Kutz M. Handbook of materials selection
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228 SELECTION OF TITANIUM ALLOYS FOR DESIGN
of these processes may strongly influence properties of titanium and its alloys,
either alone or by interacting with effects of processes to which the metal has
previously been subjected. Titanium alloy forgings are produced by all the forg-
ing methods currently available. These include open-die, closed-die, rotary forg-
ing, and others. One of the main purposes of forging, in addition to shape
control, is to obtain a combination of mechanical properties that generally does
not exist in bar or billet. Tensile strength, creep resistance, fatigue strength, and
toughness all may be better in forgings than in bar or other forms. Selection of
the optimal forging method is based on the shape desired for the forged com-
ponent as well as its desired mechanical properties and microstructure (which
largely determines properties after alloy composition is set).
Open-die forging is used to produce some shapes in titanium when volume
and/or size do not warrant the development of closed dies for a particular ap-
plication. However, closed-die forging is used to produce the greatest amount
of forged titanium alloys. Closed-die forging can be classified as blocker-type
(single die set), conventional (two or more die sets), or high definition (two or
more die sets). Precision die forging is also conducted, usually employing hot-
die/isothermal forging techniques. Conventional closed-die titanium forgings
cost more than the blocker-type, but the increase in cost is justified because of
reduced machining costs and better property control. The dies used in titanium
forging are heated to facilitate the forging process and to reduce surface chilling
and metal temperature losses that can lead to inadequate die filling and/or ex-
cessive surface cracking of the forged component. Hot-die/isothermal forging
takes the die temperature to higher levels.
Forging is more than just a shape-making process. The key to successful
forging and heat treatment is the

transus temperature. Fundamentally, there
are two principal approaches to the forging of titanium alloys:
●
Forging the alloy predominantly below the

transus
●
Forging the alloy predominantly above the

transus
Conventional
␣
–

forging is best described as a forging process in which all or
most of the deformation is conducted below the

transus. Alpha,

and trans-
formed

-phases will be present in the microstructure at some time. Structures
resulting from
␣
–

forging are characterized by deformed or equiaxed primary
␣
-phase (
␣
present during the forging process) and transformed

phase (acicular
in appearance). Beta forging is a forging technique in which the deformation of
the alloy is done above the

transus. In commercial practice,

forging actually
involves initial deformation above the

transus but final finishing with con-
trolled amounts of deformation below the

transus of the alloy. In

forging,
the influences of mechanical working (deformation) are not necessarily cumu-
lative because of the high temperature and because of the formation of new
grains by recrystallization each time the

transus is surpassed on reheating for
forging. Beta forging, particularly of
␣
and
␣
–

alloys, results in significant
reductions in forging pressures and reduced cracking tendency of the billet dur-
ing forging.
An alternative titanium die forging procedure involves the use of precision
isothermal (sometimes superplastic) forging techniques. Precision isothermal
6 MANUFACTURING PROCESSES 229
forging produces a product form that requires much less machining than con-
ventionally forged alloy to achieve final dimensions of the component. Precision-
forged titanium is a significant factor in titanium usage in the aircraft and gas
turbine engine field. Most precision-forged titanium is produced as near-net
shape (NNS) products, meaning that the forging is close to final dimensions but
that some machining is required.
Superplastic forming, a variant of superplastic isothermal forging, currently
is widely used in the aircraft industry and to a lesser extent is used in the gas
turbine industry. Advantages of superplastic forming are, among others:
●
Very complex parts can be formed.
●
Lighter, more efficient structures can be designed and formed.
●
It is performed in a single operation.
●
More than one piece may be produced in a machine cycle.
●
Pressure (force) is uniformly applied to all areas of the workpiece.
Superplastic forming coupled with diffusion bonding (SPFDB) has been used
on titanium alloys to produce complex fabricated structures.
6.4 Investment Casting
Cost factors associated with wrought alloy processing led to continual efforts to
develop and improve casting methods for titanium and its alloys. The result has
been a somewhat checkered application of titanium castings with a more wide-
spread acceptance of the practice in the last decade of the twentieth century.
Titanium castings now are used extensively in the aerospace industry and to
lesser measure in the chemical process, marine, biomedical, automotive, and
other industries. The investment casting process uses a disposable mold to create
a negative image of the desired component. Metal fills the mold and solidifies
with the desired shape and dimensions very close to final desired values. Some
machining is necessary.
Several alloys were studied at first, but soon investigators concentrated on
Ti–6Al–4V with results that supported an idea that cast-titanium parts could be
made with strength levels and characteristics approaching those of conventional
wrought alloys. Subsequently, additional titanium components have successfully
been cast from pure titanium,
␣
–

