JP2020517834A - Titanium alloy based sheet material for low temperature superplastic deformation - Google Patents

Abstract

The technical results achieved by carrying out the present invention are that of sheets from titanium alloys with a known balanced state-of-the-art production capacity of the final product with low temperature superplastic deformation properties and an optimal balanced chemical composition. Manufacturing. Such results, in weight percent, are 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 balance titanium and impurities, structural molybdenum equivalent ([Mo]eq.) 5 and the aluminum structural equivalent ([Al]eq.) is smaller than 8 and said equivalent has the following formula: [Mo]eq. =[Mo]+[V]/1.5+[Cr]1.25+[Fe]2.5+[Ni]/0.8 and [Al]eq. =[Al]+[O]10+[Zr]/6 determined by a sheet material for low temperature superplastic deformation based on a titanium alloy. [Selection diagram] None

Description

Disclosed herein are materials and products such as sheet materials and sheet-like semi-finished products, such materials and products comprising titanium alloys, which materials have a low temperature superplastic forming (SPF) at a temperature of 775°C. ) Are suitable for processing products. The materials and products can be used as a cost-effective option for sheet products made from Ti-6Al-4V alloy.

The term "superplastic forming" generally applies to processes in which a material (alloy) is superplastically formed (>500%) beyond the conventional limits of plastic strain. SPF can be applied to certain materials that exhibit superplastic properties within a limited range of temperature and strain rate. For example, titanium alloy sheets can typically undergo superplastic forming (deformation) within a temperature range of about 900 to 1010° C. at a strain rate of about 3·10 ⁻⁴ s ⁻¹ .

From a manufacturing standpoint, SPF's lower molding temperatures result in significant advantages. For example, lowering the SPF molding temperature reduces the die cost, extends the service life of the die, and enables the introduction of inexpensive steel dies. In addition, the formation and scale of the oxygen-rich layer (in the case of alpha) is reduced, thus improving product yield and reducing or eliminating the need for chemical etching. In addition, the advantage of retaining the presence of finer grains after completion of the SPF operation can result in a lower deformation temperature, which can inhibit grain growth.

Currently, there are two known approaches to improve the superplastic formability of sheet materials from titanium alloys. The first approach involves the development of a special-purpose thermomechanical process that produces granules as small as 2 μm to 1 μm and smaller, thereby enhancing grain Sakai slip. In particular, there is a known method of manufacturing a deformation sheet at a temperature lower than that of a conventional product formed of Ti-6Al-4V material (Patent No. 2243833, IPC B21B1/38, published January 10, 2005). ).

The second approach is
-Enhancement of two-phase volume fraction and morphology,
A faster diffusion method to accelerate grain Sakai slip due to the content of Fe and Ni in the alloy as a fast diffuser,
− Lower beta transus temperature (BTT)
Involves developing a new system of titanium alloy sheet material that exhibits overplasticity in coarser material grain sizes.

Therefore, in the efficient selection of the chemical composition of the alloy, it is possible to obtain satisfactory superplastic forming (deformation) characteristics at low temperatures without using any special-purpose processing technology required for the formation of ultrafine grains. It is possible.

Duplex (α+β) titanium alloys are classified into alloys with molybdenum structural equivalents ([Mo] equiv.) equal to 2.5 to 10%, depending on the level of addition of alloying elements (Kolachev BA, Polkin IS. , Talalayev VDTitanium alloys of various countries: Reference book.Moscow.VILS.2000.316 p.-p. 13-16). Such alloys are usually alloyed with aluminum and the β-stabilizing element and retain the beta phase. The amount of beta phase can vary from 5% to 50% in the annealed alloys belonging to this family. Therefore, the mechanical properties change over a relatively wide range. These alloys have been used extensively in Russia and other countries, especially the Ti-6A1-4V alloy, which has been successful in adding alloying elements. (Materials Properties Handbook: Titanium Alloys. R. Boyer, G. Welsch, E. Collings. ASM International, 1998. 1048 p.-p. 486-488). In this alloy, aluminum tends to increase strength properties and heat resistance, while vanadium is one of the few elements that not only increases strength properties but also improves plasticity. Alloys belonging to the Ti-6A1-4V group are used in the production of bars, tubes, sections, free and closed forgings, plates, sheets, strips and foils. This alloy is used in the manufacture of welded and assembled structures for aerial vehicles, numerous structural components of aeronautics and rockets, and medical implants for trauma, orthopedics and dentistry. ..

