Heat treatment of aluminium alloys

An aluminium alloy is heat treated by heating it for

a prescribed period at a prescribed temperature,

and then cooling it rapidly, usually by quenching.

The particular form of heat treatment which results

in the alloy attaining its full strength is known as

solution treatment. The alloy is raised to a temperature

of 490 °C by immersing it in a bath of molten

salt. The bath is usually composed of equal parts of

sodium nitrate and potassium nitrate contained in

an iron tank. This tank is heated by gas burners

and, except for its open top, is enclosed with the

burners in a firebrick structure which conserves the

Table 5.4Tempering colours for plain carbon steel

Colour Temperature (°C) Type of article

Pale straw 220–230 Metal turning tools,

scrapers, scribers

Dark straw 240–245 Taps, dies, reamers,

drills

Yellow-brown 250–255 Large drills, wood

turning tools

Brown 260–265 Wood working tools,

chisels, axes

Purple 270–280 Cold chisels, press

tools, small springs,

punches, knives

Blue 290–300 Springs, screwdrivers,

hand saws

Table 5.3Temperature colours for steel

Colour Temperature (°C)

Black 450–550

Very dark red 600–650

Dark red 700–750

Cherry red 800–850

Full red 850–900

Bright red 950–1000

Dark orange 1050–1100

Light orange 1150–1200

Yellow white 1270–1300

White (welding heat) 1400–1550

Metal forming processes and machines 167

heat. The temperature of the bath must be carefully

regulated, as any deviation either above or below

prescribed limits may result in the failure of the

metal to reach the required strength. The alloy is

soaked at 490 °C for fifteen minutes and then

quenched immediately in cold water.

At the moment of quenching the alloy is reasonably

soft, but hardening takes place fairly rapidly

over the first few hours. Some alloys, chiefly the

wrought materials, harden more rapidly and to a

greater extent than others. Their full strength is

attained gradually over four or five days (longer in

cold weather); this process is known as natural age

hardening. As age hardening reduces ductility, any

appreciable cold working must be done while the

metal is still soft. Working of the natural ageing

aluminium alloys must be completed within two

hours of quenching, or for severe forming within

thirty minutes. Age hardening may be delayed by

storing solution-treated material at low temperatures.

Refrigerated storage, usually at 6–10 °C, is

used for strip sheet and rivets, and work may be

kept for periods up to four days after heat treatment.

If refrigerated storage is not used to prevent

age hardening it may be necessary to repeat solution

treatment of the metal before further work is

possible.

Alloys of the hiduminium class may be artificially

age hardened when the work is finished.

Artificial ageing is often called precipitation treatment;

this refers to the precipitation of the two

inter-metallic compounds responsible for the hardening,

namely copper and manganese silicon. The

process consists of heating the work in an automatically

controlled over to a temperature in the region

of 170 °C for a period of ten to twenty hours.

Artificial ageing at this temperature does not distort

the work. The temperature of the oven must be

maintained to within a few degrees, and a careful

check on the temperature is kept by a recording

instrument. In order to ensure uniform distribution

of temperature a fan is fitted inside the oven to

keep the air in circulation. At the end of a period of

treatment the oven is opened to allow the work to

cool down. One of the chief advantages of this

process is that work of a complicated character

may be made and completed before ageing takes

place. Moreover, numerous parts may be assembled

or riveted together and will not suffer as a

result of the ageing treatment.

5.3 How metal is formed to provide

strength

It has been established that the strength of a material

is governed by its material composition and by

the method and direction of loading, i.e. tensile,

compressive, torsion, shear and bending. Generally

the majority of metals are capable of withstanding

greater loads in tension than any other type of

stress. One of the properties of steel is that, within

certain limits, it is elastic: that is, if it is distorted

by a load or force it will change shape, but it will

return to its original shape when the force is

removed. However, above a certain intensity of

load (the elastic limit) the metal will remain distorted

when the load is removed. Sheet steel, such

as is used in the manufacture of car bodies, has

reasonable strength in tension but has little resistance

to compressive and/or torsional loads. This

lack of resistance is due not so much to poor compressive

strength as to lack of rigidity. Low-carbon

steels are used extensively in the manufacture of

vehicle bodies, and the designer has to ensure that

the relatively thin sections of material are capable

of withstanding the various types of loading. In

addition to the permanent stresses present in the

material, the vehicle body as a whole is subject to

shock stresses due to road conditions, and these

must also be taken into consideration by the

designer.

With the development of deep drawing steels

and better press equipment, large streamlined panels

were designed and formed into contours that

were more attractive, gave longer life and greater

safety, and at the same time reduced the bulky

construction previously required to give similar

strength. It is known that the shape of any material

is held by the stresses set up in the material itself,

such as those given by angles, crowns, channels

and flanges. The original shape will be maintained

until the material is subject to a force sufficient to

overcome the initial stresses. Furthermore, it will

tend to return to its original shape providing it has

not been distorted beyond the point of elasticity.

Crowned surfaces

The building up of stresses at the bend or peak is

also an important consideration in the design and

manufacture of the modern car body. The most

168Repair of Vehicle Bodies

common features of the body are the curved surfaces

(Figure 5.2); these are called crowns and

may be curved either in one direction or in all

directions. A crowned surface is stronger than a

flat panel, and whilst it will resist any force tending

to change its shape, it also has the ability to

return to its original shape unless distorted

beyond its point of elasticity. These are the features

of metal sheet which has been formed in a

press into a permanent shape, with die-formed

stresses throughout its entire area tending to hold

the shape. On the bend or crown, one side of the

sheet is longer than the other; and the metal at the

surface is more dense than at the centre of the

sheet. The final action of the press is to squeeze

the surface together, thus setting up stresses and

greater strength. The greater the crown or curve

of the panel, the greater its strength and rigidity

to resist change in its shape. This is illustrated by

the fact that a low crown, i.e. a surface with very

little curve, such as a door panel, is springy and is

not very resistant to change of shape. On the

other hand high crowns, that is surfaces with a lot

of curve, like wings, edges of roofs and sill panels,

are very resistant to change in shape by an

outside force.

Angles and flanges

A further method of giving strength to metal is to

form angles or flanges along the edges of sheets

(Figure 5.3). A right-angled bend greatly increases

the strength of a sheet, as can be demonstrated by

forming a right-angled bend in a thin sheet of

metal and then trying to bend the metal across the

point of the bend. This method is used on inner

door panels and at the edges of wings, edges of

bonnets and boot lids, and wherever stiffness is

required at unsupported edges.