US2612443A - Powder metallurgy - Google Patents

Description

Sept. 30, 1952 'c. e. GOETZEL ETAL 2,612,443

POWDER METALLURGY I Filed Dec. 26, 1947 2 SHEETS-SHEET 1 INVENTORS cmus 6. 60572.42 YJOH/V L. ELL/6 ".4 rroie/vsys Sept 30, 1952 c. a. GOETZEL ETAL POWDER METALLURGY Filed Dec. 26,1947 2 SHEETSSHEET 2 FROM 6A5 INERT GAS TANK INVENTORS CZAUS G. GOETZEL JOHN LELL/S WK W A TTORfVEYS Patented Sept. 30, 1952 POWDER METALLURGY Claus G; Goetzel, Yonkers and John L. Ellis, New,York, N. Yr, assignors to Sintercast Corporation of America, New York, N. Y., a corporation of New York Application December 26, 1947, Serial No. 793,990

This invention relates to a method of form-;

ijngcompositematerial shaped bodies of metals, alloys, metal compounds, refractories, and, the like, and especially to the complete impregna tion ofa porous skeleton with a second lower melting metal or,;the like which excels by, a high resistance to corrosion andoxidation atelevated temperatures. r i

. ,In themanufacture of heat resistant metallic components. for articles such as blades, buckets,

valves and the like for jet engines, rockets, or gas turbines and the like, conventional casting or forging methods are not satisfactory. One of the reasons for this is that the temperature at which.

erainboundaries, all of which contribute to unfavorable. hot tensile strength, hot fatiguestrength, and resistance to creep at elevated-temperatures. H v i i ,Previously, composite material shaped bodies in the field of-powder metallurgy have been made by mixing the powders-comprising the same and then shaping by pressing in a suitable die, following which the, shaped body has been removed from die and-the body heated or sintered until one, of the constituents is melted 'so' that the liquid metal fills the pores of the mixture. Also in prior practice-the first or higher melting constituent hasbeen placed into a mold and a porous skeleton formed: therein, following which the, skeleton has been removed from themold, sintered, and then placed in contact with or immersed in the second or auxiliary lower melting meta-l depending only upon capillary action to fill the pores of the skeleton. In this latter case, the second metal has been brought into contact with the skeleton in afsuitable ceramic or metallic boat, and heat applied so as to liquefy the second metal and cause the same to be drawn into the skeleton by capillary action. Also the "second metal has been placed in contact with theskeleton andsmelted, the impregnation taking plac'eby capillary actioni l ;:-I n relying upon-capillary action to fully im- 1 Claim. (01. 75 22) of Metallic Surfaces 2 pregnate a skeleton, it is sometimes impossible to produce a large quantity of formed bodies having exactly the same uniformity, because of the irregularities in the size and form of granulation of particles of the skeleton. When capil lary action is dependedupomforces affectingthe penetration of the-infiltrant into the pores of the skeleton may vary from article to article, Physical examination has shown thatthe condition of the contactfaces between powder and liquid is very important for capillary action, as; is also the cross-section of the capillari es. E.. R. Parker and R. Smoluchowski. in Ca.pillarity (American Society for Metals, 13 Preprints, .1944) show that the riseof liquid in grooves depends upon thev angles of the grooves. In a sintered metal body, it is obviously impossible to determine the exact form of the capillaries, orvariation thereof, in eachbody without sectioning the .samewhich would obviously destroy the usefulness thereof. It is apparent therefor that there may be a lack of homogeneity and uniformity in many instances; in parts made where impregnation depends upon capillary action by itself. v i l a Other difficulties inherent in the process of in; filtration by capillary action have been found to consist of poor penetration, or no penetration, at all of the'infiltrant metal through the pore sys-' tem owing to lack of wetting or surface aflinitywith the skeleton material. In other instances, there havebeen found difficulties arising from. anopposite trend, with a pronounced solubilitybe tween'skeleton and infiltrant material, resulting in skeleton erosion effects at the surface aswellas drastic changes and-variations in size and/or shape. Also, it often may be considered of a disadvantage that a coverage of the surface ofthe body with the infiltrant metal may remain incomplete and discontinuous, as well as uncon trollable, as to thickness and location of the surface casing. l

Another difliculty with the use ofthe afore-, mentioned contact method or of an immersionmethod of infiltrating'the skeleton is that there may be an adherence of the molten metal on the surface of the skeleton and the possibility of dissolution of protusions of the skeleton.

