The investigation on the dynamic solidification behavior of the hybrid molten pool in layered bimetallic laser penetration welding

The dynamic solidification behavior of the hybrid molten pool in layered bimetallic DSS2205/X65 laser penetration welding has been investigated using high-speed photography and grayscale processing techniques. The results demonstrate that this welding process creates layered joints. Alongside the DSS2205 and X65 layers' interconnection, an intermediate transition layer emerges proximate to their interface, exhibiting limited solute exchange. Understanding the dynamic solidification in the hybrid molten pool is crucial. Two Marangoni eddies form in the DSS2205 and X65 layers at the posterior end of the keyhole during welding. Despite encountering less external gas convection, the central transition zone of the hybrid pool undergoes rapid cooling, prompting preferential solidification. The grains then grow towards the sides due to the Marangoni eddies, resulting in a dumbbell morphology by wider ends and a narrower middle region. Thermodynamic calculations and microstructure observations reveal fine equiaxed grains in the transition zone influenced by the greater cooling rate and element distribution, while larger grains in the base layer, and columnar grains in the cladding layer. This confirms preferential solidification in the middle region. The study uncovers the intrinsic mechanism behind this phenomenon, offering valuable insights for controlling the microstructure, optimizing parameters, and enhancing mechanical properties in layered bimetallic composite joints using laser penetration welding.