Nb-Y-Ti alloy plate, preparation method and battery shell

Abstract

The application discloses a niobium-yttrium-titanium alloy plate, a preparation method and a battery shell. Through a deep cold rolling and plasma-assisted surface nanocrystallization synergistic process, the titanium alloy plate forms a three-layer microstructure with a continuous gradient change in grain size and hardness in the thickness direction. The ultra-thin-wall titanium alloy plate realizes high strength, high plasticity, corrosion resistance and welding performance, and is suitable for the shell manufacturing of 3C product batteries.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy materials and battery casing manufacturing technology, and particularly to a niobium-yttrium titanium alloy sheet, its preparation method, and its battery casing. Background Technology

[0002] As 3C products (such as smartphones, tablets, and smartwatches) develop towards thinner, lighter designs and higher energy densities, extremely stringent comprehensive performance requirements are placed on battery casing materials. Battery casings need to simultaneously possess the following characteristics under ultra-thin wall thicknesses: high strength (to prevent bulging and deformation), high plasticity (to meet deep-drawing requirements), excellent corrosion resistance (to resist electrolyte corrosion), good weldability (to ensure sealing), and extreme lightweight (to improve energy density).

[0003] In the existing technology, although stainless steel (such as SUS316L) has high strength and good corrosion resistance, it has high density, low specific strength, low plastic strain ratio r value, poor deep drawing performance, and is difficult to manufacture deep cylindrical ultra-thin wall shells.

[0004] Although industrial pure titanium (such as TA1 / TA2) has low density and excellent corrosion resistance, it has the following technical drawbacks:

[0005] (1) Strength-plasticity contradiction: Industrial pure titanium has low strength and a very low work hardening index n value, resulting in poor bulging forming performance and easy local thinning and cracking when forming complex shapes;

[0006] (2) Insufficient weldability: Pure titanium easily absorbs hydrogen, oxygen and nitrogen gases during welding, forming pores and brittle phases, and the plasticity of the welded joint decreases significantly;

[0007] (3) Weak surface properties: Pure titanium has low surface hardness (HV180~220), poor wear resistance, and the surface oxide film is easily damaged under extreme working conditions.

[0008] To improve the properties of titanium, existing technologies attempt to add alloying elements to form titanium alloys. However, while conventional titanium alloys have high strength, their plasticity is significantly reduced (elongation after fracture is usually <15%), and their cold working performance is poor, making it difficult to produce ultra-thin sheets. In addition, surface modification techniques (such as shot peening and ion implantation) can form a hardened layer on the surface of titanium, but there are problems such as abrupt changes in the interface between the hardened layer and the substrate, poor adhesion, and easy peeling.

[0009] Therefore, it is necessary to improve the existing technology to overcome the aforementioned defects. Summary of the Invention

[0010] The purpose of this invention is to provide a niobium-yttrium titanium alloy plate, a preparation method, and a battery casing to improve the overall performance of the material.

[0011] To achieve the above-mentioned objective, in a first aspect, the present invention provides a niobium-yttrium titanium alloy sheet, which is prepared from a niobium-yttrium titanium alloy material with the following composition, expressed as a percentage by mass:

[0012] 0.08%~0.25% niobium;

[0013] 0.01%~0.08% yttrium;

[0014] 0.01%~0.04% iron;

[0015] Carbon content not exceeding 0.01%;

[0016] Nitrogen content not exceeding 0.01%;

[0017] Hydrogen content not exceeding 0.005%;

[0018] 0.05%~0.20% oxygen;

[0019] Not less than 99.5% titanium;

[0020] The titanium alloy sheet forms a three-layer microstructure with a continuous gradient in grain size and hardness along its thickness direction. The structure of each layer is as follows:

[0021] (1) Surface nanocrystalline layer: The surface layers on both sides of the thickness direction of the plate, each layer has a depth of 2~8μm from the surface of the plate inward; this layer is a nanocrystalline α-Ti structure with an average grain size of 50~200nm and a Vickers hardness of HV380~480.

[0022] (2) Intermediate ultrafine crystalline layer: connected to the two surface nanocrystalline layers, at 8~60μm inward from the surface nanocrystalline layer; it is an ultrafine α-Ti structure with an average grain size of 0.5~3μm, and Nb-Y enriched phase and Ti-C composite precipitate dispersed in the crystal. The Vickers hardness of this layer is HV280~360.

[0023] (3) Core layer: Located in the central region of the thickness direction of the plate; This layer is mainly based on equiaxed α-Ti phase, with an average grain size of 5~10μm and a hardness of HV200~250, forming a continuous hardness transition with the intermediate ultrafine grain layer.

[0024] The total thickness of the plate is 0.03mm to 0.5mm, and along the thickness direction of the plate, from the surface nanocrystalline layer to the core layer, the grain size gradually increases and the hardness gradually decreases, showing a continuous transition without abrupt interfaces.

[0025] Furthermore, the Nb-Y enriched phase is an intermetallic compound formed by Nb and Y, with a size of 20-80 nm; the Ti-C composite precipitate includes at least one of TiC and Ti-C solid solution, with a size of 10-40 nm.

[0026] Furthermore, the titanium alloy sheet has a tensile strength of 480~580MPa, a yield strength Rp0.2 of 380~480MPa, an elongation after fracture of ≥30%, a plastic strain ratio r of ≥3.5, an ultimate drawing ratio LDR of ≥4.0, and a work hardening index n of ≥0.18.

