dons and donts aluminio

8/12/2019 Dons and Donts Aluminio

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Metalworld April 2006 13

The answer is a Science for

foundriesnow dealing withcasting

Simulation softwares,

automation, robotics and other

scientific approaches. This is

recognized asatoken of maturity

and growth bytheindustry.

Buttheanswer can also bean Art,

because there are many invisible,

uncontrollable (or difficult to

control) factors playing role in

governing the output (i. e.

castings).

A s far as foundries are

concerned, wearestill in need to have

greater control over production

parameters to become a member of

theclub of highly reliableindustries.

Such control is specially needed in

non-ferrousfoundrieswhererelatively

fewer process control techniques

havebeen adopted (ascompared to

processcontrol techniquesin ferrous

foundries). Thispaper attemptsto list

various difficulties i nvolved in

aluminiumfoundry practices.

Do's and Dontsin A luminium FoundryO perations

SPSPSPSPSP. K. K. K. K. Kasi, Lasi, Lasi, Lasi, Lasi, Lakshmi Machine Wakshmi Machine Wakshmi Machine Wakshmi Machine Wakshmi Machine WorksorksorksorksorksLtd., (Foundry Division)Ltd., (Foundry Division)Ltd., (Foundry Division)Ltd., (Foundry Division)Ltd., (Foundry Division)

Courtesy:DIECASTING, Ilzda

Is metal-cIs metal-cIs metal-cIs metal-cIs metal-casting an asting an asting an asting an asting an Art?Art?Art?Art?Art?

or a Science?or a Science?or a Science?or a Science?or a Science?

Manythermodynamicfactors,

physical metallurgy principles and

foundry techniques are involved in

converting themetallic rawmaterial(s)

into castings.

A luminiumand aluminiumalloysA luminiumand aluminiumalloysA luminiumand aluminiumalloysA luminiumand aluminiumalloysA luminiumand aluminiumalloys

Aluminiumisthe20th century

metal. T wasconsidered very costly till

endof18 century. After theHal-Herold

electric extraction process became

commercial in 1911, the metal is

widely used now.

A luminium alloys have

excellent propertieslike:

Good electrical & thermal

conductivity

Lowdensity

Superior corrosion resistance

Easy fabrication and superior

recyclingnature

High strength development by

alloying

A luminiumalloy castingsare

used widely for making components

for automobiles, aeroplanes, electrical

equipment and household appliances.

Corrosion resistance is superior due

to self-healing nature of aluminium

oxide. Exposingthemetalsto ambient

conditionpromotesformationof Al2

O3

layer, which has higher volume than

the volume of aluminium from which

it is formed.

Special feature of any cast-

alloy group isthepossibilityof forming

new alloys in the group. Cast

aluminium alloys are used with or

without precipitation hardening

treatment, theonly strengtheningheat

treatment applicable to these alloys.

Heat treatable cast alloysare difficult

to weld while it is easier to weld

non-heat treatable alloys. LM 0,

LM 2, LM 4, LM 5, LM 6, LM 12, LM

13, LM 24 and LM 26 arethe major

cast alloys used for various

applications. Theseare Britishgrades,

but are also popular in India. A l-Si

(6061), Al-Cu(2024), Al-Zn(7075)

are some precipitation hardenable

alloys. Mechanical properties of cast

A luminium alloys in heat treated

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also takes place. Hence furnace

atmosphere should be slightlyreducing, and maximumtemperature

should becontrolled.

Moisture :Moisture :Moisture :Moisture :Moisture :

Dissociation of moisture

during melting produces oxygen and

hydrogen, which in turn, increases

hydrogen pick-up bythemoltenmetal,

resultingin higher pinhole porosity, a

commondefect in Aluminiumcastings.

Hence controlling moisture isimportant. A lloyingelementsand flux

must be preheated well to eliminate

chances of moisture pick-up. For Mg

addition into the melt, weshould use

inverted perforated cup to preventMg

burning on theslag.

