Microstructure And Properties Of AlCoCrFeNi High Entropy Alloy Laser Cladding Layers Enhanced By In-situ TiC And Mo

Extreme environments such as wear and corrosion reduce the service life of metal critical components,reducing the productivity of the company and increasing maintenance costs,resulting in significant wasted capacity.In order to promote energy conservation and emission reduction,achieve the goal of carbon peaking and carbon neutrality,it is urgent to develop materials with excellent properties such as wear resistance,corrosion resistance and high-temperature oxidation resistance that can protect and repair key parts.Laser cladding,as a new type of surface modification and repair technology,had the characteristics of high energy density,small deformation,and low dilution rate,providing technical support for surface protection and repair of carbon steel.The AlCoCrFeNi-based high entropy alloy as a widely used high entropy alloy system,which not only has high hardness and strength,but also has good modifiability,but its wear resistance is poor.Due to the high strength and wear resistance of ceramic phase particle reinforced metal composites,they had been widely used in the industrial field.Among many ceramic particles,TiC has high hardness,high elastic modulus,low coefficient of friction,excellent chemical stability and self-lubrication,and the excess Ti element exists in solid solution and does not change the phase structure of the AlCoCrFeNi high entropy alloy,making in-situ TiC ceramic particles an ideal reinforcing phase.The introduction of ceramic particles generally leads to a decrease in the corrosion resistance of the alloy.High melting point elements can have a significant impact on the mechanical properties and corrosion resistance of alloys.The higher melting point and Young’s modulus of Mo,among others,predicts better high-temperature properties and strength for Mo-added high-entropy alloys,and the addition of Mo can significantly improve corrosion resistance.These two enhancement methods provide new ideas for designing new wear-resistant/corrosion-resistant high entropy alloy laser cladding layers.The effect of microstructure evolution on mechanics,high-temperature oxidation and corrosion resistance was explored.The in-situ TiC and Mo element-reinforced AlCoCrFeNi high entropy alloy coatings were prepared by laser cladding technology.The microstructural evolution of the coatings was characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy spectrum analyzer（EDS）,electron backscatter diffraction（EBSD）,electron probe microanalysis（EPMA）.The mechanical properties,high-temperature oxidation performance,and corrosion resistance were analyze by the microhardness tests,nanoindentation tests,friction wear tests,high-temperature oxidation tests and electrochemical tests.The BCC phase was present in the AlCoCrFeNi,AlCoCrFeNiMo,AlCoCrFeNi（TiC）and AlCoCrFeNiMo（TiC）high entropy alloy coatings.When Mo and TiC were added separately,the coatings showedσ-phase and dendritic TiC particles,respectively;when Mo and TiC were added together,the coating showed flower-like TiC and noσ-phase appeared.The wear resistance of the coating with co-addition of Mo and TiC was 3.14and 2.6 times higher than that of Mo and TiC alone,respectively,which was greater than the sum of the individual additions,indicating that the co-addition of Mo and TiC produced a synergistic effect in improving wear resistance.The strengthening mechanism was mainly due to the solid solution strengthening effect generated by the addition of Mo and Ti,which increased the average nano-hardness of the BCC phase.Theσphase and TiC,which had high hardness and high elastic modulus,produced a second phase strengthening effect that improved the mechanical properties of the coatings.The strengthening mechanism was mainly the solid solution strengthening by the addition of Mo and Ti,the second phase strengthening by the high hardness and high elastic modulus of theσphase and TiC,and the fine grain strengthening effect due to grain refinement.The AlCoCrFeNiMo_x（TiC）_2-x high entropy alloy coatings（x=0,0.5,1,1.5,2）are all present in the BCC phase.As x increased,the size of the TiC particles decreased and the morphology appeared as small planar equiaxed crystalline,dendritic,flower-like,and near-spherical,respectively.When x=1.5 and 2,the coatings showed the first appearance of theσphase（Fe₃Mo）and FCC phase（NiCrCoMo）,respectively.The Mo promoted the formation of protective oxide layers and improved the stability of the passivation film,while the second phase increased the defects in the coatings,which determined the difference in the high-temperature oxidation and corrosion resistance of the coatings.As x increases,the high-temperature oxidation resistance and corrosion resistance of the coatings first increase and then decrease.When x=1,the coating shows the best high-temperature oxidation resistance and corrosion resistance,which are 5-12 times and 15-83 times higher than other coatings respectively.The microstructure and properties of AlCoCrFeNiMo（TiC）high entropy alloy coatings were investigated at continuous and pulsed waves（0.5,5,50,500,5000 Hz）.The BCC phase and TiC were found to be present in all coatings,theσphase（Fe₃Mo）appeared in all pulsed coatings,and the FCC phase（NiCrCoMo）appeared in the coatings.As the pulse frequency decreased,the TiC particle size and the fraction of Mo-rich phase decreased,and the microhardness,wear resistance,corrosion resistance and high-temperature oxidation resistance of the coating improved,with the best performance at a pulse frequency of 0.5 Hz.As the pulse frequency decreased,the mechanical properties of the coatings improved because the TiC particle refinement enhanced the second phase strengthening effect;the corrosion resistance and high-temperature oxidation resistance improved because the TiC particle refinement and the reduction in the proportion of Mo-rich phases synergistically reduced the tendency of the alloying elements to galvanic corrosion during corrosion and the rate of alloy diffusion during oxidation.