Explosively driven impact welding is a true example of multidisciplinary research as the phenomena associated with it fall under the various branches of engineering science. A great deal of the work in, and collaboration between various specialised fields have been expended on the subject. However, a comprehensive quantitative theory capable of giving an accurate description and prediction of the parameters and of the characteristic features of explosively welded components does not exist. Most of the investigators considered the welding process as a solid state welding process, but some believed that the process is a fusion welding process. Interfacial waves are the most discussed aspect of explosive welding. The presence of jet in the collision region, and the transient fluid-like behaviour under high pressure have led many investigators to seek an explanation and a characterisation of these waves in terms of a flow mechanics of one kind or another. In this study, part of the welding process was numerically analysed. A finite difference engineering package was used to model the oblique impact of a thin flyer plate on a relatively thick base. The results were validated by data from carefully controlled experiments using a pneumatic gun. Straight and wavy interfaces and jetting phenomena were modelled, and the magnitude of the waves and the velocity of jet predicted. The numerical analysis predicted a hump ahead of the collision point. Wave formation appears to be the result of variations in the velocity distribution at the collision point and periodic disturbances of the materials. Higher values of plastic strain were predicted in wavy interfaces. Bonding was found to be a solid state welding process. Phase changes which occur may be due to high temperatures (but less than the melting temperature) at the collision point.

Numerical and experimental studies of the mechanism of the wavy interface formations in explosive/impact welding

Results of experiments in which the collision variables were carefully controlled showed that bond zone wave formation during explosion cladding is analogous to the formation of vortex streets in fluid flow around an obstacle or in the collision of liquid streams. The fluid flow analogy explains the observed transition from a smooth metal-to-metal bond zone to a wavy bond zone above a critical collision velocity. This model is capable of predicting the minimum collision velocity necessary for bond zone wave formation in different metal systems and it also predicts correctly the strong dependence of wave size on collision angle. The magnitude of the wave size agrees with that predicted from fluid flow past a flat plate. Two other mechanisms of bond zone wave formation were explored experimentally and found to be invalid.

Mechanism of bond zone wave formation in explosion-clad metals

The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength ≈ 0.5 mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size ≈ 100–200 μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe 9.64 Ti 0.36 . Most of the observed phases do not appear in the equilibrium Fe–Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100–300 nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound.

Hierarchical microstructure of explosive joints: Example of titanium to steel cladding

Application of high velocity impact welding at varied different length scales

Magnetic pulse welding: An efficient and environmentally friendly multi-material joining technique

Strong bonds can be produced by the explosive welding process and usually the weld interface has a characteristic wavy form. In this paper the mechanism of explosive welding is discussed, the present theories of wave formation are critically examined and a new mechanism of wave formation is proposed. According to this mechanism the materials of the impacting plates in the region of collision behave in a similar manner to liquids of low viscosity. As a result the impacting or flyer plate divides into a re-entrant jet and a salient jet. Very high pressure is produced at the stagnation point of the divided jet. The parent plate deforms under the stagnation point and consequently a hump is formed in the parent plate ahead of the point of collision. The hump builds up and eventually traps the re-entrant jet. The stagnation point then transfers to the top of the hump and then descends it and starts forming a new hump, and in this manner successive waves are formed. The proposed mechanism of wave formation seems to explain the experimentally observed behaviour reasonably well. Furthermore, experiments are reported in which one of the surfaces to be bonded was copper plated and the second surface was nickel plated and by this means the movements of the surfaces being bonded were traced. These experiments gave strong support to the proposed theory of wave formation.

A. S. Bahrani

Papers

The mechanics of wave formation in explosive welding