Preparation method of high-toughness rare earth-containing titanium alloy bar

Abstract

The invention relates to a preparation method of a high-toughness rare earth-containing titanium alloy bar, and belongs to the field of non-ferrous metal processing. The method comprises the following steps: S1, weighing raw materials to press an electrode, and obtaining a titanium alloy cast ingot through a vacuum consumable electric arc furnace; s2, the titanium alloy cast ingot is subjected to homogenizing annealing; s3, the titanium alloy cast ingot is subjected to multi-heating-number cogging forging, and a primary forging stock is obtained; s4, the primary forging stock is subjected to multi-heating-number and multi-direction forging, and a fine-grain intermediate blank is obtained; s5, the intermediate blank is subjected to multi-pass rolling, and a hot-rolled bar is obtained; and S6, the hot-rolled bar is subjected to solid solution and aging treatment and air cooling. According to the method, through cooperative regulation and control of multi-element alloy strengthening, rare earth microalloying, multi-stage cooling forging and double heat treatment, good matching of high strength and high toughness is achieved. The technical problem that how to give full play to the beneficial effect of rare earth elements is solved, and the titanium alloy bar with high strength and toughness matching is obtained through reasonable component design and process regulation and control.

Description

Technical Field

[0001] This invention relates to a method for preparing high-strength and tough rare-earth titanium alloy bars, belonging to the field of non-ferrous metal processing. Background Technology

[0002] Titanium alloys are widely used in the aerospace field due to their high specific strength, excellent corrosion resistance, and good high-temperature performance. With the increasing demands for structural weight reduction and load-bearing capacity in next-generation aircraft, traditional medium-strength titanium alloys (such as TC4, with a strength grade of 900-1100 MPa) are no longer sufficient to meet the design requirements of main load-bearing components. Therefore, the development of ultra-high-strength titanium alloys with a strength ≥1300 MPa and good toughness and ductility has become a research hotspot.

[0003] Currently, the development of high-strength titanium alloys mainly follows two directions: one is to obtain high-strength β-type titanium alloys (such as Ti-5553, Ti-1023, etc.) by adding a large amount of β-stabilizing elements such as Mo, V, Cr, and Fe, whose strength can reach more than 1300MPa, but the plasticity is often low (elongation <8%), and the high content of alloying elements leads to increased costs; the other is to optimize the microstructure of α+β-type titanium alloys through fine thermomechanical processing technology to achieve a balance between strength and toughness, but traditional forging processes are prone to problems such as uneven microstructure and coarse grains.

[0004] Rare earth elements, due to their unique electronic structure and chemical reactivity, exhibit significant microalloying effects in titanium alloys. Studies have shown that the addition of appropriate amounts of rare earth elements can achieve multiple beneficial effects, including deoxidation and purification of the melt, refinement of the as-cast microstructure, inhibition of grain growth, and improvement of inclusion distribution. For example, yttrium (Y) can react with oxygen to generate fine Y₂O₃ dispersed particles, which both purify the matrix and provide dispersion strengthening; lanthanum (La) and cerium (Ce) can refine grains and improve thermal stability.

[0005] Chinese patent CN 119368741 A discloses a method for preparing high-strength, corrosion-resistant, fine-grained titanium alloy plates. By adding a reasonable ratio of various rare earth elements, costs are reduced and the consumption of specific rare earth resources is decreased. Furthermore, there is a synergistic effect between different rare earth elements; through appropriate heat treatment processes, the strengthening effect of rare earth elements can be further enhanced, promoting the formation of precipitated phases, thereby improving the alloy's strength and corrosion resistance. However, the yield strength of the final product is below 1000 MPa, and the tensile strength is below 1200 MPa.

[0006] Therefore, how to fully utilize the beneficial effects of rare earth elements and obtain high-strength and high-toughness titanium alloy rods through reasonable composition design and process control remains a technical problem that urgently needs to be solved. Summary of the Invention

[0007] The technical problem to be solved by this invention is how to fully utilize the beneficial effects of rare earth elements and obtain high-strength and high-toughness titanium alloy rods through reasonable composition design and process control.

