(HY Industrial Technology Center)
Titanium is an important structural metal developed in the 1950s. Titanium alloys are widely used in various fields due to their high strength, good corrosion resistance and high heat resistance. Many countries in the world have recognized the importance of titanium alloy materials, which have been researched and developed in succession and have been put into practical use.
The first practical titanium alloy was the Ti-6Al-4V alloy successfully developed in the United States in 1954 due to its heat resistance, strength, ductility, toughness, formability, weldability, corrosion resistance and biocompatibility. Sex. It is preferable to become an ace alloy in the titanium alloy industry, and the amount of the alloy is 75% to 85% of the total amount of the titanium alloy. Many other titanium alloys can be considered as modifications of the Ti-6Al-4V alloy.
In the 1950s and 1960s, the company developed high-temperature titanium alloys for aircraft engines and structural titanium alloys for aircraft. In the 1970s, many 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 increased from 400 ° C in the 1950s to 600 to 650 ° C in the 1990s. The appearance of the A2 (Ti3Al) and r(TiAl) based alloys allows titanium to be advanced from the cold end of the engine (fan and compressor) to the hot end (turbine) of the engine at the point of use of the engine. The structure of titanium alloy is developed in the direction of high strength, high plasticity, high strength and high toughness.
In addition, shape memory alloys such as Ti-Ni, Ti-Ni-Fe and Ti-Ni-Nb have appeared since the 1970s and are increasingly used in engineering.
Hundreds of titanium alloys have been developed in the world, and the most famous alloys are 20-30, 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.
Titanium alloys are titanium based alloys and are added to other elements. Titanium has two isomorphous crystals: below 882 ° C is a tightly packed hexagonal structure of alpha titanium, and above 882 ° C is body-centered cubic beta titanium.
Alloying elements can be classified into three categories based on their effect on the phase transition temperature:
The elements that stabilize the alpha phase and increase the phase transition temperature are alpha stabilizing elements such as aluminum, carbon, oxygen and nitrogen. Among them, aluminum is the main alloying element of titanium alloy, which has obvious effect on improving the normal temperature and high temperature strength of the alloy, reducing the specific gravity and increasing the elastic modulus.
The element that stabilizes the beta phase and lowers the phase transition temperature is a beta stabilizing element, which can be divided into two types: isomorphous and eutectoid. The former has molybdenum, niobium, vanadium, etc.; the latter has chromium, manganese, copper, iron, silicon and the like.
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 of titanium alloys. Oxygen and nitrogen have a large solubility in the α phase, and have a significant strengthening effect on the titanium alloy, but the plasticity is lowered. The content of oxygen and nitrogen in the titanium is usually specified to be 0.15 to 0.2% and 0.04 to 0.05% or less, respectively. Hydrogen has a low solubility in the alpha phase, and excessive hydrogen is dissolved in the titanium alloy to produce a hydride which makes the alloy brittle. Usually, the hydrogen content in the titanium alloy is controlled to be less than 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.
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 impurity content does not exceed 0.1%, but its strength is low and its plasticity is high. The performance of 99.5% industrial pure titanium is: density ρ=4.5g/cm3, melting point is 1725°C, thermal conductivity λ=15.24W/(mK), tensile strength σb=539MPa, elongation δ=25%, section The shrinkage ratio ψ = 25%, the elastic modulus E = 1.078 × 105 MPa, and the hardness HB 195.
1, high intensity:
The density of titanium alloy is generally around 4.51g/cm3.It is only 60% of steel. The density of pure titanium is close to that of ordinary steel. Some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength/density) of the titanium alloy is much larger 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 for aircraft engine components, skeletons, skins, fasteners and landing gear.
2 high heat intensity
The use temperature is several hundred degrees higher than that of aluminum alloy, and the required strength can be maintained at moderate temperatures. The long-term operation of these two types of titanium alloys at temperatures of 450 to 500 ° C is still high in the range of 150 ° C to 500 ° C. Specific strength, while the specific strength of aluminum alloy at 150 ° C is significantly reduced. Titanium alloys can operate at temperatures up to 500 ° C and aluminum alloys at temperatures below 200 ° C.
3, good corrosion resistance
Titanium alloy works in moist atmosphere and seawater medium, and its corrosion resistance is much better than that of stainless steel; it is particularly resistant to pitting, acid etching and stress corrosion; organic substances for alkali, chloride and chlorine, nitric acid, sulfuric acid Such as excellent corrosion resistance. However, titanium has poor corrosion resistance to a medium having a reducing oxygen and a chromium salt.
4, good low temperature performance
Titanium alloys retain their mechanical properties at low and ultra-low temperatures. Titanium alloys with good low-temperature properties and extremely low interstitial elements, such as TA7, retain a certain degree of plasticity at -253 °C. Therefore, titanium alloy is also an important low temperature structural material.
5, chemically active
Titanium has a large chemical activity and produces strong chemical reactions with O, N, H, CO, CO2, water vapor, and ammonia in the atmosphere. When the carbon content is more than 0.2%, hard TiC is formed in the titanium alloy; when the temperature is high, the TiN hard surface layer is formed by the action of N; when it is above 600 ° C, the titanium absorbs oxygen to form the hard layer with high hardness. When the hydrogen content rises, an embrittlement layer is also formed. The hard and brittle surface layer produced by absorbing gas can reach a depth of 0.1 to 0.15 mm and a degree of hardening of 20% to 30%. Titanium also has a large chemical affinity and is liable to adhere to the friction surface.
