Recent Process Development

Rotating electrode processed powder has been typically used for applications requiring a large median size (in excess of 100 /''in). New product and manufacturing approaches require REP powder characteristics, but with maximum sizes being less than 100 /'m (aerospace) and 44 /'m (metal injection molding). In order to meet these needs two new approaches have been investigated to extend REP capability. Two modifications to the REP process have been developed for the production of finer powders.

The first, liquid argon quenching (LArQ) involves impact of the molten REP droplets with a high-velocity film of liquid argon. While balance issues limit rotational speeds to approximately 625 rad/s, for brittle alloys that cannot be rotated faster, LArQ produces almost a 5 to 1 decrease in median particle size (Ref 14).

The second, involves the close-coupled gas atomization of the molten REP-produced droplets. As the droplets formed by REP are being ejected from the electrode surface they are hit by a high-velocity gas jet. The aerodynamic drag imposed on the droplets causes further subdivision and atomization of the droplets. The atomization process is driven by aerodynamics, REP simply provides a means of injecting the liquid metal into the gas flow. This approach allows the melt and the gas flow to be close coupled for reactive alloys, maximizing the atomization efficiency. Close coupling has already been shown to provide substantial yield improvement compared to remote atomization for conventional alloys. Yields of Ti-6A1-4V powder are shown in Fig. 12, with large gains being made in the -44 and -100 /'m yields (Ref 15).

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5 10 20 30 50 70 00 90 95 Percent less than

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Fig. 12 Comparison of power yields produced by REP alone and gas-assisted REP

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