Page 83 - Welding of Aluminium and its Alloys

72 The welding of aluminium and its alloys

Automation or the use of robots enables torch positioning and motion to
be controlled with the precision required for the production of quality
welds.

5.4 Welding position

Welding in the flat or downhand position is preferred for all arc welding
activities. It is easier for the welder to deposit high-quality weld metal at
high deposition rates in the flat position than in any of the other positions.
The weld pool is larger in this position with slower solidification and cooling
rates, permitting gases to evolve from the pool and reducing the amount of
porosity. The force of gravity in positions such as the horizontal–vertical,
however, means that the weld pool tends to sag, making it more difficult to
achieve an acceptable weld profile. These effects are more marked with
MIG than with TIG. Flat position welding therefore gives the best quality
weld metal at the lowest cost.
The designer should take these points into account when considering the
design of a structure. Wherever possible welding should be performed in
the flat position.This may require the fabrication of sub-assemblies that are
more easily manipulated. Manipulating equipment such as rotators or face-
plate manipulators are useful for items too large for manual handling. The
use of this equipment, however, may require the welding of temporary
attachments to the component to facilitate fitting the component to the
manipulator. As much care must be taken with the welding and removal of
these attachments as is applied to the permanent joints – formal welding
procedures should be considered in order to exercise control over this
sometimes haphazard activity.

5.5 Edge preparation and joint design
There are few more important decisions that affect the success of welding
than that of correct joint design. Problems with weld quality or performance
can often be attributed to the wrong design of edge preparation. Joint
design is determined by the strength requirements, the alloy, the thickness
of the material, the type and location of the joint, the access for welding
and the welding process to be used.
There are three fundamental forms of weld, the butt, the fillet and the
edge weld, illustrated in Fig. 5.3, from which can be developed six basic
joint types.These are the butt,T-joint, corner, cruciform, edge and lap joint,
illustrated in Fig. 5.4.
The static tensile strength of these weld types is determined by the throat
thickness (Fig. 5.5). The size of a fully penetrated butt weld is determined
by the thickness of weld metal deposited within the plane of the plate or