6, low thermal conductivity
The thermal conductivity of titanium λ = 15.24 W / (mK) is about 1/4 of nickel, 1/5 of iron, 1 / 14 of aluminum, and the thermal conductivity of various titanium alloys is less than about 50%. titanium. The elastic modulus of titanium alloy is about 1/2 of that of steel, so its rigidity is poor and it is easily deformed. It is not suitable for the manufacture of slender rods and thin-walled parts. The amount of springback of the machined surface during cutting is very large, about 2 to 3 stainless steel. Double, causing severe friction on the sides of the tool, adhesion and adhesive wear.
Titanium is an isomer of isotopes with a melting point of 1668 ° C. It is a close-packed hexagonal lattice structure below 882 ° C and is called α titanium. It is a body-centered cubic lattice structure above 882 ° C and is called β titanium. Titanium alloys with different microstructures are obtained by adding appropriate alloying elements and gradually changing the phase transition temperature and phase content by using the different characteristics of the above two structures of titanium. At room temperature, titanium alloys have three kinds of matrix structures, and titanium alloys are classified into the following three types: α alloys, (α + β) alloys and β alloys. China is represented by TA, TC, and TB respectively.
1, alpha titanium alloy
It is a single-phase alloy composed of α-phase solid solution. It is α phase at normal temperature or at a higher practical application temperature. It is structurally stable and has higher wear resistance than pure titanium and has strong oxidation resistance. At 500 ° C ~ 600 ° C temperature, its strength and creep resistance are maintained, but heat treatment can not be strengthened, the room temperature strength is not high.
2, 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. However, the thermal stability is poor and should not be used at high temperature.
3, α + β titanium alloy
It is a dual-phase alloy with good comprehensive properties, good structural stability, good toughness, plasticity and high temperature deformation properties. It can perform hot pressure processing well, and can be quenched and aged to strengthen the alloy. The strength after heat treatment is about 50% to 100% higher than that of the annealed state; the high temperature strength is high, and it can work for a long time at a temperature of 400 ° C to 500 ° C, and its thermal stability is inferior to that of the α titanium alloy.
The most commonly used of the three titanium alloys are α-titanium alloy and α+β-titanium alloy; the α-titanium alloy has the best machinability, the α+β-titanium alloy is the second, and the β-titanium alloy is the worst. The alpha titanium alloy is coded as TA, the beta titanium alloy is coded as TB, and the alpha + beta titanium alloy is coded as TC.
Titanium alloys can be classified 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). . The composition and properties of typical alloys are shown in the table.
Heat Treatment Titanium alloys can be obtained by adjusting the heat treatment process to obtain different phase compositions and microstructures. It is generally believed that the fine equiaxed structure has good plasticity, thermal stability and fatigue strength; the needle-like structure has high durability, creep strength and fracture toughness; the equiaxed and needle-like mixed structure has better comprehensive performance.
Titanium alloy has high strength and low density, good mechanical properties, and good toughness and corrosion resistance. In addition, the titanium alloy has poor processability and is difficult to cut, and it is very easy to absorb impurities such as hydrogen, nitrogen, nitrogen and carbon during hot working. There is also poor wear resistance and complicated production processes. The industrial production of titanium began in 1948. The development of the aviation industry has enabled the titanium industry to grow at an average annual growth rate of about 8%. The annual output of world titanium alloy processing materials has reached more than 40,000 tons, and 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 alloys are mainly used to make aircraft engine compressor components, followed by structural components for rockets, missiles and high-speed aircraft. In the mid-1960s, titanium and its alloys were used in the general industry for the production of electrodes for the electrolysis industry, condensers for power stations, heaters for petroleum refining and seawater desalination, and environmental pollution control devices. Titanium and its alloys have become a corrosion resistant structural material. It is also used to produce hydrogen storage materials and shape memory alloys.
China began research on titanium and titanium alloys in 1956; industrial production of titanium materials began in the mid-1960s 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 the United States in 1950, it was used for the first time on the F-84 fighter-bomber as a non-bearing member such as a rear fuselage insulation panel, an air hood, and a tail hood. In the 1960s, the use of titanium alloys moved from the rear fuselage to the middle fuselage, partially replacing structural steel to make important load-bearing components such as bulkheads, beams, and flaps.
The amount of titanium alloy used in military aircraft has increased rapidly, reaching 20% to 25% of the structural weight of the aircraft. Since the 1970s, civilian machines have begun to use titanium alloys in large quantities.
For example, the Boeing 747 passenger aircraft used more than 3,640 kilograms of titanium. Titanium for aircraft with a Mach number greater than 2.5 is mainly used to replace steel to reduce the structural weight.
