nuclei means

smaller final grain size

• Products with smaller grains have better mechanical

properties generally (except creep).

• Innoculation - Deliberate addition of small impurity particles

(that do not melt) to provide many sites for nucleation and

give grain refinement.


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• Growth - as mould extracts heat, liquid cools, nuclei grow

in size (+ more formed) and eventually consume all liquid

metal to form solid

• Direction, rate and type of growth can be controlled by the

way heat is removed

• Faster cooling tends to give less time for growth (more

nucleation) and so gives finer grains usually.


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• Study temperature of cooling metal:- thermal analysis

• Insert thermocouples into casting and study the temperature vs

time

Cooling Curves

• Superheat is the heat above

melting point

• More the superheat, more time

for metal to flow into difficult

places before freezing

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• Cooling rate is the rate at which liquid solidifies. It is the slope of

the cooling curve at a given point T/ t

• At thermal arrest heat is being removed from the mould comes

from latent heat due to solidification

Cooling Curves

• Pure metals & eutectics show

thermal arrest at Tm (plateau)

• From pouring to solidification is

the total solidification time

• From start to end of solidification

is local solidification time

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• Alloys (non-eutectic) usually have freezing range; change in

slope of T/ t.

• Now the solidification appears as a slope in the curve

Cooling Curves

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• If undercooling required for nucleation, heat of fusion increases

the temperature back to melting point this is recalescence

Cooling Curves

• Specific form of cooling curve

depends on the material poured,

type of nucleation, and rate and

means of heat removal from mould

• Faster cooling rates and short

solidification times lead to

materials with finer grains and

better mechanical properties

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• Amount of heat that must be removed from a casting for

solidification depends on the amount of superheat on the

pouring metal and volume of metal in the casting.

• The ability to remove that heat depends on the exposed

surface area through heat can be extracted and the

surrounding environment to the molten metal.

• Taking these into account, chvorinov came out with a

prediction for solidification time

Solidification Time: Chvorinov’s Rule

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ts = total solidification time

n = constant (1.5 - 2.0)

V = volume of casting

A = surface area of casting

B = mould constant (dependent on metal, mould material

etc - density, heat capacity, thermal conductivity etc).

• Establish B by casting test specimens for a given mould

material under particular conditions

n

sA

VBt


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• This value can be used for computing Ts of other castings

under similar conditions

• Since riser and casting are of same metal and in same

condition, use the rule to compare solidification time for

riser and casting

• then use rule to design casting so that casting solidifies

before riser

• This is a must as the riser will then feed the solidifying

casting


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• Structure depends on metal/alloy, cooling rate,

additions etc.

• Chill zone - Narrow band randomly oriented

along surface (touching mould) due to rapid

cooling due to nucleation

• As heat removed, grains grow inwards, process

slows down

• Preferred growth of grains with fast growth

direction oriented with heat flow.

Cast Structure

FIGURE 13.6 Cross-sectional structure of a cast metal bar showing the chill zone at the periphery, columnar grains growing toward the center, and central shrinkage cavity.

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• Columnar zone – at the end of chill zone as

the rate of heat extraction reduces, By

selection processes grains growing in other

directions are stopped, only favorably

oriented ones grow

• Grains grow longer and towards the center

• Not very desirable (anisotropic properties,

large grains).

Cast Structure

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• Equiaxed zone – in many materials nucleation

takes place inside the casting and this can grow

to form spherical randomly oriented crystals.

• low superheat, alloying, inoculation can promote this

• This produces structures with isotropic (uniform in all

directions) properties

• Preferable structure

Cast Structure

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• Liquid metals tend to be REACTIVE. (Atmosphere, crucible, mould

etc) could produce defects in castings

• Metal + Oxygen Metal Oxide which is knows as dross or slag can

be trapped inside casting, and affect

• surface finish

• machinability

• mechanical properties (strength, fatigue life etc.)

• Material from sand, furnace lining, pouring ladle contribute to

dross or slag

Molten Metal Problems

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• Dross or slag can be controlled by good foundry practice

• Use FLUXES to cover surface and prevent reactions.

• Melt under VACUUM (some alloy steel), or INERT ATMOSPHERE

(titanium).

