Study On The Microstructure And Properties Of WVTaCr_xTi_y Refractory High Entropy Alloy Coating Prepared Laser Cladding

Refractory high-entropy alloys（RHEAs）exhibit exceptional properties,including high melting points,that make them promising candidates for high-temperature applications.These alloys are attracting considerable attention both domestically and internationally as they offer superior high-temperature characteristics compared to traditional nickel-based alloys.Laser cladding（LC）is a reliable remanufacturing technique that can produce a metallurgical bond between the powder and substrate,thereby improving the surface quality,corrosion resistance,wear resistance,and high-temperature oxidation resistance of essential components.In response to the improvement of the comprehensive performance of surface modification technology,joint efforts need to be made in adjusting coating composition and optimizing technical parameters to improve the comprehensive performance of components,extend their service life,and reduce production costs.In this thesis,the aimed to investigate the development of refractory high-entropy alloys with high oxidation resistance by creating a single-pass alloy coating on the surface of 42Cr Mo steel using laser cladding.This thesis evaluated the surface quality,microstructure,and hardness of the coating and determined the optimal process parameters for the experiment.This thesis also prepared several types of multipasscoatings,includingWVTaTi_x（x=0,0.25,0.5,0.75,1）,WVTaCr_x（x=0,0.25,0.5,0.75,1）,and WVTaCr_xTi_y（x=0.25,0.5,0.75,1 y=0.25,0.5,0.75,1）,on the surface of 42Cr Mo steel using laser cladding and investigated their microstructure,phase composition,mechanical properties,high-temperature oxidation resistance,and corrosion resistance.Through comparative analysis of the coatings’compositions were able to draw several conclusions:The findings of the study suggest that the optimal laser cladding process parameters are a laser power of P=1400W,scanning speed of v=8mm/s,and spot diameter of D=6mm.Coatings produced with these parameters exhibited satisfactory surface forming quality.The microstructure of the WVTa alloy coating displayed a distinct dendritic structure with an average hardness of 450 HV.Under the optimal process parameters,WVTaTi_x（x=0,0.25,0.5,0.75,1）,WVTaCr_x（x=0,0.25,0.5,0.75,1）,and WVTaCr_xTi_y（x=0.25,0.5,0.75,1 y=0.25,0.5,0.75,1）alloy coatings were prepared by laser cladding.Each coating demonstrated a good metallurgical bond with the substrate,consisting of the cladding zone（CZ）,bonding zone（BZ）,and heat-affected zone（HAZ）.The WVTaCr_x（x=0,0.25,0.5,0.75,1）coating consisted mainly of BCC solid solution phase with a minor proportion of Laves phase,whereas the WVTaTi_x（x=0,0.25,0.5,0.75,1）alloy coating consisted primarily of BCC solid solution phase with a trace amount of MC phase.Coatings of WVTaCr_xTi_y（x=0.25,0.5,0.75,1 y=0.25,0.5,0.75,1）alloy consisted mainly of BCC solid solution phase,Laves phase,and a minute amount of MC phase.The addition of Cr and Ti elements led to grain refinement,dense organization,and induced the precipitation of the second phase.The hardness of the three alloy coatings showed a stepped distribution corresponding to the cladding zone,heat-affected zone,and substrate of the coatings,respectively.Compared to the WVTa alloy coatings,the addition of Cr and Ti elements increased the hardness of the coatings.The hardness of WVTaCr_0.5 alloy was 504.16 HV,WVTaTi_0.75 alloy was 510.13 HV,and WVTaTi Cr alloy was 627.36 HV.Experiments involving cyclic oxidation at 800°C revealed that the addition of Cr and Ti elements substantially improved the alloy’s resistance to oxidation at high temperatures.After 50 h oxidation experiments,the oxidation weight gain of the WVTa alloy coating was 10.41 mg/cm² with an oxidation rate of 0.21 mg/（cm²·h）,that of the WVTaCr alloy was 6.62 mg/cm² with an oxidation rate of 0.13 mg/（cm²·h）,that of the WVTaTi_0.5 alloy was 5.69 mg/cm² with an oxidation rate of 0.1 mg/（cm²·h）,and the oxidation weight gain of WVTaCr Ti alloy was 5.12 mg/cm²and the oxidation rate was 0.1 mg/（cm²·h）.By comparison,it was found that the oxidation resistance of WVTaCr Ti was better after the addition of Cr and Ti,and the oxidation resistance was improved by about 50%compared with that before.The formation of stable Ti O₂ on the surface of the alloy,followed by an outer layer of Cr₂O₃ with a dual protective effect,is primarily responsible for the enhancement in antioxidant performance.In addition,a minute amount of Cr Ta O₄ is also produced and is protected layer by layer.The dense oxide layer can effectively inhibit the diffusion rate of oxygen atoms,thereby enhancing the coating’s resistance to oxidation at high temperatures.Experiments on corrosion resistance in a 3.5 wt%Na Cl solution revealed that the addition of Cr and Ti elements enhanced the corrosion resistance of the alloys significantly.With a corrosion rate of 2.95 mm/a for WVTa alloy and 0.16 mm/a for WVTaCr Ti alloy,the corrosion resistance was enhanced by approximately 90%.The inclusion of Cr and Ti elements,which results in a passivation film on the alloy’s surface,is primarily responsible for the corrosion resistance enhancement.