Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Fig. 17.3 The

aluminium-rich

end of the

aluminium-copper

equilibrium

diagram.

Observ-

able

microstructural changes due to slow

cooling,

solution

treatment and

precipitation

treat-

ment are

indicated.

COPPER Wo)

liquid

°C

solution

treatment

unsaturated

solid

solution

(OC)

slowly

cooled

quenched

9 precipitated

supersaturated

solid

solution

((JC)

over

heated

precipitation

hardened

correctly

formation (submicroscopical)

coherent

intermediate phase 9'

precipitation of

non-coherent 9

has

commenced

precipitation

treatment

in the solvent metal takes place with variation in temperature (9.90) it is

more widely used in suitable aluminium-base alloys than in any others.

The phenomenon was first observed by a German research metallurgist,

Dr. Alfred WiIm who, in 1906, noticed that an aluminium-copper alloy

which had been water-quenched from a temperature of about 500

C, sub-

sequently hardened unassisted at ambient temperature over a period of

several days. The strength increased in this way, reaching a maximum

value in just under a week, and the effect was subsequently known as

age-hardening. About four years later WiIm transferred the sole rights of

his patent to the Diirener Metal Works in Germany, and the alloy pro-

duced was named Duralumin. Although this name is often used to describe

any wrought aluminium alloy of this type, it is, strictly speaking the pro-

prietary name given to a series of alloys produced by certain companies

both here and abroad. The first significant use of duralumin was during

the First World War, when it found application in the structural members

of the airships bearing the name of Graf von Zeppelin.

17.51 The 'age-hardening' process can be accelerated and higher

strengths obtained if the quenched alloy is heated at temperatures up to

180

Such treatment was originally known as artificial age-hardening but

both of these descriptions have been replaced in metallurgical nomencla-

ture by the general term precipitation hardening. Much speculation took

place over the years as to the fundamental nature of this phenomenon but

metallurgists now generally agree that the increases in strength and hard-

ness produced are directly connected with the formation of coherent pre-

cipitates (9.92) within the lattice of the parent solid solution. We will

consider the application of this principle to the precipitation-hardening of

aluminium-copper alloys.

17.52 Let us assume that we have an aluminium-copper alloy contain-

ing 4.0% copper. At temperatures above 550

C this will consist entirely of

a solid solution as indicated by the equilibrium diagram (Fig. 17.3). If we

now allow the alloy to cool very slowly to room temperature, equilibrium

will be reached at each stage and particles of the intermetallic compound

CuAl

(0) will form as a non-coherent precipitate. This precipitation will

commence at A and continue until at room temperature only 0.2% (X)

copper remains in solution in the aluminium. The resulting structure will

lack strength because only 0.2% copper is left in solution, and it will be

brittle because of the presence of coarse particles of CuAl

If the alloy is now slowly reheated, the particles of CuAl

will be gradu-

ally absorbed, until at A we once more have a complete solid solution a

(in industrial practice a slightly higher temperature, B, will be used to

ensure complete solution of the CuAl

). On quenching the alloy we retain

the copper in solution, and, in fact, produce a supersaturated solution of

copper in aluminium at room temperature. In this condition the alloy is

somewhat stronger and harder because there is more copper actually in

solid solution in the aluminium, and it is also much more ductile, because

the brittle particles of CuAl

are now absent. So far these phenomena

permit of a straightforward explanation, forthcoming from a simple study

of the microstructure. What happens subsequently is of a sub-microscopical

nature, that is, its observation is beyond the range of an ordinary optical

microscope.

17.53 If the quenched alloy is allowed to remain at room temperature,

it will be found that strength and hardness gradually increase (with a corre-

sponding reduction in ductility) and reach a maximum in about six days.

After this time has elapsed no further appreciable changes occur in the

properties. The completely a-phase structure obtained by quenching is not

the equilibrium structure at room temperature. It is in fact super-saturated

with copper so that there is a strong urge for copper to be rejected from

the solid solution a as particles of the non-coherent precipitate CuAb (0).

