Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Aluminium and its Alloys

17.10 The properties of aluminium which chiefly dictate its use as an

engineering material are its low relative density coupled with a reasonably

high tensile strength when used in one of its alloyed forms. Since its relative

density is only about one third that of steel its alloys are widely used

in aero-, automobile and constructional engineering. A combination of

alloying and heat-treatment can produce alloys which, weight-for-weight,

are in the same class as many steels.

17.11 The high affinity of aluminium for oxygen is both disadvan-

tageous and useful. It is disadvantageous in so far as it increases the cost

of extraction of the metal by making necessary a relatively expensive elec-

trolytic extraction process. Usually a metal is extracted by heating its oxide

ore with a cheap reducing agent, such as carbon (in the form of coke), and

the resulting crude metal is refined by allowing the bulk of the impurities

present in it to be oxidised. This is the basis for the production of pig iron

and its subsequent conversion to steel. The very high chemical affinity of

aluminium for oxygen means that aluminium cannot be reduced by carbon

by ordinary chemical means. Obviously any other reducing agents able

to separate aluminium from oxygen will be thermodynamically far too

expensive, so that aluminium can only be produced economically by elec-

trolytic means.

It was using an expensive reducing agent in the form of metallic potass-

ium which enabled the Danish physicist and chemist H. C. Oersted to

produce the first samples of aluminium in 1825. Consequently aluminium

was worth about £250 per kg in those days, and, even later, it is said that

the more illustrious foreign guests to the Court of Napoleon III were

privileged to use forks and spoons made from aluminium, whilst the French

nobility had to be content with mere gold plate and silver cutlery. In the

Gay Nineties aluminium was still regarded as being in the nature of a

curious precious metal, though it was in 1886 that C. M. Hall, a twenty-two-

year-old student, had discovered a relatively cheap method for producing

aluminium by electrolysing a fused mixture of aluminium oxide and the

mineral cryolite.

Aluminium cannot be purified by blowing air through it, as in the case

of iron. This treatment would oxidise the aluminium and leave behind the

impurities. Therefore the ore must be purified before being electrolysed,

and this involves an expensive chemical process. The ore of aluminium is

bauxite, which is named after Les Baux, in the south of France, where it

was discovered in 1821. Although France is still an important producer of

bauxite, Jamaica and the Republic of Guinea mine the largest quantities of

the ore followed by the CIS, Surinam and Guyana. In Europe Hungary,

Greece and Jugoslavia, in addition to France, all produce considerable

quantities of the ore. Britain is wholly dependent upon imported ore to

maintain a token small-scale production at the Kinlochleven and Lochaber

works, both of which are sited with access to water transport and relatively

cheap hydroelectric power in the Scottish Highlands; but most aluminium

is imported as the metal.

17.12 Although aluminium has a great affinity for oxygen, its corrosion-

resistance is relatively high. This is due to the dense impervious film of

oxide which forms on the surface of the metal and protects it from further

oxidation. The corrosion-resistance can be further improved by anodising

(21.91),

a treatment which artificially thickens the natural oxide film. Since

aluminium oxide is extremely hard, wear-resistance is also increased by

the oxide layer; and the slightly porous nature of the surface of the film

allows it to be coloured with either organic or inorganic dyes. In this

respect high oxygen-affinity is an asset.

The high affinity of aluminium for oxygen also makes it useful as a

deoxidant in steels and also in the Thermit process of welding (20.52).

17.13 The fact that aluminium has over 50% of the specific conduc-

tivity of copper means that, weight for weight, it is a better conductor of

electricity than is copper. Hence it is now widely used, generally twisted

round a steel core for strength, as a current carrier in the electric 'grid'

system. Large increases in the price of copper a few years ago led to the

use of aluminium as a domestic current carrier, though this type of wire is

often copper coated to increase the conductivity of the surface skin.

17.14 Unlike copper which is normally available as a high-purity metal

because of the ease of its electrolytic refinement, commercial grades of

'pure'

aluminium may contain no more than 99.0% of the metal. This is

due to the difficulty in refining crude aluminium already mentioned.

