The quality of spot welding determines the strength and safety of the vehicle body. Ultrasonic non-destructive testing methods are commonly used for spot inspection after welding. But many welding points of the structure are difficult to reach and cannot be detected when the welding is completed. In order to address the problem, ultrasonic testing technology is applied to the monitoring of welding process. Based on pulse-echo ultrasonic probe and characteristic signal analysis technology, the transient distribution of ultrasonic field inside the workpiece during spot welding process is studied. A new type of built-in ultrasonic probe electrode structure is designed. The ultrasonic probe is embedded in the water-cooled cavity of the electrode boom, and the ultrasonic on-line monitoring test of pulse resistance spot welding is carried out. The characteristics of ultrasonic time history and ultrasonic echo diagram during welding process are analyzed. The melting and solidification process of the nugget is recorded in real time by using acoustic signals, and the characteristics of the ultrasonic echo diagram of the virtual welding of the typical spot welding defects are studied, and the identification method of the virtual welding spot based on the echo diagram is analyzed. The results show that the heat affected zone, fusion zone, nugget zone and welding defects of spot welded joints can be well divided by observing the characteristics of C-scan images and A-scan signals. The virtual welding spot can be quickly identified by the characteristics of the display diagram, and then the spot welding defect detection and nugget size measurement can be realized.
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