WO2018199791A1

WO2018199791A1 - Titanium alloy-based sheet material for low-temperature superplastic deformation

Info

Publication number: WO2018199791A1
Application number: PCT/RU2017/000266
Authority: WO
Inventors: Михаил Оттович ЛЕДЕР; Игорь Юрьевич ПУЗАКОВ; Наталья Юрьевна ТАРЕНКОВА; Александр Владимирович БЕРЕСТОВ; Наталия Георгиевна МИТРОПОЛЬСКАЯ; Роберт Дэвид БРИГГС
Original assignee: Публичное Акционерное Общество "Корпорация Всмпо-Ависма"; Компания Боинг
Priority date: 2017-04-25
Filing date: 2017-04-25
Publication date: 2018-11-01
Also published as: CN111279003B; BR112019022330B1; CN111279003A; CA3062762A1; RU2691434C2; JP2020517834A; EP3617335A1; RU2017139320A3; EP3617335B1; JP7028893B2; RU2017139320A; US20200149133A1; BR112019022330A2; EP3617335A4

Abstract

The technical result achieved by performing this invention is the production of sheets from a titanium alloy, the chemical composition of which is optimally balanced with the production capabilities of the known current technologies of the final product with the low-temperature superplastic deformation properties. The result is achieved by a sheet material for low-temperature superplastic deformation, which is based on a titanium alloy comprising, wt %: 4.5-5.5 Al; 4.5-5.5 V; 0.1-1.0 Mo; 0.8-1.5 Fe; 0.1-0.5 Cr, 0.1-0.5 Ni; 0.16-0.250 the remainder being titanium and impurities, wherein the value of the structural molybdenum equivalent, [Mo]eq. is higher than 5, and the aluminium structural equivalent, [Al]eq. is lower than 8; the equivalents are determined using the following formulae: [Mo]eq.=[Mo] + [V]/1.5 + [Cr]1.25 + [Fe]2.5 +[Ni]/0.8 [Al]eq.=[Al] + [O]10 + [Zr]/6.

Description

SHEET MATERIAL BASED ON TITANIUM ALLOY FOR LOW TEMPERATURE SUPERPLASTIC DEFORMATION. The technical field to which the invention relates.

The invention relates to the field of sheet materials (semi-finished products) based on titanium alloys, which are suitable for manufacturing products by low-temperature superplastic deformation (SPD) at a temperature of 775 ° C, and can be used as a cheaper alternative to sheet semi-finished products, made - made of T.-6A1-4V alloy.

The prior art.

The term “superplastic deformation” generally refers to a process in which a material (alloy) is superplastically deformed, exceeding the usual limit of plastic deformation (over 500%). SPD can be performed with certain materials with superplastic properties in a limited range of temperatures and strain rates. For example, sheets of titanium alloys can usually be superplastically formed (deformed) in the temperature range of about (900-1010) ° C at a strain rate of about 3 · 10 ^"4 s ^{" 1} .

From a production point of view, as a result of a decrease in the temperature of the SPD molding, significant advantages arise. For example, lowering the SPD molding temperature as a result can lead to a reduction in the cost of the stamp, an increase in its service life, and the potential for using less expensive steel stamps. In addition to this, inhibition of

SUBSTITUTE SHEET (RULE 26) the use of a layer enriched with oxygen (alpha layer) and the formation of scale, which improves the yield of products and eliminates the need for chemical etching. In addition to this, lower deformation temperatures can inhibit grain growth, while maintaining the benefits of having smaller grains after SPD molding operations.

There are two approaches to improving the superplastic formability of titanium alloy sheet materials. The first approach is to develop a special thermomechanical treatment that creates small grains having sizes in the range of only 2 to 1 μm or less, which improves creep along the grain boundaries. In particular, there is a known method of manufacturing sheets for deformation at a temperature lower than during conventional molding of products from material Ti-6A1-4V (RF Patent jYs 2243833, IPC B21B1 / 38, publ. 10.01.2005).

The second approach is to develop a new system of sheet materials from titanium alloys, which demonstrates the presence of superplasticity with larger grain sizes of the material due to:

- optimization of volume fraction and morphology of two phases,

- faster diffusion, which accelerates creep along the grain boundaries due to the presence in the alloy, for example, Fe and Ni as fast diffusers,

- lower temperature polymorphic transformation (TPP). Thus, with the optimal selection of the chemical composition of the alloy, it is possible to achieve satisfactory characteristics of superplastic deformation (molding) at low temperature without resorting to special processing methods necessary

SUBSTITUTE SHEET (RULE 26) to achieve very small grain sizes.

