Mechanism analysis of the welding solidification cracks in narrow gap laser welding of high-strength steel

Welding solidification crack is dangerous defect in Narrow Gap Laser Welding (NGLW), and its mechanism is not very clear. This paper proposed an in-situ observation method for molten pool solidification in weld cross-section and a welding groove deformation test method. The correlation between the weld geometry and the solidification cracks, the deformation law of the groove, the solidification behavior of the molten pool and the phase structure of crack precipitates were studied, and the cracking mechanism of the solidification cracks in NGLW of high-strength steel was clarified. The results shows that the presence of low melting point eutectics of S and P in the tip region of solidification cracks, the weld penetration and its composition are important weld-forming geometrical factors affecting the solidification cracks in NGLW. However, the solidification behavior and solidification shrinkage deformation of the weld metal are the essential reasons of solidification cracks during the welding process. There are two solidification modes in NGLW process: sequential solidification and enclosed solidification. The latter is easy to enclose a small amount of residual liquid in the central area of the upper part of the weld, and the solidification cracks are caused by the inability of the liquid metal to feed and the shrinkage of the solidification interface. The effect of weld geometry on solidification cracks is essentially realized by changing the solidification mode of the molten pool, thus revealing the formation mechanism of solidification cracks in NGLW.