Titanium, titanium alloy
Titanium is a new type of metal. The performance of titanium is related to the content of impurities such as carbon, nitrogen, hydrogen, and oxygen. The purest titanium iodide has an impurity content of no more than 0.1%, but its strength is low and its plasticity is high. The properties of 99.5% industrial pure titanium are: density ρ=4.5g/cm3, melting point of 1725℃, thermal conductivity λ=15.24W/(mK), tensile strength σb=539MPa, elongation δ=25%, and section shrinkage Rate ψ=25%, elastic modulus E=1.078×105MPa, hardness HB195.
01. high strength
The density of titanium alloy is generally about 4.51g/cm3, which is only 60% of steel. Some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength/density) of titanium alloy is much greater than other metal structural materials, and parts with high unit strength, good rigidity and light weight can be produced. The aircraft’s engine components, skeletons, skins, fasteners and landing gear all use titanium alloys.
02. high thermal intensity
The service temperature is several hundred degrees higher than that of aluminum alloy. It can still maintain the required strength at medium temperature. It can work for a long time at a temperature of 450～500℃. These two types of titanium alloys are still very high in the range of 150℃～500℃. Specific strength, while the specific strength of aluminum alloy decreases significantly at 150°C. The working temperature of titanium alloy can reach 500℃, while that of aluminum alloy is below 200℃.
03. good corrosion resistance
Titanium alloy works in moist atmosphere and seawater medium, its corrosion resistance is far better than stainless steel; it is particularly resistant to pitting corrosion, acid corrosion, and stress corrosion; it is resistant to alkali, chloride, chlorine organic substances, nitric acid, and sulfuric acid It has excellent corrosion resistance. However, titanium has poor corrosion resistance to reducing oxygen and chromium salt media.
04. good low temperature
Titanium alloys can still maintain their mechanical properties at low and ultra-low temperatures. Titanium alloys with good low temperature performance and extremely low interstitial elements, such as TA7, can maintain a certain degree of plasticity at -253°C. Therefore, titanium alloy is also an important low-temperature structural material.
05. high chemical activit
Titanium has high chemical activity, and produces strong chemical reactions with O2, N2, H2, CO, CO2, water vapor, ammonia, etc. in the atmosphere. When the carbon content is more than 0.2%, it will form hard TiC in the titanium alloy; when the temperature is higher, it will also form a hard surface layer of TiN when it interacts with N; when the temperature is above 600℃, titanium absorbs oxygen to form a hardened layer with high hardness ; When the hydrogen content increases, an embrittlement layer will also be formed. The depth of the hard and brittle surface layer produced by absorbing gas can reach 0.1～0.15 mm, and the degree of hardening is 20%～30%. Titanium also has a high chemical affinity and is easy to adhere to the friction surface.
06. Thermal conductivity is small
The thermal conductivity of titanium λ=15.24W/(m·K) is about 1/4 of nickel, 1/5 of iron, and 1/14 of aluminum. The thermal conductivity of various titanium alloys is about 50 lower than that of titanium. %. The elastic modulus of titanium alloy is about 1/2 of that of steel, so it has poor rigidity and is easy to deform. It is not suitable to make slender rods and thin-walled parts. The springback of the machined surface during cutting is very large, about 2～3 of stainless steel. Times, causing severe friction, adhesion, and adhesive wear on the flank of the tool.
Titanium alloy has high strength, low density, good mechanical properties, toughness and corrosion resistance. In addition, titanium alloys have poor process performance and are difficult to cut. In hot processing, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen, and carbon. There is also poor abrasion resistance and complex production processes. Industrial production of titanium began in 1948. The development of the aviation industry requires the titanium industry to develop at an average annual growth rate of about 8%. The world’s annual output of titanium alloy processing materials has reached more than 40,000 tons, with nearly 30 titanium alloy grades. The most widely used titanium alloys are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).