Direct aluminium-alloy upcycling from entire end-of life vehicles

Domain: neuroscience
Published: 2026-02-13
Editorial outcome: minor_revision

Abstract
The global transition to a circular economy hinges on the development of sustainable recycling processes for end-of-life vehicles. Ongoing electrification and material choices over the recent decades hinder their integration in existing recycling pathways. This results in a large surplus of low-grade aluminium scraps and forfeits substantial energy, emissions, and cost savings, making the need for novel recycling approaches an urgent problem. This study presents a process for directly upcycling mixed end-of-life vehicles scrap into a high-performance aluminium alloy under realistic industrial conditions. It is compatible with existing infrastructure and dispenses the need for sorting, dilution or downcycling. By leveraging metallurgical principles and accelerated precipitation, the produced alloys achieve yield strengths that even surpass the commercial automotive alloy spectrum. This approach establishes a circular, low-emissions route to high-value aluminium recovery and offers a strategic model for transforming today´s and future´s critical raw material streams into next-generation structural alloys.

Introduction
In 2017 alone, 7–9 million metric tons (MT) of end-of-life vehicle (ELV) scrap was generated across the European Union1. Recycling the associated materials presents substantial challenges, driven by a complex interplay of chemical, technological, economic and regulatory factors. From a thermodynamic perspective, removal of tramp elements introduced through aluminium (Al) scrap is inherently difficult, the more noble the tramp element2–4. They dissolve in the metal Al phase2–4, demanding energy-intensive or technologically immature processes or environmentally hazardous reaction agents (e.g. Cl2-gas-mix as purge gas) for a theoretical melt purification5–7.

Current ELV recycling processes involve shredding and separation of steel and other materials from the Al fraction, but there are limits on the levels of purity reachable by these methods8. Strict regulatory alloy standards further complicate the production of high-quality wrought Al alloys via recycling9. An average European vehicle from 2019 contains 26 different Al alloys across 14 different applications10,11. In the conventional recycling process, depicted schematically in Fig. 1a (top left) below, the sorting of scrap is subject to natural limitations and often fails to completely separate the multiple Al alloys8, also because they are joined (i.e. by welding12). Therefore, the production of wrought alloys, which must typically feature low amounts of alloying elements and are subject to strict chemical standards, requires substantial amounts of primary Al to dilute the concentration of tramp elements8,13,14. Only a comparably simple melt purification step targeting a specific, limited set of tramp elements (e.g. Na, Ca, Li, Mg) is proven operational7. Hence, the largest Al scrap stream by far is currently downcycled into cast alloys14,15 for combustion engines, whose mechanical properties are greatly inferior to those of wrought alloys or primary structural cast alloys9,16,17. In 2012 alone, this stream amounted to 6.1 MT18 globally. With declining demand for the alloys involved, it is projected that by 2030 no less than 6 MT of low-quality scrap will become unrecyclable each year14,18. This trend involves substantial economic costs, estimated at ~ 15 bn $ at 2025 Al prices19, and will have severe environmental consequences, including an estimated 20,21 additional 90 MT of CO2 emissions annually. Industry and academia must urgently address the current problem arising from the cars on the roads today in the coming years. However, designing sustainable cars and alloys22,23 to prevent such new recycling dilemmas in the future is also crucial, but unfortunately seems far from global realisation.

Our one-step direct upcycling approach, presented schematically in Fig. 1a (lower left), addresses the current recycling challenge by bypassing the need for Al alloy sorting and eliminating the requirement to use primary Al to dilute impurities. Our approach disregards current alloy composition boundaries, especially those affecting silicon, iron and copper13,24–27. This contradicts the common rule in metal recycling. It posits that the purer the input materials the better their properties18. A reason for this behaviour is that many tramp elements form brittle intermetallic phases (IMPs) in Al18.

However, in the context of steels Kim et al.28 showed that brittle IMPs can be utilised to increase strength at high ductility. Recently, we discussed a heterostructure approach29 utilising brittle IMPs as the hard phase and Al as the soft phase to improve strain hardening in a 6xxx alloy27. We also assessed whether the concept could be considered for compositions generated by the Al alloys of various ELVs24. Our initial evaluation was conducted under rapid solidification conditions to control IMP morphology, conditions that are currently not applicable to the required industrial scale. Moreover, contamination with other metals from full ELVs recycling (i.e. Fe, Cu) were not considered, and the findings of that study24 indicated modest yield strengths (150-190 MPa), below current 6xxx-series automotive standards. Further, many alloys must facilitate rapid strengthening upon the formation of nanoscale precipitates within the short heat treatment cycle used in manufacturing (called paint-baking, typically 20 min at 180 °C in the automotive industry)30, which was obviously not achieved in our initial study24. However, despite these shortcomings, the initial results motivated us to address these problems. While we also explored new processes (e.g. friction extrusion for solid-phase alloying31), we chose to use the well-established mass production metallurgical infrastructure, as only this will be able to address the global problem in time and scale.