This stage is never actually reached at room temperature because of the

sluggishness of diffusion of the copper atoms within the aluminium lattice.

However, some movement does occur and the copper atoms take up pos-

itions within the aluminium lattice so that nuclei of the intermediate phase

(0') are formed. These nuclei are present as a coherent precipitate continu-

ous with the original a lattice, and in this form, cause distortion within the

a lattice. This effectively hinders the movement of dislocations (9.92) and

so the yield strength is increased. The sub-microscopical change within the

structure can be represented so:

Cu + 2Al -• &

[Al lattice] [Al lattice] [Intermediate

coherent

precipitate]

An improvement in properties over those obtained by ordinary natural

age-hardening can be attained by tempering the quenched alloy at tempera-

tures up to nearly 200

C for short periods. This treatment—called precipi-

tation treatment—increases the amount of the intermediate coherent

precipitate 0' by accelerating the rate of diffusion, and so strength and

hardness of the alloy rise still further. If the alloy is heated to a higher

temperature, a stage is reached where the structure begins to revert rapidly

to one of equilibrium and the coherent intermediate phase 0' precipitates

fully as non-coherent particles of 0 (CuAh):

0' - 0

[Intermediate [Non-coherent

coherent precipitate

precipitate] CuAl

]

When this occurs both strength and hardness begin to fall. Further

increases in temperature will cause the 0 particles to grow to a size making

them easily visible with an ordinary optical microscope, and this will be

accompanied by a progressive deterioration in mechanical properties. Fig.

17.4 illustrates the general effects of variations in time and temperature

during post-quenching treatment for a typical alloy of the precipitation-

hardening variety. At room temperature (20

C) the tensile strength

increases slowly and reaches a maximum of about 390 N/mm

after approxi-

mately 100 hours. Precipitation-treatment at temperatures above 100

TREATMENT TIME (H)

Fig.

17.4 The effects of time and temperature of precipitation-treatment on the structure

and tensile strength of a suitable alloy.

will result in a much higher maximum tensile strength being reached. Opti-

mum strength is obtained by treatment at 165°C for about ten hours, after

which, if the treatment time is prolonged, rapid precipitation of non-

coherent particles of 6 (CuAl

) will cause a deterioration in tensile strength

and hardness as shown by curve C. Treatment at 200

C, as represented by

curve Z), will give poor results because the rejection from solution of

non-coherent 9 is very rapid such that precipitation will overtake any

increase in tensile strength. This process of deterioration in structure and

properties due to faulty heat-treatment is generally termed 'reversion'.

Time and temperature of precipitation-treatment differ with the compo-

sition of the alloy, and must always be controlled accurately to give opti-

mum results.

17.54 Precipitation-hardening is not confined to the aluminium-base

alloys, but can be applied to alloys of suitable composition from many

systems in which a sloping phase boundary such as X A Y (Fig. 17.3) exists.

As far as aluminium alloys are concerned, those containing copper and

those containing magnesium and silicon (which cause hardening due to the

formation of Mg

Si) are the most important. In addition, precipitation-

hardening is utilised in a number of magnesium-base alloys (18.11),

titanium alloys (18.62), some copper-base alloys (16.70) and certain steels.

17.55 A number of alloys are sold under the general trade name of

Duralumin. Whilst some of them rely on the presence of approximately

copper to effect hardening, others contain magnesium and silicon so

that hardening will be assisted by the coherent precipitation of Mg

Si. The

original Duralumin contained 4.0 Cu; 0.5 Mg; 0.5 Si; 0.7 Fe; and 0.7 Mn,

the main function of the manganese being as a grain refiner.

