US2609576A

US2609576A - Method of making hollow shapes

Info

Publication number: US2609576A
Application number: US131310A
Authority: US
Inventors: Milton S Roush; Clyde C Clark
Original assignee: Thompson Products Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1949-12-06
Filing date: 1949-12-06
Publication date: 1952-09-09
Anticipated expiration: 1969-09-09

Description

Sept. 9, 1952 M. s.- ROUSH ETAL 2,609,576

METHOD F'MAKING HOLLOW SHAPES Filed Dec 6. 1949 E Lj [Tu En 22:7 r5

4 6. 8008/1 CLYDE C CLARK f Ff/g5.

Patented Sept. 9, 1952 METHOD OF MAKING HOLLOW sHArEs Milton S. Roush, Painesville, and Clyde 0. Clark,

Willoiighby Ohio, assignors to Thompson Products, Ina, Cleveland, Ohio, a corporation of Ohio i l eradication December 6, 1.9149, S r al, No; 13. .1 19

The present inventionirelates to a method for making hollow shapes, particularly hollow castings of complex configuration.

In the fabrication of variouselements for internal combustion or "gas engines. it is often desirable to provide such elements with hollow interiors into whichsuitable coolant materials may be inserted for the purpose of enabling the 9 Claims. (531. 22-200) element to withstand"'theeffects of high temperature operating conditions. One such element is'a poppet valve in the exhaust system of an internal combustion engine, wherein it is customaryto provide the poppetvalve head and stem with a hollow interior intowhich metallic sodium or other coolant material is. incorporated. Other elements in which use isrnade of hollow 'body structure are the blhdfi q and turbine buckets used in jet propulsion engines. v An object of the present invention is to provide a method for making such hollow shapes, and in particular for making hollow castings having a uniform wall thickness. e 7 Another objec of the prese t nven io is t provide a method for. casting a corrosion resist ant casing about a preformed core while providing means for quickly and eillciently removing the core structure after the casting has been completed. A 7

Still another object of the present invention is to provide a method for making castings having hollow interiors, with the dimensions of the void space within theicastingbeing-controllable to precise limits. 1 l f Other objects and features. of the present invention will be apparent to those skilled in the art from the following description taken in connection with the attached sheetoi drawings.

As one feature of the present invention, we have provided a method for producing hollow shapes by forming a core by powder metallurgy technique having the configuration oi the void space desired in the ultimate shape, forming the body or casingiolround such a preformed core structure, and dissolvingthe powder metal core out of the interior of the casing by means of a suitable reagent. The core material and .the body metal are so selected that-the core metal will not be melted by the body. metal which is cast upon it, although it may have a lower melting point than the body metal. The core is preserved, irrespective of its melting point, if the molten body metal is not permitted to heat the core close to its melting point, so that no buckling of the core occurs during the casting operation. In addition, it is preferred that the body metal be such that it does not wet the'oore, thus prevent ng or minimizing the amount of infiltration or body metal I in the corestructor n dditiong the c re 1.. such math. i capabl of dissolution by k id 1 a .11 time, while the, body metal is ofv such ,eom'posie tion that it is. extremel resistant to attack by such reagents, thus facilitating removal of the core from the finished castin While the powder metal core preferably has a high degree of porosity itinay be ginfiltered with an easily dissolved metal, such "as coppei to provide strength andihcre'ase its heat conductivity. The iniil'tered core, 01. purposes of thisinvention, may be .e'onsid 'd porous in that the infiltrant filling" the void. nstead of'air is easily attack d yth dissolving I it-tl a to o n up the channels or voids tho-powder metal core.

' By using a powder metal .c'ore in'a casting procedure Several advantages [are to, be realized. First of all, the poro ity. of the'eonipact is Such that it is easily 'penetrable by'flthe acid or other reagent and isquickly dissolved frdm the interior oi the cast article; rhenitoo the. Core is C article, with tho @0129 iii pnceytoai final shape before the rmovalfo the poro s core. This latter ififi lreis seiitial in t fabrication of complex shapes such as the airgfoil sections ofturbihe buckets. h 'H The core metal preferably Iiised' incon i ition with this invention is o'wdeifed ire Lcoinpact which has beeri'co'r il hcfed .to the vshapedesired in the ultimate hollow section or the casting, and sintered at the a propriate trsnsrormationjtemperaturcs according to .conventional powder metallur y techniques.- densi y Oi Sl c compact will normauy' 1b "on th order of .50 to of the density ofsql d irohp The iron par, ticles initially have-Iasiz'e'of irom ahout 8;) to 325 mesh, andar'e compacted at pressures from about 6 to SOItons 'er square .i ichl The temperature of sintel gv preferablyat theaflp agamma transformatio int? or iron at at 1600' F.) although temperaturesiup to 20010 may be used. t

.A s ere imet l core'iof 911 t pe diss nted-has the further advantage 'tha fclfifiyfil'y .flmfi pressib'le, so that nocrackms or ,th' oasti occurs upon cooling; V I I y In addition to}, ou'fsl fctallcoinpact,suitable, core materials may be 'rhade'from compacted copper, powder, aluminum powder, or titanium powder. Where the outercasing' of; the casting is to consist of a relatively low melting plastic use of powder metallurgy technique to form an easily dissolved core, and the use of a casting technique which will not melt vor buckle the powder metal core.