In this study, we utilise the full Al-stream of ELV scrap under realistic industrial conditions, such as direct chill casting, which is the central aluminium casting technology in the aluminium industry18, even more prominent for automotive products. We maximise grain refinement via particle-stimulated nucleation (PSN)32, exploit heterostructure effects, and enhance mechanical properties via an accelerated precipitation-strengthening concept. This approach enables us to achieve an exceptional strength-ductility balance in directly upcycled alloys: yield strengths exceeded 350 MPa, and high ductility is maintained. Most importantly, the process aligns with prevailing mass-production conditions, ensuring its pressing integration into existing manufacturing infrastructure.

Results
Compared to existing commercial automotive alloys, alloys directly upcycled from different vehicle types can reach an exceptional balance of strength and ductility (see Fig. 1b and Supplementary Fig. 2). The alloy derived from an average EU vehicle (EU)10,11, representing the largest ELV scrap stream, has a markedly different composition than that of standardised alloys, as does the alloy from a US pickup truck (PU)33,34. In contrast, the electric-car-oriented alloy (EC35) shows elevated iron levels but remains compositionally closer to conventional 6xxx-series alloys. Not too surprisingly, the strength-ductility plot reveals that rapid solidification at a cooling rate of 60 K/s36, characteristic of some thin-strip casting techniques37, delivers the most favourable basis for achieving good mechanical performance in further-processed sheets, consistent with previous findings26,38. What is fascinating, however, is that the performance of the directly upcycled alloy exceeds the tensile strength range (Fig. 1b) where current automotive wrought alloys (5xxx and 6xxx) are found. The key finding is that, under readily scalable industrial direct chill and continuous casting conditions (block-centre solidification rates of 1–3 K/s, higher at the ingot skins)36,37,39,40, remarkable properties can also be retained. However, details are key. At 3 K/s the strength-ductility balance remains superior in most cases, whereas at 1 K/s mechanical properties can degrade significantly. Across all cooling rates, the mechanical properties of directly upcycled alloys outperform those of typical cast alloys, with only highly specialised (primary) cast alloys exhibiting comparable elongations9,16,17.

As an example, we will examine the PU alloy during processing, as it demonstrates an exceptional strength-ductility balance while exhibiting significant sensitivity to processing conditions, such as a variation in achievable elongation at similar strength by a factor of 1.47 (NA) to 2.11 (2% plastic pre-strain) (Supplementary Fig. 2). Figure 2 presents the mechanical and microstructural characteristics in both the as-cast and homogenised states, revealing a strong dependence on cooling rate. In the as-cast condition, elongation ranges from 0.5±0.3% at 1 K/s to 4.7±1.3% at 60 K/s, while in the homogenised state, it increases from 0.6±0.4% at 1 K/s to 11.3±0.6%. A similar trend is observed in the EU and EC alloys (Supplementary Fig. 3a, b). These results correlate with microstructural observations, which show clear refinement and morphological changes on the part of IMPs at an increasing cooling rate under both as-cast (Fig. 2b–d) and homogenised (Fig. 2e–g) conditions. Greater undercooling and lower temperatures generally promote nucleation and reduce diffusion rates41,42. IMP types containing Fe are known to shift from α-AlFeSi (Chinese-scripted) to β-AlFeSi (needles) at decreasing cooling rates, as these phases form interdendritically at lower cooling rates and superheating temperatures26,43. In light of IMP morphology and literature regarding cast alloys, β-AlFeSi phases are expected to have a more detrimental effect on ductility than rounded α-AlFeSi phases26,43. This is clear if 60 K/s is compared with 3 K/s, but less clear if we consider 1 K/s.

Upon homogenisation (Fig. 2e–g), the eutectic Si network undergoes spheroidisation through disintegration, spheroidisation and particle growth, leading to enhanced ductility44,45. IMPs containing Fe typically require annealing times of more than 150 h or temperatures above 560 °C to undergo significant modification46, at which point the direct upcycling alloys are at risk of initial melting. Slow batch-cooling after homogenisation, in turn, triggers a transformation from α to β-AlFeSi40. The differences between 1 to 3 K/s are less obvious in the as-cast state than when 60 K/s is applied. However, after homogenisation for 3 K/s needle-like IMPs exist whose projected thickness (<3 µm) is smaller than that of IMPs formed upon 1 K/s (projected thickness of <6 µm). In the following, this is discussed as a parameter for improving ductility in deformed states, because thin needles are more advantageous than thicker spherical or cubic morphologies during fragmentation upon processing.