ChargingChargingChargingChargingCharging

Chargeselectionisimportant.

Clean aluminium alloy ingots and

foundryreturnsarepreferred. Turnings,borings or any metal cuttings should

be avoided as lubricants from such

scrap will spoil the melt or cause

explosion. Foundries may use pre-

alloyed ingotspurchased from quality

ingots manufacturers or prepare the

alloy at home (using hardner alloys).

While alloying at home, charging

sequence must be based on the free

energy calculationssuch that utilizing

heat evolved by reaction between twoelements is used for increasing the

heat content of newly added another

element. By this practice energy can

be preserved and elemental loss is

also minimized.

If we use charcoal cover,

aluminium may get locked into the

charcoal pieces. Skimming off the

charcoal would then result in larger

condition are comparableto cast iron

or steel, and are preferred whenstrength : weight ratio isconsidered.

Production processesProduction processesProduction processesProduction processesProduction processes

MouldingMouldingMouldingMouldingMoulding

A luminium alloys have

relatively lower meltingpoint (6600 C

or lower). Finegrain sandcan beused

for moulding. Permeability should be

at least 25-30 to allow gases

generated in the mould (during

solidification of the poured metal) toescape. Mould should not berammed

very hard. Fine-grain sand with

moderately rammed mould will

improve plasticity of the mould.

Solidified hot aluminium alloys are

lower in strength, hence if free

contractionisrestricted bythemould,

the casting may develop cracks.

Skindryingor completedrying

of the mould is important in A l-Mgalloy castingwhere Mg tendsto react

with water vapour at mould walls,

causing pinholes or subsurface

porosity. If the metal is poured

immediately after mould closing, skin

dryingisenough. For delayed pouring,

complete mould drying is necessary,

otherwise moisture from the interior

of the mould would diffuse into the

dried skin of the mould, causing

defects in the castings.

Metal moulds, with

hexachloroethaneor hexchlorobenzene

based coatings, and sand mouldswith

boric acid will givegood results.

Gating systemGating systemGating systemGating systemGating system

N on-pressurised gating

(having area of Ingate(s)> area of

Runner(s)> areaof Sprue) should be

used to permit rapid filling of the

mould with negligible turbulence, asturbulenceleadsto air aspirationwhich

may prevent mould fillingby forming

oxide layers. Gates cut in thereverse

direction, step gates or slot gates are

preferablefor smooth mould filling.

Aluminiumalloyshavehigher

coefficient of thermal expansion.

Duringmeltingmetal expandsfurther

(expansion in aluminiumismore than

that in steel iron cast iron). Henceduring solidification, the alloy

contractsheavily, resultingin shrinkage.

Higher shrinkage necessitates the use

of large size risers.

Entrapped air, slag/ dross

inclusions are more common in

aluminiumalloy castings. Entrapped air

is found beneath the cope surface.

Hencetherunner shouldhaveeffective

extension. Streamlining of moltenmetal flow is beneficial (Reynolds

number < 3500/4000).

Renoldsnumber (R) = pvd/m; m/p= n;

Where

p : Densityof molten metal;

v : Velocityofmoltenmetal ingating

system;

m : DynamicViscosity;

n : KinematicViscosity.

Viscosity is the resistance to

flowof anyfluid at given condition. As

the temperature increases, viscosity

drops.

Furnace atmosphereFurnace atmosphereFurnace atmosphereFurnace atmosphereFurnace atmosphere

During melting aluminium

tends to get readily oxidized at

elevated temperature. Loss of some

alloying elements (for instance Mg)

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F e at u r e

loss of the metal. Moreover it may

remain ineffective for protecting themelt, becausealuminiumwould come

up through it and react with air. Zinc

chloride/ AmmoniumChlorideisuseful

for cleaning. It separates the metal

from the dross by changing the

interfacial energy.

