Introduction Of Electro Hydraulic Forming (EHF) | Advantages And Application
Introduction Of Electro Hydraulic Forming | Advantages And Application
Electro Hydraulic Forming is type of high energy rate forming processes
Electrohydraulic forming is a type of metal forming in which an electric arc discharge in liquid is used to convert electrical energy to mechanical energy and change the shape of the workpiece. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short distance apart while submerged in a fluid (water or oil). The electric arc discharge rapidly vaporizes the surrounding fluid creating a shock wave. The workpiece, which is kept in contact with the fluid, is deformed into an evacuated die.
HIGH ENERGY RATE FORMING PROCESSES
The parts are formed at a rapid rate, and thus these processes are also called high – velocity forming processes. There are several advantages of using these forming processes, like die costs are low, easy maintenance of tolerances, possibility of forming most metals, and material does not show spring-back effect. The production cost of components by such processes is low. The limitation of these processes is the need for skilled personnel.
There are three main high energy rate forming processes: explosive forming, magnetic forming, and electro hydraulic forming. We shall discuss these processes.
Electro Hydraulic Forming
Electro hydraulic forming (EHF), also known as electro spark forming, is a process in which electrical energy is converted into mechanical energy for the forming of metallic parts. A bank of capacitors is first charged to a high voltage and then discharged across a gap between two electrodes, causing explosions inside the hollow work piece, which is filled with some suitable medium, generally water. These explosions produce shock waves that travel radially in all directions at high velocity until they meet some obstruction. If the discharge energy is sufficiently high, the hollow work piece is deformed.
- A sudden electrical discharge in the form of sparks is produced between electrodes and this discharge produces a shock wave in the water medium. This shock wave deforms the work plate and collapses it into the die.
- The characteristics of this process are similar to those of explosive forming. The major difference, however, is that a chemical explosive is replaced by a capacitor bank, which stores the electrical energy.
- The capacitor is charged through a charging circuit. When the switch is closed, a spark is produced between electrodes and a shock wave or pressure pulse is created. The energy released is much lesser than that released in explosive forming.
i) Stand off distance: It must be optimum.
ii) Capacitor used: The energy of the pressure pulse depends on the size of capacitor.
iii) Transfer medium: Usually water is used.
iv) Vacuum: the die cavity must be evacuated to prevent adiabatic heating of the work due to a sudden compression of air.
v) Material properties with regard to the application of high rates of strain.
i) Suitable only for smaller works
ii) Need for vacuum makes the equipment more complicated.
iii) Proper SOD is necessary for effective process.
Advantages of EHF
- A single-step process (rather than progressive stamping)
- Fine details and sharp lines can be easily formed
- Forming of male and female shapes (negative and positive)
- Only a single one-sided die is required
- Enables extremely deep forming (much more than is possible with conventional stamping)
- Even distribution and higher strength of thin material
- Extremely fast
- Equipment has small footprint
- No need for a press – the forming chamber is a self-balanced system
- Allows forming of parts up to a few square meters in size.
Accuracy of parts produced
Accuracy of electro hydraulically formed parts depends on the control of both the magnitude and location of energy discharges and on the dimensional accuracy of the dies used. With the modern equipment, it is now possible to precisely control the energy within specified limits, therefore the primary factor is the dimensional accuracy of the die. External dimensions on tubular parts are possible to achieve within ± 0.05 mm with the current state of technology.
They include smaller radar dish, cone and other shapes in thinner and small works.