Arc-furnace for the production of small investment castings of reactive or refractory metals such as titanium

An arc furnace and investment casting apparatus includes a copper base with an integrally formed crucible having a passage therethrough. A vacuum chamber is positioned on the top of the copper crucible with a non-consumable cathode projecting into the chamber to effect melting of metal placed in the crucible. A vacuum chamber is also suspended beneath the crucible for support of a mold to receive molten metal flowing through the passage.

BACKGROUND OF THE INVENTION 
This invention relates to an improved arc-furnace and investment casting 
apparatus and more particularly to an apparatus ideally suited for casting 
reactive and relatively high melting point metals such as titanium. 
Electric arc furnaces have been used for melting a variety of metals and 
alloys in order to prepare the material in the form of ingots or castings. 
Various advantages result from an arc melting process including the 
following: 
1. Almost any metal can be quickly and easily melted including those that 
are chemically reactive and have very high melting points such as tungsten 
which has a melting temperature of about 3,400.degree. C. 
2. The melting may be done in an enclosed chamber containing an inert gas 
such as argon or helium which prevents contamination of the metal by 
atmospheric gases such as oxygen or nitrogen. 
3. The molten metal may be held in a relatively nonreactive container 
thereby avoiding contamination which can be introduced if one were to use 
a graphite, refractory oxide or heated metallic crucible. 
4. Power supplies and accessory equipment are relatively inexpensive. 
Arc melting uses a non-consumable tungsten cathode which emits electrons 
through an arc under the influence of an electric field in the direction 
of an anode. The anode normally contains the material to be melted. The 
concentration of electrons at the anode causes release of heat which is 
responsible for the generation of high temperatures. This is taught in 
Campbell and Sherwood, "High Temperature Materials and Technology", John 
Wiley & Sons, New York (1967). 
Various modifications of an arc furnace have been suggested to permit 
manufacture of small castings, for example, by allowing molten metal to 
flow into a split copper mold, see Hepworth, M. T., Journal of Metals, 
volume 14, page 411 (1962) and Crow, J. E. and Sweedler, A. R. Review of 
Scientific Instruments, volume 44, page 1398 (1973). However, copper molds 
are relatively expensive and in many applications, such as for the 
manufacture of dental castings, would be uneconomical since a special mold 
is needed for each individual dental casting. 
For this reason, the "lost wax" or "investment" technique is normally used 
for the manufacture of dental castings. In this process, metal is cast 
into a mold of gypsum or other ceramic material which is preheated to 
about 1500.degree. F. (815.degree. C.) at the time the metal is cast. 
Typically, the hot mold is heated in a furnace from which it must be 
removed for placement in a casting machine. This transfer must occur 
quickly, preferably in less than one minute, so that the mold temperature 
will not decrease significantly before the casting process begins. 
Various patents suggest other melting and casting processes for the 
manufacture of dental castings. Thus, Emerick, in U.S. Pat. No. 4,150,707, 
teaches the use of an arc furnace which melts material placed in a metal 
crucible having a central bottom opening. An investment cavity is 
positioned beneath this crucible opening for receipt of the liquid metal 
upon melting thereof by the arc furnace. Emerick's relatively small 
metallic crucible is well-suited for melting gold alloys but would not be 
adequate for melting reactive metals having significantly higher melting 
temperatures such as titanium since such metals would be contaminated by 
reactions with the crucible. 
Schultheiss, in U.S. Pat. No. 3,955,612, discloses a device for melting 
metals in a closed gas pressurizable crucible. The contents of the 
crucible are heated by an induction melting process. The crucible is 
arranged on top of a molding cell which is connected by a valve controlled 
tapping passage from the crucible to the cell. The melt is tapped under 
pressure into an evacuated mold. A center tap hole in the crucible is 
opened and closed by means of a central rod which projects down through 
the crucible and the molten metal and fits into the central passage. The 
crucible and central rod, while suitable for melting gold alloys, are not 
adequate for melting more reactive metals, such as titanium, which would 
attack both the rod and the crucible. 
Ida et al in IADR Abstract No. 397 from the 1980 IADR meeting discloses a 
dental casting machine known by the tradename CASTMATIC which was used for 
the casting of titanium and titanium alloy castings. The CASTMATIC machine 
uses a graphite crucible which purportedly is non-reactive with the cast 
material. However, reactions between graphite crucibles and molten 
titanium have been described by A. D. and M. K. McQuillan "Titanium", 
Buttersworth Scientific Publications--London, pages 70-72 (1956). The 
substitution of cooled copper crucibles would eliminate problems of 
contamination, but the copper must be adequately cooled and Ida indicates 
no provision for such cooling in his design. 
