A method for preparing a large-diameter GR4 titanium alloy ingot

Abstract

The application provides a preparation method of a large-diameter GR4 titanium alloy ingot, belongs to the field of metal material processing and metallurgy technology, and aims at the solidification segregation problem of large-size ingots, and proposes a four-in-one control system of "source purification-base calibration-difference compensation-dynamic homogenization". First, the source purification of the preparation of the GR4 titanium alloy ingot is realized through the pretreatment of the sponge titanium, the calibration of the base of the raw material is realized by determining the component content of the treated raw material, the quantitative calculation of the oxygen and iron elements is carried out based on the specific difference between the actual oxygen and iron content of the raw material and the target content, the difference compensation is realized by the radial-axial double-gradient counter segregation distribution method aiming at the segregation law of the large-size titanium alloy ingot, the progressive VAR melting parameters matched with the diameter of the ingot are adopted to promote the homogenization of the solute in the molten pool, and the titanium alloy ingot with high stability of the consistency of the oxygen and iron content is obtained.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing and metallurgical technology, specifically relating to a method for preparing large-diameter GR4 titanium alloy ingots. Background Technology

[0002] GR4 titanium alloy is the strongest grade of commercially available pure titanium. Its mechanical properties mainly rely on the solid solution strengthening effect of oxygen and iron, two interstitial / substitution elements. It does not contain aluminum, vanadium, or other alloying elements that are potentially harmful to the human body, making it increasingly widely used in medical devices such as dental prostheses, dental implants, and bone implants. According to ASTM B348 standards, the typical composition range of GR4 titanium alloy is: oxygen content 0.35-0.45 wt%, iron content ≤0.25 wt%, carbon ≤0.08 wt%, nitrogen ≤0.03 wt%, hydrogen ≤0.015 wt%, with titanium as the balance. Even a small fluctuation in oxygen content (±0.03%) can cause a change in tensile strength of approximately 30-40 MPa; therefore, compositional consistency directly determines product quality stability.

[0003] However, the preparation of large-diameter (≥650mm) GR4 titanium alloy ingots faces the following key technical bottlenecks:

[0004] First, the oxygen and iron content of sponge titanium raw materials fluctuates greatly. Currently, in sponge titanium produced by the domestic magnesium reduction method (Kroll process), oxygen and iron are key impurity elements that are difficult to completely remove. During the production of sponge titanium, oxygen is mainly introduced through titanium tetrachloride raw materials, the airtightness of the reduction distillation process, and adsorption on equipment surfaces; iron mainly comes from the material of the reaction vessel, the equipment pretreatment process, and iron impurities in the raw materials. Production practice shows that even within the same batch of sponge titanium, the oxygen content difference between different particles can reach 0.02wt%-0.05wt%, and the iron content difference can reach 0.01wt%-0.03wt%. When these sponge titanium pressing electrodes are used for VAR melting, the intrinsic inhomogeneity of the raw material composition will be directly inherited by the ingot.

[0005] Secondly, the "size effect" of oxygen-iron distribution in large-sized ingots is significant. Solidification segregation is a common phenomenon in the preparation of titanium alloy ingots. Oxygen is an α-phase stable element in titanium, with an equilibrium distribution coefficient k_O (solid / liquid concentration ratio) of approximately 0.35-0.50. Iron, on the other hand, is a β-eutectoid stable element, with a distribution coefficient k_Fe of approximately 0.30-0.45. Both are less than 1, meaning that solutes enrich into the liquid phase during solidification, forming "normal segregation." For large-sized ingots with a diameter of 650 mm or more, segregation is more pronounced due to the large molten pool volume, prolonged solidification time, and complex melt convection patterns. The degree of nonlinear amplification is as follows: (1) Radial segregation: the edge of the ingot cools quickly, forming a fine-grained zone, and solute elements (Fe, O) are repelled to the center, resulting in a higher oxygen and iron content in the center; (2) Axial segregation: the top of the ingot solidifies last, enriching the repelled oxygen and iron elements, forming a "top segregation zone"; the segregation coefficient (C_max / C_0) of traditional small-diameter (≤500mm) ingots is usually controlled within 1.10, but when the diameter is increased to more than 650mm, the segregation coefficient can rise sharply to 1.20-1.35, which seriously exceeds the standard requirements for medical materials.

[0006] Therefore, developing a preparation method suitable for GR4 titanium alloy ingots with large diameters (specifically 750-900mm) of 650mm and above, and capable of systematically solving the problem of consistent oxygen and iron content, has significant industrial value and is of urgent practical importance. Summary of the Invention

[0007] The technical objective of this invention is to address the issue of uneven oxygen and iron content in 750-900mm GR4 grade titanium alloy ingots, which affects the performance of the titanium alloy ingots, as well as the problem of oxygen and iron solidification segregation caused by factors such as the "size effect" in large-sized titanium alloy ingots. This invention provides a method for preparing large-diameter GR4 titanium alloy ingots to achieve high uniformity of axial and radial composition in GR4 titanium alloy ingots.

[0008] The technical solution adopted by this invention to solve its technical problem is: a method for preparing a large-diameter GR4 titanium alloy ingot, comprising the following steps:

[0009] S1. Pre-treat the sponge titanium to remove impurities and obtain sponge titanium raw material with uniform oxygen and iron content;

[0010] S2. Calculate the oxygen and iron supplementation amounts based on the oxygen and iron content of the sponge titanium raw material and the oxygen and iron content of the target ingot.

[0011] S3. When pressing the electrode material, a radial and axial dual-gradient anti-segregation material feeding strategy is adopted to supplement oxygen and iron in the sponge titanium, and a vacuum consumable electrode is obtained after pressing.

[0012] S4. Perform multi-stage vacuum consumable arc melting, adjusting the melting parameters according to the diameter of each ingot stage.

[0013] Furthermore, the preprocessing in step S1 includes:

[0014] S1.1 Particle size sieving: The sponge titanium is divided into three grades according to particle size using a vibrating screen: +12 mesh (coarse particles), 12-30 mesh (medium particles), and -30 mesh (fine powder). The oxygen and iron content of each grade is tested separately.

[0015] S1.2 Magnetic separation for iron removal: A permanent magnet drum separator with a strength of 5000-8000 Gauss is used to perform at least two magnetic separations on the sponge titanium to remove mechanically trapped iron filings and iron-containing particles.

[0016] S1.3 Vacuum heat treatment: The sieved and magnetically separated sponge titanium is placed in a vacuum heat treatment furnace. Under the condition of vacuum degree ≤1.0×10⁻²Pa, the temperature is raised to 600-700℃ at 5-10℃ / min, held for 4-8 hours, and then cooled to below 100℃ in the furnace before being taken out.

[0017] S1.4 Homogenization and Mixing: Based on the oxygen and iron content test results of sponge titanium of each particle size grade, the materials are prepared according to the target oxygen and iron content of the ingot and mixed evenly. Sampling is carried out at no less than 5 points, with the standard deviation of oxygen content ≤0.005% and the standard deviation of iron content ≤0.003%.

[0018] Furthermore, the calculation formulas for oxygen supplementation and iron supplementation in step S2 are as follows:

[0019] m_O=(w_O^target - w_O^Ti)×M_Ti÷η_O

[0020] In the formula, m_O is the oxygen supplementation amount; w_O^target is the target oxygen content; w_O^Ti is the measured background oxygen content of the sponge titanium raw material; M_Ti is the total mass of the electrode; and η_O is the oxygen recovery rate, which is taken as 0.70-0.85.

