Inhibition Mechanism of Laser-cladding Porosity Defects Based on Steady-state Magnetic-field Damping Effect
Laser cladding,which is characterized by a small heat-affected zone,a low dilution rate,a wide range of material applications,and the ability to achieve excellent metallurgical bonding with a substrate,has gradually been applied in fields such as aerospace and energy.As one of the primary defects in laser additive manufacturing,pores significantly affect the mechanical performance under dynamic loads.A method assisted by a steady magnetic field to suppress pores without changing the laser-processing conditions is employed in this study.A multiphysics model for pore transport under a magnetic field is constructed to elucidate the inhibitory mechanism of a steady-state magnetic field on pore defects.The flow-field-distribution patterns in the molten pool,the pore-transport trajectories,the pore distribution,and the internal element distribution under various magnetic-field intensities are systematically investigated.A steady magnetic field is positioned on both sides of a ductile iron(QT-400)substrate with a carbon content of approximately 3.5 wt.％.Austenitic stainless steel(AISI 316L)powder with a particle size of 50-110 pm is used.The pores are observed using an optical microscope,and the elemental distribution is analyzed using an energy dispersive spectrometer.A two-dimensional transient finite-element model is established using the Comsol6.0® multiphysics coupling analysis software.Without applying a steady-magnetic field,the maximum surface flow velocity of the molten pool is approximately 0.137 m/s.As the magnetic flux density increases to 1.2 T,the induced Lorentz force generated by the external steady magnetic field within the molten pool enhances the viscous effect of the fluid,thus reducing the flow velocity within the molten pool to 0.054 m/s.Subsequently,the transport of pores in the molten pool model is considered and the trajectories of their movement are calculated.Without an external field,pores with diameters of 40 and 80 pm at the front of the molten pool exhibit reciprocating helical motion within the molten pool.As the pore diameter increases to 120 and 160 pm,the buoyancy exerting on them increases,and under the combined action of fluid drag force,they propagate toward the surface of the clad layer.Under steady magnetic-field conditions,as the magnetic-field intensity increases,the trajectories of the rear pores transition to a vertical-upward movement and eventually remain within the molten pool.The front pores undergo periodic motion,thus rendering it challenging for them to be expelled from the molten pool.At a magnetic field intensity of 0 to 0.6 T,the inhibitory effect of the magnetic field on the pores is not prominent.However,as the magnetic-field intensity increases to 1.2 T,the porosity decreases from 13.357％to 7.768％,thus indicating that the steady magnetic field suppresses the pores.Further analysis of the distribution patterns of the elements in the molten pool shows that under a steady magnetic field,the color gradient of Fe is more prominent compared with the case when no magnetic field is applied.Applying a magnetic field increases the disparity in elemental content between the cladding layer and substrate.This augmentation reduces the dilution effect of the substrate elements on the clad-layer composition and is primarily attributed to the magnetic damping effect,which decreases the fluid velocity,inhibits the entry of external oxygen into the molten pool,and hinders the formation of pores through the combination of carbon.Consequently,the number of pores is reduced.The results of this study can provide a theoretical foundation for controlling pore defects in processes such as magnetic-field-assisted laser cladding,welding,and repair.
laser claddingporosity defectssteady-state magnetic fielddamping effectnumerical simulation
NETL
NSTL
万方数据
