Solid State Repair Method And Means

Abstract

A method of rebuilding a jet engine compressor, fan, or turbine blade containing a worn or damaged area by trimming off the portion of such blade surrounding the area and replacing such portion with another of precisely corresponding size and metallurgical composition. Standardized templates define the size of the trimmed off portion. The replaced portion is integrally joined to the blade by solid state molecular diffusion bonding in a mold exactly duplicating the contour of the original blade when new.

Description

BACKGROUND OF THE INVENTION

Engines of turbojet or turbofan-type such as used in modern military and commercial aircraft contain many compressor and turbine blades, each of which is precision formed. Engine performance and reliability depend directly upon the detailed design contour and structural integrity of these blades. In axial flow compressors of some jet engines, as many as 20 stages of compression are accomplished by a corresponding number of blade rows, each containing from 50 to 150 individual blades. The moving rows are separated by stationary alternate rows in close proximity, whereby dimensional accuracy of the blades is crucially important. Similarly, in turbofan engines, each individual blade is contoured for maximum efficiency at the design rotation speed, whereby the angle of attack at each radial location along the blade length decreases as the tangential velocity increases. Accuracy of such contour is crucially important in achieving proper aerodynamic performance of the blade. In addition, structural integrity of the blades is absolutely essential due to the high centrifugal stresses and elevated temperatures to which they are exposed during high speed rotation under normal operating conditions.

When a blade becomes damaged due to entrance of a foreign object, erosion, temperature or load stress, such as to produce local distortion or microscopic cracking, it is unsafe for aircraft use. Accordingly, compressor and turbofan blades which are found to contain any abnormal deviation or defect in shape, dimension, structural integrity, or surface smoothness are completely discarded and replaced by new blades. Due to the extreme care and expense involved in fabrication of such blades and the costly materials used therein, replacement of such blades is the outstanding major item of expense in jet and turbofan engine maintenance.

SUMMARY OF THE INVENTION

The invention in this case provides a method for repairing metallic articles which are found to contain localized defects such as cracks, pits, dents, surface erosion, worn areas, or other dimensional deficiencies from any cause. As illustratively applied to turbofan blades, the novel method begins with a machining step to cut away that portion of the blade surrounding the defect. The foregoing step is carefully controlled to produce a cutout portion of predetermined size and shape according to a set of standardized template patterns. For economy and mass-production convenience, a large number of incremental elements of standard size and shape conforming to the mentioned templates are prefabricated for use in rebuilding damaged blades. After the mentioned machining or cutting step, the blade is placed into a two-piece mold having workfaces which define a cavity oppositely corresponding to the precise contour of a complete undamaged blade. One or more of the mentioned prefabricated incremental elements is positioned in the cutout area of the damaged blade. Heat and pressure are applied to the two-piece mold as required to effect gradual deformation of the incremental element into conformance with the stated workface contour, and to unite the incremental mass to the adjoining blade material integrally by a solid-state molecular diffusion bond.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a damaged turbofan engine blade which may be rebuilt by the novel process disclosed herein;

FIG. 2 shows the article from FIG. 1 in an intermediate state of the repair process;

FIG. 3 shows an article similar to FIG. 1 but with a different type of damage;

FIG. 4 shows a pattern of templates useful for trimming damaged portions off a turbofan blade, together with a trimmed blade;

FIG. 5 shows a finished rebuilt blade following completion of the novel process;

FIG. 6 shows a general perspective view of a retort useful in the bonding step of the novel process; and

FIGS. 7 and 8 are cross-sectional views taken along line 7-7 of FIGS. 4 and 6 showing the tooling and workpiece elements in two stages of the bonding operation used to produce the rebuilt blade of FIG. 5.