Another example is the US SR-71 high-altitude high-speed reconnaissance aircraft (with a flying Mach number of 3 and a flying height of 26,212 meters). Titanium accounts for 93% of the aircraft’s structural weight, and is known as the “all-titanium” aircraft. When the thrust-to-weight ratio of the aero-engine is increased from 4 to 6 to 8 to 10, and the compressor outlet temperature is increased from 200 to 300 ° C to 500 to 600 ° C, the low-pressure compressor disk and blades originally made of aluminum must be Use titanium alloys or titanium alloys instead of stainless steel to make high-pressure compressor discs and blades to reduce structural weight.
In the 1970s, titanium alloys used in aircraft engines generally accounted for 20% to 30% of the total weight of the structure. They are mainly used in the manufacture of compressor components, such as forged titanium fans, compressor discs and blades, cast titanium compressors, and intermediates. Machine casing, bearing housing, etc.
The spacecraft mainly utilizes 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 casings. Titanium alloy plate welded parts are also used in artificial earth satellites, lunar modules, manned spacecraft and space shuttles.
Heat treatment method：
Commonly used heat treatment methods are annealing, solid solution and aging treatment. Annealing is to eliminate internal stress, improve plasticity and structural stability to achieve better overall performance. Generally, the annealing temperature of α-alloy and (α+β) alloy is selected from 120° to 200°C below the (α+β)-→β phase transition point; solid solution and aging treatment is fast cooling from high temperature region to obtain martensite α′ The phase and the metastable β phase are then decomposed in the middle temperature region to decompose the metastable phase to obtain a finely dispersed second phase particle such as an α phase or a compound, thereby achieving the purpose of strengthening the alloy. Generally, the quenching of (α+β) alloy is carried out at 40 to 100 °C below the (α+β)-→β phase transition point, and the metastable β alloy is quenched at 40 to 80 °C above the (α+β)-→β phase transition point. 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 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 properties, facilitate reprocessing and improve dimensional and structural stability.
Solution treatment and aging: The purpose is to increase the strength, and the α-titanium alloy and the stabilized β-titanium alloy cannot be subjected to tempering heat treatment, and only annealing is performed in production. The α+β titanium alloy and the metastable β titanium alloy containing a small amount of α phase can further strengthen the alloy by solution treatment and aging.
In addition, in order to meet the special requirements of the workpiece, the industry also uses metal annealing processes such as double annealing, isothermal annealing, β heat treatment, and deformation heat treatment.
1, cutting characteristics When the hardness of titanium alloy is greater than HB350, the cutting process is particularly difficult. When it is less than HB300, the sticking phenomenon is likely to occur and it is difficult to cut. However, the hardness of titanium alloy is only one aspect that is difficult to be machined. The key is the influence of the combination of chemical, physical and mechanical properties of titanium alloy on its machinability. Titanium alloys have the following cutting characteristics:
Small deformation coefficient: This is a remarkable feature of titanium alloy machining, and the deformation coefficient is less than or close to 1. The distance of the sliding friction of the chips on the rake face is greatly increased, accelerating tool wear.
High cutting temperature: Since 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 easily transmitted. Out, concentrated in a small range near the cutting zone and the cutting edge, the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than doubled when cutting 45 steel.
The cutting force per unit area is large: the main cutting force is about 20% smaller than that when cutting steel. Since 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 liable to cause chipping. At the same time, due to the small elastic modulus of the titanium alloy, bending deformation is easily generated under the action of radial force during processing, causing vibration, increasing tool wear and affecting the accuracy of the part. Therefore, the process system is required to have better rigidity.
The chill 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 outer skin at high cutting temperature; at the same time, plastic deformation during cutting also causes surface hardening. . The chilling phenomenon not only reduces the fatigue strength of the part, but also aggravates the tool wear, which is an important feature when cutting titanium alloy.
The tool is easy to wear: the blank is processed by stamping, forging and hot rolling to form a hard and brittle uneven skin, which is easy to cause chipping, which makes the removal of the hard skin the most difficult process in the processing of titanium alloy. In addition, due to the strong chemical affinity of the titanium alloy for the tool material, the tool is prone to bond wear under conditions of high cutting temperature and large cutting force per unit area. When turning titanium alloy, sometimes the wear of the rake face is even more serious than the flank; when the feed rate f<0.1 mm/r, the wear mainly occurs on the flank face; when f>0.2 mm/r, the front Wear will occur on the flank; when using carbide cutters and semi-finished vehicles, the wear of the flank is preferably VBmax < 0.4 mm.In the milling process, since the thermal conductivity of the titanium alloy material is low, and the contact length between the chip and the rake face is extremely short, the heat generated during cutting is not easily transmitted, and is concentrated in a small range near the cutting deformation zone and the cutting edge. Extremely high cutting temperatures occur at the cutting edge during machining, which greatly reduces tool life. For titanium alloy Ti6Al4V, the cutting temperature is a key factor affecting tool life, not the cutting force, under the conditions of tool strength and machine power.
2, tool material
The cutting of titanium alloy should be based on the reduction of cutting temperature and the reduction of bonding. It is suitable for tool materials with good red hardness, high flexural strength, good thermal conductivity and poor affinity with titanium alloy. YG type hard alloy is suitable. Due to the poor heat resistance of high speed steel, tools made of hard alloy should be used as much as possible. Commonly used carbide tool materials are YG8, YG3, YG6X, YG6A, 813, 643, YS2T and YD15.
