aluminium technologies 27.10.2015

term paper

Cenk Eren ultrasonic processing of aluminium melts Murat Teke quality assesment of aluminium melts Emre Açıcı applications of aluminium foams Gazican Özkan squeeze casting of aluminium alloys Engin Kılınç foundry themes Mehmet Ağılkaya high integrity casting processes Haluk Erdemir additive manufacturing of aluminium alloys Serhan Köktaş rheo-casting of aluminium alloys and applications Alper Güneren friction stir processing of aluminium alloys Emre Baran production of aluminium foams Berkay Oral FSW automotive aluminium Erkut Özer casting of hypereutectic Al-Si alloys and applications İzzet Nahid Demir Casting of Al-Mg based foundry alloys and applications Mehmet Yasak surface treatment of aluminium alloys

Papers due on Dec 15 15 min presentations on Dec 22 & 29

castability Castability of an alloy is identified on the basis of 3

criteria:

fluidity

hot tearing tendency

die soldering

Short freezing range alloys The solidification front is planar.

Solidification is from the outside walls in towards the

centre as the metal proceeds along the mould.

The flow of metal stops when the two freezing fronts

meet

long freezing range alloys ● solidification front is no longer planar but dendritic,

and because freezing is occurring in a moving liquid,

the bulk turbulence in the liquid carries pockets of hot

liquid into the cooler regions, and thus remelting

dendrite arms and other fragments, to build up a slurry

of dendrite debris.

● flow of liquid is arrested when the volume fraction of

solid is somewhere between 25 and 50 %.

Slurry of

dendrites!

Lf

feeding distance, Lf

sand mould metal die short freezing range alloys

long freezing range alloys = 4-16 2-4

feeding distance (Lf)

● occurs when the cast aluminum alloy comes into

contact with die steel.

● Due to the natural affinity of iron and aluminum, a

reaction occurs at the surface, which results in the

formation of Al-Fe intermetallic phases.

● Over a series of shots, a significant amount of

aluminum becomes stuck to these phases at the

die surface, and the resulting cast part can begin

to miss critical tolerances or to lose integrity.

● At this point, the die must be shut down and

cleaned, which is an expensive process when it

occurs too frequently.

Die Soldering

● It is estimated that 1 to 1.5% of variable

overhead is directly attributed to die soldering in

casting plants.

● die soldering must be controlled due to a large

economic effect on the casting process.

● There are several ways to control die soldering:

melt chemistry

process conditions

die surface condition.

Die Soldering

● Fe has the greatest effect of any alloying element

in the study on reducing die soldering.

● It is well known that alloys with insufficient iron

content (< 0.8-0.9%) will solder readily to the die.

● The presence of Fe in the melt reduces the

chemical potential gradient of Fe from the steel to

the melt and slows the reactions that occur at the

surface.

● Sr also has the potential to help control die

soldering.

● In industrial trials a small Sr addition was shown to

reduce die soldering by more than 20%.

Die Soldering-melt chemistry

● High temperatures and high melt velocity

encourage soldering.

● high temperatures are more critical and must be

avoided through careful die design.

● Local overheating can be avoided via die design!

Die Soldering- process conditions

● use additional spray in the high solder areas for

extra cooling.

● use inserts with high conduction coefficients.

● control impingement velocity.

● coat die surface with lubricants (surface is likely

oxidized from prior treatment).

● A high impingement velocity can wash these

lubricants off the die surface, exposing the die

steel to the aluminum alloy erosion of the die

surface die soldering.

Die Soldering-potential remedies

● Semi-solid processing can help to reduce both the

temperature and velocities apparent in the

casting system, and should help reduce die

soldering.

● Die coatings can be useful as a diffusion barrier

between the steel in the die and the aluminum in

the cast alloy.

● An effective coating must be able to withstand the

harsh conditions at the surface of the die.

● Coatings which are sometimes used include

CrN+W, CrN, (TiAl)N and CrC.

Die Soldering-potential remedies

casting

Influence of mold filling rate ● For a clean microstructure it is necessary to

ensure non-turbulent mold filling in addition to

using clean metal.

● In order to prevent impurities (particularly oxides)

during mold filling, we need a uniform filling rate.

