Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Table

17.4

Cast

Aluminium

Alloys

Which

Are

Heat-treated

Typical mechanical properties

Tensile

strength

Hardness

Characteristics

and

uses

General purposes (sand,

gravity

and

pressure

die-casting). Withstands

moderate stresses, shock

and hydraulic pressure.

Also

used

in the

non-heat-treated

condition.

Suitable

for

intricate

castings, stressed

but not

subjected

shock. Good

corrosion-resistance

the

plain

high-silicon

alloy.

High combination

mechanical properties

and

high

resistance

corrosion.

Initially

tough

but

suffers

progressive

embrittlement

and

susceptibility

stress

corrosion cracking.

Not

recommended

for

sustained

tensile

stresses

(applied

residual).

Notable

for low

thermal

expansion.

Widely used

for

automobile

pistons.

(HbJ

90-130

100-150

65-90

Elonga-

tion

(

1.5

(NI

mm?-)

230

280

190

170

230

240

295

280

310

210

170

280

140

200

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

Sand cast

Chill cast

Sand cast

Chill

cast

Heat-treatment

Solution-treated

520

for

hours; quenched

hot

water

or oil;

precipitation-hardened

170

for 12

hours.

Solution-treated

530

for

2-4

hours; quenched

warm water;

precipitation-hardened

150-170

for 16

hours.

Solution-treated

525°C

for

hours; cooled

390

and

then quenched

oil at

160

boiling

water.

Solution-treated

520

for

4-12

hours;

oil-quenched;

precipitation-hardened

165-185°Cfor

6-12

hours.

Composition

(%)

1.5

Opt

0.3-0.7

0.2-0.6

9.5-10.5

0.8-1.5

2.0-4.0

0.7-1.5

4.0-6.0

10.0-13.0

Relevant

Specifi-

cations

1490:LM

1490:LM9.

14901M

10.

1490.LM

13.

Pistons

and

cylinder-heads for liquid-

and air-cooled engines.

Heavy duty pistons for

diesel

engines.

General

purposes.

Useful

for

intricate shapes

and pressure-tightness.

Cylinder-heads; valve

bodies; water jackets.

good general-purpose

alloy

for

sand-casting

and

gravity

die-casting. High

rigidity

and

moderate

shock-resistance.

General purpose

die-casting alloy. Used

where good

corrosion-resistance

coupled with good

mechanical properties

are

necessary. Also used

for

sand castings.

Gravity

die

cast pistons.

Very

low

coefficient

thermal expansion. High

hardness. Special pistons

for

internal combustion

engines.

90-120

100-140

TF Chill cast

270

Sand cast

170

Chill cast

230

Sand cast

230

Chill cast

280

TE-Chill

cast

200

Sand

cast

150

Chill

cast

160

Sand cast

150

Chill

cast

190

Sand cast

250

Chill

cast

280

TE-Chill

cast

210

Sand cast

120

Chill cast

190

Sand cast

120

Chill

cast

190

Solution-treated

510

precipitation-hardened

boiling water for

hours

aged

room temperature

for

days.

Solution-treated:

520°

for

hours; water-quenched;

precipitation-hardened

150-160

for

8-10

hours.

No solution-treatment

required;

precipitation-hardened

155-170

for

8-16

hours.

Solution treated

540

for

4-12

hours;

quenched;

precipitation-hardened

165°C

for

8-12

hours.

Precipitation-treated

180-190

for

8-12

hours.

Precipitation

treatment

and microscopic

examination

ensure

primary

silicon

evenly

distributed

in the

eutectic

(size shall not exceed

\im

average

and 70 |im

maximum.)

2.0

1.2

0.8-1.3

1.5

0.4^-0.6

0.2

0.2-0.6

0.5-1.5

0.8-1.3

4.0

1.0-1.5

1.8

2.0-4.0

0.8-1.3

0.3

4L35

('Y

Alloy')

4.5-5.5

2.5

6.5-7.5

8.5-10.5

22.0-25.0

1490:LM

16;

3L78.

3L51

(RR 50)

1490:LM

25.

1490:LM

26.

1490:LM

29.

adequate for their use in cylinder blocks without the inclusion of cast-iron

liners.

Both the wear-resistance and mechanical strength of these alloys

can be improved considerably by controlling the size and distribution of

the primary silicon. The finer and more evenly distributed the primary

silicon the better the wear resistance. A number of methods have been

proposed to refine this primary silicon and include the addition of small

amounts of phosphorus, arsenic or sulphur to the melt just prior to casting.

