Page 37 - Welding of Aluminium and its Alloys

Welding metallurgy 29

Increasing film
thickness giving
increasing Back filling of
sensitivity defects due to large
amount of eutectic
Crack
sensitivity Peak sensitivity
to hot tearing

Percentage of alloying element
2.14 Generalised picture of crack sensitivity.

either discontinuous or very thin. The strength of a liquid film can be
derived from
g
kA
F =
t
where F = force required to tear the liquid;
k = a constant;
g= the liquid/solid interfacial tension;
A = cross-sectional area;
t = liquid film thickness.
Therefore, as the liquid film thickness t increases, the force required to tear
the film F reduces. The force required for cracking begins to increase,
however, once there is sufficient eutectic available that it can begin to flow
into and fill any cracks that form. The crack sensitivity therefore drops, the
cracks heal themselves and a crack-free structure results. This is a very
useful feature when welding alloys that are sensitive to liquation cracking
in the HAZ. Figure 2.14 illustrates this graphically, where it can be seen that
the shape of the curve is essentially the same as that in Fig. 2.13.
The practical consequence of this is that the crack susceptibility of the
weld metal is very sensitive to changes in composition. In very many situ-
ations when welding aluminium alloys, the filler metal does not match the
parent material. It is most important that this fact is realised and that
account is taken of the composition of the resultant weld metal. There are
a number of other factors, apart from filler metal and parent metal com-
position, which affect the weld metal composition. Fit-up of the component
parts can affect the amount of dilution in a joint, dilution being the amount
of parent metal dissolved into the weld metal during welding. In the root
pass a wide gap will give low dilution, a narrow gap high dilution, as illus-
trated in Fig. 2.15.