One of the principal objects of the present in vention is the production of uniform, dense, com-.- posite bodies from powdered materials and particularly bodies having intricate shapes whereby improved physical characteristics of the finished product can be obtained, in particular at elevated temperatures. a I I In one aspect of the invention, a skeleton is uct, and then the skeleton is sintered, or heat 4 skeleton is not removed from the mold afte it is formed until followin the impregnation and other steps or treatments thereafter. Another advantage of using the sam mold is that loose tre t d, i the a d- In the compacting 5 powder packs can be used for producing the skelethe-powdered material in the mold, preferably it is pressedjunder a. pressure in combination with a pulsating, dynamic loading, or vibratory action superimposed thereon. After the skeleton has been sintered, it then can be transferred to the point of impregnation of the skeletonywith' the.

second metal.

the desired shape before it is impregnated with the infiltering metal, by heating the material ton. The skeleton thus can be oflow concentration .sothat they could notxform; a"s'tab1e body in themselves that couldbe transferred outside of the mold. If desired, a sizing'operation can be performed on the skeleton befor impregnation.

forming the porous skeleton and? pressingto thedesired density either in or outside of the infiltration mold. The density to which the skeleton'is hgtpre-ssed is less than the theoretical density attainable by suchahot pressing;

The'skeletorrl alsocan be'formed from powdered materials in othermanners and removed from the mold:uriformedwithouta mold as' conditions permits; The desired attr-ibute of the skeleton is thatfitfhave poresintercommunicating with each othr subsftantial-ly throughout the entire skeleton? I j "'If h'e'irnpregnatiorrof the skeleton with a lower meltingmetallic:material is performed in a mold inithe present invention by means of a pressure diiferential;; i n;theiporesystem of the skeleton inithe niold instead of" dependin upon capillary action-which as-explained above may have distinctidisadvantages andlimitations in the attainmer t offiuniformity and other properties. The infiltrant or auxiliary-second metal can be heated as melted-in arecep-tacle'separated from the mere and skeleton; and'brought' into contact with the. skeleton in -a liquid-on semi-liquid form and forced into allj-Ofthe poresunder a pressure differential so as to-completely-and uniformly fill ti'igsame; anpre'ssure being-{exerted on the infilre lative to-the skeleton. The pressure differential exerted is in the-same direction as the capillaryfior'ce; and can bemade high enough to overcome al-l typesof mechanical,- metallurgical, cr cheescm barriersgand' thus insur complete impregnation of all of the pores of the skeleton bye-r ing-thednfiltrantmetal in the pore system threof1;;whi eh-is necessary for uniformity, surface -and 's-ize control, as Well as to obtain the optimum-physicalcharacteristics at ordinary and elevated temperatures, and other effects which will be explained atagl-ater-point. I Inparticular; when-impregnation is carried out by akpres'siiredifferential, the time can'be made may-be 'onein which" it has been placed afterprior-treatmnt: "when; theskeleton is impreg nated iir e, mold-*or die; the exact form; ofth'e" finished-product can be attained without further machining' because the pores of the-skeletonunay just befilled under the pressure differential to thesurfaces of the-bodywhere thesesurf'aces are in contact wit-1'1 the-mold. There will then be no excess metal on the surface'of the article where in c'o'ntact with the mold. -'Ihe.shape ofthe-body' will be maintained with better accuracy if the- The sizebf the skeleton relative to the mold also can be chosen so as to obtain a casing of infiltrantmetalaround' the article to provide the 'desired casingthicknessof the. infiltrant metal.

