Methods for controlling the uniformity of microstructure in Ti55531 alloy forgings

Abstract

This invention relates to a method for controlling the microstructure uniformity of Ti55531 alloy forgings, belonging to the field of titanium alloy processing technology. The method includes the following steps: a forging step: forging the Ti55531 alloy billet using a three-stage forging method; a solution heat treatment step: performing solution treatment on the forged part, the solution treatment time being determined by a quantitative control model; the quantitative control model is based on the Mo equivalent C of the alloy. Mo The basic solution time t0 is corrected by adjusting the dislocation density ρ, and the criteria of recrystallization fraction f ≥ 95% and average grain size d ≤ 200 μm after solution treatment are met. The final solution time t is controlled within the range of 1.0 h to 3.0 h. The method of this invention is systematic, accurate, effective, and economical.

Description

Technical Field

[0001] This invention relates to a method for controlling the uniformity of microstructure in Ti55531 alloy forgings, belonging to the field of titanium alloy processing technology. Background Technology

[0002] Ti55531 (Ti-5Al-5V-5Mo-3Cr-1Zr) is a high-strength, high-toughness near-β-type titanium alloy with broad application prospects in the aerospace field (such as aircraft landing gear and structural components) due to its excellent comprehensive properties. However, controlling the microstructure uniformity during the production of forgings of this alloy is a major technical challenge. During forging, due to uneven strain distribution, even with a small overall deformation, certain areas (especially those with...) exhibit uneven microstructure. <100> Oriented β grains, due to their good deformability, cannot effectively store strain energy. While blindly increasing the deformation amount can improve uniformity, it will drastically increase mold wear and production costs. Therefore, controlling the solution time is crucial in the subsequent β solution treatment followed by aging. Insufficient solution time... <100> Oriented grains are difficult to recrystallize fully, forming a large number of tiny subgrains; while excessively long solution treatment times, although capable of recrystallization, lead to coarse grains. Both of these conditions deteriorate the final microstructure and mechanical properties of the forgings, resulting in poor batch-to-batch stability and an inability to meet the stringent requirements of high-end applications.

[0003] Patent CN120421433A discloses a forging method for large-size Ti55531 titanium alloy bars. Although it proposes refining grains by alternating forging above and below the phase transformation point, its forging process parameters are too broad and fail to precisely address the issues of strain uniformity and microstructure matching with subsequent heat treatment. A systematic and quantitative solution is lacking for the unique challenge of controlling microstructure uniformity in Ti55531 alloy. Summary of the Invention

[0004] The purpose of this invention is to provide a method for controlling the uniformity of the microstructure of Ti55531 alloy forgings.

[0005] To achieve the objective of this invention, the method includes the following steps:

[0006] Forging steps: The Ti55531 alloy billet is forged using a three-stage forging method, which includes ① a pre-forging stage: temperature 880~900℃, strain rate 0.01~0.03s. - ¹; ② Intermediate forging stage: temperature 840~860℃, strain rate 0.03~0.05s - ¹; ③ Precision forging stage: temperature 800~820℃, strain rate 0.05~0.08s - ¹;

[0007] Solution heat treatment step: The forged part is subjected to solution treatment, and the solution treatment time is determined by a quantitative control model; the quantitative control model is based on the Mo equivalent C of the alloy. Mo The basic solution time t0 was corrected by the dislocation density ρ, and the criteria of recrystallization fraction f ≥ 95% and average grain size d ≤ 200 μm after solution were met. Finally, the solution time t was controlled within the range of 1.0 h to 3.0 h.

[0008] In one specific embodiment, the deformation amount in the pre-forging stage is 20% to 25%; the deformation amount in the intermediate forging stage is 15% to 20%; the cumulative deformation amount in the pre-forging stage and the intermediate forging stage is 35% to 45%; and the deformation amount in the precision forging stage is 5% to 10%.

[0009] In one specific embodiment, a holding time of 10 to 15 minutes is set between two adjacent forging stages of the three-stage forging method, and the temperature difference is controlled within ±5℃.