● If the mold filling rate exceeds a critical value (for

molten aluminium, 0.5 m/s) separation of metal

drops or folding of the metal front occurs, leading

to formation of additional oxides.

● In the case of gravity casting, a height of only 13

mm is sufficient to reach this speed.

Critical Velocity a wave starts to form under an inertial pressure having an

approximate value of ρ·V2 , where ρ is the molten metal

density and V is the velocity of the disturbance.

Once Vcrit is exceeded, the surface of the metal will

behave in a turbulent fashion, i.e. the surface will break up into waves and droplets, causing the oxide film defects.

Limiting condition: formation of a

drop

Critical velocity

Mold filling

Influence of mold filling rate Separation of

metal drops

and folding of

metal surface

If critical

velocity

(0.5m/s) is

exceeded,

metal drops

separate!

Mold filling

Solid oxide Liquid oxide

Oxide skin

Formation of wave

Separation of

metal drops

and folding of

metal surface

Influence of filling velocity

Casting Defects ● As the molten metal is poured into the mould,

turbulence leads to the formation of layers of oxide

which gets trapped in the metal, creating

entangled oxide films mechanical weakness!

● These oxide film defects often cause castings to

fail leak tests implying that the defects are

continuous from one side to the other.

● Castings are tested by pressurizing them with air

whilst they are submerged under water so that the

defects are revealed by a stream of bubbles.

● Aluminium castings should not be top-poured.

http://nptel.ac.in/courses/112107144/metalcasting/lecture2.htm#

sprue

Casting Design Assessment

Casting Design Assessment

a section through a mould that shows four

deliberate mistakes:

a conical pouring bush

a parallel sprue

no choke

no runner (bar)

so that the metal

enters directly into

the casting.

bad design features As the metal is poured directly from the ladle into

the conical pouring bush, it is already moving quite

quickly as it enters the

top of the sprue.

Its velocity V1 will be

determined through the

height through which it

has fallen.

Thus, this basin design

is bad because it has no

decelerating effect

on the metal.

bad design features metal accelerates due to gravity and so the stream

gets thinner, reaching a velocity V2 at the bottom.

Since there is no 'choke' at the bottom of the

sprue, the sprue will not

fill up completely.

As a result, there is a

Venturi effect with air

being sucked into the

metal stream through

both the sand walls of

the sprue, thereby

forming oxides.

The metal stream then hits the bottom of the sprue,

spreads out in a relatively thin film along the

horizontal surface of the

gate with a velocity of V3

which can be significantly

greater than V2.

It therefore enters the

cavity rapidly, hitting the

far wall where it rebounds

in an uncontrolled manner, forming a splash and

creating conditions for

further oxidation.

bad design features

Pouring Basin avoid the use of a conical pouring basin since this

does not decelerate the metal and also acts as a

venturi and causes air ingress.

use an offset pouring basin which helps to

decelerate the metal stream

before it enters

the sprue.

BAD

conical basin

BETTER

offset basin

Pouring Basin The best design is to introduce a step into the basin

to give an offset stepped basin.

The step acts to stop the rapid motion of the metal

over the top of the sprue and helps to ensure that

the latter is completely filled.

BAD

conical basin

BETTER

offset basin

BEST

offset+shaped basin

● The pouring bush should be rectangular in shape so

that the upward circulation during pouring will

assist in dross removal.

● The exit from the pouring bush should be radiused

and match up with

the sprue entrance.

Pouring Basin

radiused

Tapered Sprue ● Use a tapered sprue.

● The stream of metal will accelerate from a

velocity V1 at the top of the sprue to a velocity V2

at the base of the sprue and the conservation of

matter requires that its cross-

sectional area will decrease

from A1 to A2.

● It can therefore be seen

that the sprue will remain

full if:

A1 · V1 = A2 · V2

5° taper

● The sprue should provide a 5° taper from the

controlling area.

● The cross-section of the sprue can

be round, square or rectangular.

● A rectangular shape is preferred

due to a reduced tendency to

vortex formation which could

result in air aspiration.

Sprue

Sprue Well ● A sprue well helps to decelerate the metal, cushions the stream and

allows the flow to change from vertical to horizontal with a minimum

of splashing.

