titanium-alloys

name	titanium-alloys
description	Titanium alloy classification (alpha, beta, alpha-beta), Ti-6Al-4V properties, weldability, fatigue, corrosion, machining, aerospace and biomedical applications.
metadata	{"priority":7,"promptSignals":{"phrases":["titanium alloy","Ti-6Al-4V","Ti 6Al 4V","titanium","alpha beta titanium"],"minScore":3}}

Titanium Alloys — Complete Skill

Alloy Classification

Alpha (α) Alloys

Primarily HCP structure; solid solution strengthened; not heat-treatable for strength CP titanium (Grades 1-4): S_y = 170-480 MPa (increasing Fe+O) Ti-3Al-2.5V (Grade 9): S_y = 520 MPa — tubing, bicycle frames Properties: excellent weldability, good creep resistance, used at up to 300°C

Beta (β) Alloys

BCC structure; solution + age hardenable; highest strength Ti-13V-11Cr-3Al: S_y up to 1350 MPa; used in fasteners, springs Ti-3Al-8V-6Cr-4Zr-4Mo (Beta C): S_y = 1000-1200 MPa; excellent fatigue Properties: high strength, higher density, more difficult to weld, limited corrosion

Alpha-Beta Alloys (Most Common)

Two-phase; heat treatable; best combination of strength, ductility, weldability Ti-6Al-4V (Grade 5): dominant alloy (>50% of all Ti production)

Ti-6Al-4V Properties

Standard Grade

Annealed: S_y = 880 MPa, S_u = 950 MPa, El = 14%, RA = 36% E = 114 GPa, G = 43 GPa, ν = 0.33 ρ = 4430 kg/m³ (density/weight advantage vs. steel)

Solution + Age (STA): S_y = 1100-1160 MPa, S_u = 1170-1240 MPa, El = 8-10%

ELI (Extra Low Interstitials, Grade 23): S_y = 760 MPa, S_u = 830 MPa, El = 15% — biomedical (better toughness)

Fatigue: Endurance limit (R=-1): ~550 MPa (annealed); ~700 MPa (STA) After machining: surface condition critical; Ra ≤ 0.8 μm for fatigue-critical parts Notch sensitivity moderate (Kt effects significant)

Fracture toughness: K_IC = 55-100 MPa√m (plate, L-T direction) K_IC,ELI ≥ 70 MPa√m (minimum, biomedical grade)

Ti-6Al-4V Variants

Ti-6Al-4V ELI: reduced O (<0.13%), Fe, N — improved fracture toughness + ductility — biomedical Ti-6Al-2Sn-4Zr-6Mo: S_y = 1100 MPa; compressor blades, higher temperature Ti-6Al-2Sn-4Zr-2Mo: fan blades, elevated temperature (to 450°C) Ti-5Al-2.5Sn: near-alpha; cryogenic (LH2 tanks), good weldability

High-Performance Ti Alloys

Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo): creep to 450°C; jet engine compressor Ti-1100: 500°C capability; highest service temperature alpha-beta Ti-64 PREP powder: additive manufacturing, excellent fatigue after HIP

Welding and Joining

Fusion weldable (GTAW, PAW, EBW, LBW) Critical: must shield from atmosphere — titanium reacts with O, N above 400°C EBW/LBW preferred for thin sections (minimal heat input) GTAW: trailing shields + back purge required; avoid all contamination Weld strength: near-parent if properly done; ductility may decrease slightly

FSW: effective, no shielding gas issue, good mechanical properties

Not compatible (galvanic): contact with cadmium, silver, tin in seawater — avoid

Machinability

Ti is "gummy" and work-hardens under tool: Recommendations: low cutting speed (30-50 m/min vs. 150+ for Al), high feed rate, sharp tools, abundant flood coolant Use carbide (uncoated for roughing; TiAlN coated for finishing) Ti loves to burn: coolant mandatory; never dry machine

Corrosion Resistance

Outstanding corrosion resistance due to TiO₂ passive film Seawater: immune to pitting and crevice at ambient temperature HCl: resistant at low concentration; attacked at high concentration Nitric acid: excellent Reducing acids (H₂SO₄ dilute): susceptible (unlike stainless) Not compatible with: dry chlorine gas, hot HCl, fuming red nitric acid

Applications

Application	Alloy	Reason
Airframe (fan blades)	Ti-6Al-4V	Strength/weight, fatigue
Biomedical implants	Ti-6Al-4V ELI	Biocompatibility, toughness
Pressure vessels	CP Ti or Ti-3Al-2.5V	Corrosion, weldable
Springs (high strength)	Beta C	S_y > 1200 MPa, elastic range
Chemical plant	CP Ti	Corrosion resistance
Golf club face	Ti-6Al-4V	Strength, low density

Output

Provide: alloy and condition recommendation, S_y and S_u [MPa], density [g/cm³], specific strength [kN·m/kg], K_IC [MPa√m], weld method recommendation, machining speed and feed rate, corrosion compatibility with service environment.