HEAT TREATABLE AND STRAIN HARDENING ALLOYS

Aluminium alloys may be divided into two distinct groups :

a. Alloys which may be hardened by heat treatment,

i.e. heat treatable alloys.

b. Alloys which may be hardened only be cold working

i.e. strain hardening alloys.

Heat treatable alloys

-Certain alloys are susceptible to heat treatment making them still stronger

-These alloys contain alloying constituents that have increased solid solubility at elevated temperatures.

-Heat treatable alloys contain copper, magnesium or zinc as the principle alloying element.

-Produces their full strength and corrosion resistance only if they are solution heat treated.

-Low corrosion resistance and strength in the annealed condition.

Non Heat treatable alloys ( Strain Harden Alloy)

-These alloys contain alloying constituents that remain in solid solution or are insoluble at all temperatures.

-In addition to alloying, the strength can be increased by cold working (strain hardening)

-Responds to only annealing

-Cold working (Strain hardening is done to increase its strength.)

-Used in non-structural parts such as oil tanks, fairing and engine cowling.

ALUMINIUM ALLOY TEMPER DESIGNATIONS

F-The metal is left fabricated. There has been no control over its temper

T-The metal is heat treated

T3-Solution heat treatment, followed by strain hardening. A second digit, if used, indicates the amount of strain hardening

T4-Solution heat treatment, followed by natural ageing at room temperature

T6-Solution heat treatment, followed by artificial ageing

T7-Solution heat treatment, followed by stabilization

T8-Solution heat treatment, followed by strain hardening and then artificial aging

T9-Solution heat treatment, followed by artificial ageing and then strain hardening

H-The metal cannot be heat treated but can be hardened by cold working

H1 Strain hardened by cold water

H12 Strain hardened to its 1/4 hard condition

H14 Strain hardened to its 1/2 hard condition

H16 Strain hardened to its 3/4 hard condition

H18 Strain hardened to its full hard condition

H19 Strain hardened to its extra hard hard condition

O-The metal has been anneled

Used in non-structural parts such engine cowling.