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Aluminium Scrap: Open access database for impurity levels-microstructure-property and methods to recover properties in high impurity scrap

Periodic Reporting for period 1 - AluminiumScrapDbase (Aluminium Scrap: Open access database for impurity levels-microstructure-property and methods to recover properties in high impurity scrap)

Reporting period: 2015-09-01 to 2017-08-31

Primary Al production is an energy intensive process and the world's bauxite deposits are limited, the use of Al scrap accumulated from packaging, building, automotive and engineering sectors offers a sustainable solution to secure metal supply. However, the scrap from these sectors has higher levels of impurities in particular, iron (Fe). A higher concentration of Fe is detrimental to the mechanical properties due to formation of harmful large platelet structures of Fe-based intermetallics. Considering that recycling Al scrap takes 5% of energy compared to primary alloy production, the scrap is a valuable resource to meet the demand for Al.
The objectives of this fellowship are: (i) to develop methods to tolerate higher iron impurity levels in Al scrap alloys using model alloys containing impurities (ii) to understand the solidification behaviour of scrap alloys by attempting to tackle the problem from thermodynamic analysis (iii) to build database for impurity levels in scrap alloys-property-process dependence. To meet these objectives, several Al-Si alloys with varied concentrations of Fe as impurities have been produced. Phase relationships in these alloys have been analysed with CALPHAD and studied the solidification behaviour of these alloys at wider range of cooling ranges to simulate variety of industrial processes. The influence of heat-treatment on phase transformations, influence of grain refiners on intermetallics and some specific elements such as Cu and Ni have been studied in detailed. The improvement in mechanical properties through modification of Fe-rich intermetallics has been clearly observed. The results are disseminated at international conference and published in peer-reviewed open access journals and the metadata is also available for free access.
Work Package 1: Model scrap alloys with excessive impurities
Task 1.1: Within the frame work of CALPHAD (CALculation of PHase Diagram) approach PandaT commercial software were used in order to establish the phase relationships in terms of different isopleths in these alloy systems –
i. Al-Si-Fe system with Si (up to12wt% and Fe (up to 2wt%) .
ii. Al-Si-Fe-Cu system with Cu (up to 8wt%).
iii. Al-Si-Fe-Ni system with Ni (up to 8wt%).
Solidification behaviour of these alloy system has been predicted using equilibrium model and Scheil-Gulliver model.
Task 1.2: Based on the thermodynamic analysis the following model scrap alloys has been selected –
i. Al-(6-12wt%)Si-2wt%Fe
ii. Al-6wt%Si-2wt%Fe- (0-6wt%) Cu
iii. Al-(6-12wt%)Si-2wt%Fe- (0-8wt%) Ni
Task 1.3: In order to see the effect of different cooling rates two different types of casting methodology has been adopted, namely, gravity casting (GC) and high pressure die casting (HPDC).

WP2: Establish various approaches or method(s) to enhance the Fe-tolerance limit
Task 2.1: It was found that addition of Nb-B inoculation refines the grains in Al-Si alloys which has never been reported earlier. In fact it is worth mentioning here that commercial grain refiner Ti-B does not work in Al-Si alloy. However, neither Ti-b nor Nb-B was found to refine the β-phase in Al-Si-Fe alloys.
Task 2.2: Mn is known to destabilise the β-phase by promoting the formation of α-phase. Extensive work has been carried on to find the role of other alloying elements to destabilise the β-phase. It was found that addition of Cu and Ni beyond a certain concentration destabilise the β-phase by promoting Al7Cu2Fe and Al9NiFe phases, respectively.
Task 2.3: Morphological change of β-phase in both solid state and liquid state has been tried. It was found that the heat-treatment below the eutectic temperature (572oC) fragments the β-phase and spherodize the eutectic Si phase. On the other hand, heat treatment slightly above the eutectic temperature, at 580oC, refines the β-phase but promotes the growth of the Si particle in jagged needle like structure which deteriorates the overall mechanical properties of the alloys.
Task 2.4: HPDC process in cast alloys induces fine intermetallic phases. It was found that the size of the β-phase is smaller in HPDC alloys than in GC alloys. The difference in the size for the β-phase was clearly reflected in the mechanical properties of the same alloy in comparison to the GC alloys.
WP3: Study of intermetallic micro-mechanical properties
Task 3.1: Dissolution of aluminium matrix was carried out in 10%NaOH solution at room temperature in order to get the entire three dimensional structure of the β-phase. Scanning electron microscopy and x-ray diffraction analysis was carried out to characterize the extracted β-phase.
Task 3.2: Since it was found in Task 2.1 that no inoculation agent is capable of refining the β-phase; therefore no attempt has been made to prepare the pure intermetallic phase to see the effect of different inoculating agents.
Task 3.3: Microhardness testing along with tensile testing was carried out as per ASTME92 and ASTM B557 standard, respectively.
Task 3.4: The intermetallic phases in Al-Si-Fe-X alloys (X= Ni or Cu) was found to be in micrometer size. Field emission scanning electron microscope (FESEM) along with EDS and X-ray diffraction (XRD) technique were carried out to evaluate the phases and lattice parameters and volume fraction of all the phases present in any particular alloy. A summary of all tests done on specific alloys are listed under Section WP5.
WP4: TEM investigation on these Al-Si-Fe-X cast alloys were found unnecessary because of the large size of the precipitates and intermetallic phases. However, extensive microstructural characterization was carried out using FESEM, EDS and XRD, as detailed in WP5.
WP5: Develop a comprehensive database for mechanical property/intermetallic vs alloys composition: Based on the results obtained from optical microscopy (OM), FESEM, EDS (for elemental distribution and composition in the intermetallic phases), XRD, microhardness and tensile testing a comprehensive open database has been made. Such database will provide not only the photomicrographs of the concerning alloys but also the raw experimental data, thermodynamic data (phase diagram and solidification behaviour) and processed data. The following table lists all the alloys and characterization techniques that will be provided in this open database.
Total three full length articles have been published from the present research results. Details of these publications acknowledging the support of this EU grant is mentioned below along with the highlights of the importance of results.

1. Morphological change of β-phase (via solid-state heat-treatment) in low-Si high-Fe content recyclable Al-Si alloy is preferred choice to counteract the detrimental effect of Fe content. On the other hand, elimination of Fe via gravity segregation of β-phase is ideal method for high-Si content alloy. Mat. Des. 108 (2016) 277-288, DOI: 10.1016/j.matdes.2016.06.096
2. Morphological change of β-phase in Al-6wt%Si-2wt%Fe alloy is studied with Cu addition. Cu is shown to increase the strength with a slight loss in ductility, it also destroys the Al-Si eutectic structure and Si appears as blocky particles.
Light Metals (2017) 1139-1147
3. Cu addition is shown to thermodynamically destabilize the detrimental β-Al9Fe2Si2 phase in the Al-Si-Fe system and thereby offers an alternative route to the traditional Mn addition. Nature Scientific Reports 7 (2017)