Production of quality NiAl granules: examining process features

Abstract

This paper looks at a number of different methods for producing nickel aluminide (NiAl) granules. It also examines certain factors hindering a wider application of the above material by current aircraft and engine makers. The key hindering factors that were established include an almost zero plasticity of the material under pressure forming and a poor machinability of parts made of it. Application of precise metallurgy techniques, which help obtain almost finished parts requiring minimum machining, can help eliminate the above factors. Another difficulty is related to the fact that there are no techniques that can help obtain quality ball-shaped granules using the material in view. The conducted study helped define some quality criteria applicable to the resulting granules. They include sphericity, size consistency, absence of defects such as pores or surface satellites, presence of fine-dispersed dendritic structure, etc. A number of methods was considered that enable to produce high-quality nickel aluminide granules, such as high-temperature inert gas jet atomization; Plasma Rotary Electrode Process (or, PREP); meltspinning with the help of perforated crucible. It was established that PREP delivers the optimum technique for obtaining quality NiAl granules. In the course of the study it was established that it is not that much the current I but rather the velocity of the melted electrode n that serves as the key characteristic of the process that determines the quality and diameter of granules, as well as the dendritic parameter of their microstructure. Optimal electrode RPM values were determined. They are n ≈ 15 000–16 000 min–1 at the current I ≈ 1.000–1.500 A. A process has been developed for producing quality NiAl granules, which involves the following stages: self-propagating high-temperature synthesis to make initial NiAl ingots followed by remelting, heat treatment, granule separation and PREP.