As'-is,, well known,, considerable size variations are encountered both in direction of growth and shrinkage dependington the reaction between the infiltrantand the skeleton material and also in the skeleton itself upon sintering. vRefractory metals such as tungsten, molybdenum, anticobaitcemented tungsten carbide, for example sh'rinkupto 15 as iswell known inthe-art. The

exactsize'controI requires careful calculation. It is-comm0n1y experienced that the greater the-'- shrinkage, the more will-bathe size variation of therifinished'product: Inthe process of th pres enttinvention performed-in a mold of" theexa'ct shape of the finished article, the mold willbe" completely filledand the si'deof' the article thus will 'dependon the mold and will be comparableto precision: casting processes.

.ilnsthe aspect of the invention, wheresecond mold" is' ofjlthe exact size of the finished:

product and related tothe first mold in such a way that therelwillibe a small space between the skeleton faces and the mold walls as desired.

When impregnation is-made under a pressure differential so as to fill all of the pores of'the skele 4.0 ton andjalso theentire mold exactly the skeleton bleusecl;

The mold also can be arrangedsuch that a' space-"can be left forinfiltrant metalto ffill forf' machining purposes whichma-ybe called under;

casting.

uniformly distributed throughout the skeleton.

G'O- When such a body is placed incontact with the infilt'rant metal so that it is possible for the infil-f trant-rto. flow intothe-pores, apressure diiferen-. tial created in the pores ofthe skeleton, there being a pressure; on-' the infiltrant. relative, toth'e; skeleton, a complete and uniforminipie nation.

of the skeleton will'resulti.

There are various methods. and, devices whichthepressure differential in.v the zpore. sad-'1 0,-, ten}. of. the, skeleton in. the. mold can-be created inv carrying out the:,invention. Inzoneform. of the: invention; a piston can. be-employedto; exertrthe; necessary pressure: on: the: molten metal as it is" forced into the skeleton located inr-tlreamold' on otherwi'sen In,another form;the'pressureon the the -im -Y pregnation takes place in a second mold, said" The invention can be carried out advantageouslyiwhen the skeleton'is formed from powdered; material of such a particle si'zeand 'distribution1 so asto produce-the-"desirable porous structure. Asanexample, the densityof'theskeleton should, not"'b,e.moresthan about to because-if thepore-volume is less thanabout'10 %","the par s may lose their intercommunicating characteris tics. For a uniform body, the porositysho uld be" thus completely s peratni'ospheric -'pressure applied to the infil-' tering metal may be used. l is k In still another aspect of the invention, inert gas or fluid pressure" may be applied to the surface of a body of molten metal in contact with the skeleton to force the same into the pores and I impregnate the skeleton in the mold. I

Merely byway of example,- the skeleton body may be made of tungsten, molybdenum, titanium, tantalum, columbium ch'romium, zirconium, or their alloys witheach other, orwith iron, nickel, cobalt, or their compounds of metalloidal character with carbon, boron, silicon, nitrogen, etc. some examples of theaf orementioned compounds of; metalloidal character are tungsten carbide (WC), titanium carbide (TiC) molybdenum carbide (MOzC), tantalum carbide (TaC), columblum carbide (CbC), chromium carbide (CIsCz), zirconium carbide .(ZrC), vanadium carbide (VC); tungsten boride'KwBz), titanium boride (TlBxL molybdenum boride (MoB), tantalum wpcride (TaB), columbium boride (CbB), chromium boride ('CrB), zirconium boride (ZraB4), vanadium boride WB z), thorium boride (ThBo); also stable refractory materials or compounds such as beryllium oxide, magnesium oxide, aluminiumoxide, zirconium oxide, silicon carbide, and boron carbide can be used, these being stable at elevated temperatures.

