Titanium Alloys are
classified into
Alpha alloys:
The single-phase and
near-single-phase alpha
alloys of titanium
have good weldability. The
generally high aluminum
content of this group of
alloys ensures good
strength characteristics
and oxidation resistance
at elevated temperatures
(in the range of 600 to
1,100°F). Alpha alloys
cannot be heat treated to
develop higher mechanical
properties because they
are single-phase alloys.
Alpha-beta alloys:
The addition of controlled
amounts of
beta-stabilizing alloying
elements causes the beta
phase to persist below the
beta transus temperature,
down to room temperature,
resulting in a two-phase
system. These two-phase
titanium alloys can be
strengthened significantly
by heat treatment
consisting of a quench
from some temperature in
the alpha-beta range,
followed by an aging cycle
at a somewhat lower
temperature. Beta-phase
transformation, which
would normally occur on
slow cooling, is
suppressed by the
quenching. The aging cycle
causes the precipitation
of some fine alpha
particles from the
metastable beta, imparting
a structure that is
stronger than the annealed
alpha-beta structure.
Although heat-treated
alpha-beta alloys are
stronger than the alpha
alloys, their ductility is
proportionally lower.
Beta alloys:
The high percentage of
beta-stabilizing elements
in these alloys results in
a microstructure that is
substantially beta. The
metastable beta can be
strengthened considerably
by heat treatment.
Titanium is used in
corrosive environments or
in applications that
require light weight, high
strength-to-weight ratio,
and nonmagnetic
properties. While
commercially available in
many alloys, most
requirements can be met by
a grade of commercially
pure titanium,
titanium-0.2% palladium
alloy, or by the
high-strength Ti-Al-V-Cr
(beta type) alloys. These
grades, which are
available in most common
wrought mill forms, are
covered by ASTM-AMS
specifications and, in
most cases, by a similar
ASME specification.
Beta-21S is a new beta
alloy developed as an
oxidation-resistant
aerospace material and as
a matrix for metal-matrix
composites. Composition is
Ti-15Mo-2.7Nb-3Al-0.2Si,
with molybdenum and
niobium working
synergistically to raise
corrosion resistance to
very high levels. It also
offers one of the lowest
hydrogen uptake efficiency
levels of any titanium
alloy. The combination of
high strength and high
corrosion resistance make
it an ideal candidate for
orthopedic implants, deep
sour oil wells, and
geothermal brine wells.
Like stainless steel,
titanium
sheet and plate work
harden significantly
during forming. Minimum
bend-radius rules are
nearly the same for both,
although springback is
greater for titanium.
Commercially pure grades
of heavy plate are cold
formed or, for more severe
shapes, warm formed at
temperatures to about
800°F. Alloy grades can be
formed at temperatures as
high as 1,400°F in
inert-gas atmospheres.
Tube can be cold bent to
radii three times the tube
OD, provided that both
inside and outside
surfaces of the bend are
in tension at the point of
bending. In some cases,
tighter bends can be made.
Despite their high
strength, some alloys of
titanium have superplastic
characteristics in the
range of 1,500 to 1,700°F.
The alloy used for most
superplastically formed
parts is the standard
Ti-6Al-4V alloy. Several
aircraft manufacturers are
producing components
formed by this method.
Some applications involve
assembly by diffusion
bonding.
Titanium plates
or sheets can be sheared,
punched, or perforated on
standard equipment.
Titanium and Ti-Pd
alloy plates can be
sheared subject to
equipment limitations
similar to those for
stainless steel. The
harder alloys are more
difficult to shear, so
thickness limitations are
generally about two-third
those for stainless
steel.
Titanium
and its alloys can be
machined and abrasive
ground; however, sharp
tools and continuous feed
are required to prevent
work hardening. Tapping is
difficult because the
metal galls. Coarse
threads should be used
where possible.
Titanium
castings can be produced
by investment or
graphite-mold methods.
Casting must be done in a
vacuum furnace, however,
because of the highly
reactive nature of
titanium in the presence
of oxygen. Typical
applications for titanium
castings are surgical
implants and hardware for
marine and chemical
equipment such as
compressors and valve
bodies.
Generally, titanium is
welded by gas-tungsten arc
(GTA) or plasma-arc
techniques. Metal
inert-gas processes can be
used under special
conditions. Thorough
cleaning and shielding are
essential because molten
titanium reacts with
nitrogen, oxygen, and
hydrogen, and will
dissolve large quantities
of these gases, which
embrittles the metal. In
all other respects, GTA
welding of titanium is
similar to that of
stainless steel. Normally,
a sound weld appears
bright silver with no
discoloration on the
surface or along the
heat-affected zone.
RMT is a renowned
supplier of Titanium in
the form of Pipes,
Sheets, Plates, Flanges
and Fittings
