We studied microporosity in the metallic matrix of aluminium foams produced by the powder metallurgical route both with and without application of a blowing agent. Microporosity was studied in-situ in liquid metal foams as well as ex-situ in the solidified microstructures. In-situ studies were carried out using synchrotron X-rays. Quantitative analyses of the amount and distribution of microporosity inside cell walls, Plateau borders and nodes were performed on 2D micrographs and on 3D reconstructed volumes generated by X-ray tomography. We studied the influence of alloying elements, blowing agent and holding time on the amount and type of micropores. The mechanisms of microporosity formation and the evolution of microporosity via diffusion of hydrogen and by coalescence are discussed. It was observed that alloy composition and holding time have a strong influence on microporosity. Different possible strategies to control microporosity are suggested. © 2017 Acta Materialia Inc.

Manas Mukherjee

Department of Metallurgical and Materials Engineering

F. García-Moreno

C. Jiménez

A. Rack

J. Banhart

Alloying elements

Aluminum

Blowing agents

Defects

Foams

Metallurgy

Metals

Solidification

Alloy compositions

Hydrogen diffusion

LIQUID METAL FOAMS

Metal foams

METALLIC MATRICES

Powder metallurgical

Solidified microstructures

SYNCHROTRON X RAYS

Microporosity

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IIT Madras

Acta Materialia

Pressure-induced foaming (PIF) of metals is a foaming technique in which blowing agent free compacted metal powders are foamed. The method consists of heating hot-compacted metallic precursors to above their melting temperature under gas overpressure and foaming them by pressure release. This study focuses on PIF of Al99.7 and AlSi7 alloys under both air or Ar and overpressures up to 9&nbsp;bar. In situ x-ray radioscopy allows us to follow the foaming process and to perform quantitative analyses of expansion, foam morphology, and coalescence rate. Mass spectrometry helps to identify hydrogen as the foaming gas. Adsorbates on the former powder particles are found to be the primary gas source. Various advantages of this new method are identified and discussed. © 2015 The Minerals, Metals &amp; Materials Society.

Pressure-induced foaming of metals

Pushing synchrotron x-ray radiography to increasingly higher image-acquisition rates (currently up to 100,000 fps) while maintaining spatial resolutions in the micrometer range implies drastically reduced fields of view. As a consequence, either imaging a small subregion of the sample with high spatial resolution or only the complete specimen with moderate resolution is applicable.We introduce a concept to overcome this limitation by making use of a semi-transparent x-ray detector positioned close to the investigated sample. The hard x-rays that pass through the sample either create an image on the first detector or keep on propagating until they are captured by a second x-ray detector located further downstream. In this way, a process can be imaged simultaneously in a hierarchical manner within a single exposure and a projection of the complete object with moderate resolution as well as a subregion with high resolution are obtained. As a proof-of-concept experiment, image sequences of an evolving liquidmetal foam are shown, employing frame rates of 1000 images/s (1.2 μm pixel size) and 15;000 images/s (18.1 μm pixel size) for the first and second detector, respectively. © 2013 Optical Society of America.

Fulltext

Applied Optics

Hierarchical radioscopy using polychromatic and partially coherent hard synchrotron radiation

The influence of oxides on the stabilisation of zinc foam made by foaming-compacted powder mixtures has been investigated by varying the oxide content in the zinc powder used by oxidation and reduction. Optical, scanning electron and transmission electron microscopy as well as energy dispersive X-ray mapping were used to determine the oxide distribution, morphology and structure in the foams. The study revealed that with increase in the oxide content of the foam, the maximum expansion and expansion rate increased. Small amount of nano-sized oxide particles and their cluster, which are randomly distributed, were observed within the bulk of foam. But the major fraction of oxides is observed on the surface of pores in the form of clusters. These clusters are distributed uniformly all over the surface. Effect of these oxides on the stability of foam is discussed. The formation of satellite pores, which is characteristic signature of zinc foams, and their stability, is investigated. © 2011 Springer Science+Business Media, LLC.

Journal	Data powered by TypesetActa Materialia
Publisher	Data powered by TypesetElsevier Ltd
Open Access	No