There is a known method for producing a titanium alloy sheet-like semi-finished product suitable for low temperature superplastic forming from a VT5 alloy which is an analog of Ti-6Al-4V alloy (Japanese Patent No. 2224047, IPC C22F1/18, B21B3/00). , Published on February 20, 2004). This method allows the production of titanium alloy sheet blanks with a uniform sub-microcrystalline structure (grain size less than 1 μm) suitable for low temperature superplastic forming. This method is expensive, inefficient, and requires the use of special purpose equipment.

Ti-6A1-4V alloy is known to have a sub-microcrystalline structure that exhibits superplastic properties, produced by strong strain processing (SPD) using comprehensive forging techniques. The microstructure of the alloy consists of α and β phase grains with an average size of 0.4 μm and sub-grains, high levels of internal lattice stress and elastic strain evidenced by non-uniform diffraction contrast, and structures obtained by electron microscopy. Defined by high density dislocations in the image. (S. Zherebtsov, G. Salishchev, R. Galeyev, К. Maekawa, Mechanical properties of Ti-6Al-4V titanium alloy with submicrocrystalline structure produced by severe plastic deformation. // Materials Transactions. 2005; V. 46 (9): 2020-2025). In order to produce sheet-like semi-finished products from such alloys, there is a need for decentralized, low cost SPD operations using comprehensive forging techniques that significantly increase the value of the final product.

There are known methods for making thin sheets from dual phase titanium alloys and making products from said sheets. This method comprises the following element contents in weight%: Al of 3.5-6.5, V of 4.0-5.5, Mo of 0.05-1.0, 0.5-1.5. Fe, 0.10-0.2 O, 0.01-0.03 C, 0.005-0.07 Cr, 0.01-0.5 Zr, 0.001-0.02 Of N, with the balance being titanium, including the production of semi-finished sheets; the chemical composition in this case is aluminum [Al] strength equivalent=6.0-11.55 and molybdenum [Mo] strength. Equivalent=Adjusted with a strength equivalent value of 3.5-5.6 (Patent No. 2555267, IPC C22F1/18B21B3/00, published October 7, 2015)-Prototype.

Sheet-like semi-finished products with thickness <3 mm produced within the scope of said patent may not be suitable for industrial production due to the low stability of the properties required for SPF. The reason is that even using a strength equivalent as an adjuster of the chemical composition of the alloy, it is necessary for the required regulation, the proper relationship between the alloying elements in the alloy, and the performance of SPF work on sheet-like semi-finished products. This is because the structural characteristics of the alloys used are not possible. Furthermore, the presence of Si and Zr in the alloy can lead to the formation of silicides on the surface of the grains, which prevents slippage between the grains and can lead to process instability.

Disclosed herein is the production of sheet material of (α+β) titanium alloy with superplastic formability at lower temperatures with grain sizes greater than 2 μm. This sheet material exhibits stable properties and, in the examples, is a cost-effective option for sheet-like semi-finished products made from Ti-6Al-4V alloy with finer grains.

Disclosed herein is a titanium alloy having a chemical composition that is efficiently balanced with manufacturability based on known general manufacturing techniques for end products that exhibit low temperature superplastic forming properties, It is the manufacture of sheets.

The alloy structure in the initial state is shown. The alloy structure in the initial state is shown. It is a load curve obtained during SPF. It is a load curve obtained during SPF. It is a load curve obtained during SPF. FIG. 3 is a graph showing true stress versus strain curves for [Mo] equivalents at strain levels of 0.2 and 1.1 (longitudinal direction).

