The welding of aluminium and its alloys
Power sources
The power sources are normally rated in kV A at 50% duty cycle so, if the maximum primary input power available is known, it is possible to calculate 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 equipment 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.
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9.2 Modern pedestal mounted spot welder for aerospace applications. Courtesy of British Federal. |
The spot welder may be pedestal mounted (Fig. 9.2) or portable. With portable equipment the transformer may be remote from or incorporated into the welding gun. With portable equipment the weight of the transformer makes the gun with the built-in transformer difficult to manipulate even with counterweights to ease manual handling. The gun weight may also exceed the carrying capacity of welding robots. Using a power source remote from the gun requires heavy, stiff cables to deliver the welding current, again making the gun difficult to manipulate.
9.2.2 Single phase DC units
Single phase DC units are rather more energy efficient than the single phase AC units as rectification of the current in the secondary circuit reduces losses due to inductance. Power demand across the phases is also more balanced than the single phase units. Although the rectifier adds to the weight the increased efficiency enables a lighter transformer to be used, giving an overall reduction in weight. The equipment cost is greater than the simple 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 electronics, 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 transformer 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 providing 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).
