Expulsion characterization in resistance spot welding by means of a hardness mapping technique

Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a short welding time. Expulsion has a significant impact on the final yield strength of the weld, thus the detection and characterization of expulsion events is significant for the quality assurance of resistance spot welds. In this study, hardness mapping, using a scanning hardness machine, was used as a quality assurance technique for resistance spot welding. Hardness tests were conducted on a resistance spot welded sample to prepare a hardness map. The test results showed good correlation between the hardness map and metallographic cross sections. The technique also provided further fundamental understanding of the resistance spot welding process, especially regarding the occurrence of expulsion in the nugget.