Nanopowder as sintering aid for water-atomized ferrous powder

Doctoral thesis, 2021

Press and sinter powder metallurgy steels are cost-effective solutions for structural applications. There is a constant drive for improvement in the density of these powder metallurgy steels to expand their usage in high-performance applications. In press and sinter powder metallurgy, consolidation is achieved by compaction, while sintering metallurgically bonds the metal particles. One of the promising ways to achieve improved densification during sintering is the addition of sintering activators to the conventional micrometre-sized metal powder. Nanopowder is associated with excess surface energy due to their very high surface-to-volume ratio, thus, this category of materials has enhanced reactivity. Accordingly, micro/nano bimodal powder are known to yield high densities when processed through other manufacturing routes such as metal injection moulding. This thesis explores the possibility of achieving improved densification by means of nanopowder addition as a sintering aid in water-atomized iron powder processed through the press and sinter route.

Before addressing the sintering aspects of micro/nano bimodal powder, surface, and thermal characteristics of nanopowder were investigated. Iron nanopowder was shown to be covered with an iron oxide layer of 3-4 nm. Different models were used for the estimation and the results from X-ray photoelectron spectroscopy and electron microscopy were complemented by those obtained from thermogravimetric analysis. A methodology to measure the thickness of surface oxide on the nanopowder was proposed and applied to other types of nanopowder The oxide layer underwent a single-step reduction process, and complete reduction was achieved below 600 °C when using hydrogen as a reducing agent. The progress of oxide reduction was studied using thermogravimetric and kinetic analysis, and an oxide reduction mechanism was proposed. While the surface oxide of iron nanopowder follows a single step reduction process, the actual reduction process of Fe2O3 undergoes a two-step process to form metallic iron. To study sintering, compacts from micro/nano bimodal powder mixtures were prepared to understand the influence of nanopowder addition on densification behaviour. The presence and increase in the amount of nanopowder decreased the compressibility of the blends. Still, the addition of the nanopowder produced a clear influence on sintering behaviour at temperatures as low as 600 °C compared to compacts containing only micrometre-sized powder. It was found that the sintering is activated at temperatures below 700 °C in nanopowder. Sinter response depended on the type of nanopowder used. Finally, nanopowder was added to pre-alloyed steel powder and evaluated for different characteristics, including flowability, mass loss, density, and impact strength. A detailed microstructural study of steel powder fortified with nanopowder indicated the presence of a chemically heterogenous microstructure after sintering, where presence of nanopowder is proposed to play a significant role in the microstructure development.