Machinery's Handbook, 31st Edition
1440 High-Energy Rate Metal Forming Processes The workpiece is clamped and sealed over the die cavity. A vacuum is then created in the die cavity. The die assembly is put together at the bottom of the tank. The explosive charge is placed in the intervening medium at a certain distance above the workpiece. After the detonation of the charge, the liquid buffer is instantaneously converted from a fluid of low density, low temperature, and low pressure to a fluid of high density, high temperature, and high pressure, causing the rapid forming of the blank into the cavity die. The extremely high forming velocity of explosive forming minimizes material springback, but it does not completely eliminate it. If the relative elongation of material is more than 10 percent, it can be formed without the chance of fracturing. The distance between the charge and the blank is called the “standoff distance.” The standoff distance and the amount of charge determine the amount of pressure transmitted to the workpiece. Other factors, such as the explosive type, the explosive’s shape, and the type of buffer medium, also affect the pressure. An understanding of the compression waves and rarefactions developed in detonation is extremely important in predicting the forming metal’s reaction. In the aviation and aero space industries, the explosive method of forming has been used since the 1980s. Explosive: Explosives are substances that undergo rapid chemical reaction, during which heat and large quantities of gaseous products are evolved. Explosives can be solid (TNT-trinitro toluene), liquid (nitroglycerine), or gaseous (oxygen-acetylene mixtures). Die Material: Different materials are used for the explosive forming process. For instance: kirksite and fiberglass are used for low pressure and few parts; epoxy and con crete are used for low pressure and larger numbers of parts; and ductile iron, tool steel, and cast steel are used for high pressure and many parts.
Fig. 31. Explosive Forming Set-Up Explosive forming has the following characteristic features: a) very large sheets with relatively complex shapes, although usually axisymmetric; b) low tooling costs, but high labor costs; c) long cycle times; d) suitability for low-quantity production; e) maintenance of precise tolerances; and controllable smoothness of contours. An exceptionally interesting development in the field of explosive method forming is the fabrication of spherical pressure vessels with diameters up to 13 ft, and thickness from 0.12 to 0.79 in. of high strength steel designed for water or oil storage in the chemical industry, and other uses. Electromagnetic Forming.— Electromagnetic metal forming (EMF) is a high-energy rate cold forming process that can deform metal workpieces without contact. This process uses ultrastrong pulsed magnetic fields to form metal parts rapidly. A capacitor bank is suddenly discharged by means of a switch through a coil in which the workpiece is placed. Basically, whenever an electrical current is rapidly sent through an electrical conductor, it will develop a magnetic field. This change in magnetic field will induce eddy currents in the workpiece that generally run in a direction opposite to the current in the coil (Lenz’s
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online