Figure 3a presents the mechanical performance of the PU alloy under selected processing and heat treatment conditions. Strain hardening, a key parameter for sheet formability26,29,30, is significantly greater in the PU alloy, particularly in the PA condition, than that of the standard wrought automotive 6016 alloy. Faster cooling yielded higher strain-hardening potential; the EU alloy even exhibits a slightly higher strain-hardening potential, in contrast to the EC, which has a lower one. Additional data are provided in Supplementary Figs. 4c, 5c and 6c. Besides the obvious effect that an increasing solute content increases strain hardening47–49, the superior strain hardening behaviour of EU and PU alloys originates primarily from the high volume fraction of IMPs in beneficial sizes (small) and morphology (aspect ratios of around 1)26,50, as shown in Fig. 3b. This refined structure results from the initial structure after homogenisation and fragmentation upon rolling. In this morphology, IMPs enhance strain hardening via the formation of excess geometrically necessary dislocations (GNDs) (Fig. 3b–d), thereby increasing elongation through the concept of hetero-deformation-induced (HDI) strain hardening29.

IMPs also promote PSN, thereby reducing Al grain size and improving formability. Notably, the average grain size (equivalent circle diameter) decreases with increasing cooling rate, from 21±7 µm at 1 K/s to 16±5.5 µm at 3 K/s, and further to 11±3.5 µm at 60 K/s. In regions surrounding IMPs, GNDs accumulate, creating favourable nucleation sites that facilitate grain refinement26,27,32. During recrystallisation, these sites lower the nucleation energy, while pinned grain boundaries suppress further grain growth26,27,32. This is corroborated by Kernel Average Misorientation (KAM) overlay maps of the strained samples (Fig. 3b–d), where higher misorientation values indicate a greater density of GNDs27. The KAM maps show that these elevated values are concentrated around IMPs, whereas the values at grain boundaries (marked by white lines) do not exceed the grain interiors significantly. Notably, this behaviour remains consistent regardless of cooling rate, pre-straining, or IMP chemistry (see Supplementary Information).

Refinement is most effective at 60 K/s, remains substantial at 3 K/s, and is least pronounced at 1 K/s. Changes in IMP characteristics (Figs. 2b–d and 3b–d) correlate directly with the observed decrease in ductility (19±1.1% to 11±2.3%) for the PANAPB state as the cooling rate decreases (Fig. 3a).

In conclusion, the (unexpected) superior ductility of highly contaminated alloys arises from a synergistic effect of IMPs on grain refinement and enhanced strain hardening, both parameters being physically linked to increased uniform elongation51. This finding challenges conventional expectations of ductility in heavily contaminated alloys which until now have been limited to cast alloys. Currently, no wrought alloys exist within this complex compositional space that can leverage the beneficial role of IMPs while serving as sinks for tramp elements. However, the optimal type, size, and number density of IMPs remain undefined, leaving significant opportunities for further research.

An examination of the strengthening response of the PU alloy upon paint baking revealed a moderate value for PANAPB, which is slightly below the typical value of 601622. However, adding pre-straining (PANA5%PSPB) paint baking generated a yield strength (YS) of 345±2 MPa with 400±3 MPa ultimate tensile strength (UTS) and 11±2% elongation at fracture (A) (Fig. 3a), a strengthening superior to that of automotive alloys. With values of 350±5 MPa YS, 410±2 MPa UTS and 12.3±0.9% A the 60 K/s samples range only a bit higher than the scalable direct chill casting variant at 3 K/s. The sample cooled at 1 K/s, however, shows lower strength and ductility (Supplementary Fig. 5a, b). The values for the EU alloy (Supplementary Fig. 4a, b) are only slightly lower than those of the PU alloy, although the composition differs (mainly 0.45 % more Si and 0.6 % less Cu), pointing at compositional robustness for internal combustion engine vehicles. The values of the EC range lower (Supplementary Fig. 6a, b).

