Microstructural modifications in an explosively welded Ti/Ti clad material: I. Bonding interface
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I.
INTRODUCTION
T H E application o f clad materials has been widely spread in various engineering fields and industries, such as chemical, petroleum, p o w e r plant, and shipbuilding industries. [lj Among clad materials, the cladding o f titanium to other metals and alloys has been drawing particular interest in the connection with corrosion and heat resistance o f the titanium. The industrially used cladding processes are hot-rolling, diffusion bonding, overlay welding, enshrouding casting, and explosive welding, as have been reviewed by Kawanami.[11 Any one o f these processes has both advantages and disadvantages. One o f the positive aspects o f explosive welding is the capability o f formation o f an intimate contact between two materials that cannot be bonded with conventional methods. [2,3] The principle o f explosive welding is briefly summarized as follows, t2,3] The top (flyer) plate is set parallel (constant standoff) or obliquely to the bottom (parent) one with appropriate standoff distance. The latter configuration is the preset standoff angle configuration, which was used in the present study, as illustrated in Figure 1. Explosives are put on the upper surface o f the flyer plate. As the explosive is initiated with the electrical detonator, the flyer plate is drastically accelerated by the detonation pressure and flies with high speed toward the parent plate. The standoff provides the distance across w h i c h the flyer plate can be accelerated at the necessary impact velocity. This is the most important reason for the initial standoff. Since this flight occurs with the progress of the detonation wave from the priming side to the opposite one, the dynamic bend angle is related to the flight and the detonation speed. Subsequently, the flyer plate collides with the bottom plate, and then a jet o f metal is formed M . NISHIDA, Associate Professor, A. CHIBA, Professor, and K. IMAMURA, Technician, are with the Department of Materials Science and Resource Engineering, Faculty of Engineering, Kumamoto University, Kumamoto 8 6 0 , Japan. H. M I N A T O and J. SHUDO, formerly Graduate Students, Department of Materials Science and Resource Engineering, Kumamoto University, are Research Engineers, the Nippon Steel Corporation, Kitakyushu 8 0 5 , Japan, and Daido Special Steel Corporation, Nagoya 4 7 6 , Japan, respectively. Manuscript submitted April 2 2 , 1992. METALLURGICAL TRANSACTIONS A
at the apex o f the collision, w h i c h removes the oxide l a y e r and other impurities on both surfaces to be welded. The standoff also provides an unobstructed exit path f o r the free jet and air between the plates. This is the second reason for the initial standoff. At the collision point, fresh metal surfaces enter with high impact into intimate contact and metallurgical bonding is completed along the interface within a few microseconds. Therefore, explosive welding is useful in the joining o f such active metals as titanium, w h i c h is easily oxidized. It is well known that the bonding interface has a wavy shap
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