Electric heating – Inductive heating – Metal working
Reexamination Certificate
1999-12-22
2001-01-30
Hoang, Tu Ba (Department: 3742)
Electric heating
Inductive heating
Metal working
C219S633000, C219S634000
Reexamination Certificate
active
06180932
ABSTRACT:
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.
Brown Ronald W.
Matsen Marc R.
Hammar John C.
Hoang Tu Ba
The Boeing Company
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