Method for improving strength of TC4 titanium alloy wire

Abstract

The invention discloses a method for improving the strength of a TC4 titanium alloy wire, which comprises the following steps: (1) martensite treatment: heating the TC4 titanium alloy wire to a temperature zone of 800-850 DEG C for heat preservation, and then performing water quenching to obtain an alpha + alpha'martensite initial structure; (2) tissue fibrosis treatment is conducted, specifically, the TC4 titanium alloy wire in the step (1) is subjected to drawing with medium dependent variable, and a fibrous alpha + alpha'drawn tissue is obtained; (3) martensite decomposition treatment is conducted, specifically, the TC4 titanium alloy wire in the step (2) is heated to a temperature zone of 500-600 DEG C, air cooling is conducted after short-time heat preservation, and the alpha + alpha'fibrous structure is decomposed into an alpha + (alpha s + beta) complex-phase fibrous structure; (4) non-continuous accumulated large strain drawing based on defect rearrangement treatment is conducted, specifically, non-continuous small strain drawing is conducted on the TC4 titanium alloy wire in the step (3) for multiple times, large strain drawing is achieved in an accumulated mode, and the wire is heated to 400-500 DEG C between every two adjacent passes for defect rearrangement treatment; and finally, the high-strength TC4 titanium alloy wire is obtained. The TC4 titanium alloy wire treated through the method has the ultra-high strength of 1400 MPa or above, and meanwhile the requirement that the uniform elongation of the wire is 3% or above is met.

Description

Technical Field

[0001] The present invention relates to a method for improving the strength of TC4 titanium alloy wire. Background Art

[0002] According to the tissue type, titanium alloys can be divided into different types such as α-type, near-α-type, α+β-type, β-type, etc. Among them, the α+β-type duplex titanium alloy represented by TC4 (Ti6Al4V) titanium alloy has the characteristics of both α-type titanium alloy and β-type titanium alloy. It can be heat-treated and strengthened, has good weldability, and good comprehensive mechanical properties. It is the most widely used type of titanium alloy. However, the strength and plasticity matching of TC4 titanium alloy is insufficient, and the strength usually does not exceed 1100 MPa. Therefore, it is necessary to further improve the strength of TC4 titanium alloy.

[0003] For TC4 titanium alloy wire, the widely studied strengthening method at present is "solution + aging" treatment, which uses the precipitation of the second phase to produce the precipitation strengthening effect. In this way, the strength of TC4 can reach 1300 MPa, but the uniform elongation rate is usually less than 3%. Deformation, as a common metal strengthening method, is rarely used for the strengthening of TC4 titanium alloy because the plasticity of TC4 titanium alloy is poor and it is difficult to achieve a large enough strain for strain strengthening. For example, the commonly used initial structure for the deformation of TC4 titanium alloy is equiaxed α+β structure, and its strain is usually about 0.5, and the strain strengthening effect is limited. At the same time, after deformation, it is necessary to immediately perform high-temperature annealing at above 800 °C to make the deformed structure recrystallize and soften, restoring the ability of the titanium alloy to continue to deform, but this causes the originally generated strain strengthening effect to basically disappear.

[0004] Therefore, the existing methods for improving the strength of TC4 titanium alloy have the problems of insufficient strength improvement and low uniform elongation rate of the obtained TC4 titanium alloy wire. Summary of the Invention

[0005] Object of the Invention: The object of the present invention is to provide a method for improving the strength of TC4 titanium alloy wire. The TC4 titanium alloy wire processed by this method has an ultra-high strength of more than 1400 MPa, and at the same time meets the requirement that the uniform elongation rate of the wire is more than 3%.

