Microstructures And Properties Of Medium Entropy Alloy Base Coatings On TC4 Manufactured By Laser Melting Deposition

Multi-principal alloys with broad modification space and stable properties have been received extensive attention by scholars,the Co Cr Ni medium entropy alloy was a subset of Cr Co Ni Mn Fe,which exhibited an excellent overall performance due to the low stacking fault energy.At present,Co Cr Ni medium entropy alloy is mainly prepared by traditional casting process with large grain size.The precision processing of complex parts can be achieved by laser melting deposition technology with pre-defined laser paths,improving the production efficiency and quality effectively.The high frequency shock wave generated by ultrasonic vibration,acting on the liquid high temperature laser induced pool to refine the grain and promote the elemental diffusions.The grain growth is retarded under the action of the laser melting deposition technology with the fast cooling characteristics,promoting Marangoni convection and high-temperature diffusion in the laser induced pool by the ultrasonic vibration assistance,also promoting the diffusion of corrosion-resistant elements such as Cr and Ni,enhancing the corrosion resistance of the coatings.Si₃N₄ with a low friction coefficient and high micro-hardness should have a small coefficient of thermal expansion and good thermal shock resistance,the micro-hardness and wear resistance of the coatings are enhanced effectively by adding Si₃N₄ to the Co Cr Ni medium entropy alloy base powders,improving the wear and low micro-hardness defects of TC4 titanium alloy,and further expand the application scope of titanium alloy in the aerospace field and the manufacturing industry.The TC4 titanium alloy was used as the substrate,the Co Cr Ni medium entropy alloy powders were selected as the clad powders;an ultrasonic vibration-assisted coaxial powder feeding laser melting deposition was used to clad the Tribaloy X-40-Cr-Ni-Cu-Si₃N₄mixed powders on the TC4 titanium alloy（the molar ratio of Co:Cr:Ni was 1:1:1）.The effects of the laser power,the ultrasonic vibration,the ceramics content and the productions of the multi-phase on the microstructure,the corrosion resistance,the micro-hardness and the wear resistance of the coatings were investigated by the metallographic microscope（OM）,the scanning electron microscope（SEM）,the energy dispersive spectrometer（EDS）,the electron backscattered diffraction（EBSD）,the transmission electron microscope（TEM）and the laser spectroscopic confocal microscope.The results indicated that the elemental segregation was produced in the coating of Tribaloy X-40-Cr-Ni-Cu under a laser power of 900 W or 1100 W,the coatings were composed of the dendritics also the interdendritics due to the difference of the atomic radius and mixing enthalpy among the elements.The good metallurgy bond was produced between the coating and TC4 without obvious defects,the dislocations and multi-phase were promoted by the rapid cooling rate of a laser puddle;the coatings were comprised of BCC solid solution phase,the（Ni,Co）Ti₂ intermetallics also the ceramics,the laser power had few effects on the phase constituent of the coatings.The high temperature laser induced pool existed for a long time under the laser power of 1100W,which is conducive to the diffusions of the corrosion resistant elements and the uniform distribution of the hard phases,enhancing the micro-hardness also the corrosion resistance of the coatings.The effect of ultrasonic vibration on the microstructure and corrosion resistance of the coatings was investigated.The laser induced pool flow was promoted,the dendrites were broken due to the cavitation and acoustic flow effect of the ultrasonic vibration,refining the microstructure,the average grain diameter of Tribaloy X-40-Cr-Ni-Cu-Si3N4 coating was reduced from 6.44μm to 4.22μm.The grain orientation difference was reduced due to the ultrasonic vibration,promoting the low angle grain boundaries to be formed,also enhancing the corrosion resistance of the coatings to a certain extent.Ultrasonic vibration promoted the generation of the dislocations,the geometric necessary dislocation density of the Tribaloy X-40-Cr-Ni-Cu-Si₃N₄ coating increased from 2.78×10¹⁵ m^-2 to 4.99×10¹⁵ m^-2,enhancing the micro-hardness.The uniform distribution of the corrosion-resistant elements and the grain refinement were promoted by the ultrasonic vibration,increasing the thickness of the passivation film to increase the corrosion potential and reduce the corrosion current density,further enhancing the corrosion resistance of the coatings.The effect of Si3N4 content on the micro-hardness and wear properties of the coatings was investigated.The dispersed Ti N was produced due to the in-situ reaction between Si3N4 and diluted Ti,showing the symmetrical distribution microstructure;Ti N was broken due to the ultrasonic vibration,promoting the fine dendritic/granular Ti N to be fabricated.The dislocation motion was retarded by the hard Ti N,10wt%Si3N4 reinforced coatings with an average micro-hardness of about 1055 HV0.5 and an average friction coefficient of about 0.784,and 5wt%Si3N4reinforced coatings with an average micro-hardness of about 955 HV0.5 and an average friction coefficient of about 0.89;the deep grooves and the metal debris were observed on the TC4 worn surface,exhibiting the abrasive and adhesive wear mechanism,the wear mechanism of the coatings belongs to the adhesive wear;the micro-hardness and wear resistance of the coatings were enhanced with increasing of the ceramics content.Too much ceramics reduced the corrosion resistance due to the potential difference between the ceramics and coating matrix.