Method for making titanium aluminide metallic sandwich structures

Abstract: A method for fabricating of intermetallic titanium aluminide sandwich structures in which blanks of titanium aluminide are bonded to one another, under vacuum in a retort, at selected areas to form a pack, and then, upon removal of the retort, expanded superplastically to form a desired sandwich structure. Bonding of the various preselected areas of the metal blanks is accomplished using elevated temperature gas diffusion bonding techniques. The invention contemplates a first thermal cycle during which the pack is diffusion bonded in an evacuated retort at one temperature within a predetermined range of temperatures, and a second thermal cycle during which subsequent superplastic forming of the pack of bonded blanks, with the retort removed, is carried out. Optionally, during the first thermal cycle, a step of superplastic preforming may be carried out whereby the pack is preformed against arcuate or other shaped forming members.

Description: The present invention relates to methods of making a metallic sandwich structure from titanium aluminide blanks by joining selected regions of the blanks using diffusion bonding techniques, and then superplastically expanding the bonded blanks to form a sandwich structure of the shape desired. Discussion of the Known Prior Art Titanium in its purest form is a relatively soft, weak and extremely ductile metal, but through additions of other elements, it can be converted to an engineering material exhibiting high strength and stiffness, resistance to corrosion, usable ductility, and low density. In recent years, a class of titanium alloys has evolved which exhibit excellent high temperature strength and oxidation and creep resistance, and for these reasons have found widespread utility in aerospace applications. These alloys are known as titanium aluminide alloys. Many processes for forming these metals have been attempted, but few have proven successful and consistent because titanium aluminide alloys are relatively brittle and difficult to process and/or fabricate at room or near-room temperatures. One process which has achieved prominence is superplastic forming (SPF), with or without concurrent diffusion bonding (DB). When carrying out superplastic forming using titanium aluminides and similar reactive metals, it is necessary to heat and form the materials in a controlled environment to ensure cleanliness of the titanium which is particularly sensitive to oxygen, nitrogen, and water vapor content in the air at elevated temperatures. Unless the titanium aluminide is protected, it becomes embrittled and its integrity is destroyed. Typically, fabrication of sandwich structures involves first rolling metal foil or ribbon, then forming and joining the foil sheet into a desired cellular core configuration, and then attaching the core configuration to face sheets by brazing or spot welding. Problems which have been encountered with the prior art methodology include the high cost of core fabrication due to excess material usage and the great difficulty of forming, excess time consumption, and cost of fabrication of the sandwich shape. Additionally, a separate operation is required to join a close out or attachment to the sandwich structure. Fabrication of an unusual shape for the sandwich structure, such as a taper, is nearly impossible. This is particularly true for the titanium aluminide material where foil is extremely difficult and expensive to produce, joining by welding or brazing has not been firmly established and the overall set-up cost and fabrication expenses have often proven prohibitive. GB-A-2 095 137 relates to a filamentary-reinforced metallic structure that is made by a method which uses a stack of workpieces having two principal opposed surfaces. At least one of the outer workpieces comprises at least one inner sheet made of a metal matrix material reinforced with filamentary material and two cover sheets which sandwich the inner sheets and which extend beyond the periphery of the at least one inner sheet. Selected areas of the workpieces are treated to prevent bonding at those areas. The workpieces are stacked and positioned between shaping members and are heated to a temperature sufficient to produce diffusion bonding at preselected untreateed areas. Alternately the stack can be press diffusion bonded prior to insertion into the forming apparatus. After bonding, tensile stress is induced, preferably by gas pressure, to expand super plastically at least a portion of the sheet beyond the reinforcement. The invention relates to an apparatus for effecting diffusion bonding of preselected areas of a stack of titanium aluminide blanks and to a method for forming metallic sandwich structures using titanium aluminide materials as referred to in claim 1 and claim 5, respectively. Preferred embodiments are disclosed in the dependent claims. SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide an improved method for fabrication of titanium aluminide sandwich structures that combines the processes of diffusion bonding and superplastic forming in one continuous operation. Another object of the present invention is to provide an improved method for fabricating titanium aluminide sandwich structures using diffusion bonding and superplastic preforming techniques in a first continuous operation to significantly lower the cost, difficulty and time involved, and then superplastically expanding the bonded and preformed structure. Briefly, in accordance with the invention, there is provided a method and apparatus for bonding preselected areas of titanium aluminide blanks together under vacuum in a retort using gas diffusion bonding techniques, and then superplastically expanding the diffusion bonded blanks to form a desired sandwich structure. A step of superplastic preforming may be carried out simultaneously with diffusion bonding using arcuate or other shaped forming members. The novel apparatus includes first and second tools having respective shaping surfaces, means for moving at least one of the tools into engagement with the retort surrounding the stack so as to form a first chamber, means for heating the first chamber to a first temperature within which diffusion bonding can occur, and means for delivering pressurized gas to the first chamber to press the container uniformly against the stack with a force sufficient to cause diffusion bonding of selected regions of the sheets of the stack. Following diffusion bonding of the stack, the retort is removed and pressurized gas is delivered to the interior of the stack to cause superplastic expansion of the stack interior into a sandwich structure. