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.

nanopowder

surface oxide

compaction

thermal analysis

water-atomized iron powder

sintering

master sintering curve

dilatometry

water-atomized steel powder

hydrogen

reduction kinetics

Opponent: Professor Mónica Campos Gómez, Universidad Carlos III de Madrid, Madrid, Spain

Author

Swathi Kiranmayee Manchili

Chalmers, Industrial and Materials Science, Materials and manufacture

Analysis of Iron Oxide Reduction Kinetics in the Nanometric Scale Using Hydrogen

Nanomaterials,; Vol. 10(2020)p. 1-17

Journal article

Effect of Nanopowder Addition on the Sintering of Water-Atomized Iron Powder

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science,; Vol. 51(2020)p. 4890-4901

Journal article

Investigation of surface and thermogravimetric characteristics of carbon-coated iron nanopowder

Surface and Interface Analysis,; Vol. 52(2020)p. 1045-1049

Paper in proceeding

Surface analysis of iron and steel nanopowder

Surface and Interface Analysis,; Vol. 50 (2018)p. 1083-1088-10886

Journal article

Carbon-coated nanopowder as sintering aid for water-atomized iron powder

Evolution of microstructure during sintering in nanopowder added powder metallurgy steel

Impact of nanopowder addition on the densification of Cr-Ni-alloyed powder metallurgy steel

Powder metallurgy (PM) is an interesting and innovative branch of manufacturing technologies which has risen to importance during the advent of 20th century. Though the large-scale advent of PM in the production sector happened in the 1900s, the beginnings of this technology go back to as early as at least 3000 B.C. Egyptians used iron powder for fabricating objects, making them one of the first users of PM. 1700 years ago, metallurgists in India created 7.3 meters tall and 6.5 tonnes iron pillar which was later moved to Delhi, now famously known as the Delhi iron pillar from iron powder. The pillar was made through forge welding process of iron powder and is famous for its resistance to corrosion. During the 19th century, William Coolidge designed a lamp filament using tungsten powder for Thomas Edison. During the 20th century, PM was used for the fabrication of electrical contacts, cemented carbides and porous bearings. Later, PM expanded to a spectrum of metals for various applications. In the recent times, the desire to increase the range of available materials and the resulting characteristics has led to an array of alloys and materials like tantalum, beryllium oxide, silicon carbide, rhenium, zirconium and titanium diboride which can be produced by no other conventional method.

In the modern-day context, press and sinter PM steel offers cost-effective solutions for structural applications. Properties like tensile strength, ductility, impact strength and fatigue strength are directly dependent on the density of the component. Hence, there is a constant drive for improvement of the density of these PM steels, which will expand their usage in applications demanding higher performance than what they deliver today. Of the different ways to improve the sinter density, addition of nanopowder to the conventional micrometre-sized metal powder is considered to be an effective solution. Nanopowder, owing to high surface-to-volume ratio could activate the sintering process at lower temperatures and enhances the density. Therefore, it can be said that blending nanopowder to the conventional micrometre-sized metal powder increases the sinter density by increasing the inter-particle contact area. Before venturing into the sintering aspects, surface characteristics of the nanopowder were studied as sintering being a surface phenomenon. Micro/nanopowder bimodal powder was subjected to uniaxial compaction and subsequent sintering. The presence of nanopowder reduced the green density of the compacts, however, a clear influence was observed on the sintering behaviour especially in the temperature regime as low as 500-700 °C in comparison to the compacts without nanopowder.

Nanotechnology Enhanced Sintered Steel Processing

Swedish Foundation for Strategic Research (SSF) (GMT14-0045), 2016-01-01 -- 2020-12-31.

Driving Forces

Sustainable development

Subject Categories

Materials Engineering

Nano Technology

Areas of Advance

Production

Materials Science

Roots

Basic sciences

Infrastructure

Chalmers Materials Analysis Laboratory

ISBN

978-91-7905-539-4

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5006

Publisher

Chalmers University of Technology

Online

Opponent: Professor Mónica Campos Gómez, Universidad Carlos III de Madrid, Madrid, Spain

More information

Latest update

8/12/2021