The initial hot-working of duralumin, either by hot-rolling or extrusion,

is done between 400 and 450

C. This treatment breaks up the coarse eutec-

tic to some extent, and the alloy can then be cold-worked. It is subsequently

solution-treated at 480-500

C for one to three hours in order to absorb

TENSILE STRENGTH (NW)

NON-COHERENT PRECIPITATE FORMS

the CuAl

and Mg2Si and is then water-quenched. Accurate temperature

control is essential, as the temperature of treatment must be maintained

between A and C (Fig. 17.3). After quenching, cold-working operations

can be carried out on the resulting solid-solution structure if desired. The

alloy is subsequently hardened, either by allowing it to remain at room

temperature for about five days or by heating it to some temperature

between 100 and 160

C for up to ten hours, according to the properties

required. Heating at above 160

C may cause visible particles of CuAl

to appear in the microstructure, with consequent losses in strength and

hardness.

17.56 Just as the application of heat will accelerate precipitation-

hardening so will refrigeration impede the process. This fact was utilised

extensively in aircraft production during the Second World War. An alloy

rivet must be of such a composition that it will 'age' at ambient tempera-

ture,

since it is inappropriate to precipitation-harden a completed airframe

structure by any sort of furnace treatment. Unfortunately once solution-

treated such a rivet will soon begin to harden at ambient temperature and

if this hardening had begun any attempt to drive the rivet in this hard

condition would cause it to split. Hence, immediately after quenching from

the solution-treatment temperature, rivets were transferred to a refriger-

ator at about

—

By this means age-hardening was considerably slowed

down and rivets could be stored at sub-zero temperatures until required.

17.57 Although the addition of copper forms the basis of many of the

precipitation-hardening aluminium alloys, copper is absent from a number

of them which rely instead on the presence of magnesium and silicon.

Such alloys have a high electrical conductivity approaching that of pure

aluminium, so that they can be used for the manufacture of overhead

conductors of electricity. Most commercial grades of aluminium contain

iron as an impurity, but in some of these alloys it is utilised in greater

amounts in order to increase strength by promoting the formation of FeALi,

which assists in precipitation-hardening. Titanium finds application as a

grain refiner, whilst other alloys containing zinc and chromium produce

tensile strengths in excess of 620 N/mm

in the heat-treated condition.

17.58 In the 1920s aluminium-lithium alloys were investigated as

being potentially useful in aircraft construction, since the low relative den-

sity of lithium (0.51) could mean reductions of 10% in the density of alloys

containing the metal. Subsequently interest was revived in lithium as an

alloying element and the US Navy's 'Vigilante' contains the alloy '2020',

though here lithium was used to increase the temperature range over which

existing aluminium-copper alloys could be used rather than to reduce

weight. More recent research has been based on alloys containing 2.5%

lithium along with small amounts of magnesium, copper and—as a grain

refiner—zirconium. The development of RSP (9.110) has led to experiments

with alloys containing 4% lithium since under these conditions more

lithium is retained in solution.

Since lithium is one of the chemically reactive 'alkali metals' in Group

I of the Periodic Classification (Fig. 1.2) there are difficulties encountered

in melting and casting these alloys. Moreover, early aluminium-lithium

Plate 17.1

17.1

A A wrought aluminium-manganese alloy containing 1.25% Mn in the

annealed condition, showing uniform crystals of solid solution.

Etched electrolytically and photographed under polarised light which emphasises grain

contrast. All of the crystals, however, are of the same uniform solid solution, x 100.

17.1 B An alloy of the duralumin type, in the hot-rolled condition, showing particles of

CuAI

in a matrix of solid solution, x 100. Unetched.

17.1

C The same alloy in the heat-treated condition.

Much of the

CuAI

is now in solution, x 100. Unetched.

17.1c.

17.1B.

17.1A.

Table 17.3

Wrought

Aluminium Alloys

Which

Are Heat-treated.

Typical

Mechanical

Properties

Characteristics

and

uses

Good

corrosion-resistance. Glazing

bars and window sections, windscreen

and sliding roof sections for automobile

industry.

