Method for producing TA19 large-specification titanium alloy bar with uniform structure

Abstract

The invention discloses a method for producing a TA19 large-specification titanium alloy bar with a uniform structure, the production process route is as follows: single-phase region forging, 'low-high 'forging, two-phase region upsetting and drawing forging and two-phase region drawing forging, the diameter of the bar finished product prepared by the method is 200-500mm, the structure uniformity is excellent, the comprehensive mechanical property is excellent, and the production cost is low. The method is suitable for industrial production.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of titanium alloy forging technology, in particular to a free forging method of TA19 titanium alloy bar, which is used to produce TA19 titanium alloy large-size bar with uniform structure and a specification of φ200mm-500mm. BACKGROUND

[0002] TA19 titanium alloy has a nominal composition of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (%) and belongs to near-alpha titanium alloy, and its long-time use temperature is about 500℃, and it is mainly used for middle temperature section parts in an aero-engine, such as a casing and a blade. Due to its high alpha phase content, the longitudinal microstructure of the bar produced by using a conventional forging process is prone to large or long alpha phase, and the appearance of the large or long alpha phase not only reduces the uniformity of the material, but also reduces the fatigue strength and deteriorates the plasticity, so it is extremely meaningful to solve the problem of the uniformity of the TA19 large-size titanium alloy bar. SUMMARY

[0003] In view of the above-mentioned problems in the prior art, the present application aims to provide a method for producing TA19 large-size titanium alloy bar with uniform structure, and the bar produced by the method has a diameter of 200mm-500mm, excellent uniformity and excellent comprehensive mechanical properties, and is suitable for industrial production.

[0004] To achieve the above-mentioned purpose, the production process route of the present application is as follows: single-phase zone forging→“low-high” forging→ two-phase zone upsetting and drawing forging→ two-phase zone elongation forging, and the method is realized by the following steps: Step 1) single-phase zone forging Preheating temperature ≤T β -100℃, the preheating time is 60-180min, then the temperature is increased to 1050℃-1150℃ in the furnace for 90min-210min for heat preservation, the heat preservation time is [(0.20-0.40)×the minimum cross-sectional dimension of the blank]min, the titanium alloy ingot is forged for 2-3 times of two-up two-down, the deformation amount of each upsetting or elongation is controlled to be 30%-50%, the heating and heat preservation temperature of each heat is lowered in turn (generally, the lowering range is 20~80℃), the height-diameter ratio of the forged blank after each heat is lowered in turn (generally, the lowering range is 0.25~1.00), and each heat is water-cooled after forging; Generally, the heating temperature of each heating process of the single-phase zone forging process of the titanium alloy is reduced in turn. The higher the heating temperature, the faster the atomic diffusion rate, which is beneficial to the grain boundary migration, so that the β grains are more easily coarsened and grown. The longer the holding time, the more time the β grains have to grow and merge, and the more easily the β grains are coarsened. If the β grains are too coarse, when cooled to the two-phase zone (α+β phase zone), the α phase preferentially nucleates at the grain boundary where nucleation is most favorable and grows along the specific direction of the β grain. The excessively large original β grain provides a large growth space, and finally forms a coarse α phase matching the size of the β grain. If the two-phase zone forging deformation cannot sufficiently break the coarse α phase, the final product may have coarse α phase left.

[0005] The conventional forging process generally maintains a fixed high-diameter ratio of the blank. Under the condition that the blank weight is the same, the blank cross-sectional size is basically the same, the heat capacity is the same, and the heat penetration time is also relatively fixed. Based on the above understanding, the inventors innovatively increase the high-diameter ratio of the forging blank at the high-temperature stage, especially for the forging blank with a large cross section. By increasing the high-diameter ratio, the cross-sectional area of the forging blank can be indirectly reduced. Reducing the cross-sectional area increases the specific surface area of the blank, thereby improving the radiation heat transfer efficiency. Although increasing the high-diameter ratio may prolong the axial heat conduction time, in the radiation-based electric heating furnace, the radial heat conduction is the main limiting factor of the heat penetration time. Therefore, increasing the high-diameter ratio and reducing the cross-sectional size will shorten the high-temperature holding time, which is beneficial to preventing the original β grains from being excessively coarsened. In combination with subsequent forging deformation and water cooling after forging, fine β grains can be finally obtained.

[0006] On the other hand, the high-diameter ratio of the forging blank should not be maintained at a large state for each heating process. This is because the larger the high-diameter ratio, the more likely the blank is to form "double drum" or even bend when being upset, which is not conducive to the uniformity of the structure. The reason is that the metal fluidity becomes poor and the deformation resistance becomes large as the high-diameter ratio of the forging blank is maintained at a high level and the subsequent heating temperature is reduced, and the risk of double drum is higher. The present application can overcome the above-mentioned shortcomings. This is because the present application realizes the dynamic reduction of the high-diameter ratio of the forging blank as the heating temperature is reduced, which can not only reduce the heating time of the forging blank at the high-temperature stage, but also prevent the occurrence of "double drum" at the low-temperature stage, thereby finally improving the deformation uniformity.

