Page 186 - Welding of Aluminium and its Alloys

Resistance welding processes 169

transformer AC unit but an improvement in weld quality can be used to
justify this increase.
As with the AC unit the welding gun may be pedestal mounted or
portable. The limitations in cables apply when the power source is remote
from the gun but the potential for a lighter transformer on gun welding
head eases the manipulation problems apparent with the AC units.

9.2.3 Primary rectified three phase units
Primary rectified three phase units predate the advent of solid state elec-
tronics, using ignitron or thyratron tubes for current control. They were
widely used in high-quality applications in industries such as aerospace.
Half wave rectification of the primary current is used to provide the trans-
former with DC to give a high-current, low-voltage output. It is possible to
weld a wider range of materials and thicknesses with these more efficient
units. The length of the current pulse that these units are capable of pro-
viding is limited before saturation of the transformer core occurs.Alternate
spot welds are generally made with alternate polarities.

9.2.4 Secondary rectified three phase DC units
Secondary rectified three phase DC units are very energy efficient and are
capable of delivering very high welding currents,making them ideally suited
to the welding of aluminium alloys. They do, however, use very large and
heavy transformers and diodes, making the equipment bulky and best
suited for fixed and pedestal machines.

9.2.5 Secondary rectification inverter units

As mentioned in other chapters the development of solid state electronics,
particularly inverter technology, has resulted in highly energy efficient,
compact and light power units with weight savings of up to 50% compared
with an AC unit of equivalent output. At high current output there is a risk
of overheating and up until perhaps 1997 the maximum output was limited
to some 20kA but this value has now been increased to over 50kA. This
enables these units to be used in high utilisation activities such as those
required in the automotive industry and to be mounted on robots for
continuous operation (Fig. 9.3).

9.3 Surface condition and preparation
The surface condition of the aluminium sheets is one of the most impor-
tant deciding factors in achieving consistent quality of resistance spot and