JPH03243740A - Titanium alloy excellent in superplastic workability and its manufacture as well as method for superplastic working of titanium alloy - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a titanium alloy with high strength and excellent superplastic workability, and a method for producing the same.

[Prior Art] Titanium alloys are lightweight and strong, and have recently been increasingly used as materials for aerospace equipment such as airplanes and rockets. However, titanium alloy is a material that is difficult to process, and when manufacturing a member with a complex shape, there are problems in that the product yield is extremely low and the manufacturing cost is extremely high.

Superplastic processing is known as an effective processing method for solving these problems. Superplastic processing is a processing method that utilizes superplastic phenomena, and in particular, methods that utilize fine grain superplasticity observed in fine crystal grains are industrially important. T1-6Ai) is the most widely used titanium alloy.
Even in -4V alloy, materials with fine grain structures of 5 to 10 μm are subjected to superplastic processing, but the processing temperature is as high as 875 to 950°C, and the life of the processing jig is short.
This involves many problems in terms of equipment and operation, such as the necessity of using expensive materials with high-temperature strength as the jig.

Therefore, with the aim of achieving superior superplastic properties than the T 1-6Aj7-4V alloy and lowering the superplastic processing temperature, we added Fe to the T 1-6Ap-4V alloy.
Co, or alloys to which Fe, Co, and Ni are added have been developed (US Pat. No. 4,299,626).

[Problem to be solved by the invention] However, such T 1-6AN-4V-F
Also in the e-Co-Ni alloy, the superplastic working temperature is T
i -6A (50~80℃a than 1-4V, t i
' However, it is still not enough.

Further, superplastic elongation is also insufficient.

On the other hand, even if the superplastic working temperature can be lowered sufficiently, if the deformation resistance during superplastic working is large, the working will become difficult.

This invention was made in view of such circumstances, and
A titanium alloy with excellent superplastic workability that has a low superplastic working temperature, low deformation resistance during superplastic working, and greater superplastic elongation than conventional titanium alloys, and a method for producing the same, as well as a method for producing such a titanium alloy. The purpose is to provide a plastic working method.

[Means and effects for solving the problem] The titanium alloy having excellent superplastic workability according to the present invention has Ail
: 6.5-6675%, V: 3.5-4.5%, O: 0
．． 2% or less, Cr: 0.85-3.15%, Mo:
It contains 0.85 to 3.15% and satisfies the following conditions: 3%≦f1.8x (Cr%)+(Mo%)}≦8%, the remainder consists of Ti and inevitable impurities, and the α crystal It is characterized by having a semi-average particle size of 6 μm or less.

The method for producing a titanium alloy with excellent superplastic workability according to the present invention is based on the weight percentage of AI! : 6.5-6.75%,
V: 3.5-4.5%, O: 0.2% or less, Cr: 0.
85 to 3.15%, MO: 0.85 to 3.15%, and 3%≦f1.8x (Cr%) + (Mo
%)}≦8%, and the remainder consists of Ti and unavoidable impurities, is heated at a temperature above (β transformation point -200°C) and below the β transformation point, and then heated to a temperature below the β transformation point. It is characterized by applying rolling at a rolling ratio of 3 or more at a temperature of .

The method for superplastic processing of titanium alloy according to this invention includes
So, A1Al: 6.5-6.75%, V: 3.5-4
．． 5%, O: 0.2% or less, Cr: 0. 85-3.
15%, Mo: 0.85 to 3.15%, and satisfies the condition of 3%≦(1,8X (Cr%) + (Mo%))58%, with the remainder being Ti and unavoidable impurities. A titanium alloy made of Points -20
It is characterized by performing recrystallization heat treatment at a temperature of 0° C. or higher and lower than the β transformation point, and performing superplastic working.

The inventors of the present application conducted various studies from the following viewpoints in order to develop a titanium alloy having the above characteristics, and as a result, they completed the present invention having the above structure.

In order to obtain a titanium alloy that has high strength and is capable of being superplastically worked, its microstructure must have fine equiaxed elements.

