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With new developments in manufacturing, a greater variety of metals are now being used in automotive structural engineering. High-strength steels, Usi-bore, aluminium and even magnesium are replacing conventional mild steels. Still, resistance spot welding remains a major technique to join these metals together. Destructive quality inspection of such welds is no longer practical when considering the huge production volumes, testing costs and reliability of the results. Thus, a new non-destructive spot weld characterisation technique is employed that uses an ultrasound transducer installed in the welding electrode, allowing for real-time quality evaluation of resistance spot welds. During welding, a series of A-scans is acquired through the centre of the heat affected zone and the liquid metal area. Additional reflections from the liquid metal can be observed when the base metal melts. An M-scan representation of successive A-scans is formed, which carries information regarding the total developed heat as well as melting and solidification rates; welding properties that affect joint quality. Using partial gating, pulse identification and tracking techniques, the M-scan is segmented into components used by higher-level algorithms to determine the quality of the resulting spot weld. Quality characterisation is performed quickly after welding, allowing for simple integration into existing manufacturing environments for real-time nondestructive evaluation of resistance spot welds.

This paper presents an effective means of enhancing weak acoustic reflections in real-time ultrasound signatures of a spot weld, by analyzing the propagation of acoustic waves through the electrodes and heated weld region. A new method of processing the ultrasound data is presented that removes undesired acoustical reflections in the electrode cap and enhances weak reflections at the solid-liquid interface. To facilitate this, frequency and amplitude attenuation resulting from the weld medium and acoustical interfaces of the layered weld structure are examined. Sources of echoes that destructively interfere with the desired reflections are also identified and removed by optimal filter design. Finally, basic image processing techniques are applied to a B-scan representation of the weld to improve visualization of the liquid nugget during cooling. This additionally permits the accurate measurement of the final weld thickness; an important quality control parameter.

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