Sonja Steinbach

Experimental study on interaction of fluid flow and solidification in Al–Si–Cu alloys

Abstract A hypoeutectic Al–6Si–4Cu (wt-%) alloy was used in the present study to investigate the effect of controlled fluid flow conditions on the microstructure formation in cast Al alloys experimentally. The alloy has been directionally solidified upwards under a medium temperature gradient (3 K mm−1) in an aerogel based furnace facility over a range of constant solidification velocities (0·015–0·15 mm s−1). A coil system around the sample induces a homogeneous rotating magnetic field being able to generate a controlled fluid flow in the melt. The application of rotating magnetic fields during directional solidification results in a strong segregation effect: for high magnetic induction and therefore high fluid flow velocity an enrichment of solute is observed at the axis of the sample. The primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases with increasing magnetic induction, clearly revealing the effects of convections on solidification kinetics.

In Situ Optical Determination of Fraction Solid of Al-Si-Mg Alloys

In this paper we report on a new optical technique to measure in situ the fraction solid during solidification. The technique utilizes the extreme properties of silica aerogels, being optically transparent in the visible and near infrared. From measured brightness time profiles the fraction solid can be derived using a suitable theoretical approach. The technique is tested on a technical AlSiMg alloy (A357) solidified directionally in the furnace facility Artemis. The results are compared with the well known theoretical expressions of the lever rule and the Scheil relation. The measured fraction solid as a function of temperature agrees well with model of Scheil which shows the capabilities of the new technique.

The Influence of Fluid Flow on Intermetallic Phases in Al-cast Alloys

Technical Al-Si alloys always contain sufficient amounts of Fe and Mn, especially alloys made from scrap. During casting, Fe-containing intermetallics, such as Al-Fe, Al-Fe-Si and Al-Fe- Mn-Si phases, are formed between the aluminum dendrites. Fe and Mn-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al-Si alloys. In the present work the influence of controlled fluid flow conditions on the morphology and spatial arrangement on intermetallic phases in cast Al-Si alloys is characterized. A binary Al-7wt.%Si and a ternary Al- 7wt.%Si-1wt.%Fe alloy was solidified under and without the influence of a rotating magnetic field (3mT at 50Hz) over a range of solidification velocities (0.015- 0.18mm/s) at a constant temperature gradient G of 3K/mm. The scientific results reached so far indicate a strong influence of the electromagnetic stirring on the primary dendrite and secondary dendrite arm spacings.

MICAST – Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions

The MICAST research program focuses on a systematic analysis of the effect of convection on the microstructure evolution in cast Al-alloys. The experiments of the MICAST team are carried out under well defined thermally and magnetically controlled, convective boundary conditions and analyzed using advanced diagnostics and theoretical modeling, involving phase field simulation, micro-modeling and global simulation of heat and mass transport. The MICAST team uses binary, ternary and technical alloys of the Al-Si family. This paper gives an overview on recent experimental results and theoretical modelling of the MICAST team.

ARTEX - In-situ observation of directional solidification of binary aluminium alloys on TEXUS

A binary Al-6wt. % Si alloy was directionally solidified during the TEXUS 39 mission to compare diffusive solidification conditions with convective ones. In addition, the operativeness of a new furnace technology ARTEX using fragile aerogels as a crucible material in microgravity was demonstrated. The TEXUS 39 sample is evaluated with regard to the processing parameters and microstructural features. A reduction of the secondary dendrite arm spacing and spacing of the interdendritic eutectic and an increase of the primary dendrite spacing is observed under microgravity compared with experiments under 1g conditions. To obtain a fundamental understanding of the influence of convections on the microstructure evolution the ARTEX facility is now equipped with a rotating magnetic field device being able to generate a controlled fluid flow in the melt during solidification. The forthcoming flight experiment ofARTEX+ on TEXUS 41 in November 2004 should lead to a direct comparison of pure diffusive growth conditions (TEXUS 39) and controlled convective conditions (TEXUS 41) using the same alloy and also to compare the results with extensive lab research.

