Titanium Alloy Ti 6Al-4V

FACT... Pure titanium undergoes an allotropic transformation from the hexagonal close-packed alpha phase to the bodycentered cubic beta phase at a temperature of 882.5°C (1620.5°F). Alloying elements can act to stabilize either the alpha or beta phase. Through the use of alloying additions, the beta phase can be sufficiently stabilized to coexist with alpha at room temperature. This fact forms the basis for creation of titanium alloys that can be strengthened by heat treating.

3 types of titanium alloys

Titanium alloys are generally classified into three main categories: Alpha alloys, which contain neutral alloying elements (such as Sn) and/or alpha stabilizers only and are not heat treatable; Alpha + beta alloys, which generally contain a combination of alpha and beta stabilizers and are heat treatable to various degrees; and Beta alloys, which are metastable and contain sufficient beta stabilizers (such as Mo, V) to completely retain the beta phase upon quenching, and can be solution treated and aged to achieve significant increases in strength.
Ti 6Al-4V is known as the "workhorse" of the titanium industry because it is by far the most common Ti alloy, accounting for more than 50% of total titanium usage. It is an alpha+beta alloy that is heat treatable to achieve moderate increases in strength.
Ti 6Al-4V is recommended for use at service temperatures up to approximately 350°C (660°F).
Ti 6Al-4V offers a combination of high strength, light weight, formability and corrosion resistance which have made it a world standard in aerospace applications.
Applications
Ti 6Al-4V may be considered in any application where a combination of high strength at low to moderate temperatures, light weight and excellent corrosion resistance are required. Some of the many applications where this alloy has been used include aircraft turbine engine components, aircraft structural components, aerospace fasteners, high-performance automotive parts, marine applications, medical devices, and sports equipment.

Ti 6Al-4V  and Corrosion Resistance

Ti 6Al-4V immediately and spontaneously forms a stable, continuous, tightly adherent oxide film upon exposure to oxygen in air or water. This accounts for its excellent corrosion resistance in a variety of media. Ti 6Al-4V is highly resistant to general corrosion in aqueous solutions including seawater, as well as in oxidizing acids, chlorides (in the presence of water), rocket propellants and alkalis. Conditions under which it is susceptible to general corrosion are in the presence of reducing acids or dry chlorine gas.
Stress-corrosion cracking (SCC) and crevice corrosion have been shown to occur in environments containing chlorides or other halide ions. For this reason, it is general practice to avoid the use of chlorinated solvents, cutting fluids, etc., in processing titanium.
Titanium and its alloys, including Ti 6Al-4V, are susceptible to hydrogen embrittlement. Gaseous or cathodic hydrogen can diffuse into the metal, forming brittle hydrides. Thus, it is important to minimize hydrogen pickup during processing, particularly heat treating and acid pickling. Specifications for Ti 6Al-4V mill products typically specify a maximum hydrogen limit of about 150 ppm.

Titanium Alloy Ti 6Al-4V and heat treatment

Ti 6Al-4V wrought products are typically used in either a mill annealed or solution treated and aged condition. Rapid quenching following solution treatment (water quench or equivalent) is important in order to maximize the formation of alpha' martensite phase, which in turn maximizes the aging response. Other heat treatments used on Ti 6Al-4V include stress relieving for formed or welded parts, and beta annealing, which is used for improving damage tolerance.
Ti 6Al-4V, like other titanium alloys, has a high affinity for gases including oxygen, nitrogen and hydrogen. Absorption of oxygen results in the formation of an extremely hard, brittle oxygen-stablized alpha phase layer known as alpha case upon heating in air.
Intermediate and final annealing of Ti 6Al-4V mill products is often performed in a vacuum or inert gas atmosphere to avoid alpha case formation and the associated material loss. Vacuum annealing can also be used to remove excess hydrogen pickup, a process known as vacuum degassing. Parts to be vacuum heat treated must be thoroughly cleaned.

Ti 6Al-4V Hot Working

Ti 6Al-4V can be hot worked by standard methods such as hot rolling, forging, and hot pressing. Typically, hot working is done high in the alpha/beta temperature range, at approximately 870-980 C (1600-1800°F). Care must be taken to prevent the formation of excessive alpha case, and alpha case must be removed after processing. Hot forming of sheet is typically done at temperatures around 650C (1200°F). Ti 6Al-4V has also been successfully processed by superplastic forming, using temperatures in the range of 850 C (1560 F).

Workability

"Warm" Working:
The yield strength of Ti 6Al-4V in both the annealed and STA conditions drops off rapidly with temperature, making it readily formable at intermediate temperatures. For example, heating to just 427°C (800°F) results in approximately a 40% reduction in yield strength. Warm forming is used extensively in the manufacture of many products, including fasteners, aircraft components, and medical devices.

Ti 6Al-4V Cold Working:

Ti 6Al-4V can be cold drawn and extruded, although the cold workability is somewhat limited. Cold forming is sometimes used for parts such as brackets and clips. Due to the low modulus of titanium, springback is an issue in room-temperature forming. Theoretically, it can be compensated for by over-bending; however, in practice hot sizing is often used to correct for the variability that occurs.

Ti 6Al-4V machining

Using the rating system based on AISI B1112 steel, the machinability of Ti 6Al-4V is rated at 22% of B1112. In general, low cutting speeds, heavy feed rates and copious amounts of cutting fluid are recommended. Also, because of the strong tendency of titanium to gall and smear, feeding should never be stopped while the tool and work are in moving contact. Non-chlorinated cutting fluids should be used to eliminate the possibility of chloride contamination. It should be noted that titanium chips are highly combustible and appropriate safety precautions are necessary.
 

Ti 6Al-4V welding

Ti 6Al-4V can be welded using Ti 6Al-4V filler metal. Inert gas shielding techniques must be employed to prevent oxygen pickup and embrittlement in the weld area. Gas tungsten arc welding is the most common welding process for Ti 6Al-4V. Gas metal arc welding is used for thick sections. Plasma arc welding, spot welding, electron beam, laser beam, resistance welding and diffusion welding have all been used successfully in Ti 6Al-4V welding applications.

Titanium descaling

Ti alloys in general, have a tendency to gall and are not recommended for wear applications.

Following heat treatment in air, it is extremely important to completely remove not only the surface scale but the underlying layer of brittle alpha case as well. This removal can be accomplished by mechanical methods such as grinding or machining, or by descaling (using molten salt or abrasive) followed by pickling in a nitric/hydroflouric acid mixture.
Titanium alloys are also susceptible to hydrogen embrittlement, and care must be taken to avoid excessive hydrogen pickup during heat treating and pickling/chemical milling.
Final heat treatments on finished parts must be performed in a vacuum if machining or pickling is to be avoided.
The cleanliness of parts to be vacuum heat treated is of prime importance. Oils, fingerprints, or residues remaining on the surface can result in the formation of alpha case even in the vacuum atmosphere. In addition, chlorides found in some cleaning agents have been associated with SCC of titanium. Thus, parts to be vacuum heat treated should be processed as follows: thorough cleaning using a non-chlorinated solvent or aqueous cleaning solution, followed by rinsing with copious quantities of deionized or distilled (not regular tap) water to remove all traces of cleaning agent, and finally drying. Following cleaning, parts must be handled with clean gloves to prevent recontamination of the surface.