Double Glow Plasma Surface Alloying Burn-resistant Titanium Alloy

Conventional titanium alloy may be ignited and burnt under high temperature, high pressure and high velocity gas flow condition, called titanium fire. Titanium fire limits application of titanium alloy in some work condition. For safety, some titanium parts in currently used aviation engine like lamina of high-pressure compressor are replaced by nickel based alloy parts over again in the end of 70s. in America and Great Britain. Titanium alloy are strictly prohibited to use in high-temperature parts of civil aviation engine in Russia. In 1970s, America and Russia began to study burn-resistant titanium alloy. Some primary research have been done in china.In 1990s, Ti-Cr-V series, Ti-Cu-Al series and Ti-Nb series burn-resistant titanium alloys have been developed. In these burn-resistant titanium alloys, representative alloys are Alloy C (Ti-35V-15Cr)and Ti-45Nb made in America, BTT-1(Ti-13Cu-4Al-4Mo-2Zr) and BTT-3 (Ti-18Cu-2Al-2Mo) made in Russia and Ti-40 (Ti-25V-15Cr-Si) and Til4(Ti-13Cu-1Al) made in china. In Ti-Cr-V series burn-resistant titanium alloy, large amount of element Cr and V reduces melting point and increases heat conductive rate evidently. before burning surface of the alloy may intenerate or melt, large amount of heat are absorbed, rigidity friction are avoided and friction heat decreases. In Ti-Cu-Al series burn-resistant titanium alloy, alloy element Cu and Al reduces melting point. In friction condition, liquid phase separates out ,friction coefficient decreases, large amount of heat are absorbed. In all the alloying elements of titanium, Nb has least oxidation heat, so Ti-Nb series burn-resistant titanium alloy has been producted Ti-Nb series alloys also have excellent corrosion-resistant property.Though some achievements have been taken, burn-resistant titaniumalloy is far to maturity. Either in burn-resistant mechanism study or in product development we have more to do. Now projecting problems are:1. difficult to fuse, poor process property.2. large amount of alloying elements reduce alloy's body mechanical properties, even lose the property of high ratio strengthen.3. due to large number of expensive alloying elements, cost of burn-resistant titanium alloy is higher.For solving these problems, we bring up the new idea of surface burn-resistant titanium alloy, i.e. forming burn-resistant alloying layer on the surface of conventional titanium alloy by surface alloying technology, keep the body mechanical properties. Elements with burn-resistant effect like Cu, Cr, Mo etc. have high solubility in titanium, so surface alloying is possible. Compare with body burn-resistant titanium alloy, surface burn-resistant titanium alloy have following characteristics:1. surface alloying is performed in parts made by conventional titanium alloy after molding and processing, avoid complex preparation and poor process property of body burn-resistant titanium alloy.2. burn-resistant function is fulfilled by alloying layer, mechanical properties are endured by substrate titanium alloy, so characteristic of high ratio strengthen are kept.3. thickness of alloying layer is 50-300 micron, amount of alloying element is little, so cost of burn-resistant alloy is reduced evidently.Firstly, burn-resistant mechanism is studied from three aspects: absorption and diffusion of oxygen, thermomechanics analysis and tribology. Results and other documents indicate that reasonable surface burn-resistant alloy system are: Ti-Cu(with content reaches to 10-15%), Ti-Cr(with Cr content reaches to 10-20%) and Ti-Mo.Surface alloying is performed by double glow plasma surface alloying technology. The principle of this technology is adding a source electrode made by the desired alloying elements which are introduced into the surface of the work piece in plasma nitriding device. Two high voltage DC power supplies are used for generation of glow discharges. One is connected between anode and cathode and another between anode and source electrode. The tank is evacuated to a pressure of about 5X10-2 Pa and then filled with argon to 20-50 Pa. With the two power supplies turned on both cathode andsource electrode are surrounded by glow discharge. One glow discharge heats the work piece and second glow strikes the source electrode. The desired alloying elements sputtered from the source electrode by bombardment of the ions produced in second glow discharge travel and diffuse into the surface of the work piece to form a surface alloy layer on the surface of work piece. The depth and component of the surface alloy layer may adjust by technology parameters including pressure of argon gas, voltage of the cathode, voltage of the source electrode, distance between the substrate and the source electrode, and processing temperature.The work take TA2,TC4 and TC11 as substrate materials ,Cu, Cr and Mo as alloying elements to perform surface alloying