Titanium alloy Ti-6%Al-4%V has good tensile properties at room temperature, annealed
material having a typical tensile strength of 1000-1100 MPa (145-160 ksi), and a
useful creep resistance up to 300°C of about 570 MPa (83 ksi) for 0-1% total
plastic strain in 100 hours. Heat treatment will give a guaranteed minimum tensile of
1100 MPa (160 ksi) for such applications as springs, bolts or other fasteners.
Resistance to fatigue and crack propagation is excellent. Like most titanium alloys and
grades, titanium alloy Ti-6%Al-4%V has outstanding resistance to corrosion in most
natural and many industrial process environments. Its density of 4.0-4.2 g/cm3
is even lower than that of pure titanium. It can readily be formed or forged; many
welding operations are possible.
Ti-6%Al-4%V is an alpha+beta alloy, containing 6% aluminum and 4% vanadium. The
aluminum stabilizes and strengthens the alpha phase, so raising the beta-transus
temperature, as well as reducing the density of the alloy. The vanadium is a beta
stabilizer, and provides a greater amount of the more ductile beta phase during hot
working.
On solution treatment high in the alpha+beta field, followed by rapid cooling to room
temperature, the beta phase transforms to a structure which can subsequently be
tempered to a fine dispersion of beta in an alpha matrix, with consequent strengthening
of the alloy. The chemical composition of Ti-6%Al-4%V given in the Table 1.
Table1. Chemical composition of Ti-6%Al-4%V
|
Element
|
Al
|
V
|
Fe
|
H2
|
Ti
|
|
Wt%
|
5,5-6,75
|
3,5-4,5
|
0,30 max
|
0.0125 max
|
Remainder
|
Titanium alloy Ti-6%Al-4%V is available as annealed plate and sheet, as hot worked
rod, bar and billet for further working, or as annealed rod and bar for machining.
Heat-treatable rod is available for fastener manufacturing and bard-drawn wire can be
supplied for spring applications. Pipes can be supplied as extrusions or formed and
welded from plate. More complex sections are available, made by extrusion or forming
or rolling.
Grades of Ti-Alloy of extra low interstitial content can be made available for specific
applications, which demand special ductility, fracture toughness, or resistance to
crack propagation in aqueous environments. Typical specifications for these grades are
AMS 4907 for sheet and strip, and AMS 4930 for bars, forgings, and rings.
Specification properties
Sheet is supplied in accordance with British Standard TA 10 or TA 59, and plate in
accordance with British Standard TA 56.
Rod, bar and billet are supplied in accordance with British Standards TA11 and 12. BS
TA 11 refers to bar for machining, which has the specification properties in ruling
sections up to 150 mm. BS TA 12 covers forging stock in which the specification
properties are only developed after annealing; in material greater than 150 mm ruling
section, a transverse slice may be upset forged 3:1 before annealing and testing. BS
TA 13 gives properties on annealed forgings.
Bolt stock is supplied in accordance with BS TA 28.
Forging
The beta transus temperature of Ti-6%Al-4%V is higher than that of many other alloys
of titanium, which allows a somewhat higher forging temperature to be used. To get
the optimum combination of strength and ductility in a finished component, however,
it is necessary to carry out at least a 4:1 reduction in the alpha+beta field and it
is recommended that the maximum temperature reached during preheating and forging
should not exceed 975°C. In order to guard against internal overheating by
kinetic work during rapid forging, it may be safer to use a preheating temperature
of 950°C.
For initial cogging of large or complicated pieces, or where heavy reductions can
thereby be achieved, it may be permissible to use higher temperatures in the early
stages. Some non-critical components may even be beta-forged to the finished shape,
with relatively little loss of strength, ductility or fatigue resistance; fracture
toughness may even be improved. Much depends on the power of the forging plant
available.