To further assess microstructural properties on the atomistic scale, we show atom probe tomography (APT) data in Fig. 3e–g. Starting with small Si-Mg-Cu-clusters after PA (Fig. 3e), growth is observed when solely paint baking (Fig. 3f). If pre-straining is introduced, needle-shaped Q’ precipitates are formed during paint baking, which are known to have higher strengthening capability (Fig. 3g)52. Similar to 6xxx series alloys25,53, the highest overall number density and smallest cluster size are observed in the PA condition, followed by the PANAPB material. Significantly, precipitates of 500 or more atoms or those larger than 5 nm appear only in pre-strained material (Supplementary Fig. 8a, b), indicating growth through pre-straining and paint-baking. The volume fraction of particles increases from 1.48% in the PA condition to 2.34% after PB and nearly doubles to 4.63% after 5% pre-straining. All of this supports precipitation enhancement by pre-straining18,25,54. This can be explained by the large fraction of additional excess vacancies and dislocations (see Supplementary Note 7 for the calculation and Supplementary Fig. 10), enhancing diffusion by a factor of 8.41·105 (for Mg) and even 2.35·106 for Cu. It is remarkable that the PANA5%PSPB route enables the material to achieve a structural state more comparable to a peak-aged condition rather than the underaged state typically associated with the short duration of a paint bake process, thereby resulting in the observed high yield strength30. Transmission electron microscopy (TEM) shows the increased dislocation density from PA material over the PANAPB state (see Supplementary Fig. 9a, b) towards the pre-strained sample (Fig. 3h), also indicating a higher number of dislocation-particle interactions. High-resolution TEM and STEM micrographs (inserts to Fig. 3h) show precipitate growth at pinned dislocations in the pre-strained material, a behaviour not obvious in non-pre-deformed samples; this is also demonstrated schematically in Fig. 3i. The hardening precipitates are confirmed to be Q’ phase by the diffraction pattern and its reconstruction according to literature55 (Supplementary Fig. 9c), which points to a more mature state than typically seen after a paint bake30,52.

The results gathered from our mechanical and microscopic investigations indicate a dependence on the processing path. In the as-cast condition, the alloys exhibit brittleness and low elongation at fracture, which is significantly improved by homogenisation treatment. Subsequent rolling steps promote the distribution of IMPs, favourable for generating a fine final microstructure. By optimising the sequence of heat treatments and pre-straining steps following solution annealing and quenching, impressive strength and ductility were achieved under readily scalable direct chill casting conditions, opening a pathway to explore further properties (i.e. corrosion behaviour) for high-performance applications. While achieving this superior strength requires precise control over the final thermo-mechanical processing, the results also indicate that a wider range of properties is achievable (Supplementary Fig. 2).

In summary, this study effectively addresses the largest problem in future aluminium recycling and presents a simple, direct upcycling process for ELVs that produces wrought alloys without requiring scrap sorting or dilution with primary aluminium during melting. The high content of foreign elements in these alloys offers significant advantages which outweigh the potential downsides of slightly reduced elongations. To demonstrate the robustness of our approach, we addressed a range of alloy compositions and cooling conditions, including those achievable in industrial direct chill casting processes. We also explored various heat-treatment procedures and pre-straining parameters, producing combinations of mechanical properties that, surprisingly, even outperform typical automotive alloys in some cases. The comparable property range resulting from the average European car and US pickup alloys demonstrates notable compositional robustness. Achieving this robustness seems feasible with typical shredders processing more than 100 vehicles per hour56 and typical recycling furnaces handling scrap from over 50 ELVs57. The sustainable process eliminates the need to (i) pre-sort different aluminium alloy groups, (ii) dilute with primary aluminium. Instead, it directly upcycles mixed aluminium scrap from ELVs into high-quality secondary wrought alloys. Crucially, our approach relies exclusively on a novel combination of processes already available at an industrial scale, offering a viable, immediately deployable pathway to overcome current downcycling and address the growing stream of nonferrous metal scrap.

Methods
Alloy production
The principal concept by which we calculated the composition of ELV alloys was presented in a previous study24 (see Fig. 1, Supplementary Fig. 2 and Supplementary Table 1 and the underpinning literature data9–11,33–35). For comparison, the PU alloy was updated according to the approach of Zhu et al.34. and the Fe level was increased by 0.7% to simulate contamination by unremoved steel scrap. The alloys were synthesised in two different ways: melting on a laboratory scale (100–110 g batch size, 50x42x15 mm³ ingot dimensions) in a miniaturised induction furnace (Indutherm, MC100V) and on a small scale (~8 kg batch size, 185x85x45 mm³ “filet block” dimensions with representative cooling rate) in a resistance furnace (Nabertherm). To ensure high cooling speeds of ~60 K/s36, representing thin-strip casting, a copper mould was deployed in the former. To represent direct chill casting cooling conditions, two pre-heated steel moulds were used. One of these was connected to Mettop’s58 ILTEC cooling system to raise the cooling rates from ~1 K/s (air cooling) to ~3 K/s36. Please note that this process was designed by our group in a previous work36 aiming at producing industrially relevant homogenous microstructures to represent the range of relevant cooling rates in DC casting in a technically feasible homogenous manner. Here, AlTi5B1 was also added to the melt for grain refinement at 0.1 m-%.