● It helps to ensure that the runner bar is filled.

● Recommended sizes of the sprue base are a diameter 2–3 times the

sprue exit diameter and a depth equal to twice the depth of the

runner bar.

Top vs bottom gating When the metal is introduced into the the casting cavity, by

top gating; the critical velocity is readily exceeded and the

resulting turbulence and splashing cause oxidation of the

molten metal.

The preferred technique is to use bottom gating, i.e. to

introduce the metal uphill into the casting although,

it is still important to limit the velocity with which the

metal enters the mould.

Top gating-turbulence bottom gating-prevents turbulence

Runner bar and gates

The distance between sprue and the first

gate should be maximised for effective

inclusion removal.

Non-ferrous alloys should always be cast with an

unpressurised gating system with the runner in the

drag (lower half of the mould) and the ingates in the

cope (upper half of the mould).

● Runner cross-sections should

ideally be rectangular, the wider

upper surface is to maximise the

potential of the runner bar to

collect dross and inclusions.

● Ingates should enter the mould cavity at the

lowest possible level to avoid turbulence

associated with a falling metal stream.

● As with the runner bar, ingates should be

rectangular in cross-section rather than square to

avoid a “hot-spot” and subsequent porosity at the

casting contact.

● Bottom gating means that the coldest metal is at

the top of the casting, just where the hottest

metal is needed to ensure feeding to avoid

shrinkage defects in the casting.

Runner bar and gates

● If one gate suffices: the runner bar will be a simple

parallel sided channel, arranged so that the metal

rises uphill from the sprue base, through the runner

and gate and into the casting.

● have a runner bar extension

which can be used to

receive the first metal

poured into the mould and

which often contains air

bubbles and slag particles.

Runner Bar and Gates

Runner bar extension

Uneven flow leads to an uneven T distribution and

an increased risk of turbulence-induced defects.

The runner bar should therefore have a gentle

tapered step at each gate to promote even metal

flow.

Runner Bar and Gates

The well should be the lowest point of the casting and filling

system and the metal should always progress uphill thereafter.

Uneven flow Stepped runner bar:

uniform flow

Runner Bar and Gates Another important feature is that the gating

arrangement must avoid waterfall effects:

To prevent splashing

To stay under the critical velocity

To avoid a static metal meniscus

● Where possible, gating should be into the bottom of the

casting

● Unpressurised gating should always be used, that is, the

gate areas should not limit the flow rate into the mould

cavity

● Ingates should be taken from the top of the runner to

ensure that the runner bar is always full

● The sprue should control the fill rate of the casting

● The sprue should be designed to avoid entraining air and

dross, it should be tapered downwards so that the sprue

base is the flow controlling area

● Low stream velocities should be used to avoid turbulence,

optimum stream velocities as low as 500 mm/s have been

reported for Al alloys.

Running, gating and feeding aluminium castings-summary

The widespread use of foam ceramic filters has

introduced a new dimension into the running and

gating of aluminium castings.

Filters have several important effects:

effectively trap dross and some oxide films.

control metal flow rate.

reduce turbulence.

Foam ceramic filters have a distinct advantage over

the extruded type in that there is no separation of

the initial metal stream which passes through them,

hence the possibility of reoxidation at the filter exit

face is less.

Gating with filters

Of these, the transformation from liquid to solid is

the most critical.

Types of shrinkage

7% for aluminium

linear with temperature

and can be compensated

for without much difficulty

arrangement of the

atoms from the rather

open, random close-

packed manner in a

liquid to a regular close-packed form in a solid

Solid state contraction; linear with temperature

The time interval between the start and the end of

solidification.

The interval between liquidus and solidus is

determined by a number of factors:

The solidification range of the alloy:

This is a fundamental characteristic of a particular

alloy. For a given mould material,

increasing solidification range larger interval

between start and end of solidification

Factors which influence solidification mechanisms

The thermal conductivity of the solidifying alloy:

high thermal conductivity

reduced thermal gradients within the casting

larger solidification interval.

Solidification temperature:

A low solidification temperature

reduced T gradients between mould and casting

will reduce thermal gradients within the casting

larger solidification interval.

