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Chapter 11
TECHNOLOGIC OPPORTUNITIES FOR TITANIUM
The successf ul introduction of titanium over the past three decades
as a new structural material was based on its properties (i.e., it is
light, strong, ductile, and corrosion resistant) and on the relative
abundance of its ores throughout the world. Titanium's next important
advances may well be based on property and processing improvements
achievable by introducing advanced metal-winning techniques and the
processing of metal to yield preferred fine-grained microstructures,
which give improved properties. Means also appear to be at hand for
incorporating a number of near-net-shape (NNS) technologies into the
production scheme for making titanium end items. The latter technologies
are attractive because they would permit economical parts production and
the production of parts with preferred microstructures. Other practices
that may become available in the future appear to be those that give
improved economy in the production of titanium and provide improved
properties by way of refining microstructures. This chapter considers
those processes that the panel believes are most likely to attain
significant industrial importance in the not too distant future (5 to 10
years).
Metal-Winning Processes
The foremost development in titanium metal-winning, already reduced
to semicommercial practice by the D-H Titanium Company (and, separately,
in pilot production during the past decade by TIMET), is the electrolytic
reduction of the tetrachloride to metal. Available details were
presented in Chapter 5.
Electrolytic winning of titanium is only one of many new advances.
Others include the improved and innovative techniques being incorporated
in new or renovated Kroll and Hunter processing plants for sponge
titanium production. The improved techniques are largely still
proprietary but include changes such as greatly increased batch sizes.
Continuous processes for producing sponge, and even processes f or
continuously producing consolidated metal (ingot), are now in the
research stage. The improved metal-winning processes that are now
available (e.g., larger batch sizes and electrolytic processing) or that
are being researched, are a ma jar key to extended growth in the
applications of titanium.
153
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1S4
Sponge and Alloy Metal Consolidation and Processing
Larger ingots of titanium and titanium alloys (e.g., up to 40,000
pounds at RMI) will be possible in the near future as new furnaces for
the melting of titanium now on the drawing boards or under construction
are completed and begin operation. More exact melting-parameter controls
for the production of more homogeneous ingots are featured in the new
equipment. Larger ingot availability provides not only an
economy-of-size improvement for usual mill products, but also the
possibility of applying titanium to end-items that were previously
marginal due to ingot size limitations.
Notable advances in metal melting are the electron-beam and
plasma-arc furnaces that are now or soon will be in place for the
reclamation and consolidation of titanium scrap forms that f orderly had
little recycling value. The technology for such furnac~ng may not be
regarded as an industrial technological breakthrough, but the application
of these furnaces to the melt-processing step for titanium is a
technologic opportunity. Further advances in this area (e.g., the
continued development of producing square- or rectangular-section slab
ingots suitable for direct rolling to flat-rolled products) are sure to
come. The Japanese have produced a rectangular-section ingot by
plasma-arc remelting . The Soviet s have produced small slab ingots by
electroslag remelting techniques. There is indeed opportunity for the
U. S . titanium industry to develop similar and improved capabilitie s .
The opportunities f or technological advancement in metal working do
not involve equipment so much as techniques; however, the new generation
of metal working equipment (e.g., computer-controlled rotary forging
equipment) will afford numerous advantages. Improvements in metal
processing to achieve preferred microstructures tailored to fit
particular applications for optimized properties (e.g., for needed
combinations of high strength, toughness, creep, fatigue, and
f ormability ~ are known today but not widely applied . Noteworthy
t echniques were described in Chap ter 7 .
Yttrium and Rare Earth Additions
In 1974, the RMI Company pioneered the pilot production of titanium
alloys containing 0.005 to 0.03 weight percent yttrium that refined the
microstructure and significantly improved the yield of salable mill
products from ingots. Because of the reluctance among certain users to
accept yttrium additions in titanium, production was discontinued.
Increased yield equates with increased productivity, a highly
desirable improvement worth pursuing. The apparent advantages and
disadvantages of yttrium additions to titanium alloys present a fairly
complex picture . It is an area worth studying, not only for itself, but
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155
also for the background insights on the several ways in which fine
microstructure s contribute to a number of the technologic opportunities
discussed in this chapter. Accordingly, the panel recommends research
and development on the following aspects of yttrium and rare earth
grain-refining and deoxidizing additions to titanium alloys:
1. Mechanisms for effects, and their control, on properties
produced by yttrium and rare earth additions.
2. Effects of these additions on mechanical properties.
Optimization of the variables and establishment of
specifications that will ensure the safe and full exploitation
of this potential opportunity.