Since copper, magnesium and nickel are also present (LM29) in sufficient

quantities these alloys can be further hardened and strengthened by pre-

cipitation treatment. However, despite the apparent advantages of the

high-silicon alloys production difficulties have presumably prevented them

from becoming established even though they are adequately covered in BS

1490.

A selection of the alloys representative of this group is given in Table

17.4.

Exercises

By using Fig 17.2 estimate the proportions of primary a to eutectic in an

unmodified sand cast alloy containing 6% silicon which is just below its solidifi-

cation temperature.

The figure alongside shows the

aluminium-rich end of the aluminium-

copper thermal equilibrium diagram.

(i) What are the % compositions of

phases present in an alloy containing 12%

copper which has cooled slowly to 0

(ii) In what proportions by mass will the

phases in (i) exist?

(iii) What will be the temperature range

over which an alloy containing 4% copper

could be solution treated?

(iv) What, therefore, would be an appro-

priate practical solution-treatment tem-

perature for this alloy? Give reasons,

(v) Explain, step by step, what happens

if this 4% Cu alloy is allowed to cool

slowly from its solution-treatment tem-

perature to ambient temperature,

(vi) What mechanical properties will

characterise the resultant structure in (v)?

(Atomic masses: Cu-63.53; Al-26.98)

liquid

OC 4- liquid

copper

wt J

<£ + CuAI

What characteristics of aluminium and its alloys account for its versatility in

uses? Suggest suitable competitors for two possible applications.

What is understood by the terms modification and

age-hardening

as applied to

the alloys of aluminium? What is the structural basis of each of these pro-

cedures and why are they carried out?

(17.41;

17.50)

What is meant by the terms solution-treatment and precipitation-treatment?

Illustrate your reference to a suitable aluminium-base alloy. (17.50)

6. What are the essentials of precipitation-hardening treatments? Give three dis-

tinct examples of alloys in which precipitation-hardening is found. (9.92;

13.101;

16.70; 17.50; 18.10)

Describe two methods of hardening aluminium alloys. Give the principles

underlying each method. (17.30; 17.50)

8. Which types of alloy can be hardened (a) by cold-working; (b) by precipitation-

hardening; (c) by a combination of (a) and (b)l Why is it necessary to exercise

close control of heat-treatment variables in precipitation-hardening heat treat-

ment? (17.30; 17.50;

17.53;

17.56)

9. Outline the basic theory which seeks to explain the precipitation hardening of

an aluminium alloy containing 4% copper. (9.92; 17.52)

10.

Suppose a batch of aluminium-alloy aircraft rivets had been on the shop-floor

over the week-end. Would they be usable? If not could they be salvaged—

and, if so, how? (17.56)

Bibliography

Higgins, R. A., Engineering Metallurgy (Part II), Edward Arnold, 1986.

King, F., Aluminium and its Alloys, John Wiley, 1987.

Martin, J. W.,

Precipitation

Hardening, Pergamon, 1968.

Martin, J. W., Micromechanisms in P

article-hardening

Alloys, Cambridge Univer-

sity Press, 1979.

Mondolfo, L. E., Aluminium Alloys, Structure and

Properties,

Butterworth, 1976.

Polmear, I. J., Light Alloys

(Metallurgy

of the Light Metals), Edward Arnold, 1989.

BS 1490: 1988 Aluminium and Aluminium Alloy Ingots and Castings.

BS 1470: 1987 Wrought Aluminium and Aluminium Alloys for General

Engineering

Purposes: Plate, Sheet and Strip.

BS 1471: 1972 Wrought Aluminium and Aluminium Alloys for General

Engineering

Purposes: Drawn Tube.

BS 1472: 1972 Wrought Aluminium and Aluminium Alloys for General

Engineering

Purposes: Forging Stock and Forgings.

BS 1473: 1972 Wrought Aluminium and Aluminium Alloys for General

Engineering

Purposes: Rivet, Bolt and Screw Stock.

BS 1474: 1987 Wrought Aluminium and Aluminium Alloys for General

Engineering

Purposes: Bars, Extruded Round Tubes and Sections.

BS 1475: 1972 Wrought Aluminium and Aluminium Alloys for General

Engineering

Purposes: Wire.

Other Non-ferrous

Metals And Alloys

Magnesium-base Alloys

18.10 It is perhaps surprising that magnesium-base alloys should be suit-

able for engineering purposes, since in composition they are similar to the

incendiary bombs used by the RAF and the Luftwaffe during the Second

World War. Incendiary-bomb cases contained about 7% aluminium (which

was added to introduce better casting properties) and small amounts of

manganese; the balance being magnesium. Fortunately, combustion of

such an alloy takes place only at a relatively high temperature, and this

was generated by filling the bomb case with thermit mixture (20.52). Ordi-

narily, magnesium alloys can be melted and cast without mishap, provided

that the molten alloy is not overheated.

