Study On Microstructure And Performance Of Ti-based Ti₅Si₃ Layers Prepared By Argon Arc Cladding

With the development of scientific technology and domestic economy, the demand for materials properties, especially surface properties is higher and higher in many industries. The surface modification technologies of materials are continuously developed. Among them, preparation technologies of surface coatings/films are developed quickly from plating, chemical plating, surface diffusion, surface alloying to PVD and CVD film techniques, coating techniques and ion implantation techniques. And also two or more techniques can be combined to prepare all kinds of surface coatings/films which have strong bonding to substrates and are green or without pollution and with superior properties.Titanium is an important metallic material and has merits of high strength, good corrosion resistance and low density. As a new constructive material of good properties it is being applied widely. But application of Ti and its alloys in engineering is restricted because of their high friction coefficient and poor wear resistance. Therefore, to raise the wear resistance of Ti and its alloys has become the"hot-point"of domestic and international materials researchers.The crystal structure of Ti₅Si₃ is complicated hexagonal and it has many advantages, such as atom bonds combined metallic and covalent bonds together which lead to high hardness, high melting point (2130℃) but low density (4.32g/cm³). Furthermore, Ti₅Si₃ has the highest melting point and is the most stable in thermodynamics among the five TixSiy compounds. At high temperatures, its tensile strength, creep resistance and oxidation resistance are very excellent. So the Ti₅Si₃ has become the new universal structure material at high temperatures. It will probably be applied at high temperature service condition to about 1300℃.This thesis puts forward a new method to prepare Ti₅Si₃ surface layers on pure titanium (TA2) substrates by arc cladding. Using ordinary TIG welding machine, argon is used as shielding gas. The arc directly heats the pure Ti substrate surface to bring about the reaction of 5Ti + 3Si = Ti₅Si₃ between fused Ti and Si and to obtain the Ti₅Si₃ surface cladding layers during cooling. Arc currents, traveling speeds and proportions of different powders pre-covered on the substrates were changed during experiments. The microstructures and properties of the surface cladding layers and their regular patterns were analyzed and summarized by macro morphology surveying, phase components and microstructure analyses, microhardness measurement and abrasion testing.Microstructures of cross-section from top to melting line in the layers changed from coarse to fine dendrites. Microstructures at high temperature zone in HAZ were coarserα-Ti. XRD and SEM analyses confirmed that surface cladding layers consisted of mainly Ti₅Si₃ phases, less eutectic and solid-solution between Ti and Si.The surface cladding layers with different microstructures and properties were achieved by adjusting experimental parameters such as arc currents, traveling speeds and proportions of pre-covered powders. The currents changed from 60A to 160A. With the current increasing, the Ti₅Si₃ phases increased and the surface cladding thickness increased from 0.4mm to 1.7mm. The hardness and wear resistance were raised at the same time. When traveling speeds changed from 3mm/s to 8mm/s the layer thickness decreased from 2.4mm to 1.6mm. The morphologies of Ti₅Si₃ phases became finer. The layer hardness sharply decreased and its wear resistance showed the same trend. When the currents and traveling speeds were invariable and the proportions of pre-covered powders were changed, the amount of formed fine Ti₅Si₃ dendrites in the surface cladding layers increased with the content of Ni and Fe increasing. At the same time, the microhardness and wear resistance were raised.This technique can bring about the surface cladding layers with appropriate thickness which metallurgically bonding to substrates. It is easily realized as using simple apparatus and low cost of preparation. In one word, it is a new and convenient technique to prepare Ti₅Si₃ surface cladding layers and has a potential application in future.