[0010] Preferably, the pre-forging stage is held at 880-900℃ for 50-70 minutes before forging, the intermediate forging stage is held at 840-860℃ for 20-40 minutes before forging, and the precision forging stage is held at 800-820℃ for 10-30 minutes before forging.

[0011] In the three-stage forging method, a holding time of 10 to 15 minutes is set between two adjacent forging stages, and the temperature difference is controlled within ±5℃ to ensure uniform temperature of the forging.

[0012] In one specific embodiment, the quantitative control model determines the solution treatment time in the following manner:

[0013] The basic solution treatment time t0 = 1.4h, corresponding to standard component C Mo =9.3, dislocation density ρ=7×10 12 m -2 The baseline conditions;

[0014] The solution time is corrected based on the alloy composition and dislocation density. The corrected solution time is t = t0 × k1 × k2.

[0015] Where k1 is the component correction coefficient, k1 = 1 + 0.7 × (C Mo -9.3);

[0016] k2 is the dislocation density correction factor, k2=e^(-0.04×ρ), where ρ is in units of 10. 12 m -2The final solution treatment time t is controlled within the range of 1h to 3h. When the calculated value t < 1.0h, take 1.0h; when the calculated value t > 3.0h, take 3.0h.

[0017] In one specific embodiment, the recrystallization fraction f and the solution time t satisfy the kinetic equation: f = 1 - e^(-3.1t^0.7); the average grain size d and the solution time t satisfy the growth equation: d = 34 + 66t^0.75; where the unit of d is μm and the unit of t is h.

[0018] In one specific embodiment, the Mo equivalent C of the Ti55531 alloy is... Mo The calculation formula is:

[0019] C Mo =w(Mo)+0.67×w(V)+0.44×w(Cr)-0.11×w(Al)+0.18×w(Zr); when Mo equivalent C Mo For every 0.1% increase, the solution time is prolonged by 5%–8%; for every 1×10⁻⁶ increase in dislocation density ρ, the solution time is prolonged. 12 m −2 Solution time is shortened by 3% to 5%;

[0020] w(Mo), w(V), w(Cr), w(Al), and w(Zr) represent the mass percentages of Mo, V, Cr, Al, and Zr, respectively.

[0021] In one specific embodiment, the heat preservation temperature of the solution treatment is 10 to 30°C above the β phase transformation point of the Ti55531 alloy.

[0022] In one specific embodiment, the heating rate of the solution treatment is controlled at 5 to 10 °C / min.

[0023] In one specific implementation, the dislocation density ρ is measured using X-ray diffraction, including calculating the micro-strain caused by dislocations using the Williamson-Hall method and obtaining the dislocation density ρ by combining the Scherrer formula.

[0024] In one specific embodiment, after three-stage forging, the dislocation density ρ of the Ti55531 alloy ranges from 4 × 10⁻⁶. 12 ~11×10 12 m -2 .

[0025] Beneficial effects:

[0026] Compared with the prior art, the present invention has the following significant advantages:

[0027] Systematic: For the first time, forging process and solution heat treatment are designed as a whole system, with forging preparing for heat treatment and heat treatment realizing the effective conversion of the potential introduced by forging.

[0028] Precision: The strain field distribution was precisely controlled through three-stage forging; the established quantitative model enabled dynamic and accurate calculation of solution treatment time, overcoming the limitations of traditional one-cut process.

[0029] Effectiveness: This invention can effectively solve the problem of poor microstructure uniformity in Ti55531 alloy forgings, suppress macro-region formation, and obtain a uniform microstructure with sufficient recrystallization and fine grains, thereby significantly improving the consistency of mechanical properties and batch stability of forgings.

[0030] Economic efficiency: While ensuring performance, the optimized process avoids excessive deformation and ineffective heat treatment time, which helps to reduce production costs. Attached Figure Description

[0031] Figure 1 The image shows the metallographic diagram of the forging after processing in Example 1.