Further, in order for the titanium alloy to exhibit the superplastic phenomenon, it is necessary that the volume fraction of the α phase be 40 to 60% in its microstructure. Therefore, Ti-6Ajl -4
In order to lower the superplastic processing temperature than V alloy, β
Elements that lower the transformation point, ie, C and Mo, may be added.

However, although Cr greatly contributes to an increase in strength, if its content is too high, it may form intermetallic compounds, which are brittle phases, with Ti, or a segregated phase called β flake may occur during melting. This is undesirable because it causes the mechanical properties to deteriorate as a result.

Mo also contributes to an increase in strength, but if it is added in too large an amount, it increases the specific gravity of the titanium alloy, impairing the characteristics of the titanium alloy as a material with high specific strength, and is an element that has a low diffusion rate in the β phase ψ. Therefore, it increases the deformation resistance during superplastic processing, which is undesirable.

Therefore, it is necessary to define their content within a certain range that does not cause these inconveniences.

The β phase stability of titanium alloys with these elements is 1,
It is expressed as 8x (Cr%)+(Mo%), and when this value is small, the β-transformation point is high, and conversely, when this value is large, the β-transformation point is low. As mentioned above, the optimum superplastic temperature of a titanium alloy, that is, the temperature at which superplastic phenomena can occur, is when the volume fraction of the α phase is 40.
~60%, and this temperature is closely related to the β transformation point.

In other words, if this value is too small, the advantage of a low superplasticity onset temperature cannot be obtained, and if it is too large, the temperature at which the volume fraction of the α phase is 40 to 60% will be too low, and atomic diffusion will not occur at that temperature. This results in insufficient superplastic elongation. Therefore, in order to obtain excellent superplastic workability, it is necessary to define this value within an appropriate range.

Further, the fine grain superplastic properties largely depend on the crystal grain size, and the smaller the grain size, the better the properties can be obtained. Therefore, it is necessary to define the upper limit of the crystal grain size to an extent that does not impair the superplastic properties. Such grain refinement is achieved by performing the final hot working under conditions such that a fine recrystallized structure of equiaxed α crystals can be obtained by the subsequent recrystallization heat treatment. That is, unless the final hot working conditions are appropriate, a fine recrystallized structure with zero equiaxes cannot be obtained by the subsequent recrystallization heat treatment.

Furthermore, the recrystallization heat treatment after the final hot rolling is a prerequisite for performing superplastic working, and by appropriately performing this treatment, the subsequent superplastic working can be performed satisfactorily.

Next, the reason why each component of the titanium alloy according to the present invention is limited to the above range in the present invention will be explained.

All:A1 is added as an α-phase stabilizing element to obtain an α+β structure, and contributes to an increase in strength.

However, if the content is less than 6.5%, it is insufficient to obtain the desired strength. In addition, the content is 6.75%
, the brittle phase α, phase (T i ) Al2
) is undesirable because it precipitates and deteriorates mechanical properties. Therefore 1. I'1ffi is defined in the range of 5.5 to 6.75%.

(2): v is added as a β phase stabilizing element to obtain an α+β structure, and contributes to an increase in strength without forming an intermetallic compound, which is a brittle phase, with Ti. However, if the content is less than 3.5%, it is insufficient to obtain the desired strength, and if the content exceeds 4.5%, it will reduce the superplastic elongation and Increases deformation resistance. Therefore, the v amount was defined as 3.5 to 4.5%.

O: 0 is dissolved in the α phase and contributes to an increase in strength.

However, if its content exceeds 0.2%, it will raise the β-transus point and deteriorate mechanical properties, especially ductility, at room temperature. Therefore, OHi is defined to be 0.2% or less.

Cr: Cr is added as a β-phase stabilizing element, and contributes to improving superplastic properties (increasing superplastic elongation and reducing deformation resistance) by lowering the β-transformation point, and is mainly formed as a solid solution in the β-phase. This contributes to an increase in strength at room temperature. but,
If its content is less than 0.85%, its contribution to improving these superplastic properties and increasing room temperature strength is insufficient. Also, 3
．． If it exceeds 15%, an intermetallic compound, which is a brittle phase, is formed with Ti, or β flakes are generated during melting, resulting in deterioration of ductility. Therefore, Crff1 is set to 0.
It is defined in the range of 85 to 3.15%.