According to the degree of alloying, biphasic (a +) -titanium alloys belong to the class of alloys with a structural equivalent in terms of molybdenum [Mo] equiv. equal to from 2.5 to 10%. (Kolachev B.A., Polkin I.S., Talalaev V.D. Titanium alloys from different countries: Handbook. // M .: VILS. 2000, 316 p. (P.13-16)). Such alloys are usually alloyed with aluminum and β stabilizers to fix the β phase. In alloys of this class in the annealed state, the amount of β-phase can vary from 5 to 50%. In this regard, the mechanical properties vary over a fairly wide range. These alloys are most widespread in Russia and abroad, especially the Ti-6A1-4V alloy, which is explained by its successful alloying. (Materials Properties Handbook: Titanium Alloys, R. Boyer, G. Welsch, E. Collings, ASM International, 1998, 1048p. (P. 486-488)). Aluminum in this alloy increases strength and heat-resistant properties, and vanadium is one of the few elements that increase not only strength properties, but also ductility. From alloys of the Ti-6A1-4V system, rods, pipes, profiles, forgings, stampings, plates, sheets, tape and foil are obtained. They are used for the manufacture of welded and prefabricated aircraft structures, a number of structural elements of aviation and rocket technology, as well as for the manufacture of medical implants in traumatology, orthopedics, and dentistry.

A known method of manufacturing sheet semi-finished products from titanium alloys suitable for low-temperature superplastic deformation from VT6 alloy, an analog of Ti-6A1-4V alloy, (RF Patent K “2224047, IPC C22F1 / 18, B21BZ / 00, publ. 20.02. 2004). The method allows the manufacture of sheet semi-finished products from titanium alloys with a homogeneous submicrocrystalline structure

SUBSTITUTE SHEET (RULE 26) (grain size less than 1 μm) suitable for low temperature superplastic deformation. The method is expensive, low productivity and requires specialized equipment.

The Ti-6A1-4V alloy with a submicrocrystalline structure obtained by intensive plastic deformation (IPD) by the method of comprehensive forging and having superplastic properties is known. The microstructure of the alloy is characterized by grains and subgrains of the a- and β-phases with an average size of 0.4 μm, a high level of internal stresses and elastic distortions of the crystal lattice, as evidenced by the inhomogeneous diffraction contrast and high density of dislocations in the electron-microscopic images of the structure. (S. Zherebtsov, G. Salishchev, R. Galeyev, K. Maekawa, Mechanical properties of Ti-6A1-4V titanium alloy with submicrocrystalline structure produced by severe plastic deformation. // Materials Transactions. 2005; V. 46 (9 ): 2020-2025.). For the manufacture of sheet semi-finished products from this alloy, low-productivity and costly operations of IPD by the comprehensive forging method are required, which significantly increase the price of the final product.

A known method of manufacturing thin sheets of two-phase titanium alloy and products from these sheets. The method includes the manufacture of sheet semi-finished products from an alloy containing, wt.%: 3.5-6.5 A1, 4.0-5.5 V, 0.05-1.0 Mo, 0.5-1.5 Fe, 0.10-0.2 O, 0.01-0.03 C, 0.005-0.07 Cg, 0.01-0.5 Zr, 0.001-0.02 N, the rest is titanium, while the chemical the composition is regulated by the strengths of aluminum [A1] "£ _в = 6.0 - 11.55 and molybdenum [Mo]" £ _B = 3.5 - 5.6 equivalents (RF patent 2555267, IPC C22F1 / 18 В21ВЗ / 00, publ. July 10, 2015 ) is a prototype.

SUBSTITUTE SHEET (RULE 26) The semi-finished sheet product with a thickness of <3 mm obtained according to this patent is not suitable for industrial production due to the low stability of properties for SPD. The reason is that the use of strength alloys as chemical composition regulators does not allow us to control the necessary optimal relationships between the content of alloying additives in the alloy and the necessary structural properties of the alloy during SPD operations in sheet semi-finished products. In addition, Si and Zr are present in the alloy, which form silicides on the grain surface, which hinder intergranular sliding and lead to process instability.