Factors which influence solidification mechanisms

● solidification starts at the mould interface where

heat extraction is greatest.

● The chilling action of the mould wall results in the

formation of a thin skin of solid metal surrounding

the liquid.

● With further extraction of heat through this shell of

solid metal, the liquid begins to freeze onto it and

the wall of solid metal increases in thickness.

Solidification of short freezing range alloys

● The solid and liquid portions are separated by a

relatively sharp line of demarcation – the

solidification front– which advances steadily towards

the centre of the casting.

● The crystal growth on the solidification front is

relatively short and corresponds to the start of

freeze at their apex and the end of freeze at their

bases.

● Short freezing range alloys encourage directional

solidification even at relatively low thermal

gradients.

Solidification of short freezing range alloys

Solidification of short freezing range alloys

Typical forms of porosity in short freezing range alloys

feeder

Centre-line shrinkage

Cavity at thermal

centre

small open cavities near the

end of the solidification when

the feed metal is cut off by the

merging of parallel solidification fronts

open cavities at inadequately

fed thermal centres and isolated heavy sections.

Typical forms of porosity in short freezing range alloys

Cavity at heat centre

Centreline shrinkage porosity

Casting

riser

sound metal

effect of solidification mechanism on shrinkage distribution ● With long freezing range alloys, the development of

directional solidification is difficult.

● With long freezing range alloys, feeders often show

minimal pipe as the “mushy” solidification mode will only

allow liquid flow for a part of the total solidification time.

● Finely dispersed porosity can exist throughout the entire

casting section, with coarser concentrations at parts of

slower cooling such as junctions and under feeder heads.

● Under normal foundry conditions, it is virtually impossible

to achieve absolute soundness in extremely long

freezing range alloys such as tin or phosphor bronzes.

Typical forms of porosity in long freezing range alloys

three distinct zones for

a long freezing range

alloy:

a completely liquid

zone at the thermal

centre of the casting;

a zone of solid metal

next to the mould walls

a region of partial

solidification between

the liquid and solid

zones.

Feeding Rules ● Heat transfer requirement (Chvorinov's Rule)

● Volume requirement

● Junction requirement

● Feed path requirement

● Pressure requirement

● Pressure gradient requirement

● The Zeroth Rule: you should not feed a casting

unless it is absolutely necessary!

Heat Transfer Requirement

Modulus =

volume of casting /

cooling surface area

As M increases

solidification

time increases!

Modulus of the feeder >

modulus of casting

Mf = 1.2 Mc

the freezing time of the feeder must be at least

as long as the freezing time of the casting.

Volume Requirement The feeder must contain sufficient liquid to satisfy

the volume contraction of the casting".

● when we place the feeder on the casting, a hot spot

may be created. As a result, we will inevitably get

shrinkage porosity at the base of the feeder, which is

also known as under-riser porosity.

● This problem can be overcome by enlarging the

junction between the plate and the feeder.

● However, this leads to a large feeder which is difficult

to cut off the casting.

● This difficulty can be overcome by trying to avoid

junctions altogether and one way to achieve this rather

contradictory aim is to extend the casting and to put

the feeder on that.

Junction Requirement

Junction Requirement the junction between the casting and the feeder

must not create a hot spot.

feeding a plate casting

a section through a typical flanged wheel casting

it should be possible to feed this with a single feeder

placed on one of the heavy sections (C).

It can easily be seen,

however, that the feed

path to the other heavy

section (A) will be cut

when the thinner

section B solidifies.

Feed Path Requirement

● We can use 'padding', i.e. to add extra material so

that the feed path is kept open. The extra material

then has to be removed, which adds to the

manufacturing cost.

● One alternative is to use an

extra feeder, although this is

not always feasible and

may be difficult to remove.

● A further possibility is to

apply a chill or cooling

fin to A.

Feed Path Requirement

A

A

C

Pressure Requirement

● most defects - such as porosity or hot tears -are

volume defects, that is, they are induced by the

volume changes which occur as a casting

solidifies.

● It follows that if a pressure is applied to a

solidifying liquid, it is difficult for the defects to

nucleate.