The present invention contemplates improved melting and investment casting 
of such reactive and refractory metals particularly for dental castings. 
SUMMARY OF THE INVENTION 
Briefly, the present invention comprises an improved electric arc furnace 
and investment casting apparatus. An arc furnace is arranged vertically 
above an investment casting chamber. The arc furnace includes a 
non-consumable cathode projecting into a vacuum melting chamber and over a 
crucible defined in the bottom platform of the melting chamber. The 
crucible includes a central passage which leads into the vacuum casting 
chamber. The vacuum casting chamber includes a removable floor which can 
receive a removable hot mold and can position and hold the hot mold under 
the crucible passage. An optional valve in the crucible passage may be 
utilized. 
Thus, it is an object of the invention to provide an improved arc furnace 
and investment casting apparatus. 
A further object of the present invention is to provide an improved casting 
apparatus which is adapted to utilize hot ceramic molds for the 
manufacture of castings such as dental castings. 
Still a further object of the invention is to provide an arc furnace 
utilizing a relatively cold and, therefore, inert copper crucible which is 
comprised of a generally planar plate having a bore partially therethrough 
with a passage at the center of the bore for the flow of melted metal from 
the crucible to a mold. 
Still a further object of the present invention is to provide an arc 
furnace and investment casting apparatus which is easy to operate, 
inexpensive and highly efficient. 
These and other objects, advantages and features of the invention will be 
set forth in the detailed description which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, the furnace and casting chamber of the present 
invention includes an upper arc furnace or melting chamber 10 and a lower 
casting or mold chamber 12 separated by a copper plate 16. The furnace 
chamber 10 is formed by a cylindrical furnace wall 14 which is vacuum 
sealed to the copper plate 16. Plate 16 serves as both the anode and 
crucible for the furnace. A cover 18 is vacuum sealed to the wall 14. 
An electrically insulated tungsten cathode 20 which receives power through 
a lead (not shown) projects generally along the center line axis of the 
chamber 10 from the cover 18. The cathode 20 thus projects inwardly toward 
a counterbore forming a crucible 24 in the copper plate 16. 
As shown in FIG. 1, the cathode 20 is comprised of a tungsten electrode 
which is retained by holder. The electrode 20 is inserted into a 
counterbore 124 in holder and is retained by set screw 126. An insulating 
collar 128 is attached to the holder by a set screw 130. The holder thus 
projects through a bore 132 in the collar 128 and then through a bore 134 
in a metal ring 136. The metal ring 136 is threadably secured through an 
opening 138 in the top cover 18. A rubber O-ring 140 seals the holder with 
respect to the ring 136 and also insulates the holder from the ring 136. 
The insulating collar 128 serves to orient the holder with respect to the 
bores 132, 134. In this manner the electrode 20 is electrically insulated 
from the chamber and more particularly the chamber walls or cover 18. 
Referring again to FIG. 1, a quantity of metal material 26 such as titanium 
or titanium alloy may be placed in the crucible 24 for melting. At the 
bottom surface of the crucible 24, a passage 30 is drilled through the 
plate 16 to the bottom side of that plate 16. 
A gas inlet port 32 is provided in the furnace chamber wall 14 and a vacuum 
or exhaust port 34 is also provided in the chamber wall 14. In this way, 
vacuum may be effected in the chamber 10 through the port 34. Purging 
gases such as helium or argon may then be introduced through the port 32 
or, alternatively, both functions may be effected using a single port. 
The mold chamber 12 is positioned directly beneath the crucible 24 and is 
attached to the plate 16. Thus, the mold chamber 12 is formed by a 
cylindrical wall 36 which is vacuum sealed to the lower surface 38 of the 
wall 16 by means of fasteners 40. The side wall 36 includes a lower flange 
42 with a number of slots therein for receiving support bolts 44 which 
project downwardly through openings 46 in a lower cover plate 48. Wing 
nuts 50 retain the cover plate 48 in position. The cover plate 48 may be 
retained in position to support a hot mold 52, e.g. a gypsum or other 
refractory mold form, therein for purposes of investment casting. The mold 
52 is positioned directly below the passage 30. The mold chamber 12 is 
sealed and also includes inlet port 37 and exhaust or vacuum port 39 for 
introducing or withdrawing gas from the chamber 12. 
