Ti55531 titanium alloy large-specification bar forging method

Abstract

The invention discloses a Ti55531 titanium alloy large-specification bar machining method. The method comprises the steps of cogging forging, intermediate forging and finished product forging. In the cogging forging process, the cast ingot alternately deforms at the temperature above and below the phase transformation point temperature, and beta grains are refined through static and dynamic recrystallization; and during recrystallization heating in the beta region, the temperature is controlled to be 30-50 DEG C above the phase transformation point so as to avoid excessive growth of beta grains. In the intermediate blank forging stage, controllable recrystallization annealing is added, and by controlling the forging temperature (15-25 DEG C below the phase transformation point), a part of primary alpha phase is properly reserved to limit rapid growth of beta crystal grains; through 2-3 times of'low + high 'circular forging at 40-60 DEG C and 15-25 DEG C below the beta transformation temperature, and in combination with beta-phase deformation recrystallization, the original beta grain size is remarkably refined, and the forging structure uniformity of a subsequent two-phase region is improved. The method for producing the Ti55531 titanium alloy large-specification (phi 350-600 mm) bar is advanced in process, short in preparation process, low in cost, good in product structure uniformity and high in batch stability.

Description

Technical Field

[0001] The invention relates to the technical field of titanium alloy manufacturing, and in particular to a forging method for Ti55531 titanium alloy large-size bars. Background Art

[0002] Ti55531 titanium alloy is a high-strength, high-toughness, near-β-type titanium alloy developed by VSMPO of Russia and Airbus of Europe based on BT-22. Its nominal composition is Ti-5Al-5V-5Mo-3Cr-1Zr. This alloy exhibits excellent fatigue resistance, corrosion resistance, high strength, high toughness, and other mechanical properties, is insensitive to compositional segregation, has a wide processing window, and exhibits excellent hardenability. Compared to high-strength, high-toughness alloys such as Ti1023, Ti55531 titanium alloy boasts approximately 15% higher room-temperature tensile strength. Consequently, Ti55531 titanium alloy is gaining increasing popularity in the aerospace industry, being used in the manufacture of high-stress structural components such as aircraft landing gear, blades, and wings.

[0003] As titanium alloy forgings for the aerospace industry develop toward larger and extra-large sizes, bar specifications are also expanding accordingly. During the conventional forging production of large-sized titanium alloy bars, to ensure uniformity of the bar structure, multiple fires of repeated upsetting and drawing deformation are typically performed in the α+β two-phase region to break up the α and β grains. This complex forging process, coupled with temperature drops and uneven deformation during the forging process, can easily lead to problems such as uneven structure in the finished bar and poor batch stability. Patent 201811302933.0 proposes a Ti55531 titanium alloy Φ400-Φ500mm bar processing technology. This technology, combined with a single dynamic recrystallization (80-100°C above the phase transformation point) after conventional forging, can refine the grains and reduce the number of forging fires. For titanium alloys, heating above the phase transformation point causes the β grains to grow; as the temperature increases, the growth rate accelerates (exponentially), which leads to coarsening of the dynamically recrystallized β grains. In addition, for large-sized intermediate billets, the large difference in holding time between the core and the edges after reaching temperature makes it difficult to control the uniformity of β grain size, which can easily lead to problems such as uneven microstructure between the edges and the core. This uneven β grain structure can affect the uniformity of the microstructure obtained in the subsequent two-phase forging process. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to address the shortcomings of the above-mentioned existing technologies and provide a forging method for large-scale Ti55531 titanium alloy bars with a diameter of 350 to 600 mm. The bars produced by this invention have an α+β two-phase structure with good microstructure uniformity, achieving low-cost, large-scale and stable production of large-scale Ti55531 titanium alloy bars to meet the development needs of the aerospace industry.

[0005] The technical solution of the present invention is a method for forging large-sized Ti55531 titanium alloy bars, comprising the following steps:

[0006] Step 1: Forging

[0007] Ti55531 titanium alloy ingots are forged in the following manner: forging above the phase transformation point → forging below the phase transformation point → forging above the phase transformation point;

[0008] During forging, the ingot is heated to 1000-1150℃ and subjected to 2-3 rounds of upsetting forging; after forging, it is returned to the furnace to be heated and kept at 30-50℃ below the phase transformation point, and then subjected to 1-2 rounds of upsetting forging; then it is directly returned to the furnace for recrystallization heat treatment, with the heating temperature being 30-50℃ above the β transformation temperature, and after holding for an appropriate time, it is taken out of the furnace, and then subjected to 1-2 rounds of upsetting forging, and then cooled after forging.