[0007] Step 2) "low-high" forging The heating temperature is 30-60 DEG C below the phase transition point temperature, the holding time is [(0.65-0.90) x the minimum cross-section size of the blank] min, the blank is subjected to 1-time upsetting and drawing forging, the upsetting or drawing deformation amount is controlled to be 20-40% each time, the height-diameter ratio after forging is less than or equal to the height-diameter ratio of the blank after the single-phase zone forging in step 1 (the height-diameter ratio of the forged blank after forging is 2.3-3.3) ; after forging, the hot material is reheated to 20-50 DEG C above the phase transition point temperature, the holding time is [(0.20-0.40) x the minimum cross-section size of the blank] min, then the blank is subjected to 1-time upsetting and drawing forging, the upsetting or drawing deformation amount is controlled to be 20-30% each time, the height-diameter ratio of the forged blank after forging is 1.5-2.3, and the forged blank is water-cooled after forging; the total time of each time forging in this step is less than or equal to 10 min.

[0008] Step 3) Upsetting and drawing forging in two-phase zone The heating temperature is 30-60 DEG C below the phase transition point temperature, the holding time is [(0.65-0.90) x the minimum cross-section size of the blank] min, the blank is subjected to at least 2-time reversing upsetting and drawing forging (each reversing upsetting and drawing operation needs to include four operations of "axial upsetting, lateral drawing, lateral upsetting, axial drawing"), 1-time upsetting and 1-time drawing operation are carried out each time, after the upsetting operation in the reversing upsetting and drawing time, the hot material is reheated for more than 30 min and then taken out for the drawing operation, the upsetting and drawing operation is carried out each time, and the upsetting and drawing deformation amount is controlled to be 20-40% each time; at least 1-time "normal upsetting + diagonal drawing" is inserted between the 2-time reversing upsetting and drawing, which is more beneficial to switching the edge surface, alternatingly changing the stress direction, realizing three-dimensional deformation, fully breaking the alpha phase, and being more beneficial to the uniformity of the structure; the upsetting or drawing operation time is less than or equal to 6 min each time; the height-diameter ratio of the forged blank after each time forging is 1.5-2.3, and the forged blank is air-cooled after forging.

[0009] Step 4) Drawing forging in two-phase zone The heating temperature is 30-60 DEG C below the phase transition point temperature, the holding time is [(0.65-0.90) x the minimum cross-section size of the blank] min, the blank is subjected to 2-5-time drawing forging, and is finally drawn to the finished size rod, the drawing deformation amount is controlled to be less than or equal to 30% each time, the total time of each time forging is less than or equal to 10 min, and the forged blank is air-cooled after forging.

[0010] Compared with the prior art, the application has the beneficial effects as follows: 1. In the case of a certain weight of blank, the high temperature stage of single-phase forging of the present application can reduce the cross-sectional area of the forging blank by maintaining a high height-diameter ratio, increase the specific surface area of the blank, thereby improve the radiation heat transfer efficiency, reduce the heat penetration time, and ultimately reduce the holding time at the high temperature stage, thereby preventing the excessive coarsening of β grains, and ultimately reducing the probability of large or long α phase in the finished rod; at the same time, the height-diameter of the forging blank should be reduced with the decrease of the heating temperature, the purpose is to reduce the probability of uneven deformation such as "double drum" caused by the decrease of heating temperature and the deterioration of metal fluidity during the upsetting process of the blank; 2. Although the conventional simple reversing upsetting and drawing can break the original columnar crystal, it is easy to form banded structure or local uneven deformation. In the present application, the "conventional upsetting + diagonal drawing" upsetting and drawing heat is inserted between the multiple reversing upsetting and drawing operations, 45° shear stress is introduced by diagonal drawing, and multiple reversing upsetting and drawing is used to change the stress direction alternately, which almost covers all the deformation dead angles of the blank, activates multiple slip system deformation, fully breaks the α phase, and improves the uniformity of the microstructure of the forging blank; at the same time, after the upsetting operation is completed in the reversing upsetting and drawing heat, the original warm material is reheated for more than 30 minutes before being taken out for drawing operation, which can overcome the uneven deformation phenomenon such as cracking of the TA19 titanium alloy surface caused by complex deformation mode, long deformation time and reduced surface temperature during forging. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 is the transverse axial forging state microstructure of each position of the Φ410*Lmm black skin rod prepared by the forging of the embodiment of the present application; Figure 2 is the transverse axial forging state microstructure of each position of the Φ410*Lmm black skin rod prepared by the forging of the comparative example of the present application. DETAILED DESCRIPTION

[0012] The present application will be further described in conjunction with specific embodiments. The following unexplained parts are in accordance with the existing technology in the art. Any similar or equivalent scheme without departing from the concept of the present application shall fall within the protection scope of the present application.

[0013] EMBODIMENT The raw material is TA19 titanium alloy Φ680mm ingot produced by the company (the phase transition point T β of the ingot is 1000℃). The specific process flow of the forging method is shown in Table 1 below.

