Ti4Al6V and 6061 Al dissimilar metals were butt welded by the laser oscillating welding method. The effects of laser offset, oscillation frequency, and energy distribution on the formation, microstructure, and tensile properties of dissimilar metal joints are discussed in detail. The experimental results show that the Ti6Al4V was micro melted with a laser offset of 1.1 mm, and a large number of intermetallic compounds (IMCs) were formed on the side of the Ti6Al4V. Additionally, there were some porosity defects in the fusion zone (FZ) due to an inappropriate laser oscillation frequency. When the laser offset was increased to 1.2 mm, the IMC distribution was uniform and the thickness was controlled below 2 μm. The porosity defects in the FZ decreased and the tensile strength of the joints increased significantly. The maximum value of tensile strength reached 173 MPa at a laser frequency of 28 Hz.

The currently available high-power laser shows promising opportunities for the welding of thick plates in a single pass. However, weld-root defect frequently occurs when a high-power laser is used to join thick plates in a full-penetration mode, which has a significantly adverse effect on the serviceability of the weld joint. The purpose of this work is to understand the defect formation mechanism and reduce these defects through controlling welding parameters. In this study, the characteristics of weld root defects were investigated using a 10 kW fiber laser using a program of experiment and theoretical analysis. The corresponding defect formation mechanisms were discussed based on the bottom molten pool behaviors observed by the high-speed camera. The results showed that there were four types of weld-root appearances as follows with an increase of linear heat input from 300 J/mm to 1000 J/mm: weld-root humping (30 mm/s), sound weld (25 mm/s), weld sagging (20 mm/s) and excessive weld sagging. The remedies for reducing weld-root defects were also presented to obtain sound weld bead by optimizing welding parameters. Weld-root humping was formed due to the quasi-full-penetration keyhole. Weld sagging resulted from the imbalance of the hydrostatic pressure and surface tension in the condition of a through keyhole. It was also found that the sound weld was formed when a through keyhole and a proper molten pool size were obtained. Thus, the state of the keyhole and molten pool geometry were the major factors that affect weld-root defects.