[0009] Step 2: Forging the intermediate billet

[0010] After the blanking forging, the blank is heated to 40-60℃ below the phase transformation point and upsetting at 15-25℃; then upsetting forging is carried out for 3-4 times at 30-60℃ below the phase transformation point, with the forging ratio of each time controlled between 2.0-3.0, and cooling is carried out after forging;

[0011] Step 3: Forging of Bar Billets

[0012] The billet forged in step 2 is heated to 40-60°C below the phase transformation point, and subjected to one-time drawing-rolling forging to obtain a rod billet with a forging ratio of 1.8-2.5, and then cooled after forging.

[0013] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, in step 1, after performing 1-2 rounds of upsetting and drawing forging, post-forging cooling is performed. More preferably, the post-forging cooling is post-forging air cooling.

[0014] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, in step 1, the forging ratio of each fire is controlled between 1.8 and 3.0.

[0015] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, the upsetting process in the upsetting process is uniformly accelerated and controlled at a rate of 10 to 50 mm / s, so as to avoid overheating of the core.

[0016] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, in step 2, the forging blank is subjected to one fire upsetting at 40 to 60°C below the β-phase transformation temperature; and then subjected to one fire upsetting at 15 to 25°C below the β-phase transformation temperature, and the forging ratio of each fire is controlled between 2.0 and 3.0.

[0017] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, in step 2, for large-size Φ350-600mm billets, the above process of "heating the billet after blanking forging to 40-60°C below the phase transformation point and upsetting at 15-25°C" is repeated 1-2 times.

[0018] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, in steps 1 and 2, square billets are used for deformation.

[0019] According to the Ti55531 titanium alloy large-size bar forging method of the present invention, preferably, the holding time coefficient of the blank in steps 1 to 3 is in the range of 0.6 to 0.8 min / mm.

[0020] According to the forging method of Ti55531 titanium alloy large-size bars of the present invention, preferably, the heating time coefficient of the hot blank is in the range of 0.2 to 0.4 min / mm. Each step involves melting between the firings.

[0021] Preferably, the post-forging cooling in steps 1 to 3 is post-forging air cooling.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] During the blank forging process, the ingot is alternately deformed above and below the phase transformation point temperature, and the β grains are refined by static and dynamic recrystallization; during the recrystallization heating in the β zone, the temperature is controlled at 30 to 50°C above the phase transformation point to prevent excessive growth of the β grains. During the intermediate blank forging stage, a controllable recrystallization annealing is added. By controlling the forging temperature (15 to 25°C below the phase transformation point), a portion of the primary α phase is appropriately retained to limit the rapid growth of the β grains (the pinning effect of the α phase); at the same time, the β phase is deformed and recrystallized, significantly refining the size of the original β parent phase and improving the uniformity of the structure. The refinement of the β grains will limit the precipitation space and size of the α phase. The subsequent two-phase deformation can quickly achieve the crushing of the α lamellae, thereby reducing the number of forging fires and simplifying the forging process control. The large-scale (Φ350 to 600mm) Ti55531 titanium alloy bars produced by the present invention have good structural uniformity, low cost, and high batch stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the macrostructure of the Φ400mm bar prepared in Example 1 of the present invention.

[0025] Figure 2 is the microstructure of the Φ400mm bar prepared in Example 1 of the present invention (wherein the left Figure 2a For the heart, right side Figure 2b for the edge).

[0026] Figure 3 This is the macrostructure of the Φ550mm bar prepared in Example 2 of the present invention.

