Effect of heat treatment process on microstructure and properties of TC4 titanium alloy

Titanium and its alloys are widely used in aerospace, automotive, chemical and marine industries due to their advantages of low density, high specific strength and good corrosion resistance. TC4 titanium alloy contains 6% α-phase-stable element Al and 4% β-phase-stable element V. It belongs to the typical α + β type two-phase thermally strong titanium alloy of Ti-Al-V series and has good mechanical properties and technological properties. It can be processed into the supply of semi-finished products such as bars, profiles, plates, forgings, etc., and is more and more popular. At present, the domestic research mainly focuses on the high-temperature performance, creep performance and thermal stability of TC4 titanium alloy, and there is relatively little research on how to formulate a reasonable heat treatment process to meet its actual performance. In this paper, it is of great theoretical and practical significance to study the influence of the heat treatment process on the microstructure and mechanical properties of the TC4 alloy sheet by different processes.

Sponge titanium, high-purity aluminium (99.99%) and aluminium-vanadium alloy are smelted in a vacuum water-cooled copper crucible non-consumable electric arc furnace at a certain ratio, electromagnetic field stirring, argon protection. The alloy composition after smelting is (mass fraction,%): 6.29Al, 4.14V, 0.029Fe, 0.023C, 0.19o, and the balance is Ti. In order to ensure the uniformity of the chemical composition of the sample, the TC4 alloy bar was prepared by three times of reflow melting, rolled into a plate with a thickness of 3 mm, and subjected to stress relief annealing treatment at 650 ° C × 4h. The stress-relieved and annealed sheet is processed into microstructure observation samples and tensile samples, and different heat treatments are carried out: annealing (790 ℃ × 3h), solution quenching (980 ℃ × 1h, water cooling), solution ageing ( 980 ℃ × 1h, water cooling + 580 ℃ × 8h, furnace cooling). The heat-treated samples were tested for tensile properties.

After annealing, the TC4 alloy is cooled in the furnace and both phases recrystallize. The α-phase is recrystallized, and small polygonal crystal grains are precipitated in the deformed matrix, and the secondary α is precipitated in the recrystallized β-phase, and the α-phase structure is distributed on the matrix of the β-transformed structure, and the structure is relatively uniform. Since the internal stress is eliminated, the plasticity and tissue stability are improved, but the strength and hardness are reduced. After solution quenching, the aspect ratio of the alpha sheet is reduced, the straight alpha sheet is distorted, and the continuous beta phase boundary is destroyed, forming a thin sheet or basket-like alpha, beta phase from the high-temperature region, rapid cooling is too late Forming the α phase and forming the metastable β phase. The microstructure at room temperature is martensite α '' and metastable β phase. The strength and hardness are improved, but the plasticity is reduced more. After solution aging, the martensite α ”and metastable β phases are partially decomposed and transformed into stable and dispersed α phases and β phases, whose strength and hardness are higher than that of furnace cooling, but the plasticity is lower than that of furnace cooling The overall performance of the alloy has been improved.

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High-strength and high-toughness β-type titanium alloy

The β-type titanium alloy was originally the B120VCA alloy (Ti-13v-11Cr-3Al) developed by American 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 good combination of strength and fracture toughness through solution-aging treatment. The new high-strength and high-toughness β-type titanium alloys are representative of the following types: Ti1023 (Ti-10v-2Fe- # al), which is equivalent to the 30CrMnSiA high-strength structural steel commonly used in aircraft structural parts, and has excellent forging properties ; Ti153 (Ti-15V-3Cr-3Al-3Sn), the cold workability of this alloy is better than industrial pure titanium, the room temperature tensile strength after aging can reach more than 1000MPa; β21S (Ti-15Mo-3Al-2.7Nb-0.2Si ), The alloy is a new type of oxidation-resistant, ultra-high-strength titanium alloy developed by Timet Division of American Titanium Metal Company. It has good oxidation resistance, excellent cold and hot processing properties, and can be made into a foil with a thickness of 0.064mm; SP-700 (Ti-4.5Al-3V-2Mo-2Fe) titanium alloy developed by Japan Steel Pipe Corporation (NKK) has high strength, superplastic elongation of up to 2000%, and superplastic forming temperature is higher than Ti-6Al- 4V low 140 ℃, can replace Ti-6Al-4V alloy with superplastic forming-diffusion connection (SPF / DB) technology to manufacture various aerospace components; BT-22 developed by Russia (TI-5v-5Mo-1Cr-5Al ), Its tensile strength can reach more than 1105MPA.

