Ti-6Al-2Sn-4Zr-2Mo (Grade 4)
Introduction to Ti-6Al-2Sn-4Zr-2Mo (Grade 4)
Ti-6Al-2Sn-4Zr-2Mo, also known as Grade 4 titanium in the aerospace alloy family, is a near-alpha titanium alloy designed for excellent high-temperature strength, thermal stability, and corrosion resistance. This alloy is widely used in gas turbines, airframes, and other applications requiring long-term performance under elevated temperatures.
Its reliable mechanical stability and oxidation resistance make it a preferred material for high-spec CNC machined titanium parts. For applications demanding tight tolerances and dimensional control in extreme environments, manufacturers depend on advanced CNC machining services to produce precision components from Grade 4 titanium.
Chemical, Physical, and Mechanical Properties of Ti-6Al-2Sn-4Zr-2Mo (Grade 4)
Chemical Composition (Typical)
Element | Composition Range (wt.%) | Key Role |
|---|---|---|
Titanium (Ti) | Balance | Base element with corrosion and thermal resistance |
Aluminum (Al) | 5.5–6.5 | Alpha stabilizer for high-temperature strength |
Tin (Sn) | 1.8–2.5 | Improves creep resistance |
Zirconium (Zr) | 3.6–4.5 | Enhances oxidation resistance and strength |
Molybdenum (Mo) | 1.8–2.5 | Beta stabilizer that increases hardenability |
Silicon (Si) | ≤0.25 | Strengthens matrix and enhances creep resistance |
Oxygen (O) | ≤0.15 | Strengthens but affects ductility |
Iron (Fe) | ≤0.30 | Residual element |
Hydrogen (H) | ≤0.015 | Controlled to avoid embrittlement |
Physical Properties
Property | Value (Typical) | Test Standard/Condition |
|---|---|---|
Density | 4.55 g/cm³ | ASTM B311 |
Melting Range | 1600–1660°C | ASTM E1268 |
Thermal Conductivity | 6.8 W/m·K at 100°C | ASTM E1225 |
Electrical Resistivity | 1.62 µΩ·m at 20°C | ASTM B193 |
Thermal Expansion | 8.5 µm/m·°C | ASTM E228 |
Specific Heat Capacity | 570 J/kg·K at 20°C | ASTM E1269 |
Elastic Modulus | 108 GPa | ASTM E111 |
Mechanical Properties (Annealed Condition)
Property | Value (Typical) | Test Standard |
|---|---|---|
Tensile Strength | 860–980 MPa | ASTM E8/E8M |
Yield Strength (0.2%) | 790–900 MPa | ASTM E8/E8M |
Elongation | ≥10% | ASTM E8/E8M |
Hardness | 300–340 HB | ASTM E10 |
Creep Resistance | Excellent | ASTM E139 |
Fatigue Resistance | High | ASTM E466 |
Key Characteristics of Ti-6Al-2Sn-4Zr-2Mo (Grade 4)
Elevated Temperature Strength: Grade 4 maintains high strength and creep resistance up to 500°C, making it suitable for gas turbine engines, airframes, and heat shields.
Excellent Oxidation Resistance: The addition of Zr and Sn supports the formation of a stable oxide layer, ensuring long-term durability in high-temperature and oxygen-rich environments.
Thermal Fatigue Resistance: Its alloying balance provides stability under cyclic heating and cooling, critical for aerospace and power generation components.
Superior Weldability and Stability: Despite being a near-alpha alloy, it is readily weldable and maintains phase balance after heat cycles, ideal for high-pressure assemblies.
CNC Machining Challenges and Solutions for Grade 4 Titanium
Machining Challenges
High Work Hardening Rate: Grade 4 tends to work harden during tool contact, requiring optimized chip load and tool sharpness to avoid surface distortion.
Low Thermal Conductivity: With only 6.8 W/m·K conductivity, the alloy retains heat at the tool-chip interface, reducing tool life and increasing the chance of thermal deformation.
Tool Wear and Galling: Aggressive oxide formation and beta-rich zones accelerate wear and promote chip adhesion to tools, impacting surface finish.
Elastic Recovery: With a modulus of 108 GPa, the alloy exhibits measurable spring back, especially in thin-walled parts and finishing operations.
Optimized Machining Strategies
Tool Selection
Parameter | Recommendation | Rationale |
|---|---|---|
Tool Material | Fine-grain carbide or ceramic inserts | High strength and heat resistance |
Coating | AlTiN or TiSiN (3–5 µm) | Reduces BUE and increases tool life |
Geometry | Sharp edge, high rake angle | Lowers cutting force and thermal input |
Cutting Speed | 25–60 m/min | Prevents overheating and surface hardening |
Feed Rate | 0.10–0.25 mm/rev | Supports chip formation and avoids glazing |
Coolant | Emulsion coolant, ≥100 bar | Provides effective cooling and chip flushing |
Ti-6Al-2Sn-4Zr-2Mo (Grade 4) Cutting Parameters (ISO 3685 Compliance)
Operation | Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant Pressure (bar) |
|---|---|---|---|---|
Roughing | 20–30 | 0.15–0.20 | 2.0–3.0 | 80–100 (Through-tool) |
Finishing | 45–65 | 0.05–0.10 | 0.2–0.5 | 100–150 |
Surface Treatment for Grade 4 Titanium Parts
Hot Isostatic Pressing (HIP) densifies internal microvoids and increases fatigue life, especially for flight-critical components.
Heat Treatment involves 600–675°C aging to enhance creep resistance and dimensional stability in turbine hardware.
Superalloy Welding ensures full-strength fusion welds for aerospace and propulsion assemblies using compatible filler rods.
Thermal Barrier Coating (TBC) adds oxidation and thermal fatigue protection for components in jet engines and high-heat reactors.
CNC Machining supports ±0.01 mm tolerances and is ideal for flight hardware, high-performance flanges, and brackets.
Electrical Discharge Machining (EDM) enables precise features like orifices and deep pockets in heat-treated parts.
Deep Hole Drilling provides bore depths with L/D > 30:1 and surface finish Ra ≤ 1.6 µm in structural and cooling components.
Material Testing includes tensile, creep rupture, SEM microstructure, and ultrasonic NDT to aerospace QA standards.
Material Testing and Analysis
Material validation for Grade 4 includes high-temperature tensile testing, creep rupture evaluation, microhardness, XRD, and metallographic analysis, ensuring compliance with aerospace and nuclear standards.
Industry Applications of Ti-6Al-2Sn-4Zr-2Mo (Grade 4)
Aerospace: Engine casings, afterburner components, and nozzle hardware operating above 400°C.
Power Generation: Gas turbine blades, exhaust ducting, and high-pressure combustor elements.
Defense Systems: Structural aerospace connectors and propulsion system liners requiring temperature endurance.
Industrial Equipment: Heat exchangers and high-pressure seals in corrosive and thermal cycling environments.
Aerospace Fasteners: Bolts and flange systems for high-cycle fatigue and elevated thermal load regions.
FAQs
What are the recommended CNC machining speeds for Ti-6Al-2Sn-4Zr-2Mo?
How does Grade 4 titanium perform under continuous exposure to high temperatures?
What heat treatments improve creep and fatigue resistance for this titanium alloy?
What types of aerospace components are best suited for machining from Grade 4 titanium?
What dimensional tolerances are achievable on CNC machined Grade 4 titanium components?
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