US2946681A - Method of providing a body with a porous metal shell - Google Patents

Description

July 26, 1960 R; L. PROBST ETAL METHOD OF PROVIDING A BODY WITH A POROUS METAL SHELL Filed Jan. 31, 1957 2 Sheets-Sheet 1 July 26, 1960 R. L. PROBST ET AL 2,946,681.

METHOD OF PROVIDING A BODY WITH A POROUS METAL SHELL Filed Jan. 31, 1957 2 Sheets-Sheet 2 United States;

Ann Arbor, Mich, assignors to Federal-Mogul-Bower Bearings, Inc., Detroit, Mich., a corporationof Michigan Filed Jan. 31, 1957, Ser. No. 637,471

a 'Claims.- c1. 75-408) This invention relates to improvements in transpiration-cooled turbine blades particularly for aircraft turbines and to a method of making the same.

Various structures for turbine blades have been proposed which can be classified into the following four basic categories: cast or forged blades; cermets or ceramel blades; convection cooled blades; and transpiration cooled blades.

The cast and fo'rged blades are solid in structure and are satisfactory in turbines where the operational temperatures are sufficiently below the critical temperatures of the blade material itself. Howeverbecause higher operating temperatures are desired to increase efficiency and in the case of aircraft turbines to obtain more thrust the critical temperatures of even the so-called high temperature and super alloys are surpassed so that this type of structure for turbine blades is generally unsatisfactory in the more modern turbines.

To overcome the temperature limitations of the high temperature alloys used in cast and fo'rged blades it has been proposed to use cermets or ceramels which consists of a ceramic sponge matrix impregnated with metal or metal alloys. The ceramic component while very A 2,946,681 'Pateinted July 26, 1960 inle 'ity to allow coolant to permeate uniformly through the shell and form a boundary layer of coolair on the exterior surface of the blade which protects it from the hot turbine gases impinging thereon; to provide an improved strut-supported transpiration-cooled turbine blade which comprises a load-carrying strut and a thin sintered stainless steel shell 'firmly bonded to the strut and so formed that the said turbine blade is effectively cooled in use whereby extremely high operational temperatures may-be realized in the turbine engine; to provide an improved turbine blade of the type specified which is adequately ductile and has high-temperature strength characteristics and wherewith operational efficiency of the turbine engine is increased and additionally the refractory is low in ductility and the metals or metal alloys which impregnate the ceramic are less refractory and more ductile. The objective was to combine the refractory properties of the ceramic component and the ductility of the metal component but to date such blades have not proven successful due to the lack of ductility in the finished article even though high temperature strength characteristics are obtained.

The convection cooled blades consist of a main loadcarrying strut surrounded by a no'n-porous external metal shell. This type of blade has internal passageways from root to tip lengthwise of the blade which permit the passage of a coolant, in most cases air; to pass through the internal section of the blade and dissipate the heat of the shell by heat transfer principally thro'ugh convection to the coolant which in turn is ejected at the tip of the blade. At present convection cooled turbine blades while satisfactory for operational temperatures above those used in the cast or forged blades have temperature limitations below those desired in modern turbine engines.

It has also been suggested to provide transpirationcooled turbine blades which consist of a main load-carrying strut and an external metal shell surrounding thestrut which is constructed to allow the passage of coolant therethrough in an attempt to attain the desired operational temperatures. The tip of the blades is closed off and the coolant passes transversely through the shell and this method of cooling is known as sweat-cooling or transpiration-cooling. bine blades as have been proposed leave much to be desired. 7

Accordingly, important objects of the present invention are to provide an improved strut-supported turbine blade which includes an external porous shell which is so formed that it has uniform and controlled permeabil- Such transpiration-cooled turthrustin the case of aircraft turbines; and to provide an irn'proved turbine blade of the character indicated having a sintered stainless steel shell of airfoil contour structure which is uniquely bonded to the strnt and which is forhied from stainless steel powder of special composition the particles of which are suhstantially completely spherbidal in shape and so arranged during the sintering controlled. permeability to step as to'impa'rt uniform and the said'shelll' 7 i Other important oo ec tsof theinvention are to'proe vide an' improved process of fabricating a turbine blade which yields a new and highly successful product and which includes sintering in situ on a load-carrying strut a thin stainless steel porous shell in a manner to firmly bond the same to the strut and to impart uniform and controlled permeability to the shell whereby the blade can be more efiiciently transpiration-cooled; to provide an improved process of fabricating a strut-supported transpiration-cooled turbine blade which includes the .step of treating the land areas on the load-carrying strut to facilitate bonding of the shell thereto; to provideja process of the. type indicated which incorporates the steps of releasably filling'the spacesbetweenthe land areas of the strut with a refractory material in a effectuate proper bonding of the shell to the obtain the requisite coolant passageways. N

