Lightweight structures are highly desirable for improving fuel economy and reducing carbon emissions. As key lightweight enablers, successful utilizations of advanced lightweight materials and multi-material structures rely on economical and reliable joining processes. Resistance spot welding (RSW), as one of the most extensively used technologies in the automotive industry, is not suitable for joining dissimilar materials due to their different physical properties and metallurgical differences. In addition, RSW faces challenges in joining the newest generation of advanced high strength steel and aluminum alloys. In comparison, ultrasonic spot welding (USW) shows several advantages, but its applications are limited by the output power of generally available ultrasonic transducers. This award focuses on an innovative hybrid ultrasonic resistance welding (URW) process, which effectively integrates ultrasonic and resistance welding, maximizing the advantages of each process. Preliminary results show promising enhancements of joint mechanical performance with URW. The research will deeply advance the scientific understanding of the complex coupling mechanisms between electrical and acoustic fields. Since this technology has wide applicability to the automotive, aerospace and defense industries, the project directly impacts the economic welfare and national security of the United States. The successfully-developed URW process will significantly benefit manufacturing and assembly of lightweight, and especially multi-material structures. The inherent similarities of ultrasonic and RSW system make it naturally convenient to adapt existing RSW robots and equipment, facilitating widespread industrial applications of URW. The obtained knowledge on ultrasonically induced physical phenomena will deeply contribute to developing and improving various advanced manufacturing processes. One specific example is ultrasonic resistance additive manufacturing. The URW process and microstructural evolution model enhances applications of integrated computational materials engineering (ICME) in the field of solid-state manufacturing. Finally, the multidisciplinary educational program as a result of the award will empower next-generation engineers and researchers in the fields of mechanical, material, metallurgy, control, and data processing.
The objectives of this research are to advance the knowledge of multiphysical thermo-mechanical-electrical-acoustic coupling mechanisms involved with the novel hybrid URW process, to reveal the fundamental physics of weld structure evolution and to optimize URW for joining advanced lightweight and dissimilar materials. Ultrasonic vibration is hypothesized to affect RSW in three stages: modifying contact resistance through removal of surface coatings and contaminations, influencing thermodynamic and kinetic conditions during melting through acoustic streaming and cavitation effects, and refining microstructure during solidification. The scope of this research includes: (1) A comprehensive experimental study of URW at various conditions, followed by mechanical testing and multi-scale characterizations of the welds. Process-structure-properties relationships will be established; (2) In situ analysis of the URW process through high speed and thermal imaging, as well as multiphysical modeling to determine the thermo-mechanical field during the process. Al-Fe interfacial reaction model will also be developed, which will be verified through physical simulations of the inter-metallic compounds (IMC) formation and growth under a controlled thermo-mechanical testing environment. (3) Optimization of the URW process in terms of process parameters, electrode geometry along with the synchronization between acoustic and electrical fields for desirable joint performance with the minimum amount of input process energy.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.