WO1996040466A1

WO1996040466A1 - Joining large structures using localized induction heating

Info

Publication number: WO1996040466A1
Application number: PCT/US1996/009634
Authority: WO
Inventors: Marc R. Matsen; Richard J. Poel; Wesley B. Crow; David S. Nansen
Original assignee: The Boeing Company
Priority date: 1995-06-07
Filing date: 1996-06-06
Publication date: 1996-12-19

Abstract

The processing time and energy efficiency of metal joining operations is improved, for example, by heating the structure in the vicinity of the braze joint in an induction heating press where energy goes primarily to heating the workpieces rather than to heating the tooling or being lost to the environment. Uniform localized heating at the bondline is achieved without heating the entire workpiece by using a narrow, 'smart' susceptor liner in the dies centered on the bondline.

Description

JOINING LARGE STRUCTURES USING LOCALIZED INDUCTION HEATING

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application based upon U.S. Patent Application 08/169,655, filed December 16, 1993 (which was a continuation-in-part application of U.S. Patent Application 07/777,739, filed October 15, 1991). The present application also is a continuation-in-part application based upon U.S. Patent Application

08/092,050, filed July 15, 1993 (a divisional of U.S. Patent Application 07/681,004, filed April 5, 1991, now U.S. Patent No. 5,229,562). Finally, the present application is also a continuation-in-part application based upon U.S. Patent Application 08/151,433, filed November 12, 1993, now U.S. Patent 5,420,400. We incorporate these applications and patents by reference.

TECHNICAL FIELD

The present invention is a method for to joining large metal structures using localized induction heating to heat the joint or bondline efficiently and rapidly and hydraulic rams to create high pressures at the bondline. In particular, the method involves brazing.

BACKGROUND ART

The tools or dies for metal processing typically are formed to close dimensional tolerances. They 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. These 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.

Airplanes are commonly made from metal or composite with prefabricated parts assembled and fastened or riveted together. The labor cost for fastening is a significant cost element and die fasteners add weight that limits overall performance and capability or adversely impacts operational costs. For military aircraft, weight translates to payload range which is critical with modern technology where a small advantage can mean the difference between success and failure. For commercial aircraft, while weight does not translate to survivability, it is still a significant factor because the capital cost plus the operating cost are the key elements of the airline's expense. For large structure, like the wing-carry-through structure, even fastening isn't available. Instead, these structures that must be produced to close tolerance are made by electron beam welding or diffusion bonding in a hot press. Both processes are expensive and are plagued with difficulties. Electron beam welding imposes post weld stress relief requirements, weld inspection, and warpage straightening. In addition, the electron beam welding process requires expensive tooling to create closely machined joints and large "high vacuum" chambers. Hot press diffusion bonding suffers from difficulties in repeatability as well as high tooling costs, long cycle times, and inefficient energy consumption. Technologies to reduce cost and weight, accordingly, are dear in the aircraft industry. The present invention is a brazing operation using Boeing's induction heating workcell which promises significant cost and weight savings for the manufacture of large aerospace assemblies. With diffusion bonding, the entire part is heated which is problematical because it is important to hold dimensional tolerance when the complicated assembly softens. Localized heating would reduce the problems associated with heating the entire part and would conserve energy.

Commonly, in our induction heating operations, we use a retort of sealed susceptor sheets around the entire metal workpieces to control the atmosphere around the workpiece and to achieve uniform heating, as described in greater detain in U.S. Patent 5,420,400 and U.S.

Patent Application 08/ entitled Combined Heating Cycles for Improving Efficiency in

Induction Heating Operations, which we incorporate by reference. The susceptor is heated inductively and transfers its heat principally through conduction to the preform or workpiece that is sealed within the susceptor retort. While the metals in the workpiece may themselves be susceptible to induction heating, the metal workpiece needs to be shielded in an inert atmosphere during high temperature processing to avoid oxidation of the metal, so we usually enclose the workpiece (one or more metal sheets) in a metal retort when using our ceramic tooling induction heating press.