and

-alloys. Nonetheless, the primary alloy
used for casting of titanium components is Ti–6Al–4V. Some important concepts
to remember are:
●
Hot isostatic pressing may be required to close casting porosity.
●
Heat treatment to develop properties may require close monitoring.
●
Cast component properties will tend to fall in the lower end of the scat-
terband for wrought versions of the alloy chosen (Ti–6Al–4V, unlikely
that any other conventional alloy will be cast).
●
Section thickness may affect properties generated in castings.
6.5 Machining and Residual Stresses
Machining of titanium alloys is similar to but more difficult than that of ma-
chining stainless steels. In welding or machining of titanium alloys, the effects
230 SELECTION OF TITANIUM ALLOYS FOR DESIGN
of the energy input (heat energy, deformation energy) on the microstructure and
properties of the final product must be considered, just as it must be done in
forging. Favorable residual stresses have been generated on titanium surfaces for
years. Properties measured will degrade dramatically if the favorable surface
residual stresses are reduced, for example, by chemical polishing. Shot peening
is a favorite method of increasing a titanium alloy’s fatigue strength, at least in
airfoil roots and other nongas path regions.
6.6 Joining
Components of titanium alloys are routinely welded. Titanium and most titanium
alloys can be joined by the following fusion welding techniques:
●
Gas–tungsten arc welding (GTAW)
●
Gas–metal arc welding (GMAW)
●
Plasma arc welding (PAW)
●
Electron beam welding (EBW)
●
Laser beam welding (LBW)
and by brazing or such solid-state joining techniques as diffusion bonding, inertia
bonding, and friction welding.
Just as occurs in the heat treatment of titanium and its alloys, fusion welding
processes can lead to pickup of detrimental gases. Alloys must be welded in
such a way as to preclude interstitial gases such as oxygen from being incor-
porated in the weld or weld-heat-affected area. For successful arc welding of
titanium and titanium alloys, complete shielding of the weld is necessary because
of the high reactivity of titanium to oxygen and nitrogen at welding temperatures.
Excellent welds can be obtained in titanium and its alloys in a welding chamber,
where welding is done in a protective gas atmosphere, thus giving adequate
shielding. When welding titanium and titanium alloys, only argon or helium,
and occasionally a mixture of these two gases, are used for shielding. Since it
is more readily available and less costly, argon is more widely used. Welding in
a chamber, however, is not always practical. Open-air techniques can be used
with fusion welding when the area to be joined is well shielded by an inert gas
using a Mylar bag for gas containment. Such atmospheric control by means of
a temporary bag, or chamber, is preferred. Because titanium alloy welds are
commonly used in fatigue-critical applications, a stress relief operation is gen-
erally required following welding to minimize potentially detrimental residual
stresses. The essence of joining titanium and its alloys is adhering to the follow-
ing principle conditions that need to be met:
●
Detrimental interstitial elements must be excluded from the joint region.
●
Contaminants (scale, oil, etc.) must be excluded from the joint region.
●
Detrimental phase changes must be avoided to maintain joint ductility.
When proper techniques are developed and followed, the welding of thin-to-
moderate section thickness in titanium alloys can be accomplished successfully
using all of the aforementioned processes. For welding titanium thicker than
about 2.54 mm (0.10 in.) by the GTAW process, a filler metal must be used.
7 OTHER ASPECTS OF TITANIUM ALLOY SELECTION 231
For PAW, a filler metal may or may not be used for welding metal less than
12.7 mm (0.5 in.) thick.
Titanium and its alloys can be brazed. Argon, helium, and vacuum atmo-
spheres are satisfactory for brazing titanium. For torch brazing, special fluxes
