GB2063721A - Method of bonding composite turbine wheels - Google Patents

Abstract

A method of manufacturing a composite turbine rotor assembly from dual materials includes the steps of preforming a turbine disc 18 of powdered metal by direct hot isostatic pressing; machining the external periphery 20 of the disc rim; prefabricating a cast ring 10 of air-cooled airfoil elements having a circular rim 12; shrink-fitting the cast ring 10 to the turbine disc 18; subjecting the interface to a controlled vacuum heat treatment sequence to produce sufficient diffusion bonding at the interface to seal the interface against entrance of high pressure fluids; and thereafter hot isostatically pressing the joined parts under fluid pressure directly applied to the outer surface of the parts so as to press the internal periphery of the ring into intimate contact with the external periphery of the disc until the dual materials are diffusion bonded together. <IMAGE>

Description

SPECIFICATION Method of bonding composite turbine wheels This invention relates to composite turbine rotor assemblies with parts of dissimilar metallurgical properties and more particularly to a method for fabricating such composite turbine rotors by a diffusion bonding process.

One method for manufacturing a hybrid dual property gas turbine wheel includes diffusion bonding an outer blade-containing ring to an inner disc assembly of a differing metallurgy. This arrangement is set forth in our co-pending British application, Serial No. 22687/78. In the above-described application diffusion bonding of the bladed ring and internal disc is accomplished by use of gas pressure to assist metal diffusion between the differing materials of the parts to be joined as the assembly is maintained at an elevated temperature during a high temperature isostatic pressure (HIP) sequence. In the method of the aforesaid application, the bonding force between mated components is achieved by maintenance of a braze or weld seal along the exterior joint lines at the interface between the bladed ring and disc while gas pressure is excluded from entering the interface.

The bond force in the aforesaid method thus is a bonding force which equals gas pressure times the area of the parts which is normal to the bond interface. In such arrangements, diffusion bonding requires maintenance of the parts at an intimate contact face during a predetermined process diffusion time while holding a predetermined diffusion temperature to cause metal grain growth across the interface. Our British application, Serial No. 22687/78 discloses component parts connected by a diffusion bond following shrink fit of an outer part on an inner part. The outer part is a cast blade ring with internal diameter core printouts that must be plugged prior to a HIP process sequence to form a pressure sealed interface, also sealed at externally located joint lines.

According to the present invention an improved method for assembling a composite turbine rotor assembly including a disc of one metallurgical composition and an outer ring of airfoil components of a second metallurgical composition comprises the steps of preforming outer ring of airfoils with an inner rim and airfoils of exact preshaped dimensional form to maintain desired aerodynamic flow paths therethrough; preforming a near-finished-shape metal turbine disc having a precisely machined outer periphery machining an inside periphery of the inner rim and shrink-fitting it on the outer periphery of the turbine rotor disc to form a substantially line-to-line interface joint therebetween; and thereafter sealing the interface by subjecting the joined ring and disc to a vacuum heat treatment process to a temperature level and for a time period sufficient to establish a diffusion bonded joint and sealed interface to prevent entrance of hot isostatic pressure fluid into the interface, and thereafter subjecting both the airfoil array and disc to an elevated temperature and a direct pressure thereon to eliminate anomalies in the diffusion bonded metallurgical joint between disc and airfoil ring.

The method of the present invention eliminates the need for a sealed braze or weld sealed joint at the exposed or exterior joint lines of an interface between a bladed ring and/or disc assembly that form a composite gas turbine wheel and preferably does so by the steps of preforming a bladed metal ring with airfoil blades and the blades having an exact dimensional control therebetween to maintain aerodynamic flow paths through the ring and with a fore and aft sided base rim having a premachined close tolerance internal periphery, preforming a disc of high strength having a controlled external periphery; shrink-fitting the bladed ring to the disc to form a substantially line-to-line interface therebetween with continuously circumferentially formed front and rear joint lines and with continuous contact across the full facing surface areas of both the base rim internal periphery and the controlled external periphery of the disc from the fore side to the aft side of the base rim internal periphery; subjecting the joined parts to vacuum and heat treatment to a temperature and for a time period sufficient to produce sufficient metallurgical bonding through the full extent of the full facing surface areas to maintain a diffusion bonded interface between the external periphery of the disc and the internal periphery of the bladed ring element and a sealed, pressure tight interface therebetween which prevents the entrance of fluid pressures in the order of 15 KSI (103, 421.4 kPa) through the front or rear joint lines, directly applying hot isostatic fluid on the exposed exterior surface the bladed metal ring and the disc without flow of such fluid into the interface, and thereafter hot isostatically pressing the joined bladed ring and disc by direct fluid pressure across the full exterior surface of the joined parts to form a diffused metallurgical joint without anomalies between the base rim and the disc.