Coated inserts and YT-type hard alloys will have a strong affinity with titanium alloys, which will intensify the bond wear of the tool. It is not suitable for cutting titanium alloys. For complex and multi-blade tools, high vanadium high-speed steel (such as W12Cr4V4Mo) can be used. ), high-cobalt high-speed steel (such as W2Mo9Cr4VCo8) or aluminum high-speed steel (such as W6Mo5Cr4V2Al, M10Mo4Cr4V3Al) and other tool materials, suitable for cutting titanium alloy drills, reamer, end mill, broach, tap and other tools.
The use of diamond and cubic boron nitride as a tool for cutting titanium alloys can achieve significant results. If the natural diamond tool is used to cool the emulsion, the cutting speed can reach 200 m/min; if the cutting fluid is not used, the cutting speed is only 100m/min at the same amount of wear.
3, matters needing attention
In the process of cutting titanium alloy, the following matters should be noted:
Since the elastic modulus of the titanium alloy is small, the clamping deformation and deformation of the workpiece during processing will greatly reduce the machining accuracy of the workpiece; the clamping force during the installation of the workpiece should not be too large, and the auxiliary support may be added if necessary.
If a hydrogen-containing cutting fluid is used, hydrogen will be decomposed and released at high temperature during the cutting process, and hydrogen absorption will be caused by absorption of titanium; it may also cause high-temperature stress corrosion cracking of the titanium alloy.
Chloride in cutting fluid may also decompose or volatilize toxic gas when used. It should be taken safely when used. Otherwise, it should not be used. After cutting, it should be cleaned thoroughly with chlorine-free cleaning agent to remove chlorine residue. Things.
It is forbidden to use lead or zinc-based alloys to make contact with titanium alloys. Copper, tin, cadmium and their alloys are also prohibited.
All work, fixtures or other devices in contact with the titanium alloy must be clean; the cleaned titanium alloy parts should be protected from grease or fingerprint contamination, otherwise the salt (sodium chloride) stress corrosion may occur in the future.
Under normal circumstances, there is no danger of fire when cutting titanium alloy. Only in the case of micro-cutting（Titanium-based alloy powder）, the fine chips that are cut will have a burning phenomenon. In order to avoid fire, in addition to a large amount of casting cutting fluid, it should also prevent the accumulation of chips on the machine tool, replace the tool immediately after blunt, or reduce the cutting speed, increase the feed amount to increase the chip thickness. If it is on fire, it should be extinguished with fire-fighting equipment such as talc powder, limestone powder and dry sand. It is strictly forbidden to use carbon tetrachloride, carbon dioxide fire extinguisher or watering, because water can accelerate combustion and even cause hydrogen explosion.
Titanium alloy deoxidation and pickling：
After the heat treatment and 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 surface of the bare metal, and apply a protective layer and various functional coatings on the surface of the titanium and its alloys. The surface treatment is also carried out before and during the coating process, and the coating is applied to improve the properties of the metal surface, for example, to prevent corrosion, oxidation and abrasion.
The pickling conditions of titanium and its alloys are determined by the type (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 (for example, forging, casting, welding, etc.). . Only a thin oxide layer is formed at a lower processing temperature or a heating temperature of about 600X: or less, and an oxygen-rich diffusion region is formed in the vicinity of an oxide layer under high temperature conditions, and must also be eluted by acid. In addition to this oxygen-rich diffusion layer. Various methods for removing scale can be employed: a mechanical method for removing a thick oxide layer and a hard surface layer, a scale for removing the scale in a molten salt bath, and an acid stripping and descaling in an acid solution.
In many cases, a combination of several methods may be employed, for example, mechanically removing the scale and then pickling, or first removing the scale by a salt bath followed by pickling. In the case of an oxide layer and a diffusion layer formed at a relatively high temperature, a special method is employed, but an oxide layer formed by heating at a high temperature to 600X: is mostly dissolved by a general pickling.
Problems with titanium alloys：
Titanium alloy has the advantages of light weight, high specific strength and good corrosion resistance, so it is widely used in the automotive industry. The most applied titanium alloy is the automobile engine system. There are many benefits to using titanium alloys to make engine parts.
The low density of titanium alloy can reduce the inertial mass of moving parts, while the titanium valve spring can increase free vibration, weaken the vibration of the body, and improve 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. The choice of titanium alloy can reduce the load stress of related parts and reduce the size of parts, thus reducing the quality of the engine and the whole vehicle. The reduction in the inertial mass of the components reduces vibration and noise and improves engine performance. The use of titanium alloys on other components can improve the comfort of the person and the aesthetics of the car. In the automotive industry, titanium alloys play an invaluable role in energy saving.
Despite the superior performance of titanium alloy components, there is still a large distance from titanium and its alloys in the automotive industry due to problems such as high price, poor formability and poor weldability.
The most important reason for hindering the widespread use of titanium alloys in the automotive industry is the cost.