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded view of a three-piece metal sheet assembly treated for selective diffusion bonding prior to insertion in the bonding apparatus. Figure 2A is a perspective view, with a portion broken away, showing the manner of fabrication of the retort about the pack of metal sheets prior to effecting diffusion bonding. Figure 2B is a cross-sectional view of the retort and pack of metal sheets shown in Figure 2A taken along section line 2B-2B in Figure 2A. Figure 3A is a cross-sectional view of one tool assembly used to effect diffusion bonding as contemplated by the present invention, with the sealed retort being disposed between upper and lower press assembly tools in advance of effecting the step of diffusion bonding. Figure 3B is a cross-sectional view of a second tool assembly used to effect both diffusion bonding and superplastic preforming as contemplated by the present invention, with the sealed retort being disposed between upper and lower press assembly tools in advance of effecting the step of diffusion bonding. Figure 4 is a cross-sectional elevational view of a preferred embodiment of a forming apparatus used for fabrication of metal sandwich structures with the three-piece bonded metal sheet assembly of Figure 1 inserted therein. Figure 5 illustrates the fully expanded three-piece bonded metal sheet assembly within the forming apparatus of Figure 4, with the broken lines illustrating the final position of the component metal sheets of the expanded joined assembly. Figure 6 is a detail view of an inflation tube connection for the three-piece metal sheet assembly. Figures 7 and 8 are cross-sectional elevational views of a modified forming apparatus with a two-piece bonded metal sheet assembly inserted in an initial position in Figure 7 and in a final expanded position in Figure 8. Figure 9 is a cross-sectional elevational view of a modified forming apparatus illustrating a tapered three-piece bonded metal sheet assembly in final formed position with an attachment joined to the sandwich structure at the broken lines. Figure 10 is a partial perspective view of the formed sandwich structure of Figure 9. DETAILED DESCRIPTION OF THE INVENTION In order for superplastic forming to be successful, it is necessary to use a material that will exhibit superplastic properties. The extent to which the material selected will exhibit superplastic properties is predictable in general terms from a determination of the strain rate sensitivity and a design determination of the permissible variation in wall thickness. Strain rate sensitivity can be defined as where s = stress in pounds per square inch, and e = strain rate in reciprocal minutes. Strain rate sensitivity is determined by a well-known torsion test, and it has been found that a strain rate sensitivity of about 0.5 of greater will provide satisfactory results. The larger the value, the greater the superplastic properties. Maximum strain rate sensitivity in metals is seen to occur as metals are deformed near the phase transformation temperature. For titanium aluminide, the temperature range in which superplasticity can be observed is about 1750° F. to about 1900° F. Other variables have been found to affect strain rate sensitivity and therefore should be considered in selecting a suitable metal material. Decreasing grain size results in correspondingly higher values for strain rate sensitivity. Additionally, strain rate and material texture affect the strain rate sensitivity. It has been found that for titanium aluminide, the m value reaches a peak at an intermediate value of strain rate (approximately 2 x 10⁻⁴ in./in./sec.). For maximum stable deformation, superplastic forming should be done at or near this strain rate. Too great a variance from the optimum strain rate may result in loss of superplastic properties. Diffusion bonding, through which separate elements form a single unitary mass, has been practiced in a wide variety of metals and alloys. However, the quality of the bond and the parameters employed necessarily vary for each particular choice of workpiece material. The present invention is particularly directed to reactive metals which have surfaces that would be contaminated at the elevated temperatures required for superplastic forming and diffusion bonding, as for example titanium aluminides. These metals are also particularly well-suited for the process of the present invention because they exhibit very high superplastic properties in a temperature range suitable for diffusion bonding, i.e., about 1750° - 1900° F depending on the specific alloy used. Referring now to Figure 1, there is shown an exploded view of a three-piece metal sheet assembly to be formed into a sandwich structure according to the present invention. The assembly is made up of metal blanks 10, 12, 14 all preferably in the form of sheets having upper and lower opposed principal surfaces 15 and 16, 17 and 18, and 19 and 20, respectively (see also Figure 4). The number of sheets used will vary depending on load conditions and design requirements. However, a minimum of two sheets must be used, and at least one of the sheets must exhibit superplastic properties. Any metal that exhibits suitable superplastic properties within a workable temperature range can be used for such sheet, but the present invention is particularly concerned with metals that exhibit superplastic properties within the temperature range required for diffusion bonding and that are subject to contamination at forming temperature, such as alpha-2 titanium aluminides. When using