MICAST — The effect of magnetically controlled fluid flow on microstructure evolution in cast technical Al-alloys

The MICAST research program focuses on a systematic analysis of the effect of convection on the microstructure evolution in cast Al-alloys. The experiments within MICAST are carried out under well defined thermally and magnetically controlled, convective boundary conditions and analyzed using advanced diagnostics and theoretical modeling, involving phase field simulation, micro-modeling and global simulation of heat and mass transport. The MICAST team uses as a model material the Al-Si base alloys. This paper gives a brief overview on recent experimental results of the MICAST team on the effect of rotating magnetic fields on microstructure in AlSi alloys.

Solidification of AlSi Alloys in the ARTEMIS and ARTEX Facilities Including Rotating Magnetic Fields – A Combined Experimental and Numerical Analysis

The solidification microstructure is the consequence of a wide range of process parameters, like the growth velocity, the temperature gradient and the composition. Although the influence of these parameters is nowadays considerably well understood, an overall theory of the influence of convection on microstructural features is still lacking. The application of time dependent magnetic fields during directional solidification offers the possibility to create defined solidification and flow conditions. In this work, we report about solidification experiments in the ARTEMIS and ARTEX facilities including rotating magnetic fields (RMF). The effect of the forced melt flow on microstructural parameters like the primary and secondary dendrite arm spacing is analyzed for a wide range of magnetic field parameters. The experimental analysis is supported by a rigorous application of numerical modeling. An important issue is hereby the prediction of the resulting macrosegregation, i.e., differences in the composition on the scale of the sample (macroscale) due to the RMF. For the considered configuration and parameters an axial enrichment of Si is found beyond a certain magnetic field strength. The results are compared to available theories and their applicability is discussed.

Mushy zone coarsening with fluid flow

Abstract Cylindrical samples of a near-eutectic AlCu30 alloy are annealed applying constant axial temperature gradients to directionally molten samples in an aerogel furnace. During annealing with various times and gradients also a rotating magnetic field (RMF) of 6 mT was applied leading to azimuthal and meridional flows of well known magnitude. The specific surface area of the primary phase was measured on metallographic in section perpendicular to the sample axis with a fixed amount of fraction solid. The specific surface area varies as the inverse cube root of annealing time if no RMF is applied, but varies as an inverse forth root at 6 mT. The experimental procedure and results are presented in detail and compared to isothermal coarsening measurements of Voorhees and co-workers.

Formation of intermetallic phases in AlSi7Fe1 alloy processed under microgravity and forced fluid flow conditions and their influence on the permeability

Ternary Al-6.5wt.%Si-0.93wt.%Fe alloy samples were directionally solidified on-board of the International Space Station ISS in the ESA payload Materials Science Laboratory (MSL) equipped with Low Gradient Furnace (LGF) under both purely diffusive and stimulated convective conditions induced by a rotating magnetic field. Using different analysis techniques the shape and distribution of the intermetallic phase β-Al5SiFe in the dendritic microstructure was investigated, to study the influence of solidification velocity and fluid flow on the size and spatial arrangement of intermetallics. Deep etching as well as 3-dimensional computer tomography measurements characterized the size and the shape of β-Al5SiFe platelets: Diffusive growth results in a rather homogeneous distribution of intermetallic phases, whereas forced flow promotes an increase in the amount and the size of β-Al5SiFe platelets in the centre region of the samples. The β-Al5SiFe intermetallics can form not only simple platelets, but also be curved, branched, crossed, interacting with dendrites and porosity located. This leads to formation of large and complex groups of Fe-rich intermetallics, which reduce the melt flow between dendrites leading to lower permeability of the mushy zone and might significantly decrease feeding ability in castings.

Formation of intermetallic phases in AlSi7Fe1 alloy processed onboard the ISS

This paper provides an analysis of the formation of intermetallic phases in AlSi7Fe1 alloy in samples processed onboard the ISS. Based on axial 2D cross-sections obtained from regions of pure diffusive growth and also solidified with forced melt flow, the sizes and distribution of intermetallic β-Al5FeSi phases were determined for different solidification velocities. In diffusive case the phases are larger and more homogeneously distributed than in case of induced melt flow. Additionally, especially for lower solidification velocity, the enrichment of Si and Fe in the centre part of the sample results in a few but rather large β-Al5FeSi particles.