Page 47 - Welding of Aluminium and its Alloys

Material standards, designations and alloys 39

a low temperature to soften them to the required degree of hardness
and strength.
• H3 – strain hardened and stabilised. Stabilisation is a low-temperature
heat treatment applied during or on completion of fabrication. This
improves ductility and stabilises the properties of those strain-hardened
alloys that soften with time.
• H4 – strain hardened and painted. This is for alloys that may be sub-
jected to low-temperature heat treatment as part of a paint baking or
adhesive curing operation.
The second digit after ‘H’ indicates the amount of strain hardening in the
alloy. H18 is strain hardened only and in the most heavily cold worked con-
dition. It is therefore the hardest and highest strength condition. Ductility
will be very low and further cold work may cause the component to crack.
Intermediate conditions are identified by the numbers 1 to 7 and are based
on the strength relative to that of the annealed alloy, O condition and the
H18 condition, e.g. an H14 alloy will have a strength halfway between the
annealed and fully hard condition, H12 halfway between O and H14.There
is an H9 condition in which the ultimate tensile strength exceeds that of the
2
H8 condition by a minimum of 10N/mm .
The third digit after ‘H’ is not mandatory and is used when the alloy
requires special control to achieve the specific temper identified by the
second digit or when some other characteristic of the alloy is affected.
Examples of such characteristics are exfoliation corrosion resistance, seam
welded tube or additional working after the final temper has been achieved,
e.g. by embossing.
The ‘T’ designations are applied to those alloys that are age hardened,
the first digit identifying the basic heat treatment:

• T1 – cooled from an elevated temperature-shaping treatment and
naturally aged.
• T2 – cooled from an elevated temperature-shaping process, cold worked
and naturally aged.
• T3 – solution heat treated, cold worked and naturally aged.
• T4 – solution heat treated and naturally aged.
• T5 – cooled from an elevated temperature-shaping process and artifi-
cially aged.
• T6 – solution heat treated and artificially aged.
• T7 – solution heat treated and overaged or stabilised.
• T8 – solution heat treated, cold worked and artificially aged.
• T9 – solution heat treated, artificially aged and cold worked.
More digits may be added to the designation to indicate variations in heat
treatments or cold work. For example, TX51, 510, 511, 52 or 54 all indicate