Since the introduction of titanium and titanium alloys in the early
1950s, these materials have in a relatively short time become backbone
materials for the aerospace, energy, and chemical industries.
The combination of high strength-to-weight ratio, excellent mechanical
properties, and corrosion resistance makes titanium the best material
choice for many critical applications. Today, titanium alloys are used
for demanding applications such as static and rotating gas turbine
engine components. Some of the most critical and highly-stressed
civilian and military airframe parts are made of these alloys.
The use of titanium has expanded in recent years to include applications
in nuclear power plants, food processing plants, oil refinery heat
exchangers, marine components and medical protheses.
The high cost of titanium alloy components may limit their use to
applications for which lower-cost alloys, such as aluminium and
stainless steels. The relatively high cost is often the result of
the intristic raw material cost of metal, fabricating costs and
the metal removal costs incurred in obtaining the desired final shape.
These titanium net shape technologies include powder metallurgy (P/M),
superplastic forming (SPF), precision forging, and precision casting.
Precision casting is by far the most fully developed and the most widely
used titanium net shape technology. The annual shipment of titanium
castings in the United States increased by 260% between 1979 and 1989.
As aircraft engine manufactures seek to use cast titanium at higher
operating temperatures, Ti-6Al-2Sn-4Zr-2Mo and
are being specified more frequently. Other advanced high-temperature
titanium alloys for service up to 595oC, such as Ti-1100 and IMI-834
are being developed as castings. The alloys mentioned above exhibit
the same degree of elevated-temperature superiority, as do their
wrought counterparts over the more commonly
The wrought product forms of titanium and titanium-base alloys, which
include forgings and typical mill products, constitute more than 70% of
the market in titanium and titanium alloy production. The wrought
products are the most readily available product form of titanium-base
materials, although cast and powder metallurgy (P/M) products are
also available for applications that require complex shapes or
the use of P/M techniques to obtain microstructures not achievable
by conventional ingot metallurgy.
Powder metallurgy of titanium has not gained wide acceptance and is
restricted to space and missile applications. The primary reasons for
using titanium-base products are its outstanding corrosion resistance
of titanium and its useful combination of low density (4.5 g/cm3) and
high strength. The strengths vary from 480 MPa for some grades of
commercial titanium to about 1100 MPa for structural titanium alloy
products and over 1725 MPa for special forms such as wires and springs.
Another important characteristic of titanium- base materials is the
reversible transformation of the crystal structure from alpha
(a, hexagonal close-packed) structure
to beta (b, body-centered cubic) structure
when the temperatures exceed certain level. This allotropic behavior,
which depends on the type and amount of alloy contents, allows
complex variations in microstructure and more diverse strengthening
opportunities than those of other nonferrous alloys such as copper
Pure titanium wrought products, which have minimum titanium contents
ranging from about 98,635 to 99,5 wt%, are used primarily for corrosion
resistance. Titanium products are also useful for fabrication but have
relatively low strength in service.
Titanium has the following advantages:
Commercially pure titanium with minor alloy contents include various
titanium-palladium grades and alloy Ti-0,3Mo-0,8Ni (ASTM grade 12 or
UNS R533400). The alloy contents allow improvements in corrosion
resistance and/or strength.
- Good strength
- Resistance to erosion and erosion-corrosion
- Very thin, conductive oxide surface film
- Hard, smooth surface that limits adhesion of foreign materials
- Surface promotes dropwise condensation
Titanium-palladium alloys with nominal palladium contents of about
0,2% Pd are used in applications requiring excellent corrosion resistance
in chemical processing or storage applications where the environment
is mildly reducing or fluctuates between oxidizing and reducing.
Alloy Ti-0,3Mo-0,8Ni (UNS R533400, or ASTM grade 12) has applications
similar to those for unalloyed titanium but has better strength and
corrosion resistance. However, the corrosion resistance of this alloy
is not as good as the titanium-palladium alloys. The ASTM grade 12 alloy
is particularly resistant to crevice corrosion in hot brines.
Titanium alloy compositions of various titanium alloys. Because the
allotropic behavior of titanium allows diverse changes in microstructures
by variations in thermomechanical processing, a broad range of properties
and applications can be served with a minimum number of grades.
This is especially true of the alloys with a two-phase, a+b, crystal
The most widely used titanium alloy is the Ti-6Al-4V alpha-beta alloy.
This alloy is well understood and is also very tolerant on variations
in fabrication operations, despite its relatively poor room-temperature
shaping and forming characteristics compared to steel and aluminium.
Alloy Ti-6Al-4V, which has limited section size hardenability, is most
commonly used in the annealed condition.
Other titanium alloys are designed for particular application
areas. For example:
- Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr (commonly called Ti-17) and
Ti-6Al-2Sn-4Zr-6Mo for high strength in heavy sections at elevated
- Alloys Ti-6242S, IMI 829, and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) for creep
- Alloys Ti-6Al-2Nb-ITa-Imo and Ti-6Al-4V-Eli are designed
both to resist stress corrosion in aqueous salt solutions and for
high fracture toughness
- Alloy Ti-5Al-2,5Sn is designed for weldability, and the Eli
grade is used extensively for cryogenic applications
- Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high
strength at low-to-moderate temperatures.
Welding has the greatest potential for affecting material properties.
In all types of welds, contamination by interstitial impurities such
as oxygen and nitrogen must be minimized to maintain useful ductility
in the weldment. Alloy composition, welding procedure, and subsequent
heat treatment are highly important in determining the final properties
of welded joints.
Some general principles can be summarized as follows:
Titanium and titanium alloys are heat treated for the following purposes:
- Welding generally increases strength and hardness
- Welding generally decreases tensile and bend ductility
- Welds in unalloyed titanium grades 1, 2 and 3 do not
require post-weld treatment unless the material will be
highly stressed in a strongly reducing atmosphere
- Welds in more beta-rich alpha-beta alloys such as Ti-6Al-6V-2Sn
have a high likelihood of fracturing with little or no plastic straining.
- To reduce residual stresses developed during fabrication
- To produce an optimal combination of ductility, machinability,
and dimensional and structural stability (annealing)
- To increase strength (solution treating and aging)
- To optimise special properties such as fracture toughness,
fatigue strength, and high-temperature creep strength.
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