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Metallography and Computed Tomography Analysis of the Shrinkage Cavity Formed in Advanced High-Strength Steel Resistance Spot Welds

Among various defects formed in resistance spot welds (RSW), voids or cavities formed at the center of the nugget are scarcely studied. However, it is widely accepted that such cavities significantly influence the mechanical properties of the spots, especially in advanced high-strength steels (AHSS). Nonavailability of a proper characterization technique to quantify these cavities is one of the reasons for less research in this area. Although conventional metallographic analysis portrays cavities in the nugget, the associated destructive sampling results in erroneous interpretation. To date, there is no reliable technique to quantify the size, shape, and distribution of the voids in the nugget. In this context, the present study uses a computed tomography (CT) technique to visualize and quantify the cavities in spot-welded AHSS nuggets. The images obtained using a CT scan provide a new insight into the cavity and its characteristics, such as volume and surface topology. According to the CT results, the voids formed in AHSS spots increase with the increase in base metal yield strength. Furthermore, the CT results also reveal that the conventional critical diameter of 4 × thickness1/2 to obtain plug failure in RSW is not valid for AHSS spots because of the influence of the cavity in the faying interface.