Low melting point elements

likeZn, Mgcan beadded in elemental

form, preferably with phosphorizer to

deoxidizeand capture hydrogengas(2P+ 3H

2= 2PH3), whereasother

elementsmust bebought in theform

of hardner alloys. To produce

A luminium-N ickel hardner alloy,

A luminium is melted first, then Ni

shots are taken in small crucible and

slowly agitated into the melt.

Excessive quantity of shots at a time

would produce a pasty mass. For A l-

Cu system, Cu should bemelted first,

then A l ingotsto beadded.

Melting crucibleMelting crucibleMelting crucibleMelting crucibleMelting crucible

Graphite, SiC cruciblesor Cast

Iron pots(coated with a thin layer of

refractory powder withsodiumsilicate

mixed with water asabinder) areused

for melting. Fuel economy isgoodwith

Cast Iron pots, but efficient lining is

needed to avoid excessive

contamination of the melt with Fe.

Reworkingof coatingat regular intervaland prior drying is a must, as loose

oxide pieces or moisture may affect

the melt.

TTTTTemperemperemperemperemperaturaturaturaturature ce ce ce ce controntrontrontrontrololololol

Temperature control is

essential in aluminium alloy melting.

If the dross formed by adding flux is

allowed to comein contact with air, at

temperature above 9500 C, a strong

crystalline aggregate of corundum is

formed. Itsspecific gravity of 3.9 gm/cc is higher than that of A l, hence it

settle down, exposing new melt

surfaceto further oxidation lossesand

hydrogen pick-up. Presence of A lkali

(Na, Ca, Sr, Li) of rareearthelements

like Ce or Be reduces oxidation

tendency even at higher temperature

(if sufficient content in themelt). This

is because high temperature leads to

loss of these elements. N a offersone

detrimental effect to the melt. Itencourages hydrogen pick-up from

furnace gases. Temperature control by

regular check up with pyrometer in

advisablein caseof cokefiredcrucible

furnaces. A bsorption of hydrogen is

promoted as its solubility with less

resistance for diffusion.

Drossformation (combination

ofAl2

O3

& other oxidesisbeneficial to

some extent as it prevents diffusionof hydrogen into the melt during

melting. On theother hand, it indicates

loss of alloying elements, which

necessitates remelting with flux to

recover the same.

Deoxidation & degassingDeoxidation & degassingDeoxidation & degassingDeoxidation & degassingDeoxidation & degassing

Fluxes are used to combine

with oxygen and oxides on melt

surface, enabling recovery of base

metal with alloying elements, makingviscous slag and protecting the melt

from further oxidation attack.

Therefore to homogenize the melt

stirringfrombottomupwards, without

excessively disturbing the surface,

should befollowed. Fluxesare amust

whenexcessiveproportion of foundry

scrap is used in thecharge. Foundries

operating fast-melting crucible

furnacesrarely use fluxesto minimize

thedanger from contamination.

Fluxingat low temperatureis

desirableto formoxidelayer. A sonce

corundum (crystalline aggregate) is

formed at high temperature, it would

bevery difficult to decompose it and

recover base elements.

Proper degassing will

eliminate the chances for occurrence

of defects due to gaseous elements.

In melting A luminium alloys C2Cl6(Hexachloroethane) tablets were

preferred over chlorine gas. At present

use of SF6 (Sulfur hexafluoride) with

carrier gas has become popular. It is

better than earlier practice as it does

not cause damage to environment.

C2Cl

6 not only degassesthemelt, but

also refinesthegrains(heterogeneous

nucleation). Gas flushing is more

effective than salt addition for

degassing since chloride salts needtime to decompose, liberate Chlorine

and then remove hydrogen. It may

introducegasrather than removingit,

if it isnot properlydried.

