The same is true of
aluminum, in which the oxide, alumina, will be formed, and with several
other alloys comprising elements of different melting points.
In order to eliminate these oxides, it is necessary in practical work, to
puddle the weld; this is, to have a sufficient quantity of molten metal at
the weld so that the oxide is floated away. When this is done, the two
surfaces which are to be joined are covered with a coat of melted metal on
which floats the oxide and other impurities. The two pieces are thus
allowed to join while their surfaces are protected. This precaution is not
necessary in working with steel except in extreme cases.
Another difficulty which is met with in the welding of a great many metals
is their expansion under heat, which results in so great a contraction when
the weld cools that the metal is left with a considerable strain on it. In
extreme cases this will result in cracking at the weld or near it. To
eliminate this danger it is necessary to apply heat either all over the
piece to be welded or at certain points. In the case of cast iron and
sometimes with copper it is necessary to anneal after welding, since
otherwise the welded pieces will be very brittle on account of the
chilling. This is also true of malleable iron.
Very thin metals which are welded together and are not backed up by
something to carry away the excess heat, are very apt to burn through,
leaving a hole where the weld should be.
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