Page 75 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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and cooling of the weld. In the case of materials prone to form brittle structures in
the weld an additional current cycle of magnitude I a, see Figure 2.22c, can be
useful to anneal the weld. In spot welding of thick materials (over 3 mm) the use of
several pulses of current, Figure 2.22d, is effective.

2.4.3.2 Welding Force

The increase of the welding force reduces contact resistance because, in first
analysis, it promotes the increase of contact area, due to deformation of surface
asperities and eventually the rupture of surface oxide films [36]. Electrode
clamping forces must be high, particularly when welding low resistivity metals in
order to reduce the proportion of heat generated in the interface electrode/work-
piece. Electrode force must be increased with increasing current, unless part of the
melted material of the nugget can be expelled. Other factors such as bad fit and
lack of mechanical support contributes for the material expulsion. Distance of the
weld to the edge of the sheets should be larger than 1.5 D, where D is the weld
diameter. Excessively high forces are also undesirable because they can cause large
surface indentation of the work-pieces and damage of the electrodes.

Electrode clamping force increases with increasing thickness and strength of the
work-pieces. Forces between 1000 and 15,000 N are usual for plate thicknesses up
to 3 mm, though values of 20,000 N can be used in steel sheets 6 mm thick.

Clamping force starts before the passage of current initiates and is maintained after
the current is cut off, as is illustrated in Figure 2.22. Sometimes an increase of
force is applied after current passage to forge the weld, see Figure 2.22b.

F
forge
F F
w w
I w
t t t t
w r w f
time time
a b
F w F w
I w I w
I
a

time time
c d

Figure 2.22. Timing diagrams of current and force for spot welding: Welding current – I w ;
welding time – t w ; rise time – t r ; fall time – t f ; welding force – F w ; forge force – F forge ;
annealing current