Title:
Casting of metal alloys
United States Patent 2315876


Abstract:
This invention relates to casting processes and is concerned directly with the casting of metal alloys Into shaped articles. In the casting of dentures and other shaped dental articles, which are of more or less complicated and intricate shapes, it is usual practice to melt the constituents...



Inventors:
Spencer, Sivil Cecil
Liebig, Edward O.
Clair St., Low Robert
Application Number:
US42360241A
Publication Date:
04/06/1943
Filing Date:
12/19/1941
Assignee:
BAKER & CO INC
Primary Class:
Other Classes:
419/28, 420/510
International Classes:
A61C13/20
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Description:

This invention relates to casting processes and is concerned directly with the casting of metal alloys Into shaped articles.

In the casting of dentures and other shaped dental articles, which are of more or less complicated and intricate shapes, it is usual practice to melt the constituents of the casting alloy, cast the alloy into a bar, roll the bar down to sheet of suitable thickness, cut the sheet into small pieces of uniform weight, remelt a desired number of such small pieces and cast the molten mass, either centrifugally or under pressure, into a mold of desired configuration, whereby the final shaped article is obtained. In the casting of dental articles, for instance, the various steps of the process up to and including the provision of the small pieces of uniform weight are carried 3ut by the manufacturer of the alloy and the remaining steps are carried out by the dental technician.

The casting process as heretofore carried out is subject, however, to various limitations and ;hortcomings. The melting of the alloy itself ntroduces, particularly when high temperatures ire required, risks of oxide formation and absorption of Impurities from the melting crucible, ;hereby affecting the properties of the alloys, for nstance by causing brittleness therein. Likevise, the absorption of gases from the furnace Ltmosphere may cause blisters in the cast alloys, *esulting in difficulties during the final casting nto the desired shaped articles. Furthermore, the )rocess is applicable only to such alloys which Ire sufficiently ductile and workable to permit olling thereof into sheet. The remelting of crap remaining after the blanking out of small dleces of uniform weight, carries with it dangers f composition change, as for instance by the xidation of base metal constituents, and, in the bsence of such remelting, the cost of refining high.

It Is, therefore, one object of our invention to rovide a process of casting alloys into shaped rticles of more or less complicated and intricate tiape which shall be free from the shortcomings nd limitations of casting processes as heretofore nown. It Is another object of our invention to implify such process of casting in the manner ereinafter described. It is a further object of is invention to provide a process of casting I hereby it shall be possible to cast into the form E shaped articles metals and alloys of high meltig point, in particular precious metal alloys conlining gold or a metal of the platinum group.

We have discovered that the above objects can I be achieved by utilizing and applying the principles of powder metallurgy to the casting process. In practicing the invention we prepare a sinter cake from metal powders of suitable size, comminute the sinter cake and form the resultant particles into pellets which are subsequently used in the casting of the desired dental or other shaped article of more or less intricate design.

Metal powders of the constituents of the desired alloy are thoroughly mixed, as by sieving or tumbling, in the required proportion. The intimate metal powder mixture is then treated, preferably in a hydrogen or the like atmosphere, at a temperature sufficiently high to permit diffusion alloying but low enough to produce a friable sinter cake. Where the constituents of the mass are particularly liable, at the temperatures used, to oxidize in casting, there may be added a suitable degasifying agent such as zinc, cadmium or calcium-boride. The sinter cake is then crushed and preferably screened through a suitable sieve. A small amount of lubricant, such as stearic acid, paraffin or the like, is advantageously added to the crushed sintered particles so as to coat the surface thereof. The metal powders, the surface of which is advantageously lubricated as stated, are remixed and pressed into small units of suitable shape such as tablets or pellets, whereupon they are in such shape subjected to a heat treatment in a nonoxidizing or reducing atmosphere to produce a sintered bonded mass. Where the powders were previously lubricated they are preferably heated at a relatively low temperature prior to the sintering heat treatment in order to remove the lubricant. The final sintered tablets or pellets are then ready for use by the operator who takes a suitable number of such small units, melts them at the required temperature, and casts the molten mass, centrifugally or otherwise, into a mold of desired configuration to produce the shaped article of more or less intricate design.