● Therefore, sufficient pressure must be supplied

to all parts of the solidifying metal to inhibit the

nucleation and growth of volume defects.

Pressure Requirement pressure inhibits the nucleation and growth of volume defects.

Pressure Gradient Requirement There must be sufficient pressure differential to cause

the feed metal to flow in the correct direction.

Driving force = positive pressure + negative pressure

Positive : atmospheric +

hydrostatic

Negative: generated

by solidification

Net pressure must be

higher in the feeder

than the casting

Feeding mechanisms

in skin freezing materials - such as pure metals and

eutectics - it is the only type of feeding process.

Since the liquid metal has such low viscosity (near to that of

water) this mechanism works effectively at negligibly small

pressure gradients.

If liquid feeding can be ensured in a particular casting, then

the stresses which can occur in the liquid will be maintained

at such a low level that there will be no practical difficulties.

● most “open” type of feeding mechanism generally occurs

first

● only feeding mechanism in skin-freezing alloys works well

with low pressure gradients governed by the seven feeding

rules

Liquid feeding

inadequate liquid feeding

Mass Feeding ● the flow of a slurry of liquid plus solid crystals can

occur up to about 68 % solid in some alloys.

● At that stage of freezing the dendrites start to

impinge to form a coherent network, as a three

dimensional space frame, thus gaining rigidity and

resistance to further deformation.

● Flow of slurry of solidified metal in residual liquid

improves as: - section size increases

- grain size decreases

can effectively counter the layer porosity.

Mass Feeding ● The action of mass feeding is sensitive to the

relative size of the grains and to the section

thickness of the casting.

● For instance mass feeding cannot act in thin

section castings which have not been grain

refined.

● Mass feeding improves as section thickness

increases and as grain size becomes smaller.

● This is simply because if the section is narrow and

if the grains are large, they impinge on each

other and are supported on the side walls of the

casting, and so are not free to move.

Mass Feeding ● Porosity in such sections occurs due to the difficulty

of flow of the liquid among the dendrite mesh: this is

typically layer porosity - shrinkage porosity which

grows among the fixed network of dendrites.

● As the section size grows and grains become smaller

and the interior semi-solid slurry is free to flow, thus

more easily feeding the more distant regions of the

casting.

● Layer porosity disappears in such sections.

● grain refinement is clearly an important way of

facilitating this feeding mechanism.

Interdendritic Feeding As the dendrite mesh thickens, the interdendritic channels

become progressively narrower, and progressively more

resistant to the flow of the residual liquid.

No eutectic feeding problems microporosity

eutectic present elimination of narrowest part of path no microporosity

dendrites

liquid

eutectic

Solidification direction

Feeding mechanisms in Al alloys Aluminium alloys shrink by 3.5–6.0% during

solidification, so that without feeding, castings will

contain porosity defects.

The feeding requirements are dependent to a large

extent on the freezing range of the alloy being cast.

Feeding aluminium alloy castings ● For satisfactory feeding of short freezing range

alloys, feeders must be placed over thermal

centres of the casting; they must solidify after that

part of the casting to which they are connected.

● Insulating feeding aids are used to ensure effective

feeding and to improve yield.

● Feeders must be of sufficient volume to compensate

for the liquid and solidification shrinkage of the

alloy which is influenced by the alloy composition,

the degree of pouring superheat, the shape of the

casting and the gas content of the alloy.

● The concept of directional solidification has little

relevance with long freezing range alloys.

● The goal in feeding such alloys is not to eliminate

porosity totally but to ensure that it is dispersed

as evenly as possible throughout the casting

section.

● It is often desirable for feeders to compensate only

for superheat and a portion of solidification

shrinkage so as not to extend the solidification time

excessively.

Feeding aluminium alloy castings

● The shrinkage volume for which the feeders

must compensate is again influenced by

the alloy constitution,

the degree of pouring superheat,

the section thickness of the casting and

the gas content of the alloy.

● Long freezing range alloys have virtually no feeding

range and under normal foundry conditions,

achieving a high degree of soundness is virtually

impossible.

Feeding aluminium alloy castings

Simulation modelling ● software packages are now available which model the

flow of metals into dies or moulds and allow the

solidification of the casting to be simulated.