alpha-2 titanium aluminides, the forming temperature is preferably approximately 1800° F. The initial thickness of metal blanks 10, 12, 14 is determined as a function of the dimensions of the part to be formed. A suitable stop-off material is applied to selected areas within the stack of sheets where no attachment between the sheets is desired. Thus, as shown in Figure 1, areas 30, 32 and 34 are covered with a stop-off material to prevent bonding in those areas. Other areas on surfaces 15, 18 and 20 could also be covered with stop-off if desired. Alternatively, the metal sheet structure could be spot welded or brazed at those areas where joining is desired. Additionally, as hereinafter explained, the metal sheet structure or stack 40 could be diffusion bonded at selected areas by selective application of pressure. Figures 2A and 2B illustrate a retort or container which has been formed about the stack of metal sheets for the purpose of insuring that when the diffusion bonding technique is carried out at a sufficiently high enough temperature suitable for such a process, such bonding process can be effected in a controlled, oxygen-free and contaminent-free environment. As shown in Figures 2A and 2B, the retort 400 comprises a top sheet 402 of metal, preferably stainless steel, and a bottom sheet 404 of metal, also preferably stainless steel. Preferably, the top and bottom sheets of the retort are secured in a substantially air-tight manner, such as by welding, about their periphery to sealingly envelope the metal sheet structure or stack 40. Vacuum grooves 412 and 414 are formed in the bottom sheet 404 of the retort and are disposed on opposite sides of the metal sheet stack 40. The grooves are positioned such that they extend from a respective stack edge within the retort outwardly to and through an adjacently located sealed edge of the retort. The vacuum grooves are important because they facilitate the creation of a vacuum within the sealed retort and within the stack. In this manner, a contamination- free and oxygen-free environment such as that referred to above can be maintained within the retort during the heat-up phase and the diffusion bonding phase of the process contemplated by the present invention. To create a vacuum within the sealed retort, a vacuum pump 432, which as shown in Figure 2A is interconnected with the vacuum grooves 412 via a vacuum line or tube 422, is actuated to apply negative pressure (suction) to the interior of the retort. Pump 432 is operated until the pressure guage 442 shows attainment of a predetermined vacuum level, optimally between 10⁻⁵ Torr and 10⁻⁶ Torr. A second gauge 444 is used to ensure that a vacuum has been attained throughout the retort. This is accomplished by coupling the gauge to the interior of the retort via tube 424 and vacuum grooves 414, and using the gauge to verify a vacuum throughout the entire retort. A metal seal frame 500 fabricated from stainless steel strips 502 is disposed atop the top sheet 402 of the retort in such a manner as to be spaced from, yet parallel to, the edges of the metal stack 40. The seal frame 500 is attached to the top sheet 402 of the retort 400 (note Figure 2B) and each strip 502 includes an inner side 552, an outer side 554 and a top surface 556. The purpose of the seal frame 500 will be described below in connection with the apparatus used to achieve diffusion bonding of the stack 40 of metal sheets. While the size of the seal frame 500 depicted in the Figures is not accurately portrayed, it is intended to be a true representation of the manner of construction of the retort. In actual practice, it has been found that the most advantageous height of the seal frame strips 502 is preferably on the order of between 1/4" and 1/2". Referring now to Figure 3A, there is shown one apparatus 600 by which the process of diffusion bonding of the stack 40 of metal sheets may be accomplished. The apparatus includes an upper press member 610 and a lower press member 620. Mounted on the exposed face of the upper press member is a platen member 612, and an upper tool 614 is mounted on the exposed face of the upper platen member. Likewise, a lower platen member 622 is mounted on the exposed face of the lower press member 620, and a lower tool 624 is mounted on the exposed face of the lower platen member. The upper and lower platens are fabricated from ceramic materials. Preferably, the upper and lower tools are made from metals which possess excellent heat transfer characteristics. A horizontally extending gas transfer passage 630 is provided in the upper tool 614 running from one side surface to a generally central location thereof. Gas transfer passage 630 is connected with a second, vertically extending gas transfer passage 632 which runs from the central location in the upper tool to a central location in the exposed face of the upper tool. A pressure line 635 connects the gas transfer passage 630 with a source (not shown) of bonding gas under high pressure, and appropriate valve means for initiating and stopping the flow of gas to the gas transfer passage 630 for a purpose described in detail below. When the retort assembly 400 (as depicted in Figure 2A) is positioned in an appropriate location on the lower tool of the press assembly, the upper and lower tools are moved relative to one another so that the spacing between the two tools is decreased. Preferably, the press assembly is fabricated such that the upper tool moves relative to the lower tool (i.e., where the lower tool is supported in a fixed position and the upper tool is movable relative to the lower tool). As the upper tool moves downwardly toward the lower tool, the exposed face of the upper tool engages the top surface 556 of the metal strips of which the seal frame is composed. The upper tool is caused to move into engagement with the top surfaces of the seal frame 500 with sufficient downward pressure to create a sealed volume "V" defined between the upper surface of the retort top sheet 402, the inner sides 552 of the seal frame, and the lower exposed surface of the upper tool. Throughout the process, pump 432 is maintaining