[0021] m_Fe=(w_Fe^target-w_Fe^Ti)×M_Ti

[0022] In the formula, m_Fe is the amount of iron added; w_Fe^target is the target iron content; w_Fe^Ti is the background iron content of the sponge titanium raw material; and M_Ti is the total mass of the electrode.

[0023] Furthermore, the radial and axial dual-gradient anti-segregation fabrication strategy in step S3 is as follows: the radial gradient fabrication divides the electrode cross-section into a central region, a transition region, and an edge region, with compensation coefficients α_r for each region being 0.65-0.80, 1.0, and 1.3-1.5, respectively; the axial gradient fabrication divides the electrode length into a bottom end face region, a lower region, a middle region, an upper region, and a top end face region, with compensation coefficients β_h for each region being 1.3-1.5, 1.0-1.1, 1.0, 1.1-1.2, and 1.4-1.6, respectively.

[0024] Furthermore, the central region, transition region, and edge region are three concentric regions that divide the cross-section of the electrode mold according to the following ranges: central region: ≤30% R; transition region: 30% R < transition region ≤70% R; edge region: >70% R, where R is the radius of the electrode cross-section.

[0025] Furthermore, the bottom end face area, lower area, middle area, upper area, and top end face area are five regions divided along the electrode length / height direction according to the following ranges: bottom end face area: ≤5%L / H; lower area: 5%L / H < lower area ≤30%L / H; middle area: 30%L / H < middle area ≤70%L / H; upper area: 70%L / H < upper area ≤95%L / H; top end face area: >95%L / H; L / H represents electrode length / height.

[0026] Further, the specific fabrication method for the dual-gradient anti-segregation fabric in step S3 is as follows: the electrode is divided into 15 segments according to the radial and axial dual-gradient anti-segregation fabrication strategy: bottom end face center area, bottom end face transition area, bottom end face edge area, lower center area, lower transition area, lower edge area, middle center area, middle transition area, middle edge area, upper center area, upper transition area, upper edge area, top end face center area, top end face transition area, and top end face edge area. The comprehensive compensation coefficient γ = α_r × β_h for any segment, and the addition amount for that segment = γ × average addition amount, where the average addition amount = total addition amount ÷ sum of comprehensive compensation coefficients γ for each segment.

[0027] Furthermore, oxygen supplementation uses TiO2 as an oxygen supplement agent, and iron supplementation uses iron wire as an iron supplement agent. The total amount of oxygen supplement agent added is m_TiO2 = m_O ÷ w_O^TiO2, where w_O^TiO2 is the oxygen content in TiO2.

[0028] Furthermore, in step S4, the multi-stage vacuum self-consuming arc melting is a three-stage VAR melting, and the melting parameters are as follows:

[0029] VAR1: First-grade ingot diameter 500-650mm; melting current 18-28kA; melting rate 6-10kg / min;

[0030] VAR2: Secondary ingot diameter 650-800mm; melting current 22-36kA; melting rate 7-12kg / min;

[0031] VAR3: Finished ingot diameter 750-900mm; melting current 28-42kA; melting rate 8-15kg / min.

[0032] Furthermore, in step S4, the multi-stage vacuum consumable arc melting is a three-stage VAR melting, and the magnetic field stirring parameters are:

[0033] VAR1: Uses an alternating magnetic field with a magnetic field strength of 5-15mT and a stirring frequency of 2-8Hz, changing direction every 15-30 minutes;

[0034] VAR2: Uses an alternating magnetic field with a magnetic field strength of 10-20mT and a stirring frequency of 3-10Hz, changing direction every 15-30 minutes;

[0035] VAR3: Uses an alternating magnetic field with a magnetic field strength of 15-25mT and a stirring frequency of 5-15Hz, changing direction every 15-30 minutes.

[0036] Furthermore, the preparation method also includes adding a grain refiner during electrode pressing in step S3. The grain refiner contains boron, and the amount of boron added is 0.005-0.015 wt%, which is added in the form of B-Ti master alloy. The grain refiner adopts a radial and axial dual-gradient anti-segregation distribution strategy and is added to the sponge titanium during electrode pressing.

[0037] Furthermore, the ingots prepared by the method have a diameter range of 750-900 mm, an oxygen content range of ≤0.03%, an iron content range of ≤0.02%, and a segregation coefficient of ≤1.08.

[0038] Furthermore, the as-cast grain size of the ingot prepared by the method is ≥ASTM grade 6.

[0039] This invention provides a method for preparing large-diameter GR4 titanium alloy ingots. Addressing the solidification segregation problem in large-diameter ingots (750-900mm), it proposes a four-in-one control system: "source purification - background calibration - difference compensation - dynamic homogenization." First, source purification is achieved through pretreatment of sponge titanium. Then, the composition of the treated raw material is measured to calibrate the raw material background. Based on the specific difference between the actual oxygen and iron content and the target content, the oxygen and iron elements are quantitatively calculated. Next, a radial-axial dual-gradient anti-segregation distribution method is used to compensate for the segregation patterns of large-diameter titanium alloy ingots. Finally, progressive VAR melting parameters matched to the ingot diameter are used to promote the homogenization of the solute in the molten pool, achieving dynamic homogenization of the final titanium alloy ingot and obtaining a GR4-grade titanium alloy ingot with highly stable oxygen and iron content.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] 1. This invention achieves source purification in the preparation of GR4 titanium alloy ingots through pretreatment, effectively reducing the background fluctuation of oxygen and iron impurities in sponge titanium and obtaining sponge titanium raw materials with stable composition.

[0042] 2. This invention proposes a quantitative compensation method for oxygen and iron elements based on the specific difference between the actual oxygen and iron content of the raw material and the target content, thereby determining the specific amount of oxygen and iron elements to be added to compensate, thus avoiding the blind addition based on experience in the prior art.

[0043] 3. This invention achieves differential compensation for the segregation pattern of large-size titanium alloy ingots by using a radial-axial dual-gradient anti-segregation material distribution method. During the electrode pressing process, radial and axial dual-gradient compensation material distribution is implemented. The compensation coefficient is higher at the radial edge than at the center, and the radial compensation coefficient gradually increases from the inside to the outside along the radial direction. The compensation strength at both ends of the axial direction is higher than at the middle, and the axial compensation coefficient gradually increases from the center to both ends along the axial direction.

[0044] 4. This invention promotes the homogenization of solute in the molten pool by using progressive VAR melting parameters matched with the ingot diameter, thereby achieving dynamic homogenization of the final titanium alloy ingot and obtaining a GR4 grade titanium alloy ingot with highly stable oxygen and iron content.

[0045] 5. This invention achieves grain refinement of large-size ingots by adding trace amounts of grain-refining elements (B and / or C) and optimizing the heat treatment process.

[0046] 6. The Φ750-900mm GR4 ingots prepared by the method of the present invention have an oxygen content range of ≤0.03%, an iron content range of ≤0.02%, a segregation coefficient of ≤1.08, and a grain size of ≥ASTM 6. Their mechanical properties meet the ASTM F67 standard, and their compositional uniformity reaches the international advanced level. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] At present, the following prominent problems exist in the preparation of large-size GR4 ingots: (1) Raw material processing is crude: there is a lack of batch homogenization pretreatment process for sponge titanium, and sponge titanium from different sources and batches is directly mixed and used; (2) Compensation strategy is simple: the method of adding TiO2 powder or pure iron wire as a whole is mostly adopted, without considering the difference in raw material background and the burn-off / distribution law during the smelting process; (3) Process parameters are vague: key smelting parameters (smelting current, melting rate, magnetic field stirring intensity) lack quantitative design basis; (4) Segregation control means are limited: there are no effective means to suppress radial / axial segregation of ingots with a diameter of 650mm or more.