The metal powders utilized in the preparation of the diffusion alloy may be obtained by suitable comminution, electrolytic or chemical precipitation, or any other suitable method. Likewise, the metal powders may consist of individual metals as such, or of suitable pre-alloys. Another extremely satisfactory method, in particu0i lar insofar as platinum metals are concerned, is the employment for the production of metal powder of reducible or otherwise decomposable metal compound, for instance ammonium platinic chloride, palladium ammino nitrite, copper 5 oxide, etc. Insofar as precious metal alloys are concerned, in particular alloys containing substantial quantities of gold or of metals of the platinum group, the metal powders so used should be finer than about 200 mesh in order to assure most satisfactory results; in fact, the greatest degree and highest speed of diffusion in the case of mixtures comprising substantial quantities of gold and metals of the platinum group, all of which have a high melting point and some of which have a relatively slow rate of diffusion, are obtained with metal powders finer than 300 mesh.

The following example, dealing with an alloy of 75% gold-14% silver-8% copper and 3% platinum, will more fully illustrate the nature of our invention and the method of practicing it. 3.75 troy ounces of gold, 0.7 troy ounce of silver, 0.4 troy ounce of copper, all in the form of powders finer than 325 mesh, are mixed by tumbling for several hours. The mixed powders are then heat treated in an atmosphere of hydrogen at a temperature gradually increasing from 400' C. to 500' C. to promote diffusion and produce the sinter cake. The sinter cake is crushed and screened through a 200 mesh sieve. 0.342 troy 2. ounce platinic chloride crystals, (N14) Pt Cle, equal to 0.15 troy ounce platinum, are crushed to a fine powder which is then thoroughly mixed with the crushed particles of the sinter cake.

The mixture is heat treated in hydrogen for 3( about 2 hours in a temperature of 350" C., whereupon it is subjected to another heat treatment for about 2 hours at a temperature of 450* C. gradually increasing to 500° C. In a variation df theiabove method, we may first mix and heat 3, treat,ior about 2 hours in hydrogen at a temperature of 4000-450" C., the powders of gold and silver, then crush the cake, add the required amount of ammonium platinic chloride, heat treat the mixture for 1 hour in hydrogen 4 at 350°-400° C., crush the resultant cake, thoroughly admix the copper powder and again heat treat at 400*-500" C. The sinter cake of the final mixture is again crushed and passed through a 200 mesh sieve. The screened metal 4 powder is then lubricated and finally pelleted.

We may use, for instance, as lubricant, stearic acid in acetone, such as /e gram stearic acid per cc. acetone, to produce a pasty sludge, the acetone being evaporated during stirring. After s the powder is thus dried it is again mechanically mixed in a tumbler or the like. The pellets are heat treated, preferably in hydrogen, for about 2 hours at a temperature gradually rising from 250' C. to 400" C. to remove the lubricant, and 5 are then sintered. for 1 hour at a temperature of about 800* C. A suitable number of pellets thus obtained are then melted and cast to produce the desired shaped article. If a denser pellet is desired, the pellet may be again subjected to a compressing action followed, If necessary, by a further sintering heat treatment.

The particular temperatures for the heat treatments herein described to produce sintering and diffusion may vary with different alloys. The temperatures employed in the heat treatment to produce the sinter cake should be high enough to be capable of initiating diffusion and producing the cake, usually in excess of 300* C. in the case of mixtures comprising a substantial quan- tity of gold or platinum metal, but below that temperature, taking into consideration the time of the heat treatment, at which the cake would be such that it is no longer in such friable state as to 'be capable of being crushed and comminuted to powder. In the case of the casting alloy inolved in the above example, the maximum temperature for the production of a friable sinter cake is approximately 500* C. for a short time heat treatment. The final sintering heat treatment takes place at a suitable elevated temperature. The lower limit of such temperature is controlled by the speed of diffusion of the constituents of the powder at elevated temperatures and the length of time of such heat treatment.

Ordinarily, the temperature should not be below about 60% of the melting point of the alloy of the powders subjected to the heat treatment.

Thus, In the case of the alloy involved in the above specific example, having a melting point of approximately 980* C., the lower limit of the sintering temperature lies at about 600* C. for any practical sintering heat treatment. In general the temperature of such heat treatment will Sremain below the melting point of the alloy involved.