a vacuum within the retort 400. A heated environment is created about retort 400 while the retort is secured in the press assembly 600. As the retort is heated to a temperature within the range of approximately 1700° F. to 1900° F, gas is pumped through line 635 into the volume "V" so as to create enormous pressure within the volume "V" and thereby cause various surfaces of the metal sheets in stack 40 to be pressed together and thereby experience diffusion bonding. After maintaining this high pressure and high heat environment for a predetermined period of time (of sufficient duration to accomplish diffusion bonding), the gas pressure is released, the temperature environment about the press assembly is cooled, the press assembly is opened and the retort is cooled to a sufficiently low enough temperature to permit removal of the top and bottom sheets of stainless steel, of which the retort has been fabricated, from about the now-bonded stack of metal sheets. A second forming apparatus 700 is shown in Figure 3B by which the processes of diffusion bonding and superplastic preforming of the stack 40 of metal sheets may be accomplished. This apparatus includes an upper press member 710 and a lower press member 720. Mounted on the exposed face of the upper press member is a platen member 712, and an upper shaping tool 714 is mounted on the exposed face of the upper platen member. Likewise, a lower platen member 722 is mounted on the exposed face of the lower press member 720, and a lower tool 724 is mounted on the exposed face of the lower platen member. As with the first forming apparatus, the upper and lower platens of this apparatus preferably are fabricated from ceramic materials, and the upper and lower tools are made from metals which possess excellent heat transfer characteristics. However, in contrast to the first forming apparatus 600 where the upper tool 614 is formed with a substantially planar exposed face, the upper shaping tool 714 of this forming apparatus is formed with a concave recess having generally spherical or cylindrical attributes. A horizontally extending gas transfer passage 730 is provided in the upper shaping tool 714 running from one side surface to a generally central location thereof. Gas transfer passage 730 is connected with a second, vertically extending gas transfer passage 732 which runs from the central location in the upper shaping tool to a central location on the concave surface of the upper shaping tool. A pressure line 735 connects the gas transfer passage 730 of the upper tool with a source (not shown) of bonding gas under high pressure, and appropriate valve means for initiating and stopping the flow of gas to the gas transfer passage 730. The lower shaping tool 724 also is provided with a horizontally extending gas transfer passage 740 running from one side surface of the lower shaping tool to a generally central location thereof. Gas transfer passage 740 is connected with a second, vertically extending gas transfer passage 742 which runs from the central location in the upper shaping tool to a central location on the substantially planar surface of the lower shaping tool. A pressure line 745 connects the gas transfer passage 740 of the lower tool with a source (not shown) of bonding gas under high pressure, and appropriate valve means for initiating and stopping the flow of gas to the gas transfer passage 740. Use of the apparatus disclosed in Figure 3B to effect diffusion bonding and superplastic preforming is similar in many ways to use of the apparatus shown in Figure 3A. Initially, the retort assembly 400 shown in Figures 2A and 2B is modified by attaching a seal frame 800 on the downwardly facing surface of the bottom sheet 404. The seal frame 800 has a construction similar to the seal frame 500 and is provided for a function similar to that of seal frame 500. The modified retort assembly (identified in Figure 3B with the numeral 400' for sake of distinguishing over the retort assembly of Figure 3A) is then positioned in an appropriate location on the lower shaping tool 724 of the press assembly (see Figure 3B). The upper and lower tools 714, 724 are then moved relative to one another so that the spacing between the two tools is decreased. Movement of the upper tool downwardly toward the lower tool causes the lowermost faces of the upper shaping tool to engage the top surface of the seal frame metal strips in a manner similar to that described in connection with operation of the apparatus shown in Figure 3A. The lowermost faces of the upper shaping tool bear down on the top surfaces of the seal frame with sufficient pressure to move the modified retort assembly downwardly. This results in the creation of two sealed volumes "V" and "V'". The first of these volumes "V" is defined by the upper surface of the retort top sheet 402', the inner side surfaces of the seal frame 500, and the lower exposed arcuate surface of the upper shaping tool. The second of these volumes "V'" is defined by the lower surface of the retort bottom sheet 404', the inner side surfaces of the seal frame 800, and the upper exposed substantially planar surface of the lower tool. Throughout this forming process, pump 432 is maintaining a vacuum within the retort 400'. A heated environment is created about retort 400' while the retort is secured in the press assembly 700. As the retort is heated to a temperature within the range of approximately 1700° F. to 1900° F, gas is pumped through line 735 into the volume "V" so as to create enormous pressure within the volume "V" and thereby cause various surfaces of the metal sheets in stack 40 to be pressed together and thereby experience diffusion bonding. After maintaining this high pressure and high heat environment for a predetermined period of time (of sufficient duration to accomplish diffusion bonding), the gas pressure in line 735 is cut off and instead rerouted through line 745. Now, delivery of the pressurized gas to the volume "V'", causes the diffusion bonded stack 40, while still in the evacuated retort 400' and still at the diffusion bonding temperature of between approximately 1700° F and 