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.

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).

The welding of aluminium and its alloys

Alloy designations: wrought products

Table A.4 BS EN BS EN Old BS/DTD Temperature (°C) numerical chemical number designation designation Liquidus Solidus IVIdUng range Al 99.99 1 660 660 0 AW-1080A Al 99.8 1A AW-1070A …

Principal alloy designations: cast products

Table A.3 BS EN numerical designation BS EN chemical designation Old BS number ANSI designation Temperature (°C) Liquidus Solidus Melting range Al 99.5 LM0 640 658 18 AC-46100 Al Si10Cu2Fe …

Physical, mechanical and chemical properties at 20°C

Table A.2 Property Aluminium Iron Nickel Copper Titanium Crystal structure FCC BCC FCC FCC HCP Density (gm/cm3) 2.7 7.85 8.9 8.93 4.5 Melting point (°C) 660 1536 1455 1083 1670 …

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