Abstract:
An induction brazing uses a multisheet, diaphagm retort having isolated pressure zones to control the net tooling pressure at a level that avoids core crush. The isolated chambers in the multisheet diaphragm allow us to apply controlled pressure to the braze joint, especially at the elevated brazing temperature where the honeycomb core is soft and malleable.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application 60/114,267, filed Dec. 30, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to induction brazing, and particularly, to a method to control net tooling pressure in such operations. 
     BACKGROUND OF THE INVENTION 
     The tools or dies for forming, brazing, and the like typically are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delay caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. Delays are especially significant when the manufacturing run is low rate where the dies need to be changed after producing only a few parts of each kind. 
     Attempts have been made to reduce fabrication times by actively cooling the tools after forming the composite part. These attempts have shortened the time necessary to produce a part, but the time for and cost of heating and cooling remain significant contributors to overall fabrication costs. Designing and making tools with active cooling increases their cost. 
     Boeing described a process for organic matrix forming and consolidation using induction heating in U.S. Pat. No. 5,530,227. There, prepregs were laid up in a flat sheet and were sandwiched between aluminum susceptor facesheets. To ensure an inert atmosphere around the composite during curing and to permit withdrawing volatiles and outgassing from around the composite during the consolidation, we welded the facesheets around their periphery. Such welding unduly impacts the preparation time and the cost for part fabrication. It also ruined the facesheets (i.e., prohibited their reuse). U.S. Pat. No. 5,599,472 described another Boeing technique that readily and reliably sealed the facesheets without the need for welding and permitted reuse of the facesheets in certain circumstances. 
     An example of a metal forming process using the Boeing induction heating workcell is described in U.S. Pat. No. 5,420,400. The process combines brazing and superplastic forming of metal with a single induction heating cycle. In such a process, Boeing uses a metal pack or retort to contain the multiple sheets in the workpiece in a pressure zone filled with an inert atmosphere. The sheets are welded along their periphery of the retort. The welds are costly to prepare, introduce trimming as a necessary step to recover the completed part, and limit the reuse of the retort sheets since they must be shaved smaller when trimming away the weld to recover the completed part. 
     In preparing the retort, we often use temporary seals to hold the sheets until the sheets are clamped into the press. We prefer a “C” spring clamp, as described in U.S. Pat. No. 5,599,472. The clamp sandwiches the outer susceptor sheets of the retort and provides a compressive force to hold the retort together temporarily, pressing the sheets against an “O” ring gasket. Such a gasket seats between susceptor sheets in a machined groove or crimp around the periphery of adjacent susceptors. For processing below about 600° F., the gasket is generally silicone rubber. Between about 600° F. and 1300° F., the gasket is copper; above 1300° F., the gasket is stainless steel. The gasket and susceptor sheets abut and form a gas seal via the compressive force of the die set. The “C” clamp permits handling of the retort in and out of the die set. The “C” clamp also provides a current path from the top sheet to the bottom sheet (when the gasket is rubber or stainless steel). The “C” clamp can be omitted when we use a copper gasket, but handling the susceptor sheets is more difficult. The “C” clamp jumper is only required for electrical continuity when the gasket is not an electrical conductor and, then, only on the edges of the retort transverse to the induction coils since the coils induce eddy currents in the susceptor that flow parallel to the coils. 
     By “forming,” we mean shaping the composite or metal and retort in its plastic state. “Forming” may entail superplastic forming, drawing, hot pressing, or some other shaping operation. 
     The dies or tooling for induction processing are ceramic because a ceramic is not susceptible to induction heating and, preferably, is a thermal insulator. Ceramic tooling is strengthened and reinforced with fiberglass rods or other appropriate reinforcements to permit it to withstand the temperatures and pressures necessary to form, to consolidate, or otherwise to process the composite materials or metals. Ceramic tools cost less to fabricate than metal tools and also generally have less thermal mass than metal tooling. Because the ceramic tooling is not susceptible to induction heating, it is possible to use the ceramic tooling in combination with induction heating elements to heat the retort without significantly heating the tools. The method reduces the time required and energy consumed to fabricate a part. 