[0032] Figure 2 The graph shows the β solid solution recrystallization kinetics curve (855℃) of the Ti55531 alloy in Example 2.

[0033] Note: Red dots represent experimental data points, black dashed lines represent model prediction curves (f=1-e^(-3.1t^0.7)), red dashed lines represent the recrystallization qualification standard (f=95%), and the qualified solution time range is 1.0~3.0h.

[0034] Figure 3 The figure shows the β grain growth curve (855℃) of the Ti55531 alloy in Example 2.

[0035] Note: The red squares represent experimental data points, and the black dashed line represents the model prediction curve (d=34+66t^0.75).

[0036] Figure 4 The image shows the metallographic structure of Comparative Example 1 after solution treatment for 0.8 hours.

[0037] Figure 5 The image shows the metallographic structure of Comparative Example 3 after solution treatment for 3.5 hours. Detailed Implementation

[0038] To achieve the objective of this invention, the method includes the following steps:

[0039] Forging steps: The Ti55531 alloy billet is forged using a three-stage forging method, which includes ① a pre-forging stage: temperature 880~900℃, strain rate 0.01~0.03s. - ¹; ② Intermediate forging stage: temperature 840~860℃, strain rate 0.03~0.05s - ¹; ③ Precision forging stage: temperature 800~820℃, strain rate 0.05~0.08s - ¹;

[0040] Solution heat treatment step: The forged part is subjected to solution treatment, and the solution treatment time is determined by a quantitative control model; the quantitative control model is based on the Mo equivalent C of the alloy. Mo The basic solution time t0 was corrected by the dislocation density ρ, and the criteria of recrystallization fraction f ≥ 95% and average grain size d ≤ 200 μm after solution were met. Finally, the solution time t was controlled within the range of 1.0 h to 3.0 h.

[0041] In one specific embodiment, the deformation amount in the pre-forging stage is 20% to 25%; the deformation amount in the intermediate forging stage is 15% to 20%; the cumulative deformation amount in the pre-forging stage and the intermediate forging stage is 35% to 45%; and the deformation amount in the precision forging stage is 5% to 10%.

[0042] In one specific embodiment, a holding time of 10 to 15 minutes is set between two adjacent forging stages of the three-stage forging method, and the temperature difference is controlled within ±5℃.

[0043] Preferably, the pre-forging stage is held at 880-900℃ for 50-70 minutes before forging, the intermediate forging stage is held at 840-860℃ for 20-40 minutes before forging, and the precision forging stage is held at 800-820℃ for 10-30 minutes before forging.

[0044] In the three-stage forging method, a holding time of 10 to 15 minutes is set between two adjacent forging stages, and the temperature difference is controlled within ±5℃ to ensure uniform temperature of the forging.

[0045] In one specific embodiment, the quantitative control model determines the solution treatment time in the following manner:

[0046] The basic solution treatment time t0 = 1.4h, corresponding to standard component C Mo =9.3, dislocation density ρ=7×10 12 m -2 The baseline conditions;

[0047] The solution time is corrected based on the alloy composition and dislocation density. The corrected solution time is t = t0 × k1 × k2.

[0048] Where k1 is the component correction coefficient, k1 = 1 + 0.7 × (C Mo -9.3);

[0049] k2 is the dislocation density correction factor, k2=e^(-0.04×ρ), where ρ is in units of 10. 12 m -2 The final solution treatment time t is controlled within the range of 1h to 3h. When the calculated value t < 1.0h, take 1.0h; when the calculated value t > 3.0h, take 3.0h.

[0050] In one specific embodiment, the recrystallization fraction f and the solution time t satisfy the kinetic equation: f = 1 - e^(-3.1t^0.7); the average grain size d and the solution time t satisfy the growth equation: d = 34 + 66t^0.75; where the unit of d is μm and the unit of t is h.

