Laser-melting/spin-atomization method for the production of titanium alloy powders
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Laser-Melting/Spin-Atomization Method for the Production of Titanium Alloy Powders
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T. C. PENG, S. M. L. SASTRY, and J. E. O'NEAL /---- Laser beam
Recent studies of the rapid solidification of titanium alloys have demonstrated that significant microstructural refinement and property improvements can be achieved in titanium alloys solidified at cooling rates >10 3 K per second.~-5 However, methods for producing rapidly solidified titanium powders have received little attention. Most titanium alloy powders have been produced by the hydride-dehydride process or by spin-atomization from rotating electrodes, ingots, or crucibles. 6-~~ Except for the hydride-dehydride process, all the powder-production methods involve various degrees of rapid solidification, which have not yet been quantified. Recently Konitzer et al. ~ used laser melting and spin atomization to produce rapidly solidified Ni-Mo-A1 and Ti-6A1-4V alloy powders. In the present investigation, laser-melting/spin-atomization was used to produce and characterize rapidly solidified powders of a beta titanium alloy Ti-15-3 (Ti-15V-3A1-3Sn3Cr) and a rare-earth-modified Ti (Ti-1.0Y) alloy. The objectives of this study were to quantify the dependence of powder diameter on process variables and the dependence of cooling rate on powder diameter. The as-solidified dendrite structures are revealed in both alloys by simple etching, and the dendrite-arm spacings (DAS) can be used to estimate the cooling rates from a previously established correlation between DAS and cooling rate. t2 Because rapid solidification of the Ti-I.0Y alloy results in homogeneously distributed fine dispersoids in the Ti matrix, ~-5 dispersoid sizes can also be used to quantify the cooling rates produced in the lasermelt/spin-atomization process. A schematic diagram of the experimental arrangement is shown in Figure 1. The apparatus provides an inert gas environment and simple mechanism for controlling the melting and spin-atomization. A 6.2-kW continuous wave CO2 laser beam is directed at a glancing angle of 3 to 5 deg to one end of a rotating titanium alloy rod. High speed video recordings indicated that a thin molten layer forrps on the end surface and spreads radially under the influence of the melt viscosity and the rod rotation. At the circumference of the rod, the molten layer breaks into droplet s as the centrifugal force balances the surface tension according to
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Fig. 1 - - S c h e m a t i c diagram of laser-melting/spin-atomization apparatus for producing Ti-alloy powders.
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where D is the droplet diameter, N is the rod rotational speed, cr is the melt surface tension, p is the density, and R
Fig. 2 - - Powder diameter as a function of rod rotational speed. Calculated value
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