[0008] The technical solution adopted by this invention to solve its technical problem is: a method for preparing high-strength and tough rare-earth titanium alloy rods, comprising the following steps: S1. Alloy smelting: According to the weight percentage of the chemical composition of the bar: Al 5.5%~7.0%, Mo 2.0%~6.0%, V 2.5%~5.5%, Cr 1.5%~3.5%, Fe 0.5%~1.5%, Zr 1.0%~4.0%, rare earth element RE 0.1%~0.5%, with the balance being Ti and unavoidable impurities, the sponge titanium, alloying elements and rare earth elements are weighed, the electrodes are pressed, and the titanium alloy ingot is obtained by smelting 2 to 3 times in a vacuum arc furnace; S2. Homogenization treatment: The titanium alloy ingot is homogenized by holding it at 100℃~150℃ above the β phase transformation point for 10~30 hours. S3. Initial forging: The homogenized titanium alloy ingot is subjected to multiple forging processes at 50℃~100℃ above the β phase transformation point, with a forging ratio of ≥2.5 per process and a total forging ratio of ≥8, to obtain the primary forging billet. S4. Multi-stage cooling forging: The primary forging billet is subjected to 3 to 5 multi-directional forgings in a temperature range of 40°C below the β phase transformation point to 30°C above the β phase transformation point, and the forging temperature decreases with each forging. The deformation amount of upsetting and drawing in each forging is not less than 40%, and the forging temperature of the last forging is controlled at 30°C to 50°C below the phase transformation point to obtain a fine-grained intermediate billet. S5. Rolling and forming: The intermediate billet is heated to the two-phase region temperature T. β -60℃~T β At -20℃, multiple rolling passes are performed, with a total deformation of 60% to 85%, to obtain hot-rolled bars; S6. Double heat treatment: Hot-rolled bars undergo solution treatment and aging treatment, first heated to T... β -20℃~T β After holding at +20℃ for 1 to 3 hours, water or oil cooling is performed, followed by aging treatment at 500℃ to 650℃ for 4 to 12 hours, and then air cooling.

[0009] Furthermore, the rare earth element RE in step 1 is at least one of Y, La, Ce, Nd, and Gd, and is added in the form of a pure metal or intermediate alloy.

[0010] Furthermore, in step 1, the oxygen content of the titanium alloy ingot is controlled below 0.12%, and the amount of rare earth elements added is 0.15% to 0.35%.

[0011] Furthermore, during the billet forging process in step 3, the deformation amount per cycle is controlled between 25% and 45%, and the billet needs to be reheated and kept warm between adjacent forging cycles.

[0012] Furthermore, in step 4, the billet after roughing is placed in a T... β ~T β Heating to +30℃ for the first multi-directional forging; reheating in the furnace to T... β -20℃~T β Then, perform a second forging; then cool down to T. β -40℃~T β The third forging is carried out at -20℃; finally, it is carried out at T as needed. β -50℃~T β Forging is carried out at -30℃ for the fourth or fifth time.

[0013] Furthermore, in step 4, during the multi-stage cooling forging process, the temperature drop between forging stages is 20℃ to 50℃, and each forging stage is followed by air cooling to room temperature before the next heating stage.

[0014] Furthermore, the multi-directional forging in step 4 includes upsetting and drawing deformation in at least two mutually perpendicular directions.

[0015] Furthermore, in step 5, the rolling process uses a transverse rolling mill or a continuous rolling mill, with 5 to 9 rolling passes and a final rolling temperature of not less than 750°C.