• Let oxides float on surface; take liquid metal from below so that the

oxide stays back and does not go into the casting. (figure 13.7)

• Use ceramic filters to trap particles.

• Gating system designed to trap particles as well


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• Gas Porosity – liquid metals contain

dissolved gas. more gas (hydrogen,

oxygen, etc.) can dissolve in liquid

metal than solid

• When metal solidifies, gas comes

out of solution to form bubbles –

gas porosity

• Bad for mechanical properties,

gas tightness, surface finish after

machining etc.


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• Prevention of gas porosity can be done

using different techniques

• Prevent gas entering liquid metal

• Melt under vacuum.


• Melt in inert gas or under flux coating to prevent

atmospheric contact

• Minimize superheat to minimize gas solubility

• Reduce turbulence, splashing etc during pouring.

Streamline the flow

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• Remove dissolved gas from molten metal before pouring.

• Vacuum degassing - spray molten metal through low pressure

environment

• Gas flushing – passing small bubbles of inert or reactive gas (nitrogen,

argon, chlorine in Al). Dissolved gas enters this flushing gas and is

carried away.

• React with gas to form low density solid (slag/dross) e.g. Al or Si to

deoxidize steel, Phosphorous in copper to remove oxygen. The oxides

stay on top of the molten metal and can be removed by skimming


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• Some gases enter liquid and diffuse into bulk (hydrogen in al) but

some react to form surface films.

• Usually from reaction with oxygen, moisture, hydrocarbons.

• Tin, gold, platinum usually free of films

• Lead - forms pbo on surface. Interferes with soldered joints (“dry”

joint - non-wetting) use fluxes/pre-tinning/non-lead solders.

• Ductile cast iron - more difficult than gray cast iron due to Mg.

• High Temp. alloys (many elements which can form oxides Al etc.)

Surface Films

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• POURING -This should be carried out to minimize turbulence.

• Prevent entrainment of oxide film

• Prevent further reaction/oxidation/gas entrainment.

• Low pouring height.

• Use filters.

• Casting rate must not be:

• too slow; laps, folded surface films.

• too fast; jetting, surface turbulence.

Surface Films

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Figure 1.11 The effect of increasing

height on a falling stream of liquid

illustrating: ( a) the oxide film remaining

intact; (b) the oxide film being detached

and accumulating to form a dross ring;

and (c) the oxide film and air being

entrained in the bulk melt.

Figure 1.14 Confluence geometries: (a) at the side of a

round core; (b) randomly irregular join on the top of a

bottom-gated box; and ( c) a straight and reproducible join

on the top of a bottom-gated round pipe ( Campbell, 1988) .

Surface Films

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• Machining - Oxide particles in Al alloys

and steels drag out and leave grooves.

• Tool tip is blunted

• Defects - Entrapped folded oxide films are “cracks” in the

liquid and carried into casting.

• Leak-tightness - leaking through walls of thin casting is due

to collections of defects such as entrapped films. Reduces

pressure-tightness of casting (eg. Cylinder heads etc).

Effect of Surface Films

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• Mechanical Properties

• increases scatter in property values, reduced fatigue

resistance.

• Fluidity

• “Cleaner” melts are more fluid and can be cast at lower temps.

• Repeated remelting/stirring of melt can cause problems if

oxide not removed.

Effect of Surface Films

Mech 423 #2 30

• require good flow of molten metal to all

parts of the mould and freeze in

required shape - in proper sequence

• If freezing before filling defects (misruns

& cold shuts) occur

• Ability of the metal to flow is fluidity and

this affects the minimum section

thickness of cast, length and fine details

• Measure of fluidity by standard castings

Fluidity

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• Fluidity depends on composition, melting point and freezing

range and surface tension of oxide films

• Pouring temperature affects fluidity (superheat)

• high enough for good filling

• too high - penetration into mould wall (sand mould)

• affects interactions

• between metal and mould

• between metal and atmosphere

Pouring Temperature

Mech 423 #2 32

Gating Systems

• Gating system distributes molten metal to all parts of cavity

• Speed of filling is important

• Slow – misruns and cold shuts (material solidifies before filling)

