The research objective of this award is to test the hypothesis that joint bond area in friction stir spot welding of ultra-high strength steel is determined largely by interface pressure, interface temperature, and weld time. Welding speeds in the range of 5,000 to 10,000 revolutions per minute will be considered, and the welding process will be constrained to a maximum vertical welding load of eight kilonewtons, with a goal of achieving a desired level of joint strength. A novel approach to modeling friction stir spot welding will be used within a Lagrangian finite element framework, where the joint interface will be represented virtually and where a proposed relationship between joint contact pressures, temperatures, and weld time will be used to estimate bond area. Archard's law will be used within the model to estimate tool wear. An experimental program will measure joint strength and tool wear over time, while microstructural evaluation of the joints will provide insight into the effect of tool wear on bonding mechanisms.
If successful, this project will enable the use of friction stir spot welding for joining ultra-high strength steel sheet in many industrial settings. The modeling approach will provide increased understanding of the role of tool design and process conditions on joint strength and tool life. Finding a solution to spot joining of ultra-high strength steels is important because these alloys have the potential to make car frames lighter and stiffer, improving vehicle fuel economy and reducing carbon emissions. The widespread use of ultra-high strength steel is currently limited by brittle weld microstructures produced by traditional resistance spot welding.