Examples of sheet materials for low temperature superplastic forming include the following element contents in weight percent: Al of 4.5-5.5, V of 4.5-5.5, V of 0.1-1.0. Mo, 0.8-1.5 Fe, 0.1-0.5 Cr, 0.1-0.5 Ni, 0.16-0.25 O, balance titanium and residual Is an element and has a molybdenum structural equivalent greater than 5 ([Mo]eqiv.) and an aluminum structural equivalent less than 8 ([Al]equiv.); the value of said equivalent being:
[Mo]eqiv. =[Mo]+[V]/1.5+[Cr]×1.25+[Fe]×2.5+[Ni]/0.8
[Al]eqiv. =[Al]+[O]×10+[Zr]/6
It can be made from a titanium alloy calculated from

The sheet material for low temperature superplastic forming has a structure composed of crystal grains having a size of less than 8 μm.

Sheet materials for low temperature superplastic forming can exhibit superplastic properties at temperatures of 775±10°C.

Sheet materials for low temperature superplastic forming at temperatures of 775±10° C. exhibit an α/β phase ratio of 0.9 to 1.1.

に基づいて決定され、式中：
Ｑ − ＳＰＦの間の材料中で拡散する合金元素の量（重量％）であり、
ｎ − 材料中の合金元素の量であり、
｜Δｍ｜ − ＳＰＦ過程の間の、α及びβ相の合金元素含有量の絶対変動値（重量％）である。
｜Δｍ｜は、式：
｜Δｍ｜＝（ｍβ１−ｍα１）−（ｍβ２−ｍα２），（重量％）
から計算され、
式中：
ｍβ１ − ＳＰＦの前の、β相の合金元素の含有量（重量％）であり、
ｍβ２ − ＳＰＦの後の、β相の合金元素の含有量（重量％）であり、
ｍα１ − ＳＰＦの前の、α相の合金元素の含有量（重量％）であり、
ｍα２ − ＳＰＦの後の、α相の合金元素の含有量（重量％）である。 Sheet material for low temperature superplastic forming, the amount of alloying elements diffusing between α and β phases during the SPF process is equal to a minimum of 0.5%, which is related to the relational expression:

In the formula:
Q-the amount (% by weight) of alloying elements diffusing in the material between the SPF,
n-is the amount of alloying elements in the material,
|Δm| − Absolute variation (% by weight) of alloying element content of α and β phases during SPF process.
|Δm| is an expression:
|Δm|=(mβ1-mα1)-(mβ2-mα2), (wt%)
Calculated from
In the formula:
The content (% by weight) of the β-phase alloying element before mβ1-SPF,
The content (% by weight) of the β-phase alloying element after mβ2-SPF,
content of α-phase alloying elements (wt%) before mα1-SPF,
It is the content (% by weight) of alloying elements in the α phase after mα 2 -SPF.

The provided sheet material exhibits a set of high processing and structural properties in the examples herein. This is achieved by the efficient selection of alloying elements and their ratio in the material alloy.

Group of α-stabilizing elements

Aluminum, which is used in virtually all commercial alloys, is the most efficient strengthener and improves the strength and heat resistance properties of titanium. Oxygen raises the temperature of the titanium allotropic transformation. The presence of oxygen in the range of 0.16% to 0.25% increases the strength of the alloy and has no significant adverse effect on plasticity.

The group β-stabilizing elements (V, Mo, Cr, Fe, Ni) are widely used in commercial alloys.

Vanadium in an amount of 4.5% to 5.5%, iron in an amount of 0.8% to 1.5%, and chromium in an amount of 0.1% to 0.5% increase alloy strength, Has relatively little or no effect on plasticity.

The introduction of molybdenum in the range of 0.1% to 1.0% guarantees its almost complete or complete diffusion in the α phase, so that the required strength properties are, in the examples, relative to the plastic properties. Can be achieved with little or no adverse effect.

The provided alloy comprises iron in an amount of 0.8% to 1.5 or 1.0% to 1.5% and nickel in an amount of 0.1% to 0.5%. These elements are the most diffusive β-stabilizing elements that have a positive effect on intergranular slip in SPF.

Among the structural factors affecting SPF efficiency, the first outstanding is the grain size which does not exceed 8 μm (experimental data) for the provided material.