● Fluid flow software, Magmasoft being one of the best

known, uses physics-based modelling to allow mould

filling to be studied and its effects on casting

soundness to be assessed.

● Ideally such modelling should enable the onset of

turbulence during mould filling to be predicted and the

effect of gating systems on the temperature

distribution within the casting to be studied.

● While flow modelling is not yet perfect, it does enable

possible danger areas in the casting to be predicted.

the understanding of the origins of

defects in aluminium castings and their

reduction by attention to

degassing,

metal treatment and

filtration

has greatly improved the general quality

of castings in recent years.

Casting defects

Casting defects Because of potential casting defects, aluminium

castings, like all castings, suffer from variable

mechanical properties which can be described by a

distribution curve.

Conventional

sand casting

Advanced

high integrity

casting

Mean value

is the same,

but the

scatter with

sand casting

is more!

● The variability of properties of castings must be

reduced to allow designers to have greater confidence

in castings so that thinner sections and lower weight

components can be used.

● If, for example, the mean tensile strength for a cast

alloy is 200 MPa, the designer must use a lower figure,

say 150 MPa, as the strength of the alloy to take into

account the variability of properties.

● If the spread of the distribution curve can be

reduced, then a higher design strength, say 170 MPa

can be used, even though the process mean for the

alloy and the casting process stays the same.

Casting defects

Effect of filtration

Design

stress

Design

stress

Design

stress

Design

stress

Top filled AlSi7Mg/filtered unfiltered

bottom filled AlSi7Mg/filtered unfiltered

● The unfiltered castings show a few but very

significant low strength test pieces, known as

outliers.

● A design strength below 200 MPa would have to be

used for unfiltered castings because of the

occasional outliers.

● Examination of the fracture surface of the low

strength outliers showed massive oxide fragments

indicating that inclusions in the unfiltered castings

were responsible for the low tensile strength.

● Filtration eliminates inclusions allowing the design

strength to be increased to 230 MPa.

Effect of filtration

quality of castings

● porosity

gas

shrinkage

● Oxide inclusions

● Hot tears/cold cracks

● grain structure

● dendrite arm spacing (SDAS)

● intermatallic compound particles

● eutectic (and primary) silicon

● extent of segregation

● composition/alkalis/Fe/Mg/additives (Ti-B-Sr)

Casting defects

Chill casting (into metal moulds) has inherently a

greater possibility of producing higher quality than

sand casting because the higher solidification rate

reduces pore size and refines grain size.

The highest quality components are produced

using filtered metal,

non-turbulently introduced into metal moulds

and

solidified under high external pressure to

minimise or totally avoid porosity.

Properties of castings

Sources of gas porosity ● gas held in solution in the molten metal can be

precipitated as the metal solidifies, simply as a

result of the reduced solubility on freezing.

● if the mould is filled under very poor conditions,

air can be entrained in the metal stream and

then trapped as the metal solidifies.

● the sand binders used to make the moulds and

cores often break down when in contact with the

molten metal and the gaseous decomposition

products can force their way into the solidifying

metal, leading to defects which are normally

known a 'blows‘.

Gas porosities

Hydrogen is increasingly

enriched in liquid aluminium

during solidification.

There is strong link between

the gas pore size and DAS

values!

Uniform distribution of

pores with diameters

between 0.05-0.5mm çap

1-2mm from the surface is

pore-free

● If the mould is a metal die, then the environment is

likely to be dry and thus relatively free from water

vapour and its decomposition product, hydrogen.

The liquid metal may lose hydrogen to this

environment.

● In contrast, if the mould is made from sand (either

chemically-bonded or especially if bonded with a

clay-water mixture as in a greensand mould), then

the environment all around the metal will contain

nearly pure steam at close to one atmosphere

pressure.

hydrogen pores in castings

Gas porosities Factors that impact the porosity features

Melting operations

humid foundry atmosphere

refractories with moisture

moist tools /gelberi vb

mould material

melting practices

charge materials (scrap-returns etc)

hydrocarbon based fuels used in furnaces

insufficient degassing/fluxing

insufficient filtration

3H2O + 2Al = 3H2 + Al2O3

Casting operation

turbulent mold filling:

melt transfer with cascading

mould and core (decomposition of the organic

binders) gases

air inside the mould, runners

improper venting

mould and gating design

Solidification rate

Alloy composition

Sr level

Grain refinement

Gas porosities

● When a metal is poured into a sand mould

containing cores, the resin binders start to break

down and generate additional gas.

