Investigation of the use of power ultrasound to improve manufacturing processes in fluid phase

Abstract

Advanced manufacturing techniques play a crucial role in ensuring the efficiency and reliabil-ity of electric vehicle (EV) battery packs. In particular, the assembly of busbars and connect-ors—key components for current distribution—relies on joining dissimilar metals like copper and Aluminum. Aluminium-Copper (Al-Cu) alloys are often selected as best choice due to the combination of lightweight and effective conductivity. However, conventional joining methods of these metals often lead to the formation of brittle intermetallic compounds, compromising electrical performance and mechanical integrity. Despite the progressive interest in ultrasonic vibration–assisted laser welding for joining dissimilar metals in EV busbars and connectors, there remains a restriction in foundational understanding of how ultrasonic frequency, ampli-tude, and energy coupling interact with laser–material dynamics to control melt pool physics, intermetallic compound formation, and joint integrity. There is a rare integration of systematic transducer design and tuning with welding process optimization by existing studies, leaving the relationship between transducer characteristics and weld quality largely unexplored. Con-sequently, there is a critical knowledge gap in establishing quantitative, process–structure–property relationships that link ultrasonic transducer parameters to mechanical, electrical, and metallurgical performance of dissimilar-metal laser welds in EV applications. Therefore, this study addresses the use of ultrasound vibration-assisted laser welding as a manufacturing tech-nique, to join dissimilar metals such as Al-Cu, particularly for electric vehicles (EV) battery assembly. The research considers both theoretical and experimental approaches to the mecha-nism involved in formation of cavitation microbubbles and the effects of these agitated mi-crobubbles on the process-structure-property relationship arising from the integration of high-power ultrasonic vibration technology with laser welding. Owing to optimal cavitation bubble formation at ultrasonic frequencies between 20 kHz and 40 kHz – ranges widely adopted in industry for both performance and safety – the study presents the design and development of different transducer types operating at varying frequencies of 20, 28, and 40 kHz. Considera-tion of different experimental setups and varying sets of processing parameters, like transducer angle, transducer distance from metal plate, electrical impedance, and are put in place, to obtain optimal displacement and acceleration amplitudes. The initial section of the work focuses on comprehensive review of metal joining processes; the advancements and challenges associated with EV batteries in recent times and introduces power ultrasound in industrial applications. Also, it delves into the theoretical modelling of formation and collapse of microbubbles caused by the introduction of ultrasonic waves into the solidifying phase of the weld pool. In addition, experimental investigation of structural vibra-tion of lap and butt joints is discussed. Furthermore, the latter section elucidates the mechanical and microstructural analyses of ultra-sonic vibration-assisted laser welded joints. Results showed that the application of high-inten-sity ultrasound significantly disrupted epitaxial dendrite growth, refined grain structure, mini-mized plasma cloud formation, and altered the shape of intermetallic compounds from linear to spherical. The mechanical strength of vibration-assisted joints welded at 28 kHz and at weld-ing speed of 40 mm/s showed a significant increase of 24.5% against the non-vibration-assisted counterpart. The application of high-intensity ultrasound significantly improved weld quality by straightening the weld profile, reducing differential weld width-to-root by 14 - 62%, refining grain structure, and reducing defects such as spatter and plasma cloud formation. Also, SEM results showed that the ultrasonic-assisted laser welded joint was characterized by smaller in-termetallic formations and appeared mostly in the shape of spheres. Furthermore, it was ob-served that the volume density of the secondary phases within the grain boundaries reduced with the application of ultrasonic vibration. The pull test results indicated an approximate 10% increase in load capacity and a 27% increase in extension when vibration was introduced, com-pared to welds produced without vibration. This demonstrates that vibration-assisted welding enhances joint strength and ductility, contributing to improved mechanical performance and reliability of the weld. Overall, the study provides an innovative methodology to address critical issues in industrial and manufacturing processing of dissimilar metals, mitigating the detrimental effects of the formation and presence of hard, brittle intermetallic compounds associated with Al-Cu alloys. This method is highly effective in contemporary material processing of busbars and connectors of EV batteries.