Short time high energy milling: Mechanical aspects related to the processing of iron-aluminium for low ball to powder weight ratio

The processing of Fe-Al powders by high-energy milling has been investigated as a function of milling time. Mechanical aspects related to the changes in the powder morphology and particle size distribution were studied using SEM technique. Milling was performed in two steps: a) at 400 rev. per min for 1 hour and 4:1 ball to powder weight ratio; and b) at 300 rev. per min for 1, 2, 3, 4, 5, 6, 7 h and 8:1 ball to powder weight ratio. Zinc stearat as solid lubricant was systematically added to avoid sticking of the powders to the balls and the container wall. Commercially iron and aluminium powders pre-classified to 100 mum average particle size were used to achieve Fe-10wt.%Al starting mixture. At the highest rotation speed the milling could be performed only up to 1 hour as a consequence of the low ball to powder ratio used (4:1). It was found that the average particle size deeply decreases after 5 hours (for 300 rev. per min.). X-Ray Diffractometry results indicate plastic deformation of the powders for 5 h milling time. For the highest time, an excessive particles cold welding event was verified probably as a consequence of the non-adequate use of lubricant.