Elsevier

Corrosion Science

Volume 100, November 2015, Pages 396-403
Corrosion Science

Influence of cooling rate on the microstructure and corrosion behavior of Al–Fe alloys

https://doi.org/10.1016/j.corsci.2015.08.017Get rights and content

Highlights

  • Increasing the cooling rate from 0.1 to 500 °C/s, mass loss rate decreased by 6 times.

  • Increase in corrosion resistance was attributed to the refined Fe-intermetallics.

  • Increased cooling rate resulted in increased Fe content in solid solution.

  • Direct strip casting can produce alloys with higher acceptable content of impurities.

  • Direct Strip Casting is a potential candidate to improve recyclability of Al alloys

Abstract

The effect of Fe in Al is technologically important for commercial Al-alloys, and in recycled Al. This work explores the use of the novel rapid solidification technology, known as direct strip casting, to improve the recyclability of Al-alloys. We provide a comparison between the corrosion and microstructure of Al–Fe alloys prepared with wide-ranging cooling rates (0.1 °C/s to 500 °C/s). Rapid cooling was achieved via direct strip casting, while slow cooling was achieved using sand casting. Corrosion was studied via polarisation and immersion tests, followed by surface analysis using scanning electron microscopy and optical profilometry. It was shown that the corrosion resistance of Al–Fe alloys is improved with increased cooling rates, attributed to the reduced size and number of Fe-containing intermetallics.

Introduction

Aluminium and its alloys are prone to pitting corrosion due to the presence of secondary phases (constituents, precipitates, and dispersoids) formed during casting and subsequent thermo-mechanical processing [1], [2], [3], [4], [5]. The influence of secondary phases on the corrosion of Al alloys largely depends upon their morphology and chemical composition [4], [6], [7], [8], [9], [10], [11], [12], [13]. Fe is present as an impurity in Al-alloys and tends to form coarse intermetallics (e.g., Al3Fe, AlFeMnSi, Al7Cu2Fe, and Al2CuFeMn) that are cathodic in nature and are known to cause severe pitting [2], [14], [15], [16], [17], [18], [19], [20], [21]. Fe cannot be avoided in commercial Al alloys, and its content increases during recycling [20]. Thus, Fe build-up limits the recyclability of high performance Al-alloys in which the Fe content is required to be less than 0.1 wt.% [22]. As a consequence, it is necessary to develop new technologies which could decrease the detrimental effect of Fe in Al-alloys [23], leading to the improved recyclability of these alloys [24].

Faster solidification is expected to result in smaller Fe-intermetallics and increased solid solubility in Al alloys [25] which is deemed to improve pitting susceptibility [8], [26], [27]. Any casting technology enabling a more rapid cooling rate is known to result in refined microstructure and therefore, a positive influence on the corrosion resistance is envisaged. Indeed, processing routes with high cooling rates are expected to offset the deleterious influence of Fe on corrosion of Al leading to an increase in tolerance limit of Fe in high performance Al alloys and consequently result in significantly increased recyclability of these alloys [28], [29], [30]. Various rapid solidification techniques such as melt spinning or splat quenching have been developed in the past, however such methods have limited potential for upscaling. In this paper, we investigate an industrial scale rapid solidification technique, direct strip casting (DSC), for Al alloys. Alloys were also produced with conventional casting techniques in order to provide a comparison of the effect of cooling rate on the alloys’ microstructures/properties.

The DSC technology with a vertical twin-copper roll apparatus has been recently implemented in the steel industry and provides a practical, industrially applicable, means for rapid solidification [31]. In the present study, we used DSC to rapidly solidify Al–Fe sheet samples and characterise the impact of increasing cooling rate on the intermetallics and corrosion properties. In DSC, extremely high cooling rates in excess of 500 °C/s are encountered. In such rapid cooling condition, the morphology and volume fraction of intermetallics have been poorly investigated [23]. Two alloy compositions, Al-0.1 wt%Fe and Al-2 wt%Fe, have been tested in the present study. Both cooling rate and composition influenced the composition, morphology and number density of the intermetallics. The refined intermetallics after DSC caused a major improvement in the corrosion resistance. As a result, we propose that rapid solidification technologies, such as direct strip casting, could be used to increase the tolerance limit of Fe in high performance Al alloys, thus improving their recyclability.

Section snippets

Alloy production

The two alloy compositions, Al-0.1 wt%Fe and Al-2 wt%Fe, were cast using three distinct casting routes:

  • 1)

    The highest cooling rate (∼500 °C/s) was achieved using a strip casting simulator, known as a dip tester, which is designed to simulate the initial contact between the melt and the copper rolls during the twin-copper roll casting process [32]. This design resulted in 35 × 35 × 2 mm samples.

  • 2)

    Small sand molds containing two steel plates separated by a 12 mm gap were used to achieve the intermediate

Thermal analysis

The cooling rates in the three casting techniques used herein were empirically measured, at a frequency of 10,000 Hz. The cooling rates were determined to be 0.1, 1, and 500 °C/s (over the range of 600 °C to 300 °C) for the three casting conditions used in this study. The solidification curves, for each casting condition, were measured during casting of Al-2 wt%Fe and are displayed in Fig. 1a. An exothermic event followed by a plateau at 642 °C can be identified in the slow cooling condition. These

Discussion

This study revealed that there is a significant effect of cooling rate on the Fe-intermetallic formation in Al-0.1 wt.%Fe and Al-2 wt.%Fe alloys. The high cooling rate, achieved via direct strip casting, resulted in the decrease of size and volume fraction of Fe-intermetallics, which caused a significant improvement of the corrosion performance.

Conclusions

  • 1)

    Cooling rate was found to have a strong influence on the microstructure and properties of Al–Fe alloys. The faster cooling rate decreased the size and surface area fraction of intermetallics, which resulted in a significant improvement in the corrosion performance of Fe containing Al alloys.

  • 2)

    Potentiodynamic polarisation and constant immersion tests revealed that increasing the cooling rate from 0.1 to 500 °C/s, for the Al-2 wt%Fe alloy, resulted in (1) a decrease in corrosion current density by an

Acknowledgements

The work conducted in this paper was funded by the Internal Deakin Impact grant. The authors would like to sincerely acknowledge David Gray who carried out the casting for this project. Laboratory assistant James Maxwell is warmly thanked for his involvement in the project.

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