DETAILED DESCRIPTION

Referring to the drawings described above, use of the novel process disclosed herein may be seen in connection with repairing or rebuilding turbofan blades such as blade 10 in FIG. 1. In a typical case, blade 10 is provided with a strong base portion 12 with flanges or similar means to key the same securely in a rotating hub or the like (not shown). Blade 10 is an airfoil with a progressively changing angle of attack between base 12 and distal end or tip 14 in the familiar manner of propeller blades. The leading and trailing edges of blade 10 are designated 16 and 18, respectively. Intermediate blade portions 12 and 14, blade 10 is provided with stabilizing means in the form of platform projections extending in opposite directions from both sides of the blade, only one of which is visible in FIG. 1 and denoted by reference numeral 20. When many blades identical to blade 10 are mounted in a complete circle around a hub, the mentioned intermediate platforms 20 contact each other between each pair of adjoining blades to form a circular segmented flange, the segments of which comprise platforms 20. Thus, each of the platforms is designed to bear firmly against the next adjacent platform and thereby stabilize the same against shock or vibration effects in the individual blades during high speed rotation of the turbofan.

In turbofans of conventional type, blade 10 may have an overall length on the order of 12 to 15 inches or more. The airfoil contours of blade 10 are precision engineered and fabricated to produce maximum efficiency of engine performance at the particular speed and pressure conditions at which the engine incorporating such blades is designed to operate. Any dents, punctures or other localized deformations in the contours of blade 10 seriously compromise the aerodynamic properties of the blade and are completely intolerable. In the illustrative case of FIG. 1, blade 10 contains damaged portion 22 such as frequently experienced in aircraft due to entrance of a foreign object in the inlet airflow of a turbofan engine during operation thereof. Under present commercial airline engine overhaul procedures, such damage would require blade 10 to be discarded and replaced by a new blade in the absence of the inventive process disclosed herein.

Referring to FIG. 2, the process for repairing blade 10 begins by cutting or other suitable operations whereby the portion of blade 10 containing damaged area 22 is removed from the blade. The portion thus removed has a predetermined size and shape conforming to a template 24. In the illustrative case of blade 30 shown in FIG. 3, erosion damage along substantially the entire length of leading edge 16 such as may be encountered in a typical case is shown. Blade 30 may be seen to contain other damaged portions 32 and 34 in addition to leading edge erosion area 36. Any change in normal dimensions, such as due to wear of a blade from any cause during service use thereof may be overcome by the repair method disclosed herein. In addition to template 24, it may be seen particularly from FIG. 4 that additional templates 25, 26, 27, and 28 are adapted to define those portions of blade 30 requiring removal from the blade in order to insure that no damaged areas remain on blade 30 after the cutting operation is completed. The portions of blade 30 to be thus removed are determined by placing templates 24--28 in the locations identified by reference numerals 44--48, respectively, and scribing or otherwise marking blade 30 to trace the outline of the templates thereon.

It will be understood by those skilled in the art that templates 24--28 represent standardized areas of predetermined size and location which define those portions of blades 10 and 30 which most frequently sustain damage during service use of the same, and that the templates are adapted for repair of blades 10 and 30 on a mass production basis. Thus, those portions of blades 10 and 30 which are removed during the stated cutting operations will be replaced by incremental masses of the same material used in the mentioned blades, such masses having essentially the same shape and surface area as templates 24--28, but not necessarily the same thickness. Viewing FIG. 4, for example, the incremental mass required to replace the material removed from blade 30 and defined by area 46 will have the same shape and location as area 46, and is designated by reference numeral 60 in FIG. 7.

Referring to FIGS. 7 and 8, the novel method in this case involves use of restraining die or mold means such as illustratively shown by mating dies 50 and 52. Dies 50 and 52 may be fabricated by any suitable process known to the prior art, such as by casting from plaster patterns or cutting operations on metallic blocks. Dies 50 and 52 when in completely mated relationship enclose a cavity 54 therebetween defined by the workfaces 56 and 58 of dies 50 and 52, respectively. Cavity 54 has a contour oppositely and precisely corresponding to the surfaces of a complete new blade identical to blade 10 or 30 but without any damage or defects. Following the cutting operations described in connection with FIG. 4, for example, blade 30 is positioned in lower die 52 with incremental masses corresponding in size and location to areas 44--48 shown in FIG. 4 preplaced in the manner thus suggested. Incremental mass 60, for example, seen in FIG. 7 occupies the area of the void produced by removal of material defined by area 46 in FIG. 4. However, while mass 60 has substantially the same peripheral dimensions as area 46, the thickness t of mass 60 is uniform throughout the mass and is of particular significance in practicing the method disclosed herein.