Sample processing and mechanical characterisation
Nabertherm circulating air furnaces were deployed for the homogenisation process. This started with 10 h of heating from RT to 450 °C, after which the temperature was held for 10 h. After further heating at 10 K/s, the final temperature of 520 °C was held for 10 h, followed by furnace cooling.

For hot rolling, the ingots were milled to 12 mm in thickness for the laboratory scale procedure and to 18 mm for the small-scale procedure. They were then heated in the circulating air furnace to 430 °C within 1 h, and this temperature was held for 3 h before the first step. The ingots were hot rolled in a miniaturised rolling mill to 3.4 mm in thickness with step sizes of ~1 mm. Cold rolling was conducted from 3.4 mm to 2.4 mm with step sizes of 0.2 mm. Intermediate annealing in the circulated air furnace included heating to 370 °C within 4 h, holding this temperature for 10 h and furnace cooling. Final cold-rolling to 1.2 mm sheet thickness was again performed with step sizes of 0.2 mm. Standardised A30 (30 mm initial gauge length) tensile samples were then milled from the finished sheets.

Solution annealing of tensile test samples was also conducted in the circulating air furnace, where a contact heater was placed. This ensured that the samples would reach the solution annealing temperature of 520 °C within one minute (mimicking industrial continuous heat treatment lines). The whole cycle took 10 min and was followed by subsequent water quenching.

The following heat treatment steps were carried out in circulating oil baths (Lauda): Pre-aging treatment, which took 5 h at 100 °C; and paint-baking for 20 min at 180 °C. Natural aging was performed in a Peltier-cooled incubator for 14 d at 25 °C.

For tensile tests under as-cast and homogenised conditions, for the small-scale ingots, 5 mm-thick slices were cut from the original ingot and milled to 2 mm in thickness and further to A30 samples. For the laboratory scale ingots, where the initial gauge length was 25 mm, the same procedure was applied.

The device used for the pre-straining and tensile testing was a Z100 from Zwick/Roell with a load cell of 50 kN and pneumatic sample holders. The vertical speed of the crosshead was 0.0025 s-1 after Rp0.2; for a more accurate determination of Rp0.2 the speed was lowered to 0.00025 s-1 around it. For pre-deformation, the machine was stopped at strains of 2 and 5 %. For a summary of the parameters see Supplementary Table 2.

Microstructure analysis
SEM/EBSD. A 10 mm slice was cut off the ingots before and after homogenising. The slices were then ground and polished mechanically during the last step using an OPS agent.

Scanning electron microscopy (SEM) was conducted with an FE-SEM Jeol 7200F instrument. Back-scattered-electron (BSE) micrographs were taken for the alloys investigated, and energy-dispersive X-ray (EDX) analysis of their six main alloying elements was performed. An Oxford Symmetry S2 detector was utilised for the EBSD. To enable a comparison between these different conditions, the results were determined as the equivalent circle diameter (ECD)59. Grain boundaries were defined as boundaries with a misorientation greater than 5°. Al grains larger than 10 pixels were considered for grain size analysis. The kernel average misorientation was determined for the PU alloy.

TEM. For TEM investigations, samples of 20 × 10 mm were cut from the remaining sheets for analysis without pre-deformation. For pre-deformed samples, they were cut from the tensile samples. They were then ground to 80 µm in thickness, and 3 mm cylindrical samples were stamped from the ground sheets. Electropolishing was then performed using a 25% HNO3-75% methanol electrolyte as a polishing agent at a temperature range of −18 to −25 °C and voltages between 12 and 14 V. A Thermo Fisher ScientificTM Talos F200X G2 machine was utilised at 200 kV to analyse the samples.

APT. For APT investigations, needle-shaped specimens (blanks) measuring 0.7 × 0.7 × 20 mm3 were prepared from the head or gauge (mandatory for pre-strained samples) of a tensile sample after the final heat treatment. Further preparation involved a two-stage standard electropolishing process (1st stage: 25% HNO3 in methanol; 2nd stage: 2% HClO4 in 2-butoxyethanol) to minimise potential natural aging.

