| As a new type of alloy,high entropy alloy has excellent properties such as good plasticity and toughness,high temperature stability,high strength,high wear resistance and corrosion resistance.Laser cladding as a rapid prototyping technology can realize the metallurgical combination of coating and substrate,save the manufacturing cost and so on,which is widely used in the industrial field.However,there are still many problems in laser cladding,such as uneven microstructure,large residual stress and poor forming quality,which affect the performance of the cladding layer.As a means of surface finishing and strengthening,ultrasonic impact technology is widely used in the fields of residual stress relief,metal surface nanocrystalline layer preparation,parts repair and strengthening.Aiming at the existing problems of cladding layer,laser cladding technology and ultrasonic impact technology are combined to further improve the performance of cladding layer on the basis of solving the existing problems of cladding layer.Therefore,in this paper,laser cladding technology was used to prepare CoCrFeNiMox（x=0,0.25,0.5,0.75）high entropy alloy,and its surface was treated by ultrasonic impact treatment.The strengthening mechanism of CoCrFeNiMox high entropy alloy cladding layer was explored by studying the changes of microstructure and properties before and after ultrasonic impact treatment.The thermodynamic parameters ofΔSmix、δ、ΔHmix of CoCrFeNiMox（x=0,0.25,0.5,0.75）high entropy alloy system were calculated.It was found that the high entropy alloy of the system met the conditions for forming solid solution from the thermodynamic point of view.Meanwhile,the solid solution structure was formed by laser cladding of CoCrFeNiMox（x=0,0.25,0.5,0.75）high entropy alloy cladding,among which Mo0 and Mo0.25were FCC single phase structures,Mo0.5 and Mo0.75 was FCC+σstructure,and the lattice constant increased from 3.5809（?）to 3.6235（?）with the increase of Mo element content.The four Mo content high entropy alloy cladding layers formed a good metallurgical combination with the matrix.The top and middle structure of the cladding layer was equiaxed crystal,the bottom structure was columnar crystal and dendrite crystal which was perpendicular to the fusion line direction,and Mo element segregation occurred at the grain boundary.The average microhardness of cladding layer increased from 178 HV0.1 to 474 HV0.1 with the increase of Mo content.Due to the increase of hardness,the wear resistance of the cladding layer was also improved correspondingly.The wear mechanism changed from the composite wear mechanism of abrasive wear and fatigue wear to single abrasive wear.The corrosion resistance of the cladding layer of high entropy alloy increased significantly with the increase of Mo content,and passivation occurred after adding Mo element.When x=0.75,Ecorr=-0.400 V,Icorr=5.67μA/cm2,it had better corrosion resistance.After ultrasonic impact,the phase of the cladding layer did not change,but the microstructure on the surface of the cladding layer became finer and denser,and the reticular distribution ofσphase was broken to a certain extent.In the cross-section structure,a certain thickness of plastic deformation layer can be observed from top to bottom,and the thickness gradually decreased with the increase of the hardness of the cladding layer.Banded folds similar to Lüders bands were observed.The experimental results of EBSD and TEM showed that the dislocation density on the surface of the cladding layer increased after ultrasonic impact,the crystal grain orientation turns to"soft orientation",and the dislocation slip is easier.After ultrasonic impact,the surface roughness of the cladding layer was greatly reduced,Ra reached0.209-0.281μm.With the increase of the microhardness of the high entropy alloy cladding layer,the hardness improvement effect after ultrasonic impact decreased from 119.61%to 16.71%,and the hardness gradually decreased with the increase of the distance from the surface.After ultrasonic impact,the friction coefficient and the amount of wear of the high entropy alloy cladding layer were reduced,but the wear mechanism did not change.After ultrasonic impact,the corrosion resistance was improved.After ultrasonic impact treatment,the surface grains of CoCrFeNiMox（x=0,0.25,0.5,0.75）high entropy alloy cladding layer were deformed,the dislocation density inside the deformed grains increased,and the dislocations continued to entangle and transformed into small angle grain boundaries,and the dislocations continue to accumulate at the small angle grain boundaries,and finally transformed into large angle grain boundaries,thus dividing and refining the grains.The strengthening mechanism of high entropy alloy cladding layer was mainly dislocation strengthening and fine grain strengthening. |
