Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
2000-04-21
2002-01-08
Hoang, Tu Ba (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C219S635000, C219S645000, C219S659000
Reexamination Certificate
active
06337471
ABSTRACT:
TECHNICAL FIELD
The present invention relates to superplastic forming (SPF) combined with adhesive bonding (AB) to form multisheet, expanded sandwich structures, especially 4-sheet aluminum alloy panels bonded with a polyimide adhesive.
BACKGROUND ART
Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits. This property, called superplasticity, is exhibited by certain metals and alloys within limited ranges of temperature and strain rate. For example, titanium and its alloys are superplastic in the temperature range from about 1450-1850° F. (785-1010° C.).
Superplastic forming (SPF) is a technique for expanding or stretching metal that relies on superplasticity. A typical SPF process involves placing one or more sheets of metal in a die, heating the sheets to an elevated temperature within the superplastic range for that metal, and superplastically forming the sheet(s) at the SPF temperature. Expansion can and often does exceed 100%. Generally, a differential forming pressure from a gas manifold is injected between sealed sheets and is used as the driving force to stretch the sheet(s) into the desired shape against the shaped surfaces of supporting dies. SPF can be called “blow molding” insofar as it uses differential gas pressure to form the material. The differential pressure is selected and controlled to strain the material at a strain rate that is within its superplastic range. The following patents are illustrative of SPF processes and equipment:
PATENT
TITLE
ISSUE DATE
3,920,175
Method of SPF of Metals with
November 18, 1975
Concurrent Diffusion Bonding
3,927,817
Method for Making Metallic
December 23, 1975
Sandwich Structures
3,605,477
Precision Forming of Titanium
September 29, 1971
Alloys and the Like by Use of
Induction Heating
4,141,484
Method of Making a Metallic
February 27, 1979
Structure by Combined Flow
Forming and Bonding
4,649,249
Induction Heating Platen for Hot
March 10, 1987
Metal Working
4,117,970
Method for Fabrication of
October 3, 1978
Honeycomb Structures
5,024,369
Method to Produce
June 18, 1991
Superplastically Formed
Titanium Alloy Components
We incorporate these patents by reference.
One advantage of SPF is the forming of complex shapes from sheet metal while reducing the time and eliminating the waste of milling. SPF sandwich panel production results in a considerable cost saving and reduces total part count over conventional “built up” assemblies that are arranged and fastened together. In addition, the SPF process is generally applicable to single and multisheet fabrication. For multisheet fabrication, SPF is combined with joining processes, such as diffusion bonding, brazing, or laser welding, to produce complex sandwich structures. In the present invention, we join the sheets with adhesive bonding. The SPF process produces lighter, lower cost parts that use fewer fasteners. Use of SPF is accelerating for the manufacture of parts for aircraft, missiles, and spacecraft. In the present invention, we combine SPF with adhesive bonding to make multisheet sandwich panels, especially panels made from aluminum or its SPF alloys.
Titanium superplastically-formed/diffusion-bonded (SPF/DB) panel structures can cost 50% less than conventional honeycomb construction. The SPF/DB process can produce tailored rib or integral hard point and fastener through-hole structures, such as those described in published PCT Application US96/20115, which we also incorporate by reference.
In a typical prior art SPF process for titanium or its alloys, the sheet metal is placed between dies at least one of which has a contoured surface corresponding to the shape of the product. The dies are placed on platens, which are heated, generally using embedded resistive heaters. The platens heat the dies to about 1650° F. (900° C.). Because the titanium will readily oxidize at the elevated temperature, an inert gas, such as argon, surrounds the die and workpiece. The dies heat the sheet metal to the temperature range where the sheet metal is superplastic. Then, under applied differential pressure, the sheet metal deforms against the contoured surface.
The platens and dies have a large thermal mass. They take considerable time and energy to heat and are slow to change their temperature unless driven with high heat input or with active cooling. To save time and energy, the platens must be held near the forming temperature throughout a production run (i.e., the production of a number of parts using the same dies), so loading raw materials and unloading completed parts is a challenge. The raw sheet metal must be inserted onto the dies, and formed parts removed, at or near the elevated forming temperature. The hot parts must be handled carefully at this temperature to minimize bending. Within the SPF range, the SPF metals have the consistency of taffy, so bending can easily occur unless the operators take suitable precautions. Bending generally ruins the part because the part assumes the wrong aerodynamic shape or has unintended areas of stress concentration.
U.S. Pat. Nos. 4,622,445 and 5,683,608 describe improvements for an SPF process coupling the use of ceramic dies with induction heating. With an inductively heated SPF press or workcell, the sheet metal workpiece (or a susceptor surrounding the workpiece) is preferentially heated using an oscillating magnetic field without heating the platens or dies significantly. The ceramic dies are an insulator and retain heat induced in the part. Heating is easily controlled by stopping the induction. The part can cool relatively quickly even before removing it from the die. In Boeing's induction heating workcell, less energy is wasted because we do not heat significantly the large thermal mass of the platens and dies. Press operators need not work around hot dies and platens. Boeing also saves time and energy when changing dies to set up manufacture of different parts. The dies and platens are significantly cooler than those in a conventional SPF press, so they can be handled sooner, reducing the die change 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. We also incorporate these patents by reference.
U.S. Pat. Nos. 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, especially for aluminum, 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 corrupting the bond. In addition, diffusion bonds are plagued with microvoids, which are difficult to detect nondestructively, but, if present, significantly diminish the structural performance of the joint. Even when it works, diffusion bonding is a time consuming process. The part typically must be held at elevated temperature and elevated pressure (about 400 psi) for several hours. For example, in U.S. Pat. No. 3,920,175, the diffusion bonding operation takes five hours at 1650° F. (900° C.), making the complete cycle forming and bonding each part six hours. In U.S. Pat. No. 3,927,817, diffusion bonding occurs prior to forming, but still requires four to five hours, resulting in a six hour bonding/forming cycle where the temperature must be held 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 eight hours or more 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. The equipment and facilities to house it are expensive. The consumption of resources is large. The process limits the rate of production and is far from lean or agile.
Another diffusion bonding process uses a CRES template to apply pressure i
Kistner Matthew G.
Will Jeffrey D.
Hammar John C.
Hoang Tu Ba
The Boeing Company
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