must be used on the titanium. Fluxes for titanium are primarily mixtures of
fluorides and chlorides of the alkali metals, sodium, potassium, and lithium.
Vacuum and inert-gas atmospheres protect titanium during furnace and
induction-brazing operations. Titanium assemblies frequently are brazed in high-
vacuum, cold–wall furnaces. A vacuum of l0
⫺
3
torr, or more, is required to braze
titanium. Ideally, brazing should be done in a vacuum at a pressure of about
10
⫺
5
–10
⫺
4
torr or done in a dry inert-gas atmosphere if vacuum brazing is not
possible.
7 OTHER ASPECTS OF TITANIUM ALLOY SELECTION
7.1 Corrosion
Although titanium and its alloys are used chiefly for their desirable mechanical
properties, among which the most notable is their high strength-to-weight ratio,
another important characteristic of the metal and its alloys is titanium’s outstand-
ing resistance to corrosion. CP titanium offers excellent corrosion resistance in
most environments, except those media that contain fluoride ions. Unalloyed
titanium is highly resistant to the corrosion normally associated with many nat-
ural environments, including seawater, body fluids, and fruit and vegetable
juices. Titanium exposed continuously to seawater for about 18 years has un-
dergone only superficial discoloration. Titanium is more corrosion resistant than
stainless steel in many industrial environments, and its use in the chemical pro-
cess industry has been continually increasing. Titanium exhibits excellent resis-
tance to atmospheric corrosion in both marine and industrial environments.
The major corrosion problems with titanium alloys appear to be crevice cor-
rosion, which occurs in locations where the corroding media are virtually stag-
nant. Pits, if formed, may progress in a similar manner. Other problem areas are
with a potential for stress–corrosion, particularly at high temperatures, resulting
in hot-salt stress–corrosion cracking (HSSCC). HSSCC has been observed in
experimental testing and an occasional service failure. Stress–corrosion cracking
(SCC) is a fracture, or cracking, phenomenon caused by the combined action of
tensile stress, a susceptible alloy, and a specific corrosive environment. Another
important characteristic of SCC is the requirement that tensile stress be present.
Aluminum additions increase susceptibility to SCC; alloys containing more than
6% Al generally are susceptible to stress–corrosion.
HSSCC of titanium alloys is a function of temperature, stress, and time of
exposure In general, HSSCC has not been encountered at temperatures below
about 260
⬚C (500⬚F). The greatest susceptibility occurs at about 290–425⬚C
(about 550–800
⬚F) based on laboratory tests. Time-to-failure decreases as either
temperature or stress level is increased. All commercial alloys, but not unalloyed
titanium, have some degree of susceptibility to HSSCC. Alpha alloys are more
susceptible than other alloys.
7.2 Biomedical Applications
Titanium alloys have become standards in the orthopedic industry where hip
implants, for example, benefit from several characteristics of titanium:
232 SELECTION OF TITANIUM ALLOYS FOR DESIGN
●
Excellent resistance to corrosion by body fluids
●
High specific strength owing to good mechanical strengths and low density
●
Modulus about 50–60% of that of competing cobalt-base superalloys
Corrosion resistance benefits would seem to be evident. High specific strength,
however, enables a lighter implant to be made with attendant improvement in
patient response to the device. Lastly, the modulus of bone is very low, about
10% of that of stainless steel or cobalt-base alloys. The degree of load transfer
from the implant to the bony structure in which it is implanted and which it
replaces is a direct function of the modulus. By reducing the elastic modulus,
the bone can be made to receive a greater share of the load. The result with
lower modulus titanium alloys is a longer operating time before breakdown of
the implant–bone assembly.
7.3 Cryogenic Applications
Many of the available
␣
and
␣
–

