Defects of Titanium Alloys

Titanium alloys are widely used in important load-bearing components of aircraft and aircraft engines because of their high specific strength, good medium temperature performance, corrosion resistance, and good welding performance, and are an important metal structural material. According to statistics, the weight ratio of titanium alloys used in foreign aircraft has reached about 30%, which shows that the application of titanium alloys in the aviation industry has a broad future.

However, titanium alloy flange also have some disadvantages, such as large deformation resistance, poor thermal conductivity, large notch sensitivity (about 1.5), and changes in microstructure have a significant impact on mechanical properties, which leads to smelting, forging and heat treatment. Complexity, prone to defects in titanium alloy forgings.

Introduction of various new processes for titanium alloy processing

A few days ago, the technical project of "production process of titanium heat exchanger pipe" passed the evaluation of scientific and technological achievements. This technical process is mainly improved for the traditional hot rolling process of seamless steel pipes, and transplanted to the production of titanium alloy seamless pipes, which is similar to the traditional "extrusion forming, bar drilling and boring and cross-rolling and piercing followed by cold rolling and cold drawing" process. The finished product rate of pipes is significantly improved, reaching 97%.

Through the characteristics of titanium alloy tubes, the project has made targeted improvements in the production process. The addition of thermal insulation tunnels and rapid transfer devices to the power of the main motor is innovative to a certain extent. Large titanium alloy tubes up to 12m in length.

 The cutting of titanium and titanium alloy pipes should be carried out mechanically, and the cutting speed should be low; when cutting or grinding Titanium Alloy Seamless Rectangular Pipe with grinding wheels, special grinding wheels should be used; flame cutting should not be used. The groove should be processed mechanically. Titanium alloy processing and welding should use inert gas shielded welding or vacuum welding, not oxygen-acetylene welding or carbon dioxide gas shielded welding, nor ordinary manual arc welding. When installing titanium and titanium alloy pipes, iron tools and materials should not be used to knock and squeeze; between carbon steel supports, hangers and titanium and titanium alloy pipes, rubber sheets or soft plastic sheets should be placed to prevent Titanium and titanium alloy pipes are in direct contact.

 When the titanium and titanium alloy pipes pass through the wall and floor, the casing should be installed, the gap should not be less than 10mm, and the insulation should be filled, and the insulation should not contain iron impurities. Titanium alloy pipes should not be directly welded to other metal pipes. When a connection is required, a looper flange connection can be used. The non-metallic gaskets used are generally rubber gaskets or plastic gaskets, and the chloride ion content should not exceed 25ppm.

How to weld copper and titanium plates?

The lattice type, melting point, thermal conductivity, coefficient of linear expansion and chemical composition of Titanium Clad Copper Plates are very different, so it is very difficult to weld.

 1) The weld is easy to form pores

(1) Copper and titanium have strong hydrogen absorption capacity at high temperature, and hydrogen has a large solubility in liquid copper and titanium.

(2) Gas is generated in the molten pool of pyrometallurgical reaction.

(3) Oxygen and nitrogen gases around the welding zone are immersed in the molten pool.

2) Welded joints tend to have large cracks

When copper and titanium are welded, eutectic and hydride can be formed on the metal side of the two base metals, and cracks are easily generated under the action of welding stress.

(1) Copper and bismuth form a (Cu+Bi) eutectic with a eutectic point of 270°C.

(2) Copper and aluminum form a (Cu+Pb) eutectic with a eutectic point of 326 °C.

(3) Copper and ferrous sulfide form a eutectic (Cu+Cu2O) with a eutectic point of 1067°C.

(4) Sheet-like hydride TiH2 is formed on the metal side of the titanium base material, resulting in hydrogen embrittlement.

(5) The coefficient of linear expansion of copper and titanium is more than 1 times different, and greater stress will be generated during welding.

 3) Low mechanical properties of welded joints

(1) The oxide film can weaken the intergranular bond between copper and titanium. For example, when the oxygen content in the weld reaches 0.38%, the bending angle of the joint decreases from 180° to 120°.

