| Titanium alloys have the advantages of specific strength,low density,excellent oxidation resistance and good biocompatibility,and have been widely used in aerospace,automotive transportation,biomedical and other fields.However,the poor wear resistance of titanium alloys severely limits their application in the field of turbine blades.In this paper,CeO2/Ni60A coatings were prepared on the surface of Ti6Al4V by laser cladding.The interaction mechanism between laser and powder and the thermodynamic behavior of the molten pool were analyzed;The influences of different CeO2 content on the microstructure,wear resistance and corrosion resistance of Ni60A coating were investigated.The mapping relationships between different process parameter combinations and coatings forming quality were pointed out,and the influences of process parameters on coating solidification behavior were analyzed.The process parameters were optimized based on multi-objective genetic algorithm,and the optimized parameters were applied to strengthen the turbine blades to improve the surface performance of the blades.The main work was as follows:（1）Study on temperature field distribution and solidification characteristics in laser cladding.Based on the thermodynamic theory,the flow characteristics and solidification behaviors of the molten pool of CeO2/Ni60A were analyzed by numerical simulation,and the changes of temperature field,temperature gradient field and flow velocity field of the coating were investigated.The results showed that the metal fluid was driven by the Marangoni effect.From the bottom to the top of the molten pool,the temperature gradient and shape control factor were decreased,the solidification rate and cooling rate were increased.The solidification track showed that the plane crystal at the bottom gradually evolved into cellular crystal,columnar crystal and equiaxed crystal at the top,and the grain size was decreased.The transition from columnar crystal to equiaxed crystal occurred in the middle area of the coating.（2）Effect of CeO2 on microstructure and properties of Ni60A coating.The CeO2/Ni60A coatings with five proportions of 0 wt.%～4 wt.%CeO2 were successfully prepared on TC4surface by laser cladding.The microstructure and composition of the coating with different CeO2 content were investigated by SEM and XRD,and the influences of CeO2 on the wear resistance and corrosion resistance of the coating were analyzed to reveal the correlation between microstructure and performance.The research showed that CeO2 effectively improved the laser absorption of powder,increased the width and depth of the molten pool,improved the thermophysical properties difference between the coating and the substrate,and reduced the initiation probability of cracks and pores in the coatings.When the mass percentage of CeO2 was 3 wt.%,the coating had no cracks and pores,good forming quality with the lowest wear rate of 4.10×10-6 mm3·N-1·m-1,the highest average microhardness of1035.61 HV0.3,the lowest self corrosion current density of 2.110×10-7 A·cm-2,which were attributed to the dense passivation films such as Cr2O3,Al2O3 and Ti O2 generated by fine grain strengthening.（3）CeO2 strengthening mechanism and precipitation behavior of Ti C-Ti B2 eutectic structure.Based on the XRD diffraction pattern,the lattice distortion caused by the deviation of the diffraction peak of the coatings with different CeO2 content was analyzed.The width of the primary dendrite arm in the coating was measured,the influence of CeO2 on the grain size was explored,and the dependence of Ti C-Ti B2 heterogeneous nucleation was explored based on two-dimensional lattice mismatch theory.The results showed that CeO2 was easy to cause lattice distortion in the coating.The lattice distortion of 3C coating was the largest,with the size of Ti C microcrystals of 8.15 nm,but the size of Ti C microcrystals without CeO2 coating was 15.14 nm.The CeO2 content affected the size of hard phase Ti B2 and the width of primary dendrite arm of intermetallic compounds Ti2Ni and Ti Ni.The average size of Ti B2was decreased from the maximun 11.95μm to the minimun 4.76μm,and the primary dendrite arm spacing was decreased from the maximun 2.61μm to the minimum 1.71μm.The effect of crystal refining was remarkable.Based on two-dimensional lattice mismatch theory,the lattice mismatch ratio between（0001）Ti B 2 and（111）Ti C was 1.08%,which was highly interdependent and easy to form Ti B2 and Ti C eutectic.（4）Influence mechanitsm of laser power and scanning speed on forming quality,microstructure and properties of coatings.The experiments of different laser power and scanning speed were designed to analyze the influence of different laser power and scanning speed on the microstructure,wear resistance and corrosion resistance of the coatings.The results showed that when the laser power was 2100 W,the wear rate of CeO2/Ni60A coating was reduced by 90.89%compared with the substrate,and when the scanning speed was12 mm·s-1,the wear rate was reduced by 91.37%compared with the substrate.With the increase of laser power,the corrosion resistance of the coatings first was increased and then was decreased,the scanning speed also was the same trend.At the power of 2100 W and scanning speed of 12 mm·s-1,the wear mechanism of the coating was mainly slight groove wear,and the wear resistance had been greatly improved.With the increase of laser power,the corrosion resistance of the coating showed the trend of decreased after the first increased,and it also showed the same trend under different scanning speeds.When the laser power was2100 W and the speed was 12 mm·s-1,the prepared passive film on the coating surface was more compact.After the passive film was broken down by the applied voltage,the self repair ability of the passive film on the coating surface was stronger,and the corrosion resistance was better.The corrosion efficiency was 81.05%.（5）Study on the influence mechanism of process parameters on the microhardness,wear resistance and corrosion resistance of coatings.Based on response surface method,with laser power,scanning speed and the rare earth oxide CeO2 content as dependent variables,response surface model of average microhardness,wear rate and self corrosion current density was established to analyze the influence law of single factor and multi-factor interaction on coating properties.The results showed that the laser power was preferably 2132 W～2392 W,the scanning speed was 8 mm·s-1～16 mm·s-1,and the content of CeO2 was 3.1 wt.%～4.0 wt.%.Under the optimized process parameters,the performance index ranges were:average microhardness was 912.29 HV0.3～1085.36 HV0.3,wear rate was 3.15×10-6 mm3·N-1·m-1～7.74×10-6 mm3·N-1·m-1,self corrosion current density was 1.707×10-7 A·cm-2～2.716×10-7A·cm-2.The test verified that the error between the optimization target of CeO2/Ni60A coatings and the test detection was not more than 5%,and the Pareto optimization model was accurate and reliable.（6）Research on lapping planning of multi lapping coatings for turbine blades.Based on the mathematical model of the quantitative relationship between the characteristic parameters of the single channel coating and the laser power,scanning speed and the mass fraction of rare earth oxide CeO2,the wear resistance and corrosion resistance of the coatings at different overlapping rates were analyzed.The results showed that the optimal overlapping rates was50%.The equal arc length scanning mode and the equal interval scanning mode were used for multi-channel lapping of surfaces.The test results showed that the equal arc length scanning had better forming quality,the wear rate was reduced by 90.17%,the self corrosion current density was 1.866×10-7 A·cm-2,and the corrosion efficiency was 81.13%.All performance indexes were significantly improved,which had a strong guiding significance for practical machining. |
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