     Most operations require a susceptor in or adjacent to the workpiece to achieve the necessary heating. The susceptor is heated inductively and transfers its heat principally through conduction to the workpiece that is sealed within the susceptor envelope or retort. Metals in the workpiece may themselves be susceptible to induction heating, but the metal workpiece usually needs to be shielded in an inert atmosphere during the high temperature processing to avoid oxidation of the metal. We enclose the workpiece (one or more metal sheets) in a metal retort when using our ceramic tooling induction heating press. Enclosed in the metal retort, the workpiece does not experience the oscillating magnetic field which instead is stopped in the retort sheets, so heating occurs by conduction from the retort to the workpiece. 
     Induction focuses heating on the retort and workpiece rather than on the entire tool and eliminates wasteful, inefficient heat sinks. Because the ceramic tools in our induction heating workcell do not heat to as high a temperature as the metal tooling of conventional, prior art presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced. 
     To consolidate or to form organic matrix composite materials, an organic matrix composite preform is placed adjacent a metal susceptor. The susceptor heats inductively, and in turn, heats the preform. A consolidation and forming pressure is applied to consolidate and, if applicable, to form the preform at its curing temperature. 
     The retort often includes three susceptor sheets, typically aluminum, an aluminum SPF alloy, or a ‘smart’ alloy, sealed around their periphery to define two pressure zones. The first pressure zone surrounds the workpiece and is evacuated and maintained under vacuum. The second pressure zone is pressurized (i.e., flooded with gas) to help form the composite panel or workpiece. The shared wall of the three layer sandwich acts as a diaphragm in this situation. In the present invention, we use such a retort and control the tooling pressure across the diaphragm to make delicate, brazed parts. The retort is placed in an induction heating press on the forming surfaces of dies having the desired shape of the molded composite part. After the retort and preform are inductively heated to the desired elevated temperature, pressure is applied (while maintaining the vacuum in the pressure zone around the preform) to consolidate the preform against the die into the desired shape of the completed part. 
     The susceptor sheets, at least on the outside of the retort, might be a ‘smart’ material that has a Curie point at a desired temperature. For example, for consolidating BMI, we might use INVAR36 and for consolidating thermoplastic polyimides, PERMALLOY and KOVAL. The inner diaphragm sheet typically will be aluminum because it does not intereact with the magnetic field and aluminum generally is less expensive and more readily available than the ‘smart’ materials. 
     Brazing usually is done in a vacuum furnace. This process involves large facilities costs (it requires significant space in a specialized building), high tooling costs, and long cycle times. The use of induction heating reduces facility cost due to reduced cycle time. Many more panels can be brazed in the same amount of time using induction brazing over the standard vacuum furnace. Also, the same tools used for induction brazing can be used to hot form the facesheets. This eliminates the requirement for a separate high cost tool. Finally, better control of the thermal cycle using induction brazing affords better braze quality. Vacuum furnaces with the characteristic long thermal cycles cannot tailor the thermal cycles to avoid detrimental reactions between the brazing alloys and the base materials. The induction heating process with its rapid heat-up and cool down rates and good thermal control at the critical processing temperatures can tailor the thermal cycle to avoid these detrimental reactions. 
     When induction brazing honeycomb panels, waviness of the panels can occur if the tooling pressure is insufficient to hold the panel in intimate contact with the die. Usually, 15 to 20 psi is sufficient to keep the susceptor and panel on the die surface. If the core is thin or when brazing occurs at high temperatures (1500-1800° F.), 15-20 psi tooling pressure can cause core crush. The present invention controls the pressure using several, adjustable pressure zones in the retort design to hold the retort (i.e., the part and the susceptor) against the die without crushing the core. 
