Page 184 - Welding of Aluminium and its Alloys

Resistance welding processes 167

Pressure

Weld nugget

Lap joint
Welding current

Electrodes
Pressure

9.1 Principles of spot welding process.

As with conventional fusion welding, resistance welding suffers from
similar problems of oxide entrapment and hot cracking, the latter not being
helped by the lack of a more crack-resistant filler metal, and porosity.

9.2 Power sources

The power sources are normally rated in kVA at 50% duty cycle so, if the
maximum primary input power available is known, it is possible to calcu-
late the maximum power output from the welding machine. There are five
types of power source commonly used for the welding of aluminium.These
comprise single phase AC or DC machines; three phase DC machines with
either primary or secondary rectification and inverter units with secondary
rectification.The choice of equipment depends on a number of factors such
as the primary current available, the output current required, the amount
of space required between and around the electrodes, whether the equip-
ment is required to be portable and the equipment cost.

9.2.1 Single phase AC units
The power source is simple, robust, relatively inexpensive and maintenance
free, comprising little more than a transformer and a suitable timer. For
these reasons this form of power source has been popular for low-volume
applications. However, the equipment is not very energy efficient as the
secondary circuit suffers from substantial inductive losses. Demand on the
primary supply is also high and unbalanced between the phases: welding
current can be high but the voltage is normally low, between about 3 and
20 volts.