Induction focuses heating on the retort and workpiece 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. Furthermore, we are energy efficient because significantly higher percentages of our input energy goes to heating the workpiece than occurs with conventional presses. Our reduced thermal mass and ability to focus the heating energy permits us to change the operating temperature rapidly which improves the products we produce. Finally, our shop environment is not heated as significantly from the radiation of the large thermal mass of a conventional press.

We can perform a wide range of manufacturing operations in our induction heating press. These operations liave opt^' ^)': operating temperatures ranging from about 350°F (175° C) to about 1950°F (1066°C). F^ each operation, we usually need to hold the temperature relatively constant for several mii-.:ιt' J to several hours while we complete the operations. While we can achieve temperature control by controlling the input power fed to the induction coil, we have discovered a better and simpler way that capitalizes on the Curie temperature. By judicious selection of the metal or alloy in the retort's susceptor facesheets, we can avoid excessive heating. With improved control and improved temperature uniformity in the workpiece, we produce better products.

As described to some degree in U.S. Patent 4,622,445 and in co-pending U.S. Patent Application 07/777,739, we discovered an improvement for an SPF process coupling the use of ceramic dies with inductive heating. With our inductively heated SPF press or workcell, we can heat preferentially the sheet metal workpiece with induction heating without heating the platens or dies significantly and can use the ceramic dies as an insulator to hold the induced heat in the part. We can stop the heating at any time and can cool the part relatively quickly even before removing it from the die. We do not waste the energy otherwise required to heat the large thermal mass of the platens and dies. We do not force the press operators to work around the hot dies and platens. With our inductive heating workcell, we also save time and energy when changing dies to set up to manufacture different parts because the dies and platen are significantly cooler than those in a conventional SPF press. We shorten the operation to change dies by several hours. Therefore, the induction heating process is an agile work tool for rapid prototyping or low rate production with improved efficiency and versatility. U.S. Patents 3,920, 175 and 3,927,817 describe typical combined cycles for SPF forming and diffusion bonding. Diffusion bonding is a notoriously difficult and temperamental process that has forced many SPF fabricators away from multisheet manufacturing or to "clean room" production facilities and other processing tricks to eliminate the possibility of oxidation in the bond. Oxides foul the integrity of the bond. In addition, diffusion bonds are plagued with microvoids which are difficult to detect nondestructively, but, if present, significantly d minish the structural performance of the joint. Diffusion bonding also is a time consuming process. The part typically must be held at elevated temperature and elevated pressure (about - 00 psi) for several hours. For example, in U.S. 3,920, 175, the diffusion bonding operation uϋ es five hours at 1650° F (900° C), making the forming/bonding operation six hours. In U.S. 3,927,817, diffusion bonding occurs prior to forming, still requires four to five hours, and forces a six hour bondmg/forming cycle at 1650° F (900° C) for the entire period. Typically a hot press diffusion bonding process for common titanium alloys used in aerospace applications will require over eight hours at 2500 psi and 800° C (1472° F), about six hours at 400 psi and 900° C (1650° F), or about two hours at 250-300 psi and 950° C (1742° F). Producing this heat and pressure for this length of time is expensive. Localized heating with higher localized pressure achieve a higher bonding force and a better bond in the process of the present invention.

The present invention is a timesaving process that promises higher quality parts at lower production costs with significant energy savings in shorter production times. The problems of hot press diffusion bonding are eliminated and more efficient manufacturing cycle is possible. Manufacturers have greater assurance in the integrity of the brazed bond so it prefer it. To achieve a satisfactory brazed bond quickly and reliably, we focus the heating on the part we are forming using an induction heater. We hold the part within insulating ceramic dies that are transparent to the time- varying magnetic field that our induction heater produces. We significantly reduce cycle time in manufacturing large aerospace parts.