[0016] Furthermore, in step 6, after the solution treatment and before the aging treatment, the bar needs to undergo cold deformation processing, with the cold deformation amount being 5% to 15%. The beneficial effects of this invention are as follows: This method, through the synergistic effect of multi-element alloy strengthening and rare earth microalloying, combined with multi-stage cooling forging and double heat treatment, yields bars with a tensile strength ≥1300MPa, elongation ≥10%, and fracture toughness KIC ≥70MPa·m¹ / ², overcoming the traditional problem of balancing strength and toughness in high-strength titanium alloys. The microstructure is uniform and fine; the purification and refining effect of rare earth elements, combined with multi-stage cooling forging, effectively breaks down grains, resulting in a uniform bimodal or basketweave microstructure with an α-lamellae thickness ≤0.5μm and a discontinuous distribution of the α-phase at grain boundaries, which is beneficial for improving strength and toughness. Rare earth elements exist in the form of dispersed oxides, both purifying the matrix and providing dispersion strengthening, while simultaneously inhibiting grain growth and improving thermal stability. The method also boasts strong process adaptability: it is compatible with both forging and rolling processes, enabling the production of bars of different specifications to meet the diverse needs of aerospace structural components. Detailed Implementation

[0017] The present invention will be further described below with reference to embodiments and comparative examples.

[0018] A method for preparing high-strength and high-toughness rare-earth titanium alloy rods includes the following steps: S1. Alloy smelting: According to the weight percentage of the chemical composition of the bar: Al 5.5%~7.0%, Mo 2.0%~6.0%, V 2.5%~5.5%, Cr 1.5%~3.5%, Fe 0.5%~1.5%, Zr 1.0%~4.0%, rare earth element RE 0.1%~0.5%, with the balance being Ti and unavoidable impurities, the sponge titanium, alloying elements and rare earth elements are weighed, the electrodes are pressed, and the titanium alloy ingot is obtained by smelting 2 to 3 times in a vacuum arc furnace; S2. Homogenization treatment: The titanium alloy ingot is homogenized by holding it at 100℃~150℃ above the β phase transformation point for 10~30 hours. S3. Initial forging: The homogenized titanium alloy ingot is subjected to multiple forging processes at 50℃~100℃ above the β phase transformation point, with a forging ratio of ≥2.5 per process and a total forging ratio of ≥8, to obtain the primary forging billet. S4. Multi-stage cooling forging: The primary forging billet is subjected to 3 to 5 multi-directional forgings in a temperature range of 40°C below the β phase transformation point to 30°C above the β phase transformation point, and the forging temperature decreases with each forging. The deformation amount of upsetting and drawing in each forging is not less than 40%, and the forging temperature of the last forging is controlled at 30°C to 50°C below the phase transformation point to obtain a fine-grained intermediate billet. S5. Rolling and forming: The intermediate billet is heated to the two-phase region temperature T. β -60℃~T β At -20℃, multiple rolling passes are performed, with a total deformation of 60% to 85%, to obtain hot-rolled bars; S6. Double heat treatment: Hot-rolled bars undergo solution treatment and aging treatment, first heated to T... β -20℃~T βAfter holding at +20℃ for 1 to 3 hours, water or oil cooling is performed, followed by aging treatment at 500℃ to 650℃ for 4 to 12 hours, and then air cooling. Those skilled in the art should know that the technical principle of this method is: (1) Multi-element alloy strengthening: Al, Mo, V, Cr, Fe, Zr multi-element alloy system is adopted. Al solid solution strengthens the α phase; Mo, V, Cr, and Fe are β-stabilizing elements, which improve hardenability and aging strengthening effect; Zr solid solution strengthens and improves thermal stability; (2) Rare earth microalloying: Rare earth elements (Y, La, Ce, etc.) play multiple roles in titanium alloys: react with impurities such as oxygen and nitrogen in the melt to generate high-melting-point rare earth oxides / nitrides, which play a purifying role; refine the as-cast structure and inhibit β grain growth; form fine rare earth oxide dispersed phases to provide dispersion strengthening; improve the morphology and distribution of inclusions and improve toughness and plasticity; (3) Multi-stage cooling forging: adopt the step-by-step cooling forging process from the β region to the (α+β) region, which utilizes the dynamic recrystallization of the high-temperature region to break the as-cast structure, and accumulates deformation energy at a lower temperature to obtain fine equiaxed or bi-phase structures; (4) Dual heat treatment: control the α / β phase and morphology through solid solution treatment, and precipitate dispersed secondary α phase through aging treatment to achieve precipitation strengthening. In step S2, the titanium alloy ingot is preferably homogenized by holding it at 100°C–150°C above the β phase transformation point for 10–30 hours to eliminate dendritic segregation and ensure uniform distribution of alloying elements and rare earth elements. Too short a holding time results in insufficient homogenization, while too long a time leads to grain coarsening. In step S3, the homogenized titanium alloy ingot is preferably subjected to multi-stage forging at 50°C–100°C above the β phase transformation point. Deformation in the high-temperature zone breaks down the as-cast structure and improves the original grain boundary state. The forging ratio per stage is controlled to be no less than 2.5, with a single deformation amount of 25%–45%, and a total forging ratio ≥8, ensuring sufficient fragmentation of the as-cast structure. Step S4 is the key process of this invention, employing a "gradual cooling multi-directional forging" strategy to obtain a refined α+β structure. In step S5, the intermediate billet after multi-stage forging is heated to T... β -60℃~T β The two-phase region temperature is -20℃, followed by holding at this temperature and then multi-pass rolling, with the total deformation controlled between 60% and 85%. Rolling in the α+β two-phase region allows both the α and β phases to deform and recrystallize simultaneously, forming a fine biphasic structure or basketweave structure. Step S6 employs a "solution + aging" strengthening heat treatment; solution treatment: heating to T... β -20℃~T β +20℃, hold for 1–3 hours, then water-cool or oil-cool. The aim is to obtain a supersaturated β phase and some primary α phase, and to regulate the α / β ratio and morphology. Aging treatment: Hold at 500℃–650℃ for 4–12 hours, then air-cool. This causes the precipitation of a dispersed secondary α phase in the supersaturated β phase, producing a strong precipitation-enhancing effect. The term T appears in the text. β The temperature at which the ingot undergoes a β-phase transformation is denoted as α.

