What Is Titanium alloy?
Titanium alloy refers to a variety of alloy metals made of titanium and other metals. Titanium is an important structural metal developed in the 1950s. Titanium alloys have high strength, good corrosion resistance and high heat resistance. From the 1950s to 1960s, the main focus was the development of high-temperature titanium alloys for aero-engines and structural titanium alloys for airframes.
A number of corrosion-resistant titanium alloys were developed in the 1970s. Since the 1980s, corrosion-resistant titanium alloys and high-strength titanium alloys have been further developed. Titanium alloy is mainly used for the production of aircraft engine compressor components, followed by the structural parts of rockets, missiles and high-speed aircraft
The Development History Of Titanium Alloys
Titanium is an important structural metal developed in the 1950s. Titanium alloys are widely used in various fields because of their high strength, good corrosion resistance, and high heat resistance. Many countries in the world have recognized the importance of titanium alloy materials, have conducted research and development on them, and have been practically applied.
The first practical titanium alloy was the Ti-6Al-4V alloy successfully developed by the United States in 1954. Due to its heat resistance, strength, plasticity, toughness, formability, weldability, corrosion resistance and biocompatibility. It is better, and it has become the flagship alloy in the titanium alloy industry, and the amount of this alloy used has accounted for 75% to 85% of all titanium alloys. Many other titanium alloys can be regarded as modifications of Ti-6Al-4V alloy.
In the 1950s and 1960s, the main focus was on the development of high-temperature titanium alloys for aero-engines and structural titanium alloys for airframes. In the 1970s, a number of corrosion-resistant titanium alloys were developed. Since the 1980s, corrosion-resistant titanium alloys and high-strength titanium alloys have been further developed. development of. The use temperature of heat-resistant titanium alloys has increased from 400°C in the 1950s to 600-650°C in the 1990s. The emergence of A2 (Ti3Al) and r (TiAl)-based alloys has caused titanium to be pushed from the cold end of the engine (fan and compressor) to the hot end (turbine) of the engine in the use part of the engine. Structural titanium alloys are developing in the direction of high strength, high plasticity, high strength, high toughness, high modulus and high damage tolerance.
In addition, since the 1970s, shape memory alloys such as Ti-Ni, Ti-Ni-Fe, Ti-Ni-Nb have also appeared, and they have been increasingly used in engineering.
There are hundreds of titanium alloys developed in the world, and the most famous alloys are 20-30 kinds, such as Ti-6Al-4V, Ti-5Al-2.5Sn, Ti-2Al-2.5Zr, Ti-32Mo, Ti- Mo-Ni, Ti-Pd, SP-700, Ti-6242, Ti-10-5-3, Ti-1023, BT9, BT20, IMI829, IMI834, etc. [2,4].
According to relevant statistics, the amount of titanium used in my country’s chemical industry reached 25,000 tons in 2012, a decrease from 2011. This is the first time that my country’s chemical titanium market has experienced negative growth since 2009. In recent years, the chemical industry has been the largest user of titanium processing materials, and its consumption has remained at more than 50% of the total consumption of titanium materials, and it accounted for as high as 55% in 2011. However, as the economy has fallen into a downturn, the chemical industry will not only significantly reduce new projects, but will also face adjustments in the industrial structure.
The new production capacity of some products will be controlled, and outdated production capacity will be phased out. Affected by this, the shrinkage of the amount of titanium processed materials has also become a matter of course. Before that, some insiders predicted that the amount of titanium used in the chemical industry would reach its peak between 2013 and 2015. Judging from the current market performance, the overall economic weakness in 2012 may advance the decline of chemical titanium.
The Synthesis Principle Of Titanium Alloy
Titanium alloys are alloys based on titanium added with other elements. Titanium has two isomorphic crystals: close-packed hexagonal α titanium below 882°C, and body-centered cubic β titanium above 882°C.
Alloying elements can be divided into three categories according to their influence on the phase transition temperature:
- ①The elements that stabilize the α phase and increase the phase transition temperature are α stabilizing elements, such as aluminum, carbon, oxygen, and nitrogen. Among them, aluminum is the main alloy element of titanium alloy, which has obvious effects on improving the alloy’s normal temperature and high temperature strength, reducing specific gravity, and increasing elastic modulus.