SUMMARY OF THE INVENTION

Because of our capability with induction heating to rapidly heat and cool, we significantly reduce the processing time over conventional bonding operations in fabricating large metal aerospace structures by localizing heating on the bondline with a "smart" susceptor bond. We can shorten the joining operation from hours to no more than about 35-50 min. We generally join prefabricated metal parts having close overall dimensions and significant mass, like an aircraft forward boom, with a braze alloy. We heat d e "smart" susceptor bond that we fabricate into d e surface of d e die in die proximity of die braze joint to a uniform temperature. This hot susceptor radiates heat to the bondline. When die workpiece reaches a temperature where the braze alloy in d e bondline melts, we rapidly increase pressure witii hydraulic rams. We focus heat on d e bondline and obtain braze bonds comparable to electron beam welding at a fraction of me cost both in capital and cycle time widiout heating d e entire workpiece and wimout losing overall dimensions so we avoid many of d e problems of me prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of our induction heating workcell. Figure 2 is a schematic cross-sectional view of me apparatus of FIGURE 1. Figure 3 is a schematic sectional view of our induction heating workcell adapted for brazing of large aerospace structure. Figure 4 is anodier schematic sectional view, similar to FIGURE 3, but rotated 90°. Figure 5 is a perspective view of a structure we braze in die process of the present invention.

Figure 6 is a typical temperature/pressure cycle for the brazing process of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Before describing our particular process, we will give some general background on the operation of our induction heating workcell.

1. The Induction Heating Process

In FIGURES 1 and 2 an induction heating workcell 10 includes tools or dies 20 and 22 mounted witiiin an upper 24 and a lower 26 strongback. The strongbacks are each d readed onto four ti readed column supports or jackscrews 28 or they float free on d e columns and are fixed witii nuts. We can turn d e jackscrews to move one strongback relative to the otiier. The strongbacks 24 and 26 provide a rigid, flat backing surface for d e upper and lower dies 20 and 22 to prevent die dies from bending and cracking during manufacturing operations. Preferably, die strongbacks hold d e dies to a surface tolerance of ± 0.003 inches per square foot of d e forming surface. Such tolerances are desirable to achieve proper part tolerances. The strongbacks may be steel, aluminum, or any other material capable of handling d e loads present during forming or consolidation, but we prefer materials tiiat are non-magnetic to avoid any distortion to d e magnetic field mat our induction coils produce. In some circumstances, die dies may be strong enough themselves tiiat strongbacks are unnecessary. The strongbacks transfer pressure input dirough die columns evenly to die dies.

The dies 20 and 22 are usually ceramic and are reinforced witii a plurality of fiberglass rods 32 tiiat are held witii bolts 74 and that extend botii longitudinally and transversely in a grid dirough each die. Each die usually is framed witii phenolic reinforcement 72 as well to maintain a compressive load on die die. Each die may be attached to its strongback by any suitable fastening device such as bolting or clamping. In die preferred embodiment, bom dies are mounted on support plates 76 which are held in place on d e respective strongbacks through die use of clamping bars 77. The clamping bars 77 extend around die periphery of the support plates 76 and are bolted to die respective strongbacks dirough the use of fasteners (not shown).

The dies should not be susceptible to inductive heating so mat heating is localized in die retort. We prefer a ceramic that has a low coefficient of tiiermal expansion, good diermal shock resistance, and relatively high compression strength, such as a castable fused silica ceramic.

We embed portions of an induction coil 35 in die dies. In d e illustrated embodiment, we use four separate induction segments, but the number can vary. Each segment is formed from a straight tubing section 36 tiiat extends along the length of each die and a flexible coil connector 38 mat joins me straight tubing sections 36 in the upper die 20 to die corresponding straight tubing section in the lower die 22. Connectors 40 located at d e ends of die induction coil 35 connect the induction coil 35 to an external power source or coil driver 50 and to a coolant source.

Each die surrounds and holds d e straight sections 36 of d e induction coil in proper position in relationship to die tool insert 46 or 48. In the preferred embodiment, die interior 70 of the dies is formed of a castable ceramic and die exterior sides from precast composite phenolic resin blocks 72. Pin holes (not shown) in d e dies vent gas trapped between die retort 60 and die forming surface 58 as the retort deforms. Such pin holes can be coupled to a flow meter to monitor the progress of the deformation.

When me operation is complete, the induction coil 35 is de-energized and die pressure relieved. The tool inserts and dies are separated. We remove the formed retort 60 from the press and recover die composite part from between die susceptor facesheets.

