This question describes a classic case of introducing magnetic material into the throat of an AC welder. As magnetic material enters the throat it causes the impedance to increase which absorbs energy and the weld current will decrease. This decreasing current will result in smaller weld nuggets. To counteract this one has the option of using a constant current control or use different schedules for the different welds. Impedance is not an issue for nonmagnetic materials or when using DC or MFDC welders.
MAGNETIC METAL IN THE AC THROAT AREA INCREASES
IMPEDANCE WHICH WILL DECREASE CURRENT
For a more complete discussion of this topic refer to the articles:
“WHY DO MY WELDS GET SMALLER AS I WELD ACROSS THE PART”
“WHAT IS IMPEDANCE IN RESISTANCE WELDING”
Reference: RWMA – Resistance Welding Manual 4th Edition