In FIG. 7, blade 30 with the damaged portions removed therefrom is positioned between mating dies 50 and 52 within a surrounding retort 62. Mass 60 is positioned within cavity 54 in the void resulting from removal of area 46 from blade 30, other incremental masses being similarly preplaced in the same pattern suggested by area 44--48 in FIG. 4. Each of the stated incremental masses has substantially uniform thickness, as suggested by thickness t of mass 60, but not necessarily the same thickness as between different masses. Thus, the thickness of mass 60, for example, is predetermined to result in a total volumetric content of the mass which will coincide almost exactly with the volume of the void within cavity 54 resulting from removal of area 46 of blade 30 when the blade is positioned between dies 50 and 52, and the dies are fully mated together in the relationship shown by FIG. 8. Ideally, the volumetric content of mass 60 should equal that of the mentioned void. In no case should mass 60 ever be sized to result in less volume than such void. Therefore, any tolerance allowed for dimensional inconsistencies in the manufacture of masses 60 must be limited to oversizing thereof rather than undersizing, with respect to the mentioned ideal volumetric content. As a practical matter, very slight oversizing of mass 60, as well as all the other incremental masses discussed above, on the order of 0.01 percent to 10 percent in excess of the stated ideal volumetric content, is preferable in order to compensate for any inconsistencies in the cutting operations necessary to prepare blade 30 for repair. It is, of course, the purpose of templates 24--28 to standardize the cutting patterns and minimize such inconsistencies, but some slight variations during lengthy mass production runs will inevitably occur.

With the workpiece and tooling elements arranged within retort 62 as shown by FIG. 7 and discusses above, a vacuum source (not shown) is preferably connected to the retort through conduit 64 provided for this purpose. The retort and its contents are heated to a suitable temperature for diffusion bonding to occur between the material in blade 30 and the incremental masses adjacent thereto such as mass 60. The amount of such heating will depend upon the amount of pressure to which retort 62 and its contents will be subjected and the duration of its exposure to the conditions identified with solid-state molecular diffusion bonding of the workpiece materials. Of the various materials suitable for use in turbofan blades, titanium or an alloy thereof is widely used because of its high strength and light weight, although other and different materials may be used in various other types of article repaired by the novel process disclosed herein. Where the workpiece material is titanium, complete bonding together with the necessary creep deformation required to reshape mass 60 into conformity with the contours of die cavity 54, may be accomplished at 1600.degree. F. and 1000 p.s.i. compressive force continuously maintained for 24 hours in the direction suggested by arrows 66 and 68. in FIG. 7, for example.

It will be understood that the inventive concept in this case may be practiced with a wide variation of metals and alloys and with articles of different size and shape, and that the parameters for achieving solid-state diffusion bonding will necessarily vary for each particular choice of workpiece material. Among the metals or alloys which may be joined by solid-state diffusion bonding are aluminum, stainless steel, titanium, nickel, tantalum, molybdenum, zirconium, and columbium. Diffusion bonding is characterized by intermolecular exchange between contacting surfaces of the workpiece at suitable pressures and at temperatures below the melting point of the workpiece material. In some cases, a thin interleaf material, or eutectic former, is provided while in other forms of solid-state bonding no interleaf material is necessary. The prior art involving solid-state or intermolecular diffusion bonding includes issued U.S. Pat. Nos. 3,145,466; 3,180,022; 3,044,160; 2,850,798; and 3,170,234. The precise values of time-temperature and pressure utilized in connection with bonding workpiece materials is not a critical or limiting feature of the broad concept disclosed herein, but specific materials and parameters are stated for illustration only. Similarly, many different metals or alloys for dies 50 and 52 could be used to practice the inventive principles taught herein, although hard tool steel such as those high in nickel and cobalt content are preferable due to the relatively severe temperatures and load conditions to which the dies are subjected. Illustratively, 4130 steel widely used commercially is suitable for the dies.

