| 1. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Comparative study on the densification of chromium pre-alloyed powder metallurgy steel through nanopowder addition using design of experiments
- 2021
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Ingår i: Results in Materials. - : Elsevier BV. - 2590-048X. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- There is a constant demand for high density press and sinter powder metallurgical components for automotiveapplications. Steel powder pre-alloyed with chromium is an attractive material for such applications, but newways to further increase the sinter density are required for successful processing of these powders to high density.Nanopowder could be used as a potential sintering aid in order to boost the densification of the steel powdercompact. In this study, steel powder pre-alloyed with chromium, without and with admixed nickel, is used as basepowder, to which nanopowder was added. Surface oxide removal, crucial for successful sintering of such mate-rials, was studied by thermogravimetry analysis in order to understand the influence of nanopowder addition onthe oxide reduction. Powder compacts containing nanopowder showed higher mass loss in comparison to the oneswithout nanopowder. Linear shrinkage obtained from dilatometric curves increased with the addition of nano-powder. To depict the influence of the critical parameters; sintering temperature, powder size, addition ofnanopowder and composition (with or without nickel), a design of experiment approach was applied. The criticalparameters were then adjusted at 2 different values (categorical parameters) and a‘full factorial design model’was used involving 16 experiments, with sinter density and hardness as output measures of the experimentsdetermined. The results were analyzed using polynomialfit to determine which of the parameter exerts themaximum influence. Presence of nickel increased the hardness whereas sintering temperature and presence ofnanopowder enhanced the sinter density. This led to the tentative design of optimum conditions that resulted inincrease in sinter density from 7.25 g/cm3(92.5% of the theoretical density) to 7.4 g/cm3(94% of the theoreticaldensity) with an addition of 5% nanopowder to Ni-containing grade when sintered at 1350 °C instead of 1250 °C.
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| 2. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Analysis of Iron Oxide Reduction Kinetics in the Nanometric Scale Using Hydrogen
- 2020
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Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 10:7, s. 1-17
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Tidskriftsartikel (refereegranskat)abstract
- Iron nanopowder could be used as a sintering aid to water-atomised steel powder to improve the sintered density of metallurgical (PM) compacts. For the sintering process to be efficient, the inevitable surface oxide on the nanopowder must be reduced at least in part to facilitate its sintering aid effect. While appreciable research has been conducted in the domain of oxide reduction of the normal ferrous powder, the same cannot be said about the nanometric counterpart. The reaction kinetics for the reduction of surface oxide of iron nanopowder in hydrogen was therefore investigated using nonisothermal thermogravimetric (TG) measurements. The activation energy values were determined from the TG data using both isoconversional Kissinger–Akahira–Sunose (KAS) method and the Kissinger approach. The values obtained were well within the range of reported data. The reaction kinetics of Fe 2 O 3 as a reference material was also depicted and the reduction of this oxide proceeds in two sequential stages. The first stage corresponds to the reduction of Fe 2 O 3 to Fe 3 O 4, while the second stage corresponds to a complete reduction of oxide to metallic Fe. The activation energy variation over the reduction process was observed and a model was proposed to understand the reduction of surface iron oxide of iron nanopowder
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| 3. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Carbon-coated iron nanopowder as a sintering aid for water-atomized iron powder
- 2022
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- The paper examines the influence of carbon coating on iron nanopowder used as a sintering aid for water-atomized iron powder. Iron nanopowder without such a coating was used as a reference sintering aid to isolate the influence of the carbon coating. Both nanopowder variants were characterised using XPS and HRTEM. The results showed a core–shell structure for both variants. The iron nanopowder is covered by a 3–4 nm thick iron oxide layer, while the carbon-coated iron nanopowder is encapsulated with several nanometric carbon layers. Thermogravimetry conducted in a pure hydrogen environment shows a multipeak behaviour for the carbon-coated iron nanopowder, while a single peak behaviour is observed for the iron nanopowder. Two types of micro/nanobimodal powders were obtained by mixing the nanopowder with water-atomized iron powder. Improved linear shrinkage was observed during sintering when the carbon-coated iron nanopowder was added. This can be explained by the reduction in surface diffusion in the nanopowder caused by the carbon coating, which allows the nanopowder to sinter at higher temperatures and improves densification. Carbon and oxygen analysis, density measurements, optical microscopy and JMatPro calculations were also performed.
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| 4. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Effect of Nanopowder Addition on the Sintering of Water-Atomized Iron Powder
- 2020
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 51:9, s. 4890-4901
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Tidskriftsartikel (refereegranskat)abstract
- A promising method of improving the densification of powder metallurgical steel components is to blend nanopowder with the otherwise typically used micrometre-sized powder. The higher surface-to-volume ratio of nanopowder is hypothesized to accelerate the sintering process and increase the inter-particle contact area between the powder particles. This is supposed to enhance the material transport and improve the densification. In the present investigation, water-atomized iron powder (− 45 μm) was mixed separately with pure iron and low-carbon steel nanopowder, each at a ratio of 95 to 5 pct. These powder mixes were compacted at different pressures (400, 600 and 800 MPa) and then sintered at 1350 °C in a pure hydrogen atmosphere. The sintering behavior of the powder blend compacts was compared to that of the compact with micrometre-sized powder only. Densification commenced at much lower temperatures in the presence of nanopowder. To understand this, sintering at intermittent temperatures such as 500 °C and 700 °C was conducted. The fracture surface revealed that the nanopowder was sintered at between 500 °C and 700 °C, which in turn contributed to the densification of the powder mix at the lower temperature range. Based on the sintering experiments, an attempt was made to calculate the activation energy and identify the associated sinter mechanism using two different approaches. It was shown that the first approach yielded values in agreement with the grain-boundary diffusion mechanism. As the nanopowder content increased, there was an increase in linear shrinkage during sintering.