1900° F, to deform upwardly and engage with the arcuate shaping surface of the upper shaping tool 714. In this manner, the stack 40 undergoes diffusion bonding as well as preforming, all in a single thermal cycle. Once this diffusion bonding/preforming process is completed, the gas pressure in the volume "V'" is released, the temperature environment about the press assembly 700 is cooled, the press assembly is opened and the retort is cooled to a sufficiently low enough temperature to permit removal of the top and bottom sheets of stainless steel, of which the retort has been fabricated, from about the now-diffusion bonded and superplastically preformed stack 40 of metal sheets. With reference now to Figure 4, there is illustrated a preferred apparatus 42 for effecting superplastic forming after the process of diffusion bonding, or diffusion bonding coupled with superplastic preforming, has been accomplished. As shown, the preferred apparatus includes an upper tool 44 has preferably integral side walls 45 in the form of a ring which can be of any desired shape. A lower tool 46, which preferably has the same outer area dimension as upper frame 44, can be flat and act as a base as illustrated for supporting the stack of metal blanks 40. For purposes of the claims, both upper tool 44 and lower tool 46 are considered shaping members as both cooperate to form the desired shaped structure. The inner surface of upper tool 44 defines an inner chamber 48 and a female die surface. One or more male die members (not shown) can be provided in the chamber 48 to vary the shape of the part to be formed. The stack of metal blanks 40, which is supported on the lower tool 46, covers the chamber 48. The metal blanks of the stack must all be of a material suitable for joining such as by welding, brazing or diffusion bonding. At least one of the outer metal blanks, and most likely the inner blanks, must have an effective strain rate sensitivity for exhibiting the superplastic properties at a desired forming temperature and preferably within a temperature range required for diffusion bonding of the stack. This is shown in Figure 5 by the expanded stack 40 where both sheets 12 and 14 have been superplastically expanded, while sheet 10 of the formed stack has been substantially unchanged. Initial thicknesses of the sheets of stack 40 are determined by the dimensions of the parts to be formed. The joining method to be used, namely diffusion bonding, depends on the material selected for the metal blanks, the temperature required for superplastic forming, and the desired strength. However, especially for titanium aluminide, diffusion bonding is preferred as this results in the strongest joining, in contrast to other conventional methods of joining such as welding or brazing. Furthermore, the bonding temperature is generally suitable for superplastic forming. As previously mentioned, stop-off can be used at areas 30, 32, 34 to prevent bonding at those areas. The particular stop-off selected must prevent bonding and be compatible with the metal or metals of the stack (nonreactive with the stack metals and minimal diffusion into the stack metals). For titanium aluminide metal blanks, suitable stop-off materials are graphite, boron nitride and yttria. Typically, when using yttria stop-off, the stop-off pattern on the blanks is applied using a silkscreen process with a solution of yttria and a binder therefor. The binder holds the yttria in position during bonding and eventually vaporizes below the metal forming temperature. Joining of the stack 40 at selected locations is normally accomplished outside of the forming apparatus 42 before the stack is placed therein. The weight of upper tool 44 acts as a clamping means for the stack 40. A single continuous edge of the stack 40 is effectively constrained between the upper tool 44 and the lower tool 46. This insures that the portions of the blanks of the stack to be formed will be stretched rather than drawn. Where desired, additional tightening means such as hydraulic jacks (not shown) can be employed to more effectively constrain the stack 40. Another additional tightening means that could be employed is a press (not shown), preferably hydraulic, having platens 50. Forming apparatus 42 is positioned between platens 50 and compressed thereby assuring that stack 40 is effectively constrained and chamber 48 sealed from the air. This arrangement is particularly advantageous as the platens 50 can be made of ceramic material and resistance heated wires 52 can be provided therein for heating stack 40 to the forming temperature. Other heating methods could be used with the forming apparatus 42 ordinarily surrounded by a heating means if the heating platens are not used. Alternatively, an environmental control system could be provided for contamination prevention and diffusion bonding of the stack 40 when the latter is unjoined within the forming apparatus 42. The purpose of this system is to ensure that the stack 40 is exposed only to inert gas during heating, bonding and forming by fluid pressure. The metal blanks of stack 40 will not react with inert gas due to the nature of such gas, even at the elevated bonding and forming temperatures. Thus, in this environment, contamination of the stack will be minimized. Line 52 is connected to a source of pressurized inert gas at one end (not shown) and to chamber 48 through orifice 54 in the upper tool 44. A valve 56 for governing the flow of inert gas through line 52 into chamber 48 and a pressure gauge 58 to indicate pressure are provided. The inert gas used is preferably argon in gaseous form. Line 52 also functions as an outlet for inert gas in chamber 48 and could also be connected to a source of vacuum, such as a suction pump (not shown) for creating vacuum in chamber 48. When line 52 is used as an outlet, valve 56 governs the flow of inert gas from the chamber 48. An additional line 60 is optimally provided on the opposite side of the tool 44. This line 60 is connected to the chamber 48 through orifice 62 in the upper tool 44 and functions as an outlet for inert gas in chamber 48. A valve 64 is provided in line 