[0006] Technical Solution: The method for improving the strength of TC4 titanium alloy wire described in the present invention includes the following steps:

[0007] (1) Martensitic treatment: Heat the TC4 titanium alloy wire to a temperature range of 800 °C to 850 °C for heat preservation, and then water quench to obtain an α+α′ martensite initial structure;

[0008] (2) Tissue fibrosis treatment: Perform drawing with a medium strain on the TC4 titanium alloy wire in step (1) to obtain a fibrous α+α′ drawn structure;

[0009] (3) Martensite decomposition treatment: Heat the TC4 titanium alloy wire in step (2) to the temperature range of 500°C to 600°C, keep it warm for a short time and then air-cool it to decompose the α+α′ fibrous structure into an α+(α s +β) duplex fibrous structure;

[0010] (4) Discontinuous cumulative large-strain drawing based on defect rearrangement treatment: Perform discontinuous and multiple small-strain drawings on the TC4 titanium alloy wire in step (3), cumulatively achieve large-strain drawing, and heat the wire to 400°C to 500°C between adjacent passes for defect rearrangement treatment; obtain high-strength TC4 titanium alloy wire.

[0011] Among them, in step (1), the heat preservation time is 30 min to 90 min.

[0012] Among them, in step (1), in the α+α′ martensite initial structure, the content of α′ martensite is 30% to 60%.

[0013] Among them, in step (2), the strain is 0.4 to 0.6.

[0014] Among them, in step (2), in the fibrous α+α′ drawing structure, both α and α′ phases are oriented along the drawing direction.

[0015] Among them, in step (3), the heat preservation time is 2 min to 10 min.

[0016] Among them, in step (4), for each small-strain drawing in each pass, the strain per pass is 0.2 to 0.4, and the cumulative total strain is 1.2 to 2.0.

[0017] Among them, in step (4), the heat preservation time for each defect rearrangement treatment by heating to 400°C to 500°C is 30 min to 60 min.

[0018] Principle of the present invention: Through the mutual connection and joint action among various steps, the present invention achieves the effects of large-strain drawing, microstructure refinement, and strength improvement of TC4 titanium alloy wire. Martensitic treatment of wire microstructure: Different from the conventional wire that usually adopts equiaxed α+β initial microstructure, the present invention controls the heat treatment temperature in the two-phase region and water quenching treatment, and adjusts to obtain an α+α′ martensite initial microstructure with a specific content (30% - 60%) of α′ martensite. Compared with the usual equiaxed α+β initial microstructure, the above microstructure state can endow the TC4 titanium alloy wire with good initial plasticity, laying an initial microstructure foundation for subsequent medium-strain drawing and microstructure fibrosis treatment. Microstructure fibrosis treatment: The above wire is subjected to medium-strain drawing (strain 0.4 - 0.6) to obtain a fibrous α+α′ drawn microstructure; in this fibrous microstructure, both α and α′ phases are oriented along the drawing direction, rather than the traditional equiaxed and randomly embedded arrangement, which can avoid stress concentration, further improve the drawing performance of the wire, and provide microstructure guarantee for subsequent large-strain drawing of the wire. Martensite decomposition treatment: Although microstructure fibrosis treatment is beneficial to large-strain drawing, the α′ martensite therein will cause the initial yield strength to decrease; therefore, the wire is heated briefly in the temperature range of 500°C - 600°C, and on the basis of retaining the beneficial fibrous oriented microstructure generated in the previous step, the α′ martensite decomposes into fine submicron α s +β precipitation phase, and the fibrous α+α′ microstructure generated in the previous step is transformed into a fibrous α+(α s +β) duplex microstructure. The unique morphology (fibrous) and phase composition (simultaneous presence of α, α s +β duplex) of this microstructure can not only produce obvious precipitation strengthening and heterogeneous strain hardening effects, significantly improve the initial yield strength of the wire, but also retain the advantage of good deformation plasticity of the fibrous microstructure. Discontinuous cumulative large-strain drawing based on defect rearrangement treatment: The above TC4 titanium alloy wire with fibrous α+(α s +β) duplex microstructure and high initial yield strength is subjected to discontinuous cumulative large-strain drawing to achieve ultra-high strength of the TC4 titanium alloy wire; although the fibrous duplex microstructure formed in the previous step can enable the TC4 titanium alloy wire to withstand a greater ultimate strain, excessive single-pass strain is likely to cause defects in the wire prematurely and lead to abnormal drawing fracture. Therefore, the present invention adopts a discontinuous cumulative large-strain drawing method based on defect rearrangement treatment to solve the contradiction between large-strain drawing and premature cracking: the single-pass strain is 0.2 - 0.4, and the cumulative total strain can reach an ultra-large strain of 1.2 - 2.0; and a defect rearrangement treatment at 400°C - 500°C is adopted between adjacent two passes, enabling dislocation rearrangement and increasing the dislocation free path. While retaining the large-strain fibrous microstructure and drawing deformation strengthening effect, the cold working performance of the wire is restored to the greatest extent, and the ultra-high strength of the TC4 titanium alloy is achieved by using ultra-large strain.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: The method of the present invention uses a variety of means to be interrelated and act together, and can obtain TC4 titanium alloy wires with fine grains, composite precipitation phases arranged in a fibrous orientation along the axial direction of the wire, and a large number of dislocation proliferations. Based on the synergistic effect of various strengthening mechanisms such as grain refinement, precipitation strengthening, dislocation strengthening, and heterogeneous strain hardening, the treated TC4 titanium alloy wires can obtain an ultra-high strength of more than 1400 MPa, and the uniform elongation rate is more than 3%. Description of the Drawings

