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LACK OF FUSION
Lack of fusion, also called cold lapping
or cold shuts, occurs when there is no fusion between the weld metal
and the surfaces of the base plate.
This defect can be seen in Figure 10-2. The most common cause of lack
of fusion is a poor welding technique.
Either the weld puddle is too large (travel speed too slow) and/or the
weld metal has been permitted to roll
in front of the arc. Again, the arc must be kept on the leading edge of
the puddle. When this is done, the
weld puddle will not get too large and cannot cushion the arc.
Another cause is the use of a very
wide weld joint. If the arc is directed down the center of the joint, the
molten weld metal will only flow and
cast against the side walls of the base plate without melting them. The
heat of the arc must be used to melt
the base plate. This is accomplished by making the joint narrower or by
directing the arc towards the side
wall of the base plate. When multipass welding thick material, a split bead
technique should be used whenever possible
after the root passes. Large weld beads bridging the entire
gap must be avoided.
Lack of fusion can also occur in the
form of a rolled over bead crown. Again, it is generally caused by a very
low travel speed and attempting to
make too large a weld in a single pass. However, it is also very often
caused by too low a welding voltage.
As a result, the wetting of the bead will be poor. When
welding aluminum, the common cause of this type of defect is the presence of aluminum
oxide. This oxide
is a refractory with a melting point of approximately 35000F
(19270C). It is also insoluble in molten
aluminum. If this oxide is present
on the surfaces to be welded, fusion with the weld metal will be hampered.