Microstructural study of the interface in laser-clad Ni-Al bronze on Al alloy AA333 and its relation to cracking
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INTRODUCTION
L A S E R cladding is a process where the injected powder is melted to form a molten pool on the surface of a substrate by laser illumination. The solidified clad track has metallurgical bonding to the substrate and a composition very close to the powder with a fine microstructure resulting from high cooling rates. The extended solid solution of Hf in Ni-based superalloys, m21 improved wear resistance in Fe-Cr-Mn-C alloys, t31 hightemperature oxidation resistance in Ni-Cr-AI-Hf alloys, 141 and high-temperature wear resistance by cladding Ni alloy on A! alloy I51 have been obtained by this technique. Recently, systematic research has been conducted for laser cladding Ni-A1 bronze on AI alloy AA333.161 The microstructure of the cladding has been intensively studied by microdiffraction (MD), 17] convergent beam electron diffraction, and high-resolution electron microscopy, ts,9] The two materials were chosen because the Ni-A1 bronze has a good wear resistance up to a temperature of 250 ~ and is easily machined, while A1 alloy AA333 has a low density and good casting properties. In laser cladding, melting a thin layer of the substrate is necessary to form a metallurgical bonding Y. LIU, formerly Visiting Research Associate, Department of Mechanical and Industrial Engineering, Center for Laser Aided Material Processing, University of Illinois at Urbana-Champaign, Urbana, IL 61801, is Research Assistant Professor, with the Department of Mechanical Engineering, University of Nebraska at Lincoln, Lincoln, NE 68588-0656. J. MAZUMDER, Professor, is with the Department of Mechanical and Industrial Engineering, Center for Laser Aided Materials Processing, University of Illinois at Urbana-Champaign, Urbana, IL 61801. K. SHIBATA, Senior Scientist, is with Nissan Research Center, 1, Natsushima-Cho, Yokosuka, 237 Japan. Manuscript submitted February 8, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
between the clad and the substrate. There are two undesirable situations: one situation is that the clad is not metallurgically bonded to the substrate and that no continuous interface is formed, and the other is that a large amount of the substrate is melted to cause dilution to the clad. Large dilution resulting from melting of the substrate is not desirable because the properties of the clad may degrade due to composition change. Also, intermetallic compounds may form at the interface and make the interface brittle as will be shown in this study. In a good clad, the amount of the substrate melted is just enough to create a continuous interface between the clad and the substrate. The effects of processing parameters on clad formation of Ni-AI bronze on A1 alloy AA333 were presented in an earlier article, t61 However, the significant difference in melting temperature between the cladding material Ni-A1 bronze (melting point = 1063 ~ and the substrate A1 alloy AA333 (melting point = 577 ~ creates a strong tendency toward large dilution. Large dilution can cause cracking at the interface in the following cases: (1) c
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