Laser Butt Welding of 6.6-mm Thick D406A Ultrahigh Strength Steel: Part I—Porosity and Lower Surface Depression

D406A steel is commonly used to fabricate the shell of aerospace craft and therefore rigorous requirements have been set for welding quality of the steel. Laser welding has low heat input and small welding-induced residual deformation, while it is likely to incur defects such as undercut and insufficient reinforcement. A narrow-gap groove with the root face being 5 mm high was designed for the butt joints of D406A steel with the thickness of 6.6 mm. At first, the conventional bottom-up welding (scheme A) was adopted for laser welding. Serious pore defects are found in the weld, the reinforcement on the backside of the weld is small, and even surface depression is observed on backside of the weld. This is mainly because the height and volume of molten pool formed around the root face region during laser autogeneous welding is relatively small, so that metals in the molten pool are rapidly cooled, and the melt tend to flow upwards driven by the surface tension. Afterward, a new top-down welding process (scheme B) was proposed, that is, laser oscillating wire-filling welding in the upper groove at first and then high-power laser autogeneous welding penetrating the full thickness of steel sheets. Results show that the new welding scheme can effectively inhibit pores and depression of backside of the weld. The scheme improves the uniformity of weld metals, compositions, and microhardness along the thickness direction. The tensile strength of joints welded using the new scheme is 1,745 MPa after quenched-tempered heat treatments. The joints are fractured at the fusion zone (FZ), and the fracture is full of dimples. However, the joints show poor plasticity. The quenched and tempered Joint-B has an elongation of 3% and a bending angle of 33.3°. Relevant improvement measures need to be further studied.