A typical composition of a homogeneous skeleton" material as employed by this invention consists of 70% by weightof tungsten carbide (WC), 25% titanium carbide (TiC), and 5% cobalt (Co), the tungsten carbide (WC) and titanium carbide (TiC) components being combined as a solid solution. Another typical composition of a homogeneous skeleton materialas employed by this invention consists. off4 5% by weight of tungsten carbide (WC), 26% titanium carbide (T10), 25% chromium carbide (CrsC-z) and 5% cobalt (Co), he tungsten carbide (WC) and titanium carbide (TiC), and chromium carbide (CraCz) components again being combined as a solid solution. y

Again merely byway "of example, the impregnatine .materialmay be iron, nickel, cobalt, chromium and their alloys with each other, or their alloys with the previously mentioned refractory metals, ormteal compounds as minor constituents; Itis to be understood that the ap propriate, infiltrant having a dissimilar lower melting point relativeto the skeleton can be used. A typical composition of impregnating material employed successfully by this invention comprises an alloy containing 69% by weight of cobalt, 25%],chromium and, 6% molybdenum.

Another exampleis a material containing 50% by weight of cobalt, 29% chromium, 15% nickel and 6%. molybdenum. Still another example is one containing 52% by. weight of cobalt, 28% chromium, 11% nickel, and19%-tungsten. Another example contains ,60% by. weight of nickel, 16% molybdenum, 14% chromium, tungsten and 15%. iron, :while still another contains 60% by weight of chromium, %mol ybdnum and 15%iron. w Certain combinations of skeleton and inflltrant are particularly difficult to impregnate com pletely by depending upon capillary action alone such as for example impregnation of an iron; nickel, or cobalt skeleton by aluminum i y i The intermetallic compound AlNi of; aluminum and nickelwhich melts at 1640C. contains 68.5% by weight of nickel. The'intermtallic compound AlCo of aluminum and cobalt which melts' at 1628 C. contains 68.5% by weight'of'coba'lt. skeletonof pure nickel melts atl452 C. and'a skeleton of pure cobalt melts at 1490C. "After infiltration of either of the aforementloned'ske1etons with liquid aluminum having" a" melting point of 660 C. there results aconiposite strucure which after a subsequent diffusion heat treatment is converted to the above'mentioned intermetallic compounds AlNi' and A-lCorespe'c-g tively. These cases illustratetwo examples of the diffusion heat treatment of infiltrated skeletons wherein an intermetallic' compound formed having ahigher melting point than the original skeleton or infiltrant' metal. H As mentioned, an efiicacious method of introducing the lowmelting aluminumjphase into the skeleton is to utilize a'pressure differentialin the interconnected pore system sucha's can "be done, for example, ina'die castingni'achine." Various presintering heattreatments, difiusion alloying heat treatments after impregnation, or

precipitation hardening heat treatments can be performed in the carrying out of the invention. These and otherobjects, advantages and f'eatures of the invention will become apparent n the following description and drawings 'whicli are merely exemplary. V 1 Inthedrawingsr L Figure l is a schematic sectional view'oi a mold for an article such asf turbine blade which can be made advantageolfily by the' present-"invn j tion. f Figure 2, is a sectional line 22 of Figure 1. H

' Figure 3 is a sectional vieWwherein mechani-f cally applied pressure is used: tojc'rea'tea pr'es sure differential during impregnation of "the skeleton. Figure 4 is a schematic sectional viewof one manner in which the impregnation may be ef fected by apressure difierentialoi-eatedchtrifugally. I Figure 5 is a schematic view of one form of apparatus wherein gas pressure can be applied intermittently to impregnate the skeleton. Figure 6 is a schematic sectional view of one form of apparatus forapplying a vacuum to the skeleton for creating the pressure differential in impregnation of the skeleton. Figure 7 is a diagrammatic view of'o'ne form of apparatus which may be used for applying pressure and dynamic loading to the mold in formation of the skeleton therein. I f; ,Theinvention is especially adapted for use'iin producing objects having intricate or complex shapes wherein the'cavity of the mold is'so shaped that it is difiicult to obtain' uniform 'de'nsities by prior methods. i As previously mentioned, the invention is illustrated in conjunction withi the vaca e site th manufactureof a turbine blade but it iStOI'DG 2&1254143 7. hmbmzbezohaiceramici; graphite. heat resistant alloy steel, or any other suitable material :or geombinationofmaterials. Themoldmaybeiformed, forrexampleigby investment. ;mold techniques; conaentionally .zusediin .;pre.cision-. casting-or may .be

made or; constructed-ins.anyisuitable manner.