The aim of the present invention is to obtain a sheet material based on an (a +) -titanium alloy having the properties of low-temperature superplastic deformation with a grain size of more than 2 μm. This sheet material has stable properties and is a cheaper alternative to sheet semi-_ ^ apricots made of Ti-6Al-4Vc alloy with a smaller grain size.

The technical result achieved during the implementation of the invention is the production of titanium alloy sheets in which the chemical composition is optimally balanced with the production capabilities based on known standard technologies of the final product having the properties of low-temperature superplastic deformation.

Disclosure of the invention.

The specified technical result is achieved by the fact that the sheet material for low-temperature superplastic deformation based on a titanium alloy containing wt.% 4,5-5, 5A1, 4,5-

SUBSTITUTE SHEET (RULE 26) 5.5V, 0.1-Ι, ΟΜο, 0.8-l, 5Fe, 0, l-0.5Cr, 0, l-0.5Ni, 0.16-0.250, the rest is titanium and impurities, in which structural molybdenum equivalent [Mo] equiv> 5, and aluminum structural equivalent [A1] equiv <8, equivalents are determined by the expressions:

[Mo] equiv = [Mo] + [V] / l, 5 + [Cr] x 1, 25 + [Fe] * 2.5 + [Ni] / 0.8

[A1] equiv = [A1] + [O] x lO + [Zr] / 6.

The sheet material for low-temperature superplastic deformation has a structure with a grain size not exceeding 8 microns.

Sheet material for low-temperature superplastic deformation has superplastic properties at a temperature of 775 ± 10 ° С.

Sheet material for low-temperature superplastic deformation at a temperature of 775 ± 10 ° C has an α / β phase ratio from 0.9 to 1.1.

Sheet material for low-temperature superplastic deformation, in which the number of alloying elements diffusing between the a and β phases in the SPD process is at least 0.5% and is determined by the following ratio:

Where:

Q is the number of diffusing alloying elements in the material during SPD, wt.%.

p is the number of alloying elements in the material,

| Δτη | - the absolute value of the change in the content of the alloying element in β- and a-phases, wt.% in the SPD process.

| At | - calculated by the formula:

\ At \ = (πιβΐ - mal) - (πιβ2 - t 2), wt.%

SUBSTITUTE SHEET (RULE 26) Where:

πιβΐ - content of the alloying element in the β-phase before SPD, wt.%, ιτιβ2 - content of the alloying element in the β-phase after SPD, mass. %

mod is the content of the alloying element in the α phase before SPD, wt.%, ta2 is the content of the alloying element in the α phase after SPD, mass. %

The proposed sheet material has a complex of high technological and structural properties. This is achieved due to the optimal selection of alloying elements and their ratio in the alloy of the material.

A group of stabilizers.

Aluminum, which is used in almost all industrial alloys, is the most effective hardener, improving the strength and heat-resistant properties of titanium. When the aluminum content in the alloy is less than 4.5%, the required alloy strength is not achieved, when the content is more than 5.5%, an undesirable decrease in ductility and an increase in TIP occur.

Oxygen increases the temperature of the allotropic transformation

0

titanium. The presence of oxygen in the range of 0.16-0.25% increases the strength and does not have a noticeable effect on the decrease in ductility.

The group of β stabilizers that are represented in the present invention (V, Mo, Cr, Fe, Ni) are widely used in industrial alloys.

Vanadium in an amount of 4.5-5.5%, iron in an amount of 0.8-1, 5% and chromium in an amount of 0.1-0.5% increase the strength of the alloy and practically

SUBSTITUTE SHEET (RULE 26) skiing does not reduce ductility.

The introduction of molybdenum in the range of 0.1-1.0% ensures its complete solubility in the α-phase, which allows to obtain the necessary strength characteristics without reducing the plastic properties.

The proposed alloy contains iron in an amount of 1.0-1.5% and nickel in an amount of 0.1-0.5%, which are the most diffusion-mobile β-stabilizers that favorably affect the intergranular slip during SPD.

Among the structural factors affecting the effectiveness of SPD, it is necessary first of all to single out the grain size, which should not exceed 8 microns for the claimed material (experimental data).