It is known that superplastic flow of materials can occur due to phase transformation in dual phase titanium alloy when the ratio of α/β phase at SPF temperature is close to 1 (Kaibyshev O. Superplastic properties of commercial alloys. Moscow . Metallurgy. 1984. p. 179-218.). This promotes the formation of an equiaxed structure that contributes to intergranular slip. The driving force for structural spheronization is the tendency for the surface energy to decrease. The growth of grain boundary boundaries due to the increase of β phase leads to a change in surface energy level at the grain boundary boundaries, which in turn activates spheroidization. The value of the molybdenum structural equivalent ([Mo] equiv.) must be greater than 5 to have the required amount of beta phase during the SPF process at α/β ratio close to 1, and the aluminum structural equivalent ( The value of [Al] equiv.) must not exceed 8. In addition, aluminum equivalent values above the above result in an increase in BTT and consequently an increase in SPF temperature.

The optimum temperature for affecting the superplastic properties of the provided material is equal to 775±10°C.

The amount of alloying elements diffusing between the α phase and the β phase must be 0.5% or more. This is due to the fact that the activation energy of grain boundary diffusion is less than the activation energy of volume diffusion, and the diffusion transport of atoms is performed at the grain boundaries. Such areas of the grain boundaries exhibit an increased concentration of vacancies under the influence of normal tensile stress. Those areas under the influence of compressive stress exhibit less void concentration: differences in concentration cause direct diffusion of voids. The movement of vacancies is accompanied by the exchange of atoms, and the latter moves in the opposite direction, which causes intergranular slip enhancement.

For the purpose of investigation, a sheet-shaped semi-finished product having a thickness of 2 mm was used. Six experimental alloys of various chemical compositions shown in Table 1 were melted to produce sheet material.

A sheet material having a thickness of 2 mm was manufactured for the purpose of superplastic forming by comparing with a known manufacturing method. Prior to testing for superplastic properties, the material was annealed at a temperature of 720° C. for 30 minutes and then air cooled. After completion of the treatment process, the specimens are taken from the sheet in longitudinal and transverse directions for room temperature and elevated temperature tensile strength tests, and then at room temperature for general testing of the samples to determine strength, elasticity and plastic properties. went.

An evaluation of the material structure in the initial state (FIGS. 1 and 2) shows that the structure resembles an equiaxed structure, predominantly appearing as darker (α) or brighter (β) elements in the α phase. And β-phase alternating crystal grains. With the increase of [Mo] equiv in the alloy, the volume fraction of β-phase crystal grains from the α/β ratio 2/1 expected in alloy 2 to a value close to 1/1 in alloy 3 and alloy 4 Note the rise. The average size of the phase crystal grains measured on the microstructure photograph by the cutting method is [Mo] equiv. , And tends to increase with an increase in .gamma., which is in the range of 2.8 to 3.8 .mu.m (minimum grain size has been determined for Alloy 2). Note that the grain structure homogeneity of material 1 in its initial state is low compared to other experimental alloys. In addition to equiaxed grains, Material 1 exhibits areas consisting of elongated grains with sufficient bulk. Also note that the morphology of the β-phase varies somewhat with the alloy. Alloy 2 has the least amount of alloying elements, the β phase is mainly located as a separate group between the α phase particles; however, starting from alloy 5, the β phase has a well-defined coherency and grain In addition to its texture, it is formed as a relatively thin layer between α-phase grains. [Mo] equiv. As the thickness increases, the thickness of these layers tends to increase.

Comparative Example Material structure in the forged (reduced part) state and the unforged (head part area) state after SPF (at a temperature of 775° C. and a strain rate of 3×10 ⁻⁴ s ⁻¹ , in the length direction of the sheet). A comparative analysis of the above showed that the deformation in the reduced part induced some grain growth and the formation of agglomerates from the more complex α and β phase grains compared to the nearly unforged top. Was shown.

Grain size was evaluated by the fact that the addition of alloying elements does not significantly affect the size of phase grains in the alloy with the maximum addition of β-stabilizing element, and the range is 3.5±0.5 μm ( It was shown to be between the non-forged part) and 4±0.5 μm (forged part). At the same time, in the case of alloy 2 containing the alloying elements in the minimum content, the size of the crystal grains in the reduced portion is increased up to 5 μm almost twice or more as compared with the initial state.