[0027] In a second aspect, the present invention provides a method for preparing the niobium-yttrium titanium alloy sheet described in the first aspect, which controls the thickness gradient structure of the sheet through a synergistic process of cryogenic rolling and plasma-assisted surface nano-sizing, comprising the following steps:

[0028] (1) Alloy melting and homogenization treatment: The raw material powder is mixed and then melted in vacuum self-consuming electric arc to obtain an ingot. The ingot is homogenized at 900~1000℃ for 4~8h and then air-cooled to obtain an alloy ingot with uniform composition.

[0029] (2) Thermomechanical deformation: The ingot obtained in step (1) is forged at 800~900℃ to form a billet, and then hot rolled at 700~800℃ in multiple passes to a thickness of 3~5mm; then recrystallization annealing is carried out at 550~650℃ for 1~2h to obtain an intermediate billet with a uniform equiaxed structure.

[0030] (3) Deep cryogenic rolling: The intermediate billet obtained in step (2) is subjected to deep cryogenic rolling in a liquid nitrogen environment of -150℃ to -196℃ for multiple passes to the total thickness of the target plate. The single pass reduction rate is 10% to 25%, the total reduction rate is 85% to 95%, the rolling speed is 5 to 20 m / min, and the roll temperature is controlled at -100℃ to -150℃. By suppressing dynamic recovery at low temperature, high dislocation density is accumulated to form a gradient structure that gradually changes along the thickness direction.

[0031] (4) Plasma-assisted surface nanostructuring: The thin plate obtained in step (3) is subjected to high-density plasma bombardment treatment, using pure argon or helium as the working gas, with a gas flow rate of 20~40 L / min and a plasma power density of 500~1500 W / cm². 2 The processing time is 10~30min and the processing temperature is ≤150℃. High-energy plasma particle bombardment induces severe plastic deformation of the surface layer, further refining the surface grains to the nanoscale. At the same time, it promotes the segregation of Nb and Y atoms to the grain boundaries to form nano-precipitates, and finally obtains the target titanium alloy plate with a continuous gradient of grain size and hardness along the thickness direction.

[0032] Furthermore, in the deep cryogenic rolling process described in step (3), the billet is immersed in liquid nitrogen for 15 to 30 minutes before each rolling pass to ensure that the billet temperature is uniformly reduced to below -150°C; during the rolling process, liquid nitrogen is directly injected to cool the contact area between the roll and the billet to maintain a low temperature environment.

[0033] Furthermore, after step (4), the surface roughness Ra of the titanium alloy plate is ≤0.2μm, the thickness of the surface nanocrystalline layer is 2~8μm, the width of the hardness transition zone between the surface nanocrystalline layer and the intermediate ultrafine crystalline layer is ≤5μm, and there is no obvious interface abrupt change.

[0034] Furthermore, the cryogenically rolled sheet obtained in step (3) was subjected to non-destructive testing and found to have no internal cracks or pores, no coarse second-phase particles in the crystals, and a matrix grain size uniformity coefficient ≥0.90.

[0035] Thirdly, the present invention provides a battery casing, which is made of the titanium alloy material described in the first aspect, or made of the niobium-yttrium titanium alloy plate described in the second aspect, or made of the niobium-yttrium titanium alloy plate prepared by the method described in the second aspect.

[0036] Furthermore, the housing is cylindrical with one end open; or, the housing is annular with both ends open.

[0037] Furthermore, the wall thickness of the shell is 0.03mm to 0.5mm.

[0038] Furthermore, the helium leak detection rate of the casing is ≤1×10⁻⁶. -10 Pa·m 3 The corrosion rate in LiPF6 / EC+DMC electrolyte at 60℃ is ≤0.0015mm / year, and there are no pitting corrosion, intergranular corrosion, or stress corrosion cracking on the surface.

[0039] Furthermore, after the shell is cyclically tested 3000 times within a temperature range of -40℃ to 150℃, there are no cracks, leaks, or bulges, and the shell burst pressure is ≥3.5MPa.

[0040] Furthermore, the shell is not subjected to stress-relief annealing after welding, or it is subjected to low-temperature stress-relief annealing at 180~220℃ for 0.5~1h. After annealing, the deformation of the shell is ≤0.03%, and the hardness reduction in the weld area is ≤10%. This shell is suitable for the preparation of 3C battery shells.

[0041] Titanium alloy materials contain the above-mentioned mass percentages of niobium and yttrium, and limit the content of other components (iron, carbon, nitrogen, hydrogen, and oxygen). The appropriate ratio of each component is beneficial for titanium alloy materials to have the excellent properties of adding niobium and yttrium.

[0042] In a first aspect, the present invention achieves gradient structure strengthening through a synergistic process of cryogenic rolling and plasma-assisted surface nano-sizing, resulting in the following technical effects:

[0043] 1. Synergistic improvement of strength and plasticity

[0044] Cryogenic rolling is performed at temperatures ranging from -150℃ to -196℃, which suppresses the dynamic recovery process, accumulates high dislocation density, and achieves a synergistic effect of fine-grain strengthening and work hardening. The surface nanocrystalline layer (HV380~480) provides high surface strength, while the core layer (HV200~250) maintains coarse-grain toughness. The overall tensile strength is increased to 480~580MPa, the yield strength is increased to 380~480MPa, and the elongation after fracture is maintained at ≥30%, achieving an excellent match between high strength and high plasticity.