APT experiments were conducted in voltage mode with a pulse fraction of 20%, a frequency of 200 kHz and a detection rate of 1% at a temperature of 30 K using a LEAP 5000 XR. Data reconstruction60 was performed using IVAS 3.8.14, with the maximum separation algorithm employed for cluster identification61–63. The 10th-nearest-neighbour distance distribution method was deployed to determine the maximum separation of solutes within a cluster. The volumes of the APT reconstructions were quantified using the built-in “Alpha-Shape” function64 in MATLAB™. The volume fraction was calculated as the ratio of cluster atoms to the total number of atoms within each APT specimen. The corresponding error margins for number densities and solute concentrations in matrix and clusters65 were computed according to the methodology described in literature and the Guinier radii66. The composition of solute clusters was extracted from cluster search data using customised scripts67, and the matrix composition by subtracting the number of atoms of solute elements (e.g., Mg, Si, Cu, Zn) in clusters and/or precipitates from the total number of atoms. For radial distribution function (RDF) analysis, Al, AlH1 and AlH2 represent Al. Random comparator curves are drawn around the existing atom positions 40 times within the set and the nearest-neighbour distribution68,69. The RDF can then be calculated according to Eq. (1), with Ri representing the position vector of a specified atom68:

RDF = Hist ||Ri − Rj|| / (i≠j) (1)

Data availability
The raw data generated in this study have been deposited in the Zenodo repository under the accession code Doi: 10.5281/zenodo.1719276670.