The above and related objects of the invention will appear more fully during the course of the following description taken in conjunction with the accompanying drawings. t V

In the drawings:

strut and to 'Fig. 1 is a photograph of stainless steel powdered start- Figs. 2A and 2B are diagrammatic views illustrating initial steps in the process embodying the invention, Fig. 2A being 'a' view in perspective of a load-carrying metal strutto be treated; g a a v Fig. 3 is a transverse sectional view of the strut of Fig. 2A substantially alongthe line 3- 3, thereof'with parts in elevation omitted; I i

Fig. 4 is a front .elevational' view of an intermediate product'obtained in the process embodying the invention; Fig. 5 is a transverse sectional view of the structure of Fig. 4 along the line 55 thereof with parts in elevation omitted; i

Figs. 6, 7A and 7B are views whichfurtherillustrate the process embodying theinvention; Fig. '6" being a perspective view of a .pair of sintering mold halves;

Fig. 7A being a front elevational view of a sintering Fig. 9 is a transverse sectional view of the structure manner to of Fig. 8 along the line 9-9 thereof with parts in elevation omitted.

In accordance with the present invention it has now been found that a superior transpiration-cooled turbine blade may be provided by sintering in situ on a loadcarrying metal strut a thin stainless steel porpns shell which is specially formed so as to have uniform and controlled permeability and which is bonded to the strnt in the manner to be described.

The process of fabricating the improved turbine blade embodying the invention is illustrated in the accompanying drawings and commences with a load carrying strut indicated generally at 9 as in Figs. 2A and 3. The strut 9 may be formed of various ma erials which are high temperature alloys, one example being a cobalt-base alloy identified in this art as type $7816. This alloy c sis s of th foll wi g e ements in the pro.- portions listed:

Per e t Carbon 0.38 Chromium 20 Nickel 20 Cobalt 44 Tungsten 4 Molybdenum 4 Columbium plus Ta 4 Manganese max 1.80 Silicon max 0.90 Iron max 5.00

The strut includes a body portion 11 having the airfoil contour in transverse cross section as shown in the particular example illustrated. Spaced lands 13 are formed integrally with the body portion 11 so as to project from opposite faces thereof and in the instance shown the lands 13 extend longitudinally of the strut 9 from its root 15 to its tip 17. The spaces indicated at 19 between the lands 13 have normal surfaces which coincide with the faces of the body portion 11. The strut 9 has formed at its root '15 an integral base 21 provided with legs 22. The base 21 has formed through its top plate 23 an arcuately shaped opening 25 which conforms generally to the shape in transverse cross section, of the body portion 11 but as can be seen is slightly greater in width and breadth than the body portion 11. The lands 13 are disposed substantially parallel to the transverse axis of the base 21 and span the breadth of the opening 25.

It has been found in the case of the above cobalt-base alloy that if the outermost surfaces of the lands, i.e. at 13a, are coated with a pure metal a stronger and more secure bond is attained during the subsequent step to be described of sintering the shell to the strut. Thus nickel is an example of a metal which has been found tobe satisfactory for this purpose and it may be applied by electroplating preferably in a manner to deposit the pure metal on the surfaces 13a without plating the spaces 19 and the sides of the lands 13. It is to be understood that this plating operation used for the cobalt-base alloy strut is optional for other alloys such as nickel or ironbase high strength alloys.

To prepare the strut 9 for direct bonding thereto of the shell the spaces 19 are completely filled with a refractory material and the refractory material is so disposed within the spaces 19 that it extends outwardly from the faces of the body portion 11 to lie in the air foil plane of the plated surfaces 13a. The spaces 19 are preferably filled with the refractory material by a pressure injection step which may be carried out as illustrated in Figs. 2A and 2B wherein a multi-part mold indicated generally at 26 is shown partly open which consists of mating side section dies 27 and 29 and injection header section die 31. The internal surfaces of the die-sections 27 and 29 are cooperatively shaped so that when they are fitted together their internal surfaces define a mold cavity, indicated generally at 33, which has a contour in transverse cross section at the root of the strut conforming substantially to the opening 25 in the base 21. Over the length of the strut the internal surfaces of the die-sections 27 and 29 which define the cavity 33 closely engage the surfaces 13a and the shape of the cavity 33 is thus determined by the shape of the lands 13. Outwardly of the outermost lands the cavity 33 may taper as predeterminedly fixed by the Size and c n u t on o the p irs o oolant P s desired in the final turbine blade at the sides of the strut. The final shape desired for the internal surfaces of the to-be-fonned shell is accordingly predetermined by the eontour of the cavity 33. The sides of the mold 26 are closed by fitting together the die sections 27 and 29 and the strut 9 is inserted. into the ravity 33 with the base 21 closing ofli one end of the cavity. 'The injection header die section 31 is fitted to the sections 27 and 29 and closes off the other end of the cavity 33. The injection header die-section 31 on its back face is provided with a suitable opening (not shown) through which the refractory material is pressure injected into the. cavity 33' to fill the spaces 19 between lands 13 and along each side of the outermost pair of lands, i.e. along the leading and trailing edges. The refractory material is preferably a ceramic mixture and an example of a satisfactory ceramic mixture is the following:

