Page 14 - Welding of Aluminium and its Alloys

6 The welding of aluminium and its alloys

1.3 Product forms

Aluminium is available in both wrought and cast forms.The wrought forms
comprise hot and cold rolled sheet, plate, rod, wire and foil. The ductility
and workability of aluminium mean that extrusion is a simple method of
producing complex shapes, particularly for long, structural members such
as I and H beams, angles, channels,T-sections, pipes and tubes. Forging, both
hot and cold, is used extensively as a fast, economical method of producing
simple shapes. Precision forging is particularly suitable for aluminium
alloys, giving advantages of good surface finish, close tolerances, optimum
grain flow and the elimination of machining.
The four most commonly used methods of casting are sand casting, lost
wax casting, permanent steel mould casting and die-casting. The require-
ment for high fluidity in a casting alloy means that many are based on
aluminium–silicon alloys although heat-treatable (age-hardening) alloys
are often used for sand, lost wax and permanent mould castings. Lost wax
and die-casting give products with smooth surfaces to close tolerances and
are processes used extensively for aerospace products. A number of alloys,
their product forms and applications are listed in Table 1.1.

1.4 Welding: a few definitions
Before dealing with the problems of welding aluminium alloys there are a
few definitions required, not least of which is welding itself.Welding can be
described as the joining of two components by a coalescence of the surfaces
in contact with each other.This coalescence can be achieved by melting the
two parts together – fusion welding – or by bringing the two parts together
under pressure, perhaps with the application of heat, to form a metallic
bond across the interface. This is known as solid phase joining and is one
of the oldest of the joining techniques, blacksmith’s hammer welding having
been used for iron implement manufacture for some 3500 years. The more
modern solid phase techniques are typified by friction welding. Brazing,
also an ancient process, is one that involves a braze metal which melts at a
temperature above 450°C but below the melting temperature of the com-
ponents to be joined so that there is no melting of the parent metals. Sol-
dering is an almost identical process, the fundamental difference being that
the melting point of the solder is less than 450°C. The principal processes
used for the joining of aluminium are listed in Table 1.2. Not all of these
processes are covered in this book as they have a very limited application
or are regarded as obsolescent.
Welding that involves the melting and fusion of the parent metals only
is known as autogenous welding, but many processes involve the addition