[0045] 2. Significantly optimized surface properties

[0046] Plasma-assisted surface nanostructuring refines the surface grains to 50–200 nm, forming a surface nanocrystalline layer with a thickness of 2–8 μm. This layer exhibits a 90%–140% increase in hardness compared to the core, significantly improving surface wear resistance and resistance to fatigue crack initiation. The surface roughness Ra ≤ 0.2 μm meets the stringent surface quality requirements of precision electronic devices.

[0047] 3. Forming properties have been further improved.

[0048] Deep cold rolling promotes the development of texture on the {0001} base surface, increasing the plastic strain ratio r from ≥3.0 to ≥3.5 and the ultimate drawing ratio LDR from ≥3.5 to ≥4.0. The work hardening index n increases from ≥0.1 to ≥0.18, enhancing strain dispersion and preventing local necking and cracking during deep drawing, thus adapting to precision machining of ultra-thin specifications (0.03~0.5mm).

[0049] 4. Synergistic effect of gradient structure

[0050] The grain size gradually changes from 50-200 nm on the surface to 5-10 μm in the core along the thickness direction, and the hardness gradually changes from HV380-480 to HV200-250, with a transition zone width ≤5 μm and no abrupt interface. This design avoids the interface stress concentration problem of traditional surface strengthening layers, achieves gradient load transfer during service, and improves the overall structural reliability.

[0051] 5. Strengthening and stabilizing effects of nano-precipitated phases

[0052] The Nb-Y enriched phase (20~80nm) and Ti-C composite precipitates (10~40nm) dispersed within the intermediate ultrafine crystalline layer pin grain boundaries via the Orowan mechanism, inhibiting grain growth at high temperatures. This feature enables the substrate to maintain microstructural stability under low-temperature annealing at 180~220℃ or temperature cycling at -40℃~150℃, meeting the thermal management requirements of the battery casing.

[0053] 6. Maintaining the performance of welded joints

[0054] The high dislocation density and nano-precipitates pre-stored in the gradient structure effectively suppress grain coarsening in the weld heat-affected zone. The hardness reduction in the weld area after welding is ≤10%, far superior to the reduction of more than 20% for conventional titanium alloys, ensuring that the joint strength matches the base material. The deformation after welding is ≤0.03%, meeting the dimensional tolerance requirements of high-precision battery casings.

[0055] 7. Cumulative improvement in corrosion resistance

[0056] The surface nanocrystalline layer promotes the rapid formation of a dense TiO2 film, and the nanograin boundaries provide numerous rapid diffusion channels, accelerating the self-repair of the passivation film; the Nb-Y enriched phase in the intermediate layer optimizes the chemical stability of the grain boundaries. The synergistic effect results in a corrosion rate of ≤0.0015 mm / year in LiPF6 / EC+DMC electrolyte at 60℃, which is further reduced compared to the untreated alloy.

[0057] The principle of gradient layer formation in this invention is as follows:

[0058] (1) Deep cryogenic rolling stage—constructing pre-gradient microstructure:

[0059] During rolling at cryogenic temperatures (-150℃ to -196℃), the dislocation mobility is significantly reduced and dynamic recovery is substantially suppressed because the temperature is much lower than the dynamic recovery temperature of titanium (~0.3Tm≈450℃). The large number of dislocations introduced by rolling deformation remain stable and accumulate at low temperatures, forming a high dislocation density microstructure. Simultaneously, the deformation mechanism at low temperatures shifts from slip-dominated at room temperature to a slip-twin synergy, activating multiple twinning systems, refining grains, and introducing orientation gradients.

[0060] Because the surface shear deformation is greater than the central layer plane strain deformation during rolling, a natural deformation gradient is formed along the thickness direction of the sheet: the surface layer has the largest deformation (most significant grain refinement), which gradually decreases towards the center. This forms the basis for the pre-organized gradient structure.

[0061] (2) Plasma-assisted surface nanostructuring stage—strengthening the surface layer and regulating the precipitated phase:

[0062] The cryogenically rolled sheet already possesses a grain size gradient along the thickness direction, but the surface grains are only refined to the submicron level (0.5~3μm). Through high-density plasma bombardment, high-energy particles (energy 50~200eV) induce intense plastic deformation within 10μm of the surface layer, further refining the surface grains to the nanometer level (50~200nm) through a dislocation multiplication-annihilation-rearrangement mechanism.

[0063] Simultaneously, the localized high-temperature-rapid-cooling effect generated by plasma bombardment (instantaneous surface temperature rise ≤80℃, but with a very large temperature gradient) and the surge in vacancy / dislocation density promote the rapid diffusion and segregation of Nb and Y atoms towards the grain boundaries, forming nanoscale Nb-Y enriched phases (such as intermetallic compounds like NbY and Nb4Y, with sizes of 20~80 nm). Furthermore, C atoms in the matrix combine with Ti to form TiC and Ti-C solid solutions (sizes of 10~40 nm), which are dispersed throughout the ultrafine grain layers.

[0064] These nanoprecipitates hinder dislocation movement and grain growth, stabilize the ultrafine / nanocrystalline structure, and further enhance hardness.

[0065] (3) Synergistic effect of gradient structure—solving the core contradiction of ultrathin wall panels:

[0066] Through the aforementioned synergistic process, this invention achieves a gradient performance distribution of "hard surface and tough core," specifically addressing the technical challenges of ultra-thin-walled titanium alloy sheets.

