Spray deposition of a Sn-40 Wt Pct Pb alloy with uniform droplets
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INTRODUCTION
S P R A Y deposition combines production o f a molten alloy spray usually by gas atomization and its deposition on a substrate to form dense, rapidly solidified bulk alloys with fine, equiaxed microstructures, t~,2m With appropriate control, preforms o f relatively simple near-net shapes, such as cylinders, disks, and sheets, can be sprayformed directly from the melt.I4-8] This ability to produce near net-shaped preforms with fine microstructures makes spray deposition an attractive industrial manufacturing process. The microstructure o f deposited materials depends on the mass and enthalpy fluxes o f the spray, the substrate conditions, and the heat extraction after deposition. Several workers have modeled the spray deposition process based on macroscopic parameters to predict the thermal states during spray deposition, t9,1°,11] While these models provide reasonably accurate quantitative descriptions o f the macroscopic thermal states o f both the spray and the deposited material, so far only qualitative arguments have been advanced to explain the evolution o f the microstructures in spray deposition. The inability o f applying these models to the understanding o f spray-deposited microstructures stems primarily from the complexity associated with the evaluation o f the mass and enthalpy fluxes o f gas atomized sprays which inevitably consist o f droplets of varying sizes. The thermal and solidification states o f atomized droplets at CHRISTIAN H. PASSOW, Research Assistant, and JUNG-HOON CHUN, Edgerton Assistant Professor of Mechanical Engineering, are w i t h the Laboratory for M a n u f a c t u r i n g a n d Productivity, Massachusetts Institute of Technology, Cambridge, MA 02139. TEIICHI A N D O , R e s e a r c h Associate Professor, is with the Department of Manufacturing Engineering, Boston University, Boston, MA 02215. Manuscript submitted August 7 , 1992. METALLURGICAL TRANSACTIONS A
the time o f deposition depend strongly on the droplet size, not only through the size dependence o f heat transf e r from the droplet to the ambient gas but also through the size dependence o f the acceleration and deceleration o f the droplets during their flight to the substrate. Differences in droplet size directly translate into differences in the relative velocity between the droplet and the gas and also in the time a droplet takes to reach the substrate. The droplets consequently have wide ranges o f thermal and solidification states when they impact the substrate. Such complexstates o f an atomized spray make rigorous modeling o f the gas-atomized spray deposition process very difficult. Difficulties arising from differences in droplet size are removed in the analysis o f spray deposition, with uniform droplets impacting the substrate at identical velocities. In such a spray deposition process, all the uniform droplets have identical thermal and solidification states and travel at identical velocities, dependent only on the flight distance, so that the enthalpy flux is proportional to the deposition
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