Frequency domain signal analysis based index for expulsion quantification in resistance spot welding

Resistance spot welding (RSW) is a widespread technique for joining sheet metals. In industrial applications, working closely to expulsion conditions is common practice to maximize the welding size and, thus, the mechanical performance. On the other hand, expulsions lead to the worsening of mechanical characteristics, and the joint becomes visually unappealing. Manufacturing disturbance is another source of expulsion. Abnormal conditions like initial gap and edge proximity are one of the reasons for defects in mass production. This work proposes a procedure to quantify expulsion in RSW through an innovative index obtained from frequency domain analysis of the force signals acquired during the welding process. Furthermore, an indicator extracted from the displacement signals has been compared to the suggested expulsion index. An important finding of this study is that the presented index, in addition to identifying expulsion, indicates how close the process is to the expulsion limit, providing the possibility of recognizing possible shifts and trends that could arise from the actual process conditions. This leads to an effective tool for online process monitoring. Data from an experimental campaign performed to reproduce expulsion in case of inappropriate choice of process parameters, initial gap, and edge proximity validate the effectiveness of the proposed technique.