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SCOUT SP Portable Welder Users Guide
How a Spot Weld is Formed
Spot welding relies on metal resistivity to heat and fuse metal. A large current is passed through the workpiece
metal. When energy passes through a material, the resistance it meets creates heat, which melts and then fuses
the weld materials together.
There are two phases to the melting process. The welder must overcome both the material contact resistance
(the resistance at those points where the two materials meet) and the bulk resistance (the cumulative resis-
tance of the two materials being welded together) of the material.
Figure 4.1 shows an example of a micro-scale surface prole. On the micro-scale, material surfaces are rough
and only contact each other in a limited number of locations. In the rst few milliseconds of weld formation,
the high resistance metal bridges melt, allowing other bridges to come into contact to continue the melting
process. When all the bridges have fused, the contact resistance is zero. The bulk resistance of the metal then
plays the nal role in the weld formation.
Factors Contributing to a Successful Resistance Weld
WELD PRESSURE
The amount of electrode pressure at the weld spot plays a part in the contact resistance. The more contact
resistance the energy encounters, the hotter the resultant weld. On the micro-scale, contact resistance is
reduced when more metal bridges or contact points are formed (see Figure 4.1). Using more electrode pressure
creates more metal bridges, resulting in lower contact resistance and therefore less resistive heat in the weld
spot.
Conversely, light electrode pressure results in less metal contact, higher resistance, and a hotter weld. In fact,
too little pressure can lead to arcing between the contact points and cause unwanted sparking or blow out.
An appropriate amount of pressure in conjunction with the appropriate amount of energy discharge should be
determined to ensure good weld strength and strong weld nugget formation.
ELECTRODE CONFIGURATIONS
The placement of electrodes in relation to each other can aect weld results as well. Figure 5.1 shows the typi-
cal electrode congurations used in resistance welding.
• Direct or Opposed Conguration: Current is passed from one electrode through both workpieces
and out through an opposing electrode. This is the typical desired conguration if access is available to
both top and bottom materials.
Figure 4.1. On the micro scale, surface roughness limits
surface-to-surface contact. More contact points result in
lower contact resistance.