[0049] To address the aforementioned technical problems, this invention provides a method for preparing large-diameter GR4 titanium alloy ingots. For the solidification segregation problem of large-diameter ingots (750-900mm), a four-in-one control system of "source purification - background calibration - difference compensation - dynamic homogenization" is proposed.

[0050] Oxygen in sponge titanium mainly exists in the interstitial solid solution form at the octahedral interstitial sites of the titanium lattice, while iron exists in the form of substitution. The distribution of these impurities is random and non-uniform, which is the root cause of background fluctuations. This invention obtains a stable raw material background through particle size sieving, magnetic separation for iron removal, and vacuum heat treatment of sponge titanium, reducing background fluctuations at the source and creating conditions for subsequent precise compensation. A quantitative compensation formula for TiO2 and pure iron wire based on background differences is established.

[0051] This invention proposes a radial-axial dual-gradient anti-segregation material distribution method, in which the radial edge compensation intensity is higher than the center, and the axial end compensation intensity is higher than the middle. In the area where segregation is most severe during ingot solidification, the solute concentration is increased in advance to compensate, and the reverse effect of solidification segregation is used to make the actual content in the final solidified structure tend to be uniform.

[0052] This invention employs progressive VAR melting parameters matched to the ingot diameter. The alternating magnetic field induces electromagnetic force in the molten pool, driving the melt to generate forced convection. This has the homogenization effect of breaking and refining dendrites, promoting solute redistribution at the solid-liquid interface, and breaking up temperature stratification in the molten pool. In each stage of VAR, this invention uses progressive magnetic field stirring parameters (the intensity increases with the ingot diameter) to ensure that even large molten pools can obtain sufficient stirring effect.

[0053] This invention proposes a method for preparing large-diameter GR4 titanium alloy ingots, the method specifically including the following steps:

[0054] S1: Pretreatment of Titanium Sponge

[0055] Select 0Ti-1 grade or higher grade sponge titanium as the base material, and process the sponge titanium in the following steps:

[0056] S1.1 Particle Size Sieving: A vibrating screen was used to separate the sponge titanium into three grades: +12 mesh (coarse particles), 12-30 mesh (medium particles), and -30 mesh (fine powder). The oxygen and iron content of each grade was tested. Statistical analysis showed that, due to its larger specific surface area, the fine powder typically had a 0.01-0.03 wt% higher oxygen content and a 0.005-0.015 wt% higher iron content than the medium particles.

[0057] S1.2 Magnetic separation for iron removal: A permanent magnet drum separator with a strength of 5000-8000 Gauss is used to perform 2-3 magnetic separations on the sponge titanium to remove mechanically trapped iron filings and iron-containing particles; the iron content of the sponge titanium can be reduced by 0.005-0.01wt% after magnetic separation.

[0058] S1.3 Vacuum Heat Treatment: The sieved and magnetically separated sponge titanium is placed in a vacuum heat treatment furnace, and the vacuum degree is ≤1.0×10⁻⁶. -2 Under Pa conditions, the temperature is increased to 600-700℃ at 5-10℃ / min, held for 4-8 hours, and then cooled to below 100℃ in the furnace before being removed from the furnace. Vacuum heat treatment can remove hydrogen, moisture and some low-melting-point chloride impurities adsorbed on the surface of sponge titanium, while promoting the stabilization of the internal structure of sponge titanium.

[0059] S1.4 Homogenization Mixing: Based on the oxygen and iron content test results of sponge titanium of each particle size grade, mix in a V-type mixer or double cone mixer for 2-4 hours according to the target ratio to make the overall composition more uniform; take no less than 5 samples of the mixed sponge titanium to test the oxygen and iron content, and calculate the mean and standard deviation; the standard deviation of oxygen content ≤0.005% and the standard deviation of iron content ≤0.003% are required before it can be used for subsequent processes.

[0060] S2: Calculation of oxygen and iron content compensation

[0061] S2.1 Determination of the background composition of sponge titanium: Take the pretreated sponge titanium raw material sample and determine the oxygen content by inert gas melting-infrared absorption method (GB / T 4698.7-2011). Determine the iron content by inductively coupled plasma atomic emission spectrometry (GB / T4698.2-2011). Each sample is measured in parallel 3 times and the arithmetic mean is taken. The measured oxygen content of sponge titanium is recorded as w_O^Ti (%) and the measured iron content is recorded as w_Fe^Ti (%).

[0062] S2.2 Target component determination:

[0063] Based on the performance requirements of GR4 titanium alloy products, target oxygen content w_O^target and iron content w_Fe^target are set for the ingot. For medical-grade GR4, the recommended target range is:

[0064] Oxygen content: 0.37-0.43% (center value 0.40%)

[0065] Iron content: 0.17-0.23% (center value 0.20%)

[0066] S2.3 Oxygen Supplementation Calculation

[0067] Using anatase TiO2 powder (purity ≥99.5%, particle size ≤10μm) as an oxygen supplement, the reaction of TiO2 in the titanium melt is: TiO2 + Ti → 2TiO (gas / slag) or Ti + O (dissolved), ultimately entering the titanium matrix as dissolved oxygen. Based on mass balance, the amount of oxygen supplement added is calculated using the following formula:

[0068] m_O = (w_O^target - w_O^Ti) × M_Ti ÷ η_O

[0069] m_TiO2 = m_O ÷ w_O^TiO2, where w_O^TiO2 is the oxygen content in TiO2.

[0070] In the formula:

[0071] m_O: Oxygen supplementation

[0072] m_TiO2: TiO2 addition amount

[0073] w_O^target: Target oxygen content (%)

[0074] w_O^Ti: Background oxygen content of sponge titanium (%)

[0075] M_Ti: Total electrode mass

[0076] η_O: Oxygen yield, taken as 0.70-0.85 (depending on the smelting process).

[0077] Oxygen yield η_O is affected by factors such as smelting vacuum, molten pool temperature, and smelting time. Based on extensive experimental statistics, this invention provides reference values ​​for η_O under different smelting conditions:

[0078] VAR1 (Level 1 Smelting): η_O = 0.70-0.75

[0079] VAR2 (Secondary Smelting): η_O = 0.75-0.80

[0080] VAR3 (Level 3 Smelting): η_O = 0.80-0.85

[0081] S2.4 Iron Supplementation Calculation

[0082] Using pure electrical iron wire (purity ≥99.9%, diameter 0.5-1.5mm, cut into 2-5mm long segments) as the iron supplement, the iron recovery rate is close to 100% (iron has high solubility in titanium and is not easily volatilized). The amount of iron supplemented is calculated according to the following formula:

[0083] m_Fe = (w_Fe^target - w_Fe^Ti) × M_Ti

[0084] In the formula:

[0085] m_Fe: Amount of pure iron wire added

[0086] w_Fe^target: Target iron content (%)

[0087] w_Fe^Ti: Base iron content of sponge titanium (%)

[0088] M_Ti: Total electrode mass

[0089] S2.5 Grain structure control and grain refiner calculation:

[0090] To refine the grain structure of large-diameter ingots, trace amounts of grain-refining elements are added during the electrode pressing stage. Boron (B) is selected as the grain refiner, and the amount added is controlled within the following range:

[0091] Boron: Added in the form of B-Ti master alloy (B content 5-10wt%), with the final ingot boron content controlled at 0.005-0.015wt%;

[0092] Boron forms high-melting-point compound particles such as TiB in titanium, which act as heterogeneous nucleation sites during solidification, promoting grain refinement. For large ingots with a diameter ≥800mm, adding boron can achieve a grain size of ASTM grade 6 or finer.

