ABSTRACT
A common phenomenon in resistance spot welding (RSW) is expulsion-the ejection of molten metal during welding. Expulsion, which can be observed frequently during spot welding, happens at either the faying surface or the electrode-workpiece interfaces, as shown in Figures 7.1 and 7.2, respectively. The latter may severely affect surface quality and electrode life, but not the strength of the weld if it is limited to the surface. On the other hand, expulsion at the faying surface may compromise a weld’s quality, as it involves loss of liquid metal from the nugget during welding. The risk of expulsion is especially high in spot welding aluminum as well as magnesium alloys because of the very dynamic and unstable nature of the process, which is related to application of a high current in a short welding time, compared to welding steels. The causes of expulsion are both technical and human related. Expulsion is often used as a visual indicator of a correct welding process in steel welding. In order to achieve a weld size as large as possible to meet certain requirements, a prevalent practice is to use a large welding current, often close to or beyond expulsion limits. Expulsion limits are also often deliberately exceeded in production to reduce variations in weld quality caused by random factors. However, because of the loss of metal during expulsion, defects such as voids and porosity, which may reduce weld strength, are introduced to the nugget. In addition, expulsion has a negative in˜uence on adhesive bonding, if it is used in conjunction with spot welding (so-called weld bonding), by damaging the adhesive layer.