When deformation of mass 60 is complete, dies 50 and 52 will be in the completely mated position suggested by FIG. 8 wherein cavity 54 is fully occupied by the rebuilt blade 70. In the foregoing relationship, a slight gap between the dies may be seen to exist as indicated by reference numeral 72. Compressive force denoted by arrows 66 and 68 is transmitted through the dies and directly to the workpiece elements contained within cavity 54. It is this compressive force which produces deformation of incremental masses such as mass 60 as required to conform the same into the final desired shape of the rebuilt blade 70. Moreover, application of compressive force vertically through mass 60, for example, will be understood to cause lateral or horizontal force to be exerted by mass 60 against the contacting portions of blade 30 and die cavity 54. Thus, in the absence of lateral restraint offered by blade 30 and cavity 54, the compressive force applied to mass 60 would produce considerable increase in the length and width thereof simultaneous with reduction in its thickness t. Since lateral deformation is restrained, a considerable reaction force transversely through mass 60 occurs due to vertical pressure applied to retort 62. It is this lateral component of force which is essential for solid-state molecular diffusion bonding to occur between the contacting surfaces of blade 30 and mass 60. If the volumetric content of mass 60 were insufficient to occupy completely the void produced by removal of material from blade 30, complete deformation of mass 60 would occur without creation of the necessary lateral force. It is the foregoing consideration which makes the dimensions of the incremental masses such as mass 60 so critical in practicing the process disclosed herein.

Following completion of the bonding steps discussed above, retort 62 is opened and dies 50 and 52 are separated from each other to permit removal of blade 70. It will be understood that blade 70 is a single solid unitary mass of material completely homogenous throughout with regard to metallurgical properties and composition. The shape of blade 70 oppositely corresponds in every detail with the contours of cavity 54 when dies 50 and 52 are fully mated as seen in FIG. 8. The foregoing feature completely avoids any necessity for machining operations on blade 70, resulting in a very significant savings of time, money, and material, the importance of which increases with use of costly materials such as titanium. In practicing the process thus disclosed, manual deburring of edges is occasionally required to finish rebuilt blades, but every such blade is identical to all others formed within the same set of matched dies 50 and 52.

In further connection with the illustrative case of turbofan blade repair using the method disclosed herein, use of the alloy known as Ti-6A1-4V in such blades is common. This alloy has the following approximate composition by weight:

Aluminum 51/2 to 61/2 percent

Vanadium 31/2 to 41/2 percent

Carbon 0.08 to 0.1 percent

Hydrogen 0.010 to 0.012 percent

Titanium balance

When the foregoing material is used in both blade 30 and mass 60 as well as all other incremental masses required to occupy cavity 54 completely in FIG. 7, for example, retort 62 and its contents are heated to a temperature of from 1650.degree. to 1725.degree. F. with a vacuum of 1 .times. 10.sup..sup.-4 millimeters of Mercury or less maintained in the retort. The stated temperature is continued for a period of from about 8 to about 16 hours, while a pressure of from about 2000 to about 5000 p.s.i. is applied in the direction of arrows 66 and 68 for the stated period. Where only slight deformation of mass 60 and the other incremental masses is involved, such as by preshaping the same to the approximate aerodynamic curvature of the final part before the bonding step, the lower end of the forementioned pressure and time period ranges are applicable.