Mo: Mo is also added as a β-phase stabilizing element, which contributes to improving superplastic properties (lowering the superplasticity onset temperature) by lowering the β-transformation point, and increases strength by mainly dissolving in the β-phase. Contribute to However, if the content is less than 0.85%, these effects are insufficient. Moreover, if it exceeds 3.15%, MO is a heavy metal, which increases the density of the alloy, impairing the high specific strength of titanium alloy, and also because MO has a low diffusion rate in titanium. This increases the deformation stress during superplastic forming.

Therefore, the amount of MO is defined in the range of 0.85 to 3.15%.

1.8X (Cr%) + (Mo%) is β of titanium alloy
It indicates phase stability, and as mentioned above, when these values are small, the β-transformation point is high, and conversely, when these values are large, the β-transformation point is low.

As mentioned above, the optimal superplastic temperature of a titanium alloy, that is, the temperature at which superplastic phenomena can occur, is when the volume fraction of the α phase is 40 to 40.
60%, and this temperature is closely related to the β transformation point.

If this value is less than 3%, the characteristic of the present invention that the superplasticity development temperature is low will be impaired, and if it exceeds 8%, the temperature at which the volume fraction of the α phase becomes 40 to 60% will be too low, and the temperature In this case, atomic diffusion becomes insufficient and sufficient superplastic elongation cannot be obtained.

Therefore, this value is 3%≦f1.8X (Cr%)+(
Mo%)}≦8%.

Next, the reason for limiting the α crystal grain size will be explained.

The α grain size has a close relationship with superplastic properties, and this shows small rolled up superplastic properties. In the titanium alloy according to the present invention, if the average grain size of the α-crystal exceeds 6 μm, not only the superplastic elongation decreases but also the deformation stress increases, which is not preferable. Therefore, the zero-product particle size is defined as 6 μm or less.

In order to reduce the zero particle size to 6 μm or less, the following method is adopted.

First, by hot forging, hot rolling, or both,
A titanium alloy ingot is processed and produced into a slab, and then this slab is reheated and hot worked to form a final 4-piece. In the final hot working, it is necessary to satisfy the following three conditions.

(2) The heating temperature is set to be at least (β-transformation temperature - 200°C) and below the β-transformation point. ■Prevent the temperature of the workpiece from rising above the β transformation point during processing. ■The rolling reduction ratio during processing should be 3 or more.

The reason for defining the conditions in this way is as follows.

Regarding (2), if the heating temperature is above the β transformation point, even if recrystallization annealing is performed after processing, a rod-shaped α crystal structure will be formed instead of an equiaxed structure suitable for superplastic forming, and furthermore, the former β grain boundaries will be α crystals grow and deteriorate superplastic properties and ductility at room temperature. Further, if the heating temperature is lower than the β-transus point -200°C, the temperature is too low and results such as cracking occur during processing.

As for (2), when the temperature of the workpiece becomes higher than the β transformation point during processing, it becomes a rod-shaped α structure as in (2).

Regarding (2), if the reduction ratio is small (less than 3), sufficient strain for recrystallization will not be stored in the α-crystals, and even if recrystallization heat treatment is performed, the microstructure will not form a fine-grained equiaxed structure suitable for superplastic forming, but will have a rod-like structure. This results in an α crystal structure or a coarse α structure, which is undesirable.

Next, the reasons for limiting the recrystallization heat treatment conditions performed prior to superpharyngeal processing will be explained.

If the recrystallization heat treatment is less than (β-transus point −200° C.), the temperature is too low and recrystallization is not performed sufficiently, so that equiaxed orientation cannot be obtained, which is not preferable. Furthermore, if the heat treatment temperature exceeds the β transformation point, the zero equiaxed product disappears and becomes a β single phase, which is not preferable because superplasticity cannot be obtained. Therefore, the recrystallization heat treatment temperature was specified to be higher than (β transformation point 200° C.) and lower than β transformation point.

Note that the superplastic working may be performed subsequent to the recrystallization heat treatment, or may be performed simultaneously with the recrystallization heat treatment.

[Example] Examples of the present invention will be described in detail below.