(2) A large amount of eutectic and hydride significantly reduces the plasticity and toughness of welded joints.

(3) The mutual solubility of copper and titanium is very small, and it is easy to form intermetallic compounds at high temperatures. Such as Ti2Cu, TiCu, Ti3Cu4, Ti2Cu3, TiCu2, TiCu4, increase brittleness, reduce plasticity, and significantly reduce the corrosion resistance of weld metal.

Copper and titanium or titanium alloys can obtain excellent welded joints by vacuum diffusion welding, argon arc welding, plasma arc welding, brazing and electron beam welding.

After the titanium base metal (TA2) was cleaned with trichloroethylene, it was etched in an aqueous solution with a volume fraction of 2% HF and a volume fraction of 50% HNO3 by vibration for 4 minutes to remove the oxide film, and then cleaned with water and alcohol .

(4) Assemble the cleaned two base metals according to the process requirements, and then put them into a vacuum furnace for welding. The welding parameters are: the welding temperature is 810℃±10℃, the pressure is 5~10MPa, the time is 10min, and the vacuum degree is 1.3332×10-8~ 1.3332×10-9MPa. An intermediate diffusion layer can be added between the two base metals. Usually, the material of the diffusion layer is niobium metal, or no intermediate diffusion layer is required. After welding, carefully clean the joint surface.

If argon arc welding is used to weld copper and titanium, the selection of cerium tungsten electrodes can improve the welding quality and benefit human health. For example, when welding copper alloy (QCr0.5) and titanium alloy (TC2), niobium can be used as the transition layer material, and the purity of argon gas is 99.8% to obtain high-quality joints.

Machining of titanium alloy structural parts

The processing difficulties of titanium alloy structural parts and the influencing factors of processing deformation of weakly rigid structures are presented. The control method of processing deformation of weakly rigid structural parts is proposed from the selection of machine tools, tool selection, and effective cooling. Titanium alloy materials have excellent properties such as lightweight, high strength, and high-temperature resistance. For example, using TC18 titanium alloy instead of high-strength structural steel for landing gear can reduce the weight of the aircraft structure by about 15%, so it is the main bearing of advanced foreign aircraft. A large number of new high-strength titanium alloys are used in the force parts. For example, Titanium Alloy Threaded Rod accounts for about 21% of the body structure materials of the American B-1 bomber; the titanium content of the Russian Il-76 aircraft reaches 12.5% ​​of the body structure weight. , From the development trend, the use of titanium alloys in Europe and the United States is gradually increasing, and it also shows that a large number of titanium alloys are used, especially some new titanium alloys have become the development direction of aviation design. However, most aerospace products use thin-walled parts, which are relatively complex in structure and require high precision. Due to the thin wall, the rigidity of the parts is poor. Under the action of cutting force, it is easy to produce bending deformation during processing, and the wall thickness is inconsistent up and down, resulting in out of tolerance. . At present, the commonly used method in enterprises is repeated milling in finishing. Due to the small thermal conductivity of titanium alloy, low elastic modulus (about 1/2 of steel) and high chemical activity, the small allowance cannot be milled at all, often The phenomenon of "less cutting" occurs. In order to ensure the size of the parts, it can only be polished by hand, which greatly increases the processing cycle of the parts, and may cause the surface of the parts to burn.

Application of Titanium and Titanium Alloys in Vacuum Salt Production Equipment

In the vacuum salt making system, the raw material is brine, which contains a large amount of corrosive medium. Its main equipment, such as brine preheaters, heaters, and evaporation tanks, are exposed to high-concentration brine for a long time at relatively high temperatures, and are constantly impacted by salt and sand. Under such conditions, the equipment must operate normally for a long time, and anti-corrosion is the key. question.

There are many reasons for corrosion, among which the presence of O2, high temperature, Cl- and solid salt washout are the main factors. Commonly used anti-corrosion methods include deoxidation, pH adjustment, electrochemical protection, coating protection, and the use of anti-corrosion materials. Among them, the use of anti-corrosion materials is a very effective way of anti-corrosion, and the application of titanium and titanium alloys in vacuum salt production is one example.

