Preparation method of NbTi alloy ingot for vacuum consumable melting

Abstract

The preparation method comprises the following steps: (1) selecting a high-purity titanium raw material, and preparing a titanium-based hollow ingot of which the density is greater than or equal to 99.9% and the total impurity content is less than or equal to 50ppm through an electron beam cold bed furnace; (2) an Nb rod with the purity larger than or equal to 99.9% and the hollow ingot are accurately assembled, it is ensured that the gap is smaller than or equal to 0.1 mm, the concentricity deviation is smaller than or equal to 0.1 mm, and a composite consumable electrode is formed; (3) performing vacuum consumable smelting for three times, and combining with a cooling process of more than or equal to 3 hours / stage; and (4) post-processing scalping, and sawing a riser to obtain a finished cast ingot. The method has the advantages that the deviation of main element components of the cast ingot is controlled within + / -0.5%, the smelting success rate is greatly increased, the comprehensive production cost is reduced, the method is used for industrial production of the high-purity NbTi alloy cast ingot in the fields of aerospace, superconducting magnets, medical instruments and the like, and the core process can be popularized to preparation of other titanium-based alloys.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of titanium-based alloy smelting process, and particularly relates to a preparation method of a NbTi alloy ingot for vacuum consumable smelting. BACKGROUND

[0002] The NbTi alloy is widely applied in high-end fields such as superconducting magnets, aero-engine parts, medical implant parts and the like due to excellent superconducting performance, mechanical properties and corrosion resistance. The vacuum consumable smelting is a core process for preparing the NbTi alloy ingot, and the structural design and preparation method of the consumable electrode directly determine the composition uniformity, purity and production cost of the ingot.

[0003] The existing NbTi alloy consumable electrode and ingot preparation method mainly has the following three types of technical defects: First, the sponge titanium splicing and welding type: the sponge titanium is pressed into a semicircular arc blank, spliced and then clamped to weld an Nb rod into a shape. However, due to the low density of the sponge titanium, usually ≤92%, pores and adsorbed gas are easily left in the sponge titanium, and the gas escaping during smelting causes the ingot to have porosity and pore defects; the splicing surface is prone to incomplete welding, slag inclusion and oxidation inclusion, forming a composition diffusion barrier, resulting in uneven mixing of Nb and Ti elements, and the composition segregation error of the ingot is ±2%~3%; and the sponge titanium has a high impurity content, the total impurity content is ≥100ppm, which directly affects the service reliability of the ingot.

[0004] Second, the pure titanium plate wrapping type: the electrode is prepared by wrapping an Nb rod with a pure titanium plate and then welding. However, the adhesion of the titanium plate to the Nb rod is poor, the gap can reach 0.5~1.0mm, the heat transfer is uneven at the initial stage of smelting, the Nb rod dissolution rate fluctuates greatly, and a local Nb enrichment area is easily formed; the weld is easily oxidized to form TiO2 inclusions during the welding process, which are left in the electrode and cause the inclusion defects of the ingot to exceed the standard; the electrode structure has poor stability, is prone to deformation and cracking due to thermal stress during vacuum consumable smelting, and the smelting success rate is only 75%~85%.

[0005] Third, the seamless titanium tube insertion type: the Nb rod is inserted into a seamless titanium tube to form a composite electrode. However, the titanium tube generally needs to be forged and perforated, has a low material yield, and has a long process; the titanium tube specification is limited by the existing pipe material standard, and it is difficult to adapt to various Nb rod sizes, and has poor universality; the assembly concentricity is difficult to control, the deviation is ≥0.3mm, the radial composition difference of the ingot is significant, and the raw material utilization rate is only 60%~70%.

[0006] Therefore, the above-mentioned traditional methods all have the problems of poor composition uniformity, high defect rate of the ingot, complex production process, high comprehensive cost, and cannot meet the high quality and low cost requirements of the NbTi alloy ingot in high-end fields. SUMMARY

[0007] The application aims at solving the problems of complex preparation process, high cost, poor component uniformity and high defect rate of the NbTi alloy ingot, and provides a preparation method of the NbTi alloy ingot for vacuum consumable smelting, which can meet the requirements of component uniformity, process stability and low cost, effectively removes high and low density inclusions, greatly reduces the defect rate, simplifies the complex process, shortens the production cycle, and improves the product performance and application range.

