Laser cladding is a surface modification technology that uses a high-energy laser beam as a heat source to deposit an alloy coating with special properties on a substrate. It has the advantages of a low dilution rate, a small heat-affected zone, high bonding strength with the substrate, and little environmental pollution. Therefore, it is widely used in surface repair and strengthening of key components such as automobile manufacturing, petrochemical industry, and mining machinery.
Laser cladding is a complex metallurgical process involving physics, chemistry, and materials. Its rapid heating and quenching solidification characteristics often cause defects such as cracks and pores in the cladding layer. In previous studies, domestic and foreign scholars mainly eliminated or reduced the defects of laser cladding coatings through material design and process parameter optimization. However, for high-hard alloy coatings, it is still difficult to eliminate structural defects by changing the existing process, which requires consideration of controlling the solidification structure of the coating by applying an external field, thereby improving the quality of the coating. As an external field auxiliary technology, the electromagnetic field has the advantages of various combinations, good controllability, and environmental friendliness. It has been applied in casting, welding, laser processing, and other fields. The electromagnetic force generated by the electromagnetic field is used to stir the melt, which can cause strong convection of the liquid metal in the molten pool, homogenize the temperature field and solute distribution of the molten pool, and play a role in reducing the degree of supercooling and refining the solidification structure.
1 Influence mechanism of electromagnetic field on laser cladding process
The electromagnetic field is a non-contact external field auxiliary means. During the laser cladding process, the electromagnetic field will interact with the metal melt in the molten pool to generate electromagnetic force. The electromagnetic force will change the convective movement and mass transfer and heat transfer process of the melt, and then affect the solidification process of the cladding layer. The influence of electromagnetic field on the movement behavior of melt is mainly reflected in several aspects such as the electromagnetic stirring effect, electromagnetic braking effect, thermal electromagnetic fluid effect, electromigration effect, and skin effect. The electromagnetic field's influence on the melt's solidification process is mainly reflected in several aspects such as grain fragmentation, atomic group fluctuation effect, and Joule heating effect.
2 Effects of different electromagnetic field forms on the microstructure and properties of laser cladding coatings
- Steady-state magnetic field: The steady-state magnetic field helps to suppress the surface ripple of the coating, reduce the number of cracks, and refine the coating structure. The steady-state magnetic field can reduce the flow velocity inside the molten pool, but it has no obvious effect on the temperature field; when the steady-state magnetic field strength is higher than a certain value, it has a significant inhibitory effect on the surface ripples of the molten layer.
- Unsteady magnetic field: The alternating magnetic field has little effect on the width and dilution rate of the cladding layer, while its height and contact angle decrease with the increase of the magnetic field strength, and the surface flatness of the cladding layer is also affected by the magnetic field strength and frequency. Compared with the alternating and rotating magnetic fields, the pulsed magnetic field can be intermittently applied to the molten pool by controlling the strength and frequency of the magnetic field. However, due to the rapid heating and rapid cooling process characteristics of laser cladding, the existence time of the molten pool is relatively short, so there are relatively few studies on pulsed magnetic field-assisted laser cladding. As shown in the figure, compared with the samples prepared without external field assistance, the four types of magnetic fields can reduce the number of cracks in the coating, refine the grains and increase the hardness of the coating. Among them, pulsed magnetic field-assisted laser cladding has the best effect, but the phenomenon of hard phase segregation appears in the coating.
A single electric field is widely used in welding and casting, but less research is done in the field of laser cladding. At present, there are two main forms of electric field used in laser cladding: alternating electric field and pulse electric field.
- Alternating electric field: The electromigration effect makes the ions in the melt move directionally, and the Joule heating effect of the current will change the temperature of the melt, thereby affecting the solidification process of the cladding layer. Alternating current can promote grain refinement and at the same time increase the height of the fine-grained region at the bottom of the coating, which helps to reduce crack formation. The introduction of alternating current will form an induced electromagnetic force with continuously changing direction in the molten pool, which will act as an electromagnetic stirring effect on the liquid metal in the molten pool, reduce the temperature gradient at the solidification front, and thus contribute to the refinement of grains.
- Pulse electric field: Pulse current has the characteristics of discontinuity, variability, and periodicity. Applying pulse current during the cladding process can change the melt flow velocity, and the shear force formed in the melt can break the formed grains, increase the nucleation rate, and refine the grains.
3 Electromagnetic field-assisted laser cladding coating material system
At present, electromagnetic field-assisted laser cladding technology has been applied in the preparation of various alloy coatings and composite coatings. For alloy coatings, the electromagnetic field helps improve the homogenization of coating components and the distribution of precipitated phases. For composite coatings, the electromagnetic stirring effect of the electromagnetic field can change the distribution characteristics of the strengthening phase in the molten pool.
- Iron-based coating: After applying the electromagnetic field, as the magnetic field strength increases, the surface roughness of the cladding layer decreases, the structure is significantly refined, and defects such as pores and cracks are reduced; the hardness, wear resistance, and corrosion resistance of the coating are improved. Compared with the coating prepared without a magnetic field, the hardness value of the coating prepared with magnetic field assistance is more stable along the depth direction.
- Cobalt-based coating: The steady-state magnetic field can inhibit the convection of the molten pool, and enrich the macro-segregation, and the matrix elements can be more distributed at the bottom of the molten pool, so it is easier to obtain a cladding layer with a composition close to that of the alloy powder. The magnetostrictive effect generated by the magnetic field can effectively reduce the thermal expansion coefficient and elastic modulus of the cladding layer, reduce the thermal stress during the cladding process, and then reduce the crack sensitivity.
- Composite coating: The constant magnetic field does not affect the phase composition of the composite coating, but has a significant impact on the microstructure of the coating and the distribution of the ceramic reinforcement phase. Certain magnetic field strength is conducive to the refinement of the structure, and the distribution of the ceramic reinforcement phase in the structure is dense. The figure shows the influence of the electromagnetic composite field combined with the steady-state magnetic field and the DC electric field on the distribution and microstructure of WC particles in the laser cladding In718/WC composite coating. The downward Lorentz force generated by the electromagnetic field can enhance the Marangoni convection in the molten pool, which contributes to the uniform distribution of WC particles in the composite coating. Direct current can increase the nucleation rate of eutectic carbides, and the enhanced Marangoni convection can break columnar dendrites, thereby refining the structure.
4 Outlook
Electromagnetic field-assisted laser cladding technology can realize control of the microstructure of the cladding layer, promote grain refinement, reduce composition segregation, make the distribution of the strengthening phase more uniform, and inhibit the initiation of defects such as holes and cracks. Therefore, coatings with excellent properties can be prepared by electromagnetic field-assisted laser cladding technology. Electromagnetic field-assisted laser cladding technology is an innovation of traditional laser processing technology. It can not only promote the application of electromagnetic theory in laser processing technology but also promote the development of laser remanufacturing technology on the surface of high-performance parts. It has broad theoretical research and engineering application prospects.