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| 5. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Influence of Iron Nanopowder Addition on Sintering of Water Atomized Iron Powder
- 2018
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Ingår i: World PM2018 Congress Proceedings, 2018. ; , s. 509-514
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Konferensbidrag (refereegranskat)abstract
- There is a constant drive for improvement of the density of powder metallurgy (PM) steels as they are known to offer cost-effective solutions for structural applications. Increase in the density of PM steels will expand their use in applications demanding higher performance than what they deliver today. One of the promising ways to improve densification is through the utilization of powder mixes with different size fractions. Admixing nanopowder can contribute to higher sinter-density of compacts having micrometer-sized powder as main constituent, typically used for manufacturing PM steel components. The higher surface to volume ratio of nanopowder is hypothesized to contribute to densification by increasing the inter-particle contact area. This is supposed to enhance the material transport path and improve the densification. In the present investigation, nanopowder (40-60 nm) was added to water atomized iron powder (-45 μm). This approach was to study the influence of nanopowder addition on the sintering of iron powder. The weight percent of nanopowder was varied to help understanding the densification provided by the addition. This mixed powder was compacted at 800 MPa followed by sintering of up to 1350 ? in pure H2 atmosphere. The sintering behavior was studied by means of dilatometry and thermal analysis followed by microstructural and fractographic characterization in order to elucidate the effect of nanopowder on densification. It was shown that the linear shrinkage increased from 1.5 to 2.5% when the nanopowder content was increased from 5 to 15 wt. %. Significant changes during the sintering of the compacts were observed between 500 to 700 ? due to the nanopowder addition.
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| 6. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Influence of iron nanopowder addition on the densification of chromium-prealloyed water-atomised powder metallurgy steel admixed with nickel
- 2023
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Ingår i: Powder Metallurgy. - : Informa UK Limited. - 0032-5899 .- 1743-2901. ; 66:4, s. 309-315
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Tidskriftsartikel (refereegranskat)abstract
- High sintered density is desired for heavy-duty applications and there are different ways through which the sintered density can be improved. In the current study, nanopowder is blended to the conventionally used micrometre-sized water-atomised steel powder to evaluate its impact on sintering. Both the powder variants, without and with nanopowder, were subjected to thermogravimetry analysis, and uniaxially compacted to the same green density of 7.15 g/cc or 90% relative density and sintered at 1250°C in pure hydrogen. A comparative analysis was performed with respect to the microstructural evolution between the micro and micro/nano bimodal powder compacts. JMatPro and electron backscattered diffraction was used to understand the microstructural evolution. An attempt was made to understand the improved linear shrinkage in the micro/nano bimodal powder compact using a combination of microstructure analysis and chemical analysis.
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| 7. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Investigation of surface and thermogravimetric characteristics of carbon-coated iron nanopowder
- 2020
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Ingår i: Surface and Interface Analysis. - : Wiley. - 1096-9918 .- 0142-2421. ; 52:12, s. 1045-1049
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Tidskriftsartikel (refereegranskat)abstract
- Demand for high-density press and sinter components is increasing day by day. Of the different ways to improve the sinter density, the addition of nanopowder to the conventional micrometer-sized metal powder is an effective solution. The present investigation is aimed at studying the surface chemistry of iron nanopowder coated with graphitic carbon, which is intended to be mixed with the conventional iron powder. For this purpose, iron nanopowder in the size range of 30 nm to submicron (less than 1 micron) was investigated using thermogravimetry at different temperatures: 400 degrees C, 600 degrees C, 800 degrees C, 1000 degrees C, and 1350 degrees C. The X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and high-resolution scanning electron microscopy (HR-SEM) were used for characterizing the powder as well as samples sintered at different temperatures. The presence of iron, oxygen, carbon, chromium, and zinc were observed on the surface of the nanopowder. Iron was present in oxide state, although a small metallic iron peak at 707 eV was also observed in the XPS spectra obtained from the surface indicating the oxide scale to be maximum of about 5 nm in thickness. For the sample treated at 600 degrees C, presence of manganese was observed on the surface. Thermogravimetry results showed a two-step mass loss with a total mass loss of 4 wt.% when heated to 1350 degrees C where the first step corresponds to the surface oxide reduction.
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| 8. |
- Manchili, Swathi Kiranmayee, 1987
(författare)
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Nanopowder as sintering aid for water-atomized ferrous powder
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
- Manchili, Swathi Kiranmayee, 1987
(författare)
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Role of nanopowder as sintering aid in the densification of water atomized ferrous powder
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 10. |
- Manchili, Swathi Kiranmayee, 1987, et al.
(författare)
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Sintering of bimodal micrometre/nanometre iron powder compacts - A master sintering curve approach
- 2021
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 391, s. 557-568
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Tidskriftsartikel (refereegranskat)abstract
- Though press and sinter powder metallurgy (PM) steel offers cost-effective solutions for structural applications, there is a constant drive for improvement in their density. Addition of nanopowder to the conventional micrometre-sized metal powder is explored to improve the sinter density. In this study, the effect of nanopowder addition in varying amounts has been studied. Carbonyl iron powder (<5 μm) and water atomized iron powder (<45 μm) were used as the base powder to which varying amounts of iron nanopowder (<100 nm) was added. Dilatometric sintering studies under pure hydrogen atmosphere were carried out to analyze the densification behavior. The results revealed that the bimodal powder mixture containing 25% nanopowder exhibited the highest green density for both carbonyl and ASC 300 compacts. Master sinter curve for compacts was developed based on the dilatometer data. The apparent activation energy for sintering decreased with an increase in nanopowder content. This is reflected in the values of work of sintering.
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