In the compleicshaped.mold'shown-in-Eigures mend-.2, ithe. cavity, H1, for. .therturbine blade; can

hayeyaafeather. edge ll .a. curved :portion, [2,; and

base -:|'3:,Of the .IdGSlI'Bd'IfQm. sidewall I 4 ..of.,the m l ica-n havewai suitable taper therein so .that the mold-v. will beyreceivableiin .a suitable mold holder. jgTheipai-rticular. shape. of the .mold and entrancezaperture willvary 1 with. the ,articleto. be formed-.andwiththe particular manner. in which theirpressure. differential is tube, exerted: thereon :a'se will lbesexplainedhhereafter.

. ;.-,In theiiorming ,of theskeletonm the :mold can have aalsuitable; powdered; material with..the .de-

.si-red:;-. particl e: size distribution ,fed therein, .it

being. ,preferable ,to soeselect ,the particle size distributionofthe materialsoas to obtain the desired .densityandedesired pore size distribution.

materialamoldashape. and, pressure used. Preferably, -.the density ..attainable .in ,compacting :shorildQriotbe.1more,than,about.85% to 90% because.-,a, -pore volume ofless than ,10% will cause the skeleton tolose its intercommunicating, character of its pores.

One manner of compactingtheskeleton; isto place .mold .14 with .powder therein in .a. mold holder [5 (Big. 7),;said mold holderbeing slidably carriedin,amoldholder support. l8 which in turn is mounted in a press bed I! of any suitable construction. .The1clamping ring Hi can hold the mold holder support in a predetermined position and be arranged so that space l 9. between the top not the .Jnold holder l.5,.and .the bottom of clamping ring; I 8. will permit a vertical reciprocating motion of mold holder [5 with mold v.l i therein. I

A .-.mold hol der-. .cap ,ring v -12!) can be .;'s crew threadedlyiengageddn thegmold, holder i 5 with a spacer ring 2| of any suitable material .there between, said spacerlring. serving .to evenly apply theypressure exerted by. cap. 20 to .the fragileelnd irregularmold -l 5.

. ,Spring guide 221s. fastened to. the, mold holder I .li and holds'spring 23 inplace between the press bed-.I-l-and flange. 24 of the 'springguide. .A jolting ram 25 is engageable with .flangeizl, spring 23;,normally. holding themold holder and mold in.; theirl-lowermost position relative .to the press bed l-l. The jolting rampanbe operated.in..,a

reciprocating fashionesotas,:toc'ausea reciprocation. of .the -molds holder .andmold Itherein.

-..'I'-h main ram =26 is enterable through ;.cap. .20 into-the L mold cavity. The. rams may. :be .fumished-with-pressure :inaany .desired zand conventionalcmannerxsozthatz-as the main pressure is applied -to'rram126;:a dynamic loading can be applied'a bysthe 'joltingrram 25, for example,v

by-applyingzafpulsatinghydraulic pressure thereto. In:this:manners.a-constant pressurefforce on theizpowderiinithermoldxcancbe, exerted :in coniunction twiths-a'ritibratoryaor: dynamic loading,

. space between '-tube 32 and outside walls ofreandathis swilliserveato .gproperly sand'suniformly If desired the .material forming thesskeleton may be, heated to-la highzztemperature .before.or after it is; placed in .the.,mo1d rand .hot pressgd in: the mold -merely .bytheuse oftaram izt opr arated under suitable pressure, or as a recipro- :ating.hammer. In. .this.-.ev.ent the mold, must be constructed .of @metale-or other material suitable to withstand the ypressureszapplied. .Asamentioned previously,thehotpressing of theskeleton I is .arranged'rso that it isestoppedgrbeforethe highest vtheoretical density: is .attained. could be accomplished, for..examp1,e, byea stop arranged in .a. predetermined,positiongin. relation to, thesize; of.- .theskeleton :so;.as. to. attain-the. d6,- sired/densitytherein. I; I Y Q;