[0067] Unlike traditional approaches such as "homogeneous reinforcement" or "surface coating", this invention optimizes the spatial distribution of performance within the same material system by gradient microstructure in the thickness direction, avoiding heterogeneous interface problems and ensuring structural integrity and reliability under ultra-thin wall conditions (0.03~0.5mm).

[0068] The table below shows a comparison of the key mechanical properties of titanium alloy materials with added niobium and yttrium in some embodiments with industrial pure titanium TA1 / TA2, as well as stainless steel 304 and stainless steel 316L.

[0069]

[0070] As can be seen from the table above:

[0071] 1. In terms of tensile strength and yield strength, the titanium alloy sheet of the present invention has improved performance compared with industrial pure titanium.

[0072] 2. In terms of elongation after fracture, stainless steel has better plasticity (total deformation capacity) than titanium materials (including industrial pure titanium and titanium alloy materials). Traditional industrial pure titanium has a low elongation after fracture. The titanium alloy plate of this invention achieves a significant improvement in elongation after fracture.

[0073] 3. In terms of work hardening index, stainless steel is significantly superior to titanium. Industrial pure titanium has the lowest work hardening index. The work hardening index is a key factor in the forming performance of a material, and it is related to the "bulging" performance of the product. A higher work hardening index generally indicates more uniform deformation, stronger resistance to necking, and greater suitability for complex stretching. Compared to industrial pure titanium, the titanium alloy sheet of this invention achieves an improved work hardening index through metallurgical optimization, enabling it to undergo complex stretching and improving its bulging performance.

[0074] 4. Regarding the plastic strain ratio, the titanium alloy sheet of this invention is significantly higher than that of stainless steel. The plastic strain ratio is another key factor in the material's formability. A higher plastic strain ratio indicates stronger resistance to thickness reduction, allowing the material to be stretched further. During stretching, the material flows more easily from the flange, making it suitable for manufacturing deeper cylindrical parts. A lower plastic strain ratio indicates generally poor deep-drawing performance, with a higher risk of tearing at critical fracture surfaces. Compared to industrial pure titanium, the overall plastic strain ratio of the titanium alloy sheet of this invention is more stable.

[0075] 5. In terms of ultimate tensile ratio, the titanium alloy sheet of this invention has the highest ratio, followed by industrial pure titanium, and the stainless steel has the lowest ratio. This indicates that titanium alloy materials can be formed into deeper cylindrical parts in one step, which is the most direct advantage in manufacturing products such as battery cases.

[0076] In summary, the titanium alloy sheet of this invention exhibits excellent comprehensive mechanical properties, with improved elongation after fracture and n-value (work hardening index), significantly enhancing the material's uniform deformation capability and resistance to localized necking, making it more reliable in complex forming processes. Furthermore, the titanium alloy sheet of this invention retains an extremely high r-value (plastic strain ratio), preserving its superior deep-drawing performance.

[0077] The present invention provides a method for cryogenic rolling of titanium alloy and plasma-assisted preparation of sheet metal, which improves the performance of titanium alloy sheet metal. The titanium alloy sheet metal has a tensile strength of 480~580MPa, a yield strength Rp0.2 of 380~480MPa, an elongation after fracture ≥30%, a plastic strain ratio r value ≥3.5, an ultimate drawing ratio LDR ≥4.0, and a work hardening index n value ≥0.18. Attached Figure Description

[0078] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0079] Figure 1 EBSD test image of the transition region between the intermediate ultrafine crystalline layer and the core layer;

[0080] Figure 2 The relationship between hardness and thickness in Example 1;

[0081] Figure 3 SEM surface morphology of the surface nanocrystalline layer;

[0082] Figure 4 Metallographic micrograph of a localized area of ​​the intermediate ultrafine crystalline layer. Detailed Implementation

[0083] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, specific embodiments of this application are described in detail. It is understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0084] Example 1

[0085] Ingredients by mass percentage: 0.15% niobium, 0.06% yttrium, 0.02% iron, 0.008% carbon, 0.008% nitrogen, 0.003% hydrogen, 0.12% oxygen, with the balance being titanium.

[0086] Preparation process:

[0087] The raw materials are made into ingots. Then, they are homogenized at 950℃ for 6 hours and air-cooled. Next, thermomechanical deformation is performed: the billet is forged to a thickness of 60mm at 850℃, then hot-rolled to a thickness of 4mm at 750℃, then recrystallized and annealed at 600℃ for 1.5 hours, and finally pickled to remove the oxide scale, resulting in an intermediate billet.

[0088] Based on the intermediate billet prepared above, deep cryogenic rolling is performed:

[0089] The rolling temperature is controlled at -180℃, and the billet is immersed in liquid nitrogen for 25 minutes. The roll temperature is controlled at -120℃, and direct liquid nitrogen injection cooling is used. A total of 8 rolling passes are performed, with a single pass reduction of 15% to 20%, and a cumulative reduction of 90%. The rolling speed is 12 meters per minute. The finished product thickness is 0.4 mm.

[0090] Thin sheets after cryogenic rolling undergo plasma treatment:

[0091] The power density was 1000 W / cm², the argon flow rate was 30 liters per minute, the treatment time was 25 minutes, and the temperature did not exceed 140℃. After treatment, the surface grain size was 65 nanometers, and the nanolayer thickness was 5 micrometers.