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Medical titanium alloy

Titanium is non-toxic, light-weight, high-strength, and has excellent biocompatibility. It is ideal medical metal material and can be used as an implant for implantation in the human body. At present, Ti-6Al-4vELI alloy is still widely used in the medical field. However, the latter will precipitate a very small amount of vanadium and aluminum ions, which reduces its cell suitability and may cause harm to the human body. This problem has long aroused widespread concern in the medical community. America as early as the mid-1980s

Beginning with the development of aluminum alloys, vanadium-free, and biocompatible titanium alloys, they are used in orthopedics. 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, the corrosion strength, fatigue strength, and corrosion resistance of these alloys are better than Ti-6Al-4v ELI. Compared with α + β titanium alloy, β titanium alloy has higher strength, better cutting performance, and toughness, and is more suitable for implantation in the human body. In the United States, 5 kinds of β titanium alloys have been recommended to the medical field, namely TMZFTM (TI-12Mo- ^ Zr-2Fe), Ti-13Nb-13Zr, Ti metal 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 titanium alloys with high strength, low elastic modulus, excellent affordability, and corrosion resistance are likely to replace the Ti-6Al-4V ELI alloy widely used in the medical field.

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Testing standards for titanium pressure vessels and titanium flanges

Because the properties of titanium and titanium alloy materials have many similarities or similarities with steel, the basic criteria for container design and three failure modes considered in GB150-2011 "Steel Pressure Vessels" are strength failure, stiffness failure, and stability Failure, it is also applicable to titanium equipment. At present, in addition to Russia's special titanium equipment strength calculation standards, the standards of other countries, such as American ASME, Japanese JIS, etc., together with steel pressure component standards,

Use the same calculation formula. Therefore, this standard stipulates: the design and calculation of all parts of titanium tube manufacturers of all-titanium, titanium-clad steel and titanium-lined steel equipment are based on GB150-2011 "Steel Pressure Vessels".

(1) Internal pressure cylinder and internal pressure spherical shell

The calculation formula of the internal pressure cylinder and internal pressure spherical shell is consistent with GB150-2011. For the theoretical explanation, please refer to the standard interpretation of GB150-2011 Chapter 5 "Internal Pressure Cylinder and Internal Pressure Spherical Shell".

(2) External pressure cylinder and external pressure spherical shell

The calculation method and steps of the external pressure cylinder and spherical shell are according to GB150-2011; the calculation chart of the wall thickness of the external pressure cylinder and a spherical shell made of titanium is obtained from domestic measurement using domestic titanium plates.

The design calculation of the external pressure cylinder reinforcement ring is based on GB150-2011. The material of the reinforcement ring can be titanium or steel. The former is connected to the shell by intermittent welding, and the latter is connected to the shell by brazing.

(3) The design formula of the titanium head is consistent with GB150-2011. For the theoretical explanation, please refer to the standard interpretation of Chapter 7 "Head" in GB150-2011. The calculation of the external pressure head can use the external pressure wall thickness calculation chart in Chapter 6 of this standard.

(4) The calculation formula of opening and opening reinforcement is consistent with GB150-2011. For the theoretical explanation, please refer to the standard interpretation of Chapter 8 "Opening and Opening Reinforcement" of GB150-2011.

The large opening diameter in GB150-2011 without additional reinforcement is canceled. It is up to the designer to decide whether and how to reinforce the container opening. Strength reduction factor in the reinforcement area required for the opening of the cylinder or spherical shell

All f = +-10

(5) The flanges used in titanium pressure vessels usually adopt the following structures, namely titanium tube flange and titanium welding ring jacket steel loose flange, titanium lined steel flange, and thick wall titanium tube The thread is connected with a threaded flange.

Designers of these structures can refer to GB150-2011 to take flanges of similar structures for strength calculation. The calculation formulas are not listed in this standard.

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Titanium alloy material grinding technology

At this stage, most of the aero-engine parts are made of titanium alloy materials. According to the assembly requirements, the precision of the surface dimensions and the surface roughness of the finished machining need to be ground to ensure the quality of the surface of the parts. Due to titanium alloy materials

Due to its own physical and mechanical properties, surface burns and reduced surface integrity are likely to occur during grinding. At present, it is urgent to solve the problem of grinding titanium alloy materials, so it is very necessary to select the appropriate grinding wheel.

1 Performance analysis of titanium alloy materials

The types of titanium alloys are roughly divided into three categories, α titanium alloys, β titanium alloys, and α + β titanium alloys. It has a small specific gravity, high specific strength, high-temperature resistance, corrosion resistance, super memory, non-magnetic, low elastic modulus, and biocompatibility.

A series of excellent ones make it used in a wide range of fields. Titanium has a melting point of 1668 ° C and a boiling point of 3400 ° C, which is higher than that of nickel-iron. Therefore, light heat resistance has become an excellent foundation, and it can work at 500 ° C for a long time. New

The long-term working temperature of titanium alloy is even higher, its strength is 300 times higher than that of an aluminum alloy at 300-350 ℃. Commonly used α + β titanium alloy has a strength of 1.2GPa, a specific gravity of 0.44MPa, and a specific strength of 23-27, which are higher than that of alloy steel. Titanium alloy

Its tensile strength can exceed 1.5GPa, and a large force must be applied to its processing, which is a typical difficult-to-process material.