Whether it is the initial smelting of metals or subsequent processing, the price of titanium alloys is much higher than other metals. The cost of titanium parts acceptable to the automotive industry is 8 to 13 US dollars per kg of connecting rod titanium, 13 to 20 US dollars per kg of titanium for gas valves, and titanium for springs, engine exhaust systems and fasteners. USD/kg or less. It is 6 to 15 times that of aluminum sheet and 45 to 83 times that of steel sheet.
Titanium alloy defect:
The main limitation of titanium and titanium alloys is the poor chemical reactivity with other materials at high temperatures. This property forces the titanium alloy to be different from conventional conventional refining, melting and casting techniques, and often causes damage to the mold; as a result, the price of the titanium alloy becomes very expensive. As a result, they were mostly used in aircraft structures, aircraft, and in high-tech industries such as the petroleum and chemical industries. However, due to the development of space technology and the improvement of people’s quality of life, titanium alloys are gradually being used to make people’s livelihood products for the benefit of the people. However, the prices of these products are still high, and they are mostly high-priced products. This is titanium alloy. Unable to carry the biggest fatal injury.
New progress in titanium alloy：
Countries are developing new low-cost and high-performance titanium alloys, and strive to make titanium alloys into the civilian industry with great market potential. The new progress in the research of global titanium alloy materials is mainly reflected in the following aspects：
1，High temperature titanium alloy
The world’s first high-temperature titanium alloy was developed Ti-6Al-4V at a temperature of 300-350 °C. Subsequently, alloys such as IMI550 and BT3-1 with a temperature of 400 ° C were used, and alloys such as IMI 679, IMI 685, Ti-6246, and Ti-6242 at a temperature of 450 to 500 ° C were used. The new high-temperature titanium alloys that have been successfully applied in military and civil aircraft engines include the British IMI829 and IMI834 alloys; the US Ti-1100 alloy; the Russian BT18Y and BT36 alloys. Table 7 shows the maximum operating temperatures of some new high-temperature titanium alloys in some countries.
In recent years, many companies have developed titanium alloys using rapid solidification/powder metallurgy technology, fiber or particle reinforced composites as the development direction of high-temperature titanium alloys, so that the use temperature of titanium alloys can be increased to above 650 °C [1,27,29, 31]. American McDonnell Douglas has successfully developed a high-purity, high-density titanium alloy using rapid solidification/powder metallurgy technology. Its strength at 760 °C is equivalent to the strength of titanium alloy used at room temperature.
2，Titanium aluminum compound
Compared with general titanium alloys, the most advantageous advantages of titanium-aluminum compounds as base sodium Ti3Al(α2) and TiAl(γ) intermetallic compounds are high temperature performance (maximum operating temperatures of 816 and 982 ° C, respectively), strong oxidation resistance and resistance. The creep properties and light weight (1/2 density of nickel-based superalloys) make them the most competitive materials for future aero engines and aircraft structural components.
Two Ti3Al-based titanium alloys Ti-21Nb-14Al and Ti-24Al-14Nb-#v-0.5Mo have been mass-produced in the United States. Other developed Ti3Al-based titanium alloys include Ti-24Al-11Nb, Ti25Al-17Nb-1Mo, and Ti-25Al-10Nb-3V-1Mo . TiAl (γ)-based titanium alloys are of interest in the composition range Ti-(46-52)Al-(1-10)M(at.%), where M is v, Cr, Mn, Nb, Mn, At least one element of Mo and W. TiAl3-based titanium alloys have begun to attract attention, such as Ti-65Al-10Ni alloys.
3，High strength and high toughness β
The β-type titanium alloy was first developed as a B120VCA alloy (Ti-13v-11Cr-3Al) by Crucible in the mid-1950s. The β-type titanium alloy has good hot and cold processing properties, is easy to forge, can be rolled and welded, and can obtain high mechanical properties, good environmental resistance and strength and fracture toughness by solid solution-aging treatment. The most representative of the new high-strength and high-toughness β-type titanium alloys are as follows:
Ti1023(10v-2Fe-3Al/UNS R56410), which has the same performance as 30CrMnSiA high-strength structural steel commonly used in aircraft structural parts, and has excellent forging performance;
Ti153 (Ti-15V-3Cr-3Al-3Sn), the cold workability of the alloy is better than that of industrial pure titanium, and the tensile strength at room temperature after aging can reach 1000 MPa or more;
β21S (Ti-15Mo-3Al-2.7Nb-0.2Si), a new type of anti-oxidation, ultra-high-strength titanium alloy developed by Timet Division of American Titanium Corporation. It has good oxidation resistance and hot and cold processing properties. Excellent, can be made into a foil with a thickness of 0.064mm;
The SP-700 (Ti-4.5Al-3V-2Mo-2Fe) titanium alloy successfully developed by Nippon Steel Tube Co., Ltd. (NKK) has high strength, superplastic elongation of up to 2000%, and superplastic forming temperature than Ti-6Al- 4V lower 140 ° C, can replace the Ti-6Al-4V alloy with superplastic forming-diffusion connection (SPF / DB) technology to manufacture a variety of aerospace components;
The BT-22 (TI-5v-5Mo-1Cr-5Al) developed by Russia has a tensile strength of more than 1105 MPA.
