Machinery's Handbook, 31st Edition
Superplastic Forming and Diffusion Bonding 1439
Sheet 1
Diffusion bonding Step 1
Die
Sheet 2
SPF phase 1: Membrane forming Step 2
Vacuum
Gas pressure p = f ( t )
Vacuum
SPF phase 2: Final part forming
Vacuum
p = f ( t ) Gas pressure
Vacuum
Finished part
Fig. 30. Schematic Illustration of DB/SPF Process However, this process also has its challenges. One of these involves how to apply the stop-off material in the proper location using the most cost-effective process. Historically, silk screening has been used to define the required pattern for the stop-off material. This process requires several pieces of equipment, including a wash booth, since the screen needs to be cleaned after each part. A masking paper and laser scribing process has also been developed for defining the stop-off pattern. There are a number of commercial applications of superplastic forming and diffusion bonding, including aerospace, ground transportation, and numerous miscellaneous other uses. Examples are wing access panels in the Airbus A310 and A320; bathroom sinks in the Boeing 737; turbo fan engine-cooling duct components in the B-1, the T-38, the C-17, and the F-15E; and external window frames in the space shuttle. High-Energy Rate Metal Forming Processes The term “high-energy rate forming processes” (HERF) refers to dynamic metal form- ing processes that form workpieces at very high velocities and extremely high pressure. HERF processes involve a short, sharp forming energy input usually of microsecond du- ration that is transmitted to the workpiece surface through a medium such as air or water. The resulting shockwave accelerates the workpiece to high velocity and, with its signifi - cant kinetic energy, impacts the die, which has the desired shape of the finished part. High-energy rate metal forming was studied fairly extensively as early as the 1950s. Several processes have been developed, including explosive forming and two-capacitor, discharge- based forming methods; they are electrohydraulic and electromagnetic forming . Explosive Forming.— Explosive forming is a manufacturing process that uses explosions to force sheet metal into dies. In this method, the explosive charge is located at some pre determined distance from the workpiece, and the energy is transmitted through an inter vening medium such as air, oil, or water. The maximum pressure at the workpiece may range from a few thousand psi to several hundred thousand psi, depending on the parame ters of the operation. Fig. 31 shows a typical explosive forming operation.
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