A preferred embodiment of the present invention improves the fabrication of an air-cooled composite turbine engine wheel assembly by eliminating a requirement for sealed core printouts for the air-cooled blades of the assembly.

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 of composite turbine wheels used in practicing the present invention; Figure 2 is an exploded perspective view of a ring of airfoil components with a base rim having a precise internal periphery and a turbine disc formed to a close approximation of its final shape including a precise external periphery for an interference fit with the precise internal periphery of the ring; Figure 3 is a longitudinal sectional view of a turbine wheel with the disc of Fig. 2 shrink-fitted to the airfoil ring therein; and Figure 4 is a photomacrograph of a vacuum heat treatment-sealed diffusion-bonded interface between metal specimens of the materials used in the wheel of Fig. 3.

In practicing the present invention, an initial step includes preforming an airfoil array either as a solid ring or by making an airfoil array from a plurality of component parts to form an airfoil blade ring with a base rim having a precise internal periphery.

The use of segmented blade or airfoil components enables directionally solidified, air cooled airfoils of the type having precisely, dimensionally controlled surfaces thereon to be utilized in a composite turbine wheel assembly made by the method of the present invention. An example of such directionally solidified air cooled airfoils is set forth in United States Patent No.

3,732,031, (E. Bowling metal).

Because of the use of a butt joint configuration used in the present invention, the existing airfoils of the type set forth in the preceding Bowling et al patent are modified by having a continuous surface at their base to define a ring having a machined inner periphery for a shrinkfit relationship to the outer periphery of a disc to be joined thereto.

An air cooled bladed ring subassembly 10 is illustrated in Fig. 2 as including a base ring 12 cast integrally on the blades of the subassembly 10 to form part of an inner ring of an airfoil array to be described. As part of the casting process, core printouts 14 are formed as holes in the internal periphery of ring 1 2.

The bladed ring subassembly 10 has aerodynamic flow paths 1 5 thereof. It also has an inside periphery 1 6 thereof machined by a suitable cutting tool to produce a precise internal periphery within the subassembly 10.

The method also includes preforming a disc or rotor hub 18 for insertion within the inside periphery 1 6 of the subassembly 10. The disc 1 8 preferably is preformed from a powdered metal composition.

The composition is hot isostatically pressed to form a consolidated disc 18 having a rim 20 of an approximate shape corresponding to that of the inside periphery 1 6 of the subassembly 10.

The disc 1 8 is slightly oversized and therefore machined to an exactly dimensioned outer periphery at rim 20 by a suitable cutting tool so that the outside perimeter of the disc rim 20 will be dimensioned to provide a substantially shrink fit between the rim 20 and the inside periphery 16 of the base ring 1 2 of the subassembly 10.

More specifically, the internal periphery 1 6 and the outer periphery of the rim 20 are sized to produce a .003 inch (0.076 mm) interference fit when the disc 18 is fitted within the subassembly 10 as shown in Fig. 3.

Following precision machining, the rim 20 and the internal periphery 16 are vacuum outgassed for one hour at 2100"F (1149"C) in preparation to joining them together as bond surfaces. Shrink-fitting of the parts is produced by first heating the subassembly 10 to approximately 400"F (204"C) and then slipping it on the rim 20 which is cooled to - 320"F (- 196"C). At ambient equilibrium temperatures, the internal periphery 16 becomes solidly locked onto the disc rim 20 preparatory for a vacuum heat treatment sequence in accordance with the present invention to prepare the joined parts for a subsequent hot isostatic pressurization of the parts to form a butt joint metallurgical bond between the facing surfaces of the interface between subassembly 10 and the disc 1 8. At this stage, the assembly has continuously circumferentially formed front and rear joint lines 21, 23 at the fore and aft sides 25, 27 of rim 20 as shown in Fig. 3.

