A Multiscale Study on Substrate Size Effect and Energy Density Regulation on Dynamic Response of Dilution Rate in Laser Cladding Iron-Based Coatings

This study systematically revealed the synergistic effects of laser power, cladding speed, and substrate diameter on the dilution rate and hardness of iron-based alloy coatings on the surface of 45 steel through the integration of finite element simulation, elemental migration analysis, and response surface methodology (RSM). The experiments showed that when the substrate diameter was greater than 50 mm, the coupling effect of thermal diffusion retardation and molten pool expansion caused a nonlinear surge in the dilution rate. The growth rate of the molten pool depth increased by 46% (from 0.28 to 0.41 μm), and the melting volume of the substrate expanded by 1.7 times. The dilution rate reached 15.6%–31.7% through a dual-regulation mechanism involving energy density (1.43–3.75 J/mm2) and substrate diameter (30–60 mm), with a significant hardness demarcation of 343–738 HV. Substrates with a small diameter (30 mm) achieved a peak hardness of 738 HV at an energy density of 2.14 J/mm2 through ultra-fast cooling (>1.5 × 104 K/s), while those with a large diameter (60 mm) exhibited a hardness drop to 426.5 HV due to grain coarsening. The multi-method integrated model constructed in this study achieved a dilution rate prediction error of less than 5% (R2 = 0.9775), with a prediction deviation of less than 2% under extreme parameters (diameter of 55 mm and power of 4800 W). The study proposed an optimized process window with a substrate diameter of 42–57 mm and an energy density of 1.43–2.14 J/mm2, providing a physically mechanism-driven intelligent parameter design strategy for laser cladding repair of shaft parts.