Spot welding can decrease in weld size as you weld across the part. One cause for this is increasing magnetic material in the throat of the welder as you move across the part. Most parts are low carbon steel which is a magnetic material. In AC welding applications, if after each weld more material is moved into the throat of the machine, the weld current will decrease.
METAL IN THE THROAT CAUSES IMPEDANCE TO INCREASE
A perfect example of this is welding a brace to the side of a metal file cabinet. On each weld you rocker arm welder or weld gun is swallowing more magnetic material into the throat of the machine as you reach toward the back of the file cabinet. This magnetic material acts as a brake to the current flow. Impedance is the technical term which is increasing as magnetic material enters the throat. As the impedence goes up current will decrease. You may or may not have acceptable nuggets if this goes too far.
To correct for this modern controls can be used in constant current mode where they adjust to assure that the same current is always delivered to the weld on each weld. Or you can manually program different welding conditions for the different welds as you progress across the cabinet surface.
The point of this exercise is to recognize that magnetic material in the weld throat can have adverse effect upon the weld current and should be avoided whenever possible.
This is true for AC alternate current welding. It is not true for DC direct current or MFDC mid frequency direct current welds. Impedance becomes zero in these cases.
Other factors could also affect the weld size as you weld are heat related, see another article in this blog:
HOW CAN HEAT CAUSE SMALLER NUGGETS AS I WELD?
For more information on Impedance refer to article in this blog:
“WHAT IS IMPEDANCE IN RESISTANCE WELDING”
Reference: RWMA – Resistance Welding Manual 4th Edition