[0019] Preferably, the rare earth element RE in step 1 is at least one of Y, La, Ce, Nd, and Gd, and is added in the form of a pure metal or an intermediate alloy. Those skilled in the art should understand that, since rare earth elements play multiple roles in titanium alloys, this method preferably uses at least one of Y, La, Ce, Nd, and Gd for the rare earth element RE; that is, it can be one of Y, La, Ce, Nd, and Gd, or a combination of multiple elements from Y, La, Ce, Nd, and Gd. Furthermore, it is further preferred that the rare earth element RE is added in the form of a pure metal or an intermediate alloy.

[0020] Preferably, in step 1, the oxygen content of the titanium alloy ingot is controlled below 0.12%, and the amount of rare earth elements added is 0.15% to 0.35%. Those skilled in the art should understand that, to avoid the formation of excessive brittle oxides, this method preferably controls the oxygen content of the titanium alloy ingot below 0.12%. Since adding too little rare earth (<0.1%) has little effect, and adding too much (>0.5%) easily forms coarse rare earth compounds, deteriorating plasticity, this method further preferably uses an amount of rare earth elements of 0.15% to 0.35%.

[0021] Preferably, in step 3, during the billet forging process, the deformation amount in a single operation is controlled within 25% to 45%, and the billet needs to be reheated and kept warm between adjacent forging operations. Those skilled in the art should understand that further limiting the deformation amount in a single operation to 25% to 45% and requiring the billet to be reheated and kept warm between adjacent forging operations ensures uniform deformation.

[0022] Preferably, in step 4, the billet after roughing is placed in T... β ~T β Heating to +30℃ for the first multi-directional forging; reheating in the furnace to T... β -20℃~T β Then, perform a second forging; then cool down to T. β -40℃~T β The third forging is carried out at -20℃; finally, it is carried out at T as needed. β -50℃~T β The fourth or fifth forging is carried out at -30℃. The temperature drop between forging passes is 20℃ to 50℃. After each forging pass, the material is air-cooled to room temperature before the next forging pass. Multi-directional forging includes upsetting and elongation deformation in at least two mutually perpendicular directions. Each forging pass employs multi-directional forging, including upsetting and elongation deformation in at least two mutually perpendicular directions, with the upsetting and elongation deformation in each pass not less than 40%. This process utilizes dynamic recrystallization in the high-temperature zone to refine the grains, while accumulating deformation energy at lower temperatures, allowing rare earth oxides to precipitate diffusely and distribute evenly. The forging temperature of the final pass is controlled at 30℃ to 50℃ below the phase transformation point to obtain a refined α+β microstructure.