It is known that the superplastic flow of a material is largely realized due to phase transformations in biphasic titanium alloys, and the ratio of phases α / β at the SPD temperature should be close to 1 (Kaybyshev O.A., Superplasticity of industrial alloys, M, Metallurgy , 1984, pp. 179-218.). This contributes to the emergence of equiaxial structure, contributing to intergranular slip. The driving force behind the spheroidization of structures is the desire to reduce surface energy. The growth of the grain boundary due to an increase in the β phase causes a change in the level of surface energy at the interface, which, in turn, leads to the activation of spheroidization. For the presence of the required amount of the β-phase in the SPD process, when the ratio α / β is close to 1, the value of the structural molybdenum equivalent [Mo] eq should be more than 5, and the value of the aluminum structural equivalent [A1] eq should not be more than 8. In addition, exceeding the aluminum equivalent above the specified value

SUBSTITUTE SHEET (RULE 26) leads to an increase in TPP, and, consequently, to an increase in the temperature of the realization of SPD.

The optimum temperature at which the superplastic properties of the claimed material are realized is 775 ± 10 ° C. Exceeding this temperature leads to grain growth, and lower to a decrease in the intensity of diffusion processes, which complicates the SPD process.

The amount of diffusing alloying elements of the alloy between the a and β phases must be at least 0.5%. This is explained by the fact that the activation energy of grain boundary diffusion is less than the activation energy of bulk diffusion, and diffusion transfer of atoms occurs along grain boundaries. In those regions of grain boundaries that are subjected to normal tensile stress, the concentration of vacancies is increased. In areas in which compressive stress acts, their concentration is reduced: the resulting difference in concentrations causes directional diffusion of vacancies. Since the migration of vacancies occurs through exchange of places with atoms, the latter will move in the opposite direction, intensifying intergrain gliding.

The invention is illustrated by drawings.

A brief description of the drawings.

In FIG. 1 and 2 show the structure of the alloys in the initial state, FIG. 3, 4 and 5 - loading curves obtained during SPD, in FIG. 6 is a graph of changes in true stress at a degree of deformation of 0.2 and 1, 1 (in the longitudinal direction) depending on [Mo] eq.

Detailed description and representative embodiments of the invention.

SUBSTITUTE SHEET (RULE 26) As a material for the study used sheet semi-finished products with a thickness of 2 mm To obtain sheet materials, six experimental alloys of various chemical composition were melted, which are presented in table 1.

Sheet materials 2 mm thick, manufactured according to the known technology for superplastic molding, were annealed at a temperature of 720 ° C for 30 minutes before being tested for superplasticity. followed by cooling in air. After this treatment, samples were cut from sheets in the longitudinal and transverse directions for mechanical tensile tests at room and elevated temperatures, which were then subjected to standard tests at room temperature to determine the strength, elastic and plastic characteristics .

Table 1. The chemical composition of the studied leaf

materials

SUBSTITUTE SHEET (RULE 26) Analysis of the structure of the materials in the initial state (Figs. 1 and 2) showed that it is close to equiaxial and consists mainly of alternating grains of a- and β-phases, which look like darker (a) or light (β) components. It should be noted that with an increase in the [Mo] equiv alloy, the volume fraction of β-phase grains tends to increase in the structure from the approximate ratio / β - 2/1 in alloy 2 to a ratio approaching 1/1 in alloys 3 ,four. The average grain size of the phases, measured on the microstructure images by the secant method, has a certain tendency to increase with increasing [Mo] eq and lies in the range of 2.8-3.8 μm (the minimum for alloy 2). It should be noted that in the material 5, the grain structure in the initial state is less uniform in comparison with other experimental alloys. In material 1, along with equiaxed grains, sections of sufficiently large elongated grains are observed. It can also be noted that the morphology of the β phase varies somewhat from alloy to alloy. If in alloy 2 with a minimum amount of alloying elements the β-phase is predominantly localized in separate volumes between particles of the α-phase, then already starting from alloy 5, it has a certain connection and, in addition to the grain structure, has the form thin interlayers between grains of the α phase. With an increase in [Mo] eq of the material, these layers tend to thicken.

Comparative analysis of the material structure in the deformed (working part) and unstrained (head region) condition at the SPD after (at a temperature of 775 ° C and a strain rate Sx 10 ^{* 4} ^'' in the longitudinal direction of the sheet) showed that the deformation in the working parts stimulates some grain growth compared to a practically undeformed head and the development of education

SUBSTITUTE SHEET (RULE 26) conglomerates from grains of a- and β-phases of a more complex shape.