Magnesium oxide (minus 200 mesh) grams 600 Polyvinyl alcohol cc..- to

Methyl cellulose (Methocel, 7,000 cps., a trade name) gram s 4.5

Before the die-sections 27, 29 are fitted together their internal surfaces are coated with a thin layer of a mold release agent, e.g. matured and hardened tetrafiuoroethylene resin. After the ceramic mixture is pressure injected into the mold cavity-33v to fill the spaces 19 and the spaces along the leading and trailing edges, the entire pressure injected assembly is allowed to. set for several hours and is then frozen for a suitable time in a low temperature heat transfer medium. For example, 45 minutes in a Dry Ice and acetone bath maintained in the temperature range of -90 F. to l()0 F. has been found to be satisfactory. The assembly is then removed from the freezing bath, the mold 26 opened and the ceramic-filled strut removed and dried in a warm air blast.

The resultant ceramic-filled strut which is an intermediate product of the process of the invention is shown in Figs. 4 and 5 As can be seen. the ceramic-filled strut provides a continuous surface alternately of different material for forming of the shell which in transverse cross section completely surrounds the body portion 11 and consists of plated surfaces 13a separated by set-up ceramic 35 over the Width of the spaces 19. The set-up ceramic which fills the spaces along the leading and trail ing edges is indicated at 35a.

The ceramic-filled strut 9 is the starting point for the step of; sintering in situ the porous permeable shell of the turbine blade and this. is. preferably accomplished by a mold, sintering operation as illustrated in- Figs. 6), 7 and 7B.

T he, stainless. steel powder used for forming the shell is preferably of the type described in copending applicacation assigned to the present assignee of Probst and Le Brasse, Serial No. 403,922, filed January 13, 1954', for Sintered Stainless Steel Metal Alloys, now Patent No. 2,826,805, issued March 18-, 1958, to which reference is made for a detailed description of the preferred stain less. steel powder starting material of the present invention. Such powder has a special composition being austenitic stainless steel powder high in carbon to lower the liquidus-sol-idus range so that optimum sintering corrditions can be accurately maintained to result for the pur-- poses of the present invention in a sintered. shell having uniform and controlled permeability, This type of austenitic stainless steel powder contains carbon preal loyed therewith in amounts from about 0.5%. to, albeit;

powder is shown in Fig.1 which is a'photograph at api proximately 85 diameters magnification. The mesh fractions of powder of this type may vary depending upon the. ultimate permeability desired for the sintered shell. The

powder is preferably screened'soJthat the individual spherical particles as at 36 in Fig. 1 are between 270 and plus 325 mesh size. Either simultaneously with the sintering operation or thereafter the carbon is removed by exposing the powder or the sintered shell to a decarburizing atmosphere which may be dry hydrogen gas the same as is used during sintering, so that any decarburization that is necessary is included within the terms sintering, sintering step or sintering operation as used throughout the present specification or in the appended claims. 7 The sintering operation is carried out by loose sintering about the ceramic-filled strut in a split type mold such as the one shown in Fig. 6 and indicated generally at 37. The sintering mold 37 comprises mold halves 41 and 43 which may be of SAE type 1010 steel. The cavity walls 42 and 44 of the mold halves 41 and 43 respectively are machined and ground as shown so that they conform to the dimensional specifications required for the external contour of the final turbine blade shell. The cavity walls 42 and 44 are coated with a thin film of a suitable isolating material such as a wash of Tam-Zircon No. 100 cement (a trade name) which prevents bonding of the shell to the cavity walls during the sintering operation. The coated sintering mold 37 and the ceramic-filled strut are oven dried at approximately 220 F. for approximately 24 hours to assure complete removal of any moisture or water vapor. The ceramic-filled strut is then centered in the sintering mold cavity to allow uniform clearance about the cross sectional periphery of the ceramic-filled strut. The clearance may vary over the length of the strut depending upon the taper desired in the blade shell. For example in one instance of sintering a shell to a turbine blade strut in the practice of the invention a clearance at the blade tip of 0.030 inch and a clearance of 0.050 inch at the root of the blade was employed. The tip section is located by small spacers and the root section is located bysintering mold nest locators. The base 21 is located within a hollow 45 of the enlargement 46 formed on the bottom of the mold half 41 which is enclosed by the enlargement 47 formed on the bottom of the mold half 43. 7

