GB2063747A - Method for manufacturing a clad turbine airfoil assembly - Google Patents

Abstract

A cast spar 12 is preformed to have a leading edge surface 16 and a trailing end 18 joined by side surfaces, and thereafter a sheet of cladding 24 is preformed across at least part of the spar outer surfaces; the clad spar is enclosed within a vacuum bag 38, 54 shaped so that a lead edge segment 56 of the bag is in contact with the cladding at the spar leading edge surface 16 and a seat 62 of the bag contacts the spar trailing end 18, so as to apply an initial mechanical loading to the cladding at the spar leading edge surface 16; at least one a deformable wall segment 58 of the vacuum bag is spaced with respect to to a the cladding, and thereafter the vacuum bag is subjected to a directly imposed hot isostatic environment with temperatures in excess of 1000 DEG F (538 DEG C) and pressures of 200-1000 psi (1379-6894 Kpa) to apply a progressive diffusion bonding pressure on the cladding initially at the spar leading edge surface 16 and thereafter progressively across the spar side surfaces to uniformly stretch and bond the cladding onto the spar. <IMAGE>

Description

SPECIFICATION Method for manufacturing a clad turbine airfoil assembly This invention relates to a method for manufacturing a clad turbine airfoil assembly and more particularly to an improved method for manufacturing an airfoil assembly comprising an airfoil-configured cast spar having an outer sheet metal layer of skin bonded thereto by use of hot isostatic bonding piocedures.

In the manufacture of airfoil-configured clad turbine blades and vanes by use of bonding steps or diffusion bonding cycles, it is known to maintain a porous outer surface of a cast spar at a pressure and temperature and for a time period sufficient to obtain diffusion bonding of porous metal to the cast spar.

An example of fabrication of airfoil configurations by bonding such an outer sheath to the outer surface of a cast strut is set forth in United States Patent No. 4,042,162 (Meginnis et al). In this arrangement, a fixture is utilized in the form of a pair of pressure blocks to apply a pressure to a scab sheet which will maintain a unit pressure to a scab sheet which will maintain a unit pressure distribution across an outer sheath as it is diffusion bonded to a cast spar while preventing puckering at leading edge portions of the sheath.

While the method set forth in United States Patent No. 4,042,162 is suitable for its intended purpose, as an alternative, it is recognized that hot isostatic pressure application to the outer surface of cladding or sheath material on an airfoil configured spar can be applied through a vacuum bag enclosure to prevent entrance of contaminants to the surfaces that are being bonded together.In such arrangements, however, it is also necessary to consider how application of loading is transmitted from the vacuum bag to the outer cladding or sheath so as to assure that the cladding or sheath material will be uniformly and evenly pressurized to maintain a pressure/temperature environment thereon so that the inner surface of the cladding or sheath material will be positively and uniformly diffusion-bonded to the outer surface area of an airfoil-configured cast spar during the manufacturing process.

A method for manufacturing a clad turbine airfoil assembly according to the present invention comprises the steps of preforming a cast spar having a leading edge surface and a trailing end joined by side surfaces, preforming a sheet of cladding and shaping said sheet to conform it to the leading edge, trailing end and side surfaces of the spar, enclosing the clad spar within a vacuum bag so as to initially locate a first segment of the bag in contact with the sheet of cladding across the leading edge surface of the spar and a second segment in contact with the trailing end of the spar to apply a preload to the joint line of the leading edge surface of the spar and the cladding formed thereacross, locating a deformable wall segment on the bag in spaced relationship to the cladding, and then subjecting the full outer surface of the vacuum bag to a directly imposed hot isostatic environment to direct the diffusion bonding pressure and temperature initially to the leading edge area of the cladding and spar and thereafter progressively applying pressure to the side surfaces of the clad spar by deforming the deformable vacuum bag wall segment until it fully conforms to the outer surface of the cladding material whereby a bonding load is applied to the cladding with pressure progressively being applied from the leading edge area of the cladding to the side surfaces of the cladding on the spar to assure initial bonding of the leading edge area of the cladding and spar prior to application of hot isostatic pressure on the cladding along the rest of the spar thereby to produce a sequential bonding of the cladding to the spar and a sequential imposition of uniform pressure loading across the entire spar from the leading edge surface to the trailing edge portions thereof.

The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 is a flow chart of a method of manufacture in accordance with the present invention; Figure 2 is a perspective view of a composite strut in the form of a turbine blade constructed by use of the method of the present invention; Figure 3 is a diagrammatic view of a preformed vacuum bag located in a hot isostatic environment for use in applying a progressive leading edge to trailing edge diffusion bond pressure across the outer surface of a sheath material conformed to a cast spar; and Figures 4 and 5 are diagrammatic views of improved methods utilizing other vacuum bag embodiments in other embodiments of the method of manufacture of the present invention.

