In-process ultrasonic evaluation of spot welds

Abstract

A new ultrasonic method for on-line prediction of spot-weld strength was developed. The method is based on real-time weld monitoring by measuring ultrasonic wave signals transmitted through the weld region. The transmitted signal is affected by temperature changes in the welded region during solidification and subsequent cooling of the weld region. It was shown that the minimum of the transmitted signal corresponds to the austenite transformation, resulting from the strong maximum in the attentuation of the ultrasonic signal. The time from weld-current termination to this minimum depends on the heat capacity of the weld pool and therefore can be related to the nugget volume and the weld strength. Cold welds, which from good acoustical contact between the welded plates but are weak as welds, can be detected by this method. The method shows equally good results for uncoated and galvanized steels and provides the potential for in-process weld-quality evaluation in both cases.