Toward prediction of microstructural evolution during laser surface alloying
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I. INTRODUCTION
LASER surface alloying (LSA) is a versatile process, through which one can modify the surface properties of metals and ceramics to obtain required properties. The properties of the surface are modified by localized melting and solidification. The LSA process alters the solidified microstructure by alloy-induced transformations, composite strengthening through production of second-phase particles, or a combination of both.[1–5] A review of the literature shows that a variety of applications have been developed for LSA processes. The most common uses are for improving resistance to corrosion, abrasion, erosion, oxidation, and wear.[6–13] Applications include modification of surface properties for medical applications.[14] In other cases, rapid cooling rates were used to produce metallic glasses.[15] The properties of the LSA region depend on the microstructure which evolves during melting and cooling. One of the most common microstructural changes is the dissolution of primary phases in powders, leading to precipitation of complex phases during melting and subsequent solidification. For example, during LSA of nickel-based alloys with silicon carbide particles, the particles melt in the liquid pool, and, during subsequent solidification, M7C3 carbides* precipitate *Here, the letter “M” corresponds to the metal content, including Fe, Cr, and other alloying additions.
from the liquid.[16] Previous work has attempted to control the LSA microstructure by modifying the laser beam parameters and by changing the alloying additions through extensive experimental research.[17,18] Adding alloying elements to the molten pool modifies the liquid pool composition. The alloying elements are added either by preplacing powders on the substrate or by injecting them into the trailing edge of the molten pool. Some processes combine both methodologies. The major alloying elements are preplaced on the sample, and secondary alloying elements or particles are added to
S.S. BABU, R&D Staff Member, and S.A. DAVID, Group Leader and Corporate Fellow, are with the M&C Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6095. Contact e-mail: [email protected]. R.P. MARTUKANITZ, Assistant Director, is with the Laser Processing Division, Applied Research Laboratory, Pennsylvania State University, University Park, PA 16802. K.D. PARKS, formerly Research Staff Member, with the Laser Processing Division, Applied Research Laboratory, is President & CEO, Duncan Parks Enterprises Inc., Lynchburg, TN 37352. Manuscript submitted March 20, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the trailing edge of the molten pool. It is important to note that the microstructural evolution in the alloyed regions is controlled by the thermal cycles experienced in these regions and by the stability of the various phases. The present work pertains to the development of models to predict the microstructural evolution during the LSA process. An ideal model would couple computational heat-transfer and fluid-flow models with computational therm
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