titanium alloys have been evaluated at subzero
temperatures, but service experience at such temperatures has been gained only
for a few alloys. Ti–5Al–2.5Sn and Ti–6Al–4V have very high strength–to–
weight ratios at cryogenic temperatures and have been the preferred alloys for
special applications at temperatures from
⫺196 to ⫺269⬚C(⫺320 to ⫺452⬚F).
Impurities such as iron and the interstitials oxygen, carbon, nitrogen, and hy-
drogen tend to reduce the toughness of these alloys at both room and subzero
temperatures. For maximum toughness, extra-low-interstitial (ELI) grades are
specified for critical applications.
8 FINAL COMMENTS
Many titanium alloys have been developed, although the total is small compared
to other metals such as steels and superalloys. A principle reason for this situ-
ation is the high cost of alloy development and of proving the worth and safety
of a new material. In the sport world, titanium made a brief fling at commercial
nongas turbine applications when the golf club market virtually tied up titanium
metal for a short time in the 1990s. Titanium bicycle frames are marketed but
are quite expensive.
The titanium market has been a roller coaster over the years, and gas turbine
applications remain the most significant part of the application market. Within
most aircraft gas turbine engine companies, only a few alloys have ever made
it to production. Admittedly this list differs from company to company in the
United States and somewhat more with alloys used outside the United States.
Nevertheless, it is apparent that, although the ability to push titanium’s operating
environment higher in temperature has resulted in significant gains, advances
have tapered off. Since the mid-1950s when Pratt & Whitney put the first tita-
nium in U.S. gas turbines, much industrial and government funding has been
used to increase alloy capabilities. Titanium aluminides have been the subject
of multidecades of study with interesting but hardly commercially viable results.
Barring discovery of some unforeseen nature, the message is that, if an existing
alloy works and a new alloy does not offer some benefit that overrides the
development cost of proving up the alloy for its new use, do not change alloys.
For the ever-shrinking cadre of developers in industry, the current status suggests
BIBLIOGRAPHY 233
Table 11 Associations Providing Titanium Information
Titanium Information Group
Trevor J. Glover, Secretary
5, The Lea
Kidderminster, DY11 6JY
United Kingdom
TEL:
⫹44 (0) 1562 60276
FAX:
⫹44 (0) 1562 824851
WEB: www.titaniuminfogroup.co.uk
E-MAIL: rayportman@talk21.com
International Titanium Association
Brian Simpson, Executive Director
350 Interlocken Blvd. Suite 390
Broomfield, CO 80021-3485
TEL: 303 404 2221
FAX: 303 404 9111
WEB: www.titanium.org
E-MAIL: bsimpson@titanium.net
Japan Titanium Society
22-9 Kanda Nishiki-Cho
Chiyoda-Ku, Tokyo ZIP 101
Japan
TEL: 081 (3) 3293 5958
FAX: 081 (3) 3293 6187
WEB: www.titan-japan.com
that efforts to tailor existing alloys and ‘‘sell’’ them for new or existing appli-
cations may have better return on investment.
If an alloy selector is starting from scratch to pick an alloy for an application,
then any commercially available alloy may be fair game. On the other hand, the
best alloy may not be available owing to corporate patent protection or insuffi-
cient market to warrant its continued production by the limited number of man-
ufacturers. Then, selection of another alloy from a producer may be necessary
but may possibly require development costs to get the product in workable form
and to determine design properties. If possible, select a known alloy that has
more than one supplier and more than one casting or forging source. In all
likelihood, unless a special need (such as formability of sheet) or maximum
high-temperature strength is required, Ti–6Al–4V might be the first choice. For
special needs such as in marine applications or biomedical orthopedic situations,
choice of other alloys may be warranted. In any event, one should work with
the suppliers and others in the manufacturing chain to acquire typical or design
properties for the alloy in the form it will be used. Generic alloys owned by
melters or developers are best for the alloy selector not associated with one of
the big corporate users of titanium alloys. Companies with proprietary interests
usually have nothing to benefit from giving up a technological advantage by
sharing design data or even granting a production release to use a proprietary