process. As representative, TC4 copperizing and TC11 Chromizing technology are studied. Effects of technological parameters like source cathode voltage, work piece voltage, distance between source cathode and workpiece, process temperature, gas pressure, process duration etc. on the thickness and component are studied. Results indicates that reasonable technological condition of TC4 copperizing is: process air pressure is 25Pa, voltage of source cathode is 800V, voltage of workpiece is 350V, distance between source cathode and workpiece is 16mm, process temperature is 870 °C, process duration is 3.5h and reasonable technological condition of TCI 1 chromozing is : process air pressure is 25Pa, voltage of source cathode is 700V, voltage of workpiece is 350V, distance between source cathode and workpiece is 18mm, process temperature is 900 °C, process duration is above 2h.Temperature field in double glow discharge zone is measured using CCD technology. The results indicates that temperature field in double glow discharge zone is even on the whole. It supports the uniformity of thickness and component of alloyed layer, border-corner effect exists near the cathodes.Surface morphology of TA2 after plasma surface alloying is studied. Results indicates that in the initial stage of chromizing, surface of TA2 becomes rough. Following process duration enlarge increasing, surface morphology is smooth gradually and fractal dimension of surface profile is keep between 1.1 -1.2.After 3.5h double glow plasma copperizing, thickness of alloying layeron the surface of TC4 reaches to 200 u m, Cu content reaches to 14% on the outer surface. Cu content keep above 12% in the distance 160 u m from surface. Cu content keep 20% in range 15 u m on the surface alloying layer of TCI 1 after 3.5h copperizing process. Microstructure of alloyed layer is substrate microstructure plus diffused Ti2Cu intermetallic. Component and microstructure of Ti-Cu alloying layer are similar to BTT-1 burn-resistant titanium alloy made in Russia.After double glow plasma chromizing, Cr content in alloying layer on the surface of TC4 and TCI 1 keep 20% in range of 50 u m. Microstructure of alloyed layer is substrate microstructure plus diffused Cr2Ti intermetallic. most Cr element solutions in ￡ phase. Component and thickness of Ti-Cr alloying layer meet the demand of design.Thickness of Ti-Mo alloying layer reaches 40 U m, Mo content reaches to 100% on the outer surface and reduce gradually along case depth. Microstructure of alloying layer is similar to that in substrate, Mo element is solutions in substrate.Thickness of Ti-Cr-Mo alloying layer reaches 100 u m, Cr content reaches to 13% and Mo content reaches to 3% on the outer surface. Microstructure of alloying layer is substrate microstructure plus little Cr2Ti intermetallic.Ball-plate wear tests reveal that ratio wear rate of titanium after surface double glow plasma copperizing, molybdenizing and chromizing is decreased markedly. After molybdenizing, ratio wear rate is 1/300 of substrate's, 1/50 after copperizing and 1/5 after chromizing. Friction coefficient decreases after molybdenizing and copperizing, where with a half decrease after copperizing.High temperature oxidation tests indicate that alloying element Cr and Mo increase the oxidation resistance whereas Cu is no contribution to oxidation resistance.Primary bun-resistance testes indicate that Ti-Cu, Ti-Cr, Ti-Mo and Ti-Cr-Mo alloyed layer all have burn-resistant properties.Scratch tests show that conjunction of alloying layer and substrate is good.The double glow plasma surface alloying is a process of solute element absorbed on the surface penetrates into the substrate via hot diffusion. Oneof the specifications is quick diffusion speed, the study indicates that the double glow plasma surface alloying process may be described by half-infinity diffusion model in condition with deposition layer. In condition without deposition layer, with high alloying element content and element content in many phase zones, it may be described through Wagner's multi-layer model. In condition without deposition layer, with low alloying element content and content in only one phase zone, it may be described through Wagner's two layers model mathematically.In ion bombardment condition, high vacancy concentration and high vacancy concentration grads are formed on the surface, high vacancy concentration reduces the diffusion activation energy, increase diffusion coefficient. High vacancy concentration grads lets metal atom stand a incurrent stress, so diffusion coefficient of double glow plasma surface alloying is more higher than conventional metal-penetrating.Effects of super-cell induced by ion bombardment on the diffusion of alloying element are studied by molecular dynamics simulation. Results indicate that diffusion coefficient of solute element in surface super-cell is far higher than average coefficient in allying layer.