Although electrolytic refining methods are now practised producing alu-

minium of 99.99% purity this is obviously costly. Available commercial

qualities covered by BS specifications contain 99.99%,

99.8%,

99.5%,

99.0%

aluminium respectively. The lower commercial grades are used

fairly widely for drawing, pressing and spinning large numbers of cooking

and other kitchen utensils to which non-stick coatings of PTFE (polyte-

trafluoroethene) can be applied where necessary. Aluminium is suitable

for both hot- and cold-working processes but unlike copper, another FCC

metal, it does not exhibit annealing twins following recrystallisation.

Pure aluminium is relatively soft and weak with a tensile strength of no

more than 90 N/mm

in the annealed condition, so that for most other

engineering purposes it is used in the alloyed form.

Alloys of Aluminium

17.20 The addition of alloying elements is made principally to improve

mechanical properties, such as tensile strength, hardness, rigidity and

machinability, and sometimes to improve fluidity and other casting pro-

perties.

17.21 One of the chief defects to which aluminium alloys are prone is

porosity due to gases dissolved during the melting process. Molten alu-

minium will dissolve considerable amounts of hydrogen if, for any reason,

this is present in the furnace atmosphere. When the metal is cast and

begins to solidify the solubility of hydrogen diminishes almost to zero, so

that tiny bubbles of gas are formed in the partly solid metal. These cannot

escape, and give rise to pinhole porosity. The defect is eliminated by

treating the molten metal before casting with a suitable flux, or by bubbling

nitrogen or chlorine through the melt. A more practical method is to use

specially prepared tablets of hexachloroethane which decompose liberating

small quantities of chlorine when the tablet is plunged beneath the surface

of the melt.

17.22 Aluminium alloys are used in both the cast and wrought con-

ditions. Whilst the mechanical properties of many of them, both cast and

wrought, can be improved by precipitation hardening, a number are used

without any such treatment being applied. Forty years ago there was a

bewildering multitude of aluminium alloys in the manufacturers' lists. For-

tunately obsolescence and rationalisation has reduced the number con-

siderably, though some new compositions have been introduced as a result

of expansion of the market for aluminium alloys outside the field of

aerospace.

17.23 Most of the aluminium alloys available commercially are sup-

plied according to British Standards Specifications and are covered by

General Engineering Specifications BS 1470/1475 (wrought materials) and

BS 1490 (cast alloys). Specialised applications are covered under a number

of supplementary specifications, whilst a few manufacturers produce alloys

not covered by a BS number. For wrought alloys the general specification

number, ie BS 1470/1475, is followed by a four-digit designation identifying

a specific alloy. The first of the four digits indicates the alloy group as

follows:

Aluminium (99.0% min. and

greater) - lxxx

Aluminium alloy groups (by

major alloying elements): Copper - 2xxx

Manganese - 3xxx

Silicon - 4xxx

Magnesium - 5xxx

Mg + Si - 6xxx

Zinc - 7xxx

Other elements - 8xxx

Unused series - 9xxx

In the first group (aluminium containing more than 99.0% Al) the last

two digits are the same as the two digits to the right of the decimal point

in the 'minimum % aluminium' when expressed to the nearest

0.01%.

The

second digit in the designation indicates modifications in impurity limits or

alloying elements. When the second digit is zero it indicates unalloyed

aluminium having natural impurity limits; integers 1 to 9, assigned consecu-

tively as required, indicate special control of one or more impurities or

alloying elements.

In Groups 2xxx to 8xxx the last two digits in the designation have no

special significance and are used only to identify different alloys in the

group. The second digit in the designation indicates modifications. Thus,

if the second digit is zero it indicates the original alloy; integers 1 to 9

assigned consecutively indicate modifications. National variations of

wrought aluminium alloys registered by another country are identified by

a serial letter after the four-digit designation.

The condition of the material may be denoted by suffix letters as follows:

M - material 'as manufactured'.

O - fully annealed to give lowest strength.

H8 - work-hardened. Material cold-worked after annealing.

Designations are in ascending order of strength and

hardness.

TB - solution-treated and 'aged' naturally. (Material

receives no cold-work after solution treatment.)

TE - cooled from an elevated-temperature shaping process

and precipitation hardened.

