Fundamental aspects of formation and stability of explosive welds
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I.
BACKGROUND
D U R I N G recent decades, the influence of various practical conditions on explosive welding has been extensively explored. ~'2 Different metal combinations, geometrical arrangements, detonation parameters, etc. have been studied in order to find practical criteria of weldahility and of high interfacial bond strength. 3,4 Methods of testing weld strengths have been devised ~ and welds have been investigated by metallographical means to determine the microstructure of the resulting interfaces. 5'6'7 Various continuum models aiming to describe the process of explosive welding have been proposed, 4,8-" and recently the physical process has been simulated by a computer model using the finite-difference technique. Jz Up to now, however, little attention has been paid to the fundamental physical conditions of interfacial bonding in explosive welds. In an idealized situation, when two perfectly clean metal surfaces are forced together in vacuum, some metal combinations "mate" more willingly than others. These differences in mating tendency are due to several reasons. (i) Compatibility effects due to nonmatching crystal structures, lattice parameters, and/or crystallographic orientations at the interface. (ii) Compatibility effects due to different electronic properties of the two metals. This part deals with the affinity for chemical bonding between the two materials. (iii) Other surface or interface effects which may occur, e . g . , interface alloying and interface segregation of alloying elements or other diffusion effects. When two metal surfaces are joined under the above ideal conditions, the lowering of the system energy caused by the elimination of the two free surface energies (surface "tensions") is normally quite enough to cause bonding between the metals. The factors i-iii will then tend either to strengthen the bonding by further lowering the system energy or weaken it by raising the system energy. /k. OBERG, formerly with Institute of Technology, Uppsala University, Sweden, is now with ASEA AB, V[isterhs, Sweden. N. MARTENSSON is Assistant Professor at the Institute of Physics, Uppsala University, Sweden. J-A SCHWEITZ is Professor at the Institute of Technology, Uppsala University, Sweden. Manuscript submitted July 16, 1984.
METALLURGICALTRANSACTIONS A
Usually, however, joint formation does not take place under such ideal conditions. In practice, the presence of various oxides or surface precipitates, rough surface topography, atmospheric influences, molten surface layers, and other disturbances will mask the effects of i-iii. In the case of explosive welding, on the other hand, most of these disturbances are not present. The joining of the two metals occurs simultaneously to the removing of the precipitated surface layers by a re-entrant material jet (Figure 1). Hence, it is not the original surfaces that are joined together, but the fresh, unaffected material below the original surface layers. The latter will effectively protect the materials to be joined from external influence until t
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