[0092] The surface hardness of the sheet is HV 465. The hardness at a depth of 2 micrometers is HV 448. The hardness at a depth of 5 micrometers is HV 395. The hardness at a depth of 10 micrometers is HV 340. The hardness at a depth of 20 micrometers is HV 285. The hardness at a depth of 40 micrometers is HV 245. The hardness at a depth of 80 micrometers is HV 230. The core (depth 200 micrometers) has a hardness of HV 228. See also... Figure 2 The thickness of each layer is determined by combining hardness and thickness with the grain detection results of the surface nanocrystalline layer, the intermediate ultrafine crystalline layer and the core layer. It is understandable that the thickness range of each layer is estimated and determined by combining hardness with microscopic detection, and there is no clear single boundary.

[0093] The surface nanocrystalline layer is located on the surface of the plate up to a depth of 5 micrometers, with an average grain size of 85 nanometers and an average Vickers hardness (HV) of 425.

[0094] The intermediate ultrafine crystalline layer is connected to the surface nanocrystalline layer, with a depth range of 5 to 45 micrometers (i.e., a layer thickness of 40 micrometers), an average grain size of 1.2 micrometers, and an average Vickers hardness (HV) of 325.

[0095] The core layer is located in the center region of the thickness direction of the plate, with an average grain size of 6.5 micrometers and an average Vickers hardness (HV) of 225.

[0096] The sheet metal has a tensile strength of 525 MPa, falling within the range of 480 to 580 MPa. Its yield strength Rp0.2 is 415 MPa, falling within the range of 380 to 480 MPa. The elongation after fracture is 32%, exceeding the requirement of 30%. The plastic strain ratio r is 3.8, exceeding the requirement of 3.5. The ultimate draw ratio LDR is 4.2, exceeding the requirement of 4.0. The work hardening index n is 0.20, exceeding the requirement of 0.18. The surface roughness Ra is 0.15 micrometers, less than the requirement of 0.2 micrometers.

[0097] Example 2

[0098] For ultra-thin battery casing applications, the process is as follows:

[0099] First, the intermediate billet from Example 1 is cryogenically rolled to a thickness of 0.15 mm. Intermediate annealing can be added: vacuum annealing at 280°C for 2 hours to eliminate some work hardening and prevent edge cracking. Furthermore, it is understood that in other embodiments of this application, cryogenic rolling can be performed multiple times, with intermediate annealing (vacuum annealing at 275-285°C for 1-4 hours) used to eliminate some work hardening and prevent edge cracking. Then, a second cryogenic rolling is performed: rolling to a thickness of 0.05 mm at -196°C. Finally, plasma surface nano-sizing is performed: power density 900 W / cm², argon flow rate 25 L / min, processing time 18 minutes.

[0100] The total thickness of the sheet is 0.052 mm. The surface nanocrystalline layer has a single-sided thickness of 2.5 micrometers. The intermediate ultrafine crystalline layer has a single-sided thickness of 12 micrometers. The core layer thickness is 23 micrometers. The surface hardness (HV) is 445, and the core layer hardness (HV) is 238.

[0101] Molding performance verification:

[0102] A deep cylindrical battery casing with a diameter of 8 mm and a height of 12 mm was fabricated, with a maximum draw ratio (LDR) of 4.0. The maximum thinning rate was 7.7%, which is within the allowable range. Surface quality inspection revealed no scratches, cracks, or orange peel defects.

[0103] Example 3

[0104] Based on the plate material of Example 2, a battery casing was prepared and welding tests were performed:

[0105] The shell is a cylindrical shell with one open end, an outer diameter of 18mm, a height of 65mm, and a wall thickness of 0.08mm. It is formed by deep drawing and thinning stretching.

[0106] The welding process uses laser welding with a power of 800 watts and a speed of 1.5 meters per minute. The protective gas is argon.

[0107] Three sets of comparative experiments:

[0108] The first group of welds underwent no post-weld annealing treatment. The shell deformation was 0.025%, the weld hardness (HV) was 405 (reaching 98% of the base material), and the helium leak detection rate was 5×10⁻⁶. -11 Pa·m / s.

[0109] The second group of welds underwent annealing at 200℃ for 0.5 hours, resulting in a shell deformation of 0.018% and a weld hardness (HV) of 398 (96% of the base metal). The helium leak detection rate was 3×10⁻⁶. -11 Pa·m / s.

[0110] The third group serves as a comparative example. After welding, conventional annealing at 600℃ for 2 hours resulted in a shell deformation of 0.35%, a weld hardness (HV) of 285 (only 69% of the base material), and a helium leak detection rate of 2×10⁻⁶. -10 Pa·m / s.

[0111] The alloy of this invention does not require high-temperature annealing after welding, and the deformation does not exceed 0.03%, meeting the requirements for precision battery casings. Conventional annealing (600℃) leads to coarsening of the gradient structure, a 31% decrease in hardness, and excessive casing deformation. All three airtightness requirements meet the standard of not exceeding 1×10⁻⁶. -10 The requirement is Pa·m³ / s.

[0112] Example 4

[0113] Electrolyte corrosion resistance test was conducted based on the ultrathin shell of Example 2:

[0114] The medium was a 1 mol / L mixture of lithium hexafluorophosphate / ethylene carbonate and dimethyl carbonate (volume ratio 1:1), at 60°C for 90 days. The corrosion rate was measured to be 0.0012 mm / year, and no pitting, intergranular corrosion, or stress corrosion cracking was observed on the surface.