- ②The element that stabilizes the β phase and reduces the phase transition temperature is the β-stabilizing element, which can be divided into two types: isomorphic and eutectoid. The former includes molybdenum, niobium, vanadium, etc.; the latter includes chromium, manganese, copper, iron, silicon and so on.
- ③ The elements that have little effect on the phase transition temperature are neutral elements, such as zirconium and tin.
Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have greater solubility in the α phase, which has a significant strengthening effect on the titanium alloy, but it reduces the plasticity. It is usually stipulated that the oxygen and nitrogen content in titanium is 0.15-0.2% and 0.04-0.05% or less, respectively. The solubility of hydrogen in the α phase is very small, and too much hydrogen dissolved in the titanium alloy will produce hydrides, which will make the alloy brittle. Generally, the hydrogen content in titanium alloys is controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.
The Properties Of Titanium Alloys
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.
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.
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℃.
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.
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 degree of plasticity at -253°C. Therefore, titanium alloy is also an important low-temperature structural material.
High chemical activity
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.
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.
The Classification Of Titanium Alloys
Titanium is an allotrope with a melting point of 1668°C. When it is lower than 882°C, it has a close-packed hexagonal lattice structure called α titanium; above 882°C it has a body-centered cubic lattice structure called β titanium. Using the different characteristics of the above two structures of titanium, adding appropriate alloying elements to gradually change the phase transformation temperature and phase content to obtain titanium alloys with different structures. At room temperature, titanium alloys have three matrix structures, and titanium alloys are divided into the following three categories: α alloys, (α+β) alloys and β alloys. China is represented by TA, TC, and TB respectively.
Alpha titanium alloy
It is a single-phase alloy composed of α-phase solid solution. It is α-phase regardless of normal temperature or higher practical application temperature, with stable structure, higher wear resistance than pure titanium, and strong oxidation resistance. At a temperature of 500°C to 600°C, it still maintains its strength and creep resistance, but cannot be strengthened by heat treatment, and its room temperature strength is not high.
Beta titanium alloy
It is a single-phase alloy composed of β-phase solid solution. It has high strength without heat treatment. After quenching and aging, the alloy is further strengthened. The room temperature strength can reach 1372 ~ 1666 MPa; but the thermal stability is poor and it is not suitable for use at high temperatures. .
α+β titanium alloy
It is a dual-phase alloy with good comprehensive properties, good structure stability, good toughness, plasticity and high-temperature deformation properties, can perform hot pressure processing well, can be quenched and aging to strengthen the alloy. The strength after heat treatment is about 50%-100% higher than the annealed state; the high-temperature strength is high, and it can work for a long time at a temperature of 400℃～500℃, and its thermal stability is inferior to that of α titanium alloy.
The most commonly used of the three titanium alloys are α titanium alloy and α + β titanium alloy; α titanium alloy has the best machinability, followed by α + β titanium alloy, and β titanium alloy is the worst. The alpha titanium alloy code is TA, the beta titanium alloy code is TB, and the alpha + beta titanium alloy code is TC.
Titanium alloys can be divided into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (titanium-molybdenum, titanium-palladium alloys, etc.), low-temperature alloys, and special functional alloys (titanium-iron hydrogen storage materials and titanium-nickel memory alloys), etc. .
Heat treatment: Titanium alloys can obtain different phase compositions and structures by adjusting the heat treatment process. It is generally believed that the small equiaxed structure has better plasticity, thermal stability and fatigue strength; the needle-like structure has higher endurance strength, creep strength and fracture toughness; the equiaxed and needle-like mixed structure has better comprehensive properties.
Uses of titanium alloys
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).
Titanium alloy is mainly used for the production of aircraft engine compressor components, followed by the structural parts of rockets, missiles and high-speed aircraft. In the mid-1960s, titanium and its alloys have been used in general industry to make electrodes in the electrolysis industry, condensers in power stations, heaters for petroleum refining and seawater desalination, and environmental pollution control devices. Titanium and its alloys have become a kind of corrosion-resistant structural materials. In addition, it is also used in the production of hydrogen storage materials and shape memory alloys.
China began research on titanium and titanium alloys in 1956; in the mid-1960s, industrialized production of titanium materials began and developed into TB2 alloys.
