Titanium alloy is an alloy based on titanium with other elements. Titanium has two homogeneous crystals; Titanium is an isomer with a melting point of 1720°C and a close-packed hexagonal lattice structure below 882°C, called titanium A. In 882°C above a body-centered cubic character structure, known as B titanium, using the two structures of titanium different characteristics, add appropriate alloying elements, make its phase change temperature and phase content gradually changed and get different microstructure of titanium alloys (itaniumalloys).

Titanium alloy - alloying element

Titanium alloy elements can be divided into three categories according to their effect on the phase transition temperature: ① stable a phase, improve the phase transition temperature of the elements for a stable elements, aluminum, magnesium, oxygen and nitrogen. Among them, aluminum is the main alloy element of titanium alloy, which has obvious effect on improving the strength of alloy at room temperature and high temperature, reducing specific gravity and increasing the amount of elasticity. ② Elements that stabilize B phase and reduce the temperature of phase transition are B stable elements. And can be divided into two types of isomorphic and eutectoid, the former has molybdenum, niobium, vanadium, etc.; the latter has chromium, manganese, copper, silicon, etc. ③ The elements that have little effect on the temperature of phase transformation are neutral elements, such as zirconium and tin.

Titanium alloys -- classification

a titanium alloy

It is a single-phase alloy composed of A-phase solid melt, which is a phase at both general temperature and high practical application temperature. It has stable microstructure, higher wear resistance than pure titanium, and strong oxidation resistance. It still maintains its strength and creep resistance at 500°C~600°C, but it cannot be strengthened by heat treatment, and its strength is not high at room temperature.

B titanium alloy

It is a single-phase alloy composed of B-phase solid melt, but has high strength without heat treatment. After quenching and aging, the alloy is further strengthened, and the strength at room temperature can reach 1372~1666MPa. But the thermal stability is poor, should not be used at high temperature.

a+B titanium alloy

It is a two-phase alloy, has good comprehensive performance, good structure stability, good toughness, plasticity and high temperature deformation properties can be better hot pressure processing, can be quenched, aging to strengthen the alloy. The strength after heat treatment is about 50%~100% higher than that in annealing state. High temperature strength, can be in 400°C~500°C temperature for a long time. Its thermal stability is inferior to that of a titanium alloy.

The most commonly used titanium alloys are a titanium alloy and a+B titanium alloy; a titanium alloy has the best cutting workpiece, followed by a+B titanium alloy and B titanium alloy. a titanium alloy is code-named TA, B titanium alloy is code-named TB, and a+B titanium alloy is code-named TC. According to the use of titanium alloy can be divided into heat resistant alloy, high strength alloy, corrosion resistant alloy (Ti-Mo, Ti-palladium alloy, etc.), low temperature alloy and special function alloy (Ti-iron hydrogen storage material and Ti-nickel memory alloy). Composition and properties of typical alloys.

Different phase composition and microstructure of heat-treated titanium alloy can be obtained by adjusting the heat treatment process. It is generally believed that fine equiaxed structures have good plasticity, thermal stability and fatigue strength. The acicular structure has higher lasting strength, creep strength and fracture toughness. Equiaxed and acicular mixed structures have better comprehensive properties.

Titanium alloys -- Properties

Titanium is a new type of metal, titanium performance and contained impurities such as carbon, hydrogen, oxygen, the purest titanium iodide impurity content is not more than 0.1%, but its low strength, high plasticity. The properties of 99.5% industrial pure titanium are as follows: density P=4.5g/cm3, melting point 1800°C, thermal conductivity λ=15.24W/(M.K), tensile strength σb =539MPa, elongation :δ =25%, section shrinkage rate ψ=25%, elastic content E=1.078×105MPa, hardness HB195.

High specific strength

The density of titanium alloy is generally about 4.5g/cm3, only 60% of steel, the strength of pure titanium is close to the strength of ordinary steel, some high strength titanium alloy exceeds the strength of many alloy structural steel, so the specific strength of titanium alloy (strength/density) is much greater than other metal structural materials, It can produce zero parts with high unit strength, good toughness and light weight. Titanium alloy is used in aircraft engine structural parts, skeleton, skin, fasteners and landing gear.

High heat intensity

The use of temperature is several hundred degrees higher than aluminum alloy, in the medium temperature can still maintain the required strength, can work in the temperature of 450-500°C for a long time these two types of titanium alloy in the range of 150°C-500°C still has a high specific strength, and aluminum alloy at 150°C specific strength decreased significantly, titanium alloy working temperature can reach 500°C, Aluminum alloys are below 200°C.

Good corrosion resistance

Titanium alloy works in humid atmosphere and seawater medium, other corrosion resistance is far better than stainless steel; Pitting corrosion, acid corrosion, stress corrosion resistance is particularly strong, alkali, chloride, chlorine of organic articles, nitric acid, sulfuric acid and so on have excellent corrosion resistance, but titanium with reducing oxygen and chromium salt media corrosion resistance is poor.

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 very low gap elements, such as TA7, can still maintain certain plasticity at -253°C. Therefore, titanium alloys are an important structural material at low temperature.

High chemical activity

The chemical activity of titanium is large, and the atmosphere of O, N, H, CO, CO2, water vapor, ammonia and other strong chemical reaction. When the carbon content is more than 0.2%, hard TiC will be formed in titanium alloy; When the temperature is higher, TiN hard surface will be formed by the action of N.

Above 600°C, titanium absorbs oxygen to form a hard layer; With the increase of hydrogen content, embrittlement layer will be formed. The depth of the hard brittle surface produced by gas absorption can reach 0.1-0.15mm, and the hardening degree is 20%·30%. Titanium chemical affinity is also large, easy to produce adhesion phenomenon with friction surface.

Small thermal conductivity, small elasticity

Titanium thermal conductivity λ=15.2W/ (M,K) about 1/4 nickel, iron 5/1, aluminum 1/14, and all kinds of titanium alloy thermal conductivity than titanium thermal conductivity about 50%, titanium alloy elastic amount is about 1/2 steel, so its poor rigidity, easy deformation, should not make slender rod and thin wall parts, cutting processing surface of the springback, large, About 2·3 times of stainless steel, resulting in intense friction, adhesion, adhesive wear after the tool surface.

Titanium alloy has high strength and low density, good mechanical properties, toughness and corrosion resistance. In addition, titanium alloy process performance is poor, machining difficulties, in heating, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon. The industrial production of titanium was started in 1948. The development of the aviation industry requires an average annual growth rate of about 8%. The world production of titanium alloy processing materials has reached more than 40 thousand tons, nearly 30 kinds of titanium alloy brands. The most widely used titanium alloys are Ti-6Al-4V (Grade 5), Ti-5Al-2.55n (TA7) and industrial pure titanium (Grade 1, Grade 2 and Grade 3).

Titanium alloy is mainly used in aircraft engine compressor components, followed by rocket, missile and high-speed aircraft structural components. In the mid-1960s, titanium and its alloy have been used in general industrial applications, used in the production of electrolytic industry electrodes, power station condenser, oil refining and seawater desalination heaters and environmental pollution control devices, titanium and its alloy has become a kind of corrosion resistant structural materials. In addition, it is also used to produce hydrogen storage materials and shape memory alloys.