[0115] Temperature cycling test:

[0116] The temperature range was -40°C to 150°C, with 3000 cycles and a heating / cooling rate of 5°C per minute. No cracks, leaks, or bulges were observed after testing, and the burst pressure was 4.2 MPa.

[0117] High-temperature storage test:

[0118] After being stored at 85℃ for 1000 hours, the shell size change rate is less than 0.05%.

[0119] The battery casing of this invention exhibits excellent reliability and safety under extreme operating conditions.

[0120] Example 5

[0121] Ingredients by mass percentage: 0.22% niobium, 0.03% yttrium, 0.035% iron, 0.006% carbon, 0.005% nitrogen, 0.002% hydrogen, 0.08% oxygen, with the balance being titanium.

[0122] Preparation process:

[0123] The raw materials are made into ingots. Then, they are homogenized at 980℃ for 8 hours and air-cooled. Next, thermomechanical deformation is performed: the billet is forged to a thickness of 70mm at 880℃, then hot-rolled to a thickness of 5mm at 780℃, then recrystallized and annealed at 550℃ for 2 hours, and finally pickled to remove the oxide scale, resulting in an intermediate billet.

[0124] Based on the intermediate billet prepared above, deep cryogenic rolling is performed:

[0125] The rolling temperature is controlled at -150°C, and the billet is immersed in liquid nitrogen for 15 minutes. The roll temperature is controlled at -100°C, and direct liquid nitrogen injection cooling is used. A total of 6 rolling passes are performed, with a single pass reduction of 12% to 18%. The rolling speed is 8 meters per minute. The finished product thickness is 0.5 mm.

[0126] Thin sheets after cryogenic rolling undergo plasma treatment:

[0127] The power density was 1500 W / cm², the helium flow rate was 40 liters per minute, the processing time was 30 minutes, and the temperature did not exceed 150℃. After treatment, the surface grain size was 120 nanometers, and the nanolayer thickness was 8 micrometers.

[0128] The surface hardness of the sheet is HV 395. The hardness at a depth of 3 micrometers is HV 380. The hardness at a depth of 6 micrometers is HV 345. The hardness at a depth of 12 micrometers is HV 295. The hardness at a depth of 25 micrometers is HV 255. The hardness at a depth of 50 micrometers is HV 238. The core (depth 250 micrometers) has a hardness of HV 225.

[0129] The surface nanocrystalline layer is located on the surface of the plate up to a depth of 8 micrometers, with an average grain size of 120 nanometers and an average Vickers hardness (HV) of 380.

[0130] The intermediate ultrafine crystalline layer is connected to the surface nanocrystalline layer, with a depth range of 8 to 60 micrometers, an average grain size of 2.5 micrometers, and an average Vickers hardness (HV) of 295.

[0131] The core layer is located in the center region of the thickness direction of the sheet, with an average grain size of 8.5 micrometers and an average Vickers hardness (HV) of 225. The core layer thickness is approximately 380 micrometers (total thickness 500 μm - 16 μm nanocrystalline layers on both sides - 104 μm ultrafine crystalline layers on both sides = 380 μm).

[0132] The sheet metal has a tensile strength of 565 MPa, falling within the range of 480 to 580 MPa. Its yield strength Rp0.2 is 455 MPa, also within the range of 380 to 480 MPa. The elongation after fracture is 31%, exceeding the requirement of 30%. The plastic strain ratio r is 3.6, exceeding the requirement of 3.5. The ultimate draw ratio LDR is 4.1, exceeding the requirement of 4.0. The work hardening index n is 0.19, exceeding the requirement of 0.18. The surface roughness Ra is 0.18 micrometers, less than the requirement of 0.2 micrometers.

[0133] Based on the above-mentioned sheet material, a battery casing was prepared and its performance was verified.

[0134] The shell is a cylindrical shell with one open end, an outer diameter of 25mm, a height of 45mm, and a wall thickness of 0.5mm. It is formed by deep drawing and thinning stretching.

[0135] The welding process uses laser welding with a power of 1000 watts and a speed of 2 meters per minute, with argon as the shielding gas. No stress-relief annealing is performed after welding. The shell deformation is 0.022%, the weld hardness (HV) is 420 (reaching 93% of the base material), and the helium leak detection rate is 8 × 10⁻¹¹ Pa·m³ / s.

[0136] Electrolyte corrosion resistance test: In a 1 mol / L mixture of lithium hexafluorophosphate / ethylene carbonate and dimethyl carbonate (volume ratio 1:1), at a temperature of 60℃ for 90 days, the corrosion rate was measured to be 0.0010 mm per year, with no pitting corrosion, no intergranular corrosion, and no stress corrosion cracking on the surface.

[0137] Temperature cycling test: temperature range from -40℃ to 150℃, 3000 cycles, heating and cooling rate of 5℃ per minute, no cracks, no leakage, no bulging after the test, burst pressure of 3.8MPa.

[0138] Comparative Example 1

[0139] Except for the rolling temperature being room temperature, the other parameters are basically the same as in Example 1.

[0140] Regarding surface grain size, cryogenic rolling yields 85 nm, while room temperature rolling yields 320 nm, representing a 3.8-fold refinement. In terms of core grain size, cryogenic rolling produces 6.5 μm, while room temperature rolling yields 4.2 μm, indicating a coarser and tougher core. For surface hardness, cryogenic rolling achieves HV 425, while room temperature rolling achieves HV 310, a 37% improvement. Regarding core hardness, cryogenic rolling produces HV 225, while room temperature rolling produces HV 245, indicating excessive work hardening. In terms of elongation after fracture, cryogenic rolling produces 32%, while room temperature rolling produces 18%, demonstrating significantly superior plasticity. Regarding the plastic strain ratio (r-value), cryogenic rolling produces 3.8, while room temperature rolling produces 2.9, indicating a superior texture. Finally, cryogenic rolling exhibits no edge cracking, while room temperature rolling results in severe edge cracking.

