| dc.description.abstract | The Corporate average fuel economy (CAFE) standard set by the National Highway Traffic Safety Administration (NHTSA) has prompted automotive manufacturers to produce increasingly fuel efficient vehicles. Lightweighting of vehicle structures to reduce carbon dioxide emissions can be enabled by advanced materials such as hot stamped ultra-high strength steel (UHSS), but requires new joining solutions for integration in future multi-material structures. Structural adhesives enable multi-material joining, and have been used to enhance the joint performance for mono-material structures to achieve improved joint strength and stiffness. However, implementation of adhesive joining for hot stamped UHSS requires an appropriate surface treatment to maximize the joint strength and to address delamination of the brittle intermetallic coating formed on the steel during processing.
The present study investigated adhesive joining (3MTM Impact Resistant Structural Adhesive 7333, 3MTM Canada Company) of a hot stamped UHSS (Usibor® 1500-AS, ArcelorMittal Dofasco) using three surface preparation techniques: degrease using acetone (ACE), grit-blast (GB) treatment, and adhesion promotor (AP) treatment following grit-blasting. Three hot stamping thermal treatments were considered with three quenching die temperatures: room temperature (RT), 400°C, and 700°C, which varied the yield strength of the steel, and created some differences in the morphology of the intermetallic coating. The overall work examined the surface treatments for adhesive joining of hot stamped UHSS, intermetallic coating delamination mechanism and the adhesive failure morphology under different adhesive joint configurations.
Adhesively joined adherends were evaluated using the single-lap shear (SLS) test to investigate the nine material conditions (three surface treatments, three steel thermal processing treatments). The measured joint strength of the GB and AP conditions were 60% and 56%, respectively, higher than the baseline ACE treatment (p <.001). The higher strength achieved from the GB treatment was attributed to removal of the intermetallic coating.
The ACE treatment did not remove the intermetallic coating and resulted in the lowest joint strength with the largest variability of the conditions tested, attributed to intermetallic coating delamination. The intermetallic coating morphology included microcracks and Kirkendall voids, which facilitated coating delamination. The intermetallic coating delamination was associated with a measured SLS joint rotation of 2.5°-2.8° for all three thermal treatments, while the measured joint strength decreased as the thermal treatment temperature increased (22 MPa to 14 MPa). This decrease in joint strength was attributed to the lower yield strength of the adherend material enabling the critical joint rotation to be achieved at a lower applied load. Plastic deformation in the SLS adherends was observed in the GB and AP treatments for the 400°C and 700°C thermal treatments. No plastic deformation was identified for the RT thermal treatment.
The fracture surfaces from four types of adhesively bonded test specimens (Mode I opening, Mode II shear, Mixed-Mode at 45° (MM45), and SLS) comprising steel adherends without any surface coating were investigated using an optical digital microscope. Analysis of the fracture surfaces revealed qualitative differences in the morphology for different modes of loading. Shear hackles were observed for Mode II loading, while Mode I demonstrated facets on the fracture surface. The fracture surfaces were quantified using the arithmetic mean roughness (Ra). Mode I demonstrated the lowest roughness (50 µm) while Mode II had the highest Ra (103 µm), attributed in part to the shear hackles. The MM45 (80 µm) and SLS (73 µm) demonstrated intermediate roughness values, corresponding to mixed mode loading. Thus, it was found that qualitative and quantitative assessment of fracture surfaces could be associated with the mode of loading, and mode mixity.
Ultra-high strength boron steel provides an important design option for vehicle structural engineers, with high strength achieved through thermal processing but resulting in a brittle intermetallic coating that present challenges for adhesive joining. The present study investigated adhesive joint strength for boron steel, and the corresponding intermetallic coating failure pathways and their effect on joint strength measured using a single-lap shear test. The importance of surface treatment to remove the intermetallic coating was critical to achieve high joint strength with low variability. | en |
