High Energy Rate Forming (HERF) involves a very hard order of the rate of energy flow for a very short interval of time. HERF is based on the principle that the kinetic energy of a moving body is proportional to the square of its velocity, and therefore, a significant amount of energy can be supplied by a relatively smaller body moving at high speed.
The following are three common High-Energy Rate Forming processes:
- Explosive Forming: In explosive forming, the metal sheet is clamped over an evacuated die and the whole assembly is kept in a tank confined within a fluid medium (e.g. water). A shock wave in the fluid medium is generated by detonating an explosive charge placed at a certain distance from the workpiece in the confined space. Typical explosives include trinitrotoluene (TNT) and dynamite for higher energy, and gun powder for lower energy. The pressure of the shock is sufficiently high to form the metal into the die cavity. There is a greater hazard of die failure in the confined operation due to the inevitable lack of control in explosives forming.
- Electro-Hydraulic Forming: Electric discharge in the form of sparks, instead of explosives, can also be used to generate a shock wave in a fluid. An operation using this principle of generating a shock wave is called electro-hydraulic forming. The process is also called underwater spark or electric-discharge forming. In this process, a capacitor bank is charged through the charging circuit; subsequently, the switch is closed, resulting in a spark within the electrode gap to discharge the capacitors.
- Electro-Magnetic Forming: In electro-magnetic forming, electrical energy is first stored in a capacitor bank. It is then discharged through a magnetic coil by closing the switch. The coil produces a magnetic field whose intensity depends on the value of the current. Since the metallic workpiece is in this dynamic magnetic field, a current is included in the job which sets up its magnetic field through eddy currents. The forces produced by the two magnetic fields result in a net repelling force between the coil and sheet, which forms the workpiece into the die. The workpiece in the process has to be electrically conductive but need not be magnetic.
- Production rates are higher for High-Energy Rate Forming process is high, as parts are made at a rapid rate and thus these process are also called high velocity forming process.
- Die costs are relatively lower compared to other processes.
- The versatility of the process – it is possible to form most metals including difficult to form metals.
- No or minimum spring back effect on the material after the HERF process.
- Production cost is low as power hammer (or press) is removed in the process.
- Complex shapes / profiles can be made much easily, as compared to conventional forming process.
- The required final shape/ dimensions of the product are obtained in a single step, thus eliminating intermediate forming steps and preforming dies.
- Suitable for a range of production volume such as small numbers, batches or mass production.
- HERF process is not suitable for highly brittle materials.
- Source of energy (chemical explosive or electrical) must be handled carefully.
- Highly skilled personnel are required for the HERF process from design to execution.
- Dies need to be much bigger in size to withstand high energy rates and shocks and to prevent cracking.
- In ship building – to form large parts (up to 25 mm thick).
- Bending thick tubes or pipes (up to 25 mm thick).
- Radar dish
- Elliptical domes used in space applications.
- Crimping of metal strips.
- The cladding of two large plates of dissimilar metals.