Good surface finish available

with

this alloy.

Air-screw

forgings, sheets, tubes

Highly-stressed components in aircraft,

e.g. stressed-skin construction. Engine

parts such as

connecting

rods. Also

supplied

in Alclad form, i.e. covered

with a thin layer of pure aluminium which

protects the alloy from corrosive media.

Bars

and sections for marine use; tubes,

bolts and wire.

Structural members, for road,

rail

and

sea transport vehicles, general

architectural

work,

ladders

and scaffold

tubes. High electrical conductivity - used

for

overhead

lines,

etc.

Elonga-

tion

(%)

Tensile

strength

(N/mm

)

150

180

250

310

385

450

510

200

215

300

155

220

325

0.2%

Proof

Stress

(N/mm

100

180

150

295

245

375

160

115

235

117

250

Condition

Heat-treatment

Solution-treated at

520

C, quenched and

precipitation-hardened

170

C for 10 hours.

Solution-treated at

530

quenched

and

precipitation-hardened

170°Cfor 10-20 hours.

Solution-treated at

510°C, quenched and

precipitation-hardened

170°Cfor 10 hours

Solution-treated at

520

C, quenched and

precipitation-hardened

170

C for 10 hours

Solution-treated

510

quenched and

precipitation-hardened at

175°Cfor

10 hours.

Composition

(%)t

Other

elements

Cr-0.1*

Ti-0.1*

Ni-0.6-1.4

Ti-0.2*

+ Ti-

0.2*

Cr-0.04-0.35

Ti-0.15*

Cr-0.25*

Ti-0.1*

0.35

0.6

1.2

0.5

0.7

0.5

0.1

0.5

0.4

1.2

0.15

0.4

1.0

0.2

0.6

0.5

1.3

0.5

0.9

0.4

0.8

0.7

1.3

0.45

0.9

0.6

1.2

0.2

0.8

1.2

0.6

1.2

0.1

1.0

2.8

3.9

5.0

0.15

0.4

0.1

Relevant

Specifi-

cation

(BS)

1471:6063

1472:6063

1474:6063

1475:6063

1472:2031

2L83|

1470:2014A

1475:2014A

3L63;

2L77f

1471:6061

1473:6061

1474:6061

1475:6061

1470:6082

1474:6082

Structural

members

for

aircraft

and

road

vehicles, tubular

furniture.

Rivets

for

built-up structures.

General purposes, stressed parts

aircraft

and

other structures.

The

original

Duralumin.

Highly-stressed aircraft components

such

booms. Other military equipment

requiring

the

highest strength/weight

ratio.

The

strongest aluminium alloy

produced commercially.

Upper wing skins

aircraft—good

creep properties

up to

175°C

Military aircraft construction.

Low

relative density (2.53).

Can be

shaped

'superplastically'.

High specific modulus

due to low

relative

density

(2.54)

430

300

400

650

570

550

400

150

275

580

485

490

Solution-treated

525

quenched

and

precipitation-hardened

170

for 10

hours.

Solution-treated

495

quenched

and

aged

room

temperature

for 5

days

Solution-treated

48O

quenched

and

aged

room

temperature for

days

Solution-treated

465°C, quenched

and

precipitation-hardened

120°Cfor

hours

Ti-0.3*

0.3*

Zn-5.0-7.5

Cr-0.15

Ti-0.3*

1.1

Cd-0.2

2.5

Zr-1.2

2.6

Zr-0.12

0.7

0.5

1.0

0.5

0.4

1.0

0.3

0.5

0.8

1.3

0.7

0.2

0.7

0.5

1.2

0.2

0.5

0.4

1.2

2.0

3.0

0.8

0.9

1.0

2.0

1.5

3.0

3.5

4.7

1.0

2.2

4.5

1.3

1.8

2L84f

3L86f

2L77f

2L88;

2L95

Alcoa

(USA)

'2020'

(RAE

8090)

(RAE

8091)

Single values

are

maxima unless otherwise stated.