Oxidation becomes progressively more serious as the temperature is raised. For this
reason, time and temperature should be kept to a minimum consistent with thorough
heating and, provided that the furnace capacity is adequate for the mass of metal
involved, a total heating time of 1 hour per 50 mm of section should be adequate.
Forming
One of the advantages of Ti-6%Al-4%V is its availability as sheet and plate as well
as rod, bar and billet. It can thus be used for sheet-metal fabrications or composite
sheet/forging assemblies.
Limiting factors, when carrying out work at room temperature, are its minimum bend
radius of 5t and the relatively narrow proof/tensile gap. Both these are improved by
moderate heating, and temperatures up to 700°C are commonly used in warm-working
the alloy. This has the added advantage that spring-back is less and dimensional
accuracy thereby improved.
Fine-grained Ti-6%Al-4%V sheet can be superplastically formed, giving very high
elongations, tight radii and negligible springback. The temperatures (900-950°C),
pressures and times required are the same as those needed for diffusion bonding, and
very complex parts can be made by combining these two processes. The equipment
required generally includes metal tools with integral heaters, and means for evacuating
the die cavities and applying argon gas pressure to deform the metal.
Heat treatment
Most of the applications of Ti-6%Al-4%V call for it in the annealed state, and the
properties specified in British Standards TA 10, 11, 12 and 13 refer to a heat
treatment at 700°C, followed by air cooling to room temperature. For sheet, it
is sufficient to soak for 20 min at temperature; for rod or forgings, normal practice
is 1 h per 25 mm of section with a minimum time of 1 h at temperature.
Annealing at 700°C gives the best combination of softening with little oxidation
temperature of 850-900°C will provide maximum ductility and
proof-stress/tensile-strength gap but with increased oxidation.
For disc quality material to yield optimum structure and properties, the heat treatment
recommended is 960°C, water quench, followed by annealing at 700°C. The
objective in the first stage of the treatment is to reach a structure containing
between 15 and 45 per cent retained alpha.
For stress relieving of, for example, complex fabrications, it is often possible to
obtain sufficient relaxation at a lower temperature, such as 500 or 600°C. As a
rough guide 1 h at 600°C may prove adequate in most cases. The stress-relieving
treatment can also act as an ageing treatment if the part has previously been solution
treated at a higher temperature as described in the following paragraph.
The tensile strength of small sections such as bolts and other fasteners can be
improved by heat treatment high in the alpha+beta field, followed by water quenching.
The high-temperature beta phase is thereby transformed into a martensitic structure,
which responds to controlled ageing, giving a useful increase in strength. This avoids
problems due to excessive contamination and the possibility of gram growth, which are
encountered at higher temperatures.
Elevated-temperature tensile tests have shown that the strength increase is
proportionally retained at temperatures up to 540°C. The strengthening effects
referred to earlier can only be obtained following a rapid quench from the
solution-treatment temperature. Solution treatment and ageing is usually restricted,
therefore, to small sections such as aircraft fasteners.
Mechanical properties
Room-temperature tensile properties
Various forms of Ti-6%Al-4%V, such as rod, bar and billet, are sold as forging stock
and are therefore left in the hot-rolled or forged condition. It is only guaranteed
to have the specification properties after annealing at 700°C; material greater
than 150 mm ruling section may be tested on an upset-forged and annealed slice.
Elevated-temperature and sub-zero tensile properties
The properties of Ti-6%Al-4%V vary smoothly with temperature, which covers the
range from minus 196°C up to 750°C. Although it retains useful short-term
properties up to 500°C, its properties over the longer term tend to limit its
useful range to 300°C, as suggested by the stress-rupture and creep curves.
Creep and Stability
Creep testing has shown that heat-treated material both metallurgical stability and
surface stability under conditions of stressed exposure for up to 500 h at
450°C.