Materials used in one working embodiment, in percentages by weight, are as follows: C Cr Co Mo W Bladed Ring 10 0.15 9.0 10.0 2.5 10.0 Disc 18 0.12 12.7 9.4 1.9 4.0 Ta Ti Al B Zr Hf Ni Bladed Ring 10 1.5 1.5 ' 5.5 0.02 0.05 1.0 Bal.

Disc 26 3.8 4.1 3.7 0.01 0.02 0.9 Bal Other suitable high alloy material examples are AF95, AF2-lDA and Astroloy.

The compositions of these other examples of high alloy materials are as follows: Alloy C Mn Si P S Cr Co Mo W Nb Ti AF95 0.06-0.17 0.15 0.20 0.015 0.015 13.00-15.00 07.00-09.00 3.30-3.70 3.30-3.70 3.30-3.70 2.30-2.70 AF2-1DA 0.30-0.35 0.10 0.10 0.015 0.015 11.50-12.50 09.50-10.50 2.50-3.50 5.50-6.50 - 2.75-3.25 Astroloy 0.03-0.09 0.15 0.20 0.015 0.015 14.00-16.00 16.00-18.00 4.50-5.50 - - 3.20-3.60 Alloy Al B Zr Fe Cu O N Ta Pb Bi Ni AF95 3.30-3.70 0.005-0.020 0.03-0.07 0.50 0.50 0.005 0.005 - - - Bal.

AF2-1DA 4.20-4.80 0.010-0.020 0.05-0.15 1.00 - 0.010 0.005 1.00-2.00 0.0002 0.00005 Bal.

Astroloy 3.85-4.15@ 0.020-0.030 0.06 0.50 0.10 - - - - Bal.

The vacuum heat treatment process proceeds after the parts are joined. The parts are maintained under vacuum at a temperature of 2240"F (1 227 C) for times ranging from one to six hours. Such vacuum heat treatment produces a significant amount of metallurgical bonding at a pressure sealed annular joint line 22 shown in Fig. 3 to both seal and bond the parts at the joint line 22. A byproduct of the vacuum diffusion cycle, however, is the-formation of thermally induced porosity (TIP) in the powdered metal product due to the coalescence and expansion of adsorbed and absorbed gases which are the product of the gas atomization and consilidation operations employed to produce the disc detail.

After sealing the joint line 22 the assembly is impervious to entrance of high pressure fluid between the joined parts at the joint lines 21, 23 and thus defines a continuous exterior surface extent 29 across the outer surface of the joined parts.

Following the vacuum heat seal step, the sealed assembly is subjected to a hot isostatic process to collapse and heal the thermally induced porosity and to complete the diffusion bonding of the surface represented by the internal periphery 1 6 to the surface represented by the rim 20. This process is accomplished at temperatures in the range of 221 5 F (121 3'C) and the parts are subjected to direct argon gas pressure of the order of 15,000 psi (103 421.4 KPa) which is maintained for a period of time up to three hours.Following the hot isostatic press, the assembly may be subjected to a coating diffusion cycle of 2050"F (1121 'C) for two hours followed by age heat cycle of 1 550 F (843"C) for four hours plus 1 400 F (760"C) for sixteen hours.

The aforesaid method for fabricating a composite turbine engine wheel assembly includes preforming the bladed ring 10 with airfoil blades having an exact dimensional control therebetween to maintain aerodynamic flow paths through-the ring and with a base rim 12 thereon from a cast, corrosion and thermally-resistant superalloy material. The bladed ring 10 and joined disc 18 form a near line-to-line interface therebetween with continuous contact across the full facing surface areas of both the base rim internal periphery and the controlled outer periphery of the disc.The full extent of the sealed and diffusion bonded joint line 22 at the interface between the outside periphery of the disc 1 8 and the inside periphery of the bladed ring 10 produced by the vacuum heat treatment sequence produces a pressure tight interface, produces thermally induced porosity in the powder metal, and defines an exposed exterior surface extent on the bladed metal ring and the disc for receiving direct application of hot isostatic fluid without flow of such fluid into the interface. Hot isostatic fluid pressure imposed across the full exterior surface extent of the joined parts is utilized to heal thermally induced porosity from the vacuum diffusion cycle and to collapse and bond localized portions of the diffusion bonded joint line 22 which may have failed to register and bond in a line-to-line fashion.