[0008] In order to achieve the above-mentioned purpose, the application adopts the following technical scheme: A preparation method of the NbTi alloy ingot for vacuum consumable smelting, which adopts an electron beam cold bed furnace to prepare a high-purity titanium-based hollow ingot, and directly uses the Nb rod as a consumable electrode after precise assembly, and obtains the NbTi alloy ingot through multi-stage vacuum consumable smelting, and the specific steps are as follows: S1, preparing a titanium-based hollow ingot: S11, selecting raw materials: selecting sponge titanium or titanium alloy return material with a purity of ≥99.7%, crushing to 5~30mm particle size, removing surface oxide scale through alkali washing and acid washing, and then removing high-density inclusions and impurities through magnetic separation and color selection; S12, adjusting the electron beam cold bed furnace: installing a special water-cooled coaxial copper crucible, and setting the gap D1 between the inner and outer crucibles to 250~280mm; S13, setting smelting parameters: dividing the crucible according to the circumferential angle, setting the scanning point number and the scanning time interval of adjacent points of each division, and adopting a line scanning mode; starting the smelting electron gun and the refining electron gun, controlling the acceleration voltage to 30~40kV, controlling the single gun power to 180~220kW, and pumping the vacuum degree in the furnace to 5×10 -4 ~1×10 -3 Pa; S14, ingot forming: after the titanium raw material is fully melted in the cold bed, the molten pool area no longer changes, and after reaching stability, the ingot is formed at a drawing speed of 5~15mm / min, the smelting rate is controlled to 700~800kg / h, and after the molten pool solidifies, the titanium-based hollow ingot with a wall thickness tolerance of ±0.5mm, a density of ≥99.9% and a total impurity content of ≤50ppm is obtained; S2, selecting and assembling the Nb rod: selecting a Nb rod with a purity of ≥99.9%, and designing the diameter of the Nb rod according to the inner diameter of the titanium-based hollow ingot, grinding the surface to remove the oxide layer, and inserting the Nb rod into the titanium-based hollow ingot to form a consumable electrode; S3, vacuum consumable smelting: S31, primary smelting: adopting a φ280~580mm crystallizer, controlling the vacuum degree before smelting to 5.0Pa, controlling the smelting voltage to 30~40V, controlling the current to 8~30kA, controlling the air leakage rate to ≤1.2Pa / min, and adopting direct current 3~18A for the arc current; after smelting, the cooling time is ≥3h, and the ingot is processed into a flat head. S32, secondary smelting: the ingot is loaded into a φ360~640mm crystallizer, the vacuum before melting is 2.0Pa, the smelting voltage is 30~45V, the current is 10~30kA, the air leakage rate is ≤1.0Pa / min, and the steady arc current is AC 5~20A; the cooling time after smelting is ≥4h, and the flat head is processed again; S33, third smelting: a φ440~720mm crystallizer is used, the vacuum before melting is 1.0Pa, the smelting voltage is 32~45V, the current is 8~30kA, the air leakage rate is ≤0.8Pa / min, and the steady arc current is AC 8~25A; a step-by-step input power reduction method is used, the steady arc period is kept at 5~300s, the cooling time after smelting is ≥5h, and the molten pool is stirred uniformly and fully. S4, post-processing: the ingot after the third smelting is taken out of the furnace, the oxidation layer is removed by surface peeling, and the finished NbTi alloy ingot is obtained after sawing the riser.

[0009] Further, in the step S12, the distance D2 from the electron beam outlet to the inner end face of the outer crucible is 1900~2100mm, and the distance D3 from the electron beam outlet to the outer end face of the inner crucible is 1700~1900mm.

[0010] Further, in the step S13, the crucible is evenly divided into eight equal parts according to the circumferential angle, each part has 25000~50000 scanning points, and the time interval between adjacent points is 0.02ms.

[0011] Further, in the step S14, the molten pool temperature is controlled at 1750~2000℃.

[0012] Further, in the step S14, the inner wall roughness Ra of the titanium-based hollow ingot obtained after the molten pool solidifies is ≤0.8μm.

