Unit-5 Unconventional Manufacturing Processes

INTRODUCTION 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 10 joules. 9

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 (c) Elcclrohydraulic forming.

(b) Electromagnetic 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 Mcch. press Drop hammer Gas actuated ram

0.03 0.03-0.73 0.24-2.4 2.4-82

Unconventional or HERF processes Explosive Magnetic EtecnohydnwUc

9-228 27-228 27-228

With this brief introduction the 'unconventional forming processes' will 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 . T N T (for higher energy) 3. Gun powder (for lower energy)

2. Dynamite (for higher 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 / m n r . 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) :->>>>>>>

Explosive charge Water medium Buffer plaics

Plunger Die

Figure 4.6

Principle of explosive compacting.

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.