Code availability
The python script for evaluating the diffusion enhancement upon pre-straining is also deposited in the Zenodo repository under the accession code Doi: 10.5281/zenodo.1719276670.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The study addresses the urgent challenge of recycling aluminum from end-of-life vehicles (ELVs). Current methods of\nrecycling often result in the downgrading of mixed aluminum scrap into low-grade cast alloys, which requires sorting or\ndilution with primary aluminum. The authors propose a scalable process that upcycles mixed aluminum scrap from ELVs\ndirectly into high-performance alloys. This work is highly meaningful and impactful because it addresses challenges in\naluminum sustainability and promotes a low-carbon pathway.\n1. When discussing the issues of existing recycling methods, it would be comprehensive if the authors could mention not\nonly the addition of primary aluminum to dilute but also the significant energy consumption and negative environmental\nimpact of the chemical processes used to remove impurities. One example is the process of removing magnesium with\nchlorine or fluorine.\nUpdate Figure 1a accordingly.\n2. Considering the significant improvement in strength and sound ductility compared to all the PU samples with 6016 (Figure\n3a), it would be nice to have an in-depth discussion here. Usually, the ductility of mixed recycled Al is poor because it\ncontains a high content of tramp elements (e.g., Fe>1%), which exist as large IMPs and are detrimental to the ductility. But\nhere, even the PU 1k/s sample shows good ductility after processing. Can authors explain more about how this method\navoids the coarse IMP in conventional methods? Provide a microstructure made via conventional methods with the same\nchemical composition, if possible. I believe it would make the outstanding results of this work more prominent.\n3. It is well known that the dimensions of castings affect the cooling rate, especially in the center of the casting. Could the\nauthor please specify the dimensions of the cast in this research work? Is there any difference/challenges when scaling up\nthis method to the industry level? E.g., conventional cast and giga-cast?\n4. Also, can authors provide the size of tensile specimens used in this study?\n5. Some grammar mistakes and non-standard terminology\na. Rewrite the sentence from lines 30-32.\nb. Rewrite the sentences from lines 57-59.\nc. Suggest using “million metric tons (MT)” instead of “Mio. t”\nd. Replace “für” with “for” at line 224\ne. Suggest using “utilized/utilizing/specialized/homogenize” instead of “utilised/utilising/specialized/homogenise”, etc.\nf. I guess ND means “non-deformed”, is that correct? Please spell out when using it for the first time.", "reviewer_2": "Thank you very much for submitting nice paper to Nature Communication. I have carefully read the manuscript. This paper\ncontains very interesting technical concepts. Unfortunately, however, I have decided that it is appropriate to reject this paper\ndue to the following reasons:\n1) It does not state a clear conclusion as an academic paper, and is judged to merely restate the basic concepts presented in\ntheir previous paper (Patrick Krall, Irmgard Weißensteiner, Stefan Pogatscher: Recycling aluminum alloys for the automotive\nindustry: Breaking the ource-sink paradigm, Resources, Conservation and Recycling, 202 (2024), 107370). If this paper had\nbeen submitted to this journal first, the decision might have been different; that is, this paper merely reaffirms the basic\nconcepts of the previous paper and presents the results of some case studies.\n2) The authors' discussion focuses on some of the mechanical properties of aluminum (tensile strength stress-strain property\netc.). Actually they are showing very interesting results. On the other hand, the properties that practical metallic materials\nmust satisfy are not limited to mechanical properties; physical and chemical properties (electric conductivity, corrosion\nresistance etc.) are also extremely important. These properties are highly dependent on chemical composition, so that it is\nessential to correlate the relationship between each property and composition. Such detailed information is best presented\nin a materials science journal rather than this journal.\n3) The biggest problem in recycling aluminum is contamination by the uncontrollable alloying elements. This problem has\nbeen discussed by many researchers (Reuter, Nakajima, Graedel etc.). but has rarely been cited. This is a topic that is\nappropriate for the Nature Communication. The essential solution of this problem lies in controlling or removal of the alloying\nelements in secondary aluminum, which are present in much higher concentrations than in other base metal materials like\nsteel. It must be said that the technology presented in this paper is merely a temporary solution (I understand the importance\nof a temporary solution). The secondary aluminum alloy developed by the authors will eventually be recycled again, which\nwill undoubtedly result in further enrichment of alloying elements. The authors should provide a solution to this problem.\n4) As a conclusion, the authors should be encouraged to revise and submit this paper to other higher materials science\njournal such as Acta Materialia, JALCOM, JOM etc. as the series papers.", "reviewer_3": "The authors describe an innovative approach for aluminium-alloy upcycling from entire end-of life (ELV) vehicles. The topic\nis definitively of interest for the international scientific community.\nThe proposed method looks extremely interesting and of high potential impact if compared with the methodologies actually\nin use both in industry and also mostly in research approaches.\nThe authors are requested to addo some information and discussion regarding the robustness of the proposed methodology\nat the varying (usual scattiring) of the composition scenarios and also for different heat treatment and processing sequences.\nAt the moment such aspects look a bit implicit in the main document and should be better clarified.\nWith such modification the paper can be definitively considered for publication on the journal."}, "author_response": "Response to Referees\nWe would like to thank the reviewers for the valuable contributions that improved the quality of\nour manuscript. In the following, we address the reviewers' comments, to which we respond\nafter careful discussion. The resulting changes in the manuscript are highlighted in yellow.\nGeneral revision:\nThe abstract was adapted to comply with the Nature Communications formatting instructions\n(150 words).\nReviewer #1 (Remarks to the Author):\nThe study addresses the urgent challenge of recycling aluminum from end-of-life vehicles\n(ELVs). Current methods of recycling often result in the downgrading of mixed aluminum scrap\ninto low-grade cast alloys, which requires sorting or dilution with primary aluminum. The\nauthors propose a scalable process that upcycles mixed aluminum scrap from ELVs directly\ninto high-performance alloys. This work is highly meaningful and impactful because it\naddresses challenges in aluminum sustainability and promotes a low-carbon pathway.\nAuthors: We thank the reviewer for the appreciation of both the importance and the urgency of\nour work. We also thank for many constructive inputs that we address in the following points.