Page 71 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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the interface and developing a molten nugget. In the final part of the welding cycle
plastic deformation occurs in the work-pieces, producing a visible and permanent
indentation of the pieces. If current or pressure is too high, melted material can be
expulsed (splashed) to the atmosphere.

The process has extensive application in welding of carbon steels because they
have higher electrical resistivity and lower thermal conductivity than the electrodes
made of copper. Aluminum alloys have an electrical resistivity and thermal
conductivity that are closer to those of the copper, making difficult the welding
operation of these materials, requiring higher levels of current, which can damage
the electrode tips [32]. Other materials such as galvanized steels, heat-resisting
alloys and reactive metals are also welded by this process. Since the process is very
competitive it is widely used in automotive and aerospace industries as well in the
manufacture of industrial and domestic equipment.

The major advantages of this process are the high welding speed and low thermal
distortion, respectively faster and lower than in conventional arc welding
processes, suitability for automation, the need of low skilled operators and the
absence of joint preparation or filler metal. Some limitations of this process are the
need for lap joints in thin materials, usually up to a thickness of 4 mm, the joints
are not tight and have low tensile and fatigue strengths. Add to this fact that the
initial equipment costs are higher than those of conventional arc welding
equipment.

2.4.2 Welding Equipment

The main welding equipment to consider in resistance spot welding are the welding
power sources and the electrodes. Those pieces of equipment will be considered
next in detail.

2.4.2.1 Power Sources

Spot welding machines are composed basically an electrical circuit, which
provides welding current, a control circuit that regulates welding current and
welding time, and a mechanical system, used to apply welding force.

The electrical circuit consists of a step-down transformer, whose secondary circuit
includes the electrodes and the work-pieces, see Figure 2.19. The transformer
changes the input AC high-voltage and low amperage current, in the primary
winding, to an AC high-amperage and low voltage current in the secondary
winding. These transformers have low internal impedance, because current
magnitude in secondary winding is inversely proportional to the impedance and
depends directly on the open voltage of the secondary circuit.

Single-phase AC machines providing current up to 50 kA are widely used. Spot
welders can also provide DC of continuous polarity, pulses of current of alternating