     Efforts to reduce tooling mass and to speed cooling apply forced gas to the vacuum furnace technology. Forced cooling gas only slightly improves the cycle time, but, unfortunately, raises the cost of an already expensive operation. Active or forced cooling often produces undesirable temperature gradients across the retort because the cooling occurs unevenly. 
     Our induction heater uses induction heating and reinforced cast ceramic tools with good durability and affordability. Ceramic dies provide precise control of the geometry of the component while eliminating the requirement to heat the entire tool. These characteristics allow for rapid, controlled heating and cooling down rates which lead to a more efficient, affordable, flexible method of forming and brazing of honeycomb panels. The basic tool design consists of an outer box usually constructed of phenolic materials. Copper tubing to form the induction coils and fiberglass reinforcement rods are held in place with this phenolic box before the ceramic is cast. After the ceramic is cast, the phenolic boards compress the dies when nuts on the end of the reinforcement rods are tightened. Compression of the die increases its durability. 
     The tool may include a metallic susceptor liner that acts as an antenna for the magnetic flux produced by the coil. The susceptor is typically a 0.045 inch thick layer of magnetic alloy which has a Curie point (temperature at which the alloy becomes nonmagnetic) that matches or closely approximates the desired processing temperature. 
     An oscillating electrical current (typically, 3 kHz) is passed through the coil, and creates an oscillating magnetic field which emanates from the coil through the ceramic. The magnetic field interacts with the susceptor, which is an alloy having high magnetic permeability. Magnetic flux experiences lower resistance inside such materials. The ‘smart’ susceptors when utilized below the Curie temperature tightly house the flux lines generated by the coil. The high density of time varying magnetic flux in the susceptor creates internal voltages which “induce” eddy currents that are constricted to a thin layer because of the influence of the magnetic permeability of the susceptor as shown by: 
     CURRENT DEPTH is proportional to          ρ     μ                 f                              
     wherein μ is permeability, ρ is resistivity, and f is frequency. 
     The current heats the susceptor. The heat is trapped inside the ceramic tool because the ceramic is highly insulative. Cooling of the induction coil tubes by coolant flowing within them also limits how much of the die that is heated. Heating is focused on the part, and the mass being heated is far smaller than the traditional approach. A high degree of the magnetic field coupling (efficient/direct energy transfer) occurs between the magnetic field and the susceptor. All these factors contribute to rapidly heat the workpiece, primarily by conduction from the susceptor to the enclosed part. 
     As the susceptor heats, any location on the susceptor that reaches the Curie point has its permeability decline significantly and effectively becomes nonmagnetic. That location expels magnetic flux. The flux and current will bend through the remaining high permeability magnetic material surrounding the hot spot. The rate of heating in the nonmagnetic ‘hot spot’ will fall because no current flows there and will increase in the remaining areas until all of the susceptor reaches the Curie point. The induction efficiency when all the material is nonmagnetic is much less. The temperature will hover around the Curie point, because, as cooling occurs anywhere in the susceptor, currents are created in the then magnetic areas. The currents reheat the susceptor to the Curie point. Therefore, if the Curie point is matched to the desired processing temperature, a powerful, repeatable, and simple thermal control mechanism is available. 
     Heat generated by the induced current in the susceptor is held inside the die cavity because the ceramic material is an insulator. A steep thermal gradient develops through the thickness of the die. The coefficient of thermal expansion for our castable ceramic material is low. Therefore, this material easily supports the thermal stresses that result from this steep gradient. Also, the reinforced nature of the die allows for it to withstand any tensile forces produced by the internal pressurization required to form the facesheets during brazing. Reinforcement is required because the ceramic material is weak in tension but strong in compression. The approach, just as in reinforced concrete, is to utilize the material in compression where it has good durability characteristics. The fiberglass reinforcement rods have good tensile strength and are a dielectric (not electrically conductive) material that does not heat inductively. During processing, tensile forces are counteracted by the compressive preload applied to the die. The net force on the ceramic never reaches tensile loads of any significance. 
