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A novel two-step spot friction stir welding (FSW) technique was developed with the purpose to improve the mechanical property of the joints by removing the keyhole. In the first step, a conventional spot FSW is conducted above a round dent previously made on the back plate. Therefore, a protuberance was formed on the back side of the joints due to the flow of the materials into the dent during the welding process. In the second step, the welding process was conducted on a smooth back plate using a rotating tool without the probe in order to remove the protuberance on the back side and the keyhole on the front side. This novel spot FSW method was applied to 6061 and 5052 Al alloys, and it reveals that the final joints have a smooth surface with no defects inside and shows improved mechanical properties.

We investigated the relationship between the material properties and axial collapse behavior of hat-section hollow columns made of highstrength steels. Dynamic collapse tests of the columns and FEM simulations were conducted using high-strength steels with various tensile strengths and n-values. Materials that exhibited yield point elongation (YPEl) underwent enhanced accordion-type deformation in axial collapse owing to the occurrence of long wave buckling on the hat walls in the early stage of collapse. In addition, it was important to avoid fracture in the initial buckling region to induce progressive crumpling behavior. A high n-value of the material was favorable for avoiding fracture by increasing the bend radii and decreasing the strain concentration at the buckling regions on the hat walls, thereby resulting in stable progression of accordion-type folds. In conclusion, steels exhibiting YPEl and high n-values are favorable for inducing stable accordion-type deformation during axial collapse.

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