COLD BED SMELTING OF TITANIUM ALLOY

 

1. Titanium alloys have been widely used in aerospace due to their high specific strength, good creep resistance and high temperature performance. However, with increasing dosage of titanium alloy, titanium alloy parts using conditions of increasingly demanding and its structure is complicated, titanium alloy and its parts damage and failure is unavoidable, which caused by the failure of the titanium alloy parts with fracture of catastrophic accidents have occurred more than. And compared with other materials such as high temperature alloy, alloy steel, titanium alloy in the process of metallurgy and parts produced in the process of manufacturing, and processing of defect of parts failure, especially the influence on the reliability of the product is great.

2. Development background of cold bed smelting
2.1 titanium application in aeroengine improve aeroengine in esteem than aircraft flight speed and flexibility is of great significance, and reduce the quality is the main way to increase the aeroengine in esteem than. This is supported not only by advanced aerodynamic design and structural design, but also by a large number of new materials and technologies. Titanium alloy has high specific strength and good corrosion resistance. It is one of the most widely used materials in aero-engine and plays an important role in improving aero-engine performance. In aero-engine, titanium alloy is mainly used for making disc, blade and casing of compressor and fan, instead of steel or high temperature alloy, which can significantly reduce the weight of engine.
2.2 common metallurgical defects of titanium alloys the common metallurgical defects of titanium alloys are component segregation and inclusion. Component segregation occurs during the smelting process, and there are gap element segregation, alloy element segregation, or both segregation. When alpha stable elements in segregation area (metal elements such as carbon, nitrogen, oxygen, or clearance) content is higher than that of matrix, often leading to higher hardness of alpha phase in the segregation area and the rise in the Numbers, even in some of the two-phase titanium alloys in single-phase alpha group. Among them, the gap element has the greatest influence on the performance of titanium alloy. Inclusion refers to the inclusion of titanium nitride, titanium oxide, tungsten, molybdenum, etc. in the alloy tissues due to the titanium sponge or the improper smelting process. These inclusions have high brittleness, which can promote the formation and development of cracks and increase the brittleness of alloys. Titanium nitride, titanium oxide inclusions is also called low density inclusion, hard alpha inclusions or Ⅰ defects, including TiN, the most damage. After conventional vacuum arc melting, it cannot be completely eliminated. In addition, the density of TiN is close to that of Ti matrix, and is in common with Ti matrix, which is not easy to be found in non-destructive testing. Titanium alloy of TiN, tungsten, molybdenum, and WC and so such as inclusions, also known as high density inclusion, because of its high melting point, itself is difficult to dissolve when conventional arc melting, and because of its high density, fast sinking into the liquid titanium low temperature of the molten pool area, in the ingot solidification and preserved.
3. Technical characteristics of cold bed smelting: vacuum arc smelting (VAR) has always been the main method of titanium alloy smelting. For aero-engine rotating parts with titanium alloy, in order to improve the uniformity of ingot composition and eliminate segregation as much as possible, generally three times VAR smelting is used. A large number of studies and practices have proved that VAR smelting has limited ability to eliminate high density inclusions and low density inclusions in titanium alloys.
3.1 working principle of cold bed smelting technology the melting process of cold bed furnace is divided into three areas: raw material melting zone, refining zone and solidification zone. After melting into liquid state, the raw material flows through the water-cooled furnace bed, that is, after refining, it enters the solidified crucible and solidifies into circular ingot or rectangular slab. As it flows through the refining zone, the high-density inclusions sink under gravity into the low-temperature condensate zone, where they are deposited and removed. The low-density inclusions rise to the surface of the molten pool and are heated at high temperatures and eliminated by dissolution. The middle density of the inclusion in the cold bed in the flow process, because of the complex flow field in the cold bed, so that it in the cold bed enough time to dissolve and eliminate. According to different heating methods, the cold bed furnace is divided into electron beam cold bed furnace and isonionic beam cold bed furnace. Electron beam as heating source, electron beam furnace under high voltage, electrons from the cathode and the anode accelerated after the electron beam formation, under the action of electromagnetic lens focusing and deflection magnetic field, bombarding the raw material, the electron kinetic energy into heat energy, melt the raw material, can melt all kinds of high melting point metal. The electron beam cold bed furnace is required to operate under high vacuum with a vacuum degree of 1 x 10-2 Pa. High vacuum can remove low melting volatile metals and impurities in titanium alloys and purify them. Isonionic beam furnace with isonionic beam as the heat source, isonionic beam and free arc are different, it is a compression arc, energy concentration, arc column slender. Compared with free arc, plasma beam has good stability, 200 mu of m200 microns cooling bed crucible ingot solidification shell material in the cooling bed heating heat source larger length and broad beam of scanning ability, so it has unique advantages in the field of smelting, casting. The plasma gun works in an inert atmosphere close to atmospheric pressure, preventing the volatilization of Al,Sn,Mn,Cr and other highly volatile elements.
3.2 advantages of cold bed smelting technology compared with vacuum self-consuming arc furnace, cold bed furnace smelting technology has many advantages. The arc furnace USES arc discharge to obtain heat melting material. In order to have enough heat to melt the material, the power must be large enough, so it is difficult to control the melting speed. The cold bed smelting technology adopts independent external heat source, and the feeding speed is completely controlled by the feeding system. Therefore, the temperature and melting speed of the molten pool can be controlled flexibly, which is beneficial to provide sufficient dissolution time for removing the inclusion in titanium alloy. Vacuum arc melting at the time of the molten pool temperature generally at about 1700 ℃, and the molten pool liquid shorter time, and makes it difficult to completely melt titanium oxide or nitride. And cooling bed melting liquid when the temperature of the molten pool in general is 1750 ℃ ~ 1800 ℃, the flexibility of controlling melting speed and temperature of the molten pool. In addition, the vacuum arc furnace feed way is single, need to press electrode block, welding electrode. The electrode shall be welded again during the second and third melting of the first smelting. Increased production costs and the possibility of inclusion. Cold bed melting can be used to press electrode or do not need to press electrode residual materials, such as recycled materials. As required, cold bed melting technology can produce different ingot sections, such as circles, squares and rectangles. It can significantly reduce the production cost and increase the rate of finished products. Cold bed smelting adopts the process of smelting, refining and solidifying at the same time. It can arrange continuous feeding of multiple feeding systems, so large size ingots can be produced.
4. The application of cold bed smelting technology has developed from material recovery at the beginning to irreplaceable advanced smelting technology for producing high-quality ingots of titanium alloy for aviation rotating parts.