[0093] S3: Electrode pressing and "anti-segregation gradient fabric"

[0094] S3.1 Radial gradient (cross-sectional direction)

[0095] The cross-section of the electrode mold (radius R) is divided into three concentric regions:

[0096] Central area (≤30% R)

[0097] Transition zone (30% < transition zone ≤ 70% R)

[0098] Edge region (>70% R)

[0099] Because solutes (O, Fe) accumulate towards the center during solidification, the compensation strength in the edge zone should be higher than that in the center zone during fabric application. Therefore, a radial compensation coefficient α_r is defined as follows:

[0100] Central region: α_r = 0.65-0.8

[0101] Transition region: α_r = 1.0

[0102] Edge region: α_r = 1.3-1.5

[0103] That is, the compensation intensity of TiO2 and Fe in the edge region is 1.3-1.5 times that in the center region.

[0104] S3.2 Axial gradient (length / height direction)

[0105] The electrode length / height (L / H) direction is divided into five regions:

[0106] Bottom end face area (≤5%L / H)

[0107] Lower region (5% L / H < lower region ≤ 30% L / H)

[0108] Central region (30% L / H < Central region ≤ 70% L / H)

[0109] Upper region (70% L / H < upper region ≤ 95% L / H)

[0110] Top end face area (>95% L / H)

[0111] Since the top of the ingot solidifies last and experiences the most severe segregation, the top and bottom end faces of the electrode should be given higher compensation strength. An axial compensation coefficient β_h is defined as follows:

[0112] Bottom end face region: β_h = 1.3-1.5

[0113] Lower region: β_h = 1.0-1.1

[0114] Central region: β_h = 1.0

[0115] Upper region: β_h = 1.1-1.2

[0116] Top end face region: β_h = 1.4-1.6

[0117] S3.3 Overall Fabric Density

[0118] Based on a radial and axial dual-gradient anti-segregation fabrication strategy, the electrode is divided into 15 segments: bottom end face center region, bottom end face transition region, bottom end face edge region, lower center region, lower transition region, lower edge region, middle center region, middle transition region, middle edge region, upper center region, upper transition region, upper edge region, top end face center region, top end face transition region, and top end face edge region. The comprehensive compensation coefficient γ = α_r × β_h for any segment; the amount of reagent added in this segment = γ × average amount added, where the average amount added = total amount of reagent added ÷ sum of the comprehensive compensation coefficients γ of each segment; the reagents include oxygen supplement TiO2 powder, iron supplement pure iron wire, and grain refiner B-Ti master alloy.

[0119] S3.4 electrode pressing

[0120] The calculated TiO2 powder, pure iron wire, and grain refiner are spread in layers and sections within the sponge titanium according to the above dual-gradient material distribution scheme. The electrode mold diameter is 400-500mm. Specific operation:

[0121] S3.4.1 First, lay a layer of sponge titanium at the bottom of the electrode mold, up to the bottom end face area of ​​the electrode mold. Add TiO2, iron wire and grain refiner according to the compensation addition amount of each section of the bottom end face center area, bottom end face transition area and bottom end face edge area.

[0122] S3.4.2 Add titanium sponge to the lower part of the mold, and add TiO2, iron wire and grain refiner according to the compensation amount of each section of the lower end face center area, lower end face transition area and lower end face edge area;

[0123] S3.4.3 Add sponge titanium to the middle area of ​​the mold, and add TiO2, iron wire and grain refiner according to the compensation amount of each section of the central area, the middle transition area and the middle edge area;

[0124] S3.4.4 Add sponge titanium to the upper area of ​​the mold, and add TiO2, iron wire and grain refiner according to the compensation amount of each section of the upper central area, upper transition area and upper edge area;

[0125] S3.4.5 Add sponge titanium to the top end face area of ​​the mold, and compensate for the amount of TiO2, iron wire and grain refiner added to each section of the upper end face center area, upper end face transition area and upper end face edge area;

[0126] S3.4.6 Add titanium sponge until the mold is full;

[0127] After the fabric is laid, a 4000-8000 ton hydraulic press is used to press the sponge titanium into dense consumable electrode blocks under a pressure of 50-80MPa. The electrode block density is required to reach 3.2-3.5g / cm³ (70-75% of the theoretical density) to ensure that the electrode does not fall off or collapse during the melting process.

[0128] S4: Multistage Vacuum Consumable Arc Melting (VAR)

[0129] According to the required length / height (L / H) of the consumable electrode, the pressed electrode blocks are welded into consumable electrodes in a vacuum environment and loaded into a vacuum consumable arc furnace for three-stage melting. The core parameters of each stage of VAR melting are related to the ingot diameter and target composition, and dynamic matching is required.

[0130] S4.1 Level 1 Smelting (VAR1)

[0131] Objective: To prepare primary ingots with a diameter of 500-650 mm, and to perform preliminary alloying and removal of gaseous impurities.

[0132] Key parameters:

[0133] Melting current: 18-28kA;

[0134] Operating voltage: 28-34V;

[0135] Melting speed: 6-10 kg / min;

[0136] Arc initiation current: 2.5-7.5kA;

[0137] Arc stabilizing current: 10-20A, frequency 2-5Hz;

[0138] Vacuum degree: ≤5.0×10 -2 Pa;

[0139] Leakage rate: ≤0.5Pa·L / s;

[0140] Magnetic field stirring parameters: an alternating magnetic field is used, with a magnetic field strength of 5-15 mT and a stirring frequency of 2-8 Hz, changing direction every 15-30 minutes.

[0141] S4.2 Secondary Smelting (VAR2)

[0142] Objective: To prepare secondary ingots with a diameter of 650-800 mm and further homogenize the composition.

[0143] Key parameters:

[0144] Melting current: 22-36kA;

[0145] Operating voltage: 30-36V;

[0146] Melting speed: 7-12 kg / min;

[0147] Arc initiation current: 3.0-8kA;

[0148] Arc current: 15-30A, frequency 2-5Hz;

[0149] Vacuum degree: ≤3.0×10 -2 Pa

[0150] Magnetic field stirring parameters: The magnetic field stirring intensity should be appropriately increased: magnetic field strength 10-20mT, stirring frequency 3-10Hz, and the direction should be reversed every 15-30 minutes.

[0151] S4.3 Level 3 Smelting (VAR3)

[0152] Objective: To prepare finished ingots with a diameter of 750-900mm to obtain the final dimensions and composition.

[0153] Key parameters:

[0154] Melting current: 28-42kA;

[0155] Operating voltage: 32-38V;

[0156] Melting speed: 8-15 kg / min;

[0157] Arc initiation current: 3.5-8.5kA;

[0158] Arc stabilizing current: 20-40A, frequency 2-5Hz;

[0159] Vacuum degree: ≤3.0×10 -2 Pa;

[0160] Magnetic field stirring parameters: Use an alternating magnetic field with a magnetic field strength of 15-25mT and a stirring frequency of 5-15Hz, changing direction every 15-30 minutes;

[0161] At the end of the melting process (the last 10-15% of the mass), the melting speed should be appropriately reduced (to 65-80% of the normal value), and top feeding should be strengthened to reduce top segregation.