Ingots of alloys having the compositions shown in Table 1 were melted in an arc furnace in an argon atmosphere, and slabs of various thicknesses were produced by hot forging.

Next, each of these slabs was reheated to the temperature shown in Table 1, and subsequently, each of these slabs was rolled at the rolling reduction ratio shown in Table 1, respectively, to form a plate material having a thickness of 5 mm.

The plate material thus finished was then subjected to recrystallization heat treatment. This recrystallization heat treatment was performed in a temperature range of (β transformation point −200° C.) or higher and lower than the β transformation point, except for experiment number C15. Following this recrystallization heat treatment, a superplastic tensile test was conducted. In addition, for each sheet material subjected to recrystallization heat treatment, the α grain size (average grain size) was measured and a room temperature tensile test was performed. α grains are measured by the line segment method.
The particle size was not measured for rod-like structures with an aspect ratio (ratio of long sleeve to short axis) of 3 or more.

Table 1 shows the recrystallization heat treatment temperature and α grain size of each plate, and Table 2 shows the results of the room temperature tensile test and superplastic tensile test.

In Table 1, experiment numbers A1 to A7 indicate examples within the scope of the present invention, and 81 to B4 indicate conventional examples (in the conventional examples (
1) is T L-6A#-4V alloy, (2) is Ti-
6AI! -4V-Co-(Ni) alloy), C1~C1
No. 5 is a comparative example outside the scope of the present invention. In addition, the β transformation points for the compositions of each experiment number are also listed in Table 1.

In addition, in the superplastic tensile test, the parallel part is 5 cm wide, 5 cm long,
The test was carried out using a test piece with a thickness of 4 mm at a vacuum ψ of 5×10 −6 Torr or less. Further, the maximum deformation stress was determined by dividing the maximum load by the initial cross-sectional area.

As shown in Tables 1 and 2, the experimental numbers A1 to A7, which are examples, have alloy compositions within the scope of the present invention, and the manufacturing conditions are in accordance with the method according to the present invention. The particle size of the 0 product is extremely fine and fully satisfies the requirement of 6 μm or less. Furthermore, it was confirmed that the tensile strength at room temperature was 105 kgf/m1 or more, and the elongation was 17% or more, which were better values than the Ti-6/'1-4V alloy.

Next, the superplastic tensile properties shown in Table 2 will be explained with reference to FIGS. 1 to 6.

FIG. 1 is a graph showing the relationship between the value of 1.8X (Cr%)+(Mo%) and superplastic elongation. As is clear from this figure, it was confirmed that a large elongation of 1500% or more can be obtained when this value is within the range of 3 to 8%, which is within the range of the present invention.

FIG. 2 is a graph showing the relationship between Cr content and superplastic elongation. As is clear from this figure, this value is within the range of the present invention, 150% in the range of 0.85% to 3.15%.
It was confirmed that there was a large elongation of 0% or more.

FIG. 3 is a graph showing the relationship between Mo content and superplastic elongation. As is clear from this figure, this value is within the range of the present invention, 150% in the range of 0.85% to 3.15%.
It was confirmed that there was a large elongation of 0% or more.

FIG. 4 is a graph showing the relationship between 0-product grain size and superplastic elongation. As is clear from this figure, it was confirmed that a large elongation of 1500% or more can be achieved if this value is 6 μm or less, which is within the range of the present invention.

As described above, it was confirmed that excellent superplastic properties can be obtained within the composition range and particle size range specified in the present invention.

Test numbers A1 to A7, which are within the scope of the present invention, not only have a good superplastic elongation of 1500% or more as described above, but also have a temperature of 800°C or less at which the maximum superplastic elongation occurs, which is T1-
It was confirmed that the deformation stress at that temperature was as small as 1.41 kg/s+ga2 or less, even though the temperature was 75 to 100°C lower than that of the 6Afl-4V alloy and was extremely low.

On the other hand, it was confirmed that the superplastic elongation values of Comparative Examples C1 to C14 were lower than those of the Examples.