\n1. When discussing the issues of existing recycling methods, it would be comprehensive if the\nauthors could mention not only the addition of primary aluminum to dilute but also the\nsignificant energy consumption and negative environmental impact of the chemical processes\nused to remove impurities. One example is the process of removing magnesium with chlorine\nor fluorine.\nUpdate Figure 1a accordingly.\nAuthors: Thank you for this input. The text was improved to address this issue at two places:\nLines 32–36: (in combination with issue 5a) They dissolve in the metal Al phase, demanding\nenergy-intensive or technologically immature processes or environmental hazardous reaction\nagents (e.g. Cl -gas-mix as purge gas) for a theoretical melt purification5-7.\nAvailable at https://international-aluminium.org/wp-content/uploads/2024/04/Mapping-\nStudy_Full-Report_Final.pdf (2023).\ntramp elements (e.g., Na, Ca, Li, Mg) is proven operational7.\nFig 1a: In revised Fig. 1a, a yellow gas bottle was included to symbolise melt purification\ntreatment. Also, a brown stream indicating energy input was added.\n[Figure Redacted]\n2. Considering the significant improvement in strength and sound ductility compared to all the\nPU samples with 6016 (Figure 3a), it would be nice to have an in-depth discussion here.\nUsually, the ductility of mixed recycled Al is poor because it contains a high content of tramp\nelements (e.g., Fe>1%), which exist as large IMPs and are detrimental to the ductility. But\nhere, even the PU 1k/s sample shows good ductility after processing. Can authors explain\nmore about how this method avoids the coarse IMP in conventional methods? Provide a\nmicrostructure made via conventional methods with the same chemical composition, if\npossible. I believe it would make the outstanding results of this work more prominent.\nAuthors: Thank you for this question. We will further discuss this discovery we report on. We\nperformed the following changes in the manuscript:\nThe sentence “Unlike Si-dominant IMPs spheroidised during homogenisation, Fe-dominant\nIMPs are primarily refined through rolling” is moved from line 198 in the original manuscript to\nline 176 in the revised manuscript in order to clarify the discussion in the paragraphs from line\n181 to 212, which extensively deals with the influence of IMPs on the final properties.\nIn the as-cast condition, the ductility is very poor for 1 and 3 K/s. This is also what the\ncommunity often expects for impurities in cast alloys. Important is also that only cast alloys –\nfor example – could accept this large amount of tramp elements: simply no wrought alloys exist\nin this compositional complex space. Nevertheless, when Fe-dominant IMPs are\n2\nmorphologically modified (small and nearly spherical) and evenly distributed in the\nmatrix via rolling, they deliver a substantial contribution to strain hardening, and\nconsequently increasing the uniform elongation. They also play a key role for the particle\nstimulated nucleation upon recrystallising. In the surrounding of the IMPs, geometrically\nnecessary dislocations are stored and are beneficial sites for the nucleation of new grains.\nFurthermore, these particles act as obstacle for grain boundaries and therefore, the overall\nmicrostructure is grain-refined (sizes between 11–21 µm in our work) than in conventional 6016\nalloys (average sizes between 22–32 µm) with a low number density of IMPs. Both increased\nstrain hardening and grain refinement mitigate the negative impact of the high-impurity content.\nHowever, coarse sludge phases formed during casting however are obligatory to avoid.\nEmerging from our research paper, the optimum combination between IMP sizes and number\ndensity is still not defined, stimulating further research in this emerging field of research.\nWe summarize this in a concluding remark on the ductility behaviour shown in Fig. 3 (see line\n214 to 221): In conclusion, the (unexpected) superior ductility of highly contaminated alloys\narises from a synergistic effect of IMPs on grain refinement and enhanced strain hardening,\nboth parameters being physically linked to increased uniform elongation51. This new finding\nchallenges conventional expectations of ductility in heavily contaminated alloys – which until\nnow – have been limited to cast alloys. Currently, no wrought alloys exist within this complex\ncompositional space that can leverage the beneficial role of IMPs while serving as sinks for\ntramp elements. However, the optimal type, size, and number density of IMPs remain\nundefined, leaving significant opportunities for further research.\n51. Dieter, G. E. Mechanical metallurgy. 2nd ed. (McGraw-Hill, New York, 1976).\n3. It is well known that the dimensions of castings affect the cooling rate, especially in the\ncenter of the casting. Could the author please specify the dimensions of the cast in this\nresearch work? Is there any difference/challenges when scaling up this method to the industry\nlevel? E.g., conventional cast and giga-cast?\nAuthors: We referred to our previous work on representative sample production in the methods\nsection, but without mentioning details that this was developed to represent DC casting. We\nnow added to the methods section alloy production. To clarify, we made the following changes:\nWe added in the dimensions of the ingots for 60 K/s (50x42x15 mm³ ingot dimensions) in lines\n322–323 and specified for 1 and 3 K/s (185x85x45 mm³ “filet block” dimensions with\nrepresentative cooling rate) in line 324–325 and in lines 329 to 332: Please note that this\nprocess was designed by our group in a previous work36 aiming at producing industrial relevant\n3\nhomogenous microstructures to represent the of relevant cooling rates in DC casting in a\ntechnical feasible homogenous manner.\nTo address the upscaling question: We added “industrial” in line 110, as direct chill casting\nrepresents the major industrial casting process. We also added “and continuous” casting in\nline 111, as it offers the same cooling speeds as direct chill casting. These facts mean that\nwe already recreated the industrial cooling conditions in this research.\nA casting block is never homogeneous. We therefore specified in lines 111 and 112 from (1–\nadded reference 39, which give an insight on solidification and further inhomogeneous cooling\nspeeds across the cross section of the castings. Giga-castings also show big inhomogeneities.\nHowever, as giga-castings are the finished product, no further processing – e.g. rolling – is\nperformed in their fabrication.\n39. Granger, D. A. Microstructure Control in Ingots of Aluminum Alloys with an Emphasis on\nGrain Refinement. In Essential Readings in Light Metals, edited by J. F. Grandfield & D.\nG. Eskin (Springer International Publishing, Cham, 2016), pp. 354–365.\n4. Also, can authors provide the size of tensile specimens used in this study?\nAuthors: An explanation of A (30 mm initial gauge length) and A (25 mm initial gauge\n30 25\nlength) is added in lines 349 and 364–365.\n5. Some grammar mistakes and non-standard terminology\nAuthors: We thank the reviewer for highlighting these issues, we will address them in detail\npoint to point:\na. Rewrite the sentence from lines 30-32.\nAuthors: This sentence is split now as follows (lines 32–36):\nFrom a thermodynamic perspective, removal of tramp elements introduced through aluminium\n(Al) scrap is inherently difficult the more noble the tramp element2-4. They dissolve in the metal\nAl phase2-4, demanding energy-intensive or technologically immature processes or\nenvironmentally hazardous reaction agents (e.g., Cl -gas-mix as purge gas) for a theoretical\n2\nmelt purification5-7.\nWe also added references 2–4 and 7 to fortify the sentences:\n4\nrecycