Effects of weld penetration modes on laser welding characteristics of a novel ultra-high strength steel for aerospace application

30Cr3 steel is a novel ultra-high strength steel with good matching of strength and toughness, which has been mainly used for aerospace applications. At present, there is still a lack of systematic research on the laser weldability of such a material. In this study, 30Cr3 ultra-high strength steel plates with the thickness of 2.5 mm were welded by fiber laser under different laser powers. By means of high-speed photography, the penetrability and dynamic behavior of keyhole during the welding process were directly observed. Further, the vibration signal on the upper surface of the molten pool was acquired by the laser vibrometer in a non-contact manner. According to the observation results, we proposed that there were three weld penetration modes within the selected laser power range, including keyhole unpenetrated fusion mode, keyhole critical penetrated fusion mode and keyhole stably penetrated fusion mode. Based on the classification of weld penetration modes, the correlation between the dynamic behavior of keyhole and the stability of welding process was thoroughly discussed. It was found that the instability of the keyhole critical penetrated fusion mode was mainly manifested in the large fluctuation of keyhole profile, while the phenomenon of keyhole necking and keyhole collapse frequently occurred, which would disturb the laser energy absorption and thereby affect the microstructure uniformity of the welded joints, resulting in the poorest impact toughness with the minimum impact absorbed energy of 9.23 J. By comparing the weld microstructure and the resultant mechanical properties under different weld penetration modes, we revealed that the joints of keyhole stably penetrated fusion mode not only had more uniform microstructure, but also possessed the maximum impact absorbed energy of 14.36 J and highest elongation of 22.8 %.