I N T R O D U C T I O N

1. Non-conventional metal-forming processes are also known as non-traditional forming processes. As these processes form parts at extremely high velocities and very high pressures, they arc also called High Energy Rate Forming (HERF) processes. The most distinguishing feature of these processes is high forming velocity rather than large energy expenditure and. therefore, these processes are also called 'high velocity forming' processes.

2. The duration of energy expenditure is short usually in micro-seconds.

3. Energy employed in HERF varies widely from 10 joules for transister enclosure to more than 1 x 109 joules.

4. Energy (a short burst) is transferred through a medium such as water or air.

5. Shock wave generated forces the work (part) into the die cavity of desired shape of finished component. The die cost is drastically reduced as the need of male die is completely eliminated.

6. Forming materials at a very high velocity make materials behave almost like a fluid and can be formed beyond their limits maintaining excellent dimensional controls. Materials which are difficult to form using conventional methods due to spring-back, undergo large amounts of plastic deformation with almost no spring-back.

7. HERF improves material density and ultimate tensile strength and produces more uniform strain throughout the part.

96

8. The following processes arc prominent in HERF or unconventional forming processes: (a) Explosive forming (b) Electromagnetic forming (c) Elcclrohydraulic forming.

9. As given in Table 4.1. the forming velocities of HERF processes are 10 to 100 times faster than the conventional process. If the forming speed of a process exceeds 150 m/s it is considered to be a part of HERF family.

TABLE 4.1 Deformation Velocities—A Comparison of Conventional and Non-conventional Forming Processes

PrOMS! Deformation velocity, m/s

Conventional processes Hydraulic pK$s/Prc$S brake 0.03 Mcch. press 0.03-0.73 Drop hammer 0.24-2.4 Gas actuated ram 2.4-82

Unconventional or HERF processes Explosive 9-228 Magnetic 27-228 EtecnohydnwUc 27-228

With this brief introduction the 'unconventional forming processes' wil l now be discussed in the following paragraphs.

4.2 EXPLOSIVE FORMING The methods used lor explosive forming as given in Figure 4.1 could be: (a) unconfmed. (b) confined, (c) direct forming using gas pressure and (d) gas actuated drop hammer.

Direct forming with fluid pressure is shown in Figure 4.1|(a) and (b» . They depict two schemes of explosive forming. In both cases, a shock wave is generated in the fluid medium by detonating an explosive charge. In a confined space the entire shock-wave energy is utilised for shaping a small component. For a large pad. formed in unconfmed space only a part of the wave front is used. I n n s the unconfined operation is less efficient hut there is a greater hazard of die failure in a confined operation due to inherent lack of control in explosive forming.

The typical explosives used include:

I . TNT (for higher energy) 2. Dynamite (for higher energy) 3. Gun powder (for lower energy)

With higher energy explosives placed just above the workpiece. pressure of upto 35,000 N/mm can be generated. Similarly for lower energy explosives, pressure is upto 350 N/mnr. In water, as a fluid medium, the peak pressure P is given by

p = cW'^/D" N/mm 2

where W m weight of ihe explosive in N D = stand -off-distance in cm n = 1.15 (typical value) <• = 4500 (for pcntolite)

= 4320 (for TNT) = 4280 (for Tetryl)

d — distance between explosive charge and free water surface (unconfined) a minimum 2D (otherwise much energy is lost)

Using various tooling set-ups, a variety of shapes can be obtained. Effect of process on material properties is similar to those in conventional forming.

(20 x 700= 14000 MPa) (a) Direct forming wilh fluid pressure (b) Bulging operation

(700 MPa) -(c) Direct forming by gas pressure (d) Gas actuated drop hammer

Figure 4.1 Explosive forming processes.

4.2.1 Gas Expanding Methods

Figure 4.1 ((c) and (d)] shows examples of expanding gas methods. Operation in Figure 4.1(d) is similar to drop hammer but much more rapid. Explosive forming is specially suitable for low quantity production runs of large parts such as in aerospace applications, pressure vessels, steel plates 25 mm thick and 3.6 m diameter have been formed by this method.

Additional comments about explosive forming are summarised as follows:

1. Explosive forming is an excellent method of using energy at a high rate because the gas pressure and rate of detonation can be controlled.

2. Both low and high pressure explosives are used for various applications. 3. Low pressure explosives called cartridge systems produce pressures uplo 700 MI'a and

the expanding gas is confined. 4. High pressure (detonating) explosives which may not be confined, produce pressures

up to 14.000 MPa (20 times those of low pressure explosives). They include dynamite, amatol. TNT (Tri-nilrololuene). RDX (cyclotrimcthylene trinilramine).

5. Explosive charges working in air or a liquid medium set-up intense shock waves in the medium hm there is decrease in intensity as the wave spreadsover more area.

