Casting and powder metallurgy are related only in that both are methods by which
metal products are fabricated from materials that are not easily machined or joined,
or fabricated into parts which, by their shape and size, are not economically
feasible with conventional processes. These practices employing titanium have been
studied extensively; however, commercial exploitation of the developmental work
performed has been rather limited.
The various refractory materials employed in casting are attacked by titanium with
such severity that sounds castings, possessing good mechanical properties are
difficult to obtain. Powder metallurgical studies have met with equal difficulties.
Most titanium metal powders currently available in commercial quantities do not
have sufficient purity to produce ductile metal compacts.
The pouring of molten metal into a mold in which solidification takes place is
termed casting. Although the term casting has been applied to ingot production,
and certain components such as welds have a cast structure, it is intended here to
deal with the casting of finished or semi-finished products.
The difficulties of casting titanium stem from inherent characteristics, such as
its high chemical reactivity, and the flow properties of the molten metal.
Conventional methods which employ such refractoriness as silica, magnesia, or
alumina and which have been successfully applied to other metals are not practical
Since titanium will likewise attack the furnace crucible material during melting of
the metal prior to casting, it has been found necessary to prevent the impingement
of the molten metal on the crucible wall. This has been accomplished by using skull
The method requires the maintenance of a solid layer of titanium metal between the
crucible wall and the molten metal. This is accomplished by directing the arc at the
center of the charge, and the careful maintenance of a temperature gradient between
the molten metal and the wall.
As in ingot production, the molten metal and, in turn, the hot casting are
susceptible to atmospheric contamination. To prevent this contamination an argon
atmosphere is sustained in the crucible and the mold. To further eliminate the
contamination, the furnace is designed to allow tapping of the melt at the bottom or
side, and the sealing of the mold to the furnace over these vents. Thus, the pouring
of the casting is also accomplished under an inert atmosphere.
This method of casting requires some skill in its operation. Not only does the skull
technique require careful control in the melting, but ventilation must also be
supplied simultaneously with pouring to allow escape of the gases present to minimize
gas porosity. At present it is the only satisfactory technique for casting
A second difficulty, one which is peculiar to titanium, is in the maintenance of
good flow over severe changes of dimension or direction within the mold. This
requires, in many instances, the redesign of the mold or the cast component. Fillets
or tapers, where dimensional or directional changes occur, have proved quite
satisfactory in minimizing the difficulty.
Shell casting. Shell casting or shell molding utilizes the bonding
of a refractory with a thermosetting plastic resin to form the mold. Stepwise,
the procedure consists of forming two metal cavities, usually from aluminum, which
are patterns of the part to be cast. One pattern coincides with one half of the
part and the other pattern with the other half. These patterns, of course, must
contain the necessary gates and risers.
One of the patterns is heated and clamped on top of a dump box or hopper, which
contains the resin-refractory mixture. The refractory material can be cither
graphite or zirconium, and the resin any one of several commercial phenolic casting
materials. The mesh size of the refractory is variable, as is the ratio of resin to
refractory, and is experimentally determined for the shape being cast.
In general the amount of resin employed should be slightly higher than that employed
in the shell castings of iron alloys. When the pattern has been securely fastened
lo the dump box, the hopper and pattern arc inverted for 30 to 60 seconds, after
which time they are returned to their original position. The pattern which is
coated with the resin-refractory mixture is removed and placed in a furnace for
The curing time and temperature are dependent on the resin employed. After curing,
the mold is stripped from the pattern. The other half of the mold is prepared in a
similar manner using the second pattern.
Investment casting. Investment casting, also termed precision
casting or lost wax casting, like shell molding is employed to cast small parts.
This method is not as adaptable to assembly line speed as shell molding, but it
is capable of producing the intricate shapes not possible with the shell
To prepare an investment mold, a pattern with the necessary gates and risers is
formed in the image of the component to be cast. The material used to form the
pattern should be something which can be melted, volatilized, or burned off, such
as wax or plastic. This pattern is coated with slurry consisting of the refractory
and a binding agent. For titanium, zirconium is used as the refractory, and a
silicate or zirconium compound such as zirconium nitrate serves as the binder.
The coated pattern is further built up by backing with coarse-mesh zirconium and
the investment is then air-dried. The air-dried investment is recoated with slurry
of coarse zirconium, the binder, and a hardening agent. The investment is again
air-dried, after which it is heated to remove the pattern. The mold is fired at
1500 to 1600°F (810 to 870°C) for one to two hours and air-cooled.
Some work has also been carried out with graphite as the refractory. This method
omits the backing and uses lower firing temperatures. Results are inconclusive, and
the superiority or inferiority of graphite to zirconium is not readily evident.
In either case the mold is sealed to the furnace and the casting poured and allowed
to solidify and cool. As in shell molding, the mold is expendable and is stripped
from the casting, after which the necessary machine work is performed.
Titanium end products have been produced by this method and have been found to
possess good mechanical properties.
Powder metallurgy is a method of fabrication in which metal powders are produced
and further utilized by compacting and sintering to form useful products. Although
the powder metallurgy branch of the metallurgical industry is limited, utilization
of titanium metal powders from scrap some day may render this field important to
the titanium industry. Powder metallurgy is employed today primarily to produce
simple shapes with good dimensional stability, to form shapes with material of
extremely high melting temperatures, and to produce parts not feasible by other
Both titanium scrap and sponge are being utilized as source material for titanium
powder. The production of titanium carbide as a cutting tool and other titanium
intermetallics such as refractory materials utilizes to advantage powder
Powder preparation. With most other metals to which powder
metallurgical techniques are applied, the prime consideration is the compacting
procedure rather than the preparation of the powder. The major powder preparation
for other metals is accomplished by the relatively simple hydriding process in which
the metal is embrittled with hydrogen and therefore more easily pulverized.
Application of this hydriding process in the preparation of titanium powders has not
been successful in producing ductile compacts, since complete removal of hydrogen
is economically impracticable. Therefore, with titanium the powder preparation
becomes the major concern, since compacting and sintering, as will be shown, follow
conventional procedures. Titanium powders are prepared by milling sponge or
The sponge is pulverized in an attrition mill employing titanium plates to minimize
contamination which would result from conventional cast-iron plates. To prevent
galling of the titanium sponge to the titanium plates, it has been found advisable
to employ water at 40°F (4°C) as a protective medium. The final particle
size of the powder should range from 30 to 100 meshes.
Additional problems are introduced in the preparation of metal powders when scrap
materials rather than sponge are used as a source for powder. Scrap material usually
contains some form of surface contamination such as grease, lubricating oil, or
oxide film which may adhere to the surface.
Compacting and sintering. Compacting of the titanium powders is
carried out in a die under extremely high pressures. The apparatus usually consists
of a die body with upper and lower movable punches. These are shaped to the desired
contours and are usually made of hardened tool steel ground and lapped to a mirror
finish. The correct volume of powder predetermined from the size, shape, and density
of the piece desired is placed in the die body.
Pressure is applied to the punches and the powder compacted to the desired shape.
The pressures employed are approximately 20 tons per square inch. The compact is
then sintered in a vacuum furnace at temperatures ranging from 1900 to 2000°F
(1040 to 1100°C) for one to two hours.
The properties of powder metallurgy compacts produced by the techniques described
above are not comparable to the properties obtained in the wrought metal. In general
at equivalent strength levels the sintered metal will have a somewhat lower
ductility than the wrought product. This is true of both the unalloyed and alloyed
Contamination in the final product should be kept to a minimum by careful selection
of material and preparation of the powders. The maximum contamination tolerable will
depend on the minimum specifications of the part required.
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