When the temperature in the molten pool rises rapidly, the convection inside the molten pool is strong, which adds to the diffusion movement of carbon atoms, so that the graphite of gray cast iron can decompose rapidly. After laser melting, the flake graphite on the surface of gray cast iron is completely decomposed, as shown in figure (a). Because the specific heat capacity of gray cast iron graphite is three times that of the matrix, under the rapid laser heating, the heat absorbed by gray cast iron graphite is much higher than that of the matrix, resulting in the rapid decomposition and dissolution of gray cast iron graphite into the molten pool. A large number of free carbon atoms in the molten pool react with iron atoms to form cementite, and a small amount of carbon atoms will be dissolved into iron atoms to form solid solution. In the heat affected zone, due to the high temperature near the molten pool, the graphite of gray cast iron decomposes rapidly. In the part near the matrix, the flake graphite decomposes, deforms and refines, as shown in figure (b).
Because the temperature near the matrix is low, and it is less affected by the convection in the molten pool, and the diffusion movement of carbon atoms is slow, the graphite of gray cast iron in the heat affected zone is not completely decomposed after the solidification of the molten pool. The refined graphite structure of gray cast iron avoids the cracking caused by the stress concentration in the heat affected zone, reduces the initiation of cracks around the undivided gray cast iron graphite, and improves the strength of the heat affected zone. The carbon atoms dissolved in the molten pool do not have time to diffuse into the iron matrix, but re precipitate around the residual austenite in the heat affected zone, forming a dotted gray cast iron graphite form between dendrites. The newly precipitated gray cast iron graphite is more concentrated, mainly around the unmelted gray cast iron graphite, as shown in figure (c).