Influence Of Alternating Magnetic Field On The Microstructures And Properties Of Iron-Based Alloy Coating Produced By Laser Cladding

In the laser cladding process, due to the differences in thermal properties between the substrate and powder, as well as the excessive cooling rate of cladding layer, defects such as cracks, pores and inclusions are very likely to appear in the cladding layer. The coarse and directional grain microstructures of the cladding layer limit the further improvement of the properties of the cladding layer. According to the theory of electromagnetic metallurgy, the paper applied the alternating magnetic field which is generated by a self-designed magnetic field device into the laser cladding process of iron-based alloy powder, to investigate the effect of alternating magnetic field on the macro morphology,microstructures and mechanical properties of the cladding layer. So as to provide a new kind of process method which can improve the quality of cladding layer, reduce the internal cracks and pores of in the coating.The type of magnetic field function corresponding to the different angles between the two magnets was simulated by the magnetic field analysis software Vizimag. And lastly a certain type of function was determined for the following experiments. The corresponding relationship between the distance of the two permanent magnets with the amplitude of the alternating magnetic field function, as well as the relationship between the rotating speed of the permanent magnet with the frequency of the alternating magnetic field function were confirmed. The measurement results of the Gauss Meter verified the accuracy of the numerical simulation.The paper researched the thermodynamic conditions for the formation of the crystal core and dynamic conditions for the growth of crystal nucleation. From the viewpoint of the change of the thermodynamic and dynamic conditions caused by the alternating magnetic field, it revealed the mechanism of the fusing of the dendrite and the increase convection in molten pool, thus making grain refining by alternating magnetic field. The paper analyzed the reason for the formation of cracks, pores and inclusions in the cladding layer, as well as the principle of separation of pores and inclusions by electromagnetic force, which helped to lay the theoretical foundation for the experiment of laser claddingunder alternating magnetic field.In the absence of an external alternating magnetic field, the iron-base coatings which is widely used in the engineering were prepared on the surface of the Q235 A substrates.The optimum process parameters of laser cladding on this substrate and powder system were confirmed by experiments. On this foundation, the effect of alternating magnetic field on the macroscopic morphology and dilution rate of cladding layer were studied, as well as the influence of different magnetic field parameters to them.The metallographic microscope was used to study the changes in microstructures of the cladding layer under the alternating magnetic field, as well as the influence of different amplitude and frequency to the coating structure and the mechanism of the transformation from isometric crystal to columnar crystal in the coating. XRD analysis of the cladding layer showed that the involvement of the external alternating magnetic field has little effect on the phase of the cladding layer.The comparative analysis of the changes before and after the action of magnetic field in the experiments showed that the tensile strength, impact toughness, hardness and wear resistance of cladding layer had been improved by a certain extent, thus proving that the refined microstructure can improve the comprehensive performance.