Page 31 - Welding of Aluminium and its Alloys

Welding metallurgy 23

2.9 Oxide entrapment in fillet weld. Courtesy of Roland Andrews.

during welding if defects such as lack of fusion and oxide film entrapment
are to be avoided. Figure 2.9 illustrates oxide filming in a fillet weld that
will obviously have a pronounced effect on joint strength.
Aluminium oxide (Al 2O 3) is a very tenacious and rapid-forming oxide
which gives aluminium its excellent corrosion resistance. Aluminium oxide
has a very high melting point, 2060°C compared with the pure metal
which melts at 660°C. The oxides of most other metals melt at tempera-
tures at or below that of their metals and during welding will float on
top of the weld pool as a molten slag. Heating aluminium to its melting
point without dispersing the oxide film will result in a molten pool of
aluminium enclosed in a skin of oxide, rather like a rubber toy balloon
filled with water. This skin has to be removed by some suitable means.
With fluxed processes, soldering, brazing, MMA and SA welding, the flux
needs to be very aggressive to dissolve the film. Failure to remove these
fluxes on completion can give rise to service failures from corrosion and, in
addition to porosity, is a further reason why MMA and SA welding are
rarely used.
Fortunately, in gas shielded arc welding there is a phenomenon known as
cathodic cleaning which can be employed to give the desired result. When
the electrode is connected to the positive pole of the power source and
direct current is passed there is a flow of electrons from the workpiece to
the electrode with ions travelling in the opposite direction and bombard-
ing the workpiece surface. This ion bombardment breaks up and disperses
the oxide film and permits the weld metal to flow and fuse with the parent
metal. The MIG welding process uses only DC electrode positive (DCEP)
current – using DC electrode negative (DCEN) results in an unstable arc,