A method for producing a TA19 large-size titanium alloy bar with uniform structure

Abstract

The application discloses a method for producing TA19 large-size titanium alloy rod with uniform structure, and the production process route is as follows: single-phase zone forging, "low-high" forging, two-phase zone upsetting and drawing forging, and the diameter of the rod product prepared by the method is 200mm-500mm, the structure uniformity is excellent, the comprehensive mechanical properties are excellent, and the method is suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy forging technology, specifically to a free forging method for TA19 titanium alloy bars, used to produce large-diameter TA19 titanium alloy bars with uniform microstructure and a diameter of φ200mm-500mm. Background Technology

[0002] TA19 titanium alloy has a nominal composition of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (%), belonging to near-α titanium alloys. Its long-term operating temperature is around 500℃, and it is mainly used in mid-temperature components of aero-engines, such as casings and blades. Due to its high α phase content, bars produced using conventional forging processes tend to exhibit large or elongated α phases in their longitudinal microstructure. The presence of these large or elongated α phases not only reduces the material's microstructure uniformity but also lowers fatigue strength and deteriorates plasticity. Therefore, solving the problem of achieving microstructure uniformity in large-size TA19 titanium alloy bars is extremely significant. Summary of the Invention

[0003] To address the shortcomings of the existing technology, the purpose of this invention is to provide a method for producing large-diameter TA19 titanium alloy bars with uniform microstructure. The bars produced by this method have a diameter of 200mm-500mm, excellent microstructure uniformity, and excellent comprehensive mechanical properties, making them suitable for industrial production.

[0004] To achieve the above objectives, the production process of this invention is as follows: single-phase forging → "low-high" forging → two-phase upsetting forging → two-phase elongation forging, specifically achieved through the following steps: Step 1) Single-phase forging Preheating temperature ≤ T β The temperature is set at -100℃, and the preheating time is 60-180 min. Then, the temperature is raised in the furnace for 90-210 min to 1050℃-1150℃ and held for a holding time of [(0.20-0.40) × minimum cross-sectional dimension of the billet] min. The titanium alloy ingot is then subjected to two upsetting and two drawing forging in 2-3 heats. The deformation amount of each upsetting or drawing is controlled to be 30%-50%. The heating and holding temperature of each heat is gradually reduced (generally, the reduction range is between 20 and 80℃). The height-to-diameter ratio of the forging billet after each heat is gradually reduced within the range of 2.3-3.3 (generally, the reduction range is between 0.25 and 1.00). The billet is water-cooled after each heat. In typical single-phase forging processes for titanium alloys, the heating temperature decreases sequentially with each pass. Higher heating temperatures result in faster atomic diffusion, which favors grain boundary migration, making it easier for β grains to coarsen and grow. Simultaneously, longer holding times allow sufficient time for β grains to grow and coalesce, further coarsening them. If the β grains are too large, upon cooling to the two-phase region (α+β phase region), the α phase preferentially nucleates at the most favorable grain boundaries and grows along a specific direction of the β grains. The excessively large initial β grains provide ample growth space, ultimately forming coarse α phases matching the β grain size. If the two-phase forging deformation cannot sufficiently break up the coarse α phase, it may leave behind large α phases on the final bar stock.

[0005] Conventional forging processes generally maintain a relatively fixed height-to-diameter ratio of the billet. Under the condition of the same billet weight, the cross-sectional size of the upsetting and drawing billets is basically the same, the heat capacity is also the same, and the heat penetration time is also relatively fixed. Based on the above understanding, the inventors have innovatively increased the height-to-diameter ratio of the forging billet in the high-temperature stage, especially for forging billets with larger cross-sections. By increasing the height-to-diameter ratio, the cross-sectional area of ​​the forging billet can be indirectly reduced. Reducing the cross-sectional area increases the specific surface area of ​​the billet, thereby improving the radiative heat transfer efficiency. Although increasing the height-to-diameter ratio may prolong the axial heat conduction time, in an electric heating furnace where radiation is the main factor, radial heat conduction is the main limiting factor for heat penetration time. Therefore, increasing the height-to-diameter ratio and reducing the cross-sectional size will still shorten the high-temperature holding time, which is beneficial to prevent the original β grains from becoming excessively coarsened. At the same time, in conjunction with subsequent forging deformation and post-forging water cooling, fine β grains can ultimately be obtained.

[0006] On the other hand, the height-to-diameter ratio of the forging billet should not be maintained at a high level in each heating cycle. This is because a larger height-to-diameter ratio makes it easier for "double bulges" or even bending to form during billet upsetting, which is detrimental to the uniformity of the microstructure. This is because if the forging billet maintains a consistently high height-to-diameter ratio, the metal fluidity will decrease as the heating temperature decreases in subsequent heating cycles, the deformation resistance will increase, and the risk of double bulges will be higher. However, this invention can overcome the above-mentioned shortcomings because it achieves a dynamic reduction in the height-to-diameter ratio of the forging billet as the heating temperature decreases. This reduces the heating time of the forging billet in the high-temperature stage and prevents "double bulges" from forming in the low-temperature stage, thereby ultimately improving the uniformity of deformation.