Stainless steel powder preferably as'descnibed above is i then vibrated into the space between the ceramic-filled strut and the cavity walls 42 and 44 to dispose the individual particles in a predetermined arrangement. The preferred arrangement is that in which the individual particles are disposed in closely packed hexagonal form wherein the particles in any one dimension which surround a particle are so located that the lines joining their centers form a hexagon. An arrangement similar to this is exemplified by the commonly known method of stacking cannon balls which permits a maximum number of such balls to occupy a given volume and in the stack'the voids between the balls are interconnecting and form continuous paths in all three dimensions over the height, width and breadth of the stack. Such an arrangement for the individual spheroidal particles of the stainless steel powder is diagrammatically illustrated in Fig. 7B which is of a plurality of particles in a single plane in closely packed hexacated at 50. Such an arrangement has been found to be highly desirable for the purposes of the present invention because the porous sintered shell which results from sin te'ring stainless steel powder having its particles thus dis posed has optimum uniform and controlled permeability. A cycle vibrator table which may be vibrated at variable intensityhas been satisfactorily used for thus arranging the powder within the space between the ceramicfilled' strut and the cavity walls 42 and 44. After the sintering mold cavity is filled with the stainless steel powder as. described,.a stainless steel screen 51 is positioned over the top of the mold 37 and retained in place by. a contoured end plate 52 having an opening 53. The entire assembly shown in Fig. 7A is then placed in a sintering furnace which may be a box-type electrical resistance high temperature muflle furnace. The screen 51 retains the powder at the top of the mold cavity during the-sintering step and with the opening 53 allows the sintering atmosphere which may be dry hydrogen gas to permeate through the powder. 7

The sintering step is preferably carried out in two stages, the first stage while the powder is confined in the sintering mold to knit the particles together and to the strut in a rigid shell and the second stage which maybe at a higher temperature after the blade has been removed from the mold. The second sintering stage may be conducted to the point where all or substantially all of the carbon content of theisintered article has been re; moved where prealloyed powder having a high carbon content is employed as previously described. During the sintering step the metal coating on the surfaces 13a of the lands 13 aids in bonding of the shell as it is being formed during the sintering step as the pure metal of the coating increases the cohesion between the particles of the shell and the surfaces 13a by difiusion and by promotion of surface liquid phase conditions. Entirely satisfactory turbine blades have been produced from type 302 stainless spherical steel powder of between minus 270 and plus 325 mesh by maintaining the following conditions for the first and second sintering stages which are set forth as illustrative. Coarser or finer mesh powder than this may be used depending upon the ultimate permeability desired. For the first stage dry hydrogen gas is passed through the muflle furnace for a period of approximately four hours at a rate of about 45 cu. ft./hour and the temperature is controlled at about 2160 F. Upon completion of the first sintering step the entire assembly is cooled to room temperature and the turbine blade consisting of the strut and sintered permeable shell bonded to the strut is removed from the sintering mold. The second stage is then carried out on the unconfined turbine blade by passing dry hydrogen gas through the mufile furnace at a temperature of about 2225 F. for a period of about four hours at a rate of approximately 45 cu. ft./hour.

Upon completion of the second sintering stage the turbine blade is cooled to room temperature and the tip of the blade is trimmed to the desired length and the setgonal form wherein it can be seen that the individual particles 48 which surround the particle 49 are disposed so that the lines joining their centers form a hexagon indi:

up ceramic mixture 35 is removed from the spaces between the strut and shell by mechanical vibration at sonic or supersonic frequencies. The completed turbine blade is shown in Figs. 8 and 9 in which the porous permeable stainless steel shell'is indicated at 55. As can be seen the shell completely surrounds the strut 9 in transverse cross section and is joined thereto at the surfaces 13a of the lands 13. Bond-shear tests performed on the shell have demonstrated that the bond strength closely approaches the strength of the porous shell itself. The spaces 19 after they have been cleared of the set-up ceramic mixture 35 form cool-ant passageways longitudinally of the blade from the root 15 to the tip 17. The tip 17 of the finished blade is capped by a solid stainless steel member as at 57 which is .welded, brazed or otherwise secured to both the strut 9 and the shell 55. In use of the turbine blade the coolant enters the blade through the opening 25 in the base 21 and passes longitudinally. between the strut 9 and the shell. 55 along the spaces therebetween. and is forced transversely through the permeable. shell 55. The coolant permeates. the porous shell 55 uniformly and forms a. boundary layer of cool air on the exterior surface of the shell which protects the same from thehot turbine gases impinging on the blade thus permitting higher operational temperatures than blades heretofore avail.- able.