[0141] Deep cold rolling, by suppressing dynamic recovery, achieves both surface grain refinement and core coarse grain toughness, which is key to achieving a synergistic improvement in strength and plasticity.

[0142] Comparative Example 2

[0143] It uses industrial pure titanium TA1 (titanium content not less than 99.5%, iron not more than 0.25%, and other impurities not more than 0.3%), and does not contain niobium or yttrium.

[0144] The same smelting, thermomechanical deformation, cryogenic rolling, and plasma treatment processes as in Example 1 were used.

[0145] In terms of tensile strength, the niobium-yttrium titanium alloy sheet of this application is superior to pure titanium, with niobium exhibiting significant solid solution strengthening and grain refinement effects. Regarding yield strength Rp0.2, the niobium-yttrium titanium alloy of this application shows a substantial improvement over pure titanium TA1. In terms of elongation after fracture, the niobium-yttrium titanium alloy is 32%, while pure titanium TA1 is 28%, with yttrium purifying grain boundaries and improving plasticity. Regarding the plastic strain ratio r, the niobium-yttrium titanium alloy is 3.8, while pure titanium TA1 is 3.2, with niobium-yttrium optimizing the texture. Regarding the ultimate drawing ratio (LDR), the niobium-yttrium titanium alloy is 4.2, while pure titanium TA1 is 3.4, resulting in a 24% improvement in deep drawing performance. Regarding weld porosity, the niobium-yttrium titanium alloy is 0.5%, while pure titanium TA1 is 3.2%, with yttrium reducing gas solubility. Regarding post-weld elongation, the niobium-yttrium titanium alloy is 26%, while pure titanium TA1 is 15%, with niobium refining the weld microstructure.

[0146] It should be noted that the α-Ti microstructure was determined by X-ray diffraction (XRD) or electron backscatter diffraction (EBSD). Furthermore, the Nb content in the alloy of this application is only 0.08~0.25%, far below the critical concentration for β stabilization, and the equilibrium microstructure at room temperature is α-Ti, which can be inferred from the thermodynamic phase diagram. The surface nanocrystalline layer (average grain size 50~200 nm): small grains, obtained by TEM bright-dark field imaging; the intermediate ultrafine crystalline layer (average grain size 0.5~3 μm): measured by electron backscatter diffraction (EBSD); the core layer has an average grain size of 5~10 μm, which can be observed using a conventional metallographic microscope or SEM. Through hardness testing, macroscopic performance testing, and the above-mentioned testing methods, it can be clearly determined that the titanium alloy possesses superior properties.

[0147] Figure 1 The image shows the EBSD test results of the transition region between the intermediate ultrafine crystalline layer and the core layer. It can be seen from the image that the upper layer is the intermediate ultrafine crystalline layer with grains smaller than 3 μm; the lower layer is the core layer with grains mostly larger than 5 μm. The grains of the intermediate ultrafine crystalline layer are significantly smaller than those of the core layer. Figure 3 The SEM morphology of the surface nanocrystalline layer is shown. After etching with titanium alloy etching solution (using conventional etching solution), it can be seen that there are uniform and extremely small etched and unetched areas on the surface, indicating that the surface grains are fine (the etched area often starts from the grain boundary). Figure 4 This is a partial metallographic microstructure image of the intermediate ultrafine-grained layer. It can be seen that the grains in the intermediate ultrafine-grained layer are below 3 μm. The bottom part is the surface region, shown as dark in the image, which actually represents even finer grain boundaries, invisible under the same field of view. Additional note: Due to the limitations of the etching solution in clearly displaying grain boundaries in every area, some grain boundaries may not be etched, but this does not mean that the grains in those areas do not conform to the characteristics of that layer.

[0148] It should be noted that, in the absence of conflict, the various embodiments described herein can be combined with each other to obtain more implementation schemes.

[0149] The above are merely specific embodiments of the present invention, and any improvements made based on the concept of the present invention shall be considered within the scope of protection of the present invention.

Claims

1. A niobium-yttrium titanium alloy sheet, characterized in that, The titanium alloy sheet is prepared from a niobium-yttrium titanium alloy material with the following composition, expressed as a percentage by mass: 0.08%~0.25% niobium; 0.01%~0.08% yttrium; 0.01%~0.04% iron; Carbon content not exceeding 0.01%; Nitrogen content not exceeding 0.01%; Hydrogen content not exceeding 0.005%; 0.05%~0.20% oxygen; Not less than 99.5% titanium; The titanium alloy sheet forms a three-layer microstructure with a continuous gradient in grain size and hardness along its thickness direction. The structure of each layer is as follows: (1) Surface nanocrystalline layer: The surface layers on both sides of the thickness direction of the plate, each layer has a depth of 2~8μm from the surface of the plate inward; this layer is a nanocrystalline α-Ti structure with an average grain size of 50~200nm and a Vickers hardness of HV380~480. (2) Intermediate ultrafine crystalline layer: connected to the two surface nanocrystalline layers, at 8~60μm inward from the surface nanocrystalline layer; it is an ultrafine α-Ti structure with an average grain size of 0.5~3μm, and Nb-Y enriched phase and Ti-C composite precipitate dispersed in the crystal. The Vickers hardness of this layer is HV280~360. (3) Core layer: Located in the central region of the thickness direction of the plate; This layer is mainly based on equiaxed α-Ti phase, with an average grain size of 5~10μm and a hardness of HV200~250, forming a continuous hardness transition with the intermediate ultrafine grain layer. The total thickness of the plate is 0.03mm to 0.5mm, and along the thickness direction of the plate, from the surface nanocrystalline layer to the core layer, the grain size gradually increases and the hardness gradually decreases, showing a continuous transition without abrupt interfaces. The Nb-Y enriched phase is an intermetallic compound formed by Nb and Y, with a size of 20-80 nm; the Ti-C composite precipitate includes at least one of TiC and Ti-C solid solution, with a size of 10-40 nm.

