Page 196 - Welding of Aluminium and its Alloys

Resistance welding processes 179

9.6.3 Electrode clamps

For the welding of steel copper alloys are generally used for the manufac-
ture of the electrode clamps. For aluminium, however, steel, sometimes
copper plated, has been found to give better results, conducting less heat
away from the weld, providing a longer life and more positive clamping. By
drawing the weld back through one of the clamps fitted with a knife edge
it is also possible to shear off the upset as part of the removal process. A
broach may be inserted into the bore of hollow components to remove any
internal flash. To achieve a clean cut and to prevent smearing of the upset
during removal the cutting edges must be kept sharp. The clamps are
machined to match the outside shape of the components and are split to
enable rapid insertion. They are also designed to clamp around 80% of the
circumference and to be of a sufficient length that slippage does not occur
during upsetting.
To prevent crushing or deformation of hollow components removable
inserts or backing devices may be used beneath the clamp area. Sufficient
distance must be left between the ends of the inserts to ensure that they do
not take part in the welding operation.

9.6.4 Quality control

Provided that the equipment is correctly set-up and maintained, flash butt
welding is a trouble-free process. Alignment of the components is vital to
achieve low rates of weld rejects. Failure to align the components can result
in ‘shelving’ where one component rides up over its partner and in uneven
flashing, producing lack of fusion defects. Insufficient heat and/or inade-
quate upset may both result in lack of fusion type defects or oxide entrap-
ment. Both of these defects can be readily detected by the use of a bend
test such as those required by the procedure approval specification BS EN
288 Part 4 – see Chapter 10, Table 10.3. Bend testing is a relatively inex-
pensive method of assuring weld quality. Those non-destructive test tech-
niques that are commonly used for interrogating arc-welded butt joints,
such as radiography or ultrasonic examination, are not suitable for flash
butt welding and the engineer is forced to consider destructive tests. Bend
testing of pre-production test pieces prior to the start and at the end of a
production period of some 8 hours is one of the most cost-effective and
easily performed techniques. When this testing is supplemented by in-
process monitoring of the welding parameters (Fig. 9.6) then it is possible
to demonstrate a 100% acceptable weld quality. While it is written for the
control of steel flash butt welding the specification BS 4204 ‘Flash Butt
Welding of Steel Tubes for Pressure Applications’ is an extremely useful
reference, full of information that may be applied to aluminium alloys. It