Optional

t BS

Aerospace Series, Section

(aluminium

and

light alloys).

alloys failed because of poor ductility and impact values. However, in

recent British alloys, these values have been maintained at satisfactory

levels along with a relative density of only 2.54, and hence an increase in

specific modulus (2.36) of up to 25%.

Precipitation hardening is due mainly to the phase S' (AbLi) a precipi-

tate coherent with the matrix. Magnesium is added to reduce density

further and also confer limited solid-solution hardening, but unfortunately

this leads to the formation of detrimental grain-boundary precipitates of

MgLi. However the addition of some copper causes the precipitation

of 6' and the phase Ti (Al

CuLi) instead. The simultaneous precipitation

of two phases gives rise to an appreciable improvement in impact toughness

when precipitation of the two phases is suitably controlled.

A selection of typical wrought heat-treatable alloys of aluminium is given

in Table 17.3.

Cast Alloys which are Heat-treated

17.60 German air-raids on England during the First World War were

carried out by 'Zeppelins', the airframes of which were constructed from

Duralumin. Britain's defence against these raids was provided by heavier-

than-air flying machines, the airframes of which were largely of non-

metallic construction. Nevertheless light-weight internal combustion

engines were required to power these aircraft and the National Physical

Laboratory carried out much of the development work on new aluminium

alloys in those days.

One of the very successful alloys developed by NPL is still known by

the series letter used to identify it during development—Y Alloy. Like

Duralumin this is of the 4% copper type but contains also about 2% nickel

and 1.5% magnesium. Precipitation-hardening is due to the combined

effects of coherent precipitates based on both CuAl

and NiALi, and whilst

fundamentally a casting alloy it can be used in the wrought condition. As

a cast alloy it is useful where reasonable strength at high temperatures is

required, as in high-duty pistons and cylinder heads for internal combustion

engines. Its relatively low coefficient of expansion also makes it useful in

this direction.

17.61 There are other casting alloys similar in composition to

Y Alloy',

but containing rather less copper. Some of these alloys (eg RR50) can be

precipitation hardened by heating at 155-170

C without the usual prelimi-

nary solution-treatment at a high temperature. This is due to sluggishness

in precipitation of phases like CuAl

and NiAl

as the casting cools, follow-

ing the initial pouring process. The 5.0 and 13.0% types of aluminium-sili-

con alloy are brought into this group by the addition of small amounts of

either copper and nickel or magnesium and manganese, which render the

alloys amenable to precipitation-hardening.

17.62 The use of aluminium-silicon alloys for automobile cylinder

blocks has obvious advantages in terms of weight saved as compared with

the usual iron casting. Other advantages include higher thermal conduc-

Plate 17.2 17.2A Y alloy in the as-cast condition, showing large amounts of intermetallic

compounds in the structure, x 100. Etched in 0.5% hydrofluoric

acid.

(Courtesy of Aluminium

Laboratories Ltd)

17.2B The same alloy in the solution-treated and precipitation-hardened condition.

Much of the intermetallic compounds are now in solution, x 100. Etched in 0.5%

hydrofluoric

acid.

(Courtesy of Aluminum Laboratories Ltd).

tivity which assists heat transfer in the water cooling and lubrication

systems; and enhanced corrosion resistance. Moreover, die-casting can be

used in the production of aluminium alloys further reducing production

costs when large numbers of castings are involved. Unfortunately the wear-

resistance of normal

11-13%

silicon alloys is inadequate and cast iron

liners need to be fitted resulting in increased costs.

Recently a range of hyper-eutectic aluminium-silicon alloys was de-

veloped. These contain 18-25% silicon and because of the presence of

large amounts of primary silicon the wear resistance of these alloys is

17.2 A.

17.2B.