WE Colloquium: Xun Liu, Advanced Friction Stir Joining of Dissimilar Materials
Growing concerns about energy consumption increase the demand for lightweighting. In this regard, multi-material structures are always desirable for reducing weight while enhancing products performance with an optimized material distribution. Accordingly, reliable, efficient and economical dissimilar materials joining techniques need to be developed. As a solid-state process, friction stir welding (FSW) and its variant friction stir spot welding (FSSW) are promising solutions for butt and spot joint configurations. However, the associated high welding force and the keyhole defect limit their further applications.
The first part of this talk focuses on FSW of aluminum alloy to advanced high strength steel. Fundamental mechanisms of this dissimilar material joining process are studied from experimental and model analysis. Effects of process parameters are analyzed with weld microstructure, particularly at the Al-Fe interface, based on the welding force and temperature measurement during the process. The weld microstructure is then related to the weld quality. Both analytical and numerical models have been developed to further reveal the material flow and process principles. In the second part, improvements of FSW are discussed. In order to reduce the welding force, one of the material softening phenomena, the electro-plastic effect, is employed and incorporated into the FSW process. The idea is further extended to the FSSW process. With an appropriate electrically assisted experimental system, the welding force shows to be consistently reduced and weld quality is improved. In order to remove the keyhole defect, an innovative key-hole refilled FSSW process is developed, which can be implemented with any universal CNC machine of simple tooling. Joint performance shows to be significantly improved in aspects of both strength and ductility. The talk will be concluded with future prospects of dissimilar material joining processes, multi-material interaction analysis and discussion of an emerging new area of 4D printing.
Dr. Xun Liu is a postdoctoral research fellow in the Department of Mechanical Engineering (Prof. Jun Ni group) and Material Science Engineering (Prof. Alan Taub group) at the University of Michigan, Ann Arbor. Her current research areas include solid state joining of dissimilar materials, incremental sheet forming and fabrication and characterization of aluminum nanocomposites. She received her bachelor’s degree in Mechanical Engineering at Huazhong University of Science and Technology, Wuhan, China in 2011, and then followed up with a Ph.D. degree in Mechanical Engineering and a second Master degree in Material Science and Engineering at the University of Michigan in 2016.