| Titanium alloy is widely used in aerospace,mechanical manufacturing and other fields because of its low density,small specific gravity,high temperature resistance,non-magnetic and weldability.However,its low surface hardness,poor high temperature oxidation resistance and wear resistance limit its application.The preparation of functional protective coating on titanium alloy surface by surface modification technology is an important means to improve the surface properties of titanium alloy.Compared with titanium alloy,ternary layered metal ceramic material has excellent properties of both metal and ceramic phases,which is a protective coating material with great application potential.In this paper,based on TiAl+TiN binary composite powder,the ratio of powder system was optimized,and Ti-Al-N Max phase composite coating was prepared on the surface of TC4 titanium alloy by laser cladding,in order to improve the surface properties of TC4 alloy.The phase composition and microstructure characteristics of the coating were analyzed by X-ray diffraction,scanning electron microscopy and energy spectrum,and carried out the hardness test,high temperature cyclic oxidation test and friction and wear test.The main research contents and conclusions of this paper are as follows:(1)Exploring the effects of different cladding process parameters on the quality of the coating,the laser power is 2100?2400W,the scanning speed is 150?210mm/min,the surface quality is good to obtain a well-quality molten coating.The coating and substrate exhibit good metallurgy binding,no porous hole and crack.The coating is composed of TiAl+Ti3Al two-phase substrate,Ti2Al N MAX phase and TiN hard strengthening phase.The average hardness of the coating is more than 670HV,which is more than 2 times higher than that of the substrate.Considering the influence of Al loss on the microstructure and properties of the coating,adding a small amount of Al powder can promote the reaction of TiN and TiAl in the molten pool,thus significantly increasing the content of Ti2Al N MAX phase in the coating.(2)The effects of different heat treatment temperatures(700?1000?)on coating microstructure and hardness distribution were studied.After heat treatment,the composite coating is composed of TiAl matrix(TiAl and Ti3Al two-phase structure),Ti2Al N MAX phase,Ti3Al N and TiN ceramics,and the Ti2Al N MAX phase content of the coating increases obviously at 700?.The hardness of the coating is significantly improved after heat treatment.The average hardness of the coating is about 925.02HV after holding at1000?for 1h,which is more than 2.8 times that of TC4 matrix and 37.3%higher than that of the coating without heat treatment.(3)The cyclic oxidation resistance of the substrate and the composite coating at600?1000?were compared.Under the same conditions,the composite coating showed better oxidation performance,and no cracking and spalling of the oxide film similar to that in the TC4 substrate was found.The oxidation weight gain of the composite coating is small and basically reaches a stable value in a short time.At 1000?,the oxidation weight gain is only 128.31mg/cm2,which is only 76.91%of the weight gain of the substrate.The combination of the oxide film and the coating surface is good.When the composite coating is cyclically oxidized at 600?and 800?,it mainly generates Ti O2oxide film and contains a very small amount of Al2O3.At 900?,the oxidation product of the coating is Ti O2+Al2O3mixed oxide,while at 1000?,the oxidation product is Ti O2and Al2O3mixed oxide layer,but mainly Al2O3.(4)The friction and wear behavior of the composite coatings in wide temperature range and the high temperature friction and wear behavior of the composite coatings with different components were studied.The wear resistance of the composite coatings was better than that of the matrix under the same conditions.At room temperature?400?,with the increase of temperature,the friction and wear surface of the composite coating,which is dominated by furrow,gradually begins to be dominated by oxide mechanical mixture layer,and the wear mechanism is transformed from abrasive wear to abrasive wear due to matrix softening and oxidation products.At high temperature(600?800?),the wear rate of the composite coating is lower than that of TC4 matrix.At 800?,the wear rate is only0.25×10-5mm3?N-1?m-1,and the wear mechanism is a mixture of abrasive wear and oxidative wear.The composite coating of different components shows excellent wear resistance at high temperature,among which the friction coefficient of C3(n(TiAl):n(TiN):n(Al)=1:1:0.1)is 0.2091,and the wear rate is 0.25×10-5mm3?N-1?m-1,the best wear resistance. |
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