Metallography of explosive welds

Abstract

Explosive welding is one of the industrially important techniques employing high energies derived from explosives. This technique produces sound metallurgical bonding between a wide variety of similar as well as dissimilar metals and alloys like aluminium, brass, stainless steel, invar, titanium, etc. which are not normally weldable by conventional methods. In this process a weighed amount of explosive is spread over a protecting plate placed on the cladding plate, which is spot-welded at a certain angle to the base plate. The included angle between the cladding and base plates and the amount of explosive used are critical for obtaining sound welds. These weld composites successfully withstand the normal metal-working processes. They find extensive use in bimetal industry, coinage, chemical process equipment, pressure vessel manufacture, rocket technology, etc. Collapse of the cladding plates during explosion generates a pressure wave moving ahead of the collision front and the material forming the colliding surfaces flows forward and is ejected in the form of a jet. Thus the surface jetting normally associated with this process gives rise to wavy interface as noticed in a number of weld combinations. High pressures are generated at the collision front resulting in localised severe plastic deformation. In order to develop sound explosive welds with good shear resistance, it is essential to understand the mechanism of bond formation between the two component metals and also the nature and composition of the interface regions. Techniques like optical and electron metallography, hot-stage microscopy, electron probe microanalysis, microhardness testing, etc. have been employed to investigate the metallurgical aspects of the explosive welds. Bond interface and the adjoining regions reveal many unusual microstructuralfeatures like complex pattern of plastic flow, formation of solidified melt zones, presence of shock-twins, entrapment of metal pockets, etc. Columnar grain structures, characteristic of cast metals, within the mixture zones indicate localised melting and subsequent rapid solidification starting from either side. High microhardness values of these zones could possibly be attributed to the high rates of cooling bringing in large amounts of microstresses and also possible leading to the formation of some non-equilibrium phases similar to those obtained in liquisol-quenched samples. Sets of fine recrystallized grains noticed at regions adjoining the interface indicate the presence of heat-affected zones. Hot stage microscopic examination of mild steel-stainless steel explosive welds shows features like the formation of parallel banded contours, presence of substructure within the twin bands, formation of decarburised layer in mild steel, diffusion of elements across the interface, etc. Electron probe analysis has also been used for the study of these welds by scanning the bond zones for elemental distribution and also by conducting point analysis for their composition. These studies generally indicate an alloy series midway between the two component metals. However, chance combinations of elements approximating to the composition of some intermetallic compounds could also occur. The normal diffusion does not seem to be operative in the explosive welding process, as sharp changes in concentrations have been reported in almost all the weld combinations. A number of explosive welds have also been investigated at our Defence Metallurgical Research Laboratory; some of the results of which are incorporated in this paper.