Modification of cryomilling process to tailor geometrical characteristics of nanostructured Al powder for cold spraying

Abstract

Cryomilling is the most commonly used method to fabricate ultra-fine grained (UFG) or nanostructured metallic powder. However, it has relatively low control over powder's geometric characteristics, particularly for the powders having FCC crystal structure. This shortcoming makes traditional cryomilling process unattractive to produce UFG/nanostructured feedstock for cold spraying process. In the present work, a modified pot vibration milling process was successfully employed to produce nanostructured powder of pure aluminum at 100 K. The geometrical characteristics of Al powder was controlled by using various combinations of the milling media (i.e. steel balls) of different sizes. The results indicate that with increase in milling time, the initial grain size of Al (~1–5 μm) was progressively refined to ~30–50 nm. Moreover, lamellar powder morphology was obtained with the addition of process control agent (PCA) while equiaxed polygonal structure was evolved without using PCA. It was also revealed that, in the absence of PCA, the particle size distribution was mainly controlled by the interstice size distribution of the milling media.