Role of nanopowder as sintering aid in the densification of water atomized ferrous powder

Sammanfattning: Press and sinter powder metallurgy (PM) steels are cost-effective solutions for structural applications. There is a constant drive for the improvement in the density of these PM steels which helps in expanding their usage in applications demanding higher performance than what they deliver today. In press and sinter PM, consolidation is primarily achieved by compaction and sintering helps in bonding the powder particles metallurgically. One of the promising ways to achieve improved densification during sintering is through the addition of sintering activators to the conventional micrometer sized metal powder. Nanopowder particles are associated with excess surface energy due to very high ‘surface-to-volume’ ratio. Therefore, there is an enhanced reactivity in this category of materials. Another consequence of the excess surface energy is the lowering of sintering temperature. For instance, powder blends containing micro and nano powders are known to yield high densities when processed through other manufacturing routes such as metal injection molding. In this thesis the possibility of achieving improved densification by means of nanopowder addition as a sintering aid is explored for the case of water atomized iron powder processed through the press and sinter route. In this study, the influence of nanopowder addition on sintering of water atomised iron powder has been investigated. Before venturing into the sintering aspects, surface and thermal characteristics of nanopowder were investigated. X-Ray photoelectron spectroscopy (XPS) was used to evaluate the surface oxide thickness and composition of both iron and steel nanopowder. Different models were used for this purpose and the results were complemented by those obtained from thermogravimetric analysis. A methodology to measure the thickness of surface oxide on the nanopowder was thus proposed. Further, surface oxide reduction and possibility of melt point depression for the nanopowder was evaluated using thermal analysis. For the sintering studies, various powder blends were prepared based on two different nano powder compositions, varying amounts of nanopowder content and with graphite addition to understand the influence of the individual constituents on the densification behaviour. Further, the blends were subjected to uniaxial compaction at varying pressures after which sintering was performed on the green compacts at varying heating rates. The presence and an increase in the amount of nanopowder decreased the compressibility of the blends. However, there was a clear influence of the nanopowder addition on the sintering behavior in the temperature regime as low as 500 to 700 °C when compared to compacts containing only micro-powder. To understand it further, sintering at intermittent temperatures and subsequent fractography were undertaken. It was found that the nanopowder sintering is activated at temperatures below 700 °C which contributed to the difference in sinter curve behaviour. Sinter response depended on the composition of the powder blend; however heating rate did not show much influence. An increase in the amount of nanopowder improved the density of the sintered compacts proportionally.