Directional solidificationDirectional solidificationDirectional solidificationDirectional solidificationDirectional solidification

Directional solidification isthe

desirable phenomenon in any casting

as it tends to improve the yield. By

laymans point view, it is a case whereuseful cast portion solidifies first,

followed by the riser/ feeder. By

technical understanding it is the

solidification processproceedingwith

and assisted by temperature gradient

pattern from cast portion towardsthe

riser. Chills and exothermic or

insulating sleeves are useful in

achievingdirectional solidification.


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F e at u r e

p : density,

g: gravityforce

h : heightof molten metal in thesprue.

Grain refining is important

and desirable in aluminium alloy

castings as it improves mechanical

propertiesof thecastings. Chill usuage

or ladle addition of elements like B,

Na, Ti and Cb are useful in refiningthegrains. Al-Ti (Ti = 3-10 %) or Al-

Ti-B ( Ti = 3-10%; B=0.2-1%; Ti/B

ratio = 0.1) can be used for grain

refining, if other proprietary agentsare

not used.

For modification of silicon

needlesin A l-Si alloy, either Na(in the

formofmixtureofNaCl + NaF orNaCl

+ NaF + KCl) or Sr (intheformof Al-

10% Sr alloy) can be used. Sr seemsto be good in maintaining the

modification for 3 hours, whileNacan

maintain it only for 20-30 minutesat

thetemperature750-780 0 C. Mnor

Be can al so be added f or

modification, in which the later

performs dual role as a deoxidizer

and a modifier.

SalvagingSalvagingSalvagingSalvagingSalvaging

Welding of aluminium alloycasting should be done before heat

treatmentasHA Z (heat affected zone)

will be subject to coarsening of

precipitates. Weldingrod should beof

similar composition. Higher heat input

is required generally to weld thick

castings due to high thermal

conductivity of aluminium.

A lternatively, the end portion of the

casting may be preheated well to

Amongthethreetypesof heat

transfer mechanisms(viz. cinduction,convection, radiation), conduction is

dominant in mould filling and

solidification bothin die-castingand

sand casting. Metals with low melting

point should havegood fluidityduring

pouring since temperature gradient

between the mould walls and the

surrounding atmosphere would be

less. i.e. Heat Transfer rate, Q = -KA

dt/dx is less, where

K : Thermal conductivityof the

mould material,

A : A rea of heat transfer,

dt/dx : Temperaturegradient over

thedistancein themouldwall.

Butthisdoesnot happen in Aluminium

alloy casting. The reasonsare:

(a) Theoxidelayer doesnot fuse, but

acts as a barrier in mould filling.

(b) Composition/Dendrites: Molten

metal gets entrapped in between

theprimary, secondary andtertiary

dendrites formed during

solidification. Very severe in case

of long freezing range (mushy

nature) alloys, resulting in

microshrinkageand interdendriticsegregation.

So, faster cooling must be

created during cooling to convert

concentrated porosity into dispersed,

isolated porosity byformingfinegrains

and to avoid shrinkage. Unfortunately

even dispersed porosity becomes

undesirable when the pores are

interconnected. It results in leakage

and losss of pressure tightness,

important in valves and pumps. A s aremedial measure, sealing of porosity

either by shot peening or hammering

or impregnation isdone.

During melting most metals

(except grey iron and bismuth)

undergovolumetric expansion, and on

cooling (i.e. during solidification)

contraction. Thismay develop internal

stresses acting on semi-solid metal.

The pulling action by solidified metalonsolidifyingzonecausecracking. This

occursmostly in mushy nature alloys

like A l alloy containing appreciable

amount of Cu or Zn (A l-Cu-Zn, Al-Si-

Zn), and leads to hot shortness (i.e.

high temperature cracking during

solidification) in intricate castings. In

such cases design modification (i.e.

filleting) inthecomponentor providing

a paddingis preferred.