Claims

I claim:

1. A method of repairing a worn or damaged metallic article, comprising the steps of:

providing a pair of mating dies, the workfaces of which are adapted to define a cavity having the precise size and shape of said article in the new and undamaged condition when said dies are in mating relationship;

removing a portion of said article to create a vacant area thereon, said portion including the worn or damaged area of said article;

placing said article in said dies with at least one separate incremental mass positioned in said vacant area, said mass being of the same material as said article, the volumetric content of said mass being at least equal to the volume of the void formed by said vacant area in said cavity when said dies are in mating relationship; and

applying sufficient heat and pressure to said dies to cause said mass both to deform into conformity with said void and to join said article by solid-state molecular diffusion bonding therebetween.

2. The method set forth in claim 1 above, further including, prior to said removing step:

providing at least one template for mass production use on a plurality of damaged articles corresponding generally with said damaged article;

placing said template in close proximity to said damaged article over said damaged area; and

tracing said template on said article to define said vacant area whereby said removed portion corresponds in plan view with said template.

3. The method set forth in claim 1 above wherein:

said mass has a volumetric content from 0.01 to 10.0 percent larger than said volume of said void.

4. The method set forth in claim 1 above wherein:

said metallic article is made of Ti-6A1-4V titanium alloy; and

said heat and pressure are from about 1600.degree. to 1725.degree. F. and from 2000 to 5000 p.s.i., respectively.

5. The method set forth in claim 4 above, wherein:

said dies containing said article and said mass are positioned within an airtight retort; and

a vacuum is maintained in said retort continuously during application of said heat and pressure.

6. A mass-production method for repairing a multitude of worn or damaged metallic articles having identical design but a variety of different types and locations of wear or damage, comprising the steps of:

forming a plurality of standardized templates having predetermined area dimensions;

prefabricating a plurality of incremental masses corresponding in area with said area dimensions of said templates;

removing portions of said articles, said removed portions containing said worn or damaged areas and corresponding in area with said templates;

placing said masses in the voids resulting in said articles from removal of said portions; and

joining said masses to said articles by solid-state molecular diffusion bonding therebetween.

7. The method set forth in claim 6 above, wherein:

said bonding is accomplished by the steps of placing said individual articles together with said masses between a pair of mating dies defining a cavity having the contour of said articles in the undamaged condition;

heating said dies and said articles to a temperature sufficient for said bonding to occur at a coordinated pressure; and

applying said pressure to said dies to compress said articles therebetween until said bonding occurs.

8. The method set forth in claim 6 above, wherein:

said masses have a total volumetric content from 0.01 to 10.0 percent larger than the total volumetric content of said removed portions.

9. The method of mass production rebuilding a plurality of worn or damaged metallic articles of identical design, comprising the steps of:

forming a plurality of standardized templates having predetermined plan area dimensions;

prefabricating a plurality of incremental masses corresponding in plan view with said templates;

trimming said articles to remove portions therefrom, said removed portions containing said worn or damaged locations and corresponding in plan area with said templates;

providing a pair of mating dies, the workfaces of which are adapted to define a cavity having the precise size and shape of said articles in the new or undamaged condition when said dies are in fully mated relationship;

successively placing each of said trimmed articles between said dies with a plurality of said masses positioned in the vacant areas resulting in said articles from removal of said portions therefrom, said masses being of the same material as said articles, the volumetric content of said masses in combination being at least equal to the volume of the voids formed in said cavity due to removal of said portions when said dies contain said trimmed articles and are in fully mated relationship;

surrounding said dies with an airtight retort;

maintaining a vacuum within said retort;

heating said retort and its contents to a temperature sufficient for solid-state molecular diffusion bonding between said incremental masses and said trimmed articles;

applying sufficient pressure to said dies to compress said articles and masses therebetween in an amount sufficient to deform said masses into conformity with said cavity and to achieve said diffusion bonding; and

said masses having an initial combined volumetric content from 0.01 to 10.0 percent larger than said volume of said voids in said cavity resulting from said removal of said portions.