Modeling of spray deposition: Measurements of particle size, gas velocity, particle velocity, and spray temperature in G
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I.
INTRODUCTION
S P R A Y deposition is a revolutionary process for manufacturing preforms of a variety of geometries, as shown schematically in Figure 1. A spray of droplets is atomized from a melt stream by the action of a concentric ring of gas jets. The spray of droplets is projected by the atomizing gas onto a substrate which is manipulated beneath the spray in one of the three configurations shown in Figure 1. There are two potential benefits of manufacturing preforms by spray deposition. First, spray deposition provides a near-finished-shape manufacturing route for articles, such as tubes, tl,2~ discs, [3,4,5] plates,/6-9] coatings, tl~ and composite materials, tu,~21 Second, significant metallurgical and mechanical property improvements have been displayed in spray-deposited materials. [3-8] Spray deposition is a complex process, and in order to harness the full potential of spray deposition, a detailed knowledge of mass transfer, fluid flow, heat transfer, momentum transfer, and solidification behavior of the product alloy is required. The fundamental parameter controlling the successful formation of a spray deposit is the degree of droplet solidification at the point of deposition. If all the droplets are solid, no preform will be formed. On the other hand, if all the droplets are fully liquid, the metallurgical properties of the spray-deposited preform will only be similar to those of a conventional casting. Since it is desirable to have the capability of
B.P. BEWLAY, formerly Doctoral Student, Department of Metallurgy and Science of Materials, Oxford University, is Staff Metallurgist, Corporate Research and Development, General Electric Company, Schenectady, NY 12301. B. CANTOR, Lecturer, is with the Department of Metallurgy and Science of Materials, Oxford University, Oxford OX1 3PK, United Kingdom. Manuscript submitted May 25, 1989. METALLURGICAL TRANSACTIONS B
generating a spray-deposited preform of any selected alloy and product geometry, the principal processing problem is to determine that combination of spray deposition operating parameters which produces the optimum mixture of liquid and solid droplets. The optimum combination of operating parameters can be determined from either a data base of experimental knowledge or a theoretical model. Temperature measurement of droplets in a gas-atomized spray is very difficult for two reasons. First, most of the droplets are 10 to 500/xm, and, second, they are moving. This precludes the use of thermocouples and makes optical pyrometry difficult. Photocalorimetryt13,14]has been used to determine the cooling rates in melt-spun ribbons, but the calibration problems are too severe to use photocalorimetry for accurate temperature measurement of droplets in a gas-atomized spray. There have been several developments of two-color pyrometer systems for measuring particle temperatures in plasma sprays, u51 but these have yet to be developed for gas-atomized sprays. These experimental difficulties have led several researchers to attempt to construct mathematical
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