The distribution of alloying elements between the α and β phases was tested in the initially investigated material by the method of electron microscopy (EMPA) after the superplastic properties were tested; the test was a longitudinal test. The forging reduction part and the head part of one piece were carried out, and the results are shown in Tables 2, 3 and 4.

から決定され、式中：
Ｑ − ＳＰＦの間の材料中で拡散する合金元素の量（重量％）であり、
ｎ − 材料中の合金元素の量であり、
｜Δｍ｜ − ＳＰＦ過程の間の、α−及びβ相の合金元素含有量の絶対変動値（重量％）である。
｜Δｍ｜は、式：
｜Δｍ｜＝（ｍβ１−ｍα１）−（ｍβ２−ｍα２），（重量％）
から計算され、
式中：
ｍβ１ − ＳＰＦの前の、β相の合金元素の含有量（重量％）であり、
ｍβ２ − ＳＰＦの後の、β相の合金元素の含有量（重量％）であり、
ｍα１ − ＳＰＦの前の、α相の合金元素の含有量（重量％）であり、
ｍα２ − ＳＰＦの後の、α相の合金元素の含有量（重量％）である。 The amount of alloying element that diffuses in the material during the SPF is calculated by the formula:

Determined from the formula:
Q-the amount (% by weight) of alloying elements diffusing in the material between the SPF,
n-the amount of alloying elements in the material,
|Δm| − Absolute variation (wt %) of alloying element content of α- and β phases during SPF process.
|Δm| is an expression:
|Δm|=(mβ1-mα1)-(mβ2-mα2), (wt%)
Calculated from
In the formula:
The content (% by weight) of the β-phase alloying element before mβ1-SPF,
The content (% by weight) of the β-phase alloying element after mβ2-SPF,
content of α-phase alloying elements (wt%) before mα1-SPF,
It is the content (% by weight) of alloying elements in the α phase after mα 2 -SPF.

Included in Table 4 are calculated data related to the amount of alloying elements that diffuse during the SPF process.

By analyzing the changes in the α and β phases in the forged sheet material under investigation, the difference in the gold element content between the α phase and β phase in the reduced portion of the test piece was found to be in the head portion of the test piece that was not subjected to plastic deformation. It was shown to be large compared to the same difference (Tables 2, 3 and 4).

The EMPA results obtained are also used for the evaluation of the phase volume fraction in the material subject to the superplastic property test at a temperature of 775° C. and are shown in Table 5.

The load curves obtained during the test are shown in Figures 3, 4 and 5.

Table 6 shows the characteristics of the alloys in the superplasticity test.

The true stress versus strain curves for the [Mo] equivalents at strain rates of 0.2 and 1.1 (longitudinal direction) are shown in FIG.

Material 1 with a minimum content of alloying elements (FIG. 3) has the most unstable SPF process at a temperature of 775° C., which is typical of the stress-strain curve caused by the formation of a floating neck. It is represented by a swell. Such material behavior in SPF is due to the relatively bulky initial grains (2.5 μm) which have a high growth rate (5 μm or less) in SPF, in which case the α/β phase ratio (2/1 ) Is not efficient and results in the activation of intergranular slip, which is undesirable for SPF, instead of efficient intergranular slip.

A β-stabilizing element was further added to the material 2 (FIG. 3 ), and therefore, the instability of the SPF process in the form of waviness of the stress-strain curve was caused by the decrease of the β-phase volume fraction in the structure. And decreased as compared with Alloy 1. Here, when the strain degree is in the range of 0.6 to 0.8, due to the progress of the dynamic recrystallization within the area of the incompletely processed structure (the existence of the elongated crystal grains). ) No significant cure was observed. This is not typical of all other alloys investigated.

