Expulsion is the forceful ejection of molten metal from the weld. Severe expulsion may eject enough material to create a through-hole in the workpiece, commonly termed “burn through.”
Expulsion is not the sign of a quality spot weld. It is a signal that the weld schedule (current and time) and force need correction — If nothing else, expulsion leads to uneven quality from weld to weld.
Expulsion may occur at any interface, i.e. at the tip-to-workpiece interface. (Fig. 1), or at any faying surface (Fig. 2). Expulsion is caused by too much heat, the magnetizing current and lack of containment of the expanding molten material between the electrode tip faces. Expulsion is undesirable.
Spikes of solidified expulsion extending from the circumference of a weld are called whiskers or burrs and are also to be avoided. Burn Through (Fig. 3) occurs when so much material is expelled that a hole is left that passes completely through the weld.
- Ejection of visible molten material during the weld.
- Post weld inspection for holes, burrs or whiskers.
Quality, Workplace Issues, Cost, Downtime, Maintenance, Throughput (cycle time; PPH) are all potentially affected by this condition. Special considerations are noted below:
Quality: Expulsion at the weld interface may displace or damage adhesives and sealers. Whiskers may prevent the installation of seals, or may damage them during installation. Corrosion is more likely to occur when burrs/whiskers are present.
Maintenance: Accumulation of weld expulsion on tooling and equipment, may cause machine malfunction.