Synergistic Mechanism Of Mechanical Properties And Corrosion Resistance Of Cr_yFeCoNiNb_x HEA Cladded Layer

Marine equipment represented by various ships and drilling platforms has been in high-salt and high-humidity environment for a long time,and corrosion and wear under high load are the main factors of its failure.Especially the bearing parts of marine equipment such as the stern shaft of ships,the combined effect of various factors such as chloride ion erosion,electrochemical corrosion,stress corrosion and wave impact has accelerated the initiation and development of its own failure:the stern shaft that is in direct contact with seawater is prone to failure.Corrosion occurs due to Cl^-attack,and the corroded area of the stern shaft easily becomes a structural defect,and fatigue fracture occurs under the action of stress concentration and fluctuating load.In this paper,laser cladding technology was used to prepare CrFeCoNiNb HEA coating on the surface of 42CrMo stern shaft to improve the mechanical properties and corrosion resistance of the stern shaft.The combination of theoretical analysis and experimental research is used to study the synergistic mechanism of strengthening and toughening and corrosion resistance of CrFeCoNiNb series to meet the performance requirements of marine equipment represented by the stern shaft of ships.First of all,the CrFeCoNiNb HEA cladded layer was prepared in the form of pre-powder on the surface of 42CrMo substrate,and the effect of laser energy density on the microstructure and properties of CrFeCoNiNb cladded layer was explored.For the preset powder types,the three parameters of laser cladding（spot diameter,scanning speed,laser power）are combined to act on the cladded layer in the form of energy density.Among them,the laser power has the greatest influence on the quality and performance of the cladded layer.Therefore,the effect of laser energy density on the microhardness,wear resistance and corrosion resistance of CrFeCoNiNb cladded layer was investigated by changing the laser power and then the laser energy density acting on the cladded layer.Research indicates:The main phases of CrFeCoNiNb cladded layer are face-centered cubic structure（FCC phase）and hexagonal close-packed structure（Laves phase）.With the increase of the laser energy density,the grains of the cladded layer are first refined and then coarsened,and the laser energy density at the turning point is 116.7 J/mm².When the laser energy density is116.7 J/mm²,the phase distribution of the high-entropy alloy cladded layer is uniform,and the wear morphology is mainly smooth furrows.The corresponding anti-friction effect is good,and the wear rate is reduced.The solid solution strengthening effect of the dual phase structure and the change of grain size are the main factors affecting the average microhardness and corrosion resistance of the cladded layer,respectively.The strengthening effect of the hard Laves phase conceals the grain refinement strengthening effect and becomes the main factor that dominates the microhardness of the cladded layer.The refined grains can form a dense passivation film,which can weaken the phenomenon of grain shedding caused by intergranular corrosion.To further improve the microstructure and properties of CrFeCoNiNb series cladded layer,the mechanism of Nb as a dissolve blocker on CrFeCoNiNb_xcladded layer was explored.Results show that the cladded layer of Nb0 is composed of single dendrites with a small amount of short rod-shaped protrusions distribute along the dendrites.The addition of Nb promotes the original dendrite structures to grow into interdendritic structures.Apart from the variations in the microstructure,the formation of the Laves phase with characteristics of both hard and corrosion resistant,is an important factor affecting the properties of the cladded layer.With the increase of Nb content,more large-sized Nb elements are dissolved into the Laves phase,resulting in an increase in the lattice constant of the Laves phase.This variation leads to the increase in the microhardness of the cladded layer,accompanying with the improvement of wear resistance property.Moreover,the anti-friction effect of the cladded layer is almost proportional to the wear resistance effect,except that x=0.75.Finally,on the basis of CrFeCoNiNb_0.5,the mechanism of Cras a passivation promoter on the microstructure evolution and performance improvement of the Cr_yFeCoNiNb_0.5（y=0,0.5,1,1.5,2）HEA cladded layer was explored.By adjusting the Crcontent to control the lamellar nano-eutectic structure of the cladded layer,to achieved the synergistic improvement of the mechanical properties and corrosion resistance of the Cr_yFeCoNiNb_0.5cladded layer.Through testing and analysis,it was found that the cladded layer was still a（FCC+Laves）dual-phase structure.With the addition of Cr,the layered eutectic structure appears,and the lamellar eutectic structure has a tendency of refinement and area increase,which are the main factors for the improvement of microhardness,wear resistance and corrosion resistance of the cladded layer.On the one hand,the microhardness of the cladded layer increases with the addition of Cr,which is the result of the combined effect of the grain refinement strengthening,the solid solution strengthening of the dual-phase structure and dispersion strengthening of the Laves phase.The wear amount decreases with the increase of Cr,and its wear resistance is proportional to the microhardness.The wear morphology shows that Cr1.5 has almost no oxide film and has the highest friction coefficient.The oxide film of Cr1 has a large lubricating area and no peeling phenomenon,and has a small friction coefficient and the best friction reduction.On the other hand,with the increase of Crcontent,the corrosion resistance of Cr_1.5FeCoNiNb_0.5cladded layer first increases and then decreases.The critical point is Cr1.5,which is due to that excessive Crcontent leading to serious element segregation and enhances the galvanic effect.Overall,the mechanical properties and corrosion resistance of the Cr_1.5FeCoNiNb_0.5cladded layer are better.