Double-ring oscillation laser treatment of QT400 ductile iron: thermal-fluid simulation, microstructure regulation and wear resistance enhancement

Ductile iron is widely used for wear resistant components in industrial equipment, while its insufficient surface hardness limits service life. Laser surface treatment (LST) with an oscillating trajectory has emerged as a promising technique for precise surface modification. Existing studies have shown that oscillation trajectory strongly affects molten pool thermal fluid behavior and the resulting microstructure and properties, while the differences among oscillating trajectories deserve further investigation. In this study, single ring oscillation (SRO) and double ring oscillation (DRO) trajectories were applied to QT400 ductile iron. A three-dimensional transient thermal fluid coupled model was established to investigate energy distribution, temperature evolution, and melt flow, combined with experimental characterization of microstructure, microhardness, and wear resistance. The results show that DRO supplements the central heat input and expands the thermal influence through alternating inner and outer rings, forming a larger and more continuous liquid region with a maximum melt velocity of 0.24–0.55 m/s. The oscillation induced molten pool stirring effect further promotes heat and solute transport and reduces spatial differences in cooling conditions, thereby producing finer and more uniform martensite. The average hardness of the remelted zone reaches 709 HV 0.2 , while the stable friction coefficient decreases to about 0.45–0.47 and the wear depth decreases to 83.15 μm. This work demonstrates that DRO effectively regulates thermal fluid behavior and microstructure evolution, significantly improves the wear resistance of ductile iron, and provides theoretical and technical support for advanced laser surface modification of key wear resistant components.