US3212885A - Mercury investment casting alloy composition - Google Patents

Description

Oct.

C. A. HOAG MERCURY INVESTMENT CASTING ALLOY COMPOSITION Filed Dec. 18. 1961 for' Power SUPP/y A40/a emova/ INVENTOR.

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United States Patent 3,212,885 MERCURY INVESTMENT CASTING ALLOY CGMPOSITION Chauncey A. Hoag, Claremont, Calif., assignor to Mertronics Corporation, Santa Monica, Calif., a corporation of Delaware Filed Dec. 18, 1961, Ser. No. 160,143 3 Claims. (Cl. 75-169) This invention relates generally to the art of forming and using investment casting patterns, and more particularly concerns novel alloy compositions and methods for forming and using patterns made therefrom.

U.S. Patent 2,857,641 describes the desirability of using intermetallic compounds of mercury and thallium for forming investment casting patterns. An alloy was indicated that is close to the thallium-mercury eutectc composition, Tl2Hg5, which contains 28.95 percent by weight of thallium and which starts to freeze at about 56 degrees F. and which is solid within one degree range. No other metallic composition that freezes :so slightly below -ordinary room temperature was then available. This alloy was attractive for the indicated use because of the potentially low refrigeration requirement. However, thallium oxidizes readily and the oxide is Very soluble in water, forming an alkaline hydrate solution that is a good conductor. Thallium is electro-positive to most common metals and decidedly so to mercury. The patent specilies the use of a protective liquid covering to suppress oxidation of the thallium and suggests ethylene glycol as the preferred liquid. But, under use conditions atmospheric moisture is condensed on metal parts and is absorbed by the ethylene glycol bringing free oxygen into contact with the alloy and starting the above chain of events.

As a result, the protective uid soon had to be replaced. Depletion of thallium from the alloy made it necessary to replace the expended thallium frequently and chemical analyses and computations had to be made to determine the amount of thallium to add each time. Since ethylene glycol costs about $4.50 per gallon and thallium costs about $12.50 per pound, the maintenance costs were very high relative to any potential savings in yrefrigeration costs. Further, as the thallium became depleted the frozen patterns made from it became increasingly diicult to book securely.

The present invention seeks to overcome the above mentioned problems through an approach which places primary reliance upon a factor other than the use of a protective liquid. As will appear, the invention broadly concerns the addition to the alloy of other metallic substance such as zinc which is electro-positive to thallium. As will be brought out, the zinc is preferably present in dissolved state in the alloy and in amount suflicient to inhibit oxidation of the thallium. Since the zinc oxidizes in preference to thallium, the dissolved zinc content of the alloy will tend to be depleted, and the invention contemplates keeping solid state zinc bodies in the liquid alloy reservoir from which pattern forming alloy is withdrawn for replenishment of the dissolved zinc metal whi-ch becomes oxidized.

Still another aspect of the invention has to do with the method by which the preferred alloy composition may be kept available for use in pattern forming and without deterioration, the method facilitating delivery of pure alloy of the correct composition at any desired time and also cleaning of the fluid alloy.

These and other objects of the invention as well as the details of an illustrative embodiment will be more fully understood from the following detailed description of the drawing, in which:

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FIG. 1 is a flow diagram illustrating the use of the alloy; and

FIG. 2 shows the manner in which the alloy may be kept for maintenance of correct composition, and for delivery of pure alloy at any desired time.

Referring rst to FIG. 1, typically the alloy withdrawn or delivered at 12 is used 4to invest dies or master molds which have previously been lled with acetone for absorbing moisture-free contact with the die interior surfaces, this step being indicated at 14. The alloy contained in the dies is then frozen as indicated at 15, after which the die is stripped from the frozen alloy pattern sections. The latter are then treated with a suitable ux for purposes of facilitating ready booking of the pattern sections as indicated at 16. The patterns are then rinsed free of excess flux solution, as by placing them in an ice water bath, serving for temporary storage to allow efective diffusion of the excess flux. Just prior to use of the patterns for molding purposes, they are dried as by successive rinsing in acetone.

Finally, the booked patterns are used to formmolds at 17, and typically ceramic spray in a Freon carrier is used to build up successive layers of the mold on the frozen alloy pattern, the spray booth being dehumidied -to prevent the condensation of atmospheric moisture on the successive layers .of ceramic mortar. Even if the films of moisture do not freeze, they impair the bonding between layers and produce non-uniform density and weak molds prone to separation during curing. (The main curing and conditioning of the molds takes place in the foundry where the molds are gradually heated to high temperature that is related to the pouring temperature of the metal to fbe used for investment casting.) The patterns and molds are then transferred at 18 to a melting chamber at 10 in FIG. 1 (22 in FIG. 2) wherein the frozen alloy is melted out of the molds. The molds are then withdrawn as indicated at 11 for subsequent use in casting, and the melted alloy is removed at 12 for reuse in process. The numeral 13 indicates replenishment of liquid alloy and zinc in such amounts as are found necessary.