FIG. 3 illustrates an alternative construction of the mold chamber and 
assembly. Referring to FIG. 3, a ceramic mold 52 is retained within a 
metal casting ring 180. The ring 180 is received by a metal cup 182 which 
has a central passage or bore 184. A spring assembly 186 is supported on 
the bottom cover plate 48. The spring assembly 186 is biased against the 
metal cup 182 and forces the metal cup 182 upward. Thus, spring 188 is a 
compression spring fitted between a support stand 190 and properly aligned 
by guide ring 192 so that it will engage the metal cup 182. 
A gasket such as a nickel metal or a ceramic gasket 194 is formed in an 
annular shape and positioned on the top of the metal casting ring 180 
against the bottom of the plate 16. The ceramic mold 52 may thus be 
appropriately positioned beneath the passage 30 for receipt of metal into 
the mold 52. In this manner the mold 52 is sealed and adapted to easily 
receive molten metal through the passage 30. Alternatively, the spring 188 
may be replaced by a screw mechanism which is operated externally of the 
plate 48 to thereby advance the ceramic mold 52 into position adjacent or 
below the passage 30. 
Again referring to FIG. 1, a lever arm 54 fits through a sealed opening 56 
in the wall 36. The lever arm 54 includes a valve member 58 at its 
interior end which is adapted to engage a tapered, frusto conical opening 
60 in passage 30 and thereby seal the passage 30. The outside of the lever 
arm 54 may be manually actuated to position the valve member 58 into or 
out of the passage opening 60. In this manner the inert gas in the furnace 
chamber 10 can be retained until the casting process begins. Thus, the 
atmosphere in the mold chamber 12 can be maintained with a different 
pressure or composition from that in the melting chamber 10. 
Referring now to FIG. 2, an alternative to the lever arm 54 and valve 
member 58 is illustrated. Specifically, the plate 16 includes a series of 
threaded screw receiving passages 160 circumferentially arranged around 
the passage 30 and more particularly the frusto conical opening 60. A disc 
of metal foil material such as aluminum foil 162 is retained on the bottom 
surface of the plate 16 by means of an annular plate 164 which is secured 
to the plate 16 by fasteners 166. The plate 164 thus includes a passage 
168. The plate 164 seals the foil 162 tightly against the plate 16 to 
provide a vacuum tight seal. 
During the casting operation, molten metal flowing from the crucible 24 
through the passage 60 will melt the thin foil 162 and flow into the mold. 
The foil 162 thus permits differential pressures between the chambers 10 
and 12 as well as maintenance of different gases in the chambers 10 and 
12. 
The crucible plate 16 is a copper plate which may be somewhat thicker than 
the copper crucible plate 16 associated with prior art arc furnaces since 
additional mass may be desired to dissipate the heat of melting. 
Alternatively, the copper crucible may contain an internal chamber for 
water cooling. As illustrated in FIG. 1, cooling water may be pumped into 
an internal cooling chamber 200 in the plate 16 through inlet port 201, 
and then exit through an outlet port. The plate not only serves as the 
crucible 24, but also defines a wall in each chamber 10, 12. 
In operation, when the mold 52 is in place and the chambers are properly 
purged, an arc is established in the usual manner. The operator will 
generally set the arc current to heat the sample 26 to a temperature just 
below the melting temperature. When he is ready to cast the metal, the arc 
current can be increased to melt the sample 26 and permit it to flow 
through the passage 30 into the mold 52. 
In a typical casting operation, the first step is to open the melting 
chamber 10 and place the metal 26 to be melted in the crucible 24. The 
chamber is then closed and sealed against the bottom plate 16. The valve 
58 is closed and the chamber 10 is evacuated using vacuum techniques. The 
chamber 10 is then flushed with an inert gas, for example, a mixture of 
argon and helium. The purging steps may be repeated. 
A hot investment mold 52 is then placed on the cover 48 and the cover 48 is 
fastened into position by use of the wing nuts 50. The chamber 12 is then 
sealed, evacuated and also flushed with an inert gas mixture. Next an arc 
to the metal 26 is initiated at a power level sufficient to melt the 
sample except for the very lower portion thereof over the opening 30. The 
valve 58 may then be removed from the casting opening 60. Immediately 
thereafter the arc current is increased to melt the metal and cause it to 
flow down through the opening 30 and into the mold 52. The mold 52 is 
allowed to partially cool within the chamber 12. It is then removed so 
that the casting may be retrieved. 
The present invention is especially useful for the manufacture of dental 
castings, particularly dental castings made from reactive, 
high-temperature metals or alloys such as titanium or titanium alloys. 
While there has been set forth a preferred embodiment of the invention, it 
is to be understood that the invention is to be limited only by the 
following claims and their equivalents.