Theoretical and experimental investigations of electron beam surface remelting and alloying
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I. INTRODUCTION
OVER several decades, electron-beam (EB) technology has developed into an important field in metal processing, as summarized by Schiller et al.[1] Due to its extraordinarily high energy density, the EB is used for metal-surface processing to produce high-quality layers, with very low impact on the unprocessed parts of the workpiece. Metal-surface remelting and alloying are metallurgical processes applied to improve material properties, especially wear resistance. Using these techniques for a variety of materials, entirely new fields of industrial application can be found. The industrial utilization of this technology today is limited by the area-related throughput rate and the quality of the processed surface layer. This quality, in turn, is determined by the beam parameter, the material properties, the convection in the melt pool, and the phenomena occurring at the resolidification front. To understand the dynamics describing the meltpool convection together with the melting and resolidification processes, experimental and theoretical investigations have been carried out. After resolidification, the processed surface is always characterized by surface ripples that occur somehow in the liquid phase and are frozen in when resolidified. Anthony and Cline[2] tried to explain this by some simplifications applied to the steady-state Navier–Stokes equations, neglecting capillary effects and ambient forces. Laser melting of pure metals (Al and Fe) was investigated numerically by Basu and Date,[3] who carried out two-dimensional calculations in a plane perpendicular to the scanning direction. Westerberg et al.[4] developed a two-dimensional steady-state numerical model for an EB vaporization system. They used a deformable mesh to track both the liquid-solid O. VELDE, Research Assistant, and R. GRUNDMANN, Professor, are with the Institute for Aerospace Engineering, Dresden University of Tech¨ DIGER, formerly Postdoctoral nology, 01062 Dresden, Germany. F. RU Fellow, Institute for Aerospace Engineering, Dresden University of Technology, is Research Engineer, Schott ML GmbH, 07745 Jena, Germany. Manuscript submitted July 21, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
as well as the liquid-vapor interface. Chen and Wu[5] simulated the behavior of the float zone of lithium niobate during a steady-state melting process. They used a finite-difference scheme in two dimensions and found that gravity plays an important role toward the forming of the melt/gas interface. A three-dimensional steady-state model of gas metal arc (GMA) welding processes was introduced by Ushio and Wu,[6] who took into consideration the weld-bead shape and weld-pool geometry. They computed maximum velocities of 0.36 m/s for GMA weld pools in a mild steel workpiece. The use of a surface-tension coefficient of mild steel, g/ T 5 21025 N/m/K, as well as the very coarse computational grid chosen (minimum grid space of 0.25 mm), may explain those relatively small velocities.
II. PROBLEM DESCRIPTION In the experiments, a 60 kV EB gun at
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