Forming metals

Abstract

A method of forming a stiffened panel in which an interior sheet of a superplastic material is placed between two face sheets and attached one to the other in alternate sequence so that, as the face sheets are moved apart, the attached regions of the interior sheet are drawn with them.

Description

This invention relates to the forming of stiffened panels of metallic alloys having super-plastic characteristics. Metallic alloys having super-plastic characteristics have a composition and microstructure such that, when heated to within an appropriate range of temperature and when deformed within an appropriate range of strain rate, they exhibit the flow characteristics of a viscous fluid. Such alloys have characteristics indicated by the formula:

Where:-

m is numerically of the order of 0.7 to 1.00,

f is applied stress (load per unit area),

h is a constant,

s is strain rate (extension per unit of original length per unit of time), and,

m is the strain rate sensitivity.

The condition in which these characteristics are attained is known as super-plasticity and large deformations are possible without fracture.

The invention has for an object the ready formation of stiffened panels with a minimum of forming operations.

According to the present invention a method of forming a stiffened panel includes the steps of positioning a metal face sheet on each side of an interior sheet of a metallic alloy having superplastic characteristics, attaching spaced regions of the said interior sheet alternately to the face sheet on one side and to the face sheet on the other side of the interior sheet, bringing the assembly to within that temperature range at which the interior sheet exhibits superplastic characteristics, and causing the face sheets to be moved apart and thus to draw the attached regions of the interior sheet with them such that the said interior sheet finally extends from one face sheet to the other in alternate sequence.

Conveniently, the metal face sheets have their peripheral edges sealingly joined together and the envelope so formed is inflated to urge the said face sheets apart.

Some preferred embodiments of the invention are now described with reference to the accompanying drawings:

FIG. 1 is a scross-sectional view of parts of the components of a panel before forming,

FIG. 2 is a similar view to that of FIG. 1 subsequent to forming,

FIG. 3 is a similar view to that of FIG. 2 but illustrating an alternative embodiment.

FIG. 4 is a similar view to that of FIG. 1 also illustrating an alternative embodiment,

FIG. 5 is a similar view to that of FIG. 4 subsequent to forming,

FIG. 6 is a similar view to that of FIG. 5 but showing an alternative embodiment, and,

FIGS. 7 and 8 illustrate a sealed peripheral edge of a panel respectively before and after forming.

Referring to FIGS. 1 and 2, a stiffened panel is formed of three metal sheets, two face sheets 1 and 2, respectively, and an interior sheet 3, which is of a superplastic alloy. The interior sheet is placed between the two face sheets 1 and 2 with spacer portions 4 in the form of strips of metal placed between each face sheet 1 and 2 and the interior sheet 3. The spacer portions are placed where the sheets are to be attached one to the other in alternate sequence, that is to say from the left hand edge of FIG. 1 the sheets 1 and 3 have a spacer portion 4 and then the sheets 2 and 3 and so on. The assembly is then subjected to heat and pressure so that those regions of the sheets at the spacer portions become diffusion bonded to the spacer portions 4 and thereby indirectly one to another.

In the alternative of FIG. 3, the elongated spacer portions 4 are replaced by spacer portions in the form of small disc-like portions 5. These are spaced in alternate sequence between the face sheet 1 and the interior sheet 3 and between the face sheet 2 and the interior sheet 3. Again, the sheets are diffusion bonded to the spacer portions 5 and hence indirectly to one another.

FIGS. 4, 5 and 6 are similar to FIGS. 1, 2 and 3, respectively, but illustrate embodiments where the face sheets 1 and 2 are attached directly to the interior sheet 3 without the spacer portions 4 or 5 being present. In this case the sheets 1 and 2 are locally attached to the sheet 3 by welded regions. The welded regions, which in FIG. 5 are in the form of lines 6 and in FIG. 6 are in the form of spots 7, are preferably provided by an electron beam welding process.

To bring the assembly from the condition of FIGS. 1 and 4 to that of FIGS. 2 and 5 or 3 and 6 respectively, it is brought to within the temperature range at which the interior sheet exhibits superplastic characteristics, if it is not already in that range, and the face sheets 1 and 2 are moved apart thus drawing the attached regions of the interior sheet (that is those regions adjacent the spacer portions 4 or 5 and adjacent the weld regions 6 or 7) with them in alternately opposite directions. The interior sheet 3 thus becomes of corrugated form (as in FIGS. 2 and 5) or of dimpled form (as in FIGS. 3 and 6). As can be seen in FIGS. 3 and 6, the sheet 3 becomes a series of alternate oppositely facing dimples. In both cases the interior sheet 3 zig-zags between the face sheets 1 and 2 bridging the void between them.

FIGS. 7 and 8 illustrate how the sheets can be subjected to a welding operation around their peripheries to form a sealed envelope. The weld region is shown at 8. The sealed envelope so formed is fed with an inert gas under pressure such that, when the interior sheet 3 is superplastic, the sheets 1 and 2 are moved apart by a predetermined amount to effect the previously described corrugation or dimpling of the interior sheet.

To maintain flat outer surfaces, the sheets 1 and 2 may be moved apart against oppositely facing plattens of a press (not shown). To further aid the maintaining of flat outer surfaces, the interior sheet may be of thinner gauge material than the face sheets.

The face sheets 1 and 2 may also be of a superplastic alloy; this arrangement has advantage where the finished panel required to be of other than a totally flat formation.

Claims

We claim:

1. A method of forming a stiffened panel including the steps of positioning a metal face sheet on each side of an interior sheet of a metallic alloy having superplastic characteristics, attaching spaced regions of the said interior sheet alternately to the face sheet on one side and to the face sheet on the other side of the interior sheet, sealing the said face sheets one to the other to form an inflatable envelope assembly, bringing the assembly to within that temperature range at which the interior sheet exhibits superplastic characteristics, and applying a differential pressure between the interior and the exterior of the envelope assembly thus causing the face sheets to move apart and draw the attached regions of the interior sheet with them so that the said interior sheet finally extends from one face to the other in alternate sequence.

2. A method according to claim 1 wherein the spaced attachment regions of the interior sheet are indirectly attached to the face sheets by means of metallic spacer portions metallurgically bonded both to a face sheet and to the interior sheet.

3. A method according to claim 1 wherein the spaced regions of the interior sheet are directly attached to the face sheets by means of a metallurgical bond.

4. A method according to claim 1 wherein the said spaced attachment regions are in the form of lines and the interior sheet is finally of generally corrugated form.

5. A method according to claim 1 wherein the said spaced attachment regions are in the form of spots and the interior sheet is finally of generally dimpled form.

6. A method according to claim 1 wherein the said face sheets are sealed one to the other by a peripherally extending metallurgical bond.

7. A stiffened panel formed by the method defined in claim 1.