Page 48 - Welding of Aluminium and its Alloys

40 The welding of aluminium and its alloys

those alloys that are stress relieved after heat treatment by some form of
cold working such as stretching or restriking cold in the finish die. These
additional digits are also used to indicate the temper condition of those
alloys designated ‘W’.
The T7, artificially aged, temper designation may be supplemented by
a second digit to indicate if the alloy is overaged and by how much. Other
numbers are used to identify underaged conditions and increasing degrees
of cold work etc.
The full details of these designations are contained in the specification
EN 515 ‘Aluminium and Aluminium Alloys – Wrought Products – Temper
Designations’.

3.4 Specific alloy metallurgy

3.4.1 Non-heat treatable alloys

3.4.1.1 Pure aluminium (1XXX series)

The principal impurities in ‘pure’ aluminium are silicon and iron, residual
elements remaining from the smelting process. Copper, manganese and zinc
may also be present in small amounts. The maximum impurity levels vary
with the specified purity, e.g. 1098 (Al99.98) contains a maximum impurity
content of 0.02%, comprising 0.010% Si max., 0.006% Fe max., 0.0035% Cu
max. and 0.015% Zn max. The 1050 (Al99.5) alloy contains a maximum of
0.05% of impurities. In the high-purity grades of these alloys the impurities
are in such low concentrations that they are completely dissolved. From the
welding viewpoint the alloys can be regarded as having no freezing range
and a single phase microstructure which is unaffected by the heat of
welding. The less pure alloys such as 1200 (Al99.0) can dissolve only small
amounts of the impurity elements and, as the metal freezes, most of the iron
comes out of solution to form the intermetallic compound FeAl 3. When
silicon is present in more than trace quantities, a ternary or three-element
compound, Al-Fe-Si phase, is formed. With higher silicon contents free
primary silicon is formed. All of these phases contribute to an increase in
strength, attributed to slight solution hardening and by a dispersion of the
phases.
The effects of welding on the structure of a fusion welded butt joint in
an annealed low-purity aluminium such as 1200 is to produce three distinct
zones. The unaffected parent material will have a fine-grained structure of
wrought metal with finely dispersed particles of Fe-Al-Si.The heat affected
zones show no significant change in microstructure except close to the
fusion boundary where partial melting of the low melting point constituents
along the grain boundaries occurs, leaving minute intergranular shrinkage