Referring to Fig. 2 of the drawing, a composite strut assembly 10 is illustrated in the form of a turbine blade having a cast spar 12 with a plurality of air cooled passages 14 therein located at spaced points along a leading edge surface 16 of the cast spar 14 to a trailing end 18 of the spar at spaced points across concave and convex airfoil surfaces 20, 22 on either side of the cast spar 12. A porous laminated sheath 24 is preformed from a high alloy material to provide a wear resistant outer surface on the composite strut assembly 10. Such material can be impervious or, as in the present embodiment, perforated to form an air-cooled outer surface on the strut assembly 10. Examples of such porous laminated sheaths are set forth in United States Patent Nos. 3,560,107 issued February 2, 1971, to H. E. Helmes and 3,584,972 issued June 15, 1971, to Meginnis Et Al.

In the fabrication of such composite assemblies, heretofore, pressure loading blocks have been used to apply a uniform loading from the leading edge to the trailing edge of the cast spar across the outer surface of the sheath material conformed thereto.

In the present invention, an improved vacuum bag configuration is used in the manufacture of the composite strut assembly 10 and as shown in Fig. 1 the improved method sequence includes the steps of preforming a cast spar 12 and thereafter conforming a porous metal sheath 24 to at least the leading edge surface 16 thereof and portions of the convex and concave surfaces thereon.

As shown in the embodiment of Fig. 3 the sheath 24 includes a first segment 26 that is conformed to locate an inner surface 28 of the segment 26 is juxtaposed relationship to the surface relief on the concave surface 20 of the cast spar 12. A second segment 30 of the sheath 24 is conformed to locate an inner surface 32 thereof in juxtaposed relationship with the outer surface configuration of the convex surface 22 of the cast spar 12. And the sheath 24 further includes a leading edge segment 34 that conforms to the leading edge surface 16 of the cast spar 12. The conformed outer sheath 24 and cast spar 12 are then placed in a specially configured preformed vacuum bag 36 for use in the method of the present invention.The vacuum bag 36 more particularly includes, in the embodiment of Fig. 3, a first bag wall 38 having a segment 40 thereon conformed initially to the outer surface of the sheath segment 30. The bag wall 38 includes end portions 42, 44 thereon joined by a braze seal 46, 48, respectively, to end portions 50, 52 on a second bag wall 54. The second bag wall 54 is configured to include a first segment 56 thereon that conforms initially to the leading edge segment 34 of the sheath 24 and a second deformable segment 58 that is initially located in spaced relationship to the outer surface of the sheath segment 26 that is shown in solid lines in Fig. 3.

The bag walls 38, 54 are joined together at a juncture 60 that defines a seat 62 into which the trailing end 18 of the cast spar fits.

The parts are located in a sealed container 64 connected by a conduit 66 to a source of hot isostatic pressure and a suitable heater 68 heats the environment within the container 64 to an elevated temperature range required for diffusion bonding of the interconnecting surfaces of the sheath 24 and the mating outer surface portion of the cast spar 12. In the illustrated arrangement, space 70 has a diffusion bonding temperature in excess of 1000 F (538 C) and pressure maintained therein in the order of 200 to 1000 psi (1379 to 6894 Kpa). Thus, the first segment 56 of the bag wall 54 and the seat 62 formed between the bag walls 38, 54 will apply an initially localized leading edge load to the sheath 24 across the leading edge surface 16 of the cast spar 12.Accordingly, when the hot isostatic environment first is imposed on the vacuum bag enclosed parts the leading edge between the sheath 24 and the cast spar 12 will be preloaded prior to pressurized contact between the sheath material with the rest of the spar. In such arrangements, this will prevent any tendency for the sheath material to pucker or take up at the leading edge surface 16 of the spar and will uniformly stretch the sheath material across the surface at the beginning of the diffusion bonding cycle.Thereafter, in accordance with the present invention, the bag wall segment 58 will collapse in a progressive fashion from the leading edge surface 1 6 to the trailing edge of the spar and thereby will stretch the sheath segments 26, 30 progressively from the leading edge surface 16 of the spar to the trailing end 18 thereof, thereby to ensure complete conformation of the sheath 24 to the cast spar 12. The progressive deformation of the second wall segment 58 is shown in dotted line in Fig. 3.

Following complete stretching of the sheath 24 and collapse of the vacuum bag wall portion 58 the bag is configured to apply a uniform pressure to the entire outer surface of the sheath 24 as well as the outer surface of the airfoil so that an assured diffusion bond joint can be formed uniformly therebetween.

In a second embodiment of the present invention, shown in Fig. 4, a cast spar 74 is covered by a sheath 76 like the sheath 24 in the first embodiment. In this arrangement, the cast spar 74 and sheath 76 have the same outer configuration as so those in the embodiment of Fig. 3 and the difference in method of manufacturing is due to the form of a vacuum bag 78 for carrying out the pressure application and stretch forming of the sheath 76 during a hot isostatic press forming process of the type set forth in Fig. 1. More particularly, in this arrangement the vacuum bag 78 includes a first wall 80 and a second wall 82, each having opposite ends 84, 86 and 88, 90, respectively, that are braze sealed, then closed by fusion welds 92 and 94 at opposite ends of the vacuum bag 78. In this embodiment, the first wall 80 has a bent segment 95 thereof that overlies the leading edge surface of the sheath 76 to imp#ose a localized load on the front end thereof to conform it to the leading edge of the cast spar 74 at initial pressure application to the parts at the beginning of the diffusion bonding cycle. Likewise, the bag wall 82 F as a bent segment 96 that conforms to the railing edge of the cast spar 74 so that at the beginning of the diffusion bonding cycle a localised load will stretch the sheath 76 into conformity with the leading edge of the cast spar 75. Continuously progressive conformity of the sheath material with the cast spar surface 74 in this embodiment of the invention is accomplished by producing a hot isostatic environment in a container 98 enclosing a space 100 around the bag-covered jointed parts, as seen in Fig.