In accordance with one phase of the invention, the thallium content of the alloy is protected against oxidation by the addition to the alloy of `a metallic substance that is electro-positive to thallium. In this regard, the thallium present in the alloy is sufficient to elevate the freezing temperature of the alloy well above the freezing temperature of mercury alone, and typically the thallium content will approximate the stoichiometric content of the eutectic intermetallic alloy Tl2Hg5 containing 28.95 percent by weight of thallium. This alloy freezes solidly rat about 56 degrees F., which is within Ithe range of ordinary refrigeration making the alloy attractive for commercial use. i

The metallic substance preferred for addition to the mercury thallium alloy is zinc, although other substances electro-positive to thallium are workable if not as prac@ tical as zinc, other such substances including magnesium. Sodium and potassium are believed impractical from the standpoint of their rapid reaction with moisture leading to rapid depletion of their content and effectiveness in the alloy to prevent oxidation of the thallium.

The zinc dissolved in the alloy may be about 3 percent by weight when prepared at near the boiling point of water, but at ordinary room temperatures it is typically close to 0.6 percent by weight in the duid alloy, this content being maintained in the process by the addition 0f solid state Zinc particles as shown at 19 in FIG. 2 to the liquid alloy pool 20 formed in a sink 21 at the bottom of the melting chamber 22. With zinc in solution in the alloy, thallium is protected against oxidation and dissolution, and no oxidation suppressing organic liquid such as ethylene glycol is needed. Zinc is oxidized to some extent, but zinc oxide is almost completely insoluble in water.

If one should ever need to increase the potential for rapid replacement of zinc in the alloy at ordinary temperatures, he may, at or near the end of a work shift, heat the pool of alloy in contact with excess zinc metal chunks for a period of about two or three hours at a temperature only a little below the boiling point of Water. He may then allow the alloy to cool to room ternperature, before the next work shift, at which time he will have a crop of tiny zinc rich alloy crystals in the upper layer of the pool, and these collectively have a vastly greater surface area than the solid pieces of zinc from which they were derived. By reason of the increased area of contact between fluid alloy and zinc plus zinc rich crystals, the potential for rapid replacement of depleted zinc is considered substantially increased.

The numeral 23 indicates a mold from which a frozen alloy pattern 24 is being melted through the application of heat to the chamber as indicated by the electrical heating system 25. A screen is shown at 26 to intercept pieces of ceramic which might otherwise fall from the mold 23 into the pool 20. At the top of the pool there is formed a layer 27 of floating impurities including zinc hydrate or hydroxide, the layer 27 also including fine ceramic debris and water covering the pool to protect it from oxidation by the atmosphere within the charnber 22. The oxygen content of the water layer reacts with the Zinc dissolved in the alloy to form the zinc hydrate, and accordingly the thallium content of the alloy is not oxidized but remains at the preferred weight percent.

As the level of the pool tends to rise in response to melting out of pattern alloy from the molds 23, the liquid alloy in a trap conduit 28 will be displaced into a reservoir 29, the alloy content 30 of which may vary. Conduit 28 is shown as communicating with opening 31 in the bottom of the sink 21, the conduit having a gooseneck portion 32 in which the level 33 of the alloy remains the same as the surface level of the alloy in the pool 20. Accordingly, the liquid alloy 20 never cornpletely drains from the sink 21, and the layer 27 is never carried over into the reservoir 29. Alloy in the gooseneck riser 34 will be displaced over into the receptacle 29 with the addition of alloy to the pool 20. A protective liquid such as water 35 is shown covering the surface 33 of the alloy in the gooseneck and also as covering the alloy 30 in the reservoir 29. The alloy liquid 30 in the reservoir 29, after mixing, may be withdrawn from the cone shaped bottom of the reservoir as by the valve apparatus shown at 36 whenever desired to ll the master dies. One form of mixing apparatus includes a rotor 37 rotatablek in the alloy pool 30 as by a suitable drive 38 and motor 39. The rotor should preferably be close to the bottom outlet 43 so that even small batches of alloy will be mixed. Also, the rotor should be driven rather slowly so as not to cause separation of the alloy into droplets, and the rotor design should be such as to give a slight upward thrust or circulation to the alloy, assuring mixing of the heavy'alloy. A perforated plate is shown at 40, typically near the outlet 31 of the sink 21 in order to strain any particle impurities from the alloy flowing downwardly from the pool 29 and into the trap conduit 28. Typically, the plate perforations may comprise holes of about 1/32 inch diameter.

The following table shows the results of freezing tests conducted upon a number of batches of alloy with varying weight percentages of thallium. In all of these tests the remaining weight percentage content of the alloy other than thallium comprises mercury containing 1 percent zinc, so that the zinc content varies from .73 percent for batch Y319 to .50 percent for batch G322.