     SUMMARY OF THE INVENTION 
     The present invention is an improved method and system for brazing honeycomb panels within a three sheet, diaphragm retort. First, we “hot form” the titanium facesheets for the honeycomb panel. We select a ‘smart’ susceptor which substantially matches the forming temperature of the facesheets. If the facesheets are Ti 6AI-4V, for example, then the susceptor should have a Curie point of about 1650° F. After the facesheets have been formed, their surfaces are cleaned outside the die with suitable etchants, degreasers, detergents, or their combination. We apply brazing foil to the clean facesheets. The core is sandwiched between the facesheets, and the panel is returned to the die. Two distinct pressure zones divided by a diaphragm are formed by the arrangement of the driver sheets in the retort that we use for brazing. A first zone contains the panel. This zone can either be purged (i.e., evacuated) or pressurized with inert gas. Pressure in this zone forces the facesheet against the core to define the outer mold line (OML) and holds the facesheet in the desired geometry. The inert gas pressure precludes any waviness or wrining of the facesheet and eliminates oxidation that otherwise might occur at the forming temperature. 
     Inert gas pressure in the second zone presses the diaphragm against the facesheet for the inner mold line (IML) to hold it against the core. This pressure also holds the core against the OML facesheet. 
     As suggested by Edward Woods and described in greater detail in U.S. patent application Ser. No. 09/187,614, ferrite blocks ( 151 , FIG. 7) at the edges of the coil improve the heating rate we can use to reach a substantially uniform temperature at the Curie point of the susceptor. The ferrite blocks focus the magnetic flux generated by the coil at the edge and align the flux for smooth entrance and exit from the susceptor, thereby creating a more uniform flux field in the workpiece. The blocks substantially eliminate the edge effect of the coil. More uniform flux throughout the workpiece decreases the time needed to reach a uniform thermal condition. Faster heating reduces processing time and is especially important both to control the metallurgical reactions present in some braze alloy - base metal combinations and to control core crush. 
     The susceptor for brazing titanium honeycomb panels depends on the brazing alloy used. Our common brazing alloy for brazing titanium is aluminum 3003. The susceptor in this case is 420 Stainless Steel, which has a Curie point of 1250° F. This Curie point falls in the middle of the allowed brazing temperature range from 1240°F. and 1265° F. Another common brazing alloy is TiCuNi (70%Ti, 15%Cu, 15%Ni). Its susceptor is a Cobalt alloy (78%Co, 11%Ni, 11%Fe) having a Curie point of about 1815° F., which produces a level temperature in operation of about 1780-1790°F. In one embodiment, appropriately shaped susceptors form a liner on the inside of the dies, similar to the concept we described in U.S. Pat. No. 5,587,098. They receive the magnetic energy and convert it into thermal energy through eddy current heating, as we previously explained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of our induction heating workcell for induction brazing forming organic matrix composite panels or metal workpieces. 
     FIG. 2 is a schematic cross-sectional view of the apparatus of FIG.  1 . 
     FIG. 3 illustrates an edge seal in the susceptor sheets used in a retort during brazing. 
     FIG. 4 illustrates pressure lines penetrating the edge strip gasket for a seal like that shown in FIG. 3 to allow pressurized gas into pressure zones in the retort. 
     FIG. 5 is a partial sectional view showing expansion of the facesheets against the die surfaces in a retort as an initial step in making a brazed honeycomb panel according to one method of the present invention. 
     FIG. 6 is another partial sectional view, similar to FIG. 5, but showing a retort having two pressure zones to control the net tooling pressure when brazing a honeycomb core to the formed facesheets. 
     FIG. 7 illustrates the use of ferrite blocks at the ends of the retort and inset into the lower die to shape the magnetic flux and to improve control of the thermal uniformity in the retort. 
     FIG. 8 is a graph showing a typical processing cycle for brazing aluminum panels. 
     FIG. 9 is another graph showing a typical processing cycle for brazing titanium panels using TiCuNibraze alloy. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing our preferred brazing process, we first will provide a brief description of the induction heating workcell in which we generally perform the process. 