[0013] Further, in the step S2, the size tolerance of the Nb rod is controlled at ±0.05mm, the assembly gap of the Nb rod inserted into the titanium-based hollow ingot is ≤0.1mm, the two ends are fixed by using a tool clamp, and the concentricity deviation is ≤0.1mm.

[0014] Further, in the step S3, the vacuum consumable smelting includes: S31, primary smelting: the crystallizer is φ360mm, the vacuum before melting is 5.0Pa, the voltage is 35V, the current is 18kA, the steady arc current is DC 10A, and the cooling time is 4h; S32, secondary smelting: the crystallizer is φ440mm, the vacuum before melting is 2.0Pa, the voltage is 38V, the current is 22kA, the steady arc current is AC 15A, and the cooling time is 5h; S33, third melting: vacuum 1.0 Pa before melting, voltage 40 V, current 20 kA, stable arc current AC 20 A, stable arc period 100 s, cooling time 6 h.

[0015] Further, in the step S3, the vacuum consumable melting comprises: S31, first melting: crystallizer φ580 mm, vacuum 5.0 Pa before melting, voltage 38 V, current 25 kA, stable arc current DC 15 A, cooling time 6 h; S32, second melting: crystallizer φ640 mm, vacuum 2.0 Pa before melting, voltage 42 V, current 28 kA, stable arc current AC 20 A, cooling time 6 h; S33, third melting: crystallizer φ720 mm, vacuum 1.0 Pa, voltage 45 V, current 26 kA, stable arc current AC 25 A, stable arc period 200 s, cooling time 8 h.

[0016] Further, in the step S1, the titanium-based hollow ingot is prepared by using sponge titanium with a purity of 99.8%, crushing to 10-20 mm, and adding after alkali washing and acid washing treatment into an electron beam cold bed furnace; the gap between the inner and outer crucibles is set to 270 mm, the electron beam scanning mode is line scanning, the number of equal division scanning points is 25000 points, the acceleration voltage is 35kV, the single gun power is 200kW, and the vacuum degree in the furnace is 3x10 -4 Pa; the molten pool temperature is controlled at 1850℃, and the ingot drawing speed is 8mm / min.

[0017] Further, in the step S1, the titanium-based hollow ingot is prepared by using sponge titanium with a purity of 99.8%, crushing to 10-20 mm, and adding after alkali washing and acid washing treatment into an electron beam cold bed furnace; the gap between the inner and outer crucibles is set to 270 mm, the electron beam scanning mode is line scanning, the number of equal division scanning points is 25000 points, the acceleration voltage is 35kV, the single gun power is 200kW, and the vacuum degree in the furnace is 3x10 -4 Pa; the molten pool temperature is controlled at 1850℃, and the ingot drawing speed is 8mm / min.

[0018] Compared with the prior art, the technical scheme of the present application has the following advantages: (1) The hollow ingot prepared by the electron beam cold bed furnace has high purity of the titanium-based matrix, the total impurity content is ≤50ppm, there is no porosity and gas residue, the close contact design with a combined assembly gap ≤0.1mm makes the diffusion of Nb and Ti elements sufficient, the main element composition deviation of the cast ingot is controlled within ±0.5%, which is much better than the ±2%~3% of the traditional method; in addition, the cold bed refining process can effectively remove high and low density inclusions, reduce the internal defect rate of the cast ingot, and greatly improve the composition uniformity and purity of the cast ingot; (2) The hollow ingot integrated forming of the application does not need welding, completely avoids welding defects and oxidation inclusion risks, has strong electrode structure integrity, and greatly improves the smelting success rate; the electron beam cold bed furnace can flexibly adjust the wall thickness and inner diameter of the hollow ingot, adapts to different specifications of Nb rods, meets the production demand of large specifications and small batch customized cast ingot of φ440-720mm, and significantly enhances the universality; (3) The method of the application simplifies complex processes such as pressing and welding, shortens the production cycle, and improves the utilization rate of titanium-based hollow ingot raw materials; at the same time, it reduces the rework and defect loss of smelting, reduces the comprehensive production cost, and has economic advantages for large-scale industrial application; (4) The cast ingot prepared by the application has uniform and fine structure, improves the consistency of mechanical properties such as strength, plasticity and toughness, and can meet the strict requirements of high-end fields such as superconducting magnets and aerospace on material performance; the core process idea can be extended to Ti-Mn, Ti-Ni and other titanium-based alloy cast ingot preparation, and has strong technology expansion. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a structure diagram of a vacuum consumable arc furnace in the prior art; Figure 2 is a radial cross-sectional view of the consumable electrode of the application; Figure 3 is an axial cross-sectional view of the consumable electrode of the application. DETAILED DESCRIPTION Example 1

