titanium alloy tubes and other titanium materials

1, small density, high strength, high specific strength

  The density of titanium is 4.51g/cm3, which is 57% of steel, which is only a little more than half of steel, less than twice that of aluminum, and three times stronger than aluminum. The specific strength of titanium alloy is the largest among the commonly used industrial alloys. The specific strength of titanium alloy is 3.5 times that of stainless steel, 1.3 times that of aluminum alloy, and 1.7 times that of magnesium alloy. Therefore, it is an indispensable structural material for the aerospace industry.

  The density and specific strength of titanium and other metals are compared in Table 1-1.

   Table 1-1 Comparison of density and specific strength between titanium and other metals

Metal Titanium (alloy) Iron Aluminum (alloy) Magnesium (alloy) High-strength steel

Density/(g·cm-3) Specific strength 4.5 (29) 7.87 - 2.7 (21) 1.74 (16) - 23

 

  2, excellent corrosion resistance

  The passivation of titanium depends on the existence of oxide film. Its corrosion resistance in oxidizing media is much better than that in reducing media, and high-rate corrosion can occur in reducing media. Titanium is not corroded in some corrosive media, such as sea water, wet chlorine, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chlorides, sulfides, and organic acids. However, in the medium that reacts with titanium to produce hydrogen (such as hydrochloric acid and sulfuric acid), titanium generally has a larger corrosion rate. However, if a small amount of oxidant is added to the acid, the titanium will form a passivation film. Therefore, titanium is corrosion-resistant in a mixture of sulfuric acid-nitric acid or hydrochloric acid-nitric acid, even in hydrochloric acid containing free chlorine. The protective oxide film of titanium is often formed when the metal touches water, even in the presence of a small amount of water or water vapor. If titanium is exposed to a strong oxidizing environment with no water at all, it can quickly oxidize and produce a violent, often spontaneous combustion reaction. This type of behavior has occurred in the reaction of titanium with fuming nitric acid containing excessive nitrogen oxides and titanium with dry chlorine gas. However, to prevent the occurrence of reactions in this state, a certain amount of water is necessary.

  3, good heat resistance

   Generally, aluminum loses its original higher mechanical properties at 150°C and stainless steel at 310°C, while titanium alloys still maintain good mechanical properties at about 500°C. When the speed of the aircraft reaches 2.7 times the speed of sound, the surface temperature of the aircraft mechanism reaches 230°C. Aluminum and magnesium alloys can no longer be used, while titanium alloys can meet the requirements. Titanium has good heat resistance. It is suitable for the turbine discs and blades of aero engine compressors and the skin of the rear fuselage of airplanes.

  4, good low temperature performance

  The strength of some titanium alloys (such as Ti-5Al-2.5SnELI) increases with the decrease of temperature, but the plasticity is not reduced much. It still has good ductility and toughness at low temperatures, and is suitable for use at ultra-low temperatures. It can be used on liquid hydrogen and liquid oxygen rocket engines, or as ultra-low temperature containers and storage tanks on manned spacecraft.

  5, non-magnetic

  Titanium is non-magnetic. It is used in submarine shells and will not cause mine explosions.

  6, small thermal conductivity

  The thermal conductivity of titanium and other metals are compared in the following table:

Metal Ti Al Fe Cu

Thermal conductivity/W·(m·K) 17 212 85 255

 

  The thermal conductivity of titanium is small, only 1/5 of steel, 1/13 of aluminum, and 1/25 of copper. Poor thermal conductivity is a shortcoming of titanium, but this feature of titanium can be used in some situations.

  7, the modulus of elasticity is small

Metal Ti Al Fe

Elastic modulus/GPa 108 72 196

 

  The modulus of elasticity of titanium is about 55% of that of iron. When used as a structural material, the low modulus of elasticity is a disadvantage.

   8. The tensile strength is close to the yield strength

  Ti-6Al-4V titanium alloy has a tensile strength of 960Mpa and a yield strength of 892MPa. The difference between the two is only 58Mpa.

  The comparison between the tensile strength and yield strength of titanium and other metals is shown in the following table:

Strength Titanium alloy (Ti-6Al-4V) 18-8 stainless steel aluminum alloy

Tensile strength 960 608 470

Yield strength 892 255 294

 

   9. Titanium is easily oxidized at high temperature

   Titanium has a strong binding force with hydrogen and oxygen, so care must be taken to prevent oxidation and hydrogen absorption. Titanium welding should be carried out under argon protection to prevent contamination. Titanium tubes and thin plates should be heat treated under vacuum, and titanium forgings should be controlled with a micro-oxidizing atmosphere during heat treatment.

  10, low damping resistance

Use titanium and other metal materials (copper, steel) to make clocks of exactly the same shape and size. If you knock each clock with the same force, you will find that the clock made of titanium oscillates and the sound lasts for a long time. The energy given to the bell by striking does not disappear easily.

  Special functions of titanium and titanium alloy processed materials:

  11, shape memory function

  Ti-50%Ni (atomic fraction) alloy, under certain temperature conditions, it has the ability to restore its original shape, so it is called titanium shape memory alloy.

  12, superconducting function

  NbTi alloy, when the temperature drops to close to absolute zero, the wire made of NbTi alloy loses resistance and can pass arbitrarily large currents. The wire does not generate heat and does not consume energy. Therefore, NbTi alloy is called a superconducting material.

  13、Hydrogen absorption function

  Ti-50%Fe (atomic fraction) alloy, has the ability to absorb hydrogen in a large amount. Using this feature of Ti-Fe alloy, hydrogen can be stored safely, that is, steel high-pressure gas cylinders are not necessarily used to store hydrogen. Under certain conditions, the hydrogen storage Ti-Fe alloy can also release hydrogen, so it is called a hydrogen storage material. 

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