[0027] Figure 4 This is the macrostructure of the Φ400mm bar prepared in the comparative example of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0029] Example 1: φ400 rod

[0030] Step 1: Forging

[0031] Step 1.1, heat the Ti55531 ingot with a diameter of 860mm to above 1150℃ (β-transform temperature is 870℃), keep it warm for 8 hours, and then use a fast forging machine to perform one-fire two-upsetting two-drawing forging to φ550mm, wherein the upsetting rate is controlled at about 20-50mm / s and the forging ratio is controlled at about 2.5;

[0032] Step 1.2: After forging, return to the furnace and keep at 1050℃ for 1.5h, then perform one round of upsetting forging to φ550mm, wherein the upsetting rate is controlled at about 20-50mm / s and the forging ratio is controlled at about 2.5;

[0033] Step 1.3: After forging, return to the furnace and keep it at 820℃ for 2.5h, then perform one round of upsetting forging to φ550mm. The forging ratio is controlled at about 2.2, and the upsetting rate is controlled at about 10-30mm / s to avoid overheating of the core.

[0034] Step 1.4: After forging, return to the furnace and keep it at 910℃ for 3h, then perform one round of upsetting forging to φ550mm, where the upsetting rate is controlled at about 20-50mm / s, the forging ratio is controlled at about 2.2, and air-cool and grind after forging.

[0035] Step 2: Forging the intermediate billet

[0036] Step 2.1: The Ti55531 forging blank completed in step 1 is subjected to one heat upsetting and drawing at 810°C to a diameter of 520 mm, with a forging ratio of approximately 2.5; then returned to the furnace and held at 850°C for 1.5 hours, and subjected to another heat upsetting and drawing to a diameter of 520 mm, with a forging ratio of approximately 2.5. The above process is repeated once.

[0037] Step 2.2: The forging blank completed in step 2.1 is subjected to three upsetting and drawing forgings at 820°C to a diameter of 520 mm, with the upsetting and drawing forging ratio of each upsetting and drawing being approximately 2.3.

[0038] Step 3: Finished product forging

[0039] The forging blank obtained in step 2 is subjected to one-time drawing-rounding forging at 820° C., first from square to octagonal, and then drawn to φ410 mm black bar; the forging ratio is about 2.0.

[0040] The Ti55531 titanium alloy black skin bar obtained by the above steps is turned into a φ400mm large-size bar after polishing, and the low-magnification structure is uniform and fuzzy crystal ( Figure 1 ); the microstructures at different locations show little difference (Figure 2); the mechanical properties match well, as shown in Table 1.

[0041] Figure 1 The following is a macroscopic microstructure image of a φ400mm bar produced using this forging process. The microstructure is uniform and shows no obvious metallurgical defects. Figure 2 shows the microstructure of the edge and core of the corresponding bar, demonstrating the very uniform microstructure. The material was heated to 800°C for 2 hours, followed by air cooling, and then to 600°C for 8 hours, followed by air cooling. The results are shown in Table 1.

[0042] Table 1 Mechanical properties at different positions

[0043]

[0044] Example 2: Φ550mm bar

[0045] Step 1: Forging

[0046] Step 1.1, heat the Ti55531 ingot with a diameter of 860 mm to above 1150°C (β phase transformation temperature 870°C), keep it warm for 8 hours, and then use a fast forging machine to perform one-fire two-upsetting two-drawing forging to φ600 mm, wherein the upsetting rate is controlled at about 20-50 mm / s and the forging ratio is controlled at about 2.5;

[0047] Step 1.2: After forging, return to the furnace and heat at 1100℃ for 1.5 hours, then perform one round of upsetting forging to φ600mm, wherein the upsetting rate is controlled at about 20-50mm / s and the forging ratio is controlled at about 2.5;

[0048] Step 1.3: After forging, return to the furnace and keep at 1050℃ for 1.5h, then perform one round of upsetting forging to φ600mm, wherein the upsetting rate is controlled at about 20-50mm / s and the forging ratio is controlled at about 2.5;

[0049] Step 1.4: After forging, return to the furnace and keep it at 820℃ for 2.5h, then perform two rounds of upsetting and drawing forging to φ600mm. The forging ratio of each round is controlled at about 2.2; the upsetting rate of each round is controlled at about 10-30mm / s to avoid overheating of the core.

[0050] Step 1.5: After forging, return to the furnace and keep it at 910℃ for 3h, then perform two-fire upsetting forging to φ600mm, and control the forging ratio at about 2.2. After forging, air cool and grind.

[0051] Step 2: Forging the intermediate billet

[0052] Step 2.1: The Ti55531 forging blank completed in step 1 is subjected to one heat upsetting and drawing at 830°C to φ600mm, with a forging ratio of approximately 2.5; then, it is returned to the furnace and held at 850°C for 2 hours, and subjected to one heat upsetting and drawing to φ600mm, with a forging ratio of approximately 2.5. The above process is repeated twice.

