Study On The Microstructure Evolution And Properties Enhancement Of Al₂O₃-reinforced Ni60A Composite Coating On Titanium Alloy Surface By Laser Cladding

Titanium alloys are widely applied in aerospace,marine engineering,automotive,petrochemical,and medical devices due to their high specific strength,low density,high corrosion resistance,and good biocompatibility,but their low hardness and poor frictional wear properties limit the scope of application.In this paper,we select"Ni60A+Al₂O₃"as the cladding material and use laser cladding technology to prepare ceramic-reinforced metal-based composite coatings with good cladding quality and excellent mechanical properties on the surface of titanium alloys to improve the surface hardness and frictional wear properties of titanium alloys.In this paper,COMSOL Multiphysics 5.6 finite element simulation software was used to simulate the changes of temperature and flow fields of the melting pool at different laser energy densities to study the heat and mass transfer behavior of the molten pool during the laser cladding of composite coatings.The simulation results show that with the increase of laser energy density,the temperature in the melting pool increases,and the melting pool area and molten depth increase.The flow field in the melting pool shows a symmetric distribution and the internal circulation is formed.The Al₂O₃-reinforced Ni60A composite coating was prepared by laser cladding under the finite element simulation process parameters,and the microstructure at the bonding area of the composite coating was observed,and it was found that the grains gradually changed from cytosolic to columnar and equiaxed crystals.According to the simulation results of temperature field,the temperature gradient decreases and the cooling rate increases from the bottom to the top of the cladding coating.Combined with the analysis of the longitudinal profile and microstructure of the front end of the melting pool,it is found that the main mass transfer mode in the melting pool is the annular flow,and the ceramic reinforced phase in the cladding coating is distributed in the direction of the annular flow movement,showing an outward convex vortex shape.Secondly,the Al₂O₃-reinforced Ni60A composite coating was prepared by using different Al₂O₃content of the cladding coating powder,and the macroscopic morphology and microstructure of the composite coating were observed.The results showed that the average microhardness value of the cladding coating prepared with the mixed powder ratio of Ni60A+12%Al₂O₃（wt.）was the largest,about 1111.8 HV,which was about 3.3 times that of TC4 alloy（340 HV）,and the wear resistance was also better.When the Al₂O₃content is low,the number of Cr（B,C）in the black lumpy and coarse flower-like microstructure in the cladding coating is high,which is easy to agglomerate and cause cracks,resulting in poor wear resistance of the cladding coating.With the increase of Al₂O₃content,the molten depth of the cladding coating increases,and a large number of Ti atoms in the matrix enter the melting pool to consume a large number of B and C atoms,so the number of black lumpy and coarse flower-like microstructure in the cladding coating gradually decreases to disappear,and the wear resistance of the cladding coating is improved.Then,the Al₂O₃-reinforced Ni60A composite coatings were prepared at different laser energy densities,and the macroscopic morphology and microstructure of the composite coatings were observed,and the microhardness and wear resistance of the composite coatings were tested to investigate the influence of laser energy density on the organization and properties of the cladding coating.The results showed that the macroscopic morphology and flatness of the cladding coating prepared at the laser energy density of 125J/mm²were better,and the average microhardness of the cladding coating was the highest,about 1132.7 HV,and the friction coefficient was the smallest and the wear resistance was the best.When the laser energy density was too low,the ceramic phase gathered and caused cracks,and when the energy was too high,the ceramic phase floated up and gathered mostly on the top of the cladding coating.The improvement of wear resistance of the composite coating is attributed to the generated Ti B₂and Ti C/Ti B₂ceramic eutectic microstructure diffusely distributed on the cladding coating,which increases the microhardness of the cladding coating.In addition,Ni-Ti-Cr and Ti Ni intermetallic compounds with excellent toughness are formed during the rapid solidification of the cladding coating,which is beneficial to improve the friction wear performance of the cladding coating.Finally,in order to improve the defects of poor uniformity of ceramic phase distribution in the cladding coating and strengthen the wear resistance of the cladding coating,the ultrasonic-assisted process was applied on the basis of a better Ni60A-Al₂O₃mixed powder ratio and laser process parameters to investigate the influence law of ultrasonic vibration on the hard phase distribution and properties of Al₂O₃-enhanced Ni60A composite coating.The results showed that the surface quality of the cladding coating prepared at an ultrasonic amplitude of 60μm was better,the flatness was higher,and the wear resistance was the best.When the applied ultrasonic amplitude is low,the strengthening effect of ultrasonic vibration on the wear resistance of the cladding coating is weak,and when the applied ultrasonic amplitude is too high,the violent melting pool convection affects the flatness of the cladding coating.The improvement of the wear resistance of the cladding coating is due to the cavitation and acoustic flow effect of ultrasonic vibration,which results in a finer organization of the clad layer and a more uniform distribution of the ceramic reinforcing phase.