Titanium alloy is a new important structural material used in the aerospace industry. Its specific gravity, strength and service temperature are between aluminum and steel, but it is stronger than aluminum and steel and has excellent seawater corrosion resistance and ultra-low temperature performance. In 1950, the United States used it for the first time on the F-84 fighter bomber as non-load bearing components such as rear fuselage heat shields, wind deflectors, and tail covers. Since the 1960s, the use of titanium alloy moved from the rear fuselage to the middle fuselage, partially replacing structural steel to make important load-bearing components such as bulkheads, beams, and flap slides. The amount of titanium alloy used in military aircraft has increased rapidly, reaching 20% to 25% of the weight of the aircraft structure. Since the 1970s, civilian aircraft began to use titanium alloys in large quantities.
For example, Boeing 747 passenger aircraft used more than 3,640 kg of titanium. Aircraft with a Mach number greater than 2.5 use titanium to replace steel to reduce structural weight. For another example, the US SR-71 high-altitude and high-speed reconnaissance aircraft (flying Mach number 3, flying height 26212 meters), titanium accounts for 93% of the weight of the aircraft structure, so-called “all-titanium” aircraft. When the thrust-to-weight ratio of the aero engine increases from 4-6 to 8-10, and the compressor outlet temperature increases from 200-300°C to 500-600°C, the original low-pressure compressor discs and blades made of aluminum must Change to titanium alloy, or use titanium alloy instead of stainless steel to make high-pressure compressor discs and blades to reduce the structural weight. In the 1970s, the amount of titanium alloy used in aero engines generally accounted for 20% to 30% of the total weight of the structure. It was mainly used to manufacture compressor components, such as forged titanium fans, compressor discs and blades, cast titanium compressor casings, and intermediaries. Case, bearing housing, etc. The spacecraft mainly uses the high specific strength, corrosion resistance and low temperature resistance of titanium alloys to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frames and rocket shells. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy sheet welded parts.
The Heat Treatment Of Titanium Alloy
Commonly used heat treatment methods are annealing, solid solution and aging treatment. Annealing is to eliminate internal stress, improve plasticity and structural stability, and obtain better overall performance. Usually the annealing temperature of α alloy and (α+β) alloy is selected at 120～200℃ below the (α+β)─→β phase transition point; the solution and aging treatment are rapid cooling from the high temperature zone to obtain martensite α′ Phase and meta-stable β-phase, and then heat preservation in the middle temperature zone to decompose these meta-stable phases to obtain α-phase or compound and other finely dispersed second-phase particles to achieve the purpose of strengthening the alloy. Usually (α+β) alloys are quenched at 40～100℃ below the (α+β)─→β phase transformation point, and metastable β alloys are quenched at (α+β)─→β phase transformation point 40～80℃. get on. The aging treatment temperature is generally 450 to 550°C.
In summary, the heat treatment process of titanium alloy can be summarized as:
- Stress relief annealing: the purpose is to eliminate or reduce the residual stress generated during processing. Prevent chemical attack and reduce deformation in some corrosive environments.
- Complete annealing: The purpose is to obtain good toughness, improve processing performance, facilitate reprocessing and increase the stability of size and structure.
- Solution treatment and aging: The purpose is to improve its strength. α titanium alloy and stable β titanium alloy cannot be strengthened heat treatment, and only annealing is carried out in production. α+β titanium alloy and metastable β titanium alloy containing a small amount of α phase can further strengthen the alloy through solution treatment and aging.
In addition, in order to meet the special requirements of the workpiece, the industry also adopts metal heat treatment processes such as double annealing, isothermal annealing, β heat treatment, and thermomechanical heat treatment.
The Cutting Of Titanium Alloys
When the hardness of titanium alloy is greater than HB350, cutting is particularly difficult, and when the hardness is less than HB300, it is prone to sticking phenomenon and difficult to cut. But the hardness of titanium alloy is only one aspect that is difficult to cut. The key lies in the influence of the combination of chemical, physical and mechanical properties of titanium alloy on its machinability. Titanium alloy has the following cutting characteristics:
- Small deformation coefficient: This is a significant feature of titanium alloy cutting processing, and the deformation coefficient is less than or close to 1. The sliding friction distance of chips on the rake face is greatly increased, which accelerates tool wear.