For example, in the case of a wheel produced in the method sequence of Fig. 1, the aforesaid powdered metal disc 18 is consolidated from argon-atomized powder, prior to its assembly to bladed ring 10. In such cases, the subsequent vacuum heat treatment and diffusion bond step is conducted at a sufficiently high temperature and for a sufficient duration to release any argon trapped in the material of the consolidated powdered metal disc 1 8. The argon transport can show up as porosity and create anomalies in the powdered metal disc. In accordance with the present invention, the hot isostatic press step eliminates this thermally induced porosity and any other anomalies such as unbonded localized regions at joint 22 following the vacuum heat treatment sequence.

The pressure tight interface at annular joint line 22, more particularly, also prevents the entrance of fluid pressures in the order of 1 5 KSI (103, 421.4 kPa) either through the front or rear joint lines or through the core printouts 14 and as a result there is no need to plug the core printouts prior to the shrink-fit step.

Claims (4)

1. A method of fabricating a composite turbine engine wheel assembly comprising: preforming a bladed metal ring from high performance material with airfoil blades and the blades having an exact dimensional control therebetween to maintain aerodynamic flow paths through the ring and with a base rim having a premachined close tolerance internal periphery, preforming a disc of high strength metal having a controlled external periphery, shrink-fitting the bladed ring to the disc to form a substantially line-to-line interface therebetween and with continuous contact across the full facing surface areas of both the base rim internal periphery and the controlled external periphery of the disc, subjecting the joined parts to vacuum heating treatment to a temperature level and for a time period sufficient to produce initial metallurgical bonding through the full extent of the interface lines to obtain a diffusion bonded interface between the external periphery of the disc and the internal periphery of the bladed ring which is a sealed, pressure tight interface there-between which prevents the entrance of hot isostatic fluid pressures into the interface, thereby to define an exposed substantially continuous exterior surface extent on the bladed metal ring and the disc for receiving direct application of hot isostatic fluid without flow of such fluid into the interface, and thereafter hot isostatically pressing the joined bladed ring and disc by heating the joined parts and directing fluid pressure across the full exterior surface extent of the joined parts to a pressure level sufficient to press the joined parts together so as to complete the bonding sequence and eliminate anomalies at the interface.

2. A method of fabricating a composite turbine engine wheel assembly according to claim 1; in which the bladed metal ring is cast from superalloy material with core printouts present in the internal periphery of said ring, which core printouts become sealed, but not filled, within said interface during said initial metallurgical bonding step.

3. A method of fabricating a composite turbine engine wheel assembly according to claim 1, in which the bladed metal ring is cast from superlloy material and has airfoil blades having aircooled passages therein, and a fore and aft sided base rim having said premachined close tolerance internal periphery with core printouts therein leading to the passages, said disc is preformed by consolidating high strength powdered metal under high pressure argon, the bladed ring is shrink-fitted to the disc to form said substantially line-to-line interface therebetween with continuously circumferentially formed front and rear joint lines and with said continuous contact across the full facing surface areas of both the base rim internal periphery and the controlled external periphery of the disc from the fore side to the aft side of the base rim internal periphery, the vacuum heating treatment step produces said diffusion-bonded sealed, pressure-tight interface which prevents, during the subsequent direct application of hot isostatic fluid, the entrance of hot isostatic fluid into said interface either through the front or rear joint lines or through the core printouts, and said direct application of hot isostatic fluid in said hot isostatic pressure step eliminates any thermally-induced porosity caused by migration of argon from the consolidated disc of powdered metal during the aforesaid vacuum heat treatment step.

4. A method of fabricating a composite turbine engine wheel assembly substantially as hereinbefore particularly described and as shown in Figs. 1 to 4 of the accompanying drawings.