[0023] Preferably, in step 5, the rolling process uses a transverse rolling mill or a continuous rolling mill, with 5 to 9 rolling passes and a final rolling temperature of not less than 750°C to prevent work hardening and cracks.

[0024] Preferably, in step 6, after solution treatment and before aging treatment, the bar needs to undergo cold deformation processing, with the cold deformation amount being 5% to 15%, to introduce dislocation defects to promote aging precipitation and further improve strength.

[0025] Example 1 The design composition is Ti-6Al-5Mo-4V-2.5Cr-1Fe-3Zr-0.2Y (weight percentage), with a phase transition point T. β It is approximately 865℃.

[0026] 1) Weigh out sponge titanium, Al-Mo master alloy, Al-V master alloy, pure Cr, pure Fe, pure Zr and Y according to the proportion, press the electrode, and obtain the ingot through three vacuum self-consumption melting processes.

[0027] 2) Homogenize the ingot at 980℃ for 20 hours.

[0028] 3) Forging: Heat to 940℃ and perform three forging cycles, with a forging ratio of 2.5 to 3.0 per cycle and a total forging ratio of 9.5.

[0029] 4) Multi-stage cooling forging: First stage upsetting at 880℃ (deformation 45%+50%), second stage upsetting at 850℃ (deformation 40%+45%), third stage upsetting at 820℃ (deformation 40%+40%), fourth stage upsetting at 800℃ (deformation 35%+40%), to obtain a 120mm square billet.

[0030] 5) Rolling: The billet is heated to 820℃ and rolled into Φ30mm bars in 7 passes with a total deformation of 75% and a final rolling temperature of 780℃.

[0031] 6) Heat treatment: The bar stock is solution treated at 860℃ for 1.5 hours and then water-cooled; then aged at 550℃ for 8 hours and air-cooled.

[0032] The finished bar has a tensile strength of 1350 MPa, a yield strength of 1250 MPa, an elongation of 11.5%, a reduction of area of ​​38%, and a fracture toughness (KIC) of 76 MPa·m¹ / ². The microstructure is a fine bimorphic structure, with a primary α phase content of approximately 25%, α lamellar thickness of 0.3–0.5 μm, and discontinuous distribution of the α phase at grain boundaries.

[0033] Example 2 The design composition is Ti-6.5Al-4.5Mo-5V-2Cr-0.8Fe-2.5Zr-0.15La-0.1Ce (by weight).

[0034] 1) Melting, homogenization, and billet forging are the same as in Example 1.

[0035] 2) Multi-stage cooling forging: first stage 890℃, second stage 860℃, third stage 830℃, fourth stage 810℃.

[0036] 3) Rolling: Heat to 815℃ and roll to Φ25mm.

[0037] 4) Heat treatment: solution treatment at 850℃ for 1 hour, followed by oil cooling and aging at 520℃ for 10 hours.

[0038] Tested, the tensile strength is 1320 MPa, the elongation is 12%, and the fracture toughness (KIC) is 73 MPa·m¹ / ².

[0039] Example 3 Based on Example 1, a 10% increase in cold drawing deformation was added after solution treatment, followed by aging at 550℃. The resulting bar exhibited a tensile strength of 1390 MPa, an elongation of 10%, and a reduction of area of ​​35%.

[0040] Comparative Example 1 The same composition and process as in Example 1 were used, but rare earth element Y was not added. The resulting bar had a tensile strength of 1280 MPa, an elongation of 9%, and a fracture toughness (KIC) of 62 MPa·m¹ / ². The microstructure was coarser, with continuous distribution of the α phase at grain boundaries, and its overall performance was inferior to that of Example 1.