60 for regulation of inert gas flow from the chamber 48. Line 60 can simply function as a vent or be connected to a source of vacuum, such as a suction pump (not shown). In the alternative method, the contamination prevention system can also function as a means for effecting gas pressure diffusion bonding of stack 40. Thus, when stack 40 is placed in forming apparatus 42 as shown in Figure 2, the stack can be heated in an inert gas atmosphere to a suitable diffusion bonding temperature (approximately 1800° F. when the metal blanks of stack 40 are of alpha-2 titanium aluminide materials) by heat generated from heating platens 50, and then applying pressure to stack 40 by increasing the pressure in chamber 48 by adding additional pressurized inert gas through line 52 while maintaining line 60 closed via valve 64. In this manner, the untreated areas of stack 40 will be diffusion bonded by the application of such pressure, which is preferably approximately 900 psi for alpha-2 materials, for a suitable forming time (which depends on the thickness of the stack 40 and may vary from 30 minutes to 12 hours). The edges of the blanks of stack 40 may also be diffusion bonded if desired by virtue of sealing pressure thereon in the form of the weight of upper tooling 44 and optionally pressure from a press and/or a clamping means. After diffusion bonding stack 40, the excess inert gas would be removed from chamber 48 through lines 52 and 60 to allow for inflation of stack 40. For expansion of stack 40 to the configuration shown in Figure 5, expansion tubes 72 and 74 are provided (see Figures 1 and 6 for details). Expansion tube 72 (and likewise expansion tube 74 located on the opposite side of stack 40) is positioned between metal blanks 10 and 14, and protrudes into a channel 75 defined by the recesses 76 and 78 as well as that portion of surface 20 of metal blank 14 which overlies the recess 78. Recesses 77 and 79 are provided on the opposite sides of blanks 10 and 12, respectively, to provide a channel for inflation tube 74. The positioning of expansion tube 72 in such a channel prevents compression of tube 72 by the tools 44 and 46. Also, by locating tube 72 so that it protrudes only partially into the channel 75, inert gas will flow fairly evenly between the metal blanks of the stack 40, in this case on either side of the blank 12 as shown by arrows 80 and 82. As shown in Figure 4, expansion tubes 72 and 74 have valves 84 and 86 therein, respectively, for governing flow of inert gas therethrough, and pressure gauges 88 and 90 for indicating pressure. Expansion tubes 72 and 74 can also serve to draw off vaporized binder. To this end, tube 72 could act as an inlet and tube 74 an outlet with inert gas flow being transmitted through stack 40 prior to expansion of the stack to draw off the vaporized binder. A pair of lateral grooves (only one groove 73 of the pair is shown in Figure 6) are provided on opposite sides of the lower tool 46 with the one groove 73 being in alignment with recess 76 and the other groove (not shown) being positioned for alignment with recess 77. The lateral grooves are provided to insure that passage of inert gas from inflation tubes 72 and 74 between the metal blanks of the stack 40 is not prevented from reaching treated areas 30 and 34 by pinching of the stack due to the pressure exerted by the upper and lower tools 44 and 46. The width of the lateral grooves are preferably the same as the recesses 76 and 77, but the lateral grooves terminate further inward in the stack so that pinching does not occur before the inert gas flow reaches the treated areas 30 and 34. As shown in Figure 1, when treating as by stop-off selected areas of stack 40 to prevent diffusion bonding thereon, additional areas such as that shown at 92 and 94 should likewise be treated to prevent bonding thereon so that the gas from inflation tubes 72 and 74 will reach the selected treated areas for expansion of the stack 40. Apertures 96 are provided in the treated areas 30, 32, 34 for further transmitting the inert gas from inflation tubes 72 and 74 inside the stack to the other treated areas and to insure equal pressure inside the stack (in this case on either side of the metal blank 12). If pressure is unequal, the resulting core of the sandwich, metal blank 12 in the configuration illustrated in Figure 5, would be distorted with a consequent effect on load-carrying ability on the final sandwich structure. In order to make the expanded metallic sandwich structure as shown in Figure 5 according to applicant's inventive method, metal blank workpieces 10, 12, 14 are provided. Both sheets 12 and 14 must be of a material having an effective strain rate sensitivity for superplastic forming. Optimally one or more of the blanks are treated at specified locations, such as at 30, 32, 34 so that when the metal blanks are arranged in a stack and diffusion bonded, only selected areas of the stack will be joined thereby. Alternately, the stack could be spot welded or brazed for selective joining. When joining by diffusion bonding, the use of the retort and vacuum is most desirable. However, use of the alternative method would necessitate placing the stack in the forming apparatus 42 before forming. The pressure in chamber 48 would be increased by flow of pressurized inert gas through line 52 into chamber 48. Once chamber 48 had an inert gas atmosphere, the stack would be heated by resistance wires 52 in heating platens 50 to a temperature which optimally would be suitable for both diffusion bonding and superplastic forming, although the temperature could later be raised or lowered if a different temperature is required for superplastic forming. The pressure in chamber 48 would be increased by additional pressurized inert gas through line 52 to a pressure suitable for diffusion bonding of stack 40. That pressure would be maintained for a time duration sufficient for diffusion bonding. When the metal blanks of the stack are of alpha-2, the temperature used would be approximately 1800° F. and the pressure about 800-900 psi. These values can, of course, be varied during forming and bonding as long