Tomoscopy - fast operando tomography for materials science and engineering

Capturing X-ray tomographic views of objects used to consume a lot of time since samples have to be rotated around their axis, during which hundreds of radiographic projection images are collected. With the developments of the past years real-time 3D movies of solidifying microstructures, expanding metal foams and of materials during processing can be obtained, currently at the rate of more than 200 per second.

Starting in 2019, the German DFG is funding the further development of the tomoscopy technique and its applications with a "Reinhart Koselleck Project" at the TU Berlin.

Examples

Experimental setup:

Tomography experiment at TOMCAT beamline of the Paul-Scherrer-Institute. The crucible is rotating at 10 revolutions per second, yielding 20 tomograms per second (20 tps) as each tomogram requires a 180° rotation. Two 150 W infrared lamps allow for heating the crucible up to 700 °C.

Pore nucleation in metal foam:

Foaming process of AlMg17.5 alloy in an early stage. 5 Tomograms are obtained per second (5 tps) and the period of observation is 10 s. Spatial resolution is 3 µm. Bubbles have been segmented and colour-coded to follow them individually.

The power of image analysis:

Foaming of AlSi8Mg4 alloy containing 0.25 wt.% TiH₂ observed for 40 s. Tomograms have been obtained at 1 tps. Spatial resolution is 3 µm. One individual Al50Mg50 particle in the sample has been extracted (grey). This particle melts first due to the low melting point of 450 °C, after which pores are formed within as shown in the tomography sequence.

Entire foaming process:

Foaming of AlSi8Mg4 alloy containing 0.25 TiH₂ has been investigated in high time resolution. 20 Tomographies per second (20 tps) were acquired. A slice through the centre of the foam is shown. White spots are TiH₂ blowing agent particles, pores are black. Total duration of experiment is 150 s, 80 s are shown.

Dendritic solidification under realistic conditions:

Temporal resolutions of 200 tomographies per second (200 tps) allow to investigate the solidification of AlGe10 melt at realistic cooling rates in the range of several Kelvin per second. The growth of several dendrites can be seen in three dimensions while primary growth rates reach several 100 µm/s.

Publications

Tomoscopy: Time-resolved tomography for dynamic processes in materials

The structure and constitution of opaque materials can be studied with X-ray imaging methods such as 3D tomography. To observe the dynamic evolution of their structure and the distribution of constituents, for example during processing, heating, mechanical loading, etc., 3D imaging has to be fast enough. In this paper, we briefly review recent developments of time-resolved X-ray tomography that have led to what we now call “tomoscopy”. A novel setup is presented and applied that pushes temporal resolution down to just 1 ms, i.e. 1000 tomograms per second (‘tps') are acquired, while maintaining spatial resolutions of micrometres and running experiments for minutes without interruption. Applications recorded at different acquisition rates ranging from 50 to 1000 tps are presented. We observe and quantify the immiscible hypermonotectic reaction of AlBi10 (in wt%) alloy and dendrite evolution in AlGe10 (in wt%) casting alloy during fast solidification. We analyse the combustion process and the evolution of the constituents in a burning sparkler. Finally, we follow the structure and density of two metal foams over a long period of time and derive details of bubble formation and bubble ageing including quantitative analyses of bubble parameters with millisecond temporal resolution.
García-Moreno, F., Kamm, P. H., Neu, T. R., Bülk, F., Noack, M. A., Wegener, M., von der Eltz, N., Schlepütz, C. M., Banhart, J. (2021). Tomoscopy: Time-resolved tomography for dynamic processes in materials. Advanced Materials. (accepted)

In situ observation with x-ray for tentative exploration of laser beam welding processes for aluminum-based alloys

In recent years, laser processes have taken an ever-increasing market share in the manufacture of components. The development of new, improved beam sources with corresponding systems technology and the decreasing investment costs of the beam sources are important keys to this success. Particularly, high frequency beam oscillation has great potential in laser beam welding and cutting. [...] This paper shows the initial results of the analysis of the melt pool behavior and seam formation as well as the formation of seam irregularities during the laser process. In the simplest case, radiographs were taken, i.e., 2D projections of the x-ray absorption coefficient distribution within a material. Thereby, recordings from 10 000 up to 40 000 fps could be generated. Furthermore, tomoscopies—the continuous acquisition of tomographic (3D) images, up to 100 tomograms per second—could be generated with proven equipment, whose main components are a high-speed rotation stage and a camera system. The findings will help to get a better understanding of keyhole phenomena as well as effects of turbulent melt flow such as pore formation and guide to solutions for preventing them. Hence, initial results of high frequency beam oscillation processes including melt pool degassing and porosity reduction will be shown and discussed.
Boerner, S., Dittrich, D., Mohlau, P., Leyens, C., García-Moreno, F., Kamm, P. H., ... & Schlepuetz, C. M. (2021). In situ observation with x-ray for tentative exploration of laser beam welding processes for aluminum-based alloys. Journal of Laser Applications, 33(1), 012026.

