Research On The Microstructure And Oxidation Resistance Of Narrowband/Broadband Laser Cladding High Entropy Alloy Coating

High-entropy alloys have the characteristics of high hardness,high wear resistance and high-temperature oxidation resistance.The cost of high entropy alloy coating prepared by laser cladding is low,and the microstructure of cladding layer is fine and compact.In this paper,a multi-principal element NiCoFeCrSiAlCuTiMoB high-entropy alloy coating was prepared on the surface of Q235 steel by narrowband and broadband laser cladding methods,using XRD,SEM,EDS,Vickers hardness tester and wear tester and other equipment to study laser cladding the influence of process,alloying elements and heat treatment on the structure and performance of this series of coatings.Firstly,the NiCoFeCrSiAlCuTiMoB_0.4 high-entropy alloy coating was compared and studied by using the narrowband and broadband laser cladding processes.The results show that the structure of the narrowband coating is a single BCC solid solution phase structure.The phase structure of the broadband coating is composed of the BCC solid solution phase and a small amount of B(Fe,Si)₃ and Cr₂B second phases.The heat treatment process of tempering at 600°C for 1h does not change the phase structure of the two coatings,but significantly improves the hardness of the coating.The hardness of the broadband coating is slightly higher than that of the narrowband single-pass coating.As the tempering temperature further increases,the hardness value decreases.The brittleness of broadband coating is higher than that of narrowband coating.Secondly,on the basis of NiCoFeCrSiAlCuTiMoB_0.4 high-entropy alloy coating,by reducing the content of easily segregated elements Cu and Ti and Si elements which are easy to form the second phase,design and prepare NiCoFeCrSiAlCu_0.5TiMoB_0.4NiCoFeCrSiAlCuTi_0.5MoB_0.4 and NiCoFeCrSi_0.5AlCuTiMoB_0.4 narrowband and broadband coating.The study found that under the narrowband process,the reduction of Cu and Si alloying elements does not change the coating phase structure,and the structure is still a single BCC solid solution phase.In addition to the BCC phase,a small amount of second phase Cr₂B precipitated in the coating after the reduction of Ti element.In the broadband process,the coating is still composed of BCC solid solution phase and the second phase after the content of Cu,Ti and Si elements is reduced.The heat treatment process of tempering at 600?for 1h has no obvious effect on the phase structure of the coating.Two cladding processes The coatings below all have good high temperature stability.The decrease of Ti content has little change in the hardness value under the wide band,and the hardness value increases under the narrowband process.The decrease of the Cu content increases the hardness value of the coating under both the narrow-band and broadband processes.After tempering at 500°C for 1h,the hardness of broadband NiCoFeCrSiAlCu_0.5TiMoB_0.4 coating reached the highest value of 925HV.The decrease of Si content reduced the hardness of both the broadband and narrowband coatings.The brittleness of the broadband coating after the reduction of Cu and Ti element content is still high,and obvious cracks will occur under the load of Vickers hardness.Finally,in order to further understand the high-temperature oxidation performance and high-temperature wear performance of the coating,the high-temperature oxidation experiment at 600??800?and the high-temperature wear experiment at 600?were carried out on the broadband coating with higher hardness value.The study found that after NiCoFeCrSiAlCu_0.5TiMoB_0.4 high-entropy alloy coating was oxidized at 800°C for 50 hours,the cross-section of the oxidation product is composed of an outer oxide layer and an inner oxide layer.The total oxide layer thickness is only less than 5?m,which has excellent oxidation resistance.Cr is the main element of the outer oxide layer,Al is the main element of the inner oxidation layer,Ti participates in both the inner and outer oxide layers,Cu participates in the outer oxidation in a small amount,Si participates in the internal oxidation in a small amount,and Fe,Co,Ni,Mo basically does not participate in oxidation.Oxidation first occurs in the interior of the dendrites,generating fine oxides,and then gradually spreading to the entire surface.The internal oxidation of HEAc-Ti_0.5 and HEAc-Si_0.5 coatings with reduced Ti and Si elements is relatively serious.The high temperature wear resistance of HEAc,HEAc-Cu_0.5 and HEAc-Ti_0.5 coatings at 600?is better,and the high temperature wear resistance of HEAc-Si_0.5 is slightly reduced.