TF - solution treated and precipitation hardened.

TH - solution treated, cold-worked and then precipitation

hardened.

TD - solution treated, cold-worked and 'aged' naturally

(tubes).

The cast aluminium alloys are covered by BS 1490 and here the suffix

letters indicate the condition of the material as follows:

M - 'as cast' with no further treatment

TS - castings 'stress relieved' only

TE - precipitation treatment after casting

TB - solution treated only

TB7 - solution treated and 'stabilised'

TF - solution treated and precipitation hardened

TF7 - solution treated, precipitation hardened and then

'stabilised'.

It is convenient to classify aluminium alloys into the following four main

groups according to the condition in which they are employed.

Table

17.1 Wrought alloys

which

are not

heat-treated

Typical

uses

High-purity

aluminium,

very high

ductility, corrosion resistance and

electrical conductivity.

Panelling and moulding, lightly

stressed and decorative

assemblies

especially

architecture, holloware, electrical

conductors,

equipment

for

chemical,

food and brewing

industries, packaging, automobile

radiator parts.

Wire for welding (this alloy is also

used in the sheet

form

for panelling

and light marine construction.)

Welding wire and rods.

Metal

boxes, bottle caps, domestic

and commercial food containers

and cooking utensils; roofing

sheets;

panelling

land-transport

vehicles; wire;

automobile

radiators.

Typical mechanical properties

Elongation

(%)

Tensile

strength

(N/mm

)

130

125

155

110

155

200

0.2% Proof

stress

(N/mm

)

Condition

Composition

(%)t

Other

elements

AI-99.80

AI-99.0

+ Ti*

0.10

0.02

0.05

0.15

0.9

1.5

0.15

+ Fe

1.0

0.6

0.7

11.0

13.0

4.5

6.0

0.5

0.02

0.1

0.2

0.3

0.03

0.05

0.3

0.1

Relevant

specifi-

cations

(BS)

1470:1080A

1475:1080A

1470:1200

1471:1200

1474:1200

1475:4047A

1475.-4043A

1470:3103

1475:3103

Marine superstructures, life boats,

panelling exposed to marine

atmospheres, fencing wire,

chemical plant, panelling for road

and rail vehicles, now used in

automobile radiators.

Shipbuilding and other marine

structures,

pressure-vessels,

storage tanks, welded structures,

deep-pressings for racing-car

bodies.

Mainly for corrosion-resistant

rivets.

Welding electrodes, rod and wire.

Unheated welded pressure

vessels,

marine, auto and aircraft

parts,

TV towers, drilling rigs,

transportation

equipment, missile

components.

'superplastic alloy'—many

components in the aerospace

industries.

the rolled condition iron has a

dispersion-hardening effect—

electrical purposes and household

foil.

185

220

265

240

270

300

255

290

315

345

375

200

130

175

165

225

125

235

270

120

Ti - 0.2*

Mn + Cr

0.1

- 0.5

Mn + Cr

0.1

-0.6

0.05 - 0.2

0.05 - 0.25

- 0.4

0.1

0.5

0.1

0.6

1.0

0.4

1.0

0.5

0.4

2.0

0.4

0.5

0.4

0.25

0.4

1.7

3.1

3.9

4.5

5.6

5.0

5.5

4.0

4.9

0.15

0.1

6.0

1470:5251

1471:5251

1472:5251

1474:5251

1475:5251

1470:5154A

1475:5154A

1473:5056A

1475:5056A

1475:5556A

1470:5083

1471:5083

1472:5083

1474:5083

(Alcan:2004)

tSingle

values are maxima unless otherwise stated.

*Optional

Wrought Alloys Which Are Not Heat-treated

17.30 The main requirements of alloys in this group are sufficient strength

and rigidity in the work-hardened state, coupled with good corrosion-

resistance. These properties are typical of the alloys shown in Table 17.1.