[0020] In order to make the application clearer, the preparation method of the NbTi alloy cast ingot for vacuum consumable smelting of the application will be further described below in combination with the drawings, and the specific embodiments described herein are only used to explain the application and not to limit the application.

[0021] Referring to Figure 1 is a structure diagram of a traditional vacuum consumable arc furnace, which indicates the position and working principle of the consumable electrode in the application, and is mainly composed of an electrode rod 1, a vacuum system 2, a furnace body 3, an auxiliary electrode 4, a consumable electrode 5, a crucible 6, an electric arc 7, a cast ingot 8 and other components.

[0022] When the device is powered on, the consumable electrode 5 will be melted and solidified from bottom to top under the action of the electric arc 7, and the metal cast ingot 8 is obtained in the crucible 6.

[0023] Referring to Figure 2 The middle part of the consumable electrode of the application is inserted into the Nb rod, and the periphery is a hollow Ti ingot, which shows the complete electrode structure in detail.

[0024] In this embodiment, taking the preparation of a φ440mm NbTi alloy cast ingot as an example, the specific steps are as follows: (I) Preparation of titanium-based hollow ingot: sponge titanium with a purity of 99.8% is crushed to 10-20 mm, and after alkali washing and acid washing treatment, it is added into the electron beam cold bed furnace; the gap between the inner and outer crucibles is set to 270 mm, the electron beam scanning mode is line scanning, the number of equal division scanning points is 25000 points, the acceleration voltage is 35 kV, the single gun power is 200 kW, and the vacuum degree in the furnace is 3x10 -4 Pa; the molten pool temperature is controlled at 1850℃, the ingot drawing speed is 8 mm / min, and a titanium-based hollow ingot with an inner diameter of φ120 mm and a wall thickness of 20 mm is obtained.

[0025] (II) Nb rod assembly: an Nb rod with a diameter of φ119.8 mm and a purity of 99.95% is selected, and after being inserted into the hollow ingot, the assembly gap is 0.08 mm, and the concentricity deviation is 0.05 mm.

[0026] (III) Vacuum consumable melting: Primary melting: the crystallizer is φ360 mm, the pre-melting vacuum is 5.0 Pa, the voltage is 35 V, the current is 18 kA, the steady arc current is direct current 10 A, and the cooling time is 4 h; Secondary melting: the crystallizer is φ440 mm, the pre-melting vacuum is 2.0 Pa, the voltage is 38 V, the current is 22 kA, the steady arc current is alternating current 15 A, and the cooling time is 5 h; Tertiary melting: the pre-melting vacuum is 1.0 Pa, the voltage is 40 V, the current is 20 kA, the steady arc current is alternating current 20 A, the steady arc period is 100 s, and the cooling time is 6 h.

[0027] (IV) Post-processing: the skin is removed to remove the surface 2 mm oxide layer, and the 100 mm sprue is sawn, to obtain a φ440 mm x 2000 mm NbTi alloy ingot. After detection, the Nb element content distribution of the ingot is 40.8%-42.0%, the Ti element content is 58.0%-59.2%, the total impurity content is ≤45 ppm, there is no pore and inclusion defects. Example 2

[0028] In this embodiment, the preparation of a φ720 mm NbTi alloy ingot is taken as an example, and the specific steps are as follows: (I) Preparation of titanium-based hollow ingot: sponge titanium and titanium alloy return material are used, and the mass ratio of the two is 7:3, which is crushed to 15-30 mm; the gap between the inner and outer crucibles in the electron beam cold bed furnace is 280 mm, the scanning point number is 30000 points, the acceleration voltage is 40 kV, the single gun power is 220 kW, and the vacuum degree is 1x10 -4 Pa; the molten pool temperature is 1950℃, the ingot drawing speed is 6 mm / min, and a titanium-based hollow ingot with an inner diameter of φ200 mm and a wall thickness of 25 mm is prepared.

