| In order to enhance the working ability of titanium alloy in the high-temperature environment above 1000 ?,laser cladding can be used to form the excellent characteristics of metallurgical combination,and the Al2O3-ZrO2 ceramic coating can be formed metal/ceramic functional gradient coating by laser cladding,so as to meet the application requirements of titanium alloy in the high-temperature field.Ultrasonic has the functions of refining grain structure,homogenizing microstructure and element distribution.Ultrasonic wave is directly introduced into the micro-zone of molten pool to improve the quality and mechanical properties of Al2O3-ZrO2 ceramic coating.This paper analyzes the characteristics of the ultrasonic wave,including the cavitation,acoustic streaming effect and heat effect,and the ultrasonic cavitation effect is simulated.Ultrasound is applied at different angles to observe the effect of acoustic flow under the direct action of ultrasound.The ultrasonic assisted experimental platform is set.Through the COMSOL Multiphysics simulation software,the temperature model and the molten pool flow model are established.The change of the temperature field,flow field and the flow field with ultrasonic assisted is explored.The simulation results show that the maximum temperature and the area of the molten pool increases with the increase of laser power.The melting time of the molten pool is about 1s.The eddy current of the molten pool flow model is symmetrical without ultrasound.The velocity at the center point of the model's surface increased,decreased and then increased under the influence of the eddy current.After 0.8s,the velocity decreased.The distribution of sound intensity on the surface is uneven with ultrasonic assisted flow model of molten pool.The sound velocity of the cross section of the molten pool is irregularly distributed,and the flow velocity of the cross section of the molten pool is increased.Firstly,the effect of laser power on the microstructure evolution and mechanical properties of Al2O3-ZrO2 ceramic coating were investigated.The experimental results show that the depth and width of the cladding layer are related to the laser power.The holes and cracks is existed in the cladding layer,and the cracks in the cladding layer aremainly related to the residual stress.When the laser power is low,the cracks in the cladding layer are mainly related to the horizontal tensile stress.When the laser power increases to 1700 W,the grain structure changes from coarse.When the laser power is more than 1600 W,the element distribution in the cladding layer is relatively uniform,and the content of Ti element increases gradually from the top of the cladding layer to the substrate.When the laser power is 1700 W,the average microhardness of the cladding layer is the largest and the fluctuation is small.When the laser power is 1700 W,the friction coefficient is the smallest and the fluctuation is relatively small.The average friction coefficient is 0.337.The wear mechanism of cladding layer is the combination of abrasive wear,adhesive wear and plastic deformation.Then,the effect of ultrasound on the microstructure evolution and mechanical properties of Al2O3-ZrO2 ceramic coating was studied.The experimental results show that the surface roughness of cladding layer increases with ultrasound assisted,but the cracks of cladding layer are restrained.When laser power is lower than 1500 W,the width of cladding layer is larger with ultrasound assisted.The grain size is finer with ultrasound assisted.Especially when laser power is 1700 W,the grain size of is smallest and more uniform.The friction coefficient and fluctuation amplitude are small,and when the laser power is 1700 W the friction coefficient is the smallest and the fluctuation is relatively stable.The wear mechanism is the combination of abrasive wear,adhesive wear and plastic deformation.Finally,the effects of powder ratio,powder size,laser scanning speed and ultrasonic power on the Al2O3-ZrO2 ceramic coating were discussed.The optimal parameters of ultrasonic-assisted Al2O3-ZrO2 ceramic coating were obtained.The mass ratio of Al2O3 powder to ZrO2 powder is 60:40.The powder size is 40-106?m(Al2O3)and 1-5?m(ZrO2).The scanning speed is 4 mm/s,and the ultrasonic power is 150 W. |
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