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explosive forming[ik′splō·siv ′fȯr·miŋ]
a method of forming metals, mainly sheet metals, in which pressure is created by the force of the blast of a high explosive, powder, or gaseous mixture through a transmitting medium. Explosive forming differs from normal stamping in the momentary application (for several milliseconds or microseconds) of high stresses that greatly exceed the plastic limit of the metal being formed. The quality of the finished article in terms of accuracy and physical and mechanical properties is not less than, and often exceeds, that of articles stamped on presses.
Explosive forming was first proposed at the Kharkov Aviation Institute in the 1940’s and was widely used in the mid-1950’s in the production of oversize parts for rockets and airplanes. Explosive forming may be carried out in several ways: by means of a liquid transmitting medium (usually water) or a gaseous medium, in a rarefied gas atmosphere, or in a vacuum chamber. The materials used for stamps (dies) for small-scale production of parts by explosive forming include mild steels, aluminum, zinc, plastics, and reinforced concrete; for large production runs, the dies are made from ordinary die and tool steels. The simplest explosive forming equipment consists of a metal-lined reinforced-concrete basin set in the ground and filled with water. The die, with the explosive charge above it, is immersed in the water, and the explosion is set off.
Explosive forming in basins has a number of disadvantages that prevent its wide use: it is necessary each time either to lower a die weighing many tons into the water or to evacuate the water from the basin and then refill it; ground tremors and the spillage of water owing to the force of the explosion make explosion forming in buildings difficult and usually make it necessary to carry it out at open-air sites. Basinless explosive forming, which is carried out in mobile or stationary chambers, does not suffer from these disadvantages; there is water only between the explosive charge and the blank, and the remainder of the reinforced chamber is filled with air, which significantly reduces the shock wave. A flat charge of a high explosive substance is placed in a miniature basin containing water and fitted with a retaining ring. There are cutouts in the end walls of a reinforced chamber; at the moment of the explosion, these are closed by fixed walls attached to the foundation by buttresses. The cutouts in the end walls make it possible to use a reinforced chamber at two or more work stations, thereby saving shop space. Basinless explosive forming is a promising process: the preparation of parts requires ten times less labor than with press stamping, capital expenditure is 20 times less, and set-up time is sharply reduced. Explosive forming of a part may be carried out by using large charges in a single explosion (so-called single-shot explosive forming) or by using a series of small charges (multishot explosive forming). Multishot explosive forming is sometimes carried out automatically (the charges are fed in from a special supply bunker).
REFERENCESPikhtovnikov, R. V., and V. I. Zav’ialova. Shtampovka listovogo metalla vzryvom. Moscow, 1964.
Stepanov, V. G., and I. A. Shavrov. Impul’snaia metalloobrabotka v sudovom mashinostroenii. Leningrad, 1968.
R. V. PIKHTOVNIKOV
The shaping or modifying of metals by means of explosions. The explosives may be of either the detonating or deflagrating type. Explosive gas mixtures or stored gas at high pressure may also provide the motive power.
Cold welds can be made between dissimilar metals by driving the two parts together under explosive impact. In other applications of explosive-forming methods, powders are pressed into solid billets. In a different application, high explosives are used to cut large blocks of metal and even to split thin sheets into two layers of exactly one-half the original thickness. Explosives can also be employed to extrude metal shapes and to punch hard metals with the aid of dies. Shapes produced explosively are very exact and free from the fine cracks that sometimes result when pressure is applied slowly. See Metal forming