Multi-materials vehicle structures, employing light-weight metals such as advanced high strength steels (AHSS), aluminum alloys, can satisfy the ever-increasing requirement of light-weighting and fuel efficiency, as well as maintaining or improving the crash resistance of vehicles. The present research provides a fundamental understanding of the process-microstructure-mechanical properties of resistance and ultrasonic spot welding of light-weight metals.
The dissertation consists of three main parts: (1) study of the relationship of process-microstructure-mechanical properties for resistance spot welded two sheets (2T) and complex stack-ups of ultra-high strength grade of AHSS, (2) development of a novel technique, namely Ultrasonic Plus Resistance Spot Welding, for dissimilar metal joining of Al to steel, and (3) investigation of the bonding mechanism of USW of Al by in-situ relative vibration measurement.
In the first part, softening in subcritical heat affected zone of resistance spot welded hot-stamped boron steels is investigated by weld microstructure characterization and tempering kinetics of martensite. The local constitutive behavior of the potential failure locations is extracted and incorporated into performance model to investigate its effect on the accuracy of deformation and failure prediction.
A major challenge for RSW of complex stack-ups with large thickness ratio is the limited nugget penetration into the thin sheet at the outside of the stack-up. The effect of welding current, electrode force, electrode material/size on nugget formation and the possible ways to improve nugget penetration into the thin sheet are investigated for 3T and 4T stack-ups of AHSS.
The second part of the dissertation is focused on the development of a new dissimilar metal joining method, namely ultrasonic plus resistance spot welding (abbreviated as U+RSW) for Al/Steel. The bonding mechanisms have been investigated through numerical simulation to validate the concept.
The third and last part of the dissertation is to understand the bonding mechanism in USW. Particularly, the relative motion of the sonotrode, aluminum specimens, and anvil in USW is investigated using an in-situ velocity measurement technique, Photonic Doppler Velocimetry (PDV). The relative motion analysis is correlated to destructive testing results, including lap-shear tensile testing and weld microstructure characterization, to understand and quantify bond formation during USW.
In summary, the present research studies the process-microstructure-property relationships of resistance spot welding of AHSS, dissimilar metal welding of Al to steel, and ultrasonic spot welding of Al. An improved fundamental understanding is developed for (1) heat conduction, electric current flow, mechanical stress and deformation, and nugget formation in 2T and complex stack-ups of resistance spot welded ultra-high strength steel, (2) tempering kinetics of martensite and SCHAZ softening, (3) microstructure-specific constitutive behaviors, (4) weldability, intermetallics, and strength of dissimilar metal joint between Al and steel in U+RSW, and (5) bonding mechanism in USW. Such new knowledge is essential to ensure sound multi-materials vehicle structures employing various light-weight metals to satisfy the ever-increasing demand for fuel efficiency and crash resistance.