In situ micro-cantilever bending tests were carried out on resistance spot welded DP1000-GI dual-phase high-strength steel in order to derive the mechanical response of the welds. Notched micro-cantilevers were milled using focused ion beam milling at the base metal, inter-critical, fine-grained and coarse-grained heat affected zones, and fusion zone. It was shown that due to large plastic yielding, linear-elastic fracture mechanics are inapplicable. To evaluate the fracture toughness of different weld zones, cyclic loading was applied to track the crack size and the conditional fracture toughness of weld zones was measured using crack tip opening displacement and J -integral methods. It was found that micro-cantilever bending method provides insight to the fracture toughness and local mechanical response of different weld zones. The results obtained can be used to make an accurate correlation between resistance spot welding process, microstructure and mechanical response of DP1000-GI dual-phase high-strength steel welds.

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In this research, physically vapor deposited Mg-Zn and ZnMg-Zn bi-layered coatings were annealed at 180 °C for different annealing times to study the origin of the adhesion loss during heat treatment. In the case of Mg-Zn bi-layered coatings, it was observed that MgZn2 and Mg2Zn11 intermetallics are formed during annealing from Zn and Mg by diffusion, which results in a reduction of the thickness of the initial pure zinc interlayer. In the case of ZnMg-Zn bi-layered coating, the "interfacial adhesion strength" at the ZnMg/Zn interface was quantified by using scratch test. The novel finding is that the adhesion strength of asdeposited coatings at the interface of ZnMg/Zn is independent of the thickness of the zinc interlayer (tZn). tZn decreases gradually during annealing at 180 °C. The "adhesion performance" of the studied coatings, as tested by BMW crash adhesion test (BMW AA-M223), drops drastically when tZn is less than a threshold (~ 500 nm). The obtained results indicate that tZn plays the significant role in the adhesion performance of ZnMg-Zn bi-layered coatings.

For many years, zinc coatings have been regarded as one of the most effective anti-corrosion protective coatings for steel. Recently, it was shown that the addition of even small amounts of magnesium (Mg) to a zinc (Zn) coating can noticeably increase its corrosion protection performance; however, it has also been observed that there is poor adhesion of zinc-magnesium (ZnMg) coatings to advanced high-strength steels. The addition of a more ductile Zn interlayer between the steel substrate and the ZnMg coating is a solution to improve the adhesion problem. In the present study, a series of ZnMg-Zn bi-layered coatings with different Mg concentrations (up to 14.1 wt.% Mg), and also different thicknesses of the Zn and ZnMg layers, were prepared by a thermal evaporation process (physical vapor deposition (PVD)) in order to investigate the adhesion performance and interfacial adhesion strength. The adhesion performance of these coatings was qualified by the BMW crash adhesion test (BMW AA-M223), while the interfacial adhesion strength at the ZnMg/Zn interface was quantified by the scratch test. It was found that the interfacial adhesion strength decreases gradually with an increase in the Mg content of the top layer. The novel finding is that the interfacial adhesion strength at the ZnMg/Zn interface is independent of the thickness of the Zn interlayer; however, the adhesion performance of a ZnMg-Zn bi-layered coating during a bending test is a complex function of different parameters, such as the thickness of the Zn and ZnMg layers, the interfacial adhesion strength and the interfacial defects density.