● If the mould and core have a low permeability, the

gas pressure will build up inside the core.

● If the pressure reaches the level where it exceeds

the opposing pressure of the molten metal, a

bubble can be formed in the metal and float up

towards the top of the casting.

● Such pores originating from core gases are rather

coarse as big as 10-100mm.

Gas coming from cores

Gas coming from cores

Such core

blows are as

large as 10-

100 mm.

Solutions Vent cores

include Use less volatile binders

Fill mould rapidly to build

up hydrostatic pressure

Air Entrapment

● surface turbulence in the metal stream as it fills

the mould, leading to a chaotic, scrambled mess

of metal and air.

● The air cannot escape easily because it is held in

place by the oxide film.

● Furthermore, as the air bubbles move through the

molten metal, they leave behind a collapsed sac

of oxide, forming a bubble trail which is another

form of defect in the casting.

Air entrapment

insufficient gates to

prevent turbulence

Conical pouring

basin

Parallel

column

No well!

Non-tapered runner bar

Bubbles trapped

along horizontal

surfaces above

ingate and under

ledges and

apertures in

casting;

irregular in size;

normally 0.5-5mm

solution:

improve running

system!

Bubbles

trapped on

horizontal

surfaces

above ingate

Forms due to bad casting design!

gas porosity

Gas porosity defects defect distribution size

Gas

precipitation

from solution

Uniform, apart from 1-2 mm

near surface

0.05-0.5mm

Air

entrainment

Above ingates, especially the

first ingate. Concentrated on

horizontal ledges.

Very close to surface.

Only revealed when casting is

shotblasted or machined.

1-5mm

Core gases At a uniform distance under

top of casting

Typically 100

mm dia.

10 mm thick

Gas porosities

B

A

1 mm

1 mm

1 mm

Gas porosities

Gas porosities

200µm 20µm

Gas porosities

20µm

100µm

200µm

Fatigue performance is

directly affected by the

gas porosities. The

largest pore across the

section will dictate the

fatigue life.

Gas-blister dissolved hydrogen transforms to hydrogen porosity

during thermal treatments in the course of down stream

processing. This occurs particularly near the surface

regions and impairs the surface quality by forming

blisters.

shrinkage

● The density of liquid aluminium is nearly %6.5

less than that of solid aluminium.

● Therefore aluminium contracts this much during

solidification.

● Castings always start to solidify at the surface

towards the centre.

● Unless we employ a sound feeding practice,

mould design, alloy selection, temperature

regime, this contraction is always manifested in

the form of shrinkage porosity.

● Si is the only element that counteracts shrinkage

as it expands as much as %8 during solidification.

● Alloys with little amount of eutectic (alloys far

from the eutectic point) exhibit either

dispersed microshrinkage or collapsing.

● Elements such as P, Na, Sb, Sr impact the porosity

shape since they change the morphology of the

silicon phase. These elements also effect the

porosity shape in alloys with higher Si.

Shrinkage porosity

shrinkage-alloy effect

Micro shrinkage macro shrinkage

spongious shrinkage collapsing

Shrinkage-microporosity

● This is particularly a problem in long freezing

range alloys and/or when the temperature

gradient is low.

● These conditions create an extensive and uniform

pasty zone which is favoured by:

● metals of high conductivity, such as aluminium

● high mould temperatures, as in investment

casting;

● thermal conductivity of the mould, as in sand,

investment or plaster low moulds.

