Performance Strengthening Process And Experimental Research Of Al₂O₃-ZrO₂ (8%Y₂O₃) Laser Cladding Coating On Titanium Alloy Surface

Due to its high specific strength,good corrosion resistance and biocompatibility,titanium alloys have been used more and more in aerospace,ocean and chemical and other fields,but their applications are further limited by the defects of high temperature oxidation resistance and poor wear resistance.Bary Al₂O₃-ZrO₂eutectic oxides have the advantages of high hardness,creep resistance and high temperature oxidation resistance due to their fine and stable microstructure and strong bonding interface.They have been widely used in aerospace and hot end parts of gas turbines and other extreme conditions.Laser cladding technology was used to cladding Al₂O₃-ZrO₂(8%Y₂O₃)ceramic coating on the surface of titanium alloy to improve the application of titanium alloy in aerospace and other high temperature environment.In this paper,laser cladding technology was used to prepare Al₂O₃-ZrO₂(8%Y₂O₃)ceramic coating on the surface of Ti-6Al-4V.Firstly,an experimental platform for laser cladding coupled with ultrasonic field was built.In order to overcome the defect that the no-load ultrasonic generator acts on the air and causes the oscillation of the air column to blow the preset powder away,the tool head of the ultrasonic generator is improved.The diameter of the front section of the tool head is 10mm.Ultrasonic vibration acts on the molten pool accurately,which enhances the effect of ultrasonic action on the molten pool and improves the utilization rate of energy.Secondly,orthogonal experiment was used to take the laser power,spot diameter and scanning speed as direct variables,and the laser energy density calculated from the three parameters as indirect variables.The microstructure,microhardness and wear resistance of Al₂O₃-ZrO₂(8%Y₂O₃)ceramic coating were used as evaluation indexes.The results show that too high scanning speed will cause serious spheroidization on both sides of the cladding path.Excessive laser power and spot diameter will increase the coating penetration and dilution rate.Under the conditions of moderate laser power and spot diameter and low scanning speed,the coating has compact microstructure,fine grains,fewer pores and cracks,high microhardness and good wear resistance.The optimal laser parameters are as follows:the laser power is 1.8KW,the scanning speed is3mm/s and the spot diameter is 4mm.Then,the ZrO₂(8%Y₂O₃)size and laser power in Al₂O₃-ZrO₂(8%Y₂O₃)powder mixture are changed.The effects of ZrO₂(8%Y₂O₃)size on the microstructure,microhardness,wear resistance and corrosion resistance of Al₂O₃-ZrO₂(8%Y₂O₃)ceramic coating were investigated under the condition of constant laser power.The results show that the absorption rate of small powder is high because of its large specific surface area.At the same laser power,the powder with with large specific surface area absorbs more energy,and the gas in the molten pool has enough time to escape.There are many nucleations in the coating,which mainly consist of fine dendritic eutectic structure.The microstructure is fine and compact,and the wear resistance and corrosion resistance are the best.The microhardness of the coating is as high as 1262HV,which is four times that of Ti-6Al-4V alloy substrate.Al₂O₃-ZrO₂(8%Y₂O₃)powders mixed with1-5?m ZrO₂(8%Y₂O₃)have the best ceramic coating quality.Finally,on the basis of better laser parameters and the size of the mixed powder,the external ultrasonic field is coupled.The effects of different ultrasonic amplitudes on the microstructure and properties of Al₂O₃-ZrO₂(8%Y₂O₃)ceramic coatings were investigated.The action mechanism of ultrasonic in molten pool is analyzed,and the feasibility of introducing no-load ultrasonic vibration into molten pool is expounded.The finite element simulation software COMSOL Multiphysics was used to simulate the flow rate of the melt in the laser cladding pool coupled with ultrasonic field.With the increase of ultrasonic amplitude,the effect of ultrasonic on the mechanical stirring and acoustic flow of the molten pool is enhanced,and the melt velocity is accelerated.The results of laser cladding experiments under different ultrasonic amplitudes show that the surface quality of the coating tends to be good at first and then becomes worse with the decrease of ultrasonic amplitudes.Ultrasonic amplitude has little effect on the variation of coating phase types.The brittleness of Al₂O₃-ZrO₂(8%Y₂O₃)ceramic coating is decreased and the toughness is enhanced by ultrasonic vibration.Under the action of ultrasonic cavitation and acoustic flow,the coating microstructure becomes finer after cladding.With the decrease of ultrasonic amplitude,the microstructure changed from coarse columnar crystal and cellular crystal to fine cellular crystal,columnar crystal disappeared,and the number of pores and cracks decreased.When the ultrasonic amplitude is 75?m,the average microhardness of the coating is as high as 1185.6HV,the minimum average friction coefficient is 0.31,and the friction coefficient curve fluctuation is the least.