Titanium Alloy Ti-2.5Cu, a binary alloy containing 2.5% copper, combines
the formability and weldability of unalloyed titanium with improved mechanical
properties, particularly at elevated temperatures. It can be used at temperatures
up to 350°C. Chemical composition of Ti-2.5Cu is given in the Table 1.
Table 1. Chemical composition of Ti - 2.5Cu alloy
It has been used since 1959 as sheet, forgings and extrusions for
fabricating components such as bypass ducts of gas-turbine engines.
In these applications it has been used in the annealed condition,
but since 1965 its use has spread to the airframe industry, following
the development of an ageing treatment that raises room-temperature
tensile properties by about 25% and nearly doubles the elevated-temperature
properties (e.g. creep at 200°C). Such a material is particularly
attractive since it can be formed in the soft condition, thus lowering
British Standards TA 21, 22 and 23 refer to sheet, bar and forging
stock in the annealed condition. DTD specifications 5233, 5243 and
5253 cover the same three forms of material, but in the ageable or
aged condition. A different heat treatment is given to solution-treated
material (suitable for ageing) from that for annealed material. It is
therefore necessary to decide, before ordering, in which condition it
is required. Specifications BS TA 24 and DTD 5263 refer to annealed
and aged forgings respectively.
Alloy metallurgy. The diagram for the binary titanium-copper
system is shown in Figure 1. The α+β / β transus temperature for a
pure Ti-2.5Cu alloy is around 850°C. The difference from the 895°C
found in commercial material is due to the oxygen and nitrogen content
in the latter.
Figure 1. Binary titanium-copper diagram
There is a maximum solid solubility of 2.1 wt% of copper in alpha
titanium at 798°C, and about 0.7 wt% at 600°C. This gives the
possibility of an age-hardening reaction by precipitation of a compound,
Ti2Cu, from a supersaturated alpha solid solution. It is also
possible to get age hardening from a martensitic alpha structure, obtained
by rapid quenching of the alloy from the beta field, but there is some loss
of ductility in this condition.
In material of commercial purity, X-ray diffraction studies indicate
that maximum supersaturation of the alpha phase with copper is obtained
by solution treatment at 790°C. However, to obtain consistent results
under normal production conditions, a solution-treatment temperature of
805 ± 10°C is used.
Decomposition of the supersaturated solid solution occurs, as in
all age-hardening systems, by a nucleation-and-growth mechanism.
Precipitation is controlled firstly by the number of nuclei available,
which decreases with rise in temperature, and secondly by the rate of
growth of nuclei by diffusion of copper, which increases with rise in
Early work indicated that maximum age hardening occurred after treatment
at 400°C, but the ageing reaction was very slow and ageing times as
long as 140 h were needed to achieve satisfactory response. Higher
temperatures produce a more rapid reaction. The optimum combination is
an initial treatment of 8-24 h at 400°C to develop the maximum number
of well distributed nuclei, followed by further ageing for 8 h at
475°C to give a more rapid growth to the optimum size for maximum
strength. This duplex treatment gives a consistent response without risk
Heat treatment. The full heat treatment consists of solution
treatment at 805 ± 10°C followed by a rapid cool and a duplex
ageing process of 8-24 h at 400°C and 8 h at 475°C. Many current
specifications were evolved at a period when it was thought that 24 h at
400°C was necessary to ensure adequate nucleation. Experience has
shown that this is unnecessarily long, and that 8h is quite sufficient.
This accounts for the wide tolerance in time (8-24 h) for the first part
of the duplex ageing process. All heating may be done in air, provided that
normal precautions are taken to restrict contamination by hydrogen.
For sheet, 1/2 h at the solution-treatment temperature is sufficient.
Rapid cooling is usually achieved by using a forced air blast, which causes
less distortion than a water quench. For thicker sections such as rod, bar,
extrusions or forgings, 1 h per 25 mm of section is recommended for thorough
Ageing, for 8-24 h at 400°C and 8 h at 475°C, should be followed
by air-cooling in each case. It may be necessary to guard against distortion
during the ageing treatment by, for instance, supporting sheet on a flat base
or by holding a complex fabrication in a suitable jig. The linear dimensional
change on ageing is very small.
On occasions, when subsequent development of the fully aged properties is not
necessary, a simple anneal at 790 ± 10°C followed by air cooling
is sufficient for both sheet and thicker sections. This treatment may also
be used to give a full recrystallisation anneal between heavy cold working
operations (e.g. during flow turning).
Stress relieving of annealed material after forming and welding is achieved
by a subsequent treatment at 600°C for 5h. For solution-treated material,
however, the duplex ageing treatment acts as a satisfactory stress-relieving
operation, and the 600°C treatment should not be used since it will cause
The oxide scale formed during annealing or solution treatment should be
removed by vapor blasting or other scale-modifying treatment, followed
by light pickling in a bath containing 4% commercial hydrofluoric acid and
20% nitric acid in water.
Creep properties. Aged Ti-2.5Cu Alloy (IMI Titanium 230) is more
creep-resistant than was IMI Titanium 317 over the range 150-320°C and
more creep-resistant than the hardest grade of commercially pure titanium at
It is important that any material whose properties make it technically
attractive for operation for long periods at elevated temperatures should
be metallurgically and mechanically stable under operating conditions.
The room-temperature tensile properties of aged Ti-2.5Cu sheet are not
affected by creep exposure for 100 h at temperatures in the range
200-350°C. Slight increase in strength and precipitate particle size
is apparent after 5000 h at 350°C but there is no evidence of surface
instability or loss of ductility, even though the post-creep tensile tests
were carried out with no further surface preparation.
Fatigue properties. Like the steels and most other titanium alloys,
Ti-2.5Cu Alloy has a fairly well defined fatigue limit and the S/N curve
becomes horizontal at 107 or 108 reversals of stress.
The fatigue ratio is particularly good; in most cases, the fatigue limit
is more than 0.6 times the static tensile strength.
Forging. Ti-2.5Cu Alloy is very easy to forge. A certain amount
of forging in the α+β field is required to develop optimum properties.
The ideal forging preheating temperature is 800-820°C, though a preheating
temperature of 850°C is commonly used. It may, on occasions, be permissible
to go as high as 875°C for initial roughing operations, provided that a
reduction of at least 2:1 or 4:1 is subsequently carried out at the lower
Forming. As the mechanical properties indicate, Ti-2.5Cu Alloy in the
annealed or solution-treated condition is capable of undergoing considerable
cold deformation without cracking. It is thus amenable to cold forming by
conventional methods, such as pressing, stretch forming, spinning, etc.,
as applied to the stainless steels.
Welding. Ti-2.5Cu Alloy can be joined by fusion, resistance, flash-butt
and pressure welding. Fusion welds can be made by both argon-arc and electron-beam
welding. Joining techniques are governed by the metal`s affinity for atmospheric
gases. At its melting point, titanium rapidly dissolves oxygen, nitrogen, hydrogen
and carbon. Oxy-acetylene, metal-arc, carbon-arc and atomic-hydrogen welding
processes are therefore unsuitable for titanium.
With adequate control of welding techniques, Ti-2.5Cu Alloy is one of the easiest
metals to join. Welds of 100% strength can be obtained, with only a slight
loss in tensile or bend ductility. When making a welded fabrication in an
age-hardenable alloy such as Ti-2.5Cu Alloy, it is best to carry out the
forming and welding on sheet already in the solution-treated condition.
Ageing can then be carried out on the fabricated component. If the alloy
is to be used in the annealed condition, then welding should be followed
by stress relieving for 1/2 h at 600°C.