Near-Net-Shape (NNS) Proces sing
The high cost of titanium and its alloys is due primarily to three
f actors: high sponge cost; relatively poor yield f ram sponge to mill
products, particularly for high-strength alloys; and large secondary
fabrication losses, especially in military and commercial aerospace
applicat ions .
The average ratio of buy weight to fly weight for aircraft
applications is estimated to be about 6 :1; some individual parts achieve
a low 1.5:1 ratio whereas others approach 20:1. Since military and
commercial aerospace uses that involve these high fabrication losses
account f or about 75 percent of total aerospace titanium sales,
processing that will improve yields has enormous potential for reducing
cost and f or effectively increasing titanium supply without installing
new sponge capacity. If it is assumed that U.S. sponge availability is
30,000 tons per year, 22,500 would go to aerospace. If the purchase to
end-use ratio for this sponge could be reduced from the estimated average
of 6:1 to 3 :1, effective capacity would be increased by 11,250 tons per
year .
Recognizing these possibilities early, the Manufacturing Technology
Division of the Air Force ~ Materials Laboratory since 1972 has been
actively developing fabrication technology to produce preforms near to
the f inal shape to reduce the enormous f inishing losses and cost s of
producing aircraft components for both airframes and engines. They have
been joined in this effort by the Naval Air Systems Command. The panel
commends these organizations for their foresight and for the excellent
development programs they continue to sponsor. Today, numerous complex
critical parts are flying in advanced aircraf t that give promi se of
achieving important savings in dollars and resources.
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156
Programs sponsored by these organizations cover a broad front
including: precision casting and precision powder metallurgy molding,
both consolidated to theoretical density by hot isostatic pressing;
isothermal shape rolling; diffusion bonding; and the combination of
superplastic forming and diffusion bonding. Each of these areas
represents a ma jor technological opportunity for titans um and should be
pursued vigorously.
Near-net-shape processing programs sponsored by the Air Force
Materials Laboratory and Naval Air Systems Command will be discussed only
in general terms in this report since detailed reports are available from
the agencies and their contractors. Much progress has been made and
reference will be made here only to outstanding problems and
opportunities .
Superplastic and Diffusion Bonding
Fine-grained titanium alloys are superplastic at low strain rates
within critical temperature ranges, generally near the beta transus where
deformation occurs primarily by grain boundary shearing. It probably is
not coincidental that alloys can be diffusion bonded under the same
time-temperature conditions. The foregoing combination of processes has
permitted a near-net-shape technology to be developed that permits the
joining of the two operations into a single superplastic f arming ( SPF )
and diffusion bonding (DB) process, designated SPF/DB. Ti-6Al-4V in the
fine-grained equiaxed condition commonly is processed using SPF/DB.
Processing generally is done at about 930 to 960°C, with about equal
quantities of alpha and beta phases present. Forming and bonding
generally take place in a vacuum or an inert atmosphere. Oxide films on
the surfaces of the titanium forging under these circumstances dissolve
into the underlying metal because of the high solubility of oxygen in
titanium. Any mating titanium surfaces will be diffusion bonded with a
joint that will have base metal characteristics because of
recrystallization across the bond interface.
In producing complex parts by the SPF/DB process, the components are
assembled in a die in the form of sheet, plate, ring, or other simple
geometric shapes. Parts to be diffusion bonded are pressed together by
mating dies at rather low pressures, about 150 psi. Argon pressure then
is applied to the sheet surfaces to be expanded into the enclosed die.
The result is a formed and bonded part that requires no machining and is
ready for service. Thus, although the SPF/DB process employs rather
expensive starting materials compared to conventional forming, the high
yield permits considerable cost and weight saving. Accordingly, SPF/DB
is expected to be used extensively in modern airframes like that for the
B-1 a ~ rcraft.
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The superplastic def ormation characteristic of fine-grained Ti-6Al-4V
permits tensile strains of as much as 500 percent to be achieved, which
~ s sufficient to permit pressing of sheet parts to fill complex die
cavities . This high superplastic capability also is suf f icient to permi t
forging billets to fill closed dies and form near-net-shape disc forgings
in a single step.
Isothermal forming is a variation of superplastic forming. In this,
the material is formed in hot dies approaching the temperature of the
work piece. Isothermal forming is generally practiced by press-forging
techniques. Isothermal shape rolling is accomplished in a similar
manner, but the problems of maintaining the rolls at the working
temperature are much greater than in isothermal press f orging.
The temperatures employed in isothermal forming are about the same as
those used in superplastic forming (i.e., near the beta transus).