4，Flame retardant titanium alloy
Conventional titanium alloys have a tendency to burn alkanes under certain conditions, which greatly limits their application. In response to this situation, countries have launched research on flame retardant titanium alloys and made some breakthroughs. Alloy c (also known as Ti-1720) developed in the United States, with a nominal composition of 50Ti-35v-15Cr (mass fraction), is a flame retardant titanium alloy that is insensitive to continuous combustion and has been used in F119 engines. BTT-1 and BTT-3 are flame retardant titanium alloys developed in Russia, all of which are Ti-Cu-Al alloys. They have quite good thermal deformation process properties and can be used to make complex parts.
5，Medical titanium alloy
Titanium is non-toxic, light in weight, high in strength and excellent in biocompatibility. It is an ideal medical metal material and can be used as an implant implanted in human body. Still widely used in the medical field is the 6Al-4V ELI/UNS R56401 ELI alloy. However, the latter will precipitate a very small amount of vanadium and aluminum ions, which reduces the adaptability of cells and may cause harm to the human body. This problem has already attracted widespread attention in the medical community. As early as the mid-1980s, the United States began to develop aluminum-free, vanadium-free, biocompatible titanium alloys for orthopedic surgery. Japan, the United Kingdom, etc. have also done a lot of research work in this area and made some new progress.
For example, Japan has developed a series of α+β titanium alloys with excellent biocompatibility, including Ti-15Zr-4Nb_4ta-0.2Pd, Ti-15Zr-4Nb-aTa-0.2Pd-0.20~0.05N, Ti-15Sn. -4Nb-2Ta-0.2Pd and Ti-15Sn-4nb-2Ta-0.2Pd-0.20, these alloys are superior to Ti-6Al-4v ELI in corrosion strength, fatigue strength and corrosion resistance. Compared with α+β titanium alloy, β titanium alloy has higher strength level, better cutting performance and toughness, and is more suitable for implanting into the human body as an implant.
In the United States, five beta titanium alloys have been recommended for the medical field, namely TMZFTM (TI-12Mo-^Zr-2Fe), Ti-13Nb-13Zr, Timetal 21SRx (TI-15Mo-2.5Nb-0.2Si), Tiadyne. 1610 (Ti-16Nb-9.5Hf) and Ti-15Mo. It is estimated that in the near future, such a titanium alloy having high strength, low modulus of elasticity, and excellent formability and corrosion resistance is likely to replace the Ti-6Al-4V ELI alloy widely used in the medical field.
Two characteristics of titanium alloy heating:
Based on the various properties of titanium alloys, compared to other metals, there are some unique features in the heating and heat treatment process. The following HY-industry summarizes two points for you:
Compared with copper, aluminum, iron and nickel, the thermal conductivity of titanium is low, and the main difficulty in heating is that the heating time is quite long when the surface heating method is used. When the large billet is heated, the cross-section temperature difference is large. The thermal conductivity of copper, iron, and nickel-based alloys decreases with increasing temperature, and the thermal conductivity of titanium alloy increases with increasing temperature.
They react strongly with air when the temperature is raised. When heated above 650 ° C, titanium reacts strongly with oxygen, while above 700 ° C, it also reacts with nitrogen while forming a deeper surface layer saturated by the two gases. For example, when the titanium billet having a diameter of 350 mm is heated to 1,100 to 1,150 by surface heating, it is necessary to keep the gettering layer having a thickness of 1 mm or more in the temperature range in which the titanium is strongly reacted with the gas for 3 to 4 hours. This gettering layer can deteriorate the deformation properties of the alloy.When heated in an oil furnace having a reducing atmosphere, hydrogen absorption is particularly strong, and hydrogen can diffuse into the interior of the alloy during heating to reduce the plasticity of the alloy. When heated in an oil furnace having an oxidizing atmosphere, the hydrogen absorption process of the titanium alloy is significantly slowed down; when heated in a conventional box type electric furnace, hydrogen absorption is slower.
It can be seen that the titanium alloy blank should be heated in an electric furnace. When heating with flame has to be used, the atmosphere in the furnace should be slightly oxidized to avoid hydrogen embrittlement. Regardless of the type of furnace heating, the titanium alloy should not interact with the refractory material, and the stainless steel plate should be placed on the bottom of the furnace. A heat resistant alloy sheet containing more than 50% nickel may not be used to prevent the billet from being welded to the board.
In order to obtain a uniform fine-grained structure and high mechanical properties for forgings and die forgings, it is necessary to ensure that the blank has the shortest residence time at high temperatures during heating. Therefore, in order to solve the problem of low thermal conductivity of the titanium alloy during heating and serious inhalation at high temperatures, sectional heating is usually employed. In the first stage, the billet is slowly heated to 650-700 ° C and then rapidly heated to the desired temperature. Since titanium inhales less at temperatures below 700 ° C, the total penetration of the segmented heating oxygen in the metal is much less than in the usual heating.
The use of segmented heating can shorten the residence time of the billet at high temperatures. Although titanium has a low thermal conductivity at low temperatures, its thermal conductivity is close to that of steel at high temperatures. Therefore, when titanium is heated to 700 ° C, it can be heated to a higher temperature than steel.
