Compaction and Densification Characteristics of Iron Powder/Coal Fly Ash Mixtures Processed by Powder Metallurgy Technique

The present work elucidates the compaction and the densification behavior of iron powder and coal fly ash (CFA) mixtures during powder metallurgy (P/M) processing. The flowability and the compressibility characteristics of the starting materials were exhibited through Hausner ratio and Carr’s index. Morphological, elemental and crystallographic characterizations of the starting materials were carried out using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy and x-ray diffraction investigations, respectively. The CFA of 0, 5, 10 and 15 wt.% was mixed with the iron powder through ball milling. Further, the cold compaction of the mixtures containing iron/CFA was performed in the hardened steel die using a uniaxial hydraulic press at pressures of 91 MPa, 138 MPa and 185 MPa, respectively. Subsequently, the preforms were sintered at 950, 1050 and 1150 °C in a tubular furnace under an inert atmosphere. The density of the preforms and the sintered pallets was estimated using weight to volume ratio and Archimedes method, respectively. The obtained mineralogy, morphology and physio-mechanical properties of the CFA are in good agreement with the ASTM standards. Further, the flowability and the compressibility characteristics of the starting materials rendered them suitable for processing through the P/M process. SEM analysis of the sintered pallets exhibited uniform distribution of CFA particulates in the iron matrix with clear and strong interfaces. An inverse effect of an increased amount of CFA inclusion has been observed on the green and sintered density of the composite. However, a linear influence of increased compacting pressure and sintering temperature has been observed on green and sintered densities, respectively. The magnitude of the green density achieved during cold compaction is considerably higher than that achieved during the sintering process. The obtained compaction data were successfully fitted using the Ge compaction equation.