Microporosity

Flow through the pasty zone

Promoted by:

Alloys with long freezing range

Low T gradients high metal thermal conductivity

high mould T

low mould thermal conductivity

Layer Porosity

Macro porosity

mold

Solidifying

metal

pipe

primary

secondary

Smooth shrinkage pipe in short

freezing range alloys

Sponge-like pipe in long freezing

range alloys

'pipe' formed as a simple

ingot of a short freezing

range alloy solidifies

Shrinkage porosity

scale macroporosity intermediate microporosity

cause Failure of liquid

feeding;

Feeding Rules not

correctly applied

Failure of

interdendritic

feeding

Failure of

interdendritic

feeding

Short

freezing

range

Smooth shrinkage

pipe

Centreline

shrinkage

Dispersed

microshrinkage

Long

freezing

range

Shrinkage sponge Layer porosity Dispersed

microshrinkage

microshrinkage

50µm 200µm

Porosity and mechanical properties

key structural features of aluminium

castings

structural features of aluminium castings

Casting defects

porosity

shrinkage

inclusions

hot tear cracks

cold cracks

residual stresses

Microstructure

grain size

DAS

eutectic Si

Fe based

Grain size Although grain size does tend to reduce somewhat

as freezing time is decreased, it is not closely

controlled by the

freezing time.

This is clearly

illustrated by

the general

scatter in grain

sizes above the

d = k·(tS)0.3 line

on this graph.

grain size

Growth morphology

changes from

planar to

cellular,

to dendritic as the

compositionally-

induced

undercooling

increases

(equivalent

to reducing

G/R)

Solidification mode

Dendritic solidification

Dendrites normally grow

from a single nucleus.

The dendrite arms all have

the same crystallographic

structure and

orientation, i.e. a

dendrite is a single crystal.

Grains can be considerably

larger than the DAS but,

the reverse is not possible.

Dendrite Arm Spacing (DAS) ● there is widespread confusion between the

concept of a grain and the concept of a dendrite.

● A grain may consist of one dendrite or a 'raft' of

thousands of dendrites.

● A grain boundary is formed where rafts of

different orientation meet.

● Although grain size is used to characterise the

scale of the microstructure of wrought alloys, it is

often more appropriate to characterise the scale

of cast microstructures by measuring the

secondary dendrite arm spacing (DAS).

Dendrite arm spacing (DAS) ● DAS increases with time as a result of coarsening,

owing to the reduction in surface energy achieved

by reducing the surface area.

● Some of the larger arms grow at the expense of

smaller ones, leading to an increasing DAS as the

dendrite gets older, and this process is controlled

by the rate of diffusion of solute in the liquid.

● Thus, the DAS, d, is largely a function of the

solidification time, tS, and the relationship is of

the approximate form:

d = k · tS 0.3

Dendrite Arm Spacing (DAS) Why is Dendrite Arm Spacing important ?

The mechanical properties of most cast alloys

depend strongly on DAS:

As dendrite arm spacing increases:

Tensile strength/hardness increase

Ductility and elongation increase

fracture toughness increases

fatigue resistance increases

A small DAS also reduces the time required for

homogenisation/solution heat treatments since the

diffusion distances are shorter.

Dendrite Arm Spacing (DAS)

● It is therefore beneficial to reduce the DAS as far

as possible and since this is almost exclusively a

function of the freezing time, any technique to

reduce this will improve the DAS.

● In the case of sand casting, metal chills will help

considerably in reducing the DAS.

● Die castings will have a finer DAS, and lower die

temperatures will assist even further.

Dendrite arm spacing DAS

DAS and

SDAS are

measured

with the

intercept

method!

DAS, SDAS

=n/l

Dendrite Arm Spacing (DAS)

This average length is usually measured by carrying out

a line count along the length of a number of primary

dendrite stems which happen to lie near to the plane

of the section.

Line length = 400 m

number of dendrite arms

crossed = 8

SDAS= 50 m

Dendrite arm spacing DAS

DAS vs grain size

DAS and d are two important yet independent structural

characteristics.

DAS = f (cooling rate/section thickness)

grain size = f(chemistry effects/(cooling rate, ….)

d nearly the same but DAS is much different!

DAS nearly the same but d is much different!

DAS Most critical structural parameters that impact the

mechanical properties

DAS

heat treatment and

morphology of the Si particles.

Ultimate tensile strength and elongation values

have increased markedly when the DAS values

were reduced from the initial 115 m to 25 m.

Fatigue life has also increased with decreasing

DAS.