One advantage of the procedure in accordance with this invention is the elimination of the tendency to form oxides or blisters during the meltIng of the initial alloys, due to the fact that the diffusion alloys are produced at temperatures considerably below those necessary for the formation of alloys by melting. The elimination of such tendency facilitates considerably the ulti0 mate casting of the alloys into shaped articles and, further, permits the utilization of metals which heretofore were avoided due to their particular tendency to form oxides or cause blisters.

Another primary advantage of our invention is the elimination of the necessity to roll bars down to sheet. Such elimination constitutes a considerable saving in cost of manufacture and in equipment and makes it possible to provide for casting into shaped articles alloys which heretofore 0 could not be used due to lack of sufficient ductility and workability. These and other advantages previously referred to are particularly applicable to alloys having a melting point in excess of about 700' C. such as alloys containing a substantial o proportion of gold or a metal of the platinum group. In fact, our invention was developed in connection with research having in view the overcoming of difficulties heretofore existing in the case of casting alloys having a melting point o in excess of 700* C., such difficulties being the greater the higher the melting point, and being particularly disturbing with alloys having a melting point in excess of 850* C. As our invention overcomes such difficulties as heretofore existed 5 in such cases, our invention Is especially applicable to casting alloys containing 40% and more of gold and platinum metals, either alone or in combination. In such cases the initial heat treatment to produce the friable sinter cake should be 60 in excess of 300* C. and the final heat treatment should be in excess of 4200 C. Another advantage of our invention is the more accurate composition of alloys which it is possible to obtain in cases where homogeneity is otherwise difficult 65 to obtain by melting. Absorption of impurities from the melting crucible in the original proportion of the casting alloy is avoided. Accurate weights of each individual tablet or pellet can likewise be obtained more easily than by the cut70 ting of individual units from sheet as heretofore practiced.

While we have described the method according to our invention with particular reference to the casting of dental articles, the method is 75 equally applicable to the casting of other articles of more or less complicated and intricate shapes, for instance in the casting of jewelry, where difficulties inherent in the prior art methods are overcome by the use of the method in accordance with our invention.

What we claim is: 1. The method of casting alloys, comprising mixing finely divided powders of metals in predetermined proportional relationship, heating the resultant powder mixture at an elevated temperature to produce a friable sinter cake, whereby initial diffusion of the powdered metals is obtained, comminuting said sinter cake, compressing said comminuted sinter cake into a plurality of shaped units of predetermined weight, subjecting said compressed units to a heat treatment at an elevated temperature to produce a coherent bonded mass of diffusion alloy, melting at least one of said units to molten alloy, and casting said molten alloy into a mold of predetermined configuration.

2. The method of casting alloys having a melting point in excess of 700° C. and containing at least 40% of metal taken from the group consisting of gold and platinum metals, comprising mixing finely divided powders of metals in predetermined proportional relationship, heating the resultant powder mixture at a temperature in excess of at least about 300' C. to produce a friable sinter cake, whereby initial diffusion of the powdered metals is obtained, comminuting said sinter cake, compressing said comminuted sinter cake to a plurality of shaped units of predetermined weight, subjecting said compressed units to a heat treatment at a temperature in excess of at least about 420* C. to produce a coherent bonded mass of diffusion alloy, melting at least one of said units to molten alloy, and casting said molten alloy into a mold of predetermined configuration.

3. The method of casting alloys, comprising mixing finely divided powders of metals in predetermined proportional relationship, heating the resultant powder mixture at an elevated temperature to produce a friable sinter cake, whereby initial diffusion of the powdered metals is obtained, comminuting said sinter cake, coating the powder particles of said comminuted sinter cake with a lubricant, compressing said lubricated powder particles into a plurality of shaped units of predetermined weight, evaporating the lubricant from said units, subjecting said compressed units to a heat treatment at an elevated temperature to produce a coherent bonded mass of diffusion alloy, melting at least one of said units to molten alloy, and casting said molten alloy into a mold of predetermined configuration.

CECIL S. SIVIL.

ROBERT ST. C. LOW.

EDWARD 0. LIEBIG.