The Influence of the Atmosphere and Impurities on the Sintering Behaviour of Aluminium

Green samples of aluminium pressed using air atomised powders of different purity were sintered under vacuum, argon, nitrogen, nitrogen/hydrogen gas mixtures. Only pure nitrogen is an active sintering atmosphere for aluminium, because it promotes shrinkage and sintered mechanical properties, as proved by bend test. A key feature in the use of nitrogen is the formation of aluminium (oxy)-nitrides. Hydrogen very strongly lowers the sinterability of aluminium, even when present in small concentrations in a nitrogen atmosphere. Auger electron spectroscopy was used to characterize the surface layers on aluminium powder particles and sintered samples. Distributions of aluminium, nitrogen, oxygen and magnesium, present as impurities, were obtained by Auger depth profiling measurements on these surfaces. These investigations revealed a strong surface segregation of Mg in the early stage of the sintering process. In addition, the sintering response of aluminium was influenced by the content of the Mg impurity; higher concentrations of Mg accelerated the sintering process. The results point, that enhanced concentration of magnesium within the surface film on powder particles may support sintering of aluminium.