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Casting and Heat Treatment of AL-4%Cu Alloy

Abstract:

Copper has been the most common alloying element almost since the

beginning of the aluminum industry, and a variety of alloys in which copper

is the major addition were developed.

In the cast alloys the basic structure consists of cored dendrites of aluminum

solid solution, with a variety of constituents at the grain boundaries or

interdendritic spaces, forming a brittle, more or less continuous network of

eutectics. Wrought products consist of a matrix of aluminum solid solution

with the other soluble and insoluble constituents dispersed within it.

Introduction:

Use of aluminium castings in automobiles has increased from nonstructural

demanding, as it is the case of cylinder heads and engine blocks, to

structural parts, such as suspension struts due to the bene!cial e"ects that

arise by combining light weight and mechanical properties. #he re$uirements

for such structural parts are internal soundness, integrity, high strength and

toughness, and, as result of this, %l&Cu cast alloys are considered to be ideal

candidates. #he parameter that exerts the highest in'uence on the levels of

strength and internal $uality re$uired is the solidi!cation rate( as the

microstructure of the material is re!ned when solidi!cation proceeds at a

higher rate. % further advantage of microstructural re!ning, when dealing

with aluminium alloys susceptible for heat treating, is the enhancement in

their mechanical properties( as particles formed during solidi!cation will be

smaller and will re$uire less time to dissolve during heat treating )*+.

Experimental rocedure:

!" and mold was prepared of known specimen.#"#he weighed amount of %l - Cu were taken as per calculated

theoretically.$" %l was melted in pit furnace.

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4" %fter achieving %l in molten form, Cu was introduced in the form

of Cu wire." Cu starts dissolving into the melt.&" olid degassing was done in order to remove all entrapped gases

to get sound casting.'" lag was removed mechanically.(" %fter achieving a uniform melt of %l/Cu alloy , it was casted

into sand mold.)" %fter solidi!cation di"erent samples were obtained and heat

treated , there procedure is also discussed.

*luxes and +egassers used in Al Casting:

+i,culties in Al elting:#he main di0culties in %l melting are porosity, blow holes and gas

absorption.

+rossing: 1olten oxide product of metal is called dross.

.as Absorption: olubility of gases in the molten metal is known as gas

absorption.

#he following reactions take place during the melting of aluminum(

2%l 3 4526 7 %l2643 452

8%l 3 4C627 2%l2643 %lC4

9%l 3 4C6 7 %l2643 %lC4

9%l 3 4627 %l2643 %l24

:rossing will be more in case of pit furnace melting than induction melting.

%s it is obvious from the above reactions that C6 - C62 is source for :ross

formation and it will be available in ;it furnace due to burning of . :ross formation always must be kept minimum. #here is 8?>?/ %l in

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dross that@s why dross comes on top surface otherwise sink. #hese kind of

inclusions are detrimental to mechanical properties.

52 gas is soluble in considerable amount in the molten %l. %s the

temperature of molten metal increases the amount of 52 solubility alsoincreases. 6n solidi!cation 52 solubility decreases and result in the

formation of pin holes and blow holes.

Hydrogen /emo0al:

:issolved hydrogen levels can be reduced by a number of methods, the most

important of which is 'uxing with dry, chemically pure nitrogen, argon,

chlorine, and freon. Compounds such as hexachloroethane are in common

use( these compounds dissociate at molten metal temperatures to providethe generation of 'uxing gas.

*lux:

% chemical substance used to re!ne metals by combining with impurities to

form a molten mixture that can be readily removed.

+egasser:

% chemical substance used to remove gas from li$uid metal.

*luxing of Aluminum Alloys:

Aluxes for melting aluminum are solid substances Bcommonly mixtures of

chloride and 'uoride salts used in aluminum foundries in order to reduce the

melt oxidation, minimiDe penetration of the atmospheric 5ydrogen, absorb

nonmetallic inclusions suspended in the melt, keep the furnaceEladle wallclean from the built up oxides, decrease the content of aluminum entrapped

in the dross, remove hydrogen dissolved in the melt, provide aluminum grain

re!ning during olidi!cation, modify silicon inclusions in silicon containing

alloys, oxidiDe excessive magnesium.

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Common practice of 'ux introduction is manual application.

1ost of the 'uxes are applied on the melt surface and stirred into the melt.

ome of the 'uxes Bdegassing, grain re!ning are plunged to the bottom by a

clean preheated perforated bell.

Aluxes may also be introduced into the melt by injection in form of a powder

in an inert gas B%rgon or

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+rossing *luxes:

:rossing 'uxes promote separation of molten aluminum entrapped in the

dross Bsometimes up to 8?/. Gesides chlorides and 'uorides drossing 'uxes

contain oxidiDing component BF

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+egassing:

Jemoval of dissolved gases from the li$uid metal is known as degasing.

:egassing of molten %luminum alloys is a foundry operation aimed to

remove 5ydrogen dissolved in the melt.