[0162] S5: Post-processing and Inspection

[0163] After the ingot is melted, it is cooled to 800℃ in the furnace (about 12-24 hours), then removed from the furnace and air-cooled to room temperature. The surface oxide scale and segregation layer are removed by turning (15-20mm of scale removal per side), and the risers at both ends of the ingot are cut off (3-5% of the total length of the ingot is removed from each end).

[0164] S5.1 Component Uniformity Test

[0165] Take no fewer than 9 points (3 sections at the beginning, middle, and end of the axial direction × 3 positions at the radial edge, half radius, and center) along the axial and radial directions of the ingot to test the oxygen and iron content. Requirements:

[0166] Oxygen content range (C_max - C_min) ≤ 0.03%

[0167] Iron content range ≤0.02%

[0168] Segregation coefficient (C_max / C_0) ≤ 1.08

[0169] S5.2 Mechanical Performance Test

[0170] Samples are cut from the ingot, forged, and hot-rolled into Φ15-25mm bars, and subjected to room temperature tensile tests according to ASTM E8 / E8M standards, requiring the following specifications to be met:

[0171] Tensile strength Rm≥550MPa

[0172] The specified plastic elongation strength Rp0.2 ≥ 480 MPa

[0173] Elongation at fracture (A) ≥ 15%

[0174] For specific preparation methods and parameters, please refer to the respective embodiments. Experimental methods not specified in the embodiments shall be carried out under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments not specified by the manufacturer are all commercially available products. Unless otherwise specified, "%" in this invention refers to the mass percentage "wt%".

[0175] Example 1:

[0176] This embodiment provides a method for preparing a large-diameter GR4 titanium alloy ingot, aiming to control the oxygen content in the titanium alloy ingot to 0.38-0.42% and the iron content to 0.18-0.22%. The specific steps of this method are as follows:

[0177] S1: Pretreatment and Background Calibration of Sponge Titanium

[0178] Select 3000 kg of 0Ti-1 grade sponge titanium; proceed as follows:

[0179] S1.1 Particle size sieving: +12 mesh accounts for 22%, 12-30 mesh accounts for 65%, and -30 mesh accounts for 13%;

[0180] S1.2 Magnetic separation for iron removal: 8000 Gauss magnetic separator, processed twice;

[0181] S1.3 Vacuum heat treatment: Hold at 650℃ for 6 hours, vacuum degree 5×10 -3 Pa.

[0182] S1.4 Five mixed sponge titanium samples were taken, and the test results are shown in Table 1:

[0183] Table 1. Results of Detection of Oxygen and Iron Content in Sponge Titanium and After Treatment in Example 1

[0184] O content 0.112% 0.108% 0.115% 0.110% 0.113% 0.1116% 0.0026% Fe content 0.035% 0.032% 0.038% 0.034% 0.036% 0.0350% 0.0022%

[0185] S2. Oxygen and Iron Content Compensation Calculation

[0186] S2.1 w_O^Ti=0.1116%, w_Fe^Ti=0.0350%;

[0187] S2.2 Total electrode mass M_Ti = 910kg; Target values ​​set: w_O^target = 0.40%, w_Fe^target = 0.20%;

[0188] S2.3 Oxygen Supplementation Calculation:

[0189] m_O = (w_O^target - w_O^Ti) × M_Ti ÷ η_O;

[0190] w_O^target = 0.40%, w_O^Ti = 0.1116%, and the comprehensive oxygen yield η_O of the three-stage smelting process is taken as 0.75;

[0191] m_TiO2=m_O÷w_O^TiO2, w_O^TiO2=40%;

[0192] TiO2 addition amount m_TiO2 = 8.75 kg;

[0193] S2.4 Iron Supplementation Calculation:

[0194] m_Fe = (w_Fe^target - w_Fe^Ti) × M_Ti;

[0195] w_Fe^target = 0.20%, w_Fe^Ti=0.0350%;

[0196] The amount of pure iron wire added, m_Fe = 1.50kg;

[0197] S2.5 grain refiner calculation

[0198] Add B-Ti master alloy (8wt% B content); target B content 0.008%;

[0199] B-Ti addition amount = 910kg × 0.008% / 8% = 0.91kg.

[0200] S3: Electrode pressing and "anti-segregation gradient fabric"

[0201] The electrode mold has a diameter of 460mm and a length of 1700mm;

[0202] S3.1 Radial gradient (α_r):

[0203] Central region: α_r = 0.75

[0204] Transition region: α_r = 1.0

[0205] Edge region: α_r = 1.4

[0206] S3.2 Axial gradient (β_h):

[0207] Bottom end face region: β_h = 1.4

[0208] Lower region: β_h = 1.05

[0209] Central region: β_h = 1.0

[0210] Upper region: β_h = 1.15

[0211] Top end face region: β_h = 1.5

[0212] S3.3 Calculate the comprehensive compensation coefficient γ for each section, and calculate the amount of TiO2, pure iron wire, and grain refiner added in each section based on γ for each section;

[0213] S3.4 The TiO2, pure iron wire, and grain refiner are distributed in the sponge titanium according to the calculated amount for each section, and the material is evenly distributed in each section.

[0214] The specific steps are as follows:

[0215] S3.4.1 First, lay a layer of sponge titanium at the bottom of the electrode mold, with a thickness of 1% of the mold height. Add TiO2, iron wire and grain refiner according to the compensation addition amount of each section of the bottom end face center area, bottom end face transition area and bottom end face edge area.

[0216] S3.4.2 Add sponge titanium to 10% of the mold height, and add TiO2, iron wire and grain refiner according to the compensation addition amount of each section of the lower end face center area, lower end face transition area and lower end face edge area;

[0217] S3.4.3 Add sponge titanium to 50% of the mold height, and add TiO2, iron wire and grain refiner according to the compensation amount of each section of the central area, central transition area and central edge area;

[0218] S3.4.4 Add sponge titanium to 80% of the mold height, and add TiO2, iron wire and grain refiner according to the compensation amount of each section of the upper central area, upper transition area and upper edge area;

[0219] S3.4.5 Add sponge titanium to 98% of the mold height, and add TiO2, iron wire and grain refiner according to the compensation amount of each section of the upper end face center area, upper end face transition area and upper end face edge area;

[0220] S3.4.6 Add titanium sponge until the mold is full.

[0221] S3.4.7 Pressing pressure 65MPa, holding pressure for 30 seconds.

[0222] S4: Level 3 VAR Melting

[0223] S4.1 Level 1 Smelting (VAR1):

[0224] Target primary ingot diameter: 600mm

[0225] Melting current: 20-24kA; Arc stabilization current: 12A;

[0226] Melting rate: 7.5 kg / min;

[0227] Magnetic stirring: intensity 10mT, frequency 5Hz, reversed every 20 minutes;

[0228] Vacuum degree: 8×10 -2 Pa;

[0229] S4.2 Secondary Smelting (VAR2):

[0230] Target secondary ingot diameter: 750mm

[0231] Melting current: 24-30kA; Arc stabilization current: 18A;

[0232] Melting rate: 8.5 kg / min;

[0233] Magnetic stirring: intensity 15mT, frequency 7Hz, reversed every 20 minutes;

[0234] Vacuum degree: 5×10 -2 Pa;

[0235] S4.3 Level 3 Smelting (VAR3):

[0236] Target finished ingot diameter: 880mm

[0237] Melting current: 28-34kA; Arc stabilization current: 25A;

[0238] Melting rate: 9.0 kg / min, decreasing to 6.5 kg / min at the end of the melting process (last 10% of the mass);

[0239] Magnetic stirring: intensity 18mT, frequency 8Hz, reversed every 15 minutes;

[0240] Vacuum degree: 4×10 -2 Pa.