[0014] Table 1:

[0015] Figure 1The transverse axial forging microstructures of the Φ410*Lmm black bar prepared by forging in this embodiment are shown. It can be seen that there are no long strips or large pieces of α phase in the axial microstructure, and the microstructure uniformity is relatively good.

[0016] Table 2 shows the transverse mechanical properties at R / 2 of the Φ410*Lmm black-skinned bar forged in this embodiment (a 25mm thick specimen was cut from the bar and tested after heat treatment at 970°C for 1 hour, air cooling, and then at 590°C for 8 hours, followed by air cooling). It can be seen that the mechanical properties, especially the plastic properties, have sufficient margins and a good match between strength and plasticity.

[0017] Table 2:

[0018] Comparative Example The raw materials are TA19 titanium alloy Φ680mm ingots produced by our company (the ingot phase transition point T β is 1000℃).

[0019] The difference from the embodiment is that the heating in the "single-phase zone forging" stage adopts a one-stage curve, and the height-to-diameter ratio of the forging billet is always maintained unchanged, which directly leads to a significant extension of the high-temperature holding time. All fires are air-cooled after forging. The "two-phase zone upsetting forging" stage does not adopt reversing upsetting and diagonal drawing, and no warming operation is performed between the upsetting and drawing operations within the fire. Other process parameters are basically consistent with the embodiment. The specific process flow of the forging method is shown in Table 3 below.

[0020] Table 3:

[0021] Figure 2 The transverse axial forging microstructures of the Φ410*Lmm black bar prepared by forging in this comparative example are shown. It can be clearly seen that there are long strips and large pieces of α phase in the axial microstructure, and the organizational uniformity is poor.

[0022] Table 4 shows the transverse mechanical properties at R / 2 of the Φ410*Lmm black-skinned bar prepared by forging in this comparative example (a 25mm thick specimen was cut from the bar and tested after heat treatment at 970°C for 1 hour, air cooling, and then at 590°C for 8 hours, followed by air cooling). It can be seen that the area reduction rate fluctuates greatly, and the strength-ductility matching deviates significantly from that of the embodiment.

[0023] Table 4: .

Claims

1. A method for producing uniformly organized TA19 large-size titanium alloy bars, characterized in that: This is achieved by the following steps: Step 1) Single-phase zone forging Preheating temperature ≤ T β -100℃, preheating time is 60-180min, then heating with the furnace for 90min-210min to 1050℃-1150℃ for holding, the holding time is [(0.20-0.40)×minimum cross-sectional size of the billet]min, and the titanium alloy ingot is subjected to 2-3 times of two-upsetting and two-drawing forging, and the deformation of each upsetting or drawing is controlled to be 30%-50%, the heating and holding temperature of each fire is gradually reduced, and the height-to-diameter ratio of the forging billet is gradually reduced within the range of 2.3-3.3 after each fire forging, and water cooling is performed after each fire forging; Step 2) "Low-High" Forging The heating temperature is 30°C-60°C below the phase transformation point temperature, the holding time is [(0.65-0.90)×the minimum cross-sectional size of the billet] min, the billet is subjected to one round of upsetting and drawing forging, the deformation of each upsetting or drawing is controlled to be 20%-40%, and the aspect ratio after forging is less than or equal to the aspect ratio of the billet after the single-phase region forging in step 1); after forging, the hot material is returned to the furnace and heated to 20°C-50°C above the phase transformation point temperature, the holding time is [(0.20-0.40)×the minimum cross-sectional size of the billet] min, and then the billet is subjected to one round of upsetting and drawing forging, the deformation of each upsetting or drawing is controlled to be 20%-30%, and the aspect ratio of the forged billet after forging is 1.5-2.3, and water cooling is performed after forging; the total forging time per round in this step is ≤10 min; Step 3) Two-phase zone upsetting forging The heating temperature is 30-60°C below the phase transition point, and the holding time is [(0.65-0.90) × the minimum cross-sectional size of the billet] min. The billet is subjected to at least two reversing upsetting and drawing forgings, with one upsetting and one drawing operation performed in each fire. After the upsetting operation is completed in the reversing upsetting and drawing forging fire, the original hot material is returned to the furnace for reheating for more than 30 minutes before being taken out of the furnace for the drawing operation. After each upsetting and drawing operation is completed, air cooling is performed. The deformation of each upsetting and drawing is controlled at 20%-40%. At least one "conventional upsetting + diagonal drawing" process must be interspersed between the two reversing upsetting and drawing. The time for each upsetting or drawing operation is ≤6 minutes. The height-to-diameter ratio of the forging billet after each forging fire is 1.5-2.3, and air cooling is performed after forging. Step 4) Two-phase zone drawing forging The heating temperature is 30-60°C below the phase transition point temperature, the holding time is [(0.65-0.90) × the minimum cross-sectional size of the billet] min, the billet is subjected to 2-5 rounds of drawing and forging, and finally drawn to the finished product specification size bar. The drawing deformation of each round of forging is controlled to ≤30%, the total forging time of each round is ≤10min, and air cooling is performed after forging.

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