[0020] Figure 1 It is the grain photograph (a) and the tensile engineering stress-strain curve (b) of the TC4 titanium alloy wire obtained in Example 1;

[0021] Figure 2 It is the grain photograph (a) and the tensile engineering stress-strain curve (b) of the TC4 titanium alloy wire obtained in Example 2;

[0022] Figure 3 It is the grain photograph (a) and the tensile engineering stress-strain curve (b) of the TC4 titanium alloy wire obtained in Example 3. Detailed Embodiments

[0023] Example 1

[0024] The method for improving the strength of TC4 titanium alloy wires of the present invention includes the following steps:

[0025] (1) Heat the TC4 titanium alloy wire to 800 °C and hold for 90 min, then water quench to obtain an α + α′ martensite initial structure, where the content of α′ martensite is about 30%;

[0026] (2) Perform drawing on the TC4 titanium alloy wire in step (1) with a strain of 0.4 to obtain a fibrous α + α′ drawn structure;

[0027] (3) Heat the TC4 titanium alloy wire in step (2) to 500 °C, hold for 10 min and then air cool to decompose the α + α′ fibrous structure into an α + (α s + β) duplex fibrous structure;

[0028] (4) Perform discontinuous cumulative large-strain drawing on the TC4 titanium alloy wire in step (3), with a single-pass drawing strain of 0.2 and a cumulative total strain of 1.2. Heat the wire to 400 °C and hold for 60 min between adjacent two passes for defect rearrangement treatment; obtain high-strength TC4 titanium alloy wires.

[0029] The grain photograph and the engineering stress-strain curve of the TC4 titanium alloy wire after being treated in Example 1 are as Figure 1 shown, and throughFigure 1 As can be seen from a, the titanium alloy structure is arranged in a fibrous shape along the drawing direction. Through Figure 1 b, it can be seen that its tensile strength reaches 1425 MPa; its uniform elongation is about 4.6%.

[0030] Example 2

[0031] The method for improving the strength of TC4 titanium alloy wire in the present invention includes the following steps:

[0032] (1) Heat the TC4 titanium alloy wire to 850 °C and hold for 30 min, then water quench to obtain an α + α′ martensite initial structure, where the α′ martensite content is about 60%;

[0033] (2) Perform drawing on the TC4 titanium alloy wire in step (1) with a strain of 0.6 to obtain a fibrous α + α′ drawn structure;

[0034] (3) Heat the TC4 titanium alloy wire in step (2) to 600 °C, hold for 2 min and then air cool to decompose the α + α′ fibrous structure into an α + (α s + β) duplex fibrous structure;

[0035] (4) Perform discontinuous cumulative large-strain drawing on the TC4 titanium alloy wire in step (3), with a single-pass drawing strain of 0.4 and a cumulative total strain of 2.0. Heat the wire to 500 °C and hold for 30 min between adjacent passes for defect rearrangement treatment; obtain high-strength TC4 titanium alloy wire.

