Page 14 - MetalForming-Dec-2018-issue

Tooling by Design
Container Water
Ring clamp
Sheet- metal
To power supply
Electrodes
Thin metal wire
Die cavity
Vacuum
than the SPF process.
With warm forming, the aluminum blank, and sometimes
the die, heats to 200 to 300 C before stamping. The heating cycle results in longer cycle times, while the electrical heating rods in the dies and/or blankholder require more complex tooling and insulation.
Further development work is required to establish robust design guidelines for warm forming, due to its complexity.
Electrohydraulic (Explosive) Forming
A high-energy-rate method used primarily in the pro- duction of aluminum alloy parts for aerospace, electro- hydraulic forming or explosive forming produces parts that exceed the size limits of conventional stamping equip- ment, or when forming pressures are unobtainable with conventional methods due to alloy strength and/or sheet thickness.
Unlike SPF, this process can be used with any aluminum alloy, with AA1100-O recognized as the most formable by explosive forming (ASM Specialty Handbook: Aluminum and Aluminum Alloys).
Cast iron and cast steel are commonly used for die mate- rials. Only a die cavity is required because the underwater shock wave generated during explosive forming acts as the punch (Fig. 3). Prior to detonation, the air between the sheetmetal and die cavity must be evacuated. So great is
Fig. 3—Electrohydraulic (explosive) forming
the forming speed in explosive-forming operations that air trapped between the workpiece and the die can prevent the workpiece from reaching the targeted die surface.
Deformation velocities are known to exceed 100 m/s compared to approximately 6 m/s maximum for a conven- tional stamping process. The high-pressure levels in explosive forming can substantially increase the yield and tensile strength of the workpiece when critical forming velocities are exceeded.
High-energy and high-velocity forming of aluminum alloys is not limited exclusively to explosive forming. Other methods such as electrohydraulic and electromagnetic form- ing also have been used successfully, on a limited basis (G. Daehn, Ohio State University). MF
12 MetalForming/December 2018
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