Inclusion control & grain refiningInclusion control & grain refiningInclusion control & grain refiningInclusion control & grain refiningInclusion control & grain refining

Inclusions (oxides, carbides,

graphite flakes or borides introduced

by grain refiners) are dangerous in

aluminium castings, especially under

fatigue load. Crack initiation,

propagation and failure are steps

involved in fatigue failure which is

favoured by surface irregularities or

cracks. Theseinclusionsaffect surface

appearance and also act asnucleatingsitefor cracks. Proper skimmingaction

before pouring, use of tea-spout ladle

for pouring, useof strainer core, skim

bob gating or filter usuage are

followed. Besides the efficiency of

filter, increasing the meallostatic

height should be considered,

P = pgh

Where,


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F e at u r e

minimize the heat loss from the

weldingzone. Cleaningactionof Al2O3layer formed during welding is

performed well by AC or DC (Reverse

Polarity). But, again salvaging by

welding hasassociated difficultieslike

porosity, liquification (solidification)

crackingand distortion. Distortion can

beovercome bylow heat input while

crackingis not due to hydrogen as in

the case of steel or cast iron but

solidification stresses, which means

rapid cooling necessary. Weldingshould befollowed byheat treatment.

Challenges for the foundry industryChallenges for the foundry industryChallenges for the foundry industryChallenges for the foundry industryChallenges for the foundry industry

Science & technology never

stops growing, till man puts an end.

A stheinnovation anddevelopment of

newer materials have been growing

alongwithscienceand technologythe

applications of these materials also

have been growing. Polymers or

polymersassociated composites have

slowlystartedto replacesomemetallic

materialsand alloysin situationswhere

the selection parameters (l ike

fabrication, properties, cost etc) for

later are more attractive. Industries

dealing with metals, including

foundries, will havetheimpact of such

replacements. A bove all, insufficient

technologicalresourcesandproduction

capacity to meet global demand of

producing exceptional castings,

inconsistent quality, continuous

increase in raw material and fuel

prices, stringent pollution control

norms, competition fromfoundriesof

other countries, uncertainty in business

expansion are the major challenges

beforethefoundry industry in India.

Concluding remarksConcluding remarksConcluding remarksConcluding remarksConcluding remarks

A sper the latest forecast, by2006 Indian aluminium foundry

industry could produce 1 lakh tones

of aluminiumalloy castings. Chinamay

produce 17 lakh tones ( which is

around 3/4th of the total A l casting

demand of nearly 25 lakh tonnes in

the U SA ). This indicates a huge

potential demand if Indian aluminium

foundries(includingdie-castingunits)

take appropriate actions.

Foundry activitiesneed notbe

secretive. Larger size foundries must

initiate more R & D and involve the

smaller unitsin gettingthe work done

byagreement. Dueto thiseven smallerunitswill havedirect or indirectroleto

play, enabling the larger units to tap

the full potential of the overseas

casting market.

There are hundreds of materials

in the materials spectrum. Let us

attract the designers to choose

aluminium in preference to other

materials.

There are many manufacturing

processes. Let us attract thedesignersto choose metal casting

process over other processes.

There are thousandsof foundries

world-wide. Let usattract global

casting users to choose Indian

foundriesin preferenceto others.

Think at micro level (e.g. grain

refining and modification) in

technology, andat macri level (e.g.

productivity) in management.

ReferencesReferencesReferencesReferencesReferences

Dr. Koch Hubert, 31 ReasonstoUseA l casting, Foundry,

Vol XV, No.6, Issue 90.

KirginH. Kenneth, CastingImportsbytheU.S. Approach

3 Million Tons, Foundry,

Vol XVI, No.6, Issue96.

Srinivasan N.K., Cast Metals,

Foundry Technology, Khanna

Publishers, pp. 168-173.

Flinn A. Richard, Non-ferrous

Metal Practice, FundamentalsOf

Metal Casting. Addison-Wesley

PublishingCompanyInc,

pp. 35-49.

WilliamH. Salmon, Eric N. Simons,

Foundry Practice.

Campbell L. Harry, Metal Casting,

J ohn Wiley & sonsInc.

www.castingsource.com

www.key-to-metal.com

other related websites.

dons and donts aluminio

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