1. Field of Invention
The invention relates to the field of metal forming, and particularly to an improvement to the method of making metallic structures by superplastic forming and diffusion bonding.
2. Description of Prior Art
It has been known for many years that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges. For example, some titanium alloys, such as Ti-6Al-4V, have been observed to exhibit superplastic characteristics.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult or, in some instances, not possible. A significant breakthrough in superplastic forming was made by Hamilton, et al., disclosed in U.S. Pat. No. 3,934,441, Controlled Environment Superplastic Forming, incorporated into this specification herewith by reference. Simplified, the process involves placing a metal blank workpiece over a cavity in a chamber. The blank is heated to a temperature where it exhibits superplastic characteristics after which pressure is applied to the blank, causing it to stretch and form into the cavity.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause co-mingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding (SPF/DB) in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No. 3,927,817, Method of Making Metallic Sandwich Structures, by Hamilton, et al., and is herein also incorporated by reference.
Basically, the Hamilton, et al. method for making metallic sandwich structures involves fabricating the structures from a plurality of metal blank workpieces. One or more of the blanks are coated with stopoff in selected areas not to be diffusion bonded. The blanks are positioned in a stacked relationship and placed in a die assembly wherein the stack is clamped at its periphery forming a seal thereabout. The stack is diffusion bonded together in the uncoated areas by the application of pressure, and at least one of the blanks is superplastically formed against one or more of the die surfaces forming the sandwich structure. The core configuration is determined by the location, size and shape of the joined areas. In the Hamilton, et al. method, means must be provided to couple the pressurized fluid to the untreated areas of the blanks during the step of superplastically forming. Hamilton, et al. use inflation tubes. For example, if the stack consists of three blanks, the middle blank would be notched while a groove would be machined into the inward facing surface of the lower outer blank. Thus, when the blanks are assembled into a stack, a recess is formed comprising the notch in the middle blank, the groove in the bottom blank, and the inward facing surface of the top blank. A stopoff path is provided to couple the treated areas to the recess. The inflation tube is inserted into this recess, and the stack is then tacked or spot welded together at its periphery.
This procedure does not lend itself to high production runs, for it is time consuming and expensive to fabricate the recess and to install the tubes. This is especially true if the sandwich structure is of a complex configuration requiring several such tubes to be installed. The tubes also make it difficult to handle the stack during subsequent forming steps. Furthermore, a good seal between the stack and the tube is not accomplished until the stack has been clamped and heated within the forming members, at which point the metal in the stack will flow about the tube forming a seal.
An additional problem is that care must be taken to ensure that localized compressive forces do not crush the tube. It is a normal procedure to seal the periphery of the stack at its top and bottom by providing sharp, hard projections that run continuously around the perimeter of the forming members. When using the inflation tube method, these projections must be interrupted or reduced in height in the areas over the inflation tubes lest the high localized loading crush the tube. Typical examples of the use of such projection is disclosed in U.S. Pat. No. 3,934,441, Controlled Environment Superplastic Forming by Hamilton, et al.
A method that eliminates inflation tubes is disclosed in British Pat. No. 1,398,927, Joining and Forming Sheet Metal Members by Summers, et al. Summers, et al. disclose a method of diffusion bonding and superplastic forming wherein a port is provided in the die aligned with a hole in the bottom blank. Fluid pressure is applied through the hole to expand the upper workpiece. Such a procedure has two distinct disadvantages: (1) it requires that a hole be drilled in the usable part of the structure, and (2) if the stack has three or more blanks and is to have both its outer blanks expanded, then the method will not work, because equal pressure will always exist on either side of the bottom blank having the hole.
Also of interest is U.S. Pat. No. 4,087,037, Method of and Tools for Producing Superplastically Formed and Diffusion Bonded Structures by J. F. Schier, et al. Schier, et al. disclose a SPF/DB method limited to two workpieces in the stack. The step of superplastic forming is accomplished first by welding the periphery of the two workpieces together and providing a port coupling the interior of the stack to a source of pressurized fluid. While a port near the periphery is disclosed, Schier, et al. do not contemplate coupling to a stopoff path. Without the use of a stopoff, the periphery of the stack, where a port would ideally be located, cannot be clamped between a pair of forming members for the pathway to the area to be superplastically formed would be blocked off.
It is, therefore, a primary object of this invention to provide a lower cost method of making metallic sandwich structures by superplastic forming and diffusion bonding.
It is another object of this invention to provide a method of making metallic sandwich structures by superplastic forming and diffusion bonding suitable for use in high production runs.
It is a further object of the invention to provide an improved method of making metallic sandwich structures by superplastic forming and diffusion bonding by eliminating the use of inflation tubes.