     Boeing&#39;s Induction Heating Workcell 
     In FIG. 1, an induction heating workcell  10  includes tools or dies  20  and  22  mounted within upper  24  and lower  26  strongbacks. The strongbacks are each threaded onto four threaded column supports or jackscrews  28 . We can turn the jackscrews using a bellows or other actuation mechanism to move one strongback up or down relative to the other. The strongbacks  24  and  26  provide a rigid, flat backing surface for the upper and lower dies  20  and  22  to prevent the dies from bending and cracking during repeated consolidation or forming operations. Preferably, the strongbacks should be capable of holding the dies to a surface tolerance of +/−0.003 inches per square foot of the forming surface in the toolbox. Such tolerances are desirable to achieve proper part tolerances. The strongbacks may be steel, aluminum, or any other material capable of handling the loads present during forming, consolidation or brazing, but we prefer materials that are nonmagnetic to avoid any distortion to the magnetic field that our induction coils produce. In some circumstances, the dies may be strong enough themselves that strongbacks are unnecessary. They also may include internal reinforcement such as described in U.S. Provisional Patent Application 60/071,765. 
     The dies  20  and  22  are usually ceramic that is reinforced with a plurality of fiberglass rods  32 . The rods are held with bolts  74  that extend both longitudinally and transversely in a grid through each die. Each die usually is framed with phenolic reinforcement  72  as well. Each die may be attached to its strongback by bolting, clamping, or any suitable fastening technique. In the preferred embodiment, both dies are mounted on support plates  76  which are held in place on the respective strongbacks through the use of clamping bars  77 . The clamping bars  77  extend around the periphery of the support plates  76  and are bolted to the respective strongbacks. 
     The dies should not be susceptible to inductive heating so that heating is localized in the retort rather than distributed throughout the press. We prefer a ceramic that has a low coefficient of thermal expansion, good thermal shock resistance, and relatively high compression strength, such as a castable fused silica ceramic. 
     A plurality of induction coils sections  35  are embedded in the dies, and are connected to form a solenoid coil. Each induction coil  35  includes a straight tubing section  36  that extends along the length of each die. A flexible coil connector  38  joins the straight tubing sections  36  in the upper die  20  to straight tubing sections in the lower die  22 . Connectors  40  located at the ends of the induction coils connect the induction coils  35  to an external power source or coil driver  50  and to a source of coolant (i.e., a water line, accumulator, or reservoir). 
     Cavities  42  and  44  can be formed in the respective dies to hold tool inserts  46  and  48 , although single piece dies generally are simpler to use. The upper tool insert  46  usually will have a contoured forming surface  58  shaped to correspond with the desired shaped of the outer mold line (OML) surface of the completed part. The lower tool insert determines the inner mold line (IML). The tool inserts preferably are formed the same castable ceramic as the dies. We prefer ceramic tooling because it provides the greatest flexibility and versatility for Boeing&#39;s induction heating workcell. In the preferred embodiment, the interior  70  of the dies is formed of a castable ceramic and the exterior sides from precast composite phenolic resin blocks  72 . Both can be reinforced with chopped fibers or nonwoven or woven reinforcing mats. 
     To increase the strength of the dies, fiberglass reinforcing rods  32  extend both longitudinally and transversely through the precast exterior side blocks  72  and the interior  70  to maintain a compressive load on the blocks  72 , interior  70  and the tool inserts  46  and  48 , if they are used. Suitable dies are described in U.S. Pat. No. 5,683,608. 
