Process monitoring of ultrasonic metal welding of AlMg3/Cu-DHP joints by structure-borne and air-borne vibration measurements

Ultrasonic welding is a versatile process for producing high-performance multi-material joints, but non-destructive prediction of joint quality remains challenging. Structure-borne sound measurements using laser vibrometers are commonly applied, yet they are costly and difficult to integrate into industrial environments. In this study, vibrations of the anvil during ultrasonic spot welding of Al/Cu composites were measured using two 2 kHz quartz sensors. The signals were analysed in the frequency domain via fast Fourier transformation of 0.1 s intervals over the entire weld duration. A temporal shift of the excitation frequency peak (20 kHz) and a concurrent change in damping were observed. Initially, the frequency increased while damping decreased degressively until both reached an almost constant limit. Although this limit was independent of joint quality, stronger bonds correlated with a shorter time required to reach it, indicating suitability for process monitoring. The same analysis was applied to air-borne vibrations measured with an ultrasonic microphone. Despite ambient noise, the observed frequency shifts closely matched the quartz sensor results, demonstrating feasibility for air-borne measurements. Additionally, ultrasonic sound camera measurements revealed a systematic movement of the sound centre during welding of cleaned specimens, while lubricated specimens showed irregular, diffuse behaviour. This study is the first to link ultrasonic welding process states with vibration localisation. Overall, structure-borne and air-borne vibration measurements proved effective for process monitoring and bond quality prediction in ultrasonic welding.