When a Spot welding is being made,the metal heats up to molten temperatures. At the same time , the metal molecules are polarized in the same way at any instant of time.This causes an extremly strong magnetic repulsion between these molten droplets to literaly lauch them away from the part. You see this as expulsion or sparks. In extreme cases metal expulsion can travel up to 10 mtrs.

SOME MORE REASONS & REMEDIES:

1) Low Electrode Force as it acts as mechanical barrier around molten metal to keep the material within nugget. You should always keep electrode force on maximum possible side or as recommended in RWMA handbook.

2) Proper Gun & Tip Alignment as proper tip face diameter will generate only the required heat if schedule is properly set. Always see that minimum tip diameter of 05-06 mm is there & both electrodes are properly aligned with help of carbon test. As this is the major cause of expulsion because tip dressing is not done according to scheduled time many a times.

3) Spot On Edge Placing a spot too close to edge of part can also cause metal expulsion in such cases even proper electrode force will not help in preventing expulsion. Even though minimum edge distance is recommended as per RWMA but some times part design forces a weld location close to an edge. In such cases use of pulsation in welding schedule can be applied to minimise expulsion.Always remember that divide the weld time by three impulse & inbetween two cool cycles. Use of pulsation not only helps in cases where spot location is on edge but can also be used other locations if expulsion is found more.

4) Welding Electrodes/Cap tips Many a times even if all the possible things as mentioned above are done, expulsion is not reduced in such cases see that you are using proper electrode material i.e. CuCrZr. This material should have hardness more than 70 & conductivity of 80 IACS. If using IT guns with shanks & cap tips see that you use cold forged caps for your guns as cold forging process improves the strength & grain structure of material which reduces the mushrooming frequency of your electrode.

(More addition will be done just check it out every month)

Regards,

Mangesh Borse

+919881689393