as they are maintained within a suitable range, i.e., in which the values would be sufficient for diffusion bonding and/or superplastic forming. The time duration will vary depending upon the alloys used, temperature, pressure, and thickness of the stack 40. Duration may vary from 30 minutes to 15 hours, but three (3) hours is a fairly representative expectation. As previously stated, bonding temperature may vary from about 1750° F. to about 1900° F. Gas bonding pressure may vary from about 800 psi to 2000 psi or more with the preferred range being from about 800 to about 900 psi. Before expanding the stack 40, the pressure in chamber 48 is reduced through lines 52 and 60. At superplastic forming temperature, which is about 1800° F. for alpha-2 titanium aluminide materials (generally 1750° F to 1900° F.), the stack is expanded by flowing pressurized inert gas through lines 72 and 74 while optimally a vacuum is applied to chamber 48 through lines 52 and 60. The pressurized inert gas, which protects the interior of the stack from contamination at the elevated forming temperatures, flows from tubes 72 and 74 into channels 75 (preferably on opposite sides of the stack) whereupon the inert gas flows within the stack. Such pressurized inert gas within the stack forces the expansion of the stack due to the pressure differential between the interior of the stack and the chamber 48. The pressure differential normally used for superplastic forming of alpha-2 is normally in a range of from about 800 to 900 psi. Metal blank 14 is initially lifted by the pressure differential and pulls with it at the selected joined areas metal blank 12. Such expansion allows the pressurized inert gas to flow through apertures 96 to provide an equal pressure within the stack so that the core (workpiece 12) is formed uniformly. The equal pressure also retains metal blank 10 of the stack in its initial position, the latter being forced against the base or lower tool 46. Figures 7 and 8 show a modified forming apparatus 130 and the use of a different technique for selective joining. The use of a two-sheet stack 132 with metal blanks 134 and 136 is also illustrated. Stack 132 could be joined as by diffusion bonding, brazing, or spot welding prior to the insertion in the forming apparatus 130. If diffusion bonded, the metal blanks 134, 136 would first be selectively treated with a suitable stop-off so that only certain predetermined areas of the stack would be joined. Forming apparatus 130 utilizes an upper tool 140 having a lower arcuate surface defined by a plurality of protuberances 142 spaced from one another by intermediate recesses or chambers 144. Lower tool 150 has an upper arcuate surface 152 complimentary to that formed by the protuberances 142. Inflation tubes 160 and 162 are positioned between the two metal blanks 134 and 136. Blanks 134 and 136 (similar to recesses 76 and 77 in metal blank 10 described in connection with the embodiment of Figure 1) are provided with aligned recesses (not shown) which define a cylindrical chamber (not shown) in which tubes 160 and 162 are located. Lines 164 and 166, similar to lines 52 and 60 shown in the Figure 2 embodiment, provide an inert gas environment in chamber 144 and act as vents or connection to sources of vacuum for drawing out the inert gas from the chambers 144 for superplastically expanding metal blank 134 within those chambers. Each of the tubes 164 and 166 would be provided with a valve (not shown) and pressure gauge (not shown) to control addition and removal of inert gas in chambers 144. Tubes 164 and 166 are connected to bores 170 and 172, respectively, which provide access to chambers 144. Depending on the thickness and desired curvature of stack 132, the stack can be preformed to such a shape prior to insertion in forming apparatus 130 by conventional sheet metal forming, such as by roll forming or superplastic forming, or subsequent to insertion in forming apparatus 130 by pressure applied to the stack by the protuberances 142 of upper tool 140 and surface 152 of lower tool 150. Preforming in the forming apparatus 130 is preferable as the preforming and selective diffusion bonding are accomplished simultaneously when the stack is also unjoined prior to insertion in the forming apparatus, thereby saving fabrication time and equipment costs. Using this forming apparatus 130, the unjoined stack can be diffusion bonded by application of pressure from upper tool 140 and lower tool 150 at required temperature for a suitable time duration. By virtue of protuberances 142, the pressure is only applied to selected areas of the stack so that only those areas are diffusion bonded, thereby allowing for expansion in the unbonded areas. After diffusion bonding (or insertion into the forming apparatus if the stack has already been joined) and preforming, stack 132 is inflated by flow of inert gas through lines 160 and 162 so that the unjoined areas of metal blank are expanded into the chambers 144. Optimally, a vacuum would be applied to chambers 144 through lines 164 and 166. As the only space provided for expansion is chambers 144, only metal blank 134 is expanded and consequently only that blank must be of a material suitable for superplastic forming. It is seen from the above that diffusion bonding, pre-forming, and superplastic expansion can all be done in the same apparatus in one operation. Figure 9 illustrates the forming of a variably shaped structure, the latter being shown as a tapered sandwich structure with an attachment joined thereto. Applicant accomplishes tapering of the sandwich structure by suitably designing the corresponding shaping surface of the upper tool. In Figure 9, the upper tool 170 has an upper shaping surface 172 which angles downwardly from one side to the other (shown as left to right). Thus, when the upper metal blank 174 of stack 176 is superplastically expanded against surface 172, it is shaped into the taper of such surface. The core formed by blank member 178 of stack 176 is also tapered by virtue of its dependence upon the movement of upper blank 174. Attachment 180, shown in an arbitrary design, is joined to