Metal effect pigments for reducing flow line visibility

Metallic appearance of injection moulded polymer parts is favoured in many industries, e.g. packaging or automotive. Filling polymers with metal effect pigments avoids additional coating steps, however, visible inhomogeneities in the metallic appearance suggest low part quality. [...] This paper aims at reporting novel tetrahedron shaped metal effect pigment particles for reducing flow lines, as well as novel particle manufacturing methods. Two manufacturing methods for tetrahedral particles were examined. [...] Particle orientation in the flow line region was examined in a 3D x-ray tomoscopy (time-resolved tomography). The examined tetrahedral particles did not create the characteristic flow lines of conventional planar pigment particles. By the described manufacturing methods, the manufacturability of tetrahedral metal particles was proved in the range of grams.
Demski, N. M., M. Jagodzinski, M., Malcher, M., Rolon, D. A., Kamm, P. H., Neu, T. R., Garcia-Moreno, F., Oberschmidt, D. (2020). Metal effect pigments for reducing flow line visibility. euspen

Nucleation and growth of gas bubbles in AlSi8Mg4 foam investigated by X-ray tomoscopy

Initiation and growth of metal foam is a complex and dynamical process, which is intrinsically three-dimensional and time-dependent. Tomoscopy –or time-resolved tomography– allows us to follow the nucleation and growth of gas bubbles in AlSi8Mg4 alloy in real time during foaming. The location, size and shape of individual bubbles was determined in steps of 1 s with spatial resolutions of a few µm. Moreover, the constituents responsible for gas evolution, namely Al-Mg phases and TiH2 particles, were identified in the series of 3D images. Automated quantitative image analysis of bubbles and gas-generated phases including their spatial correlations allowed us to break down the foaming process into two distinct steps, a first homogenous one driven by adsorbed gases and first melting microstructural components and a second attributed to the melting of the alloy and subsequent foam growth driven by hydrogen released from TiH₂ particles. The results of the study indicate that standard AlSi8Mg4 foam can be improved by tailoring the properties of the Al-Mg constituent powder.
Kamm, P. H., Neu, T. R., García-Moreno, F., & Banhart, J. (2021). Nucleation and growth of gas bubbles in AlSi8Mg4 foam investigated by X-ray tomoscopy. Acta Materialia, 206, 116583.

Simultaneous X-Ray Diffraction and Tomography Operando Investigation of Aluminum/Graphite Batteries

Rechargeable graphite dual‐ion batteries are extremely appealing for grid‐level stationary storage of electricity, thanks to the low‐cost and high‐performance metrics, such as high‐power density, energy efficiency, long cycling life, and good energy density. An in‐depth understanding of the anion intercalation mechanism in graphite is fundamental for the design of highly efficient systems. In this work, a comparison is presented between pyrolytic (PG) and natural (NG) graphite as positive electrode materials in rechargeable aluminum batteries, employing an ionic liquid electrolyte. The two systems are characterized by operando synchrotron energy‐dispersive X‐ray diffraction and time‐resolved computed tomography simultaneously, establishing a powerful characterization methodology, which can also be applied more in general to carbon‐based energy‐related materials. A more in‐depth insight into the AlCl₄^-/graphite intercalation mechanism is obtained, evidencing a mixed‐staged region in the initial phase and a two‐staged region in the second phase. Moreover, strain analysis suggests a correlation between the irreversibility of the PG electrode and the increase of the inhomogenous strain. Finally, the imaging analysis reveals the influence of graphite morphology in the electrode volume expansion upon cycling.
Elia, G. A., Greco, G., Kamm, P. H., García-Moreno, F., Raoux, S., & Hahn, R. (2020). Simultaneous X-Ray Diffraction and Tomography Operando Investigation of Aluminum/Graphite Batteries. Advanced Functional Materials, 2003913.