.Followiilg, compacting or forming-of. the, skele' .tondn. the, mold, whichlas, previously mentioned canbe accomplished invarious; manners; includingJhotpressi-ng, the .mold -with the skeleton therein, can be, moved to .a, sintering- .point,.;zoiie, I

or furnace :Where .the, sintering operationneanvbe'. carried out. By.keeping the:skeleton,.inthe-In5l. V

the, size vthereof willibe. ,controlled-ltosbetter ,ad;

vantage. p

,Before the sintering. operation, .in some.) invstances,,.the mold. with the; skeleton therein can be subjected to a...presinterin ..treatmentdingfa protective ,atmosphere .for the v,purpose-..of. dif-'- fusion ...alloying, temperatures .in ,.-the ra,nge. .of

1500.C..,to..2000.,C...being employed. Such presintering ..and. sintering Y treatments .wi l upon,.the,particular,.,object being .mad a terials employed. V

. Upon. completion .of (the Lsinteririg .joperation, the .compactedand-sintered .skeleton in the mold is ready for impregnation. 1 Ifthessintering op} eration ,has been .:ca.r ried..out;when the skeleton isnot in a. mold," the. skeleton is; placed. 1.11.33 mfl id before impregnation. .Itispossible, if..desi red,jat thispoint t0 subjectthe skeleton to sizing or mild coinin operation, either while the. skeleton ..;is in the mold or otherwise. Q

The pressure,,diff erential ,desirable to insure a complete, impregnation of .the skeleton. may-.Ibe applied indifferent, manners. as previouslyumenr tioned. One way in which, the pressureedifierential can be applied byimechanical pressureis seen in Figure 3 vwhereinmold l4 withsinteredlskeleton 2'! therein placedin.areceptaclela. ,IIIhe receptacle-2 8 ,may have a .water. cooled cox/c 28 through which pressuraplunger 30 ca,n recipro cate 'and'thecover 28 can, have asuitable packing'gland 31 therein. ".Tube'3j2 .can be,,mad 'of somesuitable material such as quanta. and, the

ceptacle 28 -filled with a suitable insulating powder. -A graphiteretainertube s3 can-be employed, tohold themold holder 34 initsdesire'd positionwithin receptacle" 28. -A "=graphite cover 35 having" :an aperture 36 can be placed on top ofithe mold-holder:34 after the :mold l4 is in placeiand a" Drotectivmgas-inlet*31 can be used iorzifurnislziingaamrotective gasx to-rthe housing.

9. A knockout an 30.4 can "be p'rovided to assist in loosening and removingthe mold when" cover 29 isremoved. 11.. a

The-infiltrant or auxiliary metal 38 is located on top of the skeleton and the auxiliary metal can be brought to a liquefying temperature by the heating coil. The graphite mold holder 34- from holder is transmitted to. the skeleton.

21 through I mold I4, the skeleton thus being brought to a temperature at .least that of the infiltrant metal.

Uponlapplication of. pressure to plunger 30 an impregnationhofthe skeleton will be caused by means of the pressure differential created in the pores of the skeleton. 1 It thus is seen that a mechanical pressure in this instance creates the pressure differential assisting the capillary action so as to insure complete impregnation of the skeleton by the auxiliary metal.

The-pressure differential also may be effectuated bycentrifugal force in an apparatus such as shown diagrammatically in Figure 4 wherein molds 40, similar to mold I4 of Figure l, are held in place by mold positioners 4|, mold positioners 4| in turn being held firmly against the central feeding member 42 by weights .43

when the centrifugal device is rapidly rotating. A cover may be provided and a protective gas atmosphere maintained in the casing. The centrifugal feeding chamber 42 hasan inlet aperture 43 an doutlet apertures for conducting the infiltrant metal to the skeletons .45 in molds 40. Heating apparatus. asdesired: can be employed in conjunctlonwith the centrifugal apparatus just described which apparatus is mounted on shaft 46, there being a suitable protecting cover 41 for the device. It is obvious that any number of molds may be employed in accordance with the particular arrangement of the apparatus.

