Transmission electron microscopy study of martensites in laser-clad ni-ai bronze on aluminum alloy AA333
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I.
INTRODUCTION
RECENT advances in laser technology have drawn increasing interest in materials processing and manufacturing. The high-energy density input of the order of 500 W / m m 2 can create a molten pool on the surface of a metal upon laser illumination. Laser cladding is a process where a stream of powder is injected into a lasergenerated molten pool to form a rapidly solidified clad layer with minimum dilution. Detailed descriptions of the laser cladding process are available in earlier publications.[~.2] Two major applications of laser cladding in material processing are as follows: (1) the ability to produce an extremely fine microstructure with extended solubility of certain desired elements ~1'2'3J and (2) metallurgical bonding of two kinds of materials which possess substantially different properties. Recently, we succeeded in cladding Ni-A1 bronze on AI alloy AA333 which, as a composite material, can extend the use of aluminum alloys for mechanical components. Evaluation of the clad by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) has shown that the composition in the clad is macroscopically uniform, while in the interface, the composition changes drastically with position. One outstanding feature of the laser cladding process is the high cooling rate. The resultant materials are usually metastable and cannot be described by the equilibrium-phase diagram. The high cooling r a t e (104 to 106 K/s) may lead to unique nonequilibrium microstructures and possibly to crystal structures different from those predicted from the equilibrium-phase diagrams. Y. LIU, Visiting Research Associate, and J. MAZUMDER, Professor, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. K. SHIBATA, is with Central Engineering Laboratories, Nissan Motor Company, Ltd., Natsushima-cho, Yokosuka 237, Japan. Manuscript submitted November 25, 1991.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The mechanical properties of the clad are directly related to the crystal structure of each phase in the clad. The microstructure and crystal structure determination will give a basic understanding of the stability and mechanical behavior of the clad region. To the best of the authors' knowledge, no data were reported for the present cladding and substrate materials. The present article describes the SEM and transmission electron microscopy (TEM) investigation on the microstructure and crystal structure of Ni-A1 bronze on AI alloy AA333, while the results on the interface region that was created between the clad and the substrate are presented in another article. [4]
II.
EXPERIMENTAL PROCEDURE
Compositions for the cladding and substrate materials are listed in Table I. An AVCO HPL 10 kW continuous wave CO~ laser was used as the energy source. The powder was delivered by a pneumatic powder delivery system with a feed screw. A copper nozzle 3 mm in diameter and 22-mm long was used for powder injection with helium gas as a medium.
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