[0007] Step 2) Low-High Forging The heating temperature is 30℃-60℃ below the phase transformation point temperature, and the holding time is [(0.65-0.90) × minimum cross-sectional size of the billet] min. The billet is subjected to one upsetting and drawing forging, and the upsetting or drawing deformation is controlled at 20%-40% each time. The height-to-diameter ratio after forging is less than or equal to the height-to-diameter ratio of the billet after the single-phase zone forging in step 1) (the height-to-diameter ratio of the forged billet after forging is 2.3-3.3). After forging, the hot material is reheated in the furnace to 20℃-50℃ above the phase transformation point temperature, and the holding time is [(0.20-0.40) × minimum cross-sectional size of the billet] min. Then, the billet is subjected to one upsetting and drawing forging, and the upsetting or drawing deformation is controlled at 20%-30% each time. The height-to-diameter ratio of the forged billet after forging is 1.5-2.3. After forging, it is water-cooled. The total forging time for each forging in this step is ≤10 min.

[0008] Step 3) Two-phase region upsetting and drawing forging The heating temperature is 30℃-60℃ below the phase transformation point temperature, and the holding time is [(0.65-0.90) × minimum cross-sectional dimension of the billet] min. The billet undergoes at least two reversing upsetting and drawing forging operations (each reversing upsetting and drawing operation must include four operations: "axial upsetting, lateral drawing, lateral upsetting, and axial drawing"). Each heating cycle includes one upsetting operation and one drawing operation. After the upsetting operation is completed within the reversing upsetting and drawing cycle, the original hot material is returned to the furnace for reheating for at least 30 minutes before being removed from the furnace. The drawing operation is performed, and after each upsetting and drawing operation, it is air-cooled. The deformation amount of each upsetting and drawing operation is controlled at 20%-40%. At least one "conventional upsetting + diagonal drawing" operation should be interspersed between two reversing upsetting and drawing operations. This is more conducive to switching facets, alternating the direction of force, achieving three-dimensional deformation, fully breaking down the α phase, and improving the uniformity of the microstructure. The time for each upsetting or drawing operation is ≤6min. The height-to-diameter ratio of the forged billet after each forging operation is 1.5-2.3, and it is air-cooled after forging.

[0009] Step 4) Two-phase zone elongation forging The heating temperature is 30℃-60℃ below the phase transformation point temperature, and the holding time is [(0.65-0.90)×min minimum cross-sectional size of the billet]. The billet is forged in 2-5 heats until it is finally drawn into a bar of the finished product size. The deformation amount of each heat forging is controlled to be ≤30%, and the total forging time of each heat forging is ≤10min. After forging, the billet is air-cooled.

[0010] Compared with the prior art, the present invention has the following beneficial effects: 1. With a fixed billet weight, the high-temperature stage of single-phase forging in this invention can reduce the cross-sectional area of ​​the forging billet and increase the specific surface area of ​​the billet by maintaining a high height-to-diameter ratio, thereby improving the radiative heat transfer efficiency, reducing the heat penetration time, and ultimately reducing the holding time in the high-temperature stage. This prevents excessive coarsening of β grains and ultimately reduces the probability of large or long α phases in the finished bar. At the same time, the height and diameter of the forging billet should decrease as the heating temperature decreases. The purpose of this is to reduce the probability of uneven deformation phenomena such as "double bulging" caused by the decrease in heating temperature and the deterioration of metal fluidity during the billet upsetting process. 2. While conventional simple reversing upsetting can break up the original columnar crystals, it is prone to forming banded structures or uneven local deformation. In this invention, "conventional upsetting + diagonal upsetting" is interspersed between multiple reversing upsetting operations. The diagonal upsetting introduces 45° shear stress, and with the multiple reversing upsetting operations, the direction of force is alternately changed, almost covering all the deformation dead angles of the billet, activating the multi-slip system deformation, fully breaking the α phase, and improving the uniformity of the forging structure. At the same time, after the upsetting operation is completed within the reversing upsetting operation, 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 elongation operation. This can overcome the uneven deformation phenomenon such as cracking on the surface of the difficult-to-deform TA19 titanium alloy due to the complex deformation mode, long deformation time, and reduced surface temperature during the forging process. Attached Figure Description

[0011] Figure 1 The transverse axial forged microstructure of a Φ410*Lmm black bar prepared by forging according to the embodiments of the present invention; Figure 2 The microstructure of the transverse axial forging state of a Φ410*Lmm black bar prepared by comparative forging according to the present invention is shown in the figure. Detailed Implementation

[0012] The present invention will now be further described with reference to specific embodiments. Parts not detailed below are based on existing technology in the field. Any similar or equivalent solutions that do not depart from the concept of the present invention should fall within the protection scope of the present invention.

[0013] Example The raw material is a 680mm TA19 titanium alloy ingot produced by our company (the ingot's phase transformation point T was tested). β (The temperature is 1000℃). The specific process flow of its forging method is shown in Table 1 below.