Among the comparative examples, C1 to CIO have compositions outside the scope of the present invention, and among these, C3, C5, and C9 show relatively large elongation of 1320% or more, but the maximum superplastic elongation was not obtained. The temperature was 825°C or higher, which is 25 to 50'C higher than that of the examples, and it was confirmed that the superplastic properties were clearly inferior to those of the examples. C7,cl
Although the temperature at which the maximum superplastic elongation was obtained for o was extremely low at 750'C, the superplastic elongation remained at 1300% and 780%, respectively, and the deformation stress was 2.85 kgr/*m', respectively.
2. The ivy has an extremely large value of 80 kgl'/g+v'. Also in C1, C2, C4, and C6,
The superplastic elongation was insufficient at around 1000%.

Note that the elongation of C8 at room temperature was as small as 6%, making it unsuitable for practical use, so a superplastic tensile test was not conducted.

For C1 to C14, the manufacturing conditions and zero particle size are outside the range of the present invention, for C1 to C14, the rolling reduction ratio is outside the range of the present invention, and for C14, the final hot rolling heating temperature is outside the range of the present invention. It was confirmed that C15 does not have a fine-grained equiaxed structure because the recrystallization heat treatment temperature is outside the range of the present invention, and the plastic elongation is 550 to 1380%, which is inferior to the values of the examples. Ta. The particle sizes of C12, C14, and C15 were not measured because the α-crystals were not equiaxed and became coarse rod-like crystals with an aspect ratio of 3 or more.

[Effects of the Invention] According to the present invention, superplastic properties such as low superplastic processing temperature, low deformation resistance during superplastic processing, and greater superplastic elongation than conventional titanium alloys are achieved while maintaining excellent strength. A titanium alloy with excellent workability and a method for manufacturing the same can be provided.

Moreover, the titanium alloy according to the present invention can be widely used as a high-strength titanium alloy with excellent superplastic workability, including as a material for aerospace equipment, by taking advantage of these excellent properties.

Furthermore, since the titanium alloy according to the present invention has a low deformation resistance, it can be processed using only a processing method that utilizes superplasticity.
It is clear that it has excellent workability in processing methods such as isothermal forging, normal hot forging, and warm forging, and can be applied to an extremely wide range of applications.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the value of 1.3x (Cr%) + (Mo%) and superplastic elongation, Figure 2 is a graph showing the relationship between Cr content and superplastic elongation, and Figure 3 is a graph showing the relationship between the Cr content and superplastic elongation. The figure is a graph showing the relationship between MO content and superplastic elongation, and FIG. 4 is a graph showing the relationship between a-crystal grain size and superplastic elongation.

Claims (3)

[Claims] (1) In weight%, Al: 5.5 to 6.75% V: 3.5 to 4.5% O: 0.2% or less Cr: 0.85 to 3.15% Mo: 0.85 to 3.15% and satisfies the conditions of 3%≦{1.8×(Cr%)+(Mo%)}≦8%, the remainder consists of Ti and inevitable impurities, and the average of α crystals A titanium alloy with excellent superplastic workability characterized by a grain size of 6 μm or less. (2) In weight%, Al: 5.5 to 6.75% V: 3.5 to 4.5% O: 0.2% or less Cr: 0.85 to 3.15% Mo: 0.85 to A titanium alloy containing 3.15% and satisfying the conditions of 3%≦{1.8×(Cr%)+(Mo%)}≦8%, with the balance consisting of Ti and unavoidable impurities ( Excellent superplastic workability, characterized by heating at a temperature of at least -200°C (beta transformation point) and less than the beta transformation point, followed by rolling at a rolling ratio of 3 or more at a temperature below the beta transformation point. Method of manufacturing titanium alloy. (3) In weight%, Al: 5.5 to 6.75% V: 3.5 to 4.5% O: 0.2% or less Cr: 0.85 to 3.15% Mo: 0.85 to A titanium alloy containing 3.15% and satisfying the conditions of 3%≦{1.8×(Cr%)+(Mo%)}≦8%, with the balance consisting of Ti and unavoidable impurities ( Heating is carried out at a temperature not less than β-transformation point -200°C and less than β-transformation point, followed by rolling with a reduction ratio of 3 or more at a temperature below β-transformation point.
℃) or above and below the β transformation point,
A method for superplastic processing of titanium alloys, which is characterized by applying superplastic processing.