6. Stand-off distance (between workpiece and explosive charge) is small for deep drawing and large for shallow.

7. Spring-back is minimum but exists. Thick metals exhibit less spring-back.

E L E C T R O M A G N E T I C F O R M I N G [. In this process the energy stored in a capacitor bank is discharged rapidly through a

magnetic coil. 2. Recent applications of this process are: embossing, blanking, forming and drawing, all

using the same power source but differently designed work coils. 3. Figure 4.2 shows a simple sketch showing how the process works. Charging voltage E

is supplied by a high voltage source into a bank of capacitors C connected in parallel.

source for £ charging

C

High voltage switch _

liank of capacitors

Coil 1

4. Amount of energy stored depends on (a) the voltage supplied and/or (b) the number of capacitors in the bank.

5. The charging is quite rapid and when complete, a high-voltage switch triggers ihe stored energy through the coils establishing a rapid high-intensity magnetic field. The intensity of this field depends upon Ihe value of the current. This magnetic field induces a current into the conductive workpiece placed in (near) the coil resulting in a force that acts on the workpiece (Flemings left hand rule) shown by small arrows in Figure 4.3. The shape of the workpiece before and after deformation is shown in Figure 4.4.

Figure 4.4 Electromagnetic forming showing vvorkpiece shape before and after (doited) deformation.

6. This force when exceeds the elastic limit of the material being formed causes permanent deformation. (Pressures are of the order of 350 MPa).

7. Since forces act on coils, they should be strong enough to withstand these forces. 8. The process could be suitably designed for compression, expansion or for forming

contours. 9. Applications include:

(a) Bulging of tubes (b) Shrinking of tubes (c) Flaring and swaging tubes over rods.

4 .4 ELECTRO-HYDRAULIC FORMING In explosive forming discussed earlier high intensity shock waves generated in water arc used to form a workpiccc. Similar effect can be obtained by discharging stored electrical energy in a bank of condensors across electrodes submerged in an electrolyte (generally water). The process is also called 'electric spark forming', 'electric discharge forming' and 'underwater spark forming' (see Figure 4.5).

The stored electrical energy is discharged either through a wire or across a gap. A voltage of 50 kV can jump a gap of 25 mm. When it occurs in water, the spark (arc) produced instantly converts water into steam. This generates high pressure shock waves, which are used to form a part.

Arrangement to remove air entrapped

Figure 4.5 Electro-hydraulic forming.

Similarly, if a potential difference of 30 kV discharges through a 1 mm diameter wire, in water, the centre of the wire is instantaneously raised to about 5100°C. The wire vaporises, resulting in the formation of bubbles (the current flow is temporarily slopped). The bubbles expand, the vapour pressure drops and an arc is then struck between the electrodes. Due to this, high pressure shock waves are generated and are used to form a workpiece. This second method is more efficient and also low voltage is needed. Typical set-ups for clectrohydraulic forming are shown in Figure 4.5.

1. Electro-hydraulic forming process is also called 'eleclrospark' or 'electric discharge forming'.

2. The process is best suited lo the production of small parts although parts up to 1.27 m dia have been produced.

3. The process is safe with low equipment cost and close control over energy rates to be used.

4. Method of discharge through a wire is more efficient than the first method.

5. The process is used to form lubular and dished shapes.

Materials wilh criiical impaci velocities less lhan 30 ni/s are noi suitable for this method of forming. The process is used for forming, bulging, beading and drawing.

The die materials 1. For small quantity production (up to say 12 pieces): Kirksite. epoxy resin, plaster of

pans provided the die is well supported 2. For large quantities: Steel.

Fields of applications 1. Forming lubular and dish shapes. Shapes impractical by conventional methods can be

made by this method. 2. Since only one die is needed, the tooling costs arc tow. Die materials arc also of low

cost. The method is specially suitable for short-run prototype production. 3. Production rates are higher than explosive forming rales. 4. Only relatively small components can be made.

4.5 EXPLOSIVE COMPACTING • I r I - '7- *,

>:->>>>>>>

Explosive charge Water medium Buffer plaics

Plunger

Die

Explosive compacting (see Figure 4.6) finds its application in powder metallurgy for compacting metal powders. During pressing of metal powders some powders are hard-to-compacl. They need extremely high pressures. Compacting such powders by an explosive charge offers certain advantages in forming a high density product: (i) This reduces sintering time as the density is already very high, ( i i ) The shrinkage of the compact during sintering is also less, (i i i) As the die designs arc simple, savings can be made on equipment cost.

Most designs that have been reported in the literature have a closed system. One or more plungers placed next to the metal powder are actuated by buffer plates against which the high explosive acts.

Another design uses water in a heavy-walled cylinder. Powders arc placed in water proof bags and put in cylinder. Hydrostatic pressure is exerted on the compacts by detonating an explosive charge at the end of the cylinder.

Figure 4.6 Principle of explosive compacting.