Alloy Classification: Commercially Pure Versus Alpha-Beta
Grade 3 titanium is an unalloyed commercially pure grade. Its chemistry is titanium plus tightly controlled residuals, and it sits at the strength end of the commercially pure family, above Grade 1 and Grade 2. It is not heat treatable to higher strength, and it is chosen for formed and welded parts that need a useful margin of strength with excellent ductility.
Grade 5 titanium is Ti-6Al-4V, the most widely consumed titanium alloy in engineering. Aluminium stabilises the alpha phase and vanadium stabilises the beta phase, producing an alpha plus beta microstructure that responds to heat treatment and that carries far more load than any commercially pure grade. The two materials are therefore not interchangeable substitutes; they solve different problems.
Chemical Composition
Element limits are taken from the annealed product specifications for each grade.
| Element | Grade 3 (CP Ti) | Grade 5 (Ti-6Al-4V) |
|---|---|---|
| Titanium | Balance, above 99.5% | Balance, about 88 to 90% |
| Aluminium | ≤ 0.10% residual | 5.50 to 6.75% alloying |
| Vanadium | ≤ 0.05% residual | 3.50 to 4.50% alloying |
| Iron | ≤ 0.30% | ≤ 0.30% |
| Oxygen | ≤ 0.35%, mills typically 0.18 to 0.25% | ≤ 0.20% |
| Carbon | ≤ 0.08% | ≤ 0.08% |
| Nitrogen | ≤ 0.05% | ≤ 0.05% |
| Hydrogen | ≤ 0.015% | ≤ 0.015% |
The alpha and beta stabilising additions in Grade 5 are the source of its strength: aluminium strengthens the alpha phase, while vanadium retains the beta phase at room temperature so that the alloy can be aged and forged to high strength.
Mechanical Properties in the Annealed Condition
| Property at room temperature | Grade 3 typical | Grade 5 typical | Comparison |
|---|---|---|---|
| Yield strength, 0.2% offset | 370 to 480 MPa (54 to 70 ksi) | 860 to 930 MPa (125 to 135 ksi) | Grade 5 is about twice as strong in yield |
| Tensile strength | 480 to 620 MPa (70 to 90 ksi) | 900 to 970 MPa (130 to 140 ksi) | Grade 5 is 50 to 60% higher |
| Elongation in 50 mm | 15 to 25% | 10 to 15% | Grade 3 is clearly more ductile |
| Hardness | 110 to 140 HB | 300 to 350 HB | Grade 5 resists wear far better |
| Density | about 4.51 g/cm³ | about 4.43 g/cm³ | Both remain light against steel |
The roughly twofold yield strength gap is the number that drives design. A pressure component sized on Grade 3 will generally be two to three times thicker than the same component in Grade 5, and that wall thickness also changes stiffness, weld volume and thermal response.
High Temperature and Corrosion Behaviour
Grade 3 has limited high temperature capability. Its strength falls away quickly above about 300°C and the passive film becomes progressively more porous, so long term service is normally kept at or below 300°C. Grade 5 holds its strength to roughly 400 to 450°C because the alpha plus beta structure is thermally stable, and it tolerates short excursions near 500°C, which is why it appears in hotter airframe and engine hardware.
In mild media the two grades behave similarly, both protected by a dense titanium dioxide film that resists atmosphere, seawater and dilute acids. Grade 3 performs better in strongly reducing or high purity chemical streams, where the unalloyed metal offers slightly more uniform corrosion resistance. Grade 5 is generally more sensitive to chloride driven localised attack and, in severe service, benefits from anodising or another surface treatment.
Fabrication, Welding and Application Fit
Grade 3 forming: excellent ductility allows bending, stamping, deep drawing and tube expansion, and it is readily welded without post weld heat treatment for most duties.
Grade 5 forming: higher strength and lower elongation make forming harder. Welding demands strict control of shielding gas and interpass temperature, and post weld stress relief is often required to restore ductility in the heat affected zone.
Grade 3 applications: chemical processing tanks, pipes and valves for dilute acid and seawater duty, marine hardware, seawater cooled heat exchanger tubes, cryogenic storage with its good low temperature toughness, and dental abutments or instrument housings.
Grade 5 applications: airframe structures, engine components and fasteners, defence hardware, load bearing orthopaedic implants such as hip stems and knee components, and motorsport suspension parts where low mass and high strength combine.
Frequently Asked Questions
Q: Is Grade 5 the same as Grade 3 with added alloying elements?
No. Grade 3 is commercially pure titanium with residual elements only, while Grade 5 is a deliberately alloyed alpha plus beta material containing about 6% aluminium and 4% vanadium. The two are produced to different specifications.
Q: How much stronger is Grade 5 than Grade 3?
In the annealed condition Grade 5 reaches roughly 860 to 930 MPa yield strength against 370 to 480 MPa for Grade 3, so it is about twice as strong in yield and 50 to 60% stronger in tensile strength.
Q: Which grade is easier to weld?
Grade 3 is easier. It welds without needing post weld heat treatment for most services, whereas Grade 5 needs careful inert gas protection and often a stress relief cycle afterwards.
Q: Can Grade 3 be used at high temperature?
Long term service should stay at or below about 300°C, because strength and oxidation resistance both degrade above that level. Grade 5 is the better choice for hotter duty.
Q: Which grade suits medical implants?
Both appear in medical devices, but for different duties. Grade 3 is used for instrument housings and abutments where formability matters, while Grade 5, or its extra low interstitial variant, is used for load bearing implants such as stems and bone screws.
Q: Is Grade 3 or Grade 5 more corrosion resistant?
In mild environments they are comparable. Grade 3 has an edge in strongly reducing or high purity chemical streams, while Grade 5 may need surface treatment where chlorides promote localised attack.