• Fast – erosion of gating or mould cavity and entrapment of mould

material in the casting

• CSA of various channels can regulate flow shape and length can control

heat loss (short channels with round CSA work well)

• Attached to heaviest section of casting to avoid shrinkage and to the

bottom to avoid turbulence and splashing

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Gating Systems

• Short sprues – reduce kinetic energy, avoid splashing

• Rectangular cups – prevent vortex or turbulence while pouring

• Sprue well – dissipate energy and prevent splashing

• Choke – smallest CSA in the sprue to regulate metal flow rate, if it

is above, the metal enters the runner without control (turbulence)

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Gating Systems

• Choke – located near the base, flow through runner is smooth, and

smaller CSA allows easier removal from casting

• Gating can also prevent dross from entering the cavity. Long flat

runners with more time for dross to raise will do it, but material will

cool faster

• Generally first metal contains dross and it can be trapped in well

• Ceramic filters can be added to trap dross and other foreign bodies

from entering the mould cavity as well

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Figure 2.8 (a) A simple funnel pouring cup, not recommended in general; (b) a weir bush of excellent design, whose upward circulation will assist in the separation of slag and dross, but which would need to be carefully matched to the entrance diameter of the sprue in the cope; and (c) an offset bush with an open base recommended for general use.

Gating Systems

Mech 423 #2 36

Figure 2.14 Various

sprue base designs

a) the first splash

problem - direct

linking of sprue to

runner;

b) steady-state vena

contracta problem

which cause air to

enter the stream

c) a well base,

avoiding the worst

effects of the first

splash and the vena

contracta problems.

Gating Systems

Figure 2.13 A cross-section of

a self-moulding sprue

a) formed integrally with the

pattern, - requires 'draw‘

negative taper. Bad design

b) A properly tapered sprue,

pattern needs to be

detachable, and be withdrawn

from the back

Mech 423 #2 37

• Liquid metal should flow into cavity smoothly

• Different gate designs depending on shape

• Gates can trap dross and slag

• Turbulent sensitive metals (Al & Mg) and low

mp metals use systems to prevent turbulence

• Turbulent insensitive metals (cast irons, some

copper alloys) and high mp metals use short

systems for quick filling

Gating Systems

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Gating Systems & Filters

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Gating System Design

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Figure 2.39 Vacuum delivery systems to pressure die-casting machines for (a) a horizontal cold chamber; and (b) a vertical injection type.

Figure 2.40 Low-pressure casting systems showing: (a)conventional low-pressure casting machine design using a sealed pressure vessel; and (b) using an electromagnetic pump in an open furnace.

Gating Systems - Pressure

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Gating Systems - Gravity

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• Three stages of shrinkage (volumetric contraction)

• Shrinkage of the Liquid (not usually a problem)

• Solidification Shrinkage as liquid turns to solid

• Shrinkage of the solid as it contracts while cooling to room

temperature

• Depends on co-eff of thermal contraction and superheat

• Liquid contraction can be compensated by liquid in the gating system

• While material changes from liquidus to solidus state, shrinkage can

occur, depends on the metal or alloy (not all metals shrink)

Solidification Shrinkage

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Solidification Shrinkages (%)

of some common engg.

metals

Aluminum 6.6

Copper 4.9

Magnesium 4.0

Zinc 3.7

Low-carbon steel 2.5-3.0

High-carbon steel 4.0

White cast iron 4.0-5.5

Gray cast iron -1.9


• Need to control shrinkage void

• Short freezing range metals and alloys tend

to form large cavities or pipes (Al ingots)

• design these to have void in riser

(feeder)

• Alloys with long freezing ranges have

mushy zone. Difficult to feed new liquid into

cavity. Dispersed porosity results, poor

properties

• Patterns need to compensate for shrinkage

when solid gets to room temperature

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• Eject casting immediately in die

casting to avoid cracking ?

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Risers and Riser Design

• Added reservoirs to feed liquid metal to solidifying casting.

• Aim to reduce solidification shrinkage & porosity.

• Filling & Feeding are different - Filling is quick, Feeding is slower

• Rules:

1. Feeder must NOT solidify before casting

2. Feeder must contain enough liquid to meet volume contraction

requirements

3. Junction of feeder & casting should not form a “hot-spot”

4. There must be a path for liquid to reach required regions

5. Sufficient pressure differential to feed liquid in right direction

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• Design casting to solidify directionally from extremities towards

riser (sometimes multiple risers required).

