B381 F2
B381 F2 is a Ti-6Al-4V titanium alloy forging (the most classic titanium alloy in American standard), which comes from ASTM B381 “Standard Specification for Titanium and Titanium Alloy Forgings” and monopolizes high-end fields such as aviation, medical care and military industry by virtue of “ultra-high specific strength × corrosion resistance× high temperature stability”. The following is systematically analyzed from five dimensions:
I. Standards and brand codes
1. Core standards
- International use: ASTM B381 (current version ASTM B381-21, which specifies the chemical composition, mechanical properties and flaw detection requirements of forgings);
- Grading correspondence: Grade 5 (commercial name, completely equivalent to B381 F2) of American standard titanium alloy;
- UNS number: UNS R56400 (American Uniform Numbering System logo);
- Domestic approximation: TA15 titanium alloy in GB/T 16598 (similar in composition, with emphasis on aviation application).
II. Chemical composition (mass fraction%, ASTM B381 standard)
B381 F2 achieves high strength through “Al solid solution+V stable β phase” and the elements have precise functions;
| element | Content range | Core role |
|---|---|---|
| Ti | margin | Matrix, providing low density (4.51 g/cm³) and foundation corrosion resistance. |
| Al | 5.5~6.5 | Forming α phase, solid solution strengthening, and improving normal temperature/high temperature strength. |
| V | 3.5~4.5 | Stabilize β phase and improve hot workability and high temperature toughness. |
| Fe | ≤0.30 | Impurity elements, exceeding the limit to reduce corrosion resistance (strict control) |
| O | ≤0.20 | Gap strengthening (O content =→ strength =, plasticity ↓, balance required) |
| C/N/H | ≤0.08/≤0.05/≤0.015 | Inhibiting embrittlement phases (titanium carbide, titanium nitride, hydride) |
III. Mechanical properties (annealed state, forgings)
B381 F2 has the properties of crushing pure titanium (B381 F1) due to α+β dual-phase structure and alloying;
| Performance index | Typical value (ASTM requirements) | Comparative pure titanium (B381 F1) |
|---|---|---|
| tensile strength | ≥895 MPa | 124% higher （F1≥400 MPa） |
| yield strength | ≥825 MPa | 200% higher （F1≥275 MPa） |
| extensibility（δ₅） | ≥10% | 50% lower（F1≥20%） |
| hardness | 300~350 HB | 200% higher （F1≤120 HB） |
| heat resistance | Tensile strength at 300℃ ≥700 MPa | 75% higher （F1 300℃≈400 MPa） |
IV. Heat Treatment Requirements (Biphasic Structure Control)
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Annealing treatment (normal state, balance performance):
- Temperature: 700~800℃ (heat preservation for 1~2 hours, α+β phase region, eliminating forging stress);
- Cooling: air cooling (keeping dual-phase structure, giving consideration to strength and plasticity);
- Application: General mechanical parts (such as offshore stents and medical implants).
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Solution plus aging (highly strengthened state, ultimate performance):
- Solid solution: 920~960℃(β phase region, water cooling after heat preservation to obtain supersaturated β phase);
- Aging: 500~600℃ (keeping the temperature for 4~8 hours, separating out fine α phase and strengthening the matrix);
- Effect: tensile strength ≥1100 MPa, hardness ≥380 HB (suitable for aviation landing gear and missile parts).
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Welding and post-treatment:
- Welding material: ERTi-6Al-4V welding wire (matching composition to ensure weld strength);
- Process: TIG welding (protected by high-purity argon), preheating at 150~200℃ before welding (to prevent cold cracks), and stress-relieving annealing at 550℃ after welding (to restore corrosion resistance).
V. Main application fields (high stress+extreme environment)
B381 F2 relies on “specific strength × corrosion resistance × high temperature stability” to monopolize the following fields (super stainless steel performance, only 1/3 of the cost of nickel-based alloy):
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Aerospace (70%):
- Structural parts: wing girder and landing gear (the specific strength is twice that of steel, and the weight is reduced by 40%);
- Engine: compressor blade, turbine disk (withstanding high temperature of 400℃, replacing nickel-based alloy to reduce cost by 50%).
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Medical health:
- Implants: hip joint and knee joint prosthesis (biocompatible+high strength, life span over 25 years);
- Surgical instruments: bone drill, orthopedic steel plate (body fluid corrosion resistance, no metal ion precipitation).
-
Ocean and energy:
- Deep-sea equipment: propeller, underwater robot frame (seawater corrosion resistance+high specific strength, life exceeding 5 times of brass);
- Energy equipment: shale gas wellhead valve and geothermal well pipeline (resistant to 200℃ high temperature and corrosion, instead of stainless steel).
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Military industry and high-end equipment:
- Weapon system: tank armor, missile body (impact resistance+lightweight, protection performance improved by 30%);
- Precision instruments: racing car suspension, UAV frame (high specific strength+fatigue resistance, 10 cycles without fracture).
Key summary
- Core advantages:
- Specific strength: strength/density ≈ 220 mpacm/g (twice that of steel and 1.5 times that of nickel-based alloy);
- Corrosion resistance: the corrosion rate in 3.5% NaCl is less than 0.01 mm/year (equivalent to pure titanium and far more than stainless steel);
- High-temperature stability: the long-term service strength at 400℃ has not obviously decreased (stainless steel has softened at 300℃).
- Limitations:
- High cost (30% more expensive than pure titanium and 5 times more expensive than steel);
- Cold working is difficult (hot working or multi-pass annealing is needed, which increases the manufacturing cost by 20%).
B381 F2(Ti-6Al-4V) is a “titanium alloy benchmark”, which breaks through the strength bottleneck of pure titanium through Al-V alloying, and becomes the “ultimate solution of high specific strength” in high-end fields such as aviation, medical care and military industry, redefining the performance boundary of lightweight structures.