Page 74 - Welding Robots Technology, System Issues, and Applications

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Welding Technology
machine are current, time and welding force. The choice of welding conditions
depends on thickness and physical properties of metals being welded and even on
the type of the welding equipment.

2.4.3.1 Welding Current and Time

Heat developed during welding is proportional to time and to square of current.
Though both parameters are responsible for heat generation, the weld heating rate
is determined only by current, because heat lost to the work-piece and to copper
electrodes increases with weld time. Heat lost to the work-piece increases heat
affected zone and thermal distortion, while heat in the electrodes can degradate
them, all being undesirable effects. The level of current required for any metal
tends to be inversely proportional to its electrical and thermal resistivities.

Weld current (A)

Weld time (cycles)

Figure 2.21. Schematic representation of current-time relationship for RSW

The size of the weld nugget increases rapidly with increasing current. When
welding a particular material and thickness, if current is increased welding time
should be decreased, see Figure 2.21, in order to prevent high surface indentation
or even expulsion of melted material and deterioration of electrodes [35]. The
expulsion of material defines the upper limit of usable current.

Welding currents range from 20 to 100 kA, mainly for light alloys, though the most
usual are between 4 and 20 kA for carbon steels. Time is defined in cycles of 50
Hz supply and it is between 5 and 100 Hz for steels and 5 and 20 Hz for light
alloys, in sheets up to 3 mm thick.

Weld current cycles may have different shapes that depend on the materials being
welded, as shown in Figure 2.22. A cycle of constant current magnitude, Figure
2.22a, represents the simplest situation and is suitable for welding mild steels. For
high strength steels sensitive to cold cracking a modulated welding current with a
rise time t r and a fall time t f, see Figure 2.22b, can be used to allow gradual heating