Page 191 - Welding of Aluminium and its Alloys

174 The welding of aluminium and its alloys

which is more easily maintained in production than the truncated cone.
Alignment is also less of an issue and is favoured particularly for portable
equipment. The truncated cone tends to be used for commercial quality
applications, mainly because electrode alignment is more critical and diffi-
cult to maintain consistently in production. Tip life, however, is markedly
better, by a factor of two to three, than can be achieved with the domed
electrode. Cone angles vary from 60° to 150° including an angle with a slight
radius on the tip which aids in alignment and reduces marking of the sheet.
The tip profile may be maintained by grinding, filing or by the use of
abrasive cloth in a shaped former. While this dressing operation may be
performed manually it is difficult to maintain the correct tip shape
and electrode alignment. The use of automatic tip dressing tools or spe-
cially designed hand-held manual or pneumatic tip dressers is strongly
recommended.
Efficient electrode cooling is also necessary to maintain tip life. Large
diameter electrodes will provide a greater heat sink but efficient water
cooling is imperative. The cooling channel should be carried as close to the
tip as possible, a distance of between 12 and 20mm being usual with water
flow rates of 5–10 litres/min. Water inlet temperature should be in the
region of 20°C and the outlet temperature in the region of 30°C.

9.4.4 Quality control

There is no specification available for the quality control of aluminium spot
welds within the UK although ASME IX includes rules for procedure
approval of spot welds.There is, however, a British Specification for the spot
welding of steel, BS 1140, which contains a multitude of recommendations
that may also be applied to aluminium. The main method of demonstrat-
ing acceptable quality is by means of the peel test, a simple, inexpensive
test, but this may be replaced or supplemented by a tension shear test or a
twist test. The test piece for all three tests is essentially the same – two
overlapping plates welded together with a single spot.An acceptable result
in the peel test is when the weld nugget is pulled out of the parent plate.
Monitoring of the welding parameters is an effective method of assuring
quality during production welding. This monitoring may be very simple,
detecting only the absence of a weld, or may be a sophisticated electroni-
cally based monitor which will both monitor and record current, number of
cycles, pressure and time. This recording can be augmented by audible or
visual warning of out-of-range welding parameters. Finally, one of the most
effective quality control methods is visual inspection where surface melting,
adhesion of the electrodes, pits, cracks, asymmetry of the weld spot and