Vaporizing foil actuator used for impulse forming and embossing of titanium and aluminum alloys

Electrically driven rapid vaporization of thin conductors is known to produce short-duration pressure pulses of high magnitude. This impulse can be used for applications such as high strain rate forming, shearing, collision welding, and springback calibration. Mechanical impulse was developed from aluminum foils of various thicknesses, which were vaporized using a capacitor bank discharge with a maximum charging voltage of 8.6 kV. Peak current was delivered on the order of 100 kA with rise times of about 12 μs. In this work, polyurethane was used as a medium to transfer pressure from the aluminum foil vaporization zone to the workpiece. Fundamental experiments, where AA 3003-H14 aluminum alloy was formed into perforated plates, show that for a given foil thickness, a limit existed over which supplying higher electrical energy from a given capacitor bank did not necessarily result in higher pressure. The magnitude of generated pressure was proportional to the excess Joule heat deposited into the foil before it burst. Although the polyurethane layer helped spread the pressure pulse over a larger area, the resulting pressure distribution remained heterogeneous. Practical applications, such as forming into cavities and embossing into shallow dies, were possible with this method. Sheets of 0.508 mm thick commercially pure titanium were nearly fully formed into a cellphone case die using a hybrid process that combined a quasistatic pre-forming step with a vaporizing foil forming step. Sheets of 0.508 mm thick AA 2024-T3 aluminum alloy were embossed into a die with features of varying depths. Aluminum foils with straight and curved active sections were used as actuators. The curved-section foils resulted in higher conformation of the workpiece to the die in the center region, while the straight-section foils produced better conformity to the die features on the ends.