An alternating oscillating electrical current in die induction coil 35 produces a time varying n_^gnetic field tiiat heats die susceptor sheets of d e retort via eddy current heating. The frequency at which the coil driver 50 drives d e coils 35 depends upon die nature of d e retort 60. We power die coil witii up to about 400 kW at frequencies of between about 3-10 kHz. Current penetration of copper at 3 kHz is approximately 0.06 inches (1.5 mm), while peiicj-ation at 10 kHz is approximately 0.03 inches (0.75 mm).

The shape of the coil has a significant effect upon die magnetic field uniformity. Field uniformity usually is important because temperature uniformity induced in die retort is directly related to die uniformity of d e magnetic field. Uniform heating insures tiiat different portions of die workpiece will reach die operating temperature at approximately die same time. Solenoid type induction coils like tiiose we illustrate provide a uniform magnetic field, and are preferred. Greater field uniformity is produced in a retort that is located symmetrically along die centerline of the surrounding coil. Those of ordinary skill can establish series/parallel induction coil combinations, variable turn spacing, and distances between die part and die induction coil by standard electrical calculations to achieve the desired heating from whatever coil configuration is used. The dies are usually substantially thermally insulating and trap and contain heat witiiin die retort. Since the dies and tool inserts are not inductively heated and act as insulators to maintain heat within die retort, d e present invention requires far less energy to achieve die desired operating temperature than conventional autoclave or resistive hot press metiiods where die metal tooling is a massive heat sink.

Bonding operations using our workcell are faster than prior art operations because we do not heat die large ti ermal mass of eitiier d e dies or tool inserts. The retort is heated, die tool is not. Thus, die necessary processing temperature is achieved more rapidly. In addition, die highly conductive materials in the retort provide rapid heat transfer to die workpiece. When the driver 50 is de-energized, die dies and die retort cool rapidly to a temperature at which we can remove the retort from the workcell, saving time and energy over conventional systems. Coolant flowing through the coil tubes functions as an active heat exchanger to transfer heat out of the workpiece, retort, and dies. In addition, die tiiermal cycle is not as limited by die heating and cooling cycle of die equipment and tools so we can tailor die thermal cycle better to die process for which we are using die induction heating workcell.

2. Joining Large Aerospace Structure

For brazing metal as shown in Fig. 3, we prefer to use a cobalt alloy "smart" susceptor 300 as described in our pending U.S. Patent Application 08/ entitled:

"Metiiod for Achieving Thermal Uniformity in Induction Processing of Organic Matrix Composites or Metals," which we incorporate by reference. This susceptor 300 forms a liner for the dies 20 & 22 in the vicinity of me braze joint 305 to provide localized, efficient, uniform heating, thereby eliminating die need to heat d e entire workpiece. We inject an inert atmosphere (i.e., Argon gas) around die parts 310 & 315 being produced (i.e., die workpiece) to protect diem against oxidation.

Using a smart susceptor liner 300 reduces die need for a uniform magnetic field because the susceptor is self-regulating in temperature at its Curie temperature. Therefore, complex shapes can be processed in the apparatus because the braze joint is subjected to a uniform temperature through an efficient heating process. Achieving a uniform temperature for the entire parts 310 & 315 would be difficult because of tiieir size, mass, and contour, but, as shown in Fig. 4, d e liner can match d e contour to provide focused heating upon die braze joint bondline. Generally d e liner is narrow, about 2-5 in centered on the bondline. We make me liner as narrow as possible while achieving proper heating at the bondline to form a uniform braze joint. We usually can accomplish tiiis goal witii a 2 in wide liner strip. As shown in Fig. 4, d e ceramic dies 20 & 22 are supported in a dielectric container 320 and dielectric strongbacks 325 & 330 tiiat hold die dimension of die part when it heats. In our equipment, for a large titanium part, die smart susceptor and braze joint will reach a temperature of about 1950° F (1066° Qwhile die temperature of die container and strongbacks will be ambient because of die insulating qualities of the ceramic and d e active cooling water network associated witii the induction coil.