     FIG. 2 shows a retort  60  in the workcell positioned between the tool inserts  46  and  48  along the centerline of the solenoid induction coil. The retort  60  includes a metal workpiece having outer facesheets covering a central honeycomb core and susceptor sheets sandwiching the facesheet—core—facesheet panel. The susceptor sheets of the retort are heated to the brazing temperature by energizing the coils  35 . When the sheets reach their Curie point at the desired brazing temperature, pressure source  52  applies pressure to the upper surface of the retort  60  through a conduit  62  that passes through the upper die  20  and upper tool insert  46 . In the induction workcell shown in FIG. 2, a pressure source  54  applies pressure to the lower surface of the retort  60  through a conduit  64  that passes through the lower die  22  and lower tool insert  48 . The pressure applied to the retort  60  is maintained until the retort has formed to the contour of the forming surface  58 . The pressure sources  52  and  54  generally apply a differential pressure to the retort  60 . Generally, the workcell does not need to include gas lines if a multizone retort is used to enclose the part. 
     Pin holes (not shown) in the tool inserts vent gas trapped between the retort  60  and the forming surface  58  as the retort deforms. Such pin holes can be coupled to a flow meter to monitor the progress of the deformation, as suggested in U.S. Pat. Nos. 5,419,170; 5,309,747 and 5,129,248. 
     When brazing is complete, the induction coil  35  is de-energized, and the pressure relieved. The tool inserts and dies are separated. We remove the formed retort  60  from the press and recover the completed part from between the susceptor sheets. 
     An alternating electrical current in the induction coils  35  produces an alternating magnetic field that heats the susceptor sheets of the retort via eddy current heating. The frequency at which the coil driver  50  drives the coils  35  depends upon the nature of the retort  60 . Current penetration of copper at 3 kHz is approximately 0.06 inches, while penetration at 10 kHz is approximately 0.03 inches (0.75 mm). The shape of the coil also has a significant effect upon the magnetic field uniformity. Field uniformity is important because temperature uniformity in the retort is directly related to the uniformity of the magnetic field. Uniform heating insures that different portions of the retort will reach the desired temperature at approximately the same time. Solenoid type induction coils provide a uniform magnetic field, and are preferred. Greater field uniformity is produced in a retort that is symmetric. Those of ordinary skill can establish series/parallel induction coil combinations, variable turn spacing and distances between the part and the induction coil by standard electrical calculations to achieve the desired heating. Temperature uniformity also is improved by using ferrite blocks at the edges of the coil along its centerline to alter the magnetic flux. 
     The brazing operation of the present invention is faster than prior art operations because we do not heat the large thermal mass of either the dies or tool inserts prior to the induction heating process. The retort is heated, the tool is not. Thus, the necessary processing temperature is achieved more rapidly. In addition, the highly conductive materials in the retort provide rapid heat transfer and product. When the driver  50  is de-energized, the dies and the retort cool rapidly to a temperature at which we can remove the retort from the workcell, saving time and energy over conventional systems. In addition, the thermal cycle is not as limited by the heating and cooling cycle of the equipment and tools so we can tailor the thermal better. 
     As shown in FIG. 3, susceptor sheets  100  and  102  are sealed around their periphery with a crimp. A first pressure zone  117  between the susceptors  100  and  102  surrounds the workpiece. A third sheet  108  of susceptor is positioned over the second sheet  102  and is edge sealed with high temperature gasket  110 . The second and third sheets together define a second pressure zone  119  that can be pressurized with argon or another suitable gas during the brazing operation to provide an overpressure or a net tooling pressure. A contact edge strip  112  acts as a compression edge seal and provides electrical continuity (i.e., acts as an electrical jumper) between the first and the third sheets  100  and  108  as well as pressing the sheets against the gasket  110 . Additional compressive force is applied when the retort is clamped in the press. The first and second sheets abut in the vicinity of the gasket  110 . Typically the contact edge strip  112  is a copper, elongated “C” because it has good conductivity, ductility, and susceptibility. Other metals could be substituted. 