alloy. Table 11 lists a few organizations chartered to provide assistance to users
of titanium products. A list of suppliers should be available from them.
BIBLIOGRAPHY
Boyer, R., G. Welsch, and E. Collings principal editors, Materials Property Handbook: Titanium
Alloys, ASM International, Materials Park, OH, 1994.
Collings, E. W., ‘‘Physical Metallurgy of Titanium Alloys,’’ in Materials Property Handbook: Tita-
nium Alloys, ASM International, Materials Park, OH, 1994, pp. 1–122.
Donachie, M., Titanium: A Technical Guide, 2nd ed., ASM International, Materials Park, OH, 2001.
Hanson, B., The Selection and Use of Titanium, The Institute of Materials, London, England, 1995.
234 SELECTION OF TITANIUM ALLOYS FOR DESIGN
International Conferences on Titanium, Proceedings of a continuing series of conferences held pe-
riodically and published by various organizations since 1968.
Metals Handbook, 10th ed., ASM International, Materials Park, OH, appropriate volumes on topics
of interest.
Titanium Information Group, The Effective Use of Titanium: A Designer and User’s Guide, brochure,
Kiddermeister, England, 1992.
235
CHAPTER 7
NICKEL AND ITS ALLOYS
T. H. Bassford
Jim Hosier
Inco Alloys International, Inc.
Huntington, West Virginia
1 INTRODUCTION 235
2 NICKEL ALLOYS 236
2.1 Classification of Alloys 236
2.2 Discussion and Applications 237
3 CORROSION 248
4 FABRICATION 252
4.1 Resistance to Deformation 252
4.2 Strain Hardening 253
5 HEAT TREATMENT 254
5.1 Reducing Atmosphere 256
5.2 Prepared Atmosphere 256
6 WELDING 256
7 MACHINING 257
8 CLOSURE 257
REFERENCES 258
1 INTRODUCTION
Nickel, the 24th element in abundance, has an average content of 0.016% in the
outer 10 miles of the earth’s crust. This is greater than the total for copper, zinc,
and lead. However, few of these deposits scattered throughout the world are of
commercial importance. Oxide ores commonly called laterites are largely dis-
tributed in the tropics. The igneous rocks contain high magnesium contents and
have been concentrated by weathering. Of the total known ore deposits, more
than 80% is contained in laterite ores. The sulfide ores found in the northern
hemispheres do not easily concentrate by weathering. The sulfide ores in the
Sudbury district of Ontario, which contain important by-products such as copper,
cobalt, iron, and precious metals are the world’s greatest single source of nickel.
1
Nickel has an atomic number of 28 and is one of the transition elements in
the fourth series in the periodic table. The atomic weight is 58.71 and density
is 8.902 g/cm
3
. Useful properties of the element are the modulus of elasticity
and its magnetic and magnetostrictive properties, and high thermal and electrical
conductivity. Hydrogen is readily adsorbed on the surface of nickel. Nickel will
also adsorb other gases such as carbon monoxide, carbon dioxide, and ethylene.
It is this capability of surface adsorption of certain gases without forming stable
compounds that makes nickel an important catalyst.
2
Reprinted from Mechanical Engineers’ Handbook, 2nd ed., Wiley, New York, 1998, by permission
of the publisher.
HandbookofMaterialsSelection.EditedbyMyerKutz
Copyright Ó 2002 John Wiley & Sons, Inc., NewYork.
236 NICKEL AND ITS ALLOYS
As an alloying element, nickel is used in hardenable steels, stainless steels,
special corrosion-resistant and high-temperature alloys, copper– nickel,
‘‘nickel–silvers,’’ and aluminum–nickel. Nickel imparts ductility and toughness
to cast iron.
Approximately 10% of the total annual production of nickel is consumed by
electroplating processes. Nickel can be electrodeposited to develop mechanical
properties of the same order as wrought nickel; however, special plating baths
are available that will yield nickel deposits possessing a hardness as high as 450
Vickers (425 BHN). The most extensive use of nickel plate is for corrosion
protection of iron and steel parts and zinc-base die castings used in the auto-
motive field. For these applications, a layer of nickel, 0.0015–0.003 in. thick, is
used. This nickel plate is then finished or covered with a chromium plate con-
sisting in thickness of about 1% of the underlying nickel plate thickness in order
to maintain a brilliant, tarnish-free, hard exterior surface.
2 NICKEL ALLOYS
Most of the alloys listed and discussed are in commercial production. However,
producers from time to time introduce improved modifications that make pre-
vious alloys obsolete. For this reason, or economic reasons, they may remove