- High cutting temperature: Because the thermal conductivity of titanium alloy is very small (only equivalent to 1/5 to 1/7 of 45 steel), the contact length between the chip and the rake face is extremely short, and the heat generated during cutting is not easy to transfer It is concentrated in a small area near the cutting area and the cutting edge, and the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than twice as high as when cutting No. 45 steel.
- The cutting force per unit area is large: the main cutting force is about 20% smaller than when cutting steel. Because the contact length between the chip and the rake face is extremely short, the cutting force per unit contact area is greatly increased, which is likely to cause chipping. At the same time, due to the small modulus of elasticity of titanium alloy, it is prone to bending deformation under the action of radial force during processing, causing vibration, increasing tool wear and affecting the accuracy of parts. Therefore, the process system is required to have good rigidity.
- Chilling phenomenon is serious: due to the high chemical activity of titanium, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle skin at high cutting temperatures; at the same time, plastic deformation during cutting will also cause surface hardening . Chilling phenomenon not only reduces the fatigue strength of parts, but also aggravates tool wear, which is a very important feature when cutting titanium alloys.
- The tool is easy to wear: After the blank is processed by stamping, forging, hot rolling and other methods, it will form a hard and brittle uneven skin, which is very easy to cause chipping, making the removal of the hard skin the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity of titanium alloy to the tool material, the tool is prone to bond wear under the conditions of high cutting temperature and high cutting force per unit area. When turning titanium alloy, sometimes the wear of the rake face is even more serious than that of the flank; when the feed rate f<0.1 mm/r, the wear mainly occurs on the flank; when f>0.2 mm/r, the front The tool face will be worn; when using carbide tools for fine turning and semi-finishing, the wear of the flank face should be less than 0.4 mm for VBmax.
In the milling process, due to the low thermal conductivity of the titanium alloy material and the extremely short contact length between the chips and the rake face, the heat generated during cutting is not easily transferred, and it is concentrated in a small range near the cutting deformation zone and the cutting edge. Extremely high cutting temperature will be generated at the cutting edge during machining, which will greatly shorten the tool life. For the titanium alloy Ti6Al4V, the cutting temperature is a key factor that affects the life of the tool, not the size of the cutting force, under the conditions of tool strength and machine power permitting.
Cutting titanium alloy should start from the two aspects of reducing cutting temperature and reducing adhesion. Choose tool materials with good red hardness, high bending strength, good thermal conductivity, and poor affinity with titanium alloys. YG cemented carbide is more suitable. Due to the poor heat resistance of high-speed steel, tools made of cemented carbide should be used as much as possible. Commonly used cemented carbide tool materials include YG8, YG3, YG6X, YG6A, 813, 643, YS2T and YD15.
Coated inserts and YT-type cemented carbide will have a violent affinity with titanium alloys, which will aggravate the bonding and wear of tools, and are not suitable for cutting titanium alloys. For complex and multi-edged tools, high-vanadium high-speed steel (such as W12Cr4V4Mo) ), high-cobalt high-speed steel (such as W2Mo9Cr4VCo8) or aluminum high-speed steel (such as W6Mo5Cr4V2Al, M10Mo4Cr4V3Al) and other tool materials, suitable for making drills, reamers, end mills, broaches, taps and other tools for cutting titanium alloys.
Using diamond and cubic boron nitride as tools for cutting titanium alloys can achieve significant results. For example, the cutting speed can reach 200 m/min under the condition of emulsion cooling with natural diamond tools; if the cutting fluid is not used, the allowable cutting speed is only 100 m/min at the same amount of wear.
In the process of cutting titanium alloy, the matters that should be paid attention to are:
- Due to the small modulus of elasticity of titanium alloy, the clamping deformation and force deformation of the workpiece during processing will reduce the processing accuracy of the workpiece; the clamping force should not be too large when the workpiece is installed, and auxiliary support can be added when necessary.
- If a cutting fluid containing hydrogen is used, it will decompose and release hydrogen at high temperatures during the cutting process, which will be absorbed by titanium and cause hydrogen embrittlement; it may also cause high-temperature stress corrosion cracking of titanium alloys.
- The chloride in the cutting fluid may also decompose or volatilize toxic gas during use. Safety protection measures should be taken when using it, otherwise it should not be used; after cutting, the parts should be thoroughly cleaned with a chlorine-free cleaning agent in time to remove chlorine residues Things.
