6
Sunstone Welders
User Manual
:
CDDP-A
WELD FORMATION
Spot welding relies on metal resistivity (resistance) to heat and fuse metal. A large current is passed
through the workpiece metal. Energy is dissipated due to metal resistance in the form of heat which
melts and fuses the weld materials. The welder must overcome both the contact resistance and the
bulk resistance of the material in order to begin the melting process.
FIGURE 2 below shows an example of a micro-scale surface profile. On the micro-scale, material
surfaces are rough and only contact in a limited number of locations. In the first 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 of the bridges have fused, the contact resistance is zero.
The bulk resistance of the metal then plays the final role in the weld formation.
FIGURE 2: On the micro-scale, surface
roughness limits surface-to-surface contact.
More contact points result in a lower contact
resistance.
Tips
WELD PRESSURE
Several other factors play a part in the contact resistance. The larger the contact resistance the
hotter the resultant weld. On the micro-scale, contact resistance is reduced when more metal bridges
or contact points are formed (see FIGURE 2). Using more electrode pressure creates more metal
bridges. This results in a lower contact resistance and a cooler weld. Conversely, light electrode
pressure results in less metal contact, higher resistance, and a hotter weld. An appropriate amount of
pressure should be used to ensure good weld strength.
ELECTRODE CONFIGURATIONS
FIGURE 3 shows several electrode configurations used in resistance welding. FIGURE 3a is called
an opposed configuration. Current is passed from one electrode through both workpieces and out
an opposing electrode. FIGURE 3b shows a step electrode configuration. This configuration is
used when there is access to only one side of the workpiece and an electrode can be placed on
both materials. FIGURE 3c is a series or parallel configuration. Electrodes can only be placed on
one metal surface from one side. Current is divided between the two parts. This weld configuration
requires more weld energy.