Assessment of grain size showed that doping does not strongly affect the grain size of phases in alloys with maximum doping with β stabilizers, and it fluctuates within 3.5 ± 0.5 μm (undeformed part), 4 ± 0.5 μm (deformed part). At the same time, in alloy 2 with a minimum content of alloying elements, the grain size in the working part increases almost 2 times to 5 microns or more compared to the initial state.

Using the method of X-ray microspectral analysis (MRSA), the distribution of alloying elements between the a and β phases in the studied materials was studied in the initial state and after testing for superplasticity of longitudinal samples in the working deformed part and in the head region, which are presented in tables 2, 3 and 4 .

SUBSTITUTE SHEET (RULE 26) Table 2. The average chemical composition of the α-phase (in wt.%) In sheet materials after various treatments according to MRSA

SUBSTITUTE SHEET (RULE 26) Table 3. The average chemical composition of the β-phase (wt.%) In sheet materials after various treatments according to MRSA

SUBSTITUTE SHEET (RULE 26) The number of diffusing alloying elements in the material during SPD is determined by the formula:

Q = Σ / = ι | Δ ™ Ι wt.%

Where:

p is the number of alloying elements in the material,

I Dttg I - the absolute value of the change in the content of the alloying element in the β- and a-phases, wt.% In the SPD process.

| Dt | - calculated by the formula:

| At | = (πιβΐ — та!) - [τηβ2— та2) wt.% where:

πιβΐ - content of the alloying element in the β-phase before SPD, wt.%, Γηβ2 - content of the alloying element in the β-phase after SPD, mass. %

mccl is the content of the alloying element in the α phase before SPD, wt.%, ta2 is the content of the alloying element in the α phase after SPD, mass. %

Table 4 shows the calculated data on the number of diffusing alloying elements in the SPD process.

An analysis of the changes in the compositions of the a and β phases in the studied sheet materials after deformation showed that in the working part of the samples the difference in alloying elements between the a and β phases is greater than in the region of the heads of the samples that did not undergo plastic deformation (table 2, 3 and 4).

The obtained MRSA data were also used to estimate the volume fraction of phases in the material at the temperature

SUBSTITUTE SHEET (RULE 26) tests for superplasticity at a temperature of 775 ° C and are given in table 5.

Table 4.

Table 5.

The loading curves obtained during the tests are shown in FIG. 3, 4 and 5.

The properties of alloys in superplastic tests are given

SUBSTITUTE SHEET (RULE 26) Table 6.

A plot of the true stress at a degree of deformation of 0.2 and 1.1 (in the longitudinal direction) depending on the [Mo] equiv of the alloy is shown in FIG. 6.

Table 6

In material 1 (Fig. 3) with a minimum content of alloying elements, the most unstable process of SPD realization at 775 ° С is recorded with a characteristic “waviness” of the tension curves caused by the formation of a floating neck. The reason for this behavior of the material during SPD is the relatively large initial grain (more than 2.5 μm), which has a high growth rate during SPD (up to 5 μm) with an non-optimal phase / β ratio (2/1), which leads to activation of intragrain glide less favorable for SPF instead of optimal intergranular slippage.

SUBSTITUTE SHEET (RULE 26) In material 2 (Fig. 3), more doped with β stabilizers, the instability of the SPD realization process, which manifests itself in the form of a wavy tensile curve, decreases compared to alloy 1 due to an increase in the volume fraction of the β phase in the structure, but not noticeable hardening to degrees of deformation of 0.6-0.8 is observed, due to the development of dynamic recrystallization in regions with an incompletely worked out initial structure (the presence of elongated grains), which was not observed in other alloys studied.

