Ni And Cu-based Ti₅Si₃ Coatings Prepared By SHS

Ti and Ti alloys have been applied widely in the aerospace industry, chemicals sector, civilian, medical and other fields for the advantages of low density, high specific strength, corrosion resistance and high temperature performance. Ti and Ti alloys can be used by the range of extreme low-temperature to 600℃, the specific strength keeps are better than other structural materials within the entire service temperatures. However, the development of Ti alloy casting process is behind of the press-working processing, wrought Ti alloys are usually chosen to test by adding different alloying elements to improve the performance currently.In Ti-Si eutectic alloy, Ti₅Si₃ phase is refractory intermetallic compound and it is suitable for usage in the surface of high-temperature working parts because of its good thermal, corrosion and oxidation resistance. Then the part surface properties can be improved and the costs can be greatly reduced as well. However, Ti₅Si₃ has not been widely used. The main reason is the bad room temperature ductility. The impact resistance is so weak that the prevailing solution is adding stable elements to improve its plasticity. Ni and Cu both belong toβstable elements which can reduce the transition temperature of Ti with same element but different crystal. Ni and Cu react with Ti to produce plastic Ti-Cu and Ti-Ni alloys wrapping the brittle Ti₅Si₃ phase and together with grain refinement effect to improve the overall alloy plasticity.Ti₅Si₃ can be prepared by many ways, such as laser cladding, pressure sintering, self-propagating and thermal spraying. The self-propagating technology is also called SHS which is a new technique to synthetize materials or products by keeping reaction from the self-generated-heat of the chemical reaction. There are a variety of SHS surface techniques such as casting surface layer technology by combining SHS with traditional processing, SHS sintering surface layer technology, SHS spraying deposition surface layer technology and self-reactive surface layer technology. Among them, SHS sintering surface layer technique prepares a layer of raw materials by slurry spraying on the substrate, artificial brushing or cold-pressed on the substrate and then they are put in hot-pressing furnace or chemical furnace or other sintering equipment to ignite the self-propagating reaction and sinter for a certain time. A large amount of reaction heat makes partial metal matrix surface melted and metallurgical combined with reactant to get the good surface layer combined with substrate.In order to improve the ductility at room temperature and the bonding condition to the substrate Cu and Ni-based Ti₅Si₃ surface layers were prepared in this thesis by SHS on the Ti alloy (TC4) surface. The macroscopic morphologies, microstructures and microhardness of the Ti₅Si₃ surface layers with the Ni contents from 10% to 50% and Cu content from 10% to 70% are observed and analyzed respectively.The results show that it is feasible to prepare Cu and Ni-based Ti₅Si₃ surface layers on titanium (TC4) substrate by SHS. But the experimental results are unexpected with the powders mixed by only Ti and Si as atom ratio of 5:1. It brings about reaction products ofα-Ti + Ti₅Si₃ but badly covering and bonding to the substrate. The thickness of the surface layers is uneven. And there are a mass of defects in microstructures, such as pores. When Cu or Ni powders were mixed in the Ti+Si powders, the surface layers are consisted of Ti₅Si₃ and TiCu or TiNi intermetallic compounds and solid solution. The microhardness and performance of the surface layers are improved obviously. But there are a mass of splash and less products left as the contents of Cu in the powders are 10%-50%. Metallographic samples of the surface layers can not be prepared because of the abruption away from the substrates. The SHS process can be observed easily when the contents of Cu are up to 60%-70%. After SHS process, liquid keeps for 2-3s. In this situation not only the extension of surface layers is improved, but also the solidification process tends to equilibrium. The microstructures inside layers are finer and the Ti₅Si₃ phase distribution in Cu and Cu–Ti alloys at the bottom and middle zone. Microhardness of the surface layers with 60% and 70% Cu is nearly the same and the highest is up to 1216HV which is over four times of microhardness of the Ti alloy substrate.The macroscopic patterns of surface layers are improved obviously by adding Ni to the powders. But when the content of Ni in powders is 60%, the surface layer can not be prepared, owing to the powder block flying away from the substrate to quartz cover by the magnetic field from the electric winding.Among them, the surface layer prepared by the powder with 40%Ni gave the best macroscopic pattern and the structures were beneficial to improve plasticity of the surface layer. Microhardness in the middle part of surface layer is the highest and is a little reduction in the top part. And microhardness reduces when increasing Ni content. The sample with 30%Ni gave the highest microhardness of 1540HV in the middle part which is the six times of microhardness of the titanium alloy substrate. Processing parameters, especially heating current, have some effect on the surface layers. Increasing heating current can increase the preheating temperature to improve the metallurgical bonding between the layer and the substrate. Higher heating temperature makes liquid keep for longer time and improve extension of surface layer on the substrate and uniform of microstructures. There is a little effect of heating currents on microhardness of surface layers. The microhardness of surface layers change in a low degree when high current is applied that means the achieved microstructures of the surface layers are comparatively uniform.