     In FIG. 3, the first and third sheets  100  and  108  of susceptors contact the dies and do not leave additional pressure zones between the outer susceptors and the dies, like the pressure zones we described with reference to FIG. 2. A system of conduits fabricated in the dies is described in U.S. Pat. Nos. 4,708,008; 5,129,249; 5,309,747; 5,419,170; 5,689,987; or 5,692,406. When a third sheet  108  of susceptor is used, the retort itself incorporates the necessary pressure zones. The dies can be porous, and are much easier to manufacture. Such dies do not need to carry or contain high pressure gases. They are lighter and are less expensive. Therefore, we prefer a system like that illustrated in FIG.  3 . With a multizone retort, pressure can be introduced and controlled inside the sealed retort envelope. 
     Forming gas to the pressure zone between the second and third sheet of the susceptors is introduced through suitable pressure lines  122  that penetrate the edge strip gasket  110  at desired locations, as shown in FIG. 4, to deliver pressurized inert gas to the second pressure zone  119 . We might also use a titanium pressure bladder in this zone, as shown in the drawings. These pressure lines  122  correspond to those used with the edge welded retorts we described in U.S. Pat. No. 5,530,227. Similar lines can also be used to allow fluid communication with the pressure zone  117  between the first and second sheet of the susceptor where the workpiece is placed. If such lines are used, they generally are used to evacuate the first pressure zone  117  or to flood it with an inert purging gas. 
     We energize the coil by the coil driver with a time varying electrical field to produce a time varying magnetic field to heat the susceptors inductively to their Curie point. Heat is transferred by conduction and/or radiation from the susceptors into the composite panel, so it, too, reaches consolidation temperature. 
     Gas is supplied to the second pressure zone between sheets  102  and  108  to force the diaphragm susceptor  102  against the workpiece. The pressure within the pressure zone is maintained until the brazing is complete. 
     Pressurizing the titanium pressure bladder enclosed within the second pressure zone  119  forces on the workpiece together during brazing. The pressure in zone  119  must be higher than the pressure in zone  117  to provide a net tooling force, but the total tooling force must be kept small to avoid crushing the core. Panels using 3003 aluminum target using a pressure of 15 psi in the first zone and a pressure of 20-25 psi in the second zone, for a net tooling pressure of 5-10 psi on the titanium core and a total of 20-25 psi tooling pressure holding the panel to the OML configuration. We prefer a net tooling pressure of about 10 psi and a total tooling pressure of 25 psi in this case. Because this brazing operation takes place below 1350° F., we are able to use this relatively large (10 psi) tooling pressure. In contrast, for induction brazing of titanium honeycomb panels using TiCuNi brazing alloy, the pressure we prefer for the first zone is 15.5 psi while the pressure for the second zone is 16 psi, for a net tooling pressure of 0.5 psi. Brazing TiCuNi takes place at a higher temperature of about 1780° F. The titanium core is quite soft and easily crushed, so the total and net tooling pressures must be controlled carefully. It also is important to keep the tooling pressure to a minimum. 
     After completing the brazing, we shut off the induction coils and cool the retort to a cool enough temperature to remove the retort from the dies. Although there is some heat transfer between the retort and the dies, it is insufficient to heat the dies substantially, because we circulate cooling water throughout the induction coil. Therefore, the retort can quickly be pulled from the press. When the retort cools sufficiently, we remove the edge strips and recover the completed part. The edge strips and the susceptor sheets usually are reusable. 
     It may be advantageous in some circumstances to maintain different pressures in different cells of the pressure zone at different locations of the workpiece. Welding the second and third sheet along one or more weld lines internal of the periphery defines separate pressure cells between these sheets that may be individually pressurized at different pressures. Of course, such welding destroys the reuse potential for these sheets. 
     We can shape the magnetic field and flus using ferrite blocks  151  (FIG.  7 ), as Woods suggested, to improve the temperature uniformity in the retort. 
     While we have described preferred embodiments, those skilled in the art will readily recognize alternatives, variations, and modifications which might be made without departing from the inventive concept. Therefore, interpret the claims liberally with the support of the full range of equivalents known to those of ordinary skill based upon this description. The examples are given to illustrate the invention and not intended to limit it. Accordingly, limit the claims only as necessary in view of the pertinent prior art.