Casting and Powder Metallurgy Titanium Alloys

Extrait:

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.

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.

Casting

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 with titanium.

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 melting techniques.

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 titanium.

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 curing.

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 technique.

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

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 means.

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 metallurgical processes.

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 scrap.

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 titanium.

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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