[0014] Table 1:

[0015] Figure 1The forged microstructures of the Φ410*Lmm black bar produced by forging in this embodiment are shown in the transverse axial forging state at various locations. It can be seen that there are no long strips or large blocks of α phase in the axial microstructure, and the microstructure has excellent uniformity.

[0016] Table 2 shows the transverse mechanical properties at the R / 2 point of the Φ410*Lmm black bar prepared by forging in this embodiment (a 25mm thick sample block was cut from the bar and tested after heat treatment of "holding at 970℃ for 1h, air cooling, holding at 590℃ for 8h, and air cooling"). It can be seen that the mechanical properties, especially the plasticity, have sufficient margin and the strength and plasticity are well matched.

[0017] Table 2:

[0018] Comparative Example The raw material is a 680mm TA19 titanium alloy ingot produced by our company (the ingot's phase transformation point T was tested). β (1000℃).

[0019] The difference from the embodiment is that the heating in the "single-phase zone forging" stage is carried out using a one-stage curve, and the height-to-diameter ratio of the forging billet remains constant, which directly leads to a significant extension of the high-temperature holding time. All forgings are air-cooled after each heat treatment. The "two-phase zone upsetting and drawing forging" stage does not use reverse upsetting or diagonal drawing. No reheating operation is performed in the upsetting and drawing operation between heat treatments. Other process parameters are basically the same as those in the embodiment. The specific process flow of its forging method is shown in Table 3 below.

[0020] Table 3:

[0021] Figure 2 The forged microstructure of Φ410*Lmm black bar stock prepared by this comparative forging process is shown in the transverse axial forging state at various locations. It can be clearly seen that there are long strips and large blocks of α phase in the axial microstructure, and the microstructure has poor uniformity.

[0022] Table 4 shows the transverse mechanical properties at the R / 2 point of the Φ410*Lmm black bar prepared by forging in this comparative example (a 25mm thick sample block was cut from the bar and tested after heat treatment of "holding at 970℃ for 1h, air cooling, holding at 590℃ for 8h, and air cooling"). It can be seen that the shrinkage rate fluctuates greatly, and the strength-plasticity matching is significantly different from that of the example.

[0023] Table 4: .

Claims

1. A method for producing large-diameter TA19 titanium alloy bars with uniform microstructure, characterized in that, This can be achieved through the following steps: Step 1) Single-phase forging Preheating temperature ≤ T β The temperature is set at -100℃ and the preheating time is 60-180 min. Then, the temperature is raised in the furnace for 90-210 min to 1050℃-1150℃ and held for a holding time of [(0.20-0.40) × minimum cross-sectional size of the billet] min. The titanium alloy ingot is subjected to two upsetting and two drawing forging in 2-3 fires. The deformation amount of each upsetting or drawing is controlled to be 30%-50%. The heating and holding temperature of each fire is reduced sequentially. The height-to-diameter ratio of the forging billet after each fire is reduced sequentially within the range of 2.3-3.

3. Water cooling is performed after each fire forging. Step 2) Low-High Forging The heating temperature is 30℃-60℃ below the phase transformation point temperature, and the holding time is [(0.65-0.90) × minimum cross-sectional dimension of the billet] min. The billet is subjected to one upsetting and drawing forging, and the upsetting or drawing deformation is controlled at 20%-40% each time. The height-to-diameter ratio after forging is less than or equal to the height-to-diameter ratio of the billet after the single-phase zone forging in step 1). After forging, the hot material is reheated in the furnace to 20℃-50℃ above the phase transformation point temperature, and the holding time is [(0.20-0.40) × minimum cross-sectional dimension of the billet] min. Then, the billet is subjected to one upsetting and drawing forging, and the upsetting or drawing deformation is controlled at 20%-30% each time. The height-to-diameter ratio of the forged billet after forging is 1.5-2.

3. After forging, it is water-cooled. The total forging time for each forging in this step is ≤10 min. Step 3) Two-phase region upsetting and forging The heating temperature is 30℃-60℃ below the phase transformation point temperature, and the holding time is [(0.65-0.90)×min minimum cross-sectional dimension of the billet]. The billet is subjected to at least two reverse upsetting and drawing forging operations. Each forging operation consists of one upsetting and one drawing operation. After the upsetting operation is completed within the reverse upsetting and drawing forging operation, the hot billet 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, the billet is air-cooled. The deformation amount of each upsetting and drawing operation is controlled at 20%-40%. At least one "conventional upsetting + diagonal drawing" process should be interspersed between the two reverse upsetting and drawing operations. The time for each upsetting or drawing operation is ≤6min. The height-to-diameter ratio of the forged billet after each forging operation is 1.5-2.

3. The billet is air-cooled after forging. Step 4) Two-phase zone elongation forging The heating temperature is 30℃-60℃ below the phase transformation point temperature, and the holding time is [(0.65-0.90)×min minimum cross-sectional size of the billet]. The billet is forged in 2-5 heats until it is finally drawn into a bar of the finished product size. The deformation amount of each heat forging is controlled to be ≤30%, and the total forging time of each heat forging is ≤10min. After forging, the billet is air-cooled.

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