ADVANCED SINTERING TECHNIQUES OF ALUMINUM: SPARK PLASMA SINTERING
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Spark plasma sintering (SPS) of air atomized aluminum powders was conducted in a fundamental investigation of processing and chemical effects on physical and mechanical properties. Five air atomized aluminum powders, one of commercial purity, one magnesium-doped (0.4 wt%), one iron-doped (1.0 wt%), one nickel-doped (1.0 wt%) and one iron (1.0 wt%) and nickel-doped (1.0 wt%) were processed by SPS means. Where applicable, powders were also processed by conventional powder metallurgy (PM). An investigation of SPS processing parameters and their effect on sinter quality were of primary concern. Applied pressure and ultimate processing temperature bore the greatest influence on processing, while heating rate and hold time showed a minor effect. Full density specimens were achieved for both powders under select processing conditions. To compliment this, large (80 mm) and small (20 mm) diameter samples were made to observe possible up-scaling effects, as well as tensile properties. Large samples were successfully processed, albeit with somewhat inferior densities to the smaller counterparts presumably due to the temperature inhomogeneity during processing. An investigation into tensile properties for SPS samples exhibited extensive ductility (~30%) at high sintering temperatures, while lower temperature SPS samples as well as all conventional PM processed samples exhibited a brittle nature. The measurement of residual oxygen and hydrogen contents showed a significant elimination of both species in SPS samples under certain processing parameters when compared to conventional PM equivalents. None of the transition metal additions had an overtly negative impact on SPS response. As such, all powders were successfully processed to the full density condition provided that an appropriate minimum SPS temperature was employed. Hardness improved as the net concentration of transition metals increased and was found to be greatest in the Al-Fe-Ni ternary powder (78 HRH). Microstructural coarsening was apparent in all alloys as a result of SPS processing. However, the consolidated products maintained desirable microstructures comprised of homogenous distributions of sub-micron intermetallics such as Al9FeNi, Al13Fe4 and AlNi3.