[0029] (II) Nb rod assembly: an Nb rod with a diameter of φ199.9 mm and a purity of 99.95% is selected, and the assembly gap is 0.06 mm, and the concentricity deviation is 0.08 mm.

[0030] (Three) vacuum consumable melting: Primary melting: crystallizer φ580mm, vacuum before melting 5.0Pa, voltage 38V, current 25kA, steady arc current direct current 15A, cooling time 6h; Secondary melting: crystallizer φ640mm, vacuum before melting 2.0Pa, voltage 42V, current 28kA, steady arc current alternating current 20A, cooling time 6h; Third melting: crystallizer φ720mm, vacuum 1.0Pa, voltage 45V, current 26kA, steady arc current alternating current 25A, steady arc period 200s, cooling time 8h.

[0031] (Four) post-processing: skinning 3mm, sawing 150mm riser, obtaining φ720mm×2500mm NbTi alloy ingot. The test results show that the composition deviation of the ingot is ±0.4%, the mechanical property consistency is improved by 22%, and the material meets the requirements of large superconducting magnet.

[0032] In addition to the above embodiments, the present application can also have other implementation manners. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present application.

Claims

1. A method for preparing a NbTi alloy ingot for vacuum consumable melting, comprising the following steps: S1, preparing a titanium-based hollow ingot: S11, selecting raw materials: selecting sponge titanium or titanium alloy return material with a purity of ≥ 99.7%, crushing to 5-30 mm particle size, removing surface oxide scale through alkali washing and acid washing, and then removing high-density inclusions and impurities through magnetic separation and color selection; S12, adjusting the electron beam cold hearth furnace: installing a special water-cooled coaxial copper crucible, and setting the gap D1 between the inner and outer crucibles to 250-280 mm; S14, ingot forming: after the titanium raw material is fully melted in the cold hearth, the molten pool area no longer changes, and reaches a stable state, the ingot is formed at a drawing speed of 5-15 mm / min, the melting rate is controlled at 700-800 kg / h, and after the molten pool solidifies, a titanium-based hollow ingot with a wall thickness tolerance of ±0.5 mm, a density of ≥ 99.9%, and a total impurity content of ≤ 50 ppm is obtained; S2, selecting and assembling Nb rods: selecting Nb rods with a purity of ≥ 99.9%, the diameter of which is designed according to the inner diameter of the titanium-based hollow ingot, and the surface is ground to remove the oxide layer, the Nb rods are inserted coaxially into the titanium-based hollow ingot to form a consumable electrode; S3, vacuum consumable melting: S31, primary melting: using a φ280-580 mm crystallizer, the vacuum degree before melting is controlled at 5.0 Pa, the melting voltage is 30-40 V, the current is 8-30 kA, the gas leakage rate is ≤ 1.2 Pa / min, and the steady arc current is direct current 3-18 A; after melting, the cooling time is ≥ 3 h, and the ingot is processed to be flat; S32, secondary melting: the ingot is inverted and loaded into a φ360-640 mm crystallizer, the vacuum degree before melting is 2.0 Pa, the melting voltage is 30-45 V, the current is 10-30 kA, the gas leakage rate is ≤ 1.0 Pa / min, and the steady arc current is alternating current 5-20 A; after melting, the cooling time is ≥ 4 h, and the ingot is processed to be flat again; S33, tertiary melting: using a φ440-720 mm crystallizer, the vacuum degree before melting is 1.0 Pa, the melting voltage is 32-45 V, the current is 8-30 kA, the gas leakage rate is ≤ 0.8 Pa / min, and the steady arc current is alternating current 8-25 A; the input power is gradually reduced, the steady arc period is kept at 5-300 s, the cooling time after melting is ≥ 5 h, and the molten pool is uniformly stirred and fully compensated; S4, post-processing: after the ingot after the third melting is taken out of the furnace, the oxide layer is removed by surface scaling, and the finished NbTi alloy ingot is obtained after sawing the riser.

2. The method for preparing a NbTi alloy ingot for vacuum consumable melting according to claim 1, wherein: in the step S12, the distance D2 from the electron beam outlet to the inner end face of the outer layer crucible is 1900-2100 mm, and the distance D3 from the electron beam outlet to the outer end face of the inner layer crucible is 1700-1900 mm.