For precision forgings requiring high surface quality, or important forgings with small margins (such as compressor blades, discs, etc.), the billet is preferably heated in a protective atmosphere (argon or helium), but the investment is large and the cost is high. It is high and there is still the danger of being polluted by air after it is discharged. Therefore, the coating is often coated with a glass-coated lubricant and then heated in a common box-type resistance furnace. The glass lubricant not only avoids the formation of scale on the surface of the blank, but also reduces the thickness of the alpha layer and can provide lubrication during the deformation process.
If the work is interrupted for a short time, the temperature of the furnace containing the billet should be lowered to 850 ° C. When the work continues, the furnace temperature will be raised to the initial forging temperature at the possible speed of the furnace power. When the work is interrupted for a long time, the billet should be discharged and placed on asbestos board or dry sand for cooling.
Free forging is mainly used for the initial processing of ingots, that is, the manufacture of semi-finished products of round, square or rectangular sections. Free forging ratio die forging in single or small batch production is also economically more reasonable, and free forging is also commonly used to make large size blanks.
From ingot to finished bar, the forging process is usually completed in three stages：
Its initial forging (opening) temperature is 150 to 250 ° C above the β transition point, at which time the plasticity of the cast structure is the best. At the beginning, the ingot should be deformed by tapping and quick-breaking until the primary coarse-grained structure is broken. The degree of deformation must be kept within the range of 20% to 30%. The ingot is forged into the desired section and then cut into a sized blank.
The plasticity increases after the fracture of the cast structure. Aggregation recrystallization is aggravated with increasing temperature, longer holding time and grain refinement. In order to prevent aggregation recrystallization, the forging temperature must be gradually reduced with grain refinement, and the heating and holding time should be strictly controlled.
It is forged at 80-120 ° C above the temperature of the β transition point, alternately 2 to 3 times of upsetting and lengthening, while alternately changing the axis and the edge. This results in a very uniform recrystallized fine grain structure having a beta zone deformation characteristic throughout the cross section of the blank. If the blank is rolled on a rolling mill, it is not necessary to perform such multi-directional drawing.
3，The second multi-directional repeated pull
It is the same as the first multi-directional repeated drawing method, but the initial forging temperature depends on whether the semi-finished product after forging is the blank of the next process or the delivery product. If it is the blank for the next process, the initial forging temperature can be 30~50 °C higher than the β transformation temperature; if it is delivered, the initial forging temperature is 20~40 °C below the β transformation temperature, because the thermal conductivity of titanium is low, in the free forging equipment When the upper billet is thick or long, if the tool preheating temperature is too low, the impact speed of the equipment is low and the degree of deformation is large, and an X-shaped shear band is often formed in the longitudinal section or the cross section. This is especially true when the hydraulic press is not isothermally thick. This is because the tool temperature is low, and the contact between the blank and the tool causes the surface of the metal blank to be chilled. During the deformation process, the deformation heat generated by the metal is too late to conduct heat to the periphery, and a large temperature gradient is formed from the surface layer to the center. As a result, the metal forms a strong flow. Strain band. The greater the degree of deformation, the more obvious the shear band, and finally the crack is formed under the opposite tensile stress. Therefore, when freely forging titanium alloy, the striking speed should be faster, try to shorten the contact time between the blank and the tool and preheat the tool to a higher temperature as much as possible, and at the same time properly control the degree of deformation in one stroke.
When forging, the corners are cooled fastest. Therefore, the blank must be turned over several times when the length is extended, and the hammering force is adjusted to avoid an acute angle. Forging on the hammer, the initial stage should be lightly beaten, the degree of deformation should not exceed 5% to 8%, and then the deformation can be gradually increased.
Die forging is usually used to make the final blank that is close to the finished product in shape and size, and then only heat treated and machined. The forging temperature and degree of deformation are the basic factors that determine the microstructure and properties of the alloy. The heat treatment of titanium alloy is different from the heat treatment of steel and does not play a decisive role in the microstructure of the alloy. Therefore, the process specification of the final step of titanium alloy die forging has a particularly important role.
In order to make the titanium alloy die forgings obtain high strength and plasticity at the same time, the overall deformation of the blank must be not less than 30%, the deformation temperature does not exceed the phase transition temperature, and the temperature and deformation degree are all in the entire deformed blank. It may be evenly distributed.
The microstructure and performance uniformity of titanium alloy die forgings is inferior to that of steel forgings. In the intense flow zone of metal, after recrystallization heat treatment, the low magnification is fuzzy crystal, and the high magnification is equiaxed fine crystal; in the difficult deformation zone, the microstructure tends to retain the state before deformation due to small deformation or no deformation. Therefore, in the forging of some important titanium alloy parts (such as compressor discs, blades, etc.), in addition to controlling the deformation temperature below TB and the appropriate degree of deformation, it is very important to control the microstructure of the original blank, otherwise, the coarse-grained structure Or some defects will be inherited into the forging, and the subsequent heat treatment can not be eliminated, which will cause the forging to be scrapped.