- It is forbidden to use lead or zinc-based alloys to make tools and fixtures in contact with titanium alloys. Copper, tin, cadmium and their alloys are also forbidden to use.
- All tools, fixtures or other devices in contact with the titanium alloy must be clean; the cleaned titanium alloy parts should be prevented from being contaminated by grease or fingerprints, otherwise it may cause salt (sodium chloride) stress corrosion in the future.
- Under normal circumstances, there is no danger of ignition when cutting titanium alloys. Only in micro-cutting, the small chips cut off will ignite and burn. In order to avoid fire, in addition to pouring a large amount of cutting fluid, it is also necessary to prevent the accumulation of chips on the machine tool. The tool should be replaced immediately after being blunt, or the cutting speed should be reduced, and the feed rate should be increased to increase the thickness of the chip. In case of fire, fire extinguishing equipment such as talcum powder, limestone powder, dry sand should be used to extinguish the fire. Carbon tetrachloride and carbon dioxide fire extinguishers are strictly prohibited, and watering is prohibited, because water can accelerate combustion and even cause hydrogen explosion.
Deoxidation and pickling
In the middle of heat treatment and after the heat treatment, most of the surface treatment is required to remove the oxide scale and various pollutants on the metal surface, reduce the activity of the bare metal surface, and coat the protective layer and various functional coatings on the surface of titanium and its alloys. Surface treatment should also be carried out before and during the coating process. The coating of this kind of coating is to improve the performance of the metal surface, for example, to prevent corrosion, oxidation and abrasion.
The pickling conditions of titanium and its alloys are determined by the types (characteristics) of the oxide layer and the existing reaction layer, and the type of this layer is affected by the high-temperature heating process and the increase of the processing temperature (such as forging, casting, welding, etc.). At a lower processing temperature or about 600X: only a thin oxide layer is formed under the following high temperature heating temperature conditions, and an oxygen-rich diffusion zone is formed near a certain oxide layer under high temperature conditions, which must also be eluted by acid In addition to this oxygen-rich diffusion layer. Various methods of removing scale can be used: a mechanical method of removing thick oxide layer and hard surface layer, a method of removing oxide scale in a molten salt bath, and a method of removing oxide scale by acid elution in an acid solution.
In many cases, a combination of several methods can be used, for example, a combination of mechanically removing oxide scale followed by pickling, or a combination of salt bath followed by pickling. In the case of the oxide layer and diffusion layer formed at a higher temperature, a special method must be used. However, when the oxide layer is heated to 600X at a high temperature, most of the oxide layer formed can be dissolved away by general pickling.
The Problem Of Titanium Alloy
Titanium alloy has the advantages of light weight, high specific strength, and good corrosion resistance, so it is widely used in the automobile industry. The most used titanium alloy is in automobile engine systems. There are many advantages to using titanium alloys to make engine parts.
The low density of titanium alloy can reduce the inertial mass of moving parts. At the same time, the titanium valve spring can increase free vibration, reduce the vibration of the vehicle body, and increase the engine speed and output power.
Reduce the inertial mass of the moving parts, thereby reducing the friction and improving the fuel efficiency of the engine. Choosing titanium alloy can reduce the load stress of related parts and reduce the size of the parts, thereby reducing the quality of the engine and the entire vehicle. The reduction of the inertial mass of the parts reduces the vibration and noise and improves the performance of the engine. The application of titanium alloy to other parts can improve the comfort of personnel and the beauty of the car. In the application of the automobile industry, titanium alloy has played an inestimable role in energy saving and consumption reduction.
Despite the superior performance of titanium alloy parts, there is still a long way to go from the widespread use of titanium and its alloys in the automotive industry. The reasons include high price, poor formability and poor welding performance.
The main reason hindering the widespread application of titanium alloys in the automotive industry is the high cost.
Whether it is the initial smelting or subsequent processing of metals, the price of titanium alloys is much higher than other metals. The cost of titanium parts acceptable to the automotive industry is 8-13 US dollars/kg for connecting rods, 13-20 US dollars/kg for air valves, and 8 for springs, engine exhaust systems and fasteners. Below USD/kg. It is 6-15 times that of aluminum sheet and 45-83 times that of steel sheet.
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