Rapidly solidified prealloyed powders by laser spin atomization
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I.
INTRODUCTION
T H E R E has been considerable interest in the recent past concerning the production of rapidly solidified prealloyed metal powders. A number o f approaches have been taken such as gas or water atomized liquid metals, 1'2 ultrasonic atomization, 3 and more recently centrifugal atomization.4 In this latter process, liquid alloys are directed onto an atomizing disk, which is rotating at speeds in the range 20,000 to 35,000 rpm, and the resultant liquid droplets are quenched by jets o f He gas. Reactive metals such as T i and its alloys are difficult to process in this way because o f the interaction of the liquid metal with its crucible. This problem has been circumvented by the rotating electrode process (REP),5 where a prealloyed consumable ingot (in the form o f a rod) is rotated at speeds in the range o f 2000 to 10,000 rpm, and the free end of the rod is melted by an arc established between the ingot and a nonconsumable electrode. The resulting droplets, ejected from the molten pool at the end o f the ingot, are cooled in static He gas. Some disadvantages o f this attractive technique involve the relatively large size of powders produced, the modest cooling rates achieved, an increase in the W concentration in the alloy due to contamination from the electrode, and the apparent lack o f instantaneous superheat in the molten pool at the end of the ingot. The superheat is required to ensure powder homogeneity through thorough elemental mixing prior to atomization. A recent modification to REP is the incorporation o f a plasma gun to replace the W electrode.5 This has eliminated the W contamination present in the powders at least to levels below normal limits o f detection, -
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