2. The niobium-containing yttrium titanium alloy plate according to claim 1, characterized in that, The titanium alloy sheet exhibits the following properties under room temperature tensile testing conditions: tensile strength 480~580MPa, yield strength Rp0.2 380~480MPa, elongation after fracture ≥30%, plastic strain ratio r ≥3.5, ultimate drawing ratio LDR ≥4.0, and work hardening index n ≥0.

18.

3. A method for preparing a niobium-yttrium titanium alloy plate as described in any one of claims 1-2, characterized in that, Controlling the thickness gradient structure of sheet metal through a synergistic process of cryogenic rolling and plasma-assisted surface nano-sizing includes the following steps: (1) Alloy melting and homogenization treatment: The raw material powder is mixed and then melted in vacuum self-consuming electric arc to obtain an ingot. The ingot is homogenized at 900~1000℃ for 4~8h and then air-cooled to obtain an alloy ingot with uniform composition. (2) Thermomechanical deformation: The ingot obtained in step (1) is forged at 800~900℃ to form a billet, and then hot rolled at 700~800℃ in multiple passes to a thickness of 3~5mm; then recrystallization annealing is carried out at 550~650℃ for 1~2h to obtain an intermediate billet with a uniform equiaxed structure. (3) Deep cryogenic rolling: The intermediate billet obtained in step (2) is subjected to deep cryogenic rolling in a liquid nitrogen environment of -150℃ to -196℃ for multiple passes to the total thickness of the target plate. The single pass reduction rate is 10% to 25%, the total reduction rate is 85% to 95%, the rolling speed is 5 to 20 m / min, and the roll temperature is controlled at -100℃ to -150℃. By suppressing dynamic recovery at low temperature, high dislocation density is accumulated to form a gradient structure that gradually changes along the thickness direction. (4) Plasma-assisted surface nano-sizing: The thin plate obtained in step (3) is subjected to high-density plasma bombardment treatment. Pure argon or helium is used as the working gas, with a gas flow rate of 20~40L / min, a plasma power density of 500~1500W / cm², a treatment time of 10~30min, and a treatment temperature of ≤150℃. High-energy plasma particle bombardment induces severe plastic deformation of the surface layer, further refining the surface grains to the nanoscale. At the same time, it promotes the segregation of Nb and Y atoms to the grain boundaries to form nano-precipitates, and finally obtains the target titanium alloy plate with a continuous gradient change in grain size and hardness along the thickness direction.

4. The preparation method according to claim 3, characterized in that, In step (3), during the deep cryogenic rolling process, the billet is immersed in liquid nitrogen for 15-30 minutes before each rolling pass to ensure that the billet temperature is uniformly reduced to below -150℃; during the rolling process, liquid nitrogen is directly injected to cool the contact area between the roll and the billet to maintain a low temperature environment.

5. The preparation method according to claim 3, characterized in that, After step (4), the surface roughness Ra of the titanium alloy plate is ≤0.2μm, the thickness of the surface nanocrystalline layer is 2~8μm, the width of the hardness transition zone between the surface nanocrystalline layer and the intermediate ultrafine crystalline layer is ≤10μm, and there is no obvious interface abrupt change.

6. The preparation method according to claim 3, characterized in that, The cryogenically rolled sheet obtained in step (3) was subjected to non-destructive testing and found to have no internal cracks or pores, no coarse second-phase particles in the crystals, and a matrix grain size uniformity coefficient ≥0.

90.

7. A battery casing, characterized in that, It is prepared by using the niobium-containing yttrium titanium alloy sheet as described in any one of claims 1 to 2, or by using the preparation method of the niobium-containing yttrium titanium alloy sheet as described in any one of claims 3 to 6.

8. The battery casing according to claim 7, characterized in that, The housing is cylindrical with one end open; or, the housing is annular with both ends open.

9. The battery casing according to claim 7, characterized in that, The wall thickness of the shell is 0.03mm to 0.5mm.

10. The battery casing according to claim 7, characterized in that, The helium leak detection rate of the casing is ≤1×10⁻⁶. - 10 The corrosion rate at 60℃ in LiPF6 / EC+DMC electrolyte is ≤0.0015mm / year, with no pitting, intergranular corrosion, or stress corrosion cracking on the surface.

11. The battery casing according to claim 7, characterized in that, After the shell is cyclically tested 3000 times in the temperature range of -40℃ to 150℃, there are no cracks, leaks, or bulges, and the burst pressure of the shell is ≥3.5MPa.

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