Except for molybdenum (alloy 5) and chromium (alloy 6), materials 3, 5 and 6 (Figs. 4 and 5) containing the maximum content of β-stabilizing element increase coherence and facilitate inter-grain slip. It is represented by a stress-strain curve with a small waviness compared to materials 1 and 2 due to the increase of β-phase in the alloy structure with; It becomes more prominent (Table 3, Figure 6). In this case, in particular, in the transverse test, the waviness is maintained at a strain degree of 0.6 or less, which is the ratio of the initial texture of the sheet and the inefficient α/β phase (3 to 3 close to 2). ) Can be attributed to. The absence of chromium in material 6 affects the stress-strain curve to a lesser extent than the absence of molybdenum in material 5 as compared to material 3. One of the reasons may be that the effect of molybdenum addition on the stability of the SPF process is strong compared to the addition of chromium (which may be 2-2.5 fold).

Material 4 contains the maximum amount of β-stabilizing element and is further alloyed with 0.3% nickel; it has a more stable superplastic behavior at a temperature of 775° C. in both transverse and longitudinal directions. Exhibited a minimal stress at the onset of flow, the absence of significant curvilinear undulations and a monotonic hardening with increasing strain. This is due to the almost efficient α/β phase ratio (1/1) at the deformation temperature and the maximum content of diffusing β-stabilizing elements (nickel, iron) compared to all alloys investigated. Therefore, it promotes the mass transfer process in the inter-grain slip (the total difference in the change of the alloying element contents of the α phase and the β phase during the SPF process exceeds 1.9 wt %).

Of the alloys investigated, Material 4 gave the best results in full compliance with the material requirements (Table 7). The tensile tests (3×10 ⁻⁴ inch/inch/second strain) at constant strain rate and temperature of (775±7)° C. are shown in Table 7 below.

A comparison of the mechanical properties of the annealed sheets is shown in Table 8.

The data shown in Tables 7 and 8 show that, as a result of one exemplary embodiment, the known general production of semi-finished products having a grain size above 2 μm and complying with the requirements applied to aerospace materials. Based on the technology, it shows that the sheet material was made from a titanium alloy with a chemical composition that was well balanced with manufacturability.

It should be noted that the products manufactured in accordance with this specification can have a variety of designs. The designs provided herein are exemplary rather than limiting, and the invention's limitations are defined by the following claims.

Claims (5)

The following element contents are included by weight%: Al of 4.5-5.5, V of 4.5-5.5, Mo of 0.1-1.0, Fe of 0.8-1.5 and 0. 0.1-0.5 Cr, 0.1-0.5 Ni, 0.16-0.25 O with the balance titanium and residual elements with molybdenum structural equivalents greater than 5 ([[ Mo]eqiv.) and an aluminum structural equivalent below 8 ([Al]equiv.); the value of said equivalent being the following formula:
[Mo]eqiv. =[Mo]+[V]/1.5+[Cr]×1.25+[Fe]×2.5+[Ni]/0.8
[Al]eqiv. =[Al]+[O]×10+[Zr]/6
Sheet material for low temperature superplastic forming, made from titanium alloy calculated from. The sheet material for low temperature superplastic forming according to claim 1, having a structure composed of crystal grains having a size of less than 8 mμ. The sheet material for low temperature superplastic forming according to claim 1, which exhibits superplastic properties at a temperature of 775±10°C. The sheet material for low temperature superplastic forming according to claim 1 or 2, which exhibits an α/β phase ratio of 0.9 to 1.1 at a temperature of 775±10°C. The amount of alloying elements diffusing between the α and β phases during the SPF process is at least equal to 0.5%, said amount having the following relational expression:

Determined from the formula:
Q is the amount (% by weight) of alloying elements diffusing in the material between the SPFs,
n is the amount of alloying elements in the material,
|Δm| is the absolute variation value (% by weight) of the alloying element content in the β phase and the α phase during the SPF process,
|Δm| is an expression:
|Δm|=(mβ1-mα1)-(mβ2-mα2) (% by weight)
Calculated from
here:
mβ1 is the content (% by weight) of the β-phase alloying element before SPF,
mβ2 is the content (% by weight) of the β-phase alloying element after SPF,
mα1 is the content (% by weight) of the α-phase alloying element before SPF,
mα2 is the content (% by weight) of the α-phase alloying element after SPF,
A sheet material for low-temperature superplastic forming according to claim 1, 2, 3, or 4.

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