Family overview
Titanium has a lower density than steel and a high strength for its weight, and the stable oxide film that forms on its surface gives good corrosion resistance, including in seawater and chloride environments. The material divides into commercially pure titanium, with almost no deliberate alloying, and titanium alloys, to which elements such as aluminum and vanadium are added to raise strength. It can still corrode in environments such as hydrofluoric acid or hot reducing acids, so the corrosive environment must be confirmed together with the grade.
Commercially pure titanium is divided into several grades, mainly by the permitted limits of impurities such as oxygen and iron. A grade with higher limits tends to have higher strength and lower ductility and formability. ASTM standards designate these classes by grade number, and the limits for each grade follow the applicable product standard.
Grade 5 in the grade list below is the ASTM designation for the Ti-6Al-4V alloy. It is an α+β alloy in which aluminum stabilizes the α phase and vanadium stabilizes the β phase; its strength can be adjusted by heat treatment, and it is widely used for machine parts and structural parts. Even within the same alloy family, the standard may define a separate grade with tighter limits on interstitial elements, so the grade number is specified exactly when ordering.
In machining, low thermal conductivity concentrates cutting heat at the tool edge, and an elastic modulus lower than that of steel makes the workpiece spring back easily during cutting. Feed, cutting speed and cooling conditions must be controlled, and fine chips can ignite, so handle them with care.
Role of the main elements
- Titanium (Ti): the base element. Its crystal structure changes between the α phase and the β phase with temperature, and alloys are classed as α, α+β or β types, and heat-treatment conditions are chosen, on the basis of this change. The surface oxide film provides the corrosion resistance.
- Aluminum (Al): an element that stabilizes the α phase. It raises strength by solid-solution strengthening, lowers the alloy density and helps high-temperature strength. Too much can cause embrittlement, so the content is limited.
- Vanadium (V): an element that stabilizes the β phase. Part of the β phase remains at room temperature, giving an α+β structure, so strength can be adjusted by heat treatment and hot workability improves.
- Oxygen, nitrogen and carbon (interstitial elements): even in small amounts they raise strength considerably but lower ductility and toughness. Standards set upper limits for these elements.
- Iron and hydrogen (impurity elements): iron tends to stabilize the β phase, and hydrogen embrittles the material. Standards set upper limits for these two as well, and commercially pure grades are distinguished mainly by the permitted limits of oxygen and iron.
Heat treatment and delivery condition
- Commercially pure titanium: its structure is mainly α phase, so unlike steel it cannot be hardened by heat treatment. Strength is set by the grade (impurity limits) and by cold work, and heat treatment is mainly used for stress relief and annealing.
- Grade 5, annealed condition: it is often used in the annealed condition. This condition suits general uses that need a balance of strength and ductility.
- Grade 5, solution treating and aging: as an α+β alloy that retains some β phase, it can be strengthened further by aging after solution treatment. The hardening depth is limited, however, so a thick section may not reach the same strength at its center. To apply this condition, it must be stated separately on the drawing or order.
- Stress relief: residual stress left after machining, straightening or welding affects deformation and straightness after machining, so a stress-relief heat treatment is applied where needed.
- Heat-treatment atmosphere: when exposed to high temperature in air, titanium absorbs oxygen, nitrogen and hydrogen and forms a hard, brittle surface layer (alpha case). Heat treatment is therefore done in vacuum or an inert atmosphere, and any layer that forms is removed by machining or similar work.
- Product form and surface: even for the same alloy, sheet, bar and tube can fall under different product standards, so the product form is specified together with the alloy. Whether the oxide layer is removed, the surface condition such as ground or turned, and the delivery condition are stated on the drawing or order.
Grades
| Group | Grade |
|---|---|
| Titanium alloy (ASTM grade designation) | Grade 5 |
Composition range
Confirm the composition range against the applicable standard.
Typical applications
- Lightweight machine parts: used for parts such as rotating and reciprocating components, where lower mass reduces inertia.
- Corrosion-resistant shafts, pins and fasteners: used for parts that must resist corrosion in seawater or chloride environments.
- Marine and chemical equipment parts: used where corrosion resistance is needed, such as the parts of valves, pumps and heat-exchange equipment that contact the fluid.
- Aerospace and medical parts: used for structural parts, fasteners and device parts that need low weight, high strength and corrosion resistance together.
- Sports and high-performance machine parts: used where weight reduction directly affects performance, such as bicycles, leisure equipment, and high-performance engine or motorsport parts.