Freezing Batch Thallium, Initial RP., Final RP Range,

Percent Deg. F. Deg. Approx.,

Deg. Range 26. 7 4s. s 26, s 12 27. 9 50. 6 42 9 28. 3 50. 9 45 0 2s. 7 51. 4 47 4 29. 1 5l. 2 48 3 30. 2 51. 7 48 4 3l. 6 5l. 2 47 4 32. 0 50. 9 37. 6 1S 33. 0 49. 7 30. 2 19 34. 0 48. 3 35. 3 13 34. 6 44. 6 34. 4 10 38.8 39. 2 30 9+ 40. 3 40. 5 30. 4 10 It was found that the thallium alloys with relatively wide freezing temperature range, that is the alloys containing 32.0 percent thallium and greater, have rough spots or tears of extruded alloys on the surface of the patterns, these rough spots being the last to freeze. Furthermore, if such alloys are used for making molds, the castings resulting from use of the molds have a rough surface, and accordingly it is determined that the alloy composition should be such that the freezing temperature range is considerably less than 13 degrees F., or alternatively that the thallium content should be no more than 31.8 weight percent.

Also, it was found that pattern sections containing 27.9 and less Weight percent thallium not only have a freezing temperature range of 9 degrees F. and greater, but also exhibit inferior booking properties. For example, whereas the alloys having freezing ranges of no more than 6 degrees F. may be booked rmly without external pressure applied to the test pieces, it is necessary to apply external pressure to the test pieces in order to book them when such pieces contain thallium outside the pre ferred freezing temperature range of no greater than 6 degrees F.

Referring. again to the dissolution of high purity electroyltic zinc metal in the mercury thallium alloy it was found that excess zinc metal may be kept floating in the fluid alloy at ordinary temperatures without putting an excess of zinc into the alloy, i.e. the zinc content remains below 1.0 percent by Weight. At the start, zinc is incorporated in the thallium-mercury alloy up to the equilibrium content at room temperature, ordinarily 0.6 to 0.7 percent by weight of zinc. This alloy starts to freeze at about 5l degrees F. and becomes solid at about 47 degrees F. Refrigeration requirements are therefore similar to those for the straight thallium-mercury alloy. Under operating conditions the zinc content of the alloy maybe somewhat depleted, in which case the initial freezing point would be higher than 5'1 degrees. However, so long as the initial freezing point of the alloy remains below 56 degrees F., an eifective zinc content in the alloy is assured.

More rapid wetting of the zinc pieces by the fluid alloy is attained by applying acetate flux solution to the pieces, facilitating the dissolution of the zinc in the alloy. Also, the application of lgentle heat and stirring lpromote dissolution as indicated by the heater 41 and power driven mixer 42. Another advantageous function of the zinc is to neutralize any acid materials, such as residues from Freon used in ceramic sprays falling into the pool 20 from the mold 23 during the melting process. A simple test to determine whether the zinc content of the liquidalloy has been exhausted consists in mixing a portion of the alloy `with water and adding phenolphthalein indicator. If zinc has been exhausted, thallium will go into solution, the water will be alkaline and the indicator will show red coloration immediately.

It should be pointed out that segregation of the components of the liquid alloy takes place due to differences in density between mercury, thallium and zinc. To assure ne structure and smooth surfaces on the frozen .patterns it is essential to thoroughly mix the iuid alloy forming a shell mold, said alloy consisting of before it is dispensed. liquid mercury,

I claim: thallium mixed with the liquid mercury, said thallium 1. A low melting investment alloy for use in forming being present in a suicient quantity to raise the a shell mold, said alloy consisting of 5 melting point of the yalloy solution into a range bemercury in arange of between 68 and 73% by weight, tween 40 F. and 56 F., and thallium in the range between 28 and 32% by weight, a metal that is electro-positive to thallium mixed in and said solution, said metal being in the range of 0.5 zinc between 0.5 and 3% by weight. up to 3% from a class consisting of zinc, magnesium, 2. A low melting investment alloy for use in forming 10 a portion of said met-al being dissolved in said alloy a shell mold, said alloy consisting of solution and the `remaining portion being in solid mercury and thallium, form. said thallium being present in a range that will cause initial freezing to begin at less than 56 and the References Cited by the Examiner final freezing to be completed above 40 F. and in 15 UNITED STATES PATENTS a range that Will cause the difference between the initial freezing and the nal freezing to be less than 2857641 10/'58 Kramer 75-169 X 6 F, and b t 0 5 nd 3l? b ht FOREIGN PATENTS zinc 1n a range e Ween a o y we1g the remainder of .said alloy being all mercury. 1010743 6/57 Germany 3. A low melting investment alloy solution for use in DAVID L. RECK, Primary Examiner.

Claims (1)

1. A LOW MELTING INVESTMENT ALLOY FOR USE IN FORMING A SHELL MOLD, SAID ALLOY CONSISTING OF MERCURY IN A RANGE OF BETWEEN 68 AND 73% BY WEIGHT, THALLIUM IN THE RANGE BETWEEN 28 AND 32% BY WEIGHT, AND ZINC BETWEEN 0.5 AND 3% BY WEIGHT.

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