3. The method for preparing a NbTi alloy ingot for vacuum consumable melting according to claim 1 or 2, wherein: ​ S13, setting smelting parameters: the crucible is equally divided by circumferential angle, and the number of scanning points and the time interval of adjacent points scanning are set for each division, and a line scanning mode is adopted; the smelting electron gun and the refining electron gun are started, the acceleration voltage is 30~40kV, the single gun power is controlled in 180~220kW, and the vacuum degree in the furnace is extracted to 5×10 -4 ~1×10 -3 Pa; ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ In the step S13, the crucible is evenly divided into eight equal parts by a circumferential angle, and 25000-50000 points are scanned in each part, and the time interval between adjacent points is 0.02ms.

4. The method according to claim 1 or 2, characterized in that: In the step S14, the temperature of the molten pool is controlled at 1750-2000℃.

5. The method according to claim 1 or 2, characterized in that: In the step S14, the inner wall roughness Ra of the titanium-based hollow ingot obtained after solidification of the molten pool is ≤0.8μm.

6. The method according to claim 1 or 2, characterized in that: In the step S2, the size tolerance of the Nb rod is controlled at ±0.05mm, the assembly gap of the Nb rod after being inserted into the titanium-based hollow ingot is ≤0.1mm, and the two ends are fixed by using a tool clamp to ensure that the concentricity deviation is ≤0.1mm.

7. The method according to claim 1 or 2, characterized in that: In the step S3, the vacuum consumable melting includes: S31, primary melting: the crystallizer is φ360mm, the pre-melting vacuum is 5.0Pa, the voltage is 35V, the current is 18kA, the steady arc current is direct current 10A, and the cooling time is 4h; S32, secondary melting: the crystallizer is φ440mm, the pre-melting vacuum is 2.0Pa, the voltage is 38V, the current is 22kA, the steady arc current is alternating current 15A, and the cooling time is 5h; S33, tertiary melting: the pre-melting vacuum is 1.0Pa, the voltage is 40V, the current is 20kA, the steady arc current is alternating current 20A, the steady arc period is 100s, and the cooling time is 6h.

8. The method according to claim 1 or 2, characterized in that: In the step S3, the vacuum consumable melting includes: S31, primary melting: the crystallizer is φ580mm, the pre-melting vacuum is 5.0Pa, the voltage is 38V, the current is 25kA, the steady arc current is direct current 15A, and the cooling time is 6h; S32, secondary melting: the crystallizer is φ640mm, the pre-melting vacuum is 2.0Pa, the voltage is 42V, the current is 28kA, the steady arc current is alternating current 20A, and the cooling time is 6h; S33, tertiary melting: the crystallizer is φ720mm, the vacuum is 1.0Pa, the voltage is 45V, the current is 26kA, the steady arc current is alternating current 25A, the steady arc period is 200s, and the cooling time is 8h.

9. The method according to claim 1 or 2, characterized in that: In the step S1, the titanium-based hollow ingot is prepared by crushing the sponge titanium with a purity of 99.8% to 10-20 mm, adding the sponge titanium into the electron beam cold hearth furnace after alkali washing and acid washing treatment; the gap between the inner and outer crucibles is set to 270 mm, the electron beam scanning mode is linear scanning, the number of equal division scanning points is 25000 points; the acceleration voltage is 35 kV, the single gun power is 200 kW, and the vacuum degree in the furnace is 3×10 -4 Pa; the temperature of the molten pool is controlled at 1850°C, and the ingot drawing speed is 8 mm / min.

10. The method according to claim 1 or 2, characterized in that: In the step S1, the titanium-based hollow ingot is prepared by using sponge titanium and titanium alloy return material, and the mass ratio of the two is 7:3, which is crushed to 15-30 mm; the gap between the inner and outer crucibles of the electron beam cold bed furnace is 280 mm, the scanning point number is 30000 points, the acceleration voltage is 40 kV, the single gun power is 220 kW, and the vacuum degree is 1×10 -4 Pa; the molten pool temperature is 1950 ℃, and the ingot pulling speed is 6 mm / min.

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