[0241] S5: Post-processing and Inspection

[0242] The ingot is cooled to 800℃ in the furnace (about 18 hours), then removed from the furnace and air-cooled; the outer skin is removed by turning 18mm on one side, and the risers at both ends are cut off by 120mm each.

[0243] S5.1 Composition analysis: Take three cross sections at the head, middle and tail of the axis. Take three points at the edge, 1 / 2R and center of each cross section, for a total of 9 points, to determine the O and Fe content and grain size. The O content test results are shown in Table 2, and the Fe content test results are shown in Table 3.

[0244] Table 2. O content test results (%) of the finished ingots from Example 1

[0245] Head section 0.402 0.408 0.412 Mid-section 0.396 0.401 0.405 Tail section 0.398 0.403 0.407

[0246] Table 3. Fe content test results (%) of the finished ingots from Example 1

[0247] Head section 0.198 0.203 0.206 Mid-section 0.195 0.199 0.202 Tail section 0.196 0.201 0.204

[0248] The range of O content is 0.016%, and the segregation coefficient is 1.040; the range of Fe content is 0.011%, and the segregation coefficient is 1.045; the as-cast microstructure rating of the grain size is ASTM level 6.5.

[0249] S5.2 Mechanical property testing: The ingot was forged, hot-rolled into Φ18mm bars, and after annealing, samples were taken to determine the tensile strength, yield strength and elongation after fracture in accordance with the national standard GB / T 228.1-2010. The test results are shown in Table 4.

[0250] Table 4. Mechanical property test results of the rolled bar stock (Φ18mm) from the finished ingot of Example 1.

[0251] Head edge 582 496 17.5 Head center 578 492 18.0 central edge 585 499 17.0 Central Center 580 494 17.5 Tail edge 579 493 18.0 Tail center 575 490 18.5

[0252] The above test results show that the titanium alloy ingots cast in Example 1 fully meet the requirements of ASTM F67 (pure titanium for surgical implants) for GR4 (Rm≥550MPa, Rp0.2≥480MPa, A≥15%).

[0253] Example 2:

[0254] This embodiment provides a method for preparing a large-diameter GR4 titanium alloy ingot, aiming to control the oxygen content in the titanium alloy ingot to 0.39-0.43% and the iron content to 0.19-0.23%. The specific steps of this method are basically the same as those in Embodiment 1, with the following differences:

[0255] S1: Pretreatment and Background Calibration of Sponge Titanium

[0256] 0Ti-1 grade sponge titanium was selected. The batch was different from that in Example 1, and the background composition fluctuated greatly. After sieving, magnetic separation and vacuum heat treatment (680℃×5 hours), samples were taken for testing. The test results are shown in Table 5.

[0257] Table 5. Results of oxygen and iron content detection in sponge titanium and treated sponge from Example 2.

[0258] O content 0.135% 0.142% 0.138% 0.140% 0.136% 0.1382% 0.0028% Fe content 0.028% 0.031% 0.033% 0.030% 0.032% 0.0308% 0.0019%

[0259] S2: Calculation of oxygen and iron content compensation

[0260] S2.1 w_O^Ti=0.1382%w_Fe^Ti=0.0308%;

[0261] S2.2 Total electrode mass M_Ti = 1200kg; Target values: w_O^target = 0.41%, w_Fe^target = 0.21%;

[0262] S2.3 Oxygen Supplementation Calculation:

[0263] m_O = (w_O^target - w_O^Ti) × M_Ti ÷ η_O;

[0264] w_O^target = 0.41%, w_O^Ti = 0.1382%, and the comprehensive oxygen yield η_O of the three-stage smelting process is taken as 0.73;

[0265] m_TiO2=m_O÷w_O^TiO2; w_O^TiO2=40%;

[0266] TiO2 addition amount m_TiO2 = 11.17kg

[0267] S2.4 Iron Supplementation Calculation:

[0268] m_Fe = (w_Fe^target - w_Fe^Ti) × M_Ti

[0269] w_Fe^target = 0.21%, w_Fe^Ti=0.0308%

[0270] The amount of pure iron wire added, m_Fe = 2.15kg

[0271] S2.5 grain refiner calculation:

[0272] Add B-Ti master alloy (8wt% B content); target B content 0.01%;

[0273] The amount of B-Ti master alloy (8wt% B content) added is 1200kg × 0.010% / 8% = 1.5kg.

[0274] S3: Electrode pressing and "anti-segregation gradient fabric"

[0275] The electrode mold has a diameter of 480mm and a length of 2000mm;

[0276] S3.1 Radial gradient (α_r):

[0277] Central region: α_r = 0.70

[0278] Transition region: α_r = 1.0

[0279] Edge region: α_r = 1.45

[0280] S3.2 Axial gradient (β_h):

[0281] Bottom end face region: β_h = 1.45

[0282] Lower region: β_h = 1.08

[0283] Central region: β_h = 1.0

[0284] Upper region: β_h = 1.18

[0285] Top end face region: β_h = 1.55

[0286] S3.3 Calculate the comprehensive compensation coefficient γ for each section, and calculate the amount of TiO2, pure iron wire, and grain refiner added in each section based on γ for each section.

[0287] S3.4 The TiO2, pure iron wire, and grain refiner are distributed in the sponge titanium according to the calculated amount for each section, and the material is evenly distributed in each section. The specific operation is the same as in Example 1.

[0288] S4: Level 3 VAR Melting

[0289] S4.1 Level 1 Smelting:

[0290] First-grade ingot diameter: 600mm

[0291] Melting current: 22-26kA; Arc stabilization current: 15A;

[0292] Melting rate: 8.0 kg / min;

[0293] Magnetic stirring: intensity 12mT, frequency 5Hz; reversed every 20 minutes;

[0294] S4.2 Secondary Smelting:

[0295] Secondary ingot diameter: 750mm

[0296] Melting current: 26-34kA; Arc stabilization current: 22A;

[0297] Melting rate: 9.5 kg / min

[0298] Magnetic stirring: intensity 18mT, frequency 7Hz; reversed every 20 minutes;

[0299] S4.3 Level 3 Smelting:

[0300] Finished ingot diameter: 880mm

[0301] Melting current: 30-38kA; Arc stabilizing current: 30A;

[0302] Melting rate: 10.5 kg / min, decreasing to 8.0 kg / min at the end of the melting process (last 10% of the mass);

[0303] Magnetic field stirring: intensity 22mT, frequency 8Hz; reverse direction every 15 minutes.

[0304] S5: Post-processing and Inspection

[0305] S5.1 Composition analysis: Take three cross sections at the head, middle and tail of the axis. Take three points at the edge, 1 / 2R and center of each cross section, for a total of 9 points, to determine the O and Fe content and grain size. The O content test results are shown in Table 6, and the Fe content test results are shown in Table 7.

[0306] Table 6. O content test results (%) of the finished ingots from Example 2

[0307] Head section 0.412 0.418 0.423 Mid-section 0.406 0.411 0.416 Tail section 0.408 0.414 0.419

[0308] Table 7. Fe content test results (%) of the finished ingots from Example 2

[0309] Head section 0.208 0.213 0.217 Mid-section 0.205 0.209 0.213 Tail section 0.206 0.211 0.215

[0310] The O element content range is 0.017%, and the segregation coefficient is 1.042; the Fe element content range is 0.012%, and the segregation coefficient is 1.047; the grain size is ASTM 7.0 grade.