4. A heater 102 is included to increase the temperature of the environmerXt to a temperature in excess of 1000 F (538 C) and pressures in the order of 200-1000 psi (1379 to 6894 KPa) are also maintained in this space 100. As a result, the vacuum bag walls 80, 82 have segments 104, 106 thereon, respectively, that progressively collapse into a concave shape as shown in a final dotted line position in Fig. 4. Likewise, segments 110, 11 2 of the bag walls 80, 82 respectively, also collapse in response to application of the diffusion pressure to conform to the convex surface of the sheath material thereby to impose a uniform pressure diffusion bond on the parts to assure a uniform diffusion bonded joint therebetween.

Fig. 5 is another embodiment of the invention including a tear-shaped airfoil spar 120 with convexly formed pressure and suction surfaces 122, 124. The surfaces 122, 124 are covered by a sheet of cladding 126 including a leading edge segment 128 thereon fitted over a leading edge 130 of the spar 120. A trailing end 132 of spar 120 is grounded on fixture plate 134. In this embodiment of the invention a vacuum bag 136 of sheet material is fitted over the cladding 126 so that a curved outer segment 138 thereon initially imposes a preload on the cladding 1 26 at the leading edge segment 128. Side walls 140, 142 of bag 136 each have end flanges 144, 146 respectively connected by welds 148, 150 to the fixture plate 134. The interfaces 152, 154 between flanges 144, 146 and fixture plate 134 are braze-sealed together. Again the bag is placed in a hot isostatic environment such as discussed above to cause walls 140, 142 to progressively collapse across the sheet of cladding 126 as shown in dotted lines in Fig. 5 to impose the diffusion bond load between the inner surface 156 of the cladding 126 and the outer surface of spar 120.

Claims (6)

1. A method for manufacturing a clad turbine airfoil assembly comprising the steps of preforming a cast spar having a leading edge surface and a trailing end joined by side surfaces, preforming a sheet of cladding and shaping said sheet to conform it to the leading edge, trailing end and side surfaces of the spar, enclosing the clad spar within a vacuum bag so as to initially locate a first segment of the bag in contact with the sheet of cladding across the leading edge surface of the spar and a second segment in contact with the trailing end of the spar to apply a preload to the joint line of the leading edge surface of the spar and the cladding formed thereacross, locating a deformable wall segment on the bag in spaced relationship to the cladding, and then subjecting the full outer surface of the vacuum bag to a directly imposed hot isostatic environment to direct the diffusion bonding pressure and temperature initially to the leading edge area of the cladding and spar and thereafter progressively applying pressure to the side surfaces of the clad spar by deforming the deformable vacuum bag wall segment until it fully conforms to the outer surface of the cladding material whereby a bonding load is applied to the cladding with pressure progressively being applied from the leading edge area of the cladding to the side surfaces of the cladding on the spar to assure initial bonding of the leading edge area of the cladding and spar prior to application of hot isostatic pressure on the cladding along the rest of the spar thereby to produce a sequential bonding of the cladding to the spar and a sequential imposition of uniform pressure loading across the entire spar from the leading edge surface to the trailing edge portions thereof.

2. A method for manufacturing a clad turbine airfoil assembly according to claim 1, in which the leading edge surface and the trailing end of the cast spar are joined on one side by a concave side surface and on the other side by a convex side surface, the deformable wall segment on the bag is located in spaced relationship to the cladding at the concave side surface of the spar and a second wall segment on the bag is located in contact with the full extent of the cladding at the convex side surface of the spar.

3. A method for manufacturing a clad turbine airfoil assembly according to claim 1, in which the clad spar is enclosed within a vacuum bag having two walls so as to initially locate a first segment of one wall of the bag in contact with the sheet of cladding across the leading edge surface of the spar and a second segment of the other wall in contact with the trailing end of the spar to apply said preload to the joint line of the leading edge surface of the spar and the cladding formed thereacross, there being deformable wall segments on each wall of the bag in spaced relationship to respective portions of the cladding, said deformable wall segments being deformed in said hot isostatic environment until they fully conform to the respective portions of the outer surface of the cladding material to apply said bonding load.

4. A method for manufacturing a clad turbine airfoil assembly substantially as hereinbefore particularly described and as shown in Figs. 1 and 3 of the accompanying drawings.

5. A method for manufacturing a clad turbine airfoil assembly substantially as hereinbefore particularly described and as shown in Figs. 1 and 4 of the accompanying drawings.

6. A method for manufacturing a clad turbine airfoil assembly substantially as hereinbefore particularly described and as shown in Figs. 1 and 5 of the accompanying drawings.