The smart susceptor 300 is 0.040 in tiiick, which makes it an efficient energy converter at one frequency. It has relatively small mass and, accordingly, heats and cools rapidly. The parts 310 and 315 typically are halves of a forward boom for a fighter aircraft, about 0.5 in tiiick titanium witii peripheral dimensions of about 42x27x19x22 in counterclockwise respectively in Fig. 4. Only a small portion is heated, which again saves energy, simplifies die operation, holds overall dimensional tolerance, and reduces die possibility of warpage.

The pressure and heat (Fig. 6) produces a superior joint where the diffusion of die braze alloy species into die bulk metal of die parts makes the joint nearly indistinguishable from the bulk metal when the part is complete. We energize the coil 36 to heat the liner 300 to its Curie temperature of about 1950° F (about 15-20 min. after energizing the coil). We maintain the temperature and pressure for about 15-20 min until we have the desired final dimensions before cooling the part while letting the pressure decay naturally to set the part dimension. We believe we can use die process to join large titanium parts of complex curvature having strict dimensional tolerances and strengtii requirements, producing quality parts tiiat are difficult or impossible to make by any other process. We join parts to form assemblies that could not be machined as a unified whole because of d e complex curvatures and interferences.

For me formed boom shown in Fig. 5, we position the halves to define die bondline using braze alloy on d e clean faces. When we attain die bonding temperature localized at d e bondline, we raise the pressure rapidly and monitor die part location looking for lateral movement of about 0.10 in as the bond forms between die 0.50 in tiiick part. We release d e pressure when we have reached die exact final dimensions of die part. The joint should be substantially indistinguishable from the bulk metal when the joining is complete. We can also diffusion bond in tiύs process, although we prefer brazing. The localized heating and high pressure forms a quality diffusion bond.

We clean die surfaces of the joint interface by using chemical etchants to provide an oxide-free (pure metal) surface for brazing or bonding.

The liner made from the "smart" susceptor functions like the retort in our earlier processes but has die added advantage tiiat it allows localized heating of the bondline.

Localized heating means that we avoid loss of dimensions in die remote portions of the parts. Because it has a uniform temperature because of die Curie temperature phenomenon, the liner can be curving sheet (i.e., a thin band or ribbon) tiiat maps the contour of the part, as shown in Fig. 4.

The bondline typically is a flat surface normal to the pressure rams 335, as shown in Fig. 3. If not a straight face a die joint, men d e parts should be profiled to provide substantially uniform pressure in d e bondline.

While we have described preferred embodiments, tiiose skilled in me art will readily recognize alterations, variations, and modifications which might be made witiiout departing from me inventive concept. Therefore, interpret d e claims liberally witii the support of die full range of equivalents known to tiiose of ordinary skill based upon tiiis description. The examples are given to illustrate the invention and not intended to limit it. Accordingly, limit die claims only as necessary in view of the pertinent prior art.

Claims

We claim: 10

1. A method for joining large metal aerospace structure using localized induction heating to heat d e structure along a bondline to a joining temperature to reduce processing time and energy consumption, comprising the steps of:

(a) assembling of at least two workpieces to define a bondline;

(b) placing die assembly in an induction heating press witii d e bondline adjacent a susceptor incorporated into the wall of d e die; (c) energizing d e induction coil of the heating press to heat d e bondline locally;

(d) applying pressure when the bondline reaches the joining temperature;

(e) monitoring the geometry of die joined part; and

(f) de-energizing the coil.

2. The meti od of claim 1 wherein me induction heating press includes ceramic dies in which a solenoid coil is embedded and wherein die heating involves flowing alternating current through the coil at a frequency of about 3-10 kHz and wherein a braze alloy is sandwiched by die workpieces at die bondline to braze d e workpieces togetiier when die braze alloy melts at me joining temperature.

3. A product obtainable by die process of claim 1.

4. An induction heating apparatus for brazing metal parts together, comprising: (a) matching cast ceramic dies having a smart susceptor liner on a narrow portion of the mating surfaces and aligned where the braze joint will form; and

(b) an induction coil embedded in die dies adjacent to die liner.