[0311] S5.2 Mechanical property testing: The ingot was forged, hot-rolled into Φ20mm bars, and after annealing, samples were taken to determine the tensile strength, yield strength and elongation after fracture in accordance with the national standard GB / T 228.1-2010. The test results are shown in Table 8.

[0312] Table 8. Mechanical property test results of the rolled bar stock (Φ20mm) from the finished ingot in Example 2

[0313] Head edge 591 505 16.5 Head center 586 500 17.0 central edge 594 508 16.0 Central Center 588 502 17.0 Tail edge 590 504 16.5 Tail center 585 499 17.5

[0314] Example 3

[0315] This embodiment provides a method for preparing a large-diameter GR4 titanium alloy ingot, aiming to control the oxygen content in the titanium alloy ingot to 0.37-0.41% and the iron content to 0.17-0.21%. The specific steps of this method are basically the same as those in Embodiment 1, with the main differences as follows:

[0316] S1: Pretreatment and Background Calibration of Sponge Titanium

[0317] A mixture of 0Ti-1 grade sponge titanium and a portion of 0Ti-0 grade sponge titanium (low oxygen) was selected, and tested after pretreatment.

[0318] Oxygen content: mean 0.0952%, standard deviation 0.0031%; Iron content: mean 0.0255%, standard deviation 0.0024%.

[0319] S2: Calculation of oxygen and iron content compensation

[0320] S2.1 w_O^Ti=0.0952%; w_Fe^Ti=0.0255%;

[0321] S2.2 Total electrode mass M_Ti = 1600kg; Target values: w_O^target = 0.39%, w_Fe^target = 0.19%;

[0322] S2.3 Oxygen Supplementation Calculation:

[0323] m_O = (w_O^target - w_O^Ti) × M_Ti ÷ η_O;

[0324] w_O^target = 0.39%, w_O^Ti = 0.0952%, and the comprehensive oxygen yield η_O of the three-stage smelting process is taken as 0.72;

[0325] m_TiO2=m_O÷w_O^TiO2; w_O^TiO2=40%;

[0326] TiO2 addition amount m_TiO2 = 16.38kg;

[0327] S2.4 Iron Supplementation Calculation:

[0328] m_Fe = (w_Fe^target - w_Fe^Ti) × M_Ti;

[0329] w_Fe^target = 0.19%, w_Fe^Ti=0.0255%;

[0330] The amount of pure iron wire added, m_Fe = 2.63kg;

[0331] S2.5 Grain Refining Agent Calculation

[0332] Add B-Ti master alloy (8wt% B content); target B content 0.012%;

[0333] B-Ti master alloy addition amount = 1600 kg × 0.012% / 8% = 2.4 kg.

[0334] S3: Electrode pressing and "anti-segregation gradient fabric"

[0335] The electrode mold has a diameter of 500mm and a length of 2500mm;

[0336] S3.1 Radial gradient (α_r):

[0337] Central region: α_r = 0.65 (Segregation is more severe in the center of large ingots, so the compensation intensity in the central region is reduced).

[0338] Transition region: α_r = 1.0

[0339] Edge region: α_r = 1.50

[0340] S3.2 Axial gradient (β_h):

[0341] Bottom end face region: β_h = 1.50

[0342] Lower region: β_h = 1.10

[0343] Central region: β_h = 1.0

[0344] Upper region: β_h = 1.20

[0345] Top end face region: β_h = 1.60

[0346] S3.3 Calculate the comprehensive compensation coefficient γ for each section, and calculate the amount of TiO2, pure iron wire, and grain refiner added in each section based on γ for each section.

[0347] S3.4 The TiO2, pure iron wire, and grain refiner are distributed in the sponge titanium according to the calculated amount for each section, and the material is evenly distributed in each section. The specific operation is the same as in Example 1.

[0348] S4: Level 3 VAR Melting

[0349] S4.1 Level 1 Smelting:

[0350] First-grade ingot diameter: 600mm

[0351] Melting current: 24-28kA; Arc stabilizing current: 18A;

[0352] Melting rate: 8.5 kg / min;

[0353] Magnetic stirring: intensity 14 mT; frequency 5 Hz; reverse direction every 20 minutes;

[0354] S4.2 Secondary Smelting:

[0355] Secondary ingot diameter: 750mm

[0356] Melting current: 28-36kA; Arc stabilizing current: 25A;

[0357] Melting rate: 10.0 kg / min

[0358] Magnetic stirring: intensity 20mT, frequency 7Hz; reversed every 20 minutes;

[0359] S4.3 Level 3 Smelting:

[0360] Finished ingot diameter: 880mm

[0361] Melting current: 34-42kA; Arc stabilization current: 35A;

[0362] Melting rate: 11.5 kg / min, decreasing to 7.5 kg / min at the end of the melting process (last 15% of the mass);

[0363] Magnetic field stirring: intensity 25mT; frequency 8Hz; reverse direction every 15 minutes.

[0364] S5: Post-processing and Inspection

[0365] S5.1 Composition analysis: Take three cross sections at the head, middle and tail of the axial direction. Take three points at the edge, 1 / 2R and center of each cross section, for a total of 9 points, to determine the O and Fe content and grain size. The O content test results are shown in Table 9, and the Fe content test results are shown in Table 10.

[0366] Table 9. O content test results (%) of the finished ingots from Example 3

[0367] Head section 0.394 0.400 0.405 Mid-section 0.387 0.393 0.398 Tail section 0.390 0.396 0.401

[0368] Table 10. Fe content test results (%) of the finished ingots from Example 3

[0369] Head section 0.191 0.196 0.201 Mid-section 0.188 0.193 0.198 Tail section 0.190 0.195 0.200

[0370] The O element content range is 0.018%, and the segregation coefficient is 1.046; the Fe element content range is 0.013%, and the segregation coefficient is 1.051; the grain size is ASTM 6.5 grade.

[0371] S5.2 Mechanical property testing: The ingot was forged, hot-rolled into Φ22mm bars, and after annealing, samples were taken to determine the tensile strength, yield strength and elongation after fracture in accordance with the national standard GB / T 228.1-2010. The test results are shown in Table 11.

[0372] Table 11. Mechanical property test results of the rolled bar stock (Φ22mm) from the finished ingot in Example 3.

[0373] Head edge 572 487 18.5 Head center 568 483 19.0 central edge 575 490 18.0 Central Center 570 485 18.5 Tail edge 569 484 19.0 Tail center 565 481 19.5

[0374] In the traditional process of preparing GR4 titanium alloy ingots using sponge titanium, the sponge titanium is not pretreated. Although oxygen and iron content compensation is performed during the preparation process, the specific compensation formula for the required oxygen and iron elements is not clearly defined. Oxygen and iron are supplemented based solely on experience. Oxygen is supplemented by uniformly sprinkling titanium dioxide into the extrusion electrode block, and iron is supplemented by uniformly adding pure iron into the electrode block through machining. Furthermore, the smelting parameters are not dynamically adjusted during the smelting process. The oxygen content range of 750mm ingots prepared by the traditional process is 0.05%-0.08%, the iron content range is 0.03%-0.05%, and the grain size is ASTM grade 3-4. The mechanical properties of the rolled bars fluctuate greatly, with tensile strength Rm of 550-580MPa, yield strength Rp0.2 of 470-510 MPa, and elongation after fracture A of 14%-18%.

[0375] The results of comparing the traditional process with those of Examples 1, 2, and 3 in terms of grain size and mechanical properties of hot-rolled bars are shown in Table 12.