The casting of hollow articles maybe facili: tated by this invention, since the porous core permits evacuation of the mold cavity through the core by merely connecting the core to a'source of vacuum. Further, the invention makes pos-- sible the use of powder metallurgy technique to form molds which can be dissolved off of the casting.

Where the finished article is to be used under conditions of high temperature corrosion, the preferred body metal which'forms the exterior casing is preferably a corrosionresistant alloy of the type known commercially as'the Vitallium alloys. A typical composition for such an alloy includes 30% chromium, 6 molybdenum, and the balance cobalt. Portions of the cobalt may be replaced by nickel, and portions of the molybdenum content re laced by tungsten. Other suitable alloys are those "of the Stellite series, which contain about 45-55% cobalt, 28-32% chromium, 10-15% tungsten, and about 2.5% carbon. These alloys have melting points around 2450 F. These alloys are extremely resistant to lead oxide corrosion and are substantially unattacked by mineral acids suchv as hydrochloric acid and nitric acid. In addition, they do, not wet the porous infiltration of the latter is avoided during the casting operation.

Another method forforming the body of the article about the preshaped porous core consists ure 3 and including the other half of the mold;

.40 iron compacts to any substantial degree. so that in chromizing the powder metal compact in a conventional chromizingprocedure.

This procedure consists in packing the material to be chromized in a closed container with chromium powder and then heating to a temperature between about 1800 and 2600 F. in an atmosphere of hydrogen.

The chromizing operation is carried out until such time that a self-sustaining outer shell or casing is provided about the porous metal core. The thickness of the casing may be conveniently controlled by controlling the time or temperature of the chromizing operation. The casing may be made more dense by coining the sintered porous compact to a high density, or by sintering the same in a carburizing atmosphere.

The case which is produced in such a chromizing operation is essentially a iron-chromium alloy having a diminishing chromium concentration throughout the body of the case. The outside layer of the case has been found to contain approximately 60% chromium, while the layer immediately adjacent to the porous compact has a chromium concentration of approximately 10%.

A more detailed description of the present invention will be made in conjunction with the description of the attached sheet of drawings, in which:

Figure 1 is a vertical cross-sectional view of a poppet valve casting around a porous compact and Figure 5 is a vertical sectional view of a hollow turbine blade'composed of a metal built up on the core as a coating, by chromizing or the like.

As shown on the drawings:

In Figure 1, the reference numeral 10 denotes generally a, permanent mold consisting of two symmetrical halves such as H mating at a partingline, While a permanent mold is used for purposes of illustration, it'will be understood that sand and plaster molds may be used where appropriate. Disposed'within therecessof the mold is a core comprising a powderjmetal compact I2 shaped to the configuration desired in the void space of the finished article, as described previ ously. In the illustration shown in Figure 1, the compact core l2 forms the neck and stem portions of a poppet valve assembly. The base of the compacted core 12 is provided with an appendage which forms a core print '-l3;held between appropriate recesses formed in the mating spaces of the mold i0, therebyholding the compacted core 12' within the mold lll. I

Asshown in-Figure l, a castingalloy which is to form thebodybf the poppet valve is cast around the compacted'core l2 while the same is held within the mold Hi. The body thus formed consists of an arcuate valve head portion 15, an elongated stem portion i6, and a base portion ll. It will be noted that the base portion ll terminates short of the lower base of the mold In so that the core printl3 extends downwardly beyond the limit of the base ll. A sprue portion 3 is also formed during the casting operation.

When the assembly shown in Figure 1 is removed from the mold 10, the sprue portion 18 may be trimmed off, and the core I! removed from the cast body. This removal may be most conveniently effected by immersing the cast valve into a solution of acidor other reagent capable of removing the porous core without substantially afiecting the body metal. 'By virtue of the high degree of Y porosity of the core, the acid will be quickly soaked up into the interstices of the core, and dissolution will be efiected very rapidly. The dissolved core may then be drained from the interior ofthe valve structure through the opening provided by the dissolution of the core print l3. Coolant material, such as sodium I9 is then inserted through the opening provided by the core print portion I3 to partially fill the hollow interior of thevalve. To" close the sodium filled chamber, a tapered plug I9 is inserted into the valve base ll, and a cap 20 applied to the valve base l'las by means of welding.

