Page 49 - Welding of Aluminium and its Alloys

Material standards, designations and alloys 41

cavities that result in a slight loss of strength. There will also be a loss of
strength in the cold work alloys where the structure has been annealed and
softened.The weld metal has an as-cast structure.When the filler metal has
the same nominal composition as the parent metal the low melting point
constituents such as Fe-Al-Si are the last to solidify and will be located at
the grain boundaries.

3.4.1.2 Aluminium–manganese alloys (3XXX series)
When iron is present as an impurity the solubility of manganese in alu-
minium is very low. The rate of cooling from casting or welding is suffi-
ciently rapid for some manganese to be left in supersaturated solution.
Further processing to provide a wrought product causes the manganese to
precipitate as FeMnAl 6, this precipitate giving an increase in strength due
to dispersion hardening. Any uncombined iron and silicon impurities may
be present as an insoluble Al-Fe-Mn-Si phase.
The weld zones are similar to those seen in pure aluminium, the only
major difference being the composition of the precipitates. The heat of
welding has the same effect on the structure as on pure aluminium, with
the precipitates arranged along the grain boundaries and a loss of strength
in the annealed regions of cold worked alloys.
The 3103 (AlMn1)alloy is more hot short (see Section 2.5) than pure alu-
minium, despite having a similar freezing range. In practice, however, hot
cracking is rarely encountered. Those alloys containing copper (alloy 3003)
or magnesium (alloys 3004, 3005 and 3105) are more sensitive to hot crack-
ing. Weld cracking may be sometimes encountered when autogenous
welding but this is easily prevented by the use of an appropriate filler metal
composition.

3.4.1.3 Aluminium–silicon alloys (4XXX series)

The aluminium silicon alloys form a binary eutectic at 11.7% silicon with a
melting point of 577°C, the two phases being solid solutions of silicon in
aluminium, 0.8% maximum at room temperature, and aluminium in silicon.
There are no intermetallic compounds. Sodium may be added in small
amounts to refine the eutectic and increase the strength by improved dis-
persion hardening. Iron, even in small amounts, can seriously degrade
toughness although manganese may be added to reduce this effect.
The 4XXX series has very high fluidity and is extensively used for casting
purposes, often being alloyed with copper and magnesium to provide some
degree of precipitation hardening and with nickel to improve high temper-
ature properties. Because of its high fluidity and low sensitivity to hot short-
ness it is commonly used as a weld filler metal.