[0376] Table 12 Comparison of ingot grain size and mechanical properties of rolled bars from traditional process and Examples 1, 2, and 3

[0377] Grain size / ASTM Level 6.5 Level 7 Level 6.5 Level 3-4 Tensile strength Rm / MPa 575-585 585-594 565-575 550-580 Yield strength Rp0.2 (MPa) 490-499 499-508 481-490 470-510 Elongation after fracture A / % 17.0-18.5 16.0-17.5 18.0-19.5 14-18

[0378] Compared with traditional processes, Examples 1, 2, and 3 show consistent oxygen and iron content in the ingots, and the rolled bars have Rm≥550MPa, Rp0.2≥480MPa, and A≥15%, meeting the requirements of ASTM F67 (pure titanium for surgical implants) for GR4. The comparative results fully demonstrate that the preparation method of a large-diameter GR4 titanium alloy ingot of the present invention can control the consistent oxygen and iron content in the ingot and stably prepare GR4 titanium alloy ingots with uniform composition, fine grains, and excellent mechanical properties within a large diameter range (750-900mm), with all indicators reaching advanced levels.

[0379] The above technical solutions illustrate the technical concept of the present invention, but should not be construed as limiting the scope of protection of the present invention. Any modifications or alterations made to the above technical solutions based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the scope of protection of the technical solutions of the present invention.

Claims

1. A method for preparing a large-diameter GR4 titanium alloy ingot, characterized in that, Includes the following steps: S1. Pre-treat the sponge titanium to remove impurities and obtain sponge titanium raw material with uniform oxygen and iron content; S2. Calculate the oxygen and iron supplementation amounts based on the oxygen and iron content of the sponge titanium raw material and the oxygen and iron content of the target ingot. S3. When pressing the electrode material, a radial and axial dual-gradient anti-segregation material feeding strategy is adopted. Oxygen and iron are added to the sponge titanium material, and a vacuum consumable electrode is obtained after pressing. S4. Perform multi-stage vacuum consumable arc melting and adjust the melting parameters according to the diameter of each ingot. In step S3, the radial and axial dual-gradient anti-segregation fabrication strategy is as follows: the radial gradient fabrication divides the electrode cross-section into a central region, a transition region, and an edge region, with compensation coefficients α_r for each region being 0.65-0.80, 1.0, and 1.3-1.5, respectively; the axial gradient fabrication divides the electrode length into a bottom end face region, a lower region, a middle region, an upper region, and a top end face region, with compensation coefficients β_h for each region being 1.3-1.5, 1.0-1.1, 1.0, 1.1-1.2, and 1.4-1.6, respectively. The specific fabric application method for the dual-gradient anti-segregation fabric is as follows: the electrode is divided into 15 segments: bottom end face center area, bottom end face transition area, bottom end face edge area, lower center area, lower transition area, lower edge area, middle center area, middle transition area, middle edge area, upper center area, upper transition area, upper edge area, top end face center area, top end face transition area, and top end face edge area. The comprehensive compensation coefficient γ = α_r × β_h for any segment, and the amount added to that segment = γ × average amount added, where the average amount added = total amount added ÷ sum of the comprehensive compensation coefficients γ of each segment.

2. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 1, characterized in that, The preprocessing in step S1 includes: S1.1 Particle size sieving: The sponge titanium was separated into three grades according to particle size using a vibrating screen: +12 mesh coarse particles, 12-30 mesh medium particles, and -30 mesh fine powder. The oxygen and iron content of each grade was tested separately. S1.2 Magnetic separation for iron removal: A permanent magnet drum separator with a strength of 5000-8000 Gauss is used to perform at least two magnetic separations on the sponge titanium; S1.3 Vacuum heat treatment: The sieved and magnetically separated sponge titanium is placed in a vacuum heat treatment furnace. Under the condition of vacuum degree ≤1.0×10⁻²Pa, the temperature is raised to 600-700℃ at 5-10℃ / min, held for 4-8 hours, and then cooled to below 100℃ in the furnace before being taken out. S1.4 Homogenization and Mixing: Based on the oxygen and iron content test results of sponge titanium of each particle size grade, the materials are prepared according to the target oxygen and iron content of the ingot and mixed evenly. Sampling is carried out at no less than 5 points, with the standard deviation of oxygen content ≤0.005% and the standard deviation of iron content ≤0.003%.

3. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 1, characterized in that, The calculation formulas for oxygen supplementation and iron supplementation in step S2 are as follows: m_O=(w_O^target - w_O^Ti)×M_Ti÷η_O In the formula, m_O is the oxygen supplementation amount; w_O^target is the target oxygen content; w_O^Ti is the measured background oxygen content of the sponge titanium raw material; M_Ti is the total mass of the electrode; and η_O is the oxygen recovery rate, which is taken as 0.70-0.

85. m_Fe=(w_Fe^target-w_Fe^Ti)×M_Ti In the formula, m_Fe is the amount of iron added; w_Fe^target is the target iron content; w_Fe^Ti is the background iron content of the sponge titanium raw material; and M_Ti is the total mass of the electrode.

4. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 1, characterized in that, The central area, transition area, and edge area are three concentric regions that divide the cross-section of the electrode mold according to the following ranges: central area: ≤30% R; transition area: 30% R < transition area ≤70% R. Edge region: The region >70% of R, where R is the radius of the electrode cross-section; The bottom end face area, lower area, middle area, upper area, and top end face area are five regions divided along the electrode length / height direction according to the following ranges: Bottom end face area: ≤5% L / H; Lower area: 5% L / H < Lower area ≤30% L / H; Middle area: 30% L / H < Middle area ≤70% L / H; Upper area: 70% L / H < Upper area ≤95% L / H; Top end face area: >95% L / H; L / H is the electrode length / height.

5. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 4, characterized in that, Oxygen supplementation uses TiO2 as an oxygen supplement and iron supplementation uses iron wire as an iron supplement. The total amount of oxygen supplement added is m_TiO2 = m_O ÷ w_O^TiO2, where w_O^TiO2 is the oxygen content in TiO2.

6. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 1, characterized in that, In step S4, the multi-stage vacuum self-consuming arc melting is a three-stage VAR melting, and the melting parameters are as follows: VAR1: First-level ingot diameter 500-650mm; melting current 18-28kA, melting rate 6-10kg / min; using alternating magnetic field, magnetic field strength 5-15mT, stirring frequency 2-8Hz, changing direction every 15-30 minutes; VAR2: Secondary ingot diameter 650-800mm; melting current 22-36kA, melting rate 7-12kg / min; alternating magnetic field with magnetic field strength 10-20mT, stirring frequency 3-10Hz, changing direction every 15-30 minutes; VAR3: Finished ingot diameter 750-900mm; melting current 28-42kA; melting rate 8-15kg / min; using an alternating magnetic field with a magnetic field strength of 15-25mT and a stirring frequency of 5-15Hz, changing direction every 15-30 minutes.

7. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 1, characterized in that, The preparation method further includes adding a grain refiner during electrode pressing in step S3. The grain refiner contains boron, and the amount of boron added is 0.005-0.015 wt%, which is added in the form of B-Ti master alloy. The grain refiner adopts a radial and axial dual-gradient anti-segregation distribution strategy and is added to the sponge titanium during electrode pressing.

8. The method for preparing a large-diameter GR4 titanium alloy ingot according to claim 7, characterized in that, The ingot diameter ranges from 750 to 900 mm, the oxygen content range is ≤0.03%, the iron content range is ≤0.02%, and the segregation coefficient is ≤1.08; the as-cast grain size of the prepared ingot is ≥ASTM grade 6.