When the hammer is forged into a complex titanium alloy forging, in the sharply deformed region where the thermal effect is locally concentrated, even if the heating temperature is strictly controlled, the temperature of the metal may exceed the TB of the alloy. For example, when a titanium alloy blank having an I-shaped cross section is forged, the hammer is excessively heavy, and the temperature in the middle portion (the web portion) is about 100 ° C higher than the edge portion due to the effect of the deformation heat. In addition, in the hard-to-deformation zone and the region having the critical deformation degree, the coarse-grained structure having relatively low plasticity and long-lasting strength is easily formed during heating after the die forging. Therefore, the mechanical properties of forgings with complex forgings on the hammer are often very unstable.
Reducing the die forging heating temperature can eliminate the risk of local overheating of the blank, but it will lead to a sharp increase in deformation resistance, increase tool wear and power consumption, and must use a higher power device for hammering and forging, using multiple tapping methods. It also reduces local overheating of the blank. However, this is a forging that has to be heated to reduce the amount of heat lost by the contact between the blank and the colder mold, and the plasticity and permanent strength requirements of the deformed metal are not too high. Hammer forging is better. However, the beta alloy should not be hammer forged, because the multiple heating during the die forging process will adversely affect the mechanical properties. Compared with the forging hammer, the working speed of the press (hydraulic press, etc.) is greatly reduced, and the deformation resistance and deformation heat effect of the alloy can be reduced. When the titanium alloy is die-wrapped on a hydraulic press, the unit die forging force of the blank is about 30% lower than that of the upper die forging, thereby improving the life of the die. The reduction in thermal effects also reduces the risk of metal overheating and temperature rise above TB.
When die-forging with a press, the heating temperature of the blank can be reduced by 50 to 100 ° C under the same conditions of unit pressure and forging die forging. In this way, the interaction between the heated metal and the periodic gas and the temperature difference between the blank and the mold are correspondingly reduced, thereby improving the uniformity of the deformation, the uniformity of the structure of the die forging is also greatly improved, and the consistency of the mechanical properties is also improved.
To reduce the deformation speed, the most obvious numerical increase is the surface shrinkage rate, which is most sensitive to the tissue defects caused by overheating.
The deformation of titanium alloys is characterized by the difficulty of flowing into deep, narrow cavities than steel. This is because the deformation resistance of titanium is high, the friction with the tool is large, and the contact surface of the blank cools too fast. To improve the fluidity of titanium alloys and improve mold life. The usual practice is to increase the forging pitch and fillet radius and use the lubricant: the height of the edging bridge on the forging die is larger than that of steel, which is generally about 2 mm.
In order to make the groove easy to fill, it is sometimes possible to use a burr groove having a non-uniform bridge size to limit or accelerate the flow of metal to a certain portion of the groove. For example, a rectangular box forging (shown in Figure 12) has thin front and rear side walls; the left and right side walls are thicker. When the burr groove shown by B-B is used around the box member, the resistance of the metal flowing into the left and right side walls is small, so that the metal is difficult to flow to the thin front and rear side walls, and the filling is not satisfactory. Later, the front and rear side walls still use the burr groove shown by B-B, and the left and right side walls are changed to the burr groove shown by A-A. Due to the wide size of the bridge and the obstruction of the damping groove, the thin front and rear side walls are completely filled.
Moreover, the metal is saved by the use of the aforementioned burr groove.
One of the most effective ways to improve the flowability of titanium alloys and reduce the deformation resistance is to increase the preheating temperature of the mold. Isothermal die forging and hot die forging developed in the past two or three decades at home and abroad provide a feasible method for solving the large and complex titanium forgings. This method has been widely used in the production of titanium alloy forgings.
When the titanium alloy is die-forged by the closed die forging method, the life of the mold is lowered due to the large pressure. Therefore, closed die forging must strictly limit the volume of the original blank, which complicates the stocking process. Whether to use closed die forging depends on both cost and process feasibility. In the case of open die forging, the burr loss accounts for 15% to 20% of the weight of the blank, and the process waste of the clamping portion (if it is required to be retained under the die forging condition) accounts for 10% of the weight of the blank. The relative loss of raw metal is usually increased with the decrease of the weight of the blank. Some structural asymmetry, large cross-sectional area difference and forgings with hard-to-fill parts can be used up to 50%. Closed die forging has no burr loss, but the blanking process is complicated, and it is necessary to add more transitional shaped grooves, which will undoubtedly increase the auxiliary cost.
Titanium alloys tend to bend at their own weight at high temperatures, which increases the tendency of the product to twist during die forging, cooling and heat treatment. Distortion is particularly noticeable in articles with sharp cross-section changes or particularly thin sections, so titanium alloys are often required. Forgings are calibrated to meet dimensional requirements.
Unlike aluminum alloys, titanium alloys are not easy to be cold-corrected. Because of their high yield strength and high modulus of elasticity, they produce a large rebound. Therefore, the calibration of titanium alloy forgings mainly depends on creep correction and thermal correction. The former is more universal. The creep correction of most titanium alloys can be completed during the annealing and aging process, and the temperature is the annealing and aging temperature. However, if the annealing time is less than about 540-650 ° C, the time required for the creep correction to complete for different alloys may be extended.
Creep calibration requires simple or complex fixtures and dies. The thermal correction in the mold is mostly used for medium-sized forgings, which are heated to an annealing or aging temperature for thermal calibration, and then subjected to stress release at a temperature lower than the thermal correction temperature.
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