| Laser cladding technology is one of the surface modification technologies which has a giant application potential in industrial application. However, the formation of cracks and pores in the cladding layer, causing by thermal stress and transformation stress, has hampered the technology applications such as large parts repairing. In addition, the corrugated surface morphology on the surface layers also have increased working hours and costs in the subsequent machining.Therefore, focusing on above two problems, the steady-state magnetic field assisted laser cladding technology has been put forward to preparing iron base alloy coatings on 45 steel in this paper. The micro-structure and composition of the composite coatings were characterized by scanning electron microscope, energy disperse spectroscopy, X-ray diffraction, the micro-hardness distribution characteristics and wear resistance of composite coatings were analyzed. Basing on the above analysis, the morphology and micro-structure changes of the composite coatings under different exciting current were analyzed, and the forming mechanism of the composite coatings were researched. The influence of Lorentz Force on molten pool convection and solidification behavior of laser molten pool was discussed. The micro-hardness and wear resistance of composite coatings improved by magnetic field were also discussed in the paper. Finally reaching the purpose of eliminating the internal defects, improving layer quality and providing a new way to promote the further application in industry process of laser cladding coating.Under the cooperation of laser cladding parameters and steady-state magnetic field parameters, the intermittent phenomena and corrugated morphology of the laser cladding coating were obviously improved at 2.0 A, and fine layers with good surface quality were obtained. The steady-state magnetic field resulted in penetration decreased from 773.5 μm to 694.1μm and 586.5 μm, but it’s not really obvious to width. Besides, a concave phenomenon appears on one side of the weld pool due to the inhibition of steady-state magnetic field.The phase composition wasn’t changed no matter whether applying the steady-state magnetic field. The experimental results showed that the micro-structure was mainly α-(Fe,Cr) and (Fe,Ni) solid solution, (Cr,Fe)23C6 carbide and Cr9.1Si0.9. The introduction of steady-state magnetic field promoted the mass transfer process of laser cladding in the molten pool in a certain extent. It was analyzed that the content of Cr element in inter dendrite was much higher than that of Cr element in dendrite, while the content of Fe element increased a little in dendrite under the same condition of magnetic field, which promoted to form a-(Fe,Cr) solid solution and M23C6 carbides.The micro-structure from the bottom of the layer to the top of the laser cladding iron-based alloy composite coatings without steady-state magnetic field are mainly composed of the plane, the cellular, columnar dendrites, small branches grain and the equiaxial grain. The micro-structure prepared under the steady-state magnetic field consisted more elongate columnar dendrites along the direction of maximum heat dissipation. And with the increase of magnetic field intensity, the electromagnetic braking effect was enhanced, giving rise to suppressing laser molten pool convection. Thus, the volume fraction of primary dendrites had a parabolic variation and reached the peak of 9.9% at the excitation current of 2.0 A, the growth of part of secondary dendrite was restrained causing by steady-state magnetic field. Therefore, the grain growth of cladding layer can be affected by controlling melt convection process using the steady magnetic field.The applying of the magnetic field can promote the transformation of the columnar crystal to equiaxed grain. And with the increase of the magnetic field current, the formation region of equiaxed grain increased, refining the cladding layer micro-structure.Under the influence of magnetizing force, the easy axis of dendrites tended to grow towards magnetic direction causing the deflection of dendrites growing. The angle between dendrites growth and horizontal direction decreased from 55.1° (without magnetic field) to 26.9° (with magnetic field). Besides, columnar crystal grain size increased after applying steady-state magnetic field. Long axes of columnar crystals with applying magnetic field are mainly distributed in 19.3～57.8 μm, while short axes varied from 1.9～5.3 μm. Long axes of columnar crystals without applying magnetic field are mainly distributed in 13.2～33.3μm, while short axes varied from 1.6μm to 4.4 μm.Lorentz force is available to inhibit the motion of molten pool toward perpendicular direction of magnetic field and change the metal molten pool movement characteristic to obtain homogeneous chemical composition, optimized micro-structure and improved mechanical properties. When the exciting current reaches 2.5 A, the mechanical properties of cladding layer has been improved well. The micro-hardness is 779.9 HV0.2 while the wear loss is 7.4x 10’3 mg. |
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