[0036] The grain photo and engineering stress-strain curve of the TC4 titanium alloy wire after being treated in Example 2 are as Figure 2 shown. As can be seen from Figure 2 a, the titanium alloy structure is arranged in a fibrous shape along the drawing direction. Through Figure 2 b, it can be seen that its tensile strength reaches 1420 MPa; its uniform elongation is about 3.5%.

[0037] Example 3

[0038] The method for improving the strength of TC4 titanium alloy wire in the present invention includes the following steps:

[0039] (1) Heat the TC4 titanium alloy wire to 820 °C and hold for 60 min, then water quench to obtain an α + α′ martensite initial structure, where the α′ martensite content is about 40%;

[0040] (2) Perform drawing on the TC4 titanium alloy wire in step (1) with a strain of 0.5 to obtain a fibrous α + α′ drawn structure;

[0041] (3) Heat the TC4 titanium alloy wire in step (2) to 550 °C, hold for 5 min and then air cool to decompose the α+α′ fibrous structure into an α+(α s +β) duplex fibrous structure;

[0042] (4) Perform discontinuous accumulative large-strain drawing on the TC4 titanium alloy wire in step (3), with the strain per pass being 0.3 and the total accumulative strain being 1.5. Heat the wire to 450 °C and hold for 40 min between adjacent passes for defect rearrangement treatment; obtain a high-strength TC4 titanium alloy wire.

[0043] The grain photos and engineering stress-strain curves of the TC4 titanium alloy wire after the treatment of Example 3 are as Figure 3 shown. As can be seen from Figure 3 a, the titanium alloy structure is arranged in a fibrous shape along the drawing direction. As can be seen from Figure 3 b, its tensile strength reaches 1401 MPa; its uniform elongation is about 3.2%.

[0044] Comparative Example 1

[0045] (1) Perform drawing with a strain of 0.4 on a TC4 titanium alloy wire with an equiaxed α+β initial structure;

[0046] (2) Heat the TC4 titanium alloy wire in step (1) to 500 °C, hold for 10 min and then air cool;

[0047] (3) Perform discontinuous accumulative large-strain drawing on the TC4 titanium alloy wire in step (2), with the strain per pass being 0.2 and the total accumulative strain being 1.2. Heat the wire to 400 °C and hold for 60 min between adjacent passes.

[0048] Mechanical property tests were carried out on the TC4 titanium alloy wire treated above, and it was found that its tensile strength was about 1250 MPa and its uniform elongation was about 2.3%.

[0049] Comparative Example 2

[0050] (1) Heat the TC4 titanium alloy wire to 800 °C and hold for 90 min, then water quench to obtain an α+α′ martensite initial structure, where the α′ martensite content is about 30%;

[0051] (2) Perform drawing with a strain of 0.4 on the TC4 titanium alloy wire in step (1) to obtain a fibrous α+α′ drawn structure;

[0052] (3) Heat the TC4 titanium alloy wire in step (2) to 500 °C, hold for 10 min and then air cool to decompose the α+α′ fibrous structure into an α+(α s +β) duplex fibrous structure;

[0053] (4) The TC4 titanium alloy wire rod in step (3) is subjected to discontinuous drawing, with the strain per single pass being 0.2 and the cumulative total strain being 0.8.

