Page 18 - Welding of Aluminium and its Alloys

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Welding metallurgy

2.1 Introduction
Ideally a weldment – by this is meant the complete joint comprising the weld
metal, heat affected zones (HAZ) and the adjacent parent metal – should
have the same properties as the parent metal.There are, however, a number
of problems associated with the welding of aluminium and its alloys that
make it difficult to achieve this ideal.The features and defects that may con-
tribute to the loss of properties comprise the following:

• Gas porosity.
• Oxide inclusions and oxide filming.
• Solidification (hot) cracking or hot tearing.
• Reduced strength in the weld and HAZ.
• Lack of fusion.
• Reduced corrosion resistance.
• Reduced electrical resistance.

This chapter deals with the first four of these problem areas, i.e. those of
porosity, oxide film removal, hot cracking and a loss of strength. Before dis-
cussing these problems, however, there is a brief introduction as to how
metals achieve their mechanical properties. Some of the terms used to
describe specific parts of a welded joint are shown in Fig. 2.1.

2.2 Strengthening mechanisms
There are five separate strengthening mechanisms that can be applied to
the aluminium alloys. These are grain size control, solid solution alloying,
second phase formation, strain hardening (cold work) and precipitation or
age hardening.

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