[0053] Step 2.2: The forging blank completed in step 2.1 is subjected to four upsetting and drawing forgings at 820°C to a diameter of 600 mm, with the upsetting and drawing forging ratio of each upsetting and drawing being approximately 2.3.

[0054] Step 3: Finished product forging

[0055] The forging blank obtained in step 2 is subjected to one-time drawing-rounding forging at 820° C., first from square to octagonal, and then drawn to Φ560mm black bar; the forging ratio is about 2.0.

[0056] The Ti55531 titanium alloy black bar obtained by the above steps is turned into a φ550mm large-size bar after polishing, and the macrostructure is uniform and fuzzy crystal ( Figure 3 ); The microstructural differences between the edge and the core of the bar are small.

[0057] Comparative Example 1: φ400 rod

[0058] Step 1, blank forging is the same as in Example 1; Step 2, intermediate blank forging does not include step 2.1; Step 2.2 is changed to 4-fire upsetting to φ520mm; Step 3, finished product forging is the same as in Example 1.

[0059] Figure 4 This is a low-magnification microstructure diagram of a φ400mm finished bar produced by forging using this process. It can be seen that the low-magnification microstructure is very uneven, with traces of unbroken large β grains remaining in some areas.

[0060] through Figure 1 and Figure 4 By comparison, it can be seen that the forging method of the present invention effectively improves the uniformity of the titanium alloy structure.

[0061] Those skilled in the art should recognize that the above embodiments are merely intended to illustrate the present invention and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A method for forging large-sized Ti55531 titanium alloy bars, characterized by: The steps include: Step 1: Forging Ti55531 titanium alloy ingots are forged in the following manner: forging above the phase transformation point → forging below the phase transformation point → forging above the phase transformation point; During forging, the ingot is heated to 1000-1150℃ and subjected to 2-3 rounds of upsetting and drawing forging. After forging, it is returned to the furnace to be heated and kept at 30-50℃ below the phase transformation point, and then subjected to 1-2 rounds of upsetting and drawing forging. Then, it is directly returned to the furnace for recrystallization heating at a temperature 30-50℃ above the β transformation temperature, kept at this temperature for an appropriate time, and then taken out of the furnace. Then, it is subjected to 1-2 rounds of upsetting and drawing forging, and cooled after forging. Step 2: Forging the intermediate billet After the blanking forging, the blank is heated to 40-60℃ below the phase transformation point and upsetting at 15-25℃; then upsetting forging is carried out for 3-4 times at 30-60℃ below the phase transformation point, with the forging ratio of each time controlled between 2.0-3.0, and cooling is carried out after forging; Step 3: Forging of Bar Billets The billet forged in step 2 is heated to 40-60°C below the phase transformation point, and subjected to one-time drawing-rolling forging to obtain a rod billet with a forging ratio of 1.8-2.5, and then cooled after forging.

2. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: In step 1, after 1 to 2 rounds of upsetting and forging, the steel is cooled after forging.

3. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: In step 1, the forging ratio of each fire is controlled between 1.8 and 3.

0.

4. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: In step 1, the upsetting process in the upsetting process is uniform, and the rate is controlled at 10 to 50 mm / s.

5. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: In step 2, the forging blank is subjected to one fire upsetting and drawing at 40 to 60° C. below the β-phase transformation temperature; and then subjected to one fire upsetting and drawing at 15 to 25° C. below the β-phase transformation temperature, with the forging ratio of each fire being controlled between 2.0 and 3.

0.

6. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: In step 2, for large-size billets of Φ350-600 mm, the above process of "heating the billet after blanking and forging to 40-60°C below the phase transformation point and upsetting at 15-25°C" is repeated 1-2 times.

7. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: In steps 1 and 2, square billets are used for deformation.

8. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: The holding time coefficient of the blank in steps 1 to 3 is in the range of 0.6 to 0.8 min / mm.

9. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, wherein: The heating time coefficient of the hot blank returned to the furnace is in the range of 0.2 to 0.4 min / mm.

10. The method for forging large-sized Ti55531 titanium alloy bars according to claim 1, characterized in that: The post-forging cooling described in steps 1 to 3 is post-forging air cooling.

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