As will be seen, these alloys are widely used in the manufacture of panels

for land-transport vehicles. Here the high corrosion-resistance of the alu-

minium-magnesium alloys is utilised, those with a higher magnesium value

having an excellent resistance to sea-water and marine atmospheres, so

that they are used extensively for marine superstructures. The desired

mechanical properties are produced by the degree of cold-work applied in

the final cold-working operation, and these alloys are commonly supplied

as soft (0), or having undergone varying degrees of work-hardening as

denoted by the BS Specification suffixes Hl, H2, H3, etc. up to H8 (full

hard).

The main disadvantage is that, once the material has been finished

to size, no further variation can be made in mechanical properties (other

than softening by annealing), whereas, with the precipitation-hardening

alloys, the properties can be varied, within limits, by heat-treatment.

17.31 As would be expected, most of these alloys (the main exception

being those of aluminium and silicon) have structures consisting entirely

of solid solutions. This is a contributing factor to their high ductility and

high corrosion-resistance. In the case of aluminium-magnesium alloys the

solubility of magnesium falls sharply from 14.9% at 451°C to about 1.5%

at 0

C (Fig. 17.1). Consequently care must be taken to cool these alloys

rapidly in order to prevent the intercrystalline precipitation of the inter-

mediate phase (3 which would lead to intercrystalline corrosion and em-

brittlement. Some of the alloys listed are occasionally used in the cast

condition.

17.32 Recently a number of superplastic alloys have been developed

for use by stretch forming techniques. Early development work was carried

out on the eutectic alloy (Al-33Cu) (4.64) but currently the most widely

used alloy is 'Alcan 2004' (Al-6Cu-0.4Zr). Some of these alloys can be

further strengthened by precipitation treatment.

17.33 Aluminium-iron alloys containing up to 2% iron are now in

MAGNESIUM (%>)

Fig.

17.1 The aluminium-magnesium thermal equilibrium diagram.

LIQUID

OC + LIQUID

+ /3

production. These alloys can be rolled to foil which has higher strength

and good electrical conductivity, the rolling technique being such that

nearly all of the iron remains out of solid solution. In this condition it has

a considerable dispersion hardening effect whilst at the same time, since

little iron is in solution, electrical conductivity remains high.

Cast Alloys which are not Heat-treated

17.40 Alloys in this group are widely used in the form of general-purpose

sand castings and die castings. They are used where rigidity, good corrosion

resistance and fluidity in casting are of greater importance than strength.

17.41 Undoubtedly the most widely used alloys in this class are those

containing between 9.0 and 13.0% silicon, with occasionally small amounts

of copper. These alloys are of approximately eutectic composition (Fig.

17.2),

a fact which makes them eminently suitable as die-casting alloys,

since their freezing range will be small. The rather coarse eutectic structure

can be refined by a process known as modification. This consists of adding

Fig.

17.2 The 'modification' of aluminium-silicon alloys.

In (i) the structure consists of primary silicon + eutectic because the composition is just to

the right of the eutectic point (11.6% Si). Note that the eutectic is very coarse and consists

of needles of silicon in a matrix of a (aluminium rich).

(ii) Here 'modification' has displaced the eutectic point to the right (14% Si) so that the

composition of the alloy under consideration is now to the left of this point and the structure

consists of primary a (light) + eutectic. The latter is now extremely fine-grained and gives

superior mechanical properties.

°c

LIQUID

OC + LIQUID

SILICON + LIQUID

displacement of the

cuteclic point as a

result of 'modification

SILICON

I8<fc silicon

Table

17.2

Cast

Aluminium

Alloys

Which

Are Not

Heat-treated

Characteristics

and

uses

Pressure

die-castings. General purposes.

Withstands moderate stresses. Good

fluidity,

hence

useful

for

thin-walled castings.

Sand-castings; gravity

and

pressure die-castings.

Good foundry characteristics

and

inexpensive.

General-purpose

alloy

where mechanical

properties

are of

secondary

importance.

Can be

heat-treated.

The

most

widely used

and

versatile

casting alloy.

Sand-castings

and

gravity die-castings. Suitable

for

moderately stressed parts. Good

corrosion-resistance

for

marine work. Takes

excellent

polish.

Food processing.

Sand-castings; gravity

and

pressure die-castings.

Excellent foundry characteristics. Large

castings

for

general

and

marine engineering.