[0051] In one specific embodiment, the Mo equivalent C of the Ti55531 alloy is... Mo The calculation formula is:

[0052] C Mo =w(Mo)+0.67×w(V)+0.44×w(Cr)-0.11×w(Al)+0.18×w(Zr); when Mo equivalent C Mo For every 0.1% increase, the solution time is prolonged by 5%–8%; for every 1×10⁻⁶ increase in dislocation density ρ, the solution time is prolonged. 12 m −2 Solution time is shortened by 3% to 5%;

[0053] w(Mo), w(V), w(Cr), w(Al), and w(Zr) represent the mass percentages of Mo, V, Cr, Al, and Zr, respectively.

[0054] In one specific embodiment, the heat preservation temperature of the solution treatment is 10 to 30°C above the β phase transformation point of the Ti55531 alloy.

[0055] In one specific embodiment, the heating rate of the solution treatment is controlled at 5 to 10 °C / min.

[0056] In one specific implementation, the dislocation density ρ is measured using X-ray diffraction, including calculating the micro-strain caused by dislocations using the Williamson-Hall method and obtaining the dislocation density ρ by combining the Scherrer formula.

[0057] In one specific embodiment, after three-stage forging, the dislocation density ρ of the Ti55531 alloy ranges from 4 × 10⁻⁶. 12 ~11×10 12 m -2 .

[0058] The specific embodiments of the present invention will be further described below with reference to examples, but the present invention is not limited to the scope of the embodiments described herein.

[0059] Example 1

[0060] The measured composition (mass fraction) of the Ti55531 alloy used in this embodiment is: Al 5.12%, Mo 4.98%, V 5.05%, Cr 3.02%, Zr 1.03%, Fe 0.05%, C 0.01%, N 0.008%, O 0.12%, with the balance being Ti. The measured β phase transformation point Tβ = 845℃, and the calculated Mo equivalent C... Mo =4.98 + 0.67 × 5.05 + 0.44 × 3.02 - 0.11 × 5.12 + 0.18 × 1.03 = 9.62. The dislocation density of the billet before forging is ρ0 = 0.8 × 10¹² m⁻², and the dislocation density after three-stage forging is ρ = 6.2 × 10¹² m⁻² (within a reasonable range of 4 × 10¹² to 11 × 10¹² m⁻²).

[0061] forging:

[0062] Pre-forging: The billet is heated to 890℃, held for 60 min, and then forged at a strain rate of 0.02 s⁻¹, with a deformation of 22%;

[0063] Temperature uniform holding: 12 min, with the temperature difference of the forging controlled within ±3℃;

[0064] Intermediate forging: After cooling to 850℃ and holding for 30 min, forging is performed at a strain rate of 0.04 s⁻¹, with a deformation of 18% and a cumulative deformation of 40%.

[0065] Temperature uniform holding: 12 min, with the temperature difference of the forging controlled within ±3℃;

[0066] Precision forging: Cool down to 810℃, hold for 20 min, and then forge at a strain rate of 0.06 s⁻¹, with a deformation of 8%.

[0067] Solution treatment:

[0068] Solution time was calculated using a quantitative model.

[0069] t = 1.4 × [1 + 0.7 × (9.62 - 9.3)] × e^(0.04 × 6.2) = 1.4 × 1.224 × e^(0.248) = 1.4 × 1.224 × 0.780 ≈ 1.34 h. Take the actual solution treatment time as 1.3 h (within the range of 1.0 to 3.0 h). Heat the forging to 855 °C (T0) at a rate of 8 °C / min. β +10℃), keep warm for 1.3 hours and then air cool.

[0070] Performance test results:

[0071] Metallographic analysis of the treated forgings revealed a recrystallization fraction of 97.2%, an average grain size of 115 μm, and the absence of mixed grains and non-recrystallized regions. The microstructure exhibited significantly better uniformity than that produced by traditional processes, as shown in Figure 1. Mechanical property tests are presented in Table 1, demonstrating excellent strength and plasticity matching, with minimal performance fluctuations between different batches and even within the same batch at different locations. The average room temperature tensile strength R0... m =791MPa, yield strength R p0 .2=731MPa, elongation after fracture A=21%, reduction of area Z=58%; the tensile strength fluctuation of different locations (core, 1 / 2 radius, 1 / 4 radius, surface) in the same batch is ±16MPa, and the yield strength fluctuation is ±13MPa.