18.11 Although pure magnesium is a relatively weak metal, its alloys

containing suitable amounts of aluminium, zinc or thorium can be strength-

ened considerably by precipitation-hardening. The thermal-equilibrium

diagrams (Fig. 18.1) indicate the changes in solid solubility of these three

metals in the respective a-phases; changes which are necessary if precipi-

tation-hardening is to be effected (9.92).

18.12 Small amounts of manganese, zirconium and the rare earth

metals are also added to some magnesium alloys. Manganese improves

corrosion-resistance, whilst zirconium is an effective grain refiner. Thorium

and the rare-earth metals (in particular, cerium) give further increase in

strength whilst allowing the alloy to retain a somewhat higher ductility,

but their most important function is to give improved creep-resistance at

high working temperatures.

The greater strength obtained by alloying, coupled with their low relative

density of about 1.8, makes these alloys particularly useful where weight

Fig.

18.1 Changes in solid solubility along XY make precipitation-hardening possible in

each case.

is a limiting factor. Such uses include various castings and forgings used in

the aircraft industry—landing-wheels, petrol tanks, oil tanks, crankcases

and airscrews, besides many engine parts in both piston and jet engines.

Those alloys which can be precipitation hardened are first solution treated

at temperatures between 300

C and 565°C—410

C for those alloys contain-

ing aluminium; 320

C for those containing zinc; and 565°C for those con-

taining thorium as indicated by the equilibrium diagrams (Fig. 18.1).

Precipitation treatment is usually carried out at between 170° and 200

for up to 12 hours. Examples of both cast and wrought alloys are given in

Tables 18.1 and 18.2.

18.13 The melting and casting of these alloys present some difficulty

in view of the ease with which molten magnesium takes fire, particularly

when overheated. They are generally melted under a flux containing the

fluorides of calcium and sodium and the chlorides of potassium and mag-

nesium. During the casting operation flowers of sulphur is often dusted on

to the stream of molten alloy. The sulphur takes fire in preference to the

magnesium, and thus protects the latter with a surrounding atmosphere of

sulphur dioxide gas. Nevertheless modern developments such as fluxless

melting and mechanical stirring have improved commercial production in

recent years.

Zinc-base Die-casting Alloys

18.20 Zinc-base alloys containing 4% aluminium and small amounts of

copper and magnesium have been used for many years for die casting a

wide range of components both in engineering industries and for domestic

appliances, and were sold initially under the trade name of 'Mazak'. These

alloys have excellent casting properties as well as reasonable strength and

rigidity and are used for many parts in washing machines, radio sets, alarm

°c

LIQUID

ALUMINIUM %

ZINC °/o

THORIUM %

Table

18.1

Cast Magnesium-base Alloys

Typical

mechanical properties.

Elongation

(%)

Chill

Sand

Chill

185

230

170

275

215

170

215

Tensile

strength

(N/mm

)

Sand

140

200

125

200

125

185

Chill

100

130

110

0.2%

Proof

stress

(N/mm

)

Sand

130

110

Condition

Rare

Earths

Composition

(%)

0.15

0.4

0.15

0.4

0.15

0.8

0.3

1.0

0.3

1.0

0.3

1.5

7.5

9.0

10.5

7.5

9.5

Relevant

specifications

2970

General purpose alloys

MAG.1

(Mgr-

AI8-Zn-Mn)

MAG.3

(Mg-

AIIO-Zn-Mrr)

MAG.7

(Mg-

AI8.5-Zn1-Mn)

215

155

185

255

265

240

200

140

185

255

265

240

135

110

155

185

135

155

185

0.75

1.75

2.5

4.0

Y-4.0 Nd and

others-3.5

and

others-2.5

2.5

4.0

1.5

2.3

0.4

1.0

0.4

1.0

0.4

1.0

0.4

1.0

0.5

0.6

3.5

5.0

0.8

3.0

1.7

2.5

5.0

6.0

2.5

Special

alloys

MAG.5

(Mg-

Zn4-RE-Zr)

MAG.6

(Mg-

RE3-Zn-Zr)

MAG.8

(Mg-

Th3-Zn2-Zr)

MAG.9

(Mg-

Zn5.5-Th2-Zr)

(WE

43)

DTD.

5035A

M—as

cast

TE—precipitation-treated

TB—solution-treated

TF—solution-treated

and

precipitation-treated.

