There are a number of processes, described below, which may be called unconventional forming processes or high energy rate forming processes. In these processes, unconventional sources of energy such as Explosive Forming, Electro-Hydraulic Forming, Magnetic Pulse Forming, Petro-Forge and Drop Hammer and Dynapak etc., are used. The release of energy is at extremely high rate and hence the name high energy rate forming.
In these processes, explosives are used as source of energy. The blank is kept over a die which is kept under water (Fig. 2.23). The space between the die and the blank is connected to a vacuum pump and evacuated. An explosive charge placed at a small distance from the upper surface of work piece and still in water, is exploded. The pressure waves produced in water deform the blank to the die shape.
The process is generally used for forming big components such as ends plates of pressure vessels, which cannot be manufactured by conventional machines because of their big size. Smaller components may also be made when the requirement is so small that it is not economical to manufacture them by conventional dies and tools.
In explosive forming the component size may vary from 50 mm to 5000 mm diameter and the sheet thickness may vary from 0.5 mm to 30 mm. The main advantage of the process is that very little investment on equipment is required. However safety norms have to be very strict and only those well trained in handling explosives should do the set up and firing.
Explosive gas mixtures may be used for decreasing the rate of forming or for forming small components which need less energy. A mixture of fuel gas and oxygen is detonated in a closed chamber over the work piece which is clamped over a die (Fig. 2.24). The space below the work piece is evacuated. The main benefit of the process is the saving on tooling cost.
The process is illustrated in Fig. 2.25. It makes use of discharge of large amount of electric energy stored in a capacitor bank. The discharge is carried out between two electrodes immersed in water and placed at a gap. A thin fuse wire is fixed between the electrodes. Due to heavy discharge the wire explodes and shock waves are produced, which deform the work piece.
The process, illustrated in Fig. 2.26, is carried out with the help of a magnetic pulse. A high rate of change of magnetic field is created by discharging a heavy current through a coil suitably placed with respect to the work piece. The eddy currents produced in the metal work piece produce an opposing magnetic field.
As a result of reaction of the two magnetic fields the work piece experiences a repulsive force which pushes the work piece at a high speed against the die. The process is generally used to form components from thin metal sheets or to deform thin tubes or to carry out an assembly.
The application of the process are (a) two tubes may be joined together by pressing their ends on to a connecting plug, (b) ends of a tube may be pressed on to a plug to make a vessel and manufacture of corrugated sheet disc, etc. These are illustrated in Fig. 2.27.
The technique is also being used in combination with the conventional sheet metal forming tools to create fine details on the formed components. The magnetic coils are imbedded in the conventional tools and magnetic forming is carried out after the bulk forming of the component by the conventional dies is over. The process may also be used as an aid to achieve higher drawability in sheet metal forming.
Petro-forge was developed by Tobias at university of Birmingham. It makes use of a mixture of fuel gas and oxygen which is ignited by a spark plug. The process, illustrated in Fig. 2.28, is similar to the one that takes place in an internal combustion engine.
The difference being that in Petro-forge the piston is brought up again for the next stroke with the help of compressed air while in the engine it is done by rotation of crank shaft. For each stroke, gases are filled and ignited. The piston speeds achieved are in the range of 18-20 m/sec.
Petro-forge may be used for high speed forging, for bar cropping, for hot forging of gears, metal powder compaction, sheet blanking etc. However, the machine is very noisy, which is a major reason for not being adapted by industry.
High velocity impact may also be produced by dropping a mass from a height. This has been used in drop hammers. Figure 2.29 shows one such device in which a large mass is dropped from a height of 4 to 10 meters.
The mass may make a direct impact on the work piece or strike a piston placed over a column of water, at the bottom of which the work piece to be deformed is kept. The pressure wave generated by the impact deforms the component. The process has generally been used to study metal response to high strain rate.
Dynapak machine makes use of compressed air as the source of energy. The machine consists of two cylinders i.e., (i) a low pressure cylinder and (ii) a high pressure cylinder (Fig. 2.30). The two are connected by a small diameter hole. The piston is pushed up by low pressure air and the interconnecting hole gets closed. The high pressure cylinder is then filled up.
The forces on the two sides of piston are balanced because high pressure acts on a small diameter (hole) while low pressure acts on the entire area on lower side of piston. The stroke may be started by exhausting the low pressure side to atmosphere. The energy of high pressure air gets converted into kinetic energy of piston. The piston speeds up to 20 m/sec may be achieved. The component to be deformed is kept on an anvil below the hammer.