certain alloys from their commercial product line. Some of these alloys have
been included to show how a particular composition compares with the strength
or corrosion resistance of currently produced commercial alloys.
2.1 Classification of Alloys
Nickel and its alloys can be classified into the following groups on the basis of
chemical composition.
3
Nickel
(1) Pure nickel, electrolytic (99.56% Ni), carbonyl nickel powder and pellet
(99.95% Ni); (2) commercially pure wrought nickel (99.6–99.97% nickel); and
(3) anodes (99.3% Ni).
Nickel and Copper
(1) Low-nickel alloys (2–13% Ni); (2) cupronickels (10–30% Ni); (3) coinage
alloy (25% Ni); (4) electrical resistance alloy (45% Ni); (5) nonmagnetic alloys
(up to 60% Ni); and (6) high-nickel alloys, Monel (over 50% Ni).
Nickel and Iron
Wrought alloy steels (0.5–9% Ni); (2) cast alloy steels (0.5–9% Ni); (3) alloy
cast irons (1–6 and 14–36% Ni); (4) magnetic alloys (20–90% Ni): (a) con-
trolled coefficient of expansion (COE) alloys (29.5–32.5% Ni) and (b) high-
permeability alloys (49–80% Ni); (5) nonmagnetic alloys (10–20% Ni); (6) clad
steels (5–40% Ni); (7) thermal expansion alloys: (a) low expansion (36–50%
Ni) and (b) selected expansion (22–50% Ni).
Iron, Nickel, and Chromium
(1) Heat-resisting alloys (40–85% Ni); (2) electrical resistance alloys (35–60%
Ni); (3) iron-base superalloys (9–26% Ni); (4) stainless steels (2–25% Ni); (5)
valve steels (2–13% Ni); (6) iron-base superalloys (0.2–9% Ni); (7) maraging
steels (18% Ni).
2 NICKEL ALLOYS 237
Nickel, Chromium, Molybdenum, and Iron
(1) Nickel-base solution-strengthened alloys (40–70% Ni); (2) nickel-base pre-
cipitation-strengthened alloys (40–80% Ni).
Powder-Metallurgy Alloys
(1) Nickel-base dispersion strengthened (78–98% Ni); (2) nickel-base mechan-
ically alloyed oxide-dispersion-strengthened (ODS) alloys (69–80% Ni).
The nominal chemical composition of nickel-base alloys is given in Table 1.
This table does not include alloys with less than 30% Ni, cast alloys, or welding
products. For these and those alloys not listed, the chemical composition and
applicable specifications can be found in the Unified Numbering System for
Metals and Alloys, published by the Society of Automotive Engineers, Inc.
2.2 Discussion and Applications
The same grouping of alloys used in Tables 1, 2, and 3, which give chemical
composition and mechanical properties, will be used for discussion of the various
attributes and uses of the alloys as a group. Many of the alloy designations are
registered trademarks of producer companies.
Nickel Alloys
The corrosion resistance of nickel makes it particularly useful for maintaining
product purity in the handling of foods, synthetic fibers, and caustic alkalies,
and also in structural applications where resistance to corrosion is a prime con-
sideration. It is a general-purpose material used when the special properties of
the other nickel alloys are not required. Other useful features of the alloy are its
magnetic and magnetostrictive properties; high thermal and electrical conductiv-
ity; low gas content; and low vapor pressure.
4
Typical nickel 200 applications are food-processing equipment, chemical ship-
ping drums, electrical and electronic parts, aerospace and missile components,
caustic handling equipment and piping, and transducers.
Nickel 201 is preferred to nickel 200 for applications involving exposure to
temperatures above 316
⬚C (600⬚F). Nickel 201 is used as coinage, plater bars,
and combustion boats in addition to some of the applications for Nickel 200.
Permanickel alloy 300 by virtue of the magnesium content is age-hardenable.
But, because of its low alloy content, alloy 300 retains many of the character-
istics of nickel. Typical applications are grid lateral winding wires, magneto-
striction devices, thermostat contact arms, solid-state capacitors, grid side rods,
diaphragms, springs, clips, and fuel cells.
Duranickel alloy 301 is another age-hardenable high nickel alloy, but is made
heat treatable by aluminum and titanium additions. The important features of
alloy 301 are high strength and hardness, good corrosion resistance, and good
spring properties up to 316
⬚C (600⬚F); and it is on these mechanical consider-
ations that selection of the alloy is usually based. Typical applications are ex-
trusion press parts, molds used in the glass industry, clips, diaphragms, and
springs.
Nickel–Copper Alloys
Nickel–copper alloys are characterized by high strength, weldability, excellent
corrosion resistance, and toughness over a wide temperature range. They have