[0072] Table 1 Mechanical properties at different locations

[0073]

[0074] Example 2

[0075] This embodiment uses the same forging process as Example 1, only changing the solution treatment time to verify the accuracy of the quantitative control model. The solution treatment temperature is fixed at 855℃, and the solution treatment times are set to 0.8h, 1.0h, 1.5h, 2.0h, 2.5h, 3.0h, and 3.5h, respectively. The aging process is the same as in Example 1.

[0076] The recrystallization fraction f / % (model prediction) is calculated based on: f = 1 - e^(-3.1t^0.7);

[0077] The average grain size d / μm (model prediction) was calculated based on d = 34 + 66t^0.75; experimental results are detailed in Table 2.

[0078] Table 2

[0079]

[0080] Model validation curve:

[0081] Based on the above experimental data, recrystallization kinetics curves and grain growth curves were plotted, as shown in Figure 2 and Figure 3, respectively.

[0082] Experimental results show that:

[0083] 1. The goodness of fit R² between the experimental values ​​and the model predictions for recrystallization fraction and grain size are 0.994 and 0.991, respectively, proving that the quantitative model of this invention has extremely high accuracy;

[0084] 2. Only when the solution treatment time is within the range of 1.0h to 3.0h can the tissue qualification criteria of f≥95% and d≤200μm be met simultaneously, which is completely consistent with the time window predicted by the model.

[0085] Example 3

[0086] Process stability verification

[0087] In this embodiment, five Ti55531 alloy billets from different melting furnaces were selected. Their composition and process parameters are shown in Table 3 below:

[0088] Table 3

[0089]

[0090] All heats employ the three-stage forging and aging process of this invention, with the solution treatment temperature being 10°C above the measured β phase transformation point for each heat.

[0091] Performance test results:

[0092] The recrystallization fraction of the forgings from all five heats ranged from 96.3% to 98.4%, and the average grain size ranged from 108 μm to 140 μm. The room temperature tensile strength was 788–801 MPa, the yield strength was 725–736 MPa, and the elongation after fracture was 19.5%–21%. The tensile strength fluctuation between different heats was only ±13 MPa, far lower than the ±50 MPa or more of the industry's conventional processes, proving that the process of this invention has excellent batch stability.

[0093] Comparative Example 1

[0094] The same forging process as in Example 1 was used, with the solution treatment time set to 0.8h (below the lower limit of the model) and 3.5h (above the upper limit of the model), respectively.

[0095] Performance test results:

[0096] Solution treatment for 0.8 h: recrystallization fraction f = 92.0%, with a large amount of residual subgrains, average grain size d = 84 μm, typical microstructure as shown in Figure 4; tensile strength Rm =701MPa, elongation A=15%, indicating severely insufficient strength and plasticity. Figure 4 shows a large number of fine subgrains and incompletely recrystallized deformed structures after 0.8h of solution treatment, with blurred grain boundaries.

[0097] Solution treatment for 3.5 hours: recrystallization fraction f = 99.8%, but abnormal grain growth, with an average grain size d = 210 μm. A typical microstructure is shown in Figure 5. Tensile strength R... m =751MPa, elongation A=16%, both strength and toughness decreased significantly. As shown in Figure 5, abnormal grain growth was observed after 3.5h of solution treatment, with coarse β grains exceeding 300μm in size and straight grain boundaries.

