| Laser cladding technology uses a high-energy density laser beam to instantly melt the alloy powder and the thin layer material on the substrate surface and quickly condense,thereby forming a dense cladding layer with good metallurgical bonding.Laser cladding technology can realize the strengthening,repairing and remanufacturing of key parts.It can also prepare high-performance coatings on the surface of low-cost carbon steel to replace the whole alloy parts,effectively reduce costs,and be environmentally friendly.It is the most valuable remanufacturing technology.However,due to the inherent characteristics of ’quick heat and fast cooling ’of the laser cladding process itself,and the differences in the physical properties(thermal expansion coefficient,elastic modulus and thermal conductivity,etc.)of the cladding layer material and the matrix material,it is difficult to avoid the formation of organizational stress and thermal stress during the cladding process,and it is easy to produce stress concentration at pores,inclusions,etc.,forming micro/macro cracks.Therefore,the high cracking sensitivity of the cladding layer,especially the high hardness cladding layer,is one of the key issues for the large-scale application of laser cladding technology.Based on the above problems,a new method to realize crack-free high-hard cladding layer was explored.Three process methods were compared and studied : laser remelting process,temperature field assisted cladding process,laser remelting and temperature field assisted synergistic assisted cladding process.In the study,NiCrSiBC-WC was selected as the cladding powder material,and the ceramic-metal composite coating was prepared with low-cost carbon steel as the substrate.The surface and cross-section crack morphology,porosity,hardness,fracture toughness,residual stress and mechanical properties of the cladding layer prepared by different treatment processes were compared and studied.The cracking sensitivity and cracking behavior of the ceramic-metal composite coating were quantitatively analyzed.The cracking mechanism was described,and the optimized process method for achieving crack-free ceramic-metal composite coating was further explored.The main research contents and conclusions include:(1)The high hardness cracking mechanism of ceramic-metal composite coating was analyzed.NiCrSiBC-WC composite coatings with WC mass fraction of 40 %,50 % and 60 %were prepared on carbon steel substrate by laser cladding technology.The macro and micro morphology and phase composition of the three coatings were compared and observed.The hardness,residual stress,fracture toughness and nanoindentation behavior of the three coatings were further tested and quantitatively calculated,and the high hardness cracking mechanism of the composite coating was analyzed.The results show that obvious cracks appear in 50 % and 60 % WC coatings,while no obvious cracks appear in 40 % WC coating.With the increase of WC content,the grains are refined and the cracks increase.The cracking of 50 % WC coating is attributed to high tensile residual stress and low toughness.Compared with 50 % WC coating,the increase of crack sensitivity of 60 % WC coating is mainly due to the further increase of residual stress.With the increase of WC content,the indentation depth of the composite coating decreases,and the yield strength and indentation hardness increase gradually with the increase of WC content.(2)The influence mechanism of laser remelting process and temperature field assisted process on WC ceramic-metal composite coating was studied.Four kinds of NiCrSiBC-based ceramic-metal composite coatings with WC content of 30 % and 40 % were prepared by laser remelting and temperature field assisted process.The macro and micro morphology,phase composition,microstructure,hardness and residual stress of the samples under two different process conditions were compared and analyzed.The results show that compared with the laser cladding layer,the laser remelting process and the temperature field assisted process can prepare a composite coating without obvious cracks on the surface and cross section,and the microhardness of the composite coating is significantly improved and the residual stress is significantly reduced.Both processes contribute to the uniform distribution of WC particles in the coating.In addition,compared with the temperature field assisted process,the composite coating under the laser remelting process has obvious grain refinement effect,and the type and quantity of carbides further increase.(3)The influence mechanism of laser remelting and temperature field assisted composite process on WC ceramic-metal composite coating was studied.Two kinds of NiCrSiBC-based ceramic-metal composite coatings with WC content of 30 % and 40 % were prepared by laser remelting and temperature field assisted composite process.The morphology,microstructure and mechanical properties of the composite coatings were compared and analyzed.The results show that compared with direct laser cladding,the composite coating with no obvious cracks on the surface and cross section can also be obtained by laser remelting and temperature field assisted composite process.Compared with single laser remelting process or single temperature field assisted process,the composite coating prepared by laser remelting and temperature field assisted composite process has higher microhardness under the same WC content.The precipitation of carbides and grain growth in the coating have the characteristics of both temperature field assisted coating and laser remelting coating. |
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