the stack 176 on the upper member 174 along bond line 183 during the superplastic expansion of the stack in the same forming apparatus, thereby lowering fabrication time, apparatus cost, and forming difficulty. An attachment 180 could be positioned within a suitably shaped groove 182 where the attachment may or may not protrude from such groove, or be located in the forming chamber, without the use of a groove so that it forms either a male or female shaping surface itself. For purposes of the claims, when a groove is used, it is considered part of the forming chamber, so that the attachment is positioned within the chamber when in the groove. In the embodiment illustrated, when the stack is superplastically expanded, it contacts attachment 180 along bond line 183. By virtue of the superplastic forming temperature, the attachment is also heated, and the pressure expanding the stack eventually forces the upper member to bear against surface 172 and the attachment. The attachment is then diffusion bonded to the stack along bond line 183 when the temperature and pressure are maintained at a required diffusion bonding level for a suitable time duration. Optimally, the temperature and pressure for superplastic forming of members 174 and 178 would also be suitable for diffusion bonding so that the pressure or temperature would not have to be increased or decreased after forming in order to diffusion bond. The material selected for the attachment should be one that is suitable for diffusion bonding to the material of member 174, and preferably of a like material. As the superplastic forming gas pressure typically used is 900 psi, a pressure considerably less than the 2000psi used normally in diffusion bonding, the bond which results may not develop full parent metal strength, but would likely be analogous to a high quality braze joint. However, once the sandwich structure is fully expanded, as shown in Figure 9, the pressure can be increased within the expanded sandwich through inflation tubes 190, 192 to a level more suitable for complete diffusion bonding. A close-out such as shown at 194 can also be formed to the expanded sandwich structure in the same operation by diffusion bonding the close out, which comprises an unexpanded end of the stack 176, by heating the stack to the diffusion bonding temperature with application of pressure by upper and lower tools 170 and 171, respectively. The expanded sandwich structure 200 of Figure 9 removed from the forming apparatus is shown at Figure 10 with the side ends cut off.

Claims: An apparatus for effecting diffusion bonding of preselected areas of a stack (40) of titanium aluminide blanks (10, 12, 14) to form a structural unit expandable to a sandwich structure, comprising: container means (400, 402, 404) for forming a sealed evacuated environment about said stack 40, first and second coacting tooling members (44, 46; 614, 624; 714, 724) each engagable with a portion of said container means (400, 402, 404) to form a sealed chamber, means for heating said first and second tooling members (44, 46; 614, 624; 714, 724) as well as said container means (400, 402, 404) to a temperature suitable for diffusion bonding of said titanium aluminide blanks (10, 12, 14), means (432) for supplying pressurized gas to said chamber defined between the container means (400, 402, 404) and the first tooling member (44; 614; 714), said pressurized gas urging said container means (400, 402, 404) toward said second tooling member (46; 624; 724) and into engagement with and against said blanks (10, 12, 14) with a pressure great enough to cause diffusion bonding of said preselected areas of said metal blanks. The apparatus of claim 1, wherein said second tooling member (46; 624; 724) includes a shaping surface against which the stack (40) of blanks may be pressed following diffusion bonding of said blanks, so that after diffusion bonding has been completed said chamber can be further pressurized to cause superplastic preforming of said stack (40) thereby imparting to it a contour corresponding to the shape of said second tooling member. The apparatus of claim 1, wherein the container means includes sealing frame means (500) on the exterior thereof engagable with the first and second coacting tooling members 44, 46; 614, 624; 714, 724) to form the sealed chamber. The apparatus of claim 1, wherein said means for supplying pressurized gas comprises a source (432) of pressurized gas and channel means (62; 630; 730) in said first tool (44; 614; 714) having one end communicating with said source of pressurized gas and another end communicating with said first chamber, and said gas supply means includes means for selectively redirecting the flow of pressurized gas between said first chamber and a second chamber for superplastic forming. A method for forming metallic sandwich structures using titanium aluminide materials, comprising: sealing a stack (40) of sheet metal blanks (10, 12, 14) of said titanium aluminide materials within a container means (400, 402, 404), said stack having stop-off materials applied to preselected regions (30, 32, 34) of at least one (12) said sheet metal blanks (10, 12, 14), creating a contamination-free, evacuated environment within said container means (400, 402, 404), forming a sealed chamber about a portion of said container means (400, 402, 404) and pressurizing said chamber with a gas to cause the blanks of said stack (40) to be pressed together with uniform pressure, whereby selected areas of said stack (40) are diffusion bonded at a first elevated temperature in said contamination-free environment, and removing said container means (400, 402, 404) and superplastically expanding said diffusion bonded stack (40) at a second elevated temperature to form a desired sandwich structure. The method of claim 5, and further comprising the step of superplastically preforming said diffusion bonded stack at said first temperature.

Inventors: STACHER GEORGE W, STACHER, GEORGE W.
Applicants: ROCKWELL INTERNATIONAL CORP, ROCKWELL INTERNATIONAL CORPORATION
Application year: 1992
Main classification: B21D26
IPC codes: B23K20, B21D26, B21D47