Preheater

The raw material needs to be preheated before entering the heater to save energy. The preheater used to be a tube-and-tube heat exchanger. The heat exchange tube material was made of carbon steel, which was easy to block and corrode through the tube. The heat exchange tube was often replaced and the production was stopped for maintenance, which brought great inconvenience to production. There are also preheaters that use red copper tubes, the corrosion still exists, the service life is extended to 1 year, but it is still very short, and the cost of copper tubes is high.

Until the advent of titanium plate heat exchangers. The heat exchange surface of the plate heat exchanger is a plate, although the strength is not as good as that of the tubular heat exchanger, but its heat transfer effect is good, the material is small, the weight is light, and it is easy to maintain, and its service life can reach 15 years. For example, and the Gr9 Ti-3Al-2.5V Titanium Sheet is still as bright as new, and has never been replaced, and the economic benefits are obvious.

heater

Since the heat exchange tubes of the heater are densely arranged on the tube sheet, maintenance and repairs are quite time-consuming, and all the heating tubes need to be replaced once a year. The temperature of effect II and effect III is lower, and the heating tube can be used for about 2 years. The temperature of the IV effect is the lowest and it can only be used for 4 years and needs to be replaced. During the overhaul, the shutdown delays the production, the output is reduced, the production is seriously affected, and the maintenance cost is high (some manufacturers use red copper for the heating room, the situation has improved, but the I effect is used. The period is no more than 3-5 years). In view of the characteristics of titanium, if titanium is used in heaters, the situation will be greatly improved.

Target performance requirements

The target restricts the physical and mechanical properties of the sputtered film and affects the quality of the coating. Therefore, the target quality evaluation is relatively strict, and the following requirements should be met:

1) Low impurity content and high purity. The purity of the target material affects the uniformity of the film.

2) High density. The high-density target has the advantages of good electrical conductivity, good thermal conductivity, and high strength. Using this target for coating, the sputtering power is small, the film-forming rate is high, the film is not easy to crack, the Ti-Al Sputtering Target has a long service life, and the resistivity of the sputtered film is high. low, high transmittance.

3) The composition and structure are uniforms. The uniform target composition is an important guarantee for stable coating quality.

4) The grain size is small. The finer the grain size of the target, the more uniform the thickness distribution of the Titanium Sputtering Target, and the faster the sputtering rate. Because of the above-mentioned special requirements on the performance of the target, its preparation process is relatively complicated.

The main characteristics of titanium rod and titanium alloy material processing

Titanium rods and titanium alloys have high chemical activity. titanium rods and titanium alloys are prone to violent reactions with oxygen, nitrogen and other oxygen-containing gases at high temperatures. When the surface of the billet is heated in air, an oxide scale and a surface getter layer are formed. Titanium rods and titanium alloys are easy to absorb hydrogen when heated, which causes difficulties in the processing of certain types of titanium alloys.

The thermal conductivity of titanium rods and titanium alloys is poor. The thermal conductivity of titanium rods and titanium alloys is usually only 1/15 of the thermal conductivity of aluminum alloys and 1/5 of steel. The low thermal conductivity leads to a large temperature difference along the section of the ingot and the billet in the hot B inch, resulting in a large thermal stress, and cracks will be formed in severe cases, so the heating speed must be limited, and the variable temperature, deformation speed, Deformation rate, deformation equipment.

Polycrystalline transformation of titanium rods and titanium alloys. Titanium has a-β phase transition. Heating to the p temperature can significantly improve the plasticity and reduce the deformation resistance, but the deformation of the β region is not enough to obtain the organization with good performance.

The cold deformation ability of titanium alloy is low. The cold working deformation of most titanium alloys is difficult, and a little preheating (to 200~300T) can significantly reduce the deformation resistance and improve the plasticity.

Titanium is easy to bond and deform molds. This tendency is easy to deteriorate the surface quality of the processed material, and puts forward more stringent requirements for deformation tools and process lubrication.

High yield ratio and low elastic modulus. Straightening in the cold state is very difficult.

The above processing characteristics should be fully considered when formulating the production process.