With the molds in place and when the device is rotating at a high rate of speed, the centrifugal force exerted on the molten impregnating metal will cause the metal to flow into the pores of the skeleton due to the pressure differential created. It is apparent that various types of centrifugal arrangements can be used.

In another form of the invention, the molten metal can be forced into the compacted skeleton in a mold under a pressure differential created by means of intermittently applied gas pressure. Mold 48 (Fig. has a compacted skeleton 49 therein, said mold being in a mold holder 59 carried on mold table 5 I. A high frequency heating coil 52 may be employed for heating the metals as necessary in the mold. An electrically resistance heated casting tube 53 extends from the mold table downwardly into a molten bath 54 of metal in the metal holding furnace 55, said furnace being heated by a suitable heating apparatus. A refractory cover 56 may be located on said holding furnace 55. Casting tube 53 extends belowthe surface of the molten metal 54 as does also the filling spout 51. The cover for the spout and cover 56 are arranged so that they will remain closed when gas pressure is present inside of the holder furnace 55 and a clamp arrangement 58 may be employed to hold the mold 48 on mold table 5| and in contact with the casting pipe.

The intermittent gas pressure system has an inert gas pressure tank 58 which may have an inert gas generator connected thereto as conventionally used in continuous casting machines. The generator through a suitable system of valves less of loss and leakages in the system. ,Gon; trol valve 59 controls the flow of inert gas through the supply gas line into the gas space fillof, the metal holding furnace 55. Aninertgasrw lease tank 62 is connected to the gas spacetl of the furnace 55 through pipe 63, control valve 64 being interposed therein. A heating coil 55 -;may be connected with the gas supply pipetll and a, cooling coil 66 associated with the outlet 53. gas pump 67 is connected in the return 68 from the inert gas release tank 62 .to-theinert aspressure tank. q i 34 In operation, after a mold48;with a compaet ed skeletontherein has been placed ontthetable; 5i and clamped thereon, ,the heating coils 5 2; can be energized as needed to bring the skeleton to the required temperature. Valve :59. then, opened to subject holding furnace space lil tothe; gas pressure of inert gas pressure tank 58. =Upon completion of the penetration of the pore of skeleton 49 in mold 48 with the infiltrant'nietal, the high frequency power on coil 52 can be interrupted so as to rapidly cool the mold assembly with the exception of the electrically heated casting tube 53. Immediately after the molten metal has somewhat solidified in the skeleton, valve-64 is opened so as to allow the accumulated gas pressure in space 6| to escape into the'release gas tank 62 thereby dropping the level of the metal inside of the casting tube to the level of the metal in the holding furnace 55. Thereafter the mold and impregnated skeleton can be removed-from the table and the article removed from themold, or a further heating treatment given to the impregnated skeleton as desired.

An automatic timer arrangement (not shown) can be connected to valve 59 so as to cause operation of valve 59 at a predetermined time after current is supplied to coil 52 and thus insure proper heating of the skeleton before impregnation takes place. The particular time delay can be determined by experiment after several casting operations of a given combination. It also is possible to have control means in the holding tank so as to de-energize the control system when the level of metal 54 in the holding furnace 55 drops below a predetermined point and thus prevent gas being forced upwardly into the casting tube.

Still another manner of applying a pressure differential for impregnation is the application of a vacuum to the skeleton while in a mold. This can be accomplished by placing the mold 69 (Fig. 6), with a compacted skeleton 10 therein into a mold holder 1|, said mold holder having a high frequency heating coil 12 associated therewith. The mold 69, similar to that shown in Figure 5, can have an aperture 13 therein to fit on to the casting pipe 14. Cover 75 has an aperture 16 therein having a vacuum pipe ll connected thereto. When a vacuum is applied to pipe 11, atmospheric pressure can be employed to cause molten metal in the casting tube 14 to flow into the skeleton 69 and impregnate the same.

As previously mentioned, various heat treatments can be employed if desired. A high temperature heat treating technique could be used for the purpose of producing a solid stable structure of optimum strength, toughness and resistance to deformation at high temperatures. It may be desirable to eliminate the liquid phase at infiltration temperature after a complete pene-

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