[0041] Examples 1-3 and Comparative Example 1 above demonstrate that the present invention successfully prepared titanium alloy bars with both high strength and high toughness through the synergistic regulation of "multi-element alloy strengthening + rare earth micro-alloying + multi-stage cooling forging + dual heat treatment", which has good prospects for industrial application.

Claims

1. A method for preparing high-strength and high-toughness rare-earth-containing titanium alloy rods, characterized in that, Includes the following steps: S1. Alloy smelting: According to the weight percentage of the chemical composition of the bar: Al 5.5%~7.0%, Mo 2.0%~6.0%, V 2.5%~5.5%, Cr 1.5%~3.5%, Fe 0.5%~1.5%, Zr 1.0%~4.0%, rare earth element RE 0.1%~0.5%, with the balance being Ti and unavoidable impurities, the sponge titanium, alloying elements and rare earth elements are weighed, the electrodes are pressed, and the titanium alloy ingot is obtained by smelting 2 to 3 times in a vacuum consumable arc furnace; S2. Homogenization treatment: The titanium alloy ingot is homogenized by holding it at 100℃~150℃ above the β phase transformation point for 10~30 hours. S3. Initial forging: The homogenized titanium alloy ingot is subjected to multiple forging processes at 50℃~100℃ above the β phase transformation point, with a forging ratio of ≥2.5 per process and a total forging ratio of ≥8, to obtain the primary forging billet. S4. Multi-stage cooling forging: The primary forging billet is subjected to 3 to 5 multi-directional forgings in a temperature range of 40°C below the β phase transformation point to 30°C above the β phase transformation point, and the forging temperature decreases with each forging. The deformation amount of upsetting and drawing in each forging is not less than 40%, and the forging temperature of the last forging is controlled at 30°C to 50°C below the phase transformation point to obtain a fine-grained intermediate billet. S5. Rolling and forming: The intermediate billet is heated to the two-phase region temperature T. β -60℃~T β At -20℃, multiple rolling passes are performed, with a total deformation of 60% to 85%, to obtain hot-rolled bars; S6. Double heat treatment: Hot-rolled bars undergo solution treatment and aging treatment, first heated to T... β -20℃~T β After holding at +20℃ for 1 to 3 hours, water or oil cooling is performed, followed by aging treatment at 500℃ to 650℃ for 4 to 12 hours, and then air cooling.

2. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: The rare earth element RE in step 1 is at least one of Y, La, Ce, Nd, and Gd, and is added in the form of pure metal or intermediate alloy.

3. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that, In step 1, the oxygen content of the titanium alloy ingot is controlled below 0.12%, and the amount of rare earth elements added is 0.15% to 0.35%.

4. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: In step 3, during the billet forging process, the deformation amount per cycle is controlled between 25% and 45%, and the billet needs to be reheated and kept warm between adjacent forging cycles.

5. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: In step 4, the billet after roughing is placed in T... β ~T β Heating to +30℃ for the first multi-directional forging; reheating in the furnace to T... β -20℃~T β Then, perform a second forging; then cool down to T. β -40℃~T β The third forging is carried out at -20℃; finally, it is carried out at T as needed. β -50℃~T β Forging is carried out at -30℃ for the fourth or fifth time.

6. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: In step 4, during the multi-stage cooling forging process, the temperature drop between forging stages is 20℃ to 50℃. After each forging stage, the forging is air-cooled to room temperature before the next heating stage.

7. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: Step 4, multi-directional forging, includes upsetting and drawing deformation in at least two mutually perpendicular directions.

8. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: In step 5, the rolling process uses a transverse rolling mill or a continuous rolling mill, with 5 to 9 rolling passes and a final rolling temperature of not less than 750℃.

9. The method for preparing a high-strength and high-toughness rare-earth-containing titanium alloy rod according to claim 1, characterized in that: In step 6, after solution treatment and before aging treatment, the bar needs to undergo cold deformation processing, and the amount of cold deformation is 5% to 15%.