Fatigue properties
Rotating bending fatigue tests on specimens machined from 20 mm annealed rod have
shown lives of >107 cycles at ±560 MPa (81 ksi). Samples of larger bar,
i.e. 60 mm dia., have given slightly lower values of ±430 MPa (62 ksi) on
smooth specimens, both plain and after anodizing. Notched specimens (Kt = 2,7) gave
values of ±230 and 210 MPa (33 and 30 ksi) respectively.
Direct-stress zero minimum fatigue tests on 25 mm diameter bar gave fatigue limits
of 690 MPa (100 ksi) on smooth specimens and 260 MPa (39 ksi) on notched specimens
(Kt = 3).
Fracture toughness
Titanium alloy Ti-6%Al-4%V has good fracture toughness, as shown by the following
properties obtained on 75 mm diameter bar.
Table2. Effect of heat treatment on tensile and fracture toughness properties
of 75 mm diameter bar.
|
Heat treatment
|
0-2% proof stress
Mpa
|
Tensile strength
Mpa
|
Elongation on 50 mm
%
|
Reduction in area
%
|
Fracture toughness
MPa√m
|
Annealed
2 h/700°C
|
890
|
980
|
17
|
39
|
84
|
|
1h/900°C.WQ+8h/500°C
|
970
|
1080
|
16
|
42
|
69
|
|
1h/960°C.WQ+2h/700°C
|
950
|
1030
|
14
|
37
|
57
|
Impact properties
A typical room-temperature Izod value of IMI 318 is 20 J, toughness varying quite
smoothly with temperature from 95 J at 500°C down to 15 J at minus 196°C.
Properties of welds
Electron-beam welds
Alloy Ti-6%Al-4%V is an excellent material for joining by electron-beam welding
techniques. Electron-beam welding has been widely adopted for critical components
such as the center wing-box for the Tornado, Concorde engine thrust struts, and the
engine spool assembly for the Rolls-Royce Gem.
Flash-butt welds
Alloy Ti-6%Al-4%V is readily joined by flash-butt welding, a process, which is
widely used for the manufacture of engine rings.
A hardness survey showed small peaks on either side of the weld, in the as-welded
condition, but these disappeared on heat treatment.
Tungsten-inert-gas welds
Ti-6%Al-4%V is less suited to TIG welding. There is little change in tensile
strength, but the tensile elongation of, for example, TIG welded 1.6 mm sheet
measured over 50 mm can drop from 14 to 5 per cent. Post-weld heat treatment in the
range 700-800°C improves ductility, but can cause surface oxidation and distortion
of sheet-based structures.
Applications
Titanium alloy Ti-6%Al-4%V is perhaps the most fully evaluated of all titanium alloys
and has been used in the widest range of finished parts. Originally developed for the
aircraft industry, it has been used as sheet fabrications, brackets and fasteners where
lightness and high strength are required.
Its easy forgeability and strength at moderate temperature has led to extensive use
as compressor blades and discs in gas-turbine engines and as fan blades in the most
recent turbofan engines. An entirely new range of cost and weight saving components for
both airframes and engines are now being developed using superplastic forming and
diffusion bonding processes, for which this alloy is ideal.
Industries other than the aircraft industry have used for steam-turbine blades and
lacing wire, axial and radial-flow gas compressor discs, springs for corrosion
resistance, data logging capsules for oil and mineral exploration, etc.
A growing use of Ti-6%Al-4%V is as an implant material. Its excellent biocompatibility
and good fatigue strength in body fluids make it ideal for the replacement of hip and
knee joints, for bone screws, and for other surgical devices.
Other uses include reciprocating and rotating parts such as compressor valve plates,
internal-combustion-engine connecting rods, rocker arms, valve springs and retaining
caps, road springs and drive shafts for racing cars, and rotors for centrifuges and
ultracentrifuges. Marine uses include armament, sonar equipment, deep-submergence
applications, hydrofoils, telephone cable repeater station capsules, etc.
Although one of the earliest titanium alloys is studied so widely, many fresh uses
are still being found for this versatile material.