Research On Fabrication,Microstructure Controlling And Properties Of Mechanically Alloyed Composite Coatings On TC4 Alloy

Mechanical alloying?MA?,being a solid-state powder processing technology,has been shown to be capable of fabricating alloys,compouds,composites,nanomaterials and non-equilibrium materials.MA also can be used to synthesize coatings on the surfaces of materials with powder mixtures.Fabrication of coatings by MA is a solid-state coating fabricating method.The advantages of this method include the low time and energy consumption,the flexibility of preparing coatings with different structures and the capability of preparing coatings with the above mentioned structures.Especially,this method is suitable for fabricating coatings on the inner walls of tubular specimens.Based on these,it is assumed that MA method has great potential to be used to fabricate coatings.However,there are very limited investigations on the fabrication of coatings using MA method by now.The interaction between powder particles and bulk materials during MA has not been clearified.In this work,we try to synthesize coatings on the TC4 alloy substrate in order to improve its wear and oxidation resistances.On the one hand,the investigations on the fabrication coatings by MA method can be enriched and the purpose for providing theoretical and practical experiences to the synthesis coatings by MA can be realized.On the other hand,we attempt to introduce a new flexible method to conduct the surface modification on titanium alloys to improve its applications at both normal and elevated temperature.In the present work,MA method was applied to synthesize composite coatings on TC4 alloy substrate with powder mixtures of alloy systems such as Al-Cr and Al-Si,as well as carbide particle?B₄C,TiC and SiC?reinforced Al matrix composites systems,respectively.The results of the investigations on the Al-Cr and Al-Si coatings showed that the increase of Al fraction in the raw powder mixture favored the improvement of coating densities.The thickness of the coatings increased with the raise of milling time,but too much milling time led to the flaking of coatings.The thickness and roughness of the coatings increased with improvement of rotational speed.The surface roughness of the coatings could be effectively reduced by friction stir processing.The use of original brittle powder with small particle sizes could improve the homogeneity of the coatings.The annealing treatment was conductive to the improvement of stress state in the coating and the release of coating residual stress.The microdefects would form in the Al-Cr coating during annealing at high temperatures,which was unfavorable to the improvement of coating properties.The Al-Si coating could form thick and continuous alloy layers through the interdiffusion between the coating and the substrate during annealing at high temperatures.The Al-Cr and the Al-Si coatings could improve the surface properties of the TC4 alloy substate and the annealing treatment to the coatings at 800°C could further improve their wear and oxidation resistances.Based on the results of the microstructures and the properties of the Al-Cr and Al-Si coatings,a mono-layer Al-Cr-Si coating and a bi-layer Al-Cr/Al-Si coating were respectively fabricated by MA.Their microstructures and properties were analyzed and compared.The relationship between the microstructure and properties of the bi-layer coating and the gradient coating process was initially clarified.Both the mono-layer Al-Cr-Si coating and the bi-layer Al-Cr/Al-Si coating could improve the surface microhardness of the substrate.The mono-layer coating had higher surface hardness and the bi-layer coating had more stable hardness distribution.The bi-layer coating had better oxidation resistance and the mono-layer coating had better wear resistance.The results of the investigations on the carbide particle?B₄C,TiC and SiC?reinforced Al matrix composite coatings indicated that both the thickness and the uniformity of the coatings were increased with the decrease of raw particle size.Continuous Al₃Ti layers could be obtained in the Al-B₄C,Al-TiC and Al-SiC coatings after annealing treatment.An outer layer with dense ceramic/metal composite structure could form in the Al-B₄C coating,but loose structures formed in both Al-TiC and Al-SiC coatings during the annealing processes.The Al-B₄C coating could improve both the mechanical properties and the high-temperature oxidation resistance of the substrate.The Al-TiC coating could protect the substrate from oxidation at elevated temperature.The Al-SiC coating could improve the surface mechanical properties of the substrate.The formation mechanism of the coating during MA was also investigated.The formation of coatings during MA was a simultaneasly coating deposition and flaking process.On the one hand,the ductile particles gradually adhered to the bulk material surface and got cold welded to it under reapeated ball impacts during MA;On the other hand,the deposited coating were peeled off by the shearing and friction effects of grinding balls.The formation of particle reinforced Al matix composite coatings on TC4 alloy substrate mainly included four stages,namely slow deposition stage,rapid deposition stage,equilibrium stage and flaking stage.