The thermal conductivity of titanium is 0.036cal, and the thermal conductivity of TC11 titanium alloy is even worse. The elastic modulus of titanium is about 1/2 of that of steel. It has a high resilience during processing and is easy to vibrate.

Titanium alloys contain oxygen, hydrogen, nitrogen, carbon, and sometimes also impurity elements such as silicon and iron. These elements react strongly and exist in the lattice in a gap form, which can increase the strength of the titanium alloy and reduce the plasticity.

Fracture toughness, low-temperature toughness, fatigue strength, corrosion resistance, cold forming, and weldability are deteriorated. Titanium alloys are highly chemical at high temperatures. At certain grinding temperatures, titanium forms oxide and nitride protective films, making the surface layer hard

It becomes brittle, reduces elasticity, and increases the degree of work hardening. It is easy to attach during grinding, block the grinding wheel, cause overheating of the grinding, and reduce the surface integrity.

2 Selection of titanium alloy grinding wheels

2.1 Titanium alloy grinding requires small adhesion of the grinding wheel, small wear, easy clogging, and low grinding temperature

This mainly includes the particle size of the abrasive, the tissue shape, and the size of the binder. The ordinary grinding wheel is composed of abrasive bond and pores. The role of the abrasive is to grind the processed material to form a surface that meets the requirements. The role of the binder is to stick the abrasive

Together, a certain shape and hardness are formed, so that the abrasive particles maintain a stable trajectory during the grinding process and can self-detach. The air holes are used for chip removal in the grinding process, cooling, and lubrication. Common abrasives include just

Jade series (alumina), and silicon carbide series. For grinding titanium alloys, silicon carbide grinding wheels should be selected.

2.2 Selection of binder

There are two types of binders: resin and ceramic:

1) The ceramic binder has strong particle capacity, good thermal stability, and chemical stability, waterproof, heat resistance, corrosion resistance, small wear, long-term grinding performance, porosity, not easy to block, and high productivity. Brittle, no

Can withstand large impact loads.

2) The resin bond grinding wheel has high strength, elasticity, good impact resistance, poor thermal stability, poor corrosion resistance, and will soften and lose strength at high temperatures.

Grinding titanium alloy should choose the ceramic bond grinding wheel.

2.3 Selection of granularity

Grinding titanium alloy usually uses 36 # -80 # particle size.

2.4 Grinding wheel structure hardness

Grinding titanium alloy should usually choose soft or medium hardness, loose structure of the large pore grinding wheel.

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What is the main trend of titanium metal materials in the future?

In terms of future development trends, the output demand for titanium metal materials increases rapidly. The application of non-ferrous metal materials is mainly used as functional materials and structural materials. If a metal is used only as a functional material, the market is relatively small, and the space as a structural material is quite big. If you want to answer where is the growth space of titanium metal in the future, where is the explosive growth time and catalyst? It depends on the development of civilian titanium, especially in the field of structural and functional titanium, the use of titanium in civil and structural and functional applications has increased It may make titanium the fourth metal.

In addition to production technology, it is necessary to consider the economic applicability of titanium production. Why titanium is used in aviation titanium is mainly because it is expensive, so the production cost of titanium may be different.

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Introduction of heat treatment commonly used in titanium alloys!

Commonly used heat treatment methods for metal materials such as titanium alloys and titanium alloy plates are annealing, solution treatment, and aging treatment. Annealing is to eliminate internal stress, improve plasticity and organizational stability to obtain better overall performance. Usually, the annealing temperature of α alloy and (α + β) alloy is selected from (α + β) ─ → β phase transition point 120 ～ 200 ℃; solid solution and aging treatment is rapid cooling from high-temperature area to obtain martensite α ′ Phase and meta-stable β phase, and then incubate in the middle-temperature zone to decompose these meta-stable phases to obtain finely dispersed second phase particles such as α phase or compound to achieve the purpose of strengthening the alloy. Normally, the quenching of (α + β) alloy is carried out at (-40 + 100 ° C) below (α + β) ─ → β phase transition point and the quenching of metastable β alloy is at 40 ～ 80 ℃ above (α + β) ─ → β phase transition point gets on. Aging treatment temperature is generally 450 ~ 550 ℃.

Introduction of heat treatment commonly used in titanium alloys

In summary, the heat treatment process of titanium alloy can be summarized as:

(1) Stress relief annealing: The purpose is to eliminate or reduce the residual stress generated during processing. Prevent chemical attacks and reduce deformation in some corrosive environments.

(2) Complete annealing: The purpose is to obtain good toughness, improve processing performance, facilitate reprocessing, and improve the stability of size and structure.

(3) Solution treatment and aging: The purpose is to improve its strength. The α titanium alloy and the stable β titanium alloy cannot be strengthened and heat-treated, and only annealing is performed in the production. The α + β titanium alloy and the metastable β titanium alloy containing a small amount of α phase can be further strengthened by 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 deformation heat treatment.

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