Claims

1. A method of controlling the microstructure uniformity of a Ti55531 alloy forging, characterized in that, Includes the following steps: Forging steps: The Ti55531 alloy billet is forged using a three-stage forging method, which includes ① a pre-forging stage: temperature 880~900℃, strain rate 0.01~0.03s. - ¹; ② Intermediate forging stage: temperature 840~860℃, strain rate 0.03~0.05s - ¹; ③ Precision forging stage: temperature 800~820℃, strain rate 0.05~0.08s - ¹; Solution heat treatment step: the forged piece after forging is subjected to solution treatment, and the solution time is determined by a quantitative control model; the quantitative control model is based on Mo equivalent C of the alloy Mo and dislocation density p to modify the base solution time t0, and meets the determination criteria of recrystallization fraction f≥95% and average grain size d≤200μm after solution, and the final solution time t is controlled in the range of 1.0h-3.0h.

2. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1, wherein, The deformation amount in the pre-forging stage is 20% to 25%; the deformation amount in the intermediate forging stage is 15% to 20%; the cumulative deformation amount in the pre-forging stage and intermediate forging stage is 35% to 45%; and the deformation amount in the precision forging stage is 5% to 10%.

3. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1 or 2, characterized in that, The three-stage forging method has a holding time of 10 to 15 minutes between two adjacent forging stages, and the temperature difference is controlled within ±5℃. Preferably, the pre-forging stage is held at 880-900℃ for 50-70 minutes before forging, the intermediate forging stage is held at 840-860℃ for 20-40 minutes before forging, and the precision forging stage is held at 800-820℃ for 10-30 minutes before forging.

4. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1 or 2, characterized in that, The quantitative control model determines the solution time in the following manner: Basic solution time t0 = 1.4 h, corresponding to standard composition C Mo = 9.3, dislocation density p = 7 x 10 12 m -2 of reference conditions; The solution time is corrected based on the alloy composition and dislocation density. The corrected solution time is t = t0 × k1 × k2. wherein k1 is a composition correction factor, k1 = 1 + 0.7 x (C Mo -9.3); k2 is a dislocation density correction coefficient, k2 = e (-0.04 x p), the unit of p is 10 12 m -2 ; the final solid solution time t is controlled in the range of 1 h to 3 h, when the calculated value t < 1.0 h, 1.0 h is taken; when the calculated value t > 3.0 h, 3.0 h is taken.

5. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1, wherein, The recrystallization fraction f and the solution time t satisfy the kinetic equation: f = 1 - e^(-3.1t^0.7); the average grain size d and the solution time t satisfy the growth equation: d = 34 + 66t^0.75; where the unit of d is μm and the unit of t is h.

6. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1, wherein, Mo equivalent C of the Ti55531 alloy Mo The calculation formula is: C Mo = w(Mo) + 0.67 x w(V) + 0.44 x w(Cr) - 0.11 x w(Al) + 0.18 x w(Zr); when Mo equivalent C Mo For each increase of 0.1%, the solid solution time is prolonged by 5% to 8%; the dislocation density p is increased by 1 x 10 12 m −2 , the solid solution time is shortened by 3% to 5%; w(Mo), w(V), w(Cr), w(Al), and w(Zr) represent the mass percentages of Mo, V, Cr, Al, and Zr, respectively.

7. The method of controlling the uniformity of the structure of a Ti55531 alloy forge piece according to claim 1 or 2, characterized in that, The solution treatment temperature is 10–30°C above the β-phase transformation point of the Ti55531 alloy.

8. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1 or 2, characterized in that, The heating rate of the solution treatment is controlled at 5–10 °C / min.

9. The method of controlling the microstructural uniformity of a Ti55531 alloy forging according to claim 1 or 2, wherein The dislocation density ρ was measured using X-ray diffraction, including calculating the micro-strain caused by dislocations using the Williamson-Hall method, and obtaining the dislocation density ρ using the Scherrer formula.

10. The method for controlling the uniformity of microstructure in Ti55531 alloy forgings according to claim 1 or 2, characterized in that, After three-stage forging, the dislocation density ρ of Ti55531 alloy ranges from 4×10¹² to 11×10¹²m⁻².

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