The Influence of Alloy Composition and Liquid Phase on Foaming of Al–Si–Mg Alloys

The foaming behaviour of aluminium alloys processed by the powder compaction technique depends crucially on the exact alloy composition. The AlSi8Mg4 alloy has been in use for a decade now, and it has been claimed that this composition lies in an “island of good foaming”. We investigated the reasons for this by systematically studying alloys around this composition by varying the Mg and Si content by a few percent. We applied in situ X-ray 2D and 3D imaging experiments combined with a quantitative nucleation number and expansion analysis, X-ray tomography of solid foams to assess the pore structure and pore size distribution, and in situ diffraction experiments to quantify the melt fraction at any moment. We found a correlation between melt fraction and expansion height and verified that the “island of good foaming” actually exists, and foams outside a preferred range for the liquid fraction—just above TS and between 40–60%—show a poorer expansion performance than the reference alloy AlSi8Mg4. A very slight increase of Si and decrease of Mg content might further improve this foam.
García-Moreno, F., Radtke, L. A., Neu, T. R., Kamm, P. H., Klaus M., Schlepütz, C. M. & Banhart, J. (2020). The Influence of Alloy Composition and Liquid Phase on Foaming of Al–Si–Mg Alloys. Metals, 10(2), 189.

Using X-ray tomoscopy to explore the dynamics of foaming metal

The complex flow of liquid metal in evolving metallic foams is still poorly understood due to difficulties in studying hot and opaque systems. We apply X-ray tomoscopy –the continuous acquisition of tomographic (3D) images– to clarify key dynamic phenomena in liquid aluminium foam such as nucleation and growth, bubble rearrangements, liquid retraction, coalescence and the rupture of films. Each phenomenon takes place on a typical timescale which we cover by obtaining 208 full tomograms per second over a period of up to one minute. An additional data processing algorithm provides information on the 1 ms scale. Here we show that bubble coalescence is not only caused by gravity-induced drainage, as experiments under weightlessness show, and by stresses caused by foam growth, but also by local pressure peaks caused by the blowing agent. Moreover, details of foam expansion and phenomena such as rupture cascades and film thinning before rupture are quantified. These findings allow us to propose a way to obtain foams with smaller and more equally sized bubbles.
García-Moreno, F., Kamm, P. H., Neu, T. R., Bülk, F., Mokso, R., Schlepütz, C. M., Stampanoni, M. & Banhart, J. (2019). Using X-ray tomoscopy to explore the dynamics of foaming metal. Nature communications, 10(1), 1-9.

Time-resolved in situ tomography for the analysis of evolving metal-foam granulates

An experimental setup has been developed that allows for capturing up to 25 tomograms s−1 using the white X-ray beam at the experimental station EDDI of BESSY II, Berlin, Germany. The key points are the use of a newly developed, precise and fast rotation stage, a very efficient scintillator and a fast CMOS camera. As a first application, the foaming of aluminium alloy granules at 923 K was investigated in situ. Formation and growth of bubbles in the liquid material were observed and found to be influenced by the limited thermal conductivity in the bulk granules. Changes that took place between two tomographic frames separated in time by 39 ms could be detected and analysed quantitatively.
García-Moreno, F., Kamm, P. H., Neu, T. R. & Banhart, J. (2018). Time-resolved in situ tomography for the analysis of evolving metal-foam granulates. Journal of synchrotron radiation, 25(5).

Fast Synchrotron X‐Ray Tomography of Dynamic Processes in Liquid Aluminium Alloy Foam

Series of fast synchrotron X‐ray tomographies are taken continuously at a rate of up to 5 Hz, while aluminium alloy precursors are foamed in an X‐ray transparent setup for several minutes using infra‐red (IR) lasers for heating. The entire foaming process from the solid precursor to the expanded liquid foam is captured. The analysis of the sequence of tomographies is done with an emphasis on nucleation and bubble growth. In early stages of foaming, bubble and crack formation and evolution are observed. We analyze the nucleation stage and obtain quantitative results for the number of nucleation centers and their distribution and derive the nucleation rate as a function of time.
Kamm, P. H., García‐Moreno, F., Neu, T. R., Heim, K., Mokso, R. & Banhart, J. (2017). Fast Synchrotron X‐Ray Tomography of Dynamic Processes in Liquid Aluminium Alloy Foam. Advanced Engineering Materials, 19(11), 1600550.

Tomoscopy

time-resolved tomography