Fine-grained, equiaxed microstructures are preferred to acicular
microstructures. The strain rates for isothermal forging are about
one-tenth to one one-hundredth those used in conventional forging but are
higher than those used in superplastic forming. The low strain rates
employed allow considerable relaxation to take place during the forming
operation even though actual dynamic recrystallization may not occur as
in superplastic f arming . Thus, high strains may be accomplished in a
si ngle f arming operation.
The main problem is die materials. For an alpha-beta alloy like
Ti-6Al-4V, molybdenum alloy dies may be required whereas for a beta alloy
like Ti-lOV-2Fe-3Al, superalloy dies are satisfactory. Isothermal
forging of f ers considerable economic advantage over conventional forging
because the initial billets are comparable in cost, but isothermal
forging results in a near-net-shape product with little or no scrap. In
contrast, isothermal forging requires much greater die cost and press
t ime . The trade-of f between the two processes varies f ram one case to
another, but much greater use is expected for isothermal f orging in the
future. Isothermal shape rolling also might be expected to be used more
commonly f or gaining similar economic advantages in cases where rolled
shape s can be applied in volume .
Precision Casting
The precision casting of complex titanium shapes has progressed
rapidly during the past decade, particularly when the castings are
subsequently hot isostatically pressed to close internal porosity. In
principle, the properties of optimized castings should approach those of
wrought products in most categories and have promise of even exceeding
one or more of these properties, one example being elevated temperature
creep. However, more validation testing of individual castings is
required. Moreover, the problem of achieving optimized mechanical
properties of castings, even after hot isostatic pressing, requires
further study before proper design allowables can be established.
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158
Additional major problems in precision casting result from surface
contamination due to mold-metal reactions; from defects such as porosity,
cold shuts, and inclusions; and from the economical reprocessing of
casting scrap, at least in some parts of the industry. Research and
development in each of these areas promises valuable payoffs.
Precision Powder Metallurgy Molding of Complex Shapes
Precision powder metallurgy molding, like precision casting, shows
promise for cost reduction and materials savings. It, too, has serious
problems that justify a further research and development effort.
Precision powder metallurgy has suffered most from the lack of
high-quality, reasonable-cost powders. Only pilot-scale titanium powder
production has been attained to date, and 1981 prices of alloy powder
have exceeded $40 per pound ( for micron-sized, free-flowing spheres of
precise chemistry, f ree f row the ever-present dirt that would be
incorporated as embri ttling inc. fusions--the kind of powder required by
present precision molding techniques). Such high prices would seem to
preclude any significant future for the production of complex-shape,
precision, titanium alloy molded parts, particularly in competition with
precision-cast part s. When hot isostatically pressed, the properties of
both precision-cast and precision-molded parts approach (their proponents
maintain that they can equal and occasionally even exceed ~ wro ught
properties. Unalloyed titanium powder, blended with master alloy
part icles, is a possible lower cost alternative to alloy spheres,
particularly for some applications. A major opportunity and challenge
for titanium powder metallurgy, therefore, is the development of much
lower cost, commercial ly pure titanium alloy powders.
In considering the application of titanium powder metallurgy, it is
important to realize that titanium and its alloys are almost unique among
metals in their suitability for powder processing. For example, titanium
pa rt s can be f armed by dif fusion bonding to provide important material
savings. Yet diffusion bonding is not used with any other metal. To
cite an exemplary case, the surface f ilm of A12O3 on aluminum is
insoluble in aluminum at any temperature; therefore diffusion bonding
will not take place. Complex oxides on alloy steels and superalloys also
prevent diffusion bonding. In the case of titanium, thin surf ace oxides
and nitrides dissolve readily in titanium above 500°C; therefore, parts
pressed together in a vacuum or an inert gas atmosphere at or above that
temperature will deform to provide intimate surf ace contact and will
diffusion bond across the interface to provide joints as strong and
ductile as the parts themselves. Indeed, when properly prepared, joints
are indistinguishable from the base metal.
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Ti tanium powder metallurgy, therefore, is simply dif fusion bonding of
small particles instead of large components. With hot isostatic pressure
assuring interparticle contact' sound, 100-percent-dense, ductile'
strong, titanium parts can be produced. As noted earlier, however, it is
not possible to take maximum advantage of the potential of this process
without the ready availability of low-cost, high-quality powder.
Re search and development in powder preparation of both pure titanium and
titanium alloys is therefore an identified need.