Figures 3 and 4 illustrate a method similar to that described in connection with Figures 1 and 2 for the production of a hollow cast turbine bucket. As shown in Figure 3, a'porous core 2! is provided within the mating faces of an appropriately recessed mold 22, the core 2| having a core print 23 extending'therefrom and serving to secure and to index the position of the core 2! isjeastsmuna it, "theco'ri'ffi is -'provide'djvwi'ith a coating 25.' 'Ihe'coating' may be applied by' casting, dipping, spraying, electroplating and similar techniques well known by those skilled in the art., This coating. may consist of a metallic coating, such as chromium or nickel, or may suitably be composed of a vitrified ceramic compositione The casting metal is poured into the mold structure, around the preformed porous core 2|, I

to define an air foil section 21 and a root portion 28 of the finished bucket. A sprue 29 remains, partially enclosing the core print 23 of the core 2!.

The cast assembly may then be removed from the mold and coined or otherwise shaped to its final configuration. To insure a proper degree of rigidity in the air-foil section during such finishing operation, it is preferable to leave the compacted core 2| within the cast body until the final shape is attained. The compacted core may then be removed by acid dissolution in-a manner already described, leaving the body metal and as much of the coating 26 as has not been dissolved by the acid.

It will be noted that in the process described above the casting of the article may be accomplished while leaving only a small outlet aperture for the removal of the porous core. Thus, an element may be cast to close tolerances initially and the closure of such outlet is all that is necessary to provide a sealed hollow chamber extending throughout the cast body.

As shown in Figure 5, a hollow turbine blade 39 can be produced by this invention by forming the blade metal as a coating on a core such as the core 24 of Fig. 3 and by dissolving out the core. No casting operation is then involved. The blade 38, if formed by chromizing, will have the exact shape and dimensions of the core, since the chromized layer is actually created by replacement of the core surface and an external building up of an added layer is not. involved. It will be of controlled porosity, depending upon the porosity of the core, but it will be less porous than the core on which it is formed. The porous blade in use in a turbo-jet engine can be sweat-cooled by oozing alcohol or some other vaporizable material through the blade.

From the foregoing it will be appreciated that we have herein provided a convenient method for producing articles having complex interior voids. The articles so produced have uniform wall thicknesses and may be precision cast to close tolerances.

It will be evident that various modifications and variations may be efiected without departing from the scope of the novel concepts of the present invention.

We claim as our invention:

1. A method of producing hollow articles which comprises compacting powdered metal into a core having the configuration desired in the hollow portion of the finished article, casting metal to form the body of the article about said core without melting or buckling the core, and dissolving out said core with a reagent capable of dissolving ew-tea -materiaL .fi u fish, fa ou a {said core, anddissolvih out said iron core ima ,3. The-method of making hollow castings' which comprises-forming a core'comprising a porous sintered powdered aluminum compact into the configuration desired in-thetfhollow portion of the finished article, castingibody metal-about said core; and dissolving out said aluminum core with an alkali.

4. The method of making hollow castings which comprises forming a core comprising a porous metal compact into a shape having the configuration of the hollow portion of the finished casting and also having an appendage forming a core print, supporting said core in a mold by means of said core print, casting an acid resistant body metal about said core while supported in said mold, removing the cast article from the mold, and dissolving the core from said cast article through said core print.

5. The method of making hollow castings which comprises forming a core comprising a sintered powdered ferrous metal compact into a shape.

having the-configuration of the hollow portion of the finished casting, coating the core with a metal coating, casting an acid-resistant body metal about said coating, and dissolving out said core with an acid capable of dissolving said ferrous core without substantially afiecting the body metal.

6. The method of making hollow castings which comprises forming a core comprising a sintered powdered ferrous metal compact into a shape having the configuration of the hollow portion of the finished casting, coating the core with a refractory coating, casting an acid resistant body metal about said coated core, and dlSSOlViIlg out said core with an acid capable of dissolving said ferrous core without substantially affecting the body metal.

7. The method of making hollow castings which comprises forming a core comprising a sintered powdered ferrous metal compact into a shape having the configuration of the hollow portion of the finished casting, chromizing the core, casting an acid resistant body metal about the chromized core, and dissolving out said core in an acid capable of dissolving the core without substantially affecting the body metal.

8. The method of making a hollow metal article which comprises forming a core comprising a porous sintered powdered metal compact, casting the body metal of the article about said core, and dissolving out said core with a reagent'capable of dissolving the core Without substantially affecting the body metal.

9. The method of making a hollow metal article which comprises forming a core comprising a porous sintered powdered metal compact, casting the body metal of the article about said core, deforming the resulting article with the core in place to produce an irregularly shaped member, and thereafter dissolving out said core with a reagent capable of dissolving the, core without substantially affecting the body metal.

MILTON S. ROUSH.

CLYDE C. CLARK.

(References on following page) REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number r I V; FQREIGN :PATENTS Sept. 22, 1932 Apr. 23,1937 Nov. 1, 1945 Gray et'al. Qct. 20, 1942 V 5 l 16999 7 5