Table

18.2

Wrought Magnesium-base Alloys

Relevant

Specification

(BS)

337O:MAG.S.1O1

3372.MAG.F.101

3373:MAG.E.1O1

3370.MAG.S.111

3373:MAG.E.111

3372.MAG.F.121

3373:MAG.E.121

3370:MAG.S.131

3372.MAG.F.131

3373:MAG.E.131

3370:MAG.S.141

3372:MAG.F.141

3373:MAG.E.141

3370:MAG.S.151

3372:MAG.F.151

3373:MAG.E.151

3372:MAG.F.161

3373:MAG.E.161

Composition

(%)

2.5-3.5

5.5-6.5

0.6-1.4

0.5-1.5

1.5-2.3

0.75-1.5

2.5-4.0

4.8-6.2

1.0-2.0

0.15-0.40

0.6-1.3

0.4-0.8

0.45-0.8

Condi-

tion

Typical mechanical properties

0.2

Proof

stress

(N/mm

)

100

120

180

160

120

170

180

230

Tensile

strength

(N/mm

)

200

250

220

270

250

220

250

265

315

Elonga-

tion

(%)

BS3370—Plate, sheet and strip

(S)

BS3372—Forgings and cast forging stock

(F)

BS3373—Extruded bars, sections and tubes

(E)

clocks, electric fires

and

accurately-scaled 'matchbox' toys; whilst

the

aver-

age motor

car

contains

multitude

zinc-base die-castings from

the

door

handles

to the

wiper-motor frames.

18.21 Difficulty was experienced

in the

development

these zinc-base

alloys owing

swelling

the casting during use, coupled with intercrystal-

line embrittlement. These faults were found

to be due to

intercrystalline

corrosion caused

small amounts

cadium,

tin and

lead.

For

this reason

high-grade zinc

four nines quality

(ie

99.99% pure)

used

for the

manufacture

these alloys,

and the

impurities thus limited

to the

small

quantities indicated

Table 18.3.

18.22 Normally, after casting, these zinc-base alloys undergo

slight

shrinkage which

complete

about five weeks. When close dimensional

limits

are

necessary

stabilising anneal

in dry air at

100±5°C

for six

hours

should

given before machining,

order

accelerate

any

volume

change that

likely

take place.

In order

test these alloys

for

growth

and

intercrystalline corrosion,

the steam test

applied. This accelerates

the

formation

these defects,

and consists

suspending

the

test piece above water

95°C

in a

closed

vessel

for ten

days.

The

test piece

then examined

for

expansion,

and

mechanically tested to reveal any intercrystalline corrosion which may have

occurred.

18.23 Of the two casting alloys generally available, alloy A is used for

engineering purposes since it has greater dimensional stability and re-

sponds consistently to the stabilising anneal. It is also more ductile and

retains its impact strength at high temperatures much better than alloy B.

The corrosion resistance of alloy A is also superior. Alloy B, however, has

greater strength and hardness as well as better castability in thin sections

and is therefore used for such components as zip-fastner slides and small

gear wheels.

18.24 In recent decades the relatively low-strength die-cast alloys have

been replaced for many purposes by plastics mouldings. This, together

with a fashion change away from chromium-plated finishes, led to a big

decline in the use of the 4% aluminium (BS1004) alloys. However a new

series of zinc-base alloys containing greater quantities of aluminium and

copper has been developed to give increased strength and hardness, and

it is expected that these will find increasing use in the automobile industries

as engine mountings and other parts where strength and toughness com-

bined with a saving in weight are involved.

Three new alloys, designated by the trade ZA8, ZA12 and ZA27 contain

respectively 8, 11 and 27% aluminium along with small amounts of copper

and magnesium (Table 18.3). As with the BS1004 alloys the impurities

Table 18.3 Zinc-base Casting Alloys

Designation

BS1004

(alloy A)

BS1004

(alloy B)

ZA8

ZA12

ZA27

Composition

(%)

BaI.

4.1

8.4

11.0

26.5

0.10

max.

1.0

0.8

2.2

0.05

0.03

0.02

Impurities

(max.)

Fe-0.10

Pb-0.004

Cd-0.003

Sn-0.002

Condition

die cast

'stabilised'

die cast

'stabilised'

sand cast

pressure

die-cast

sand cast

gravity

die-cast

sand cast

Typical

mechanical

properties

Tensile

strength

(N/mm

)

286

273

335

312

260

375

300

327

420

sand cast

(heat-treated)

gravity

die-cast

317

424

Elongation

(%)

4.5

Impact

toughness

(J)

Hardness

(Hb)

103

100

115

Relative

density

6.7

6.3

6.0

5.0