• Design riser to feed properly WITH minimum metal (scrap) -

sprue+gate+runner+riser+casting = total liquid metal required.

• Sphere is best theoretical shape (vol/S.Area is high) but

impractical for casting. Cylindrical shape is common.

• Make modulus (V/A) of feeder > modulus of casting.

• Thickest sections are usually last to freeze so attach riser to

them


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• Top Riser - sits on top of casting (short feeding

distance)

• Side Riser - sits next to casting

• Blind Riser - contained within mould (must be

vented)

• Open Riser - top of riser open to atmosphere


• Live (hot) Riser - receives last hot metal poured (metal in mould already

may have started to cool) – smaller than dead riser (part of gating

system)

• Dead (cold) Riser - filled before or concurrent with cavity by metal that

has flown through the mould. (top riser – dead riser)

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• Use Chvorinov’s Rule.

• Mould constant, B is the same, Assume n = 2.

• Make riser take 25% longer to freeze, i.e.; triser = 1.25tcasting

n

sA

VBt

22

25.1

casting

casting

riser

riser

A

V

A

V

• Insert modulus of casting and then calculate riser size.

• Note: Only use riser areas that allow heat loss - discount

common surfaces.

• Other methods exist.


Mech 423 #2 50


• Modulus of common

shapes

• Design should take into

account if there is un-

cooled based where the

riser and casting share

a surface

• Small - to reduce scrap

and low modulus to

solidify last

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• Riser has to be removed from casting (as well as runner/gate)

• Make connection small - easier to cut off

• But if too small link freezes before feeding.

• Use short connections placing riser close to casting.

Note: Risers are not always required. For alloys with large freezing

ranges feeding does not work well - fine dispersed porosity is

common.

• Die-casting, pressure casting, centrifugal casting pressure provides

feeding action to compensate for freezing.


Mech 423 #2 52

• Methods developed for risers to perform their job

• Promote directional solidification

• Reduce the number and size of riser to increase yield

• Generally done by

• Chills – speeding solidification of casting

• Sleeves or Toppings – retard the solidification in riser

Risering Aids

Mech 423 #2 53

• CHILLS - External and Internal

• Aim to speed (directional) solidification of casting

• External Chills - chunk of high-heat-capacity, high thermal

conductivity, material placed in mould wall next to casting to

accelerate cooling and promote directional solidification. (Made

from steel, graphite, copper) - reduce shrinkage defects.

• Internal Chills - Pieces of metal placed IN mould cavity to

absorb heat and promote rapid solidification. Becomes part of

casting same metal as casting.

Risering Aids

Mech 423 #2 54

• To slow cooling of a riser:

• Switch from Blind to Open riser

• Place insulating sleeves and toppings on risers

• Place exothermic material around feeder to add heat only

around the riser

Risering Aids

Mech 423 #2 55

• General design rules

for riser necks used in

iron castings;

a. general riser

b. side riser for plates

c. top round riser


Mech 423 #2 56

Figure 5.10 (a) Castings with blind feeders, F2 is correctly vented but has mixed results on sections S3 and S4. Feeder F3 is not vented and therefore does not feed at all. The unfavourable pressure gradient draws liquid from a fortuitous skin puncture in section S8. The text contains more details of the effects. (b) The plastic coffee cup analogue: the water is held up in the upturned cup and cannot be released until air is admitted via a puncture. The liquid it contains is then immediately released.


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A - gates

B - runner

C - Sprue exit (Choke)

• System is often designed to

follow ratio of (CSA) 1:2:2, or

1:4:4 WRT:

• Sprue exit CSA C : total runner

CSA B: total gate CSA A

• Gating system is un-pressurized

if area is increasing (e.g. 1:4:4)

or pressurized if there is a

constriction (4:8:3).


• Un-pressurized system reduces

metal velocity and turbulence

• Pressurized systems usually

reduce size and weight of gating

system (pressure at constriction

(gate) causes metal to completely

fill runner more quickly)

Mech 423 #2 61

Mech 423 #2 62

Surface Films

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