In materials 3,5,6 (Figs. 4, 5) having a maximum content of β-stabilizers, with the exception of molybdenum (alloy 5), chromium (alloy 6) due to an increase in the structure of the volume fraction of the β-phase, in which the cohesion increases and intergranular slippage is realized more easily, tensile curves have less waviness as compared to materials 1, 2, and hardening is more actively realized as the degree of true deformation increases (table 3, Fig. 6). But at the same time, the presence of a “wave” at strain levels up to 0.6 is preserved, especially during tests in the transverse direction, which may be due to the initial texture state of the sheets, as well as the incompletely optimal phase ratio α / β (close to 3 to the 2nd). The absence of chromium in material 6 affects the tensile curves to a lesser extent than the absence of molybdenum in material 5, compared with material 3. One of the reasons may be the stronger effect of molybdenum additives on the stability of the SPD process than chromium, which is introduced in 2 -2.5 times less.

In material 4, having a maximum number of β-stabilizers and additionally doped with 0.3% nickel,

SUBSTITUTE SHEET (RULE 26) the most stable course of superplastic deformation at 775 ° C is observed both in the transverse and longitudinal directions with a minimum stress at the beginning of the flow, the absence of a pronounced “waviness” of the curve, and with monotonic hardening with an increase in the degree of deformation. This is due to the almost optimal α / β (1/1) phase ratio at the deformation temperature, as well as the maximum content of the most diffusively mobile β-stabilizers (nickel, iron) among the studied alloys, which should facilitate mass transfer processes upon realization of intergranular slippage (the total difference in the change in the content of alloying elements between the a and β phases in the SPD process is more than 1.9 wt.%).

Of the alloys studied in the work, the best results were shown by material 4, which fully complies with the requirements for the material (table 7). Tensile tests with a constant strain rate at a temperature of (775 ± 7) ° С at a strain of 3 x 10 ^" inch / inch / second).

Table 7

SUBSTITUTE SHEET (RULE 26) The comparative mechanical properties of the sheets after annealing are shown in table 8.

Table 8

The data shown in tables 7 and 8 show that as a result of the invention, a sheet material was obtained including a titanium alloy, the chemical composition of which is optimally balanced with the production capabilities based on the well-known standard technologies of sheet semi-finished products with grain sizes more than 2 microns, corresponding to the requirements for the material used in the aerospace industry.

It should be noted that the products of the present invention can be implemented in a variety of embodiments. The embodiments described in all respects should be considered as illustrative only and not restrictive, and the boundaries of the present invention are defined by the claims.

SUBSTITUTE SHEET (RULE 26)

Claims

Claim.

1. Sheet material for low-temperature superplastic deformation based on a titanium alloy containing wt.% 4.5-5.5 A1, 4.5-5.5 V, 0.1-1, 0 Mo, 0.8- 1.5 Fe, 0.1-0.5 Cg, 0.1-0.5, 0.16-0.25 O, the rest is titanium and impurities, in which the value of the structural molybdenum equivalent [Mo] eq> 5, and aluminum structural equivalent [A1] equiv <8, equivalents are determined by the expressions:

[Mo] equiv = [Mo] + [V] / l, 5 + [Cr] x l, 25 + [Fe] x2.5 + [Ni] / 0.8

[A1] equiv = [A1] + [0] x 10+ [Zr] / 6.

2. A sheet material for low-temperature superplastic deformation according to claim 1, having a structure with a grain size not exceeding 8 microns.

3. Sheet material for low-temperature superplastic deformation according to claim 1, having superplastic properties at a temperature of 775 ± 10 ° C.

4. Sheet material for low-temperature superplastic deformation according to Claims 1 and 2, having at a temperature of 775 ± 10 ° C, the α / β phase ratio is from 0.9 to 1.1.

5. Sheet material for low-temperature superplastic deformation (SPD) according to claim 1, 2, 3 and 4, in which the number of alloying elements diffusing between the a and β phases in the SPD process is at least 0, 5% and is determined by the following ratio:

Q = Σ ι | Am | > 0.5 wt.%

Where:

SUBSTITUTE SHEET (RULE 26) n is the number of alloying elements in the material,

| At | - the absolute value of the change in the content of the alloying element in the β- and a-phases, wt.%, in the SPD process.

| Dt | - calculated by the formula:

| Δτπ | = (ττηβΐ— mal) - (m 32 – та2) mass. %

Where:

τηβΐ is the content of the alloying element in the β phase before SPD, wt.%, ιτιβ2 is the content of the alloying element in the β phase after SPD, mass. %

mat is the content of the alloying element in the α phase before SPD, wt.%, ta2 is the content of the alloying element in the α phase after SPD, mass. %

SUBSTITUTE SHEET (RULE 26)