Properties and Applications of Titanium Alloys
Titanium is a new type of metal. The properties of titanium are related to the content of impurities such as carbon, nitrogen, hydrogen, and oxygen. The purest titanium iodide contains no more than 0.1% impurities, but it has low strength and high plasticity. The properties of 99.5% industrial pure titanium are: density ρ=4.5g/cm3, melting point 1725℃, thermal conductivity λ=15.24W/(mK), tensile strength σb=539MPa, elongation δ=25%, section Shrinkage rate ψ=25%, elastic modulus E=1.078×105MPa, hardness HB195.
Property 1: Good Corrosion Resistance
Titanium alloy resists corrosion better than stainless steel when working in humid atmospheres and seawater; It is particularly resistant to pitting, acid corrosion and stress corrosion; It has excellent corrosion resistance to alkalis, chlorides, organic articles of chlorine, nitric acid, sulfuric acid, etc. However, titanium has poor corrosion resistance to media with reducing oxygen and chromium salts.
Property 2: High Strength
The density of titanium alloy is generally about 4.51g/cm3, which is only 60% of steel. The density of pure titanium is close to ordinary steel, and 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 that of other metal structural materials, and parts with high unit strength, good rigidity and light weight can be produced. Titanium alloys are used in aircraft engine components, skeletons, skins, fasteners and landing gear.
Property 3: High Thermal Strength
The operating temperature of titanium alloy is several hundred degrees higher than that of aluminum alloy. It can maintain the required strength at moderate temperatures, and work for a long time at the temperature of 450~500℃. Titanium alloy still has a high specific strength in the range of 150℃~500℃, while aluminum alloy has a significant decrease in specific strength at 150℃. Titanium alloys can operate at temperatures up to 500°C, while aluminum alloys are below 200°C.
Property 4: Good Low Temperature Performance
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 plasticity at -253 °C. Therefore, titanium alloy is also an important low-temperature structural material.
Property 5: High Chemical Activity
Titanium is chemically active and reacts strongly with O, N, H, CO, CO2, water vapor, ammonia, etc. in the atmosphere. When the carbon content is greater than 0.2%, hard TiC will be formed in the titanium alloy; At higher temperatures, TiN hard surface layers are also formed by interaction with N, titanium absorbs oxygen to form a hardened layer with high hardness above 600℃; Embrittlement layers also form when the oxygen content rises. The depth of the hard and brittle surface layer produced by absorbing gas can reach 0.1 to 0.15 mm, and the degree of hardening is 20% to 30%. The chemical affinity of titanium is also large, and it is easy to adhere to the friction surface.
Property 6: Small Thermal Conductivity
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, and 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 its rigidity is poor and it is easy to deform. It is not suitable to make slender rods and thin-walled parts. During cutting, the springback of the machined surface is very large, which is about 2 to 3 times that of stainless steel, resulting in severe friction, adhesion, and bonding wear on the flank of the tool.