What is claimed is: j/

1. The method. of providing. a longitudinally extending body with a porous metal shell comprised of substantially spherical particles and which can be supplied internally with cooling fluid that flows outwardly through the shell, said method comprising forming the outer surface of the body with longitudinally extending lands separated by ga'ps, filling said gaps with refractory material, metal-coating the outer surfaces of said lands between said gaps, confining the body in a mold cavity of a size to provide a space surrounding saidbody, filling said space witha metal powder the individual particles of which are substantially spherical in shape, vibrating the powder to pack it withinsaid space, initially sintering said powder to bond it to the metal coating on said lands, removing said body with the sintered shell thereon from said mold cavity, finally sintering said powder, and removing the refractory material from said gaps by mechanically vibrating said body and said shell.

' 2. The method of providing a longitudinally extending body with a porous metal shell comprised of substantially spherical particles and which can be supplied internally with cooling fluid that flows outwardly through the shell, said method comprising forming the outer surface of the body with longitudinally extending spaced lands having metallic outer surfaces, filling the gaps between said lands with refractory material, confining the body in a mold cavity of a size to provide a space surrounding said body, filling said space with a metal powder, the individual particles of which are substantially 8 V spherical in shape, vibrating the powder to pacle it within said space, sintering. said powder to bond it to the metal.

surfaces of said lands, removing said body with the sintered shell thereon from said mold cavity,, and removing the refractory material from saidgapsn 3 A process of fabricating. a. transpiration cooled turbine blade. comprising electroplating; the. outermost surfaces of the lands. on a load-carrying metal strut, pressure injecting, the spaces between said lands with a ceramic mixture soas to leave exposed, the electroplated surfaces on said lands, and releasably bonding said ceramic mixture to the sides of the lands and the body portion of the strut between said lands so as to. leave the exposed electroplated. surfaces on said lands, thereafter disposing the ceramic-filled strut in a si'ntering mold to center the same with respect to and spaced from the cavity walls of the sint'eringymold, and depositing stainlesssteel powder, the individual particles of which are substantially completely spheroidal in shape, into the cavity of the sintering mold so that the powder is loosely confined about the ceramic-filled strut and disposed in closely packed hexagonal form, sintering said powder to form a thin uniformly permeable stainless steel shell and to firmly bond said shell to. the electroplated surfaces on said lands, and thereafter removing the ceramic mixture from the spaces between.

said lands without smearing the particles adjacent there to and while maintaining said shell bonded to the strut.

References Cited in the file of this patent UNITED STATES PATENTS 2,554,343 Pall May 22, i

2,648,520 Schmitt Aug. 11, 1953.

2,665,881 Smith Ian. 12, 1954 2,672,415 Balke Mar. 16, 1954 2,687,278 Smith et al. Aug. 24, 1954 FOREIGN PATENTS 731,161 Great Britain June 1, 1955

Claims (1)

1. THE METHOD OF PROVIDING A LONGITUDINALLY EXTENDING BODSY WITH A POROUS METAL SHELL COMPRISED ON SUBSTANTIALLY SPHERICAL PARTICLES AND WHICH CAN BE SUPPLIED INTERNALLY WITH COOLING FLUID THAT FLOWS OUTWARDLY THROUGH THE SHELL, SAID METHOD COMPORISING FORMING THE OUTER SURFACE OF THE BODY WITH LONGITUDINALLY EXTENDING LANDS SEPARATED BY GAPS, FILLING SAID GAPS WITH REFRACTORY MATERIAL, METAL-COATING THE OUTER SURFACES OF SAID LANDS BETWEEN SAID GAPS, CONFINING THE BODY IN A MOLD CAVITY OF A SIZE TO PROVIDE A SPACE SURROUNDING SAID BODY, FILLING SAID SPACE WITH A METAL POWDER THE INDIVIDUAL PARTICLES OF WHICH ARE SUBSTANTIALLY SPHERICAL IN SHAPE, VIBRATING THE POWDER TO PACK IT WITHIN SAID SPACE, INITIALLY SINTERING SAID POWDER

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