Radiators; sumps; gear-boxes;

etc. One of the

most

widely used aluminium alloys.

Good fluidity

but

lower cost than LM.6. Good

corrosion resistance—hence used

cooking

and food utensils.

very

popular

alloy

for

pressure die-castings.

Good strength

and

machinability.

Sand-

and

gravity-die castings,

e.g.

large cambox

(sand)

and

small steering housing (gravity).

Also

good general purpose alloy.

Very

low

coefficient

thermal expansion. High

hardness. Special

applications

such

cylinder

blocks with

unlined

bores.

Typical

mechanical properties

Tensile

strength

Elongation

(%)

1.5

(N/mm

)

150

140

160

230

280

140

170

160

190

120

140

180

140

160

150

160

Condition

Sand cast

Chill

cast

Sand cast

Chill

cast

Sand cast

Chill

cast

Sand cast

Chill cast

Sand cast

Chill

cast

Sand cast

Chill cast

M—Chill cast

Sand cast

Chill cast

M—Chill cast

TS-Chill

cast

Composition

(%)

0.2-0.6

0.3-0.7

0.2-0.6

3.0-6.0

0.4^-0.7

0.7-2.5

2.0-4.0

3.0-4.0

1.5-2.5

4.0-5.0

9.0-11.5

4.0-6.0

10.0-13.0

4.5-6.0

7.5-9.5

6.0-8.0

16.0-18.0

BSS.

1490:

LM.2.

LM.4.

LM.5.

LM.6.

LM.18

LM.24

LM.27

LM.30

small amounts of sodium (about 0.01% by weight of the charge) to the

melt just before casting. The effect is to delay the precipitation of silicon

when the normal eutectic temperature is reached, and also cause a shift of

the eutectic composition towards the right of the equilibrium diagram.

Therefore, as much as 14% silicon may be present in a modified alloy

without any primary silicon crystals forming in the structure (Fig. 17.2).

It is thought that sodium collects in the liquid at its interface with the

newly-formed silicon crystals, inhibiting and delaying their growth. Thus,

undercooling occurs and new silicon nuclei are formed in large numbers

resulting in a relatively fine-grained eutectic structure. Remelting tends to

restore the original structure due to a loss of sodium by oxidation. The

modification process raises the tensile strength from 120 to 200 N/mm

and

the elongation from 5.0 to over 15.0%. The relatively high ductility of this

cast eutectic alloy is due to the fact that the a-solid-solution phase in the

eutectic constitutes nearly 90% of the total structure. As will be seen (Fig.

17.2) it is therefore continuous in the microstructure and acts as a cushion

against much of the brittleness arising from the hard silicon phase.

These aluminium-silicon alloys can therefore be produced in the

wrought form, though they are better known as materials for die-casting

and other types of casting where intricate sections are required, since high

fluidity and low shrinkage, as well as the narrow freezing range mentioned

above, make them very suitable for such purposes. Their high corrosion-

resistance makes these alloys useful for marine work, and the fact that

they are somewhat lighter than the aluminium-copper alloys makes them

suitable for aero and automobile construction.

17.42 Aluminium-copper alloys containing up to 10% copper were

popular in the early days of the aluminium industry. These contained

considerable amounts of the brittle compound CuAl

(see Fig. 17.3) and

were useful when rigidity and good casting properties were required in

components not subjected to shock. They also machined well but since

their corrosion-resistance was poor they have become completely obsolete.

However, small amounts of copper are added to some of the alu-

minium-silicon alloys in order to improve strength and machinability at

the cost of some losses in castability and corrosion resistance. One of these

alloys (LM4) though generally used in the 'as-cast' form is responsive to

heat-treatment.

17.43 The main feature of the aluminium-magnesium-manganese

alloys is good corrosion resistance which enables them to receive a high

polish. Since many of these alloys are also rigid and shock-resistant they

are very suitable for moderately-stressed parts in marine constructions.

Wrought Alloys which are Heat-treated

17.50 Without doubt the most metallurgically-important feature of the

aluminium alloys is the ability of some of them to undergo a change in

properties when suitably heat-treated. Although this phenomenon is

common to many alloys in which a change in solubility of some constituent