Titanium Mill Products by Tonnage Powder Metallurgy (TPM)
Bottlenecks in the titanium production cycle were reviewed in
Chapter 8. A major general bottleneck singled out was the
custom-job-shop nature of the titanium mill product industry. Is there
any prospect of breaking this major bottleneck? To answer this, it is
instructive to review here the basics of winning, melting, and mill
processing that were outlined in Chapters 5 and 7.
Kroll and Hunter processes win titanium from TiC14 crystal
particles that agglomerate loosely into sponge. Electrolytic processes
can produce much larger crystals. These agglomerates are compacted and
double- OF triple-melted into large ingots that are custom-job-shopped
through hot breakdown, hot rolling, and cold-hand-pack rolling to produce
alloy sheet. Each operation is accompanied by elaborate conditioning
that results in a reduction in the yield of salable sheet f ram the
starting sponge to perhaps 50 percent. A near-net-shape alternative is
t o process the as-won part icle agglomerates directly into rolled sheet,
to compact them into extrusion billets for producing rods and tubing, or
to press them into forging preforms. These powder metallurgy operations
might be accomplished with substantial savings of time and energy.
Minimum cap ital outlays, reduced energy use, and low inventory
accumulation at each operation might result. Overall yields approaching
90 perc ent might be expected f ram the initial sponge using the tonnage
powder metallurgy (TPM) technology envisioned.
Recognizing the foregoing early in titanium's industrial evolution,
du Pont undertook a major, multiyear, multimillion dollar program during
the 19 50s to bypass the titanium melting operation by processing powder
directly to mill products. At that time, du Pont was a major producer of
t itanium sponge. The near-net-shape route to mill products was to be
used to achieve the cost reduction du Pont felt was necessary for
titanium to become an important metal to the process industries. This
large effort failed because of a single, unanticipated technical problem
that has been kept as a trade secret until released by du Pont for this
NMAB report. The problem was that traces of chloride residual to the
sponge that could not be removed except by vacuum melting. During fusion
welding (of such TPM processed sheet), these chlorides volatized causing
welding arc instability that adversely affected consistent weld quality.
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Even if this problem were solved, only part of the panel ventures to
speculate that titanium tonnage powder metallurgy could ever supplant an
important portion of titanium ingot metallurgy. If the du Pant data on
its near success were not available, the potential would perhaps not
merit more than passing mention in thi s report. However, those data (see
Appendix K), plus the serious bottlenecks of current ingot metallurgy and
custom-job-shop processing and the high costs associated with these
processes, led the panel o consider whether or not the entrenched ingot
metallurgy can ever prof itably be supplanted. This led to the panel' s
recommendation that an appropriate government agency (perhaps the Air
Force) sponsor a detailed study to calculate both the energy consumption
and the complete manufacturing costs of mill products by conventional
ingot metallurgy and to compare these data with those of tonnage powder
metallurgy mill products using an assumed reasonable cost for tonnage
powder (e.g., 100, 200, and 400 percent of sponge cost). If there i s
little difference between the mill product costs of the two systems even
with an optimistic assumption for the cost of tonnage powder, the
established arc melting route wl 11 certainly prevail. If TPM appears to
off er attractive savings, it is anticipated that, as soon as these
estimates are published, one or more industrial organizations will
carefully assess the potential and take appropriate action.
Rapid Solidif icat ion tTechno logy ~ RST ~
RST is arguably the most exotic development in physical metallurgy
since precipitation hardening. Employing cooling rates in excess of one
million degrees Centigrade per second on exotic alloy compositions,
ultrafine microstructures are achieved--amorphous and microcrystalline--with
properties better than the best obtained by the major structural metals
such as iron, aluminum, and titanium. One example, exotic in both
composition and properties, is amorphous 50Ti-40Be-lOZr (atomic percent)
which gave the following combination of properties (Tanner and Ray 1977~:
Yield strength 328,000 psi
Bend ductility zero T
Density 4.13 g/cm3
RST drawbacks are, however, equally dramatic . In f act, it is
possible that RST titanium may never graduate from the laboratory. At
least a decade--probably two--of research and development will be
required to prove its usefulness. Prudence suggests that it should not
be ignored because, for example, RST aluminum is at least half a decade
in development ahead of RST titanium and today shows some promising
results. This advantage also may allow aluminum to gain back some
markets lost to titanium because unmatched strength versus weight
propert ies similar to those cited above promise important new
applications for aluminum. RST titanium may have an even greater
potential.
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REFERENCE
Tanner, L. E . and R . Ray. Physical Properties of Tis o-Be4 o-Zr
Scripta Met., 11, 1977: 783-789.
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Representative terms from entire chapter:
mill products