5ydrogen in aluminum

:egassing by 'uxes

Jotary degasser

In the case of diatomic gases such as 52, 62, or

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Mi$uid aluminum actively dissolves hydrogen, which forms as a result of

chemical reaction with water vaporK

2%l 3 4526 N %l264 3 95

olubility of gaseous hydrogen in li$uid aluminum at its melting point

B*22?.=OAE99?.OC is ?.9* in4Elb B2.2 cm4 per *?? g.olubility of gaseous

hydrogen falls sharply when aluminum solidi!esK solid aluminum at melting

point contains only ?.?* in4Elb B?.? cm4 per *?? g.

#herefore aluminium alloys release excessive amount of hydrogen during

olidi!cation. #his results in porosity defects distributed throughout the solid

metal. iDe of the hydrogen pores and their $uantity is determined by the

initial content of hydrogen, the alloy composition and the solidi!cation

conditions.

5ources of ydrogen in molten aluminum:

atmosphere humidity(

wet metallic charge(

wet furnace lining Bcrucible, transfer ladles(

wet foundry instruments(

wet 'uxes and other consumables(

furnace fuel combustion products containing hydrogen.

Heat Treatment 6f Al-4%Cu Alloy:

Precipitation hardening, or age hardening, provides one of the most widely used mechanisms for

the strengthening of metal alloys. The fundamental understanding and basis for this technique

was established in early work at the U. S. Bureau of Standards on uralumin.

The importance of theoretical suggestion for the development of new alloys is clear from the

historical record. !t the end of the "#th century, cast iron was the only important commercial

alloy not already known to western technology at the time of the $omans. %hen age hardening

of aluminum was discovered accidentally by %ilm, during the years "#&' ("#"", it quickly

became an important commercial alloy under the trade name uralumin.

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The strength and hardness of some metal alloys may be enhanced by the formation of e)tremely

small uniformly dispersed second(phase particles within the original phase matri) in a process

known as precipitation or age hardening. The precipitate particles act as obstacles to dislocation

movement and thereby strengthen the heat(treated alloys. *any aluminum based alloys, copper(

tin, certain steels, nickel based super(alloys and titanium alloys can be strengthened by age

hardening processes.

+n order for an alloy system to be able to be precipitation(strengthened, there must be a terminal

solid solution that has a decreasing solid solubility as the temperature decreases. The !l(u

-uralumin is an aluminum alloy of /// group0 phase diagram shown in 1igure " shows this

type of decrease along the solvus between the 2 and 234 regions. onsider a #5wt6!l 7 8wt

6u alloy which is chosen since there is a large degrease in the solid solubility of solid solution

2 in decreasing the temperature from 99&: to ;9:.

Figure 1:The aluminum rich end of the !l(u phase diagram showing the three steps in the age(

hardening heat treatment and the microstructures that are produced.

+n an attempt to understand the dramatic strengthening of this alloy, Paul . *erica and his

coworkers studied both the effect of various heat treatments on the hardness of the alloy and the

influence of chemical composition on the hardness. !mong the most significant of their findings

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was the observation that the solubility of u!l in aluminum increased with increasing

temperature.

!lthough the specific phases responsible for the hardening turned out to be too small to be

observed directly, optical e)amination of the microstructures provided an identification ofseveral of the other phases that were present. The authors proceeded to develop an insightful

e)planation for the hardening behavior of uralumin which rapidly became the model on which

innumerable modern high(strength alloys have been developed.

They summari

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precipitated particles or go around them. By restricting dislocation movement during

deformation, the alloy is strengthened.

Age Hardening Precipitation.The strongest aluminum alloys -))), 5))) and ;)))0 are

produced by age hardening. ! fine dispersion of precipitates can be formed by appropriate heat

treatment.

! general model for decomposition is given, followed by details of the precipitation sequences in

8 specific alloy systems= !l(u, !l(u(*g, !l(*g(Si and !l(?n(*g. The !l(u system is used

as the main e)ample of decomposition, i.e.

a& -SSSS0@ AP

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"& nm diameter copper(rich plates on E"&&F!l planes. These develop into AP

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Figure 2: Microstructure of Al-4%cu Alloy solution treated at 580oC

for 2 hours, at 100 , etched in !"#A$ reagent

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Figure : Microstructure of Al-4%cu Alloy solution treated at 580oC

for 2 hours, at 200 , etched in !"#A$ reagent

Figure 4: Microstructure of Al-4%cu Alloy solution treated at 580oC

for 2 hours, at 400 , etched in !"#A$ reagent

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Figure 5: Microstructure of Al-4%cu Alloy Arti&cial Aged at 180oC for

1 hours, at 100 , etched in !"#A$ reagent

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Figure ': Microstructure of Al-4%cu Alloy Arti&cially aged at 180oC

for 1 hours, at 200 , etched in !"#A$ reagent

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Figure (: Microstructure of Al-4%cu Alloy Arti&cially aged at 180oC

for 1 hours, at 400 , etched in !"#A$ reagent

/eferences

*.CharacteriDation of an %l&Cu cast alloy ,1.%. #alamantesilvaa, %.

JodrPgueDb, Q. #alamantesilvab, . Laltierrab, Jafael ColRsa, a Aacultad de

IngenierPa 1ecRnica y SlTctrica, Universidad %utnoma de A, 99* an