[0054] It is found that when the cumulative strain of the TC4 titanium alloy wire rod reaches about 0.8 during the treatment in step (4), wire breakage occurs, and large-strain drawing above 1.2 cannot be carried out. Measuring the properties of the TC4 titanium alloy wire rod after wire breakage, it is found that its tensile strength is about 1260 MPa and the uniform elongation is about 2.5%.

[0055] Comparative Example 3

[0056] (1) The TC4 titanium alloy wire rod is heated to 820 °C and held for 60 min, and then water quenched to obtain an α + α′ martensite initial structure, where the content of α′ martensite is about 40%;

[0057] (2) The TC4 titanium alloy wire rod in step (1) is drawn with a strain of 0.5 to obtain a fibrous α + α′ drawn structure;

[0058] (3) The TC4 titanium alloy wire rod in step (2) is subjected to discontinuous cumulative large-strain drawing, with the strain per single pass being 0.3 and the cumulative total strain being 1.5. The wire rod is heated to 450 °C and held for 40 min between two adjacent passes.

[0059] The obtained TC4 titanium alloy wire rod is subjected to a mechanical property test, with a tensile strength of about 1290 MPa and a uniform elongation of about 2.8%.

[0060] Comparative Example 4

[0061] (1) The TC4 titanium alloy wire rod is heated to 800 °C and held for 90 min, and then water quenched to obtain an α + α′ martensite initial structure, where the content of α′ martensite is about 30%;

[0062] (2) The TC4 titanium alloy wire rod in step (1) is heated to 500 °C, held for 10 min and then air cooled to decompose the α + α′ equiaxed structure into an α + (α s + β) equiaxed structure;

[0063] (3) The TC4 titanium alloy wire rod in step (2) is subjected to discontinuous drawing, with the strain per single pass being 0.25. The wire rod is heated to 400 °C and held for 60 min between two adjacent passes, and the cumulative total strain is 0.75.

[0064] It was found that during the treatment in step (3), when the cumulative strain of the TC4 titanium alloy wire reached about 0.75, wire breakage occurred and large-strain drawing above 1.2 could not be carried out. By measuring the properties of the TC4 titanium alloy wire after wire breakage, it was found that its tensile strength was about 1230 MPa and the uniform elongation was about 2.5%.

Claims

1. A method for improving the strength of TC4 titanium alloy wire, characterized in that: The steps include: (1) Martensitic treatment: The TC4 titanium alloy wire is heated to a temperature range of 800°C to 850°C and then water quenched to obtain an α+α′ martensitic initial structure; (2) tissue fibrosis treatment: the TC4 titanium alloy wire of step (1) is subjected to medium strain drawing to obtain a fibrous α+α′ drawing structure; (3) Martensite decomposition treatment: The TC4 titanium alloy wire of step (2) is heated to a temperature range of 500°C to 600°C, kept warm for a short time, and then air-cooled to decompose the α+α′ fibrous structure into α+(α s +β) complex fibrous tissue; (4) Discontinuous cumulative large strain drawing based on defect rearrangement treatment: The TC4 titanium alloy wire of step (3) is subjected to discontinuous, multiple small strain drawing to cumulatively achieve large strain drawing, and the wire is heated to 400° C. to 500° C. between two adjacent passes to perform defect rearrangement treatment; High-strength TC4 titanium alloy wire is obtained.

2. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (1), the insulation time is 30 min to 90 min.

3. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (1), in the initial α+α′ martensite structure, the content of α′ martensite is 30% to 60%.

4. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (2), the strain is 0.4 to 0.

6.

5. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (2), in the fibrous α+α′ drawn structure, both the α and α′ phases are oriented along the drawing direction.

6. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (3), the insulation time is 2 min to 10 min.

7. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (4), small strain drawing is performed in each pass, the strain of a single pass is 0.2 to 0.4, and the accumulated total strain is 1.2 to 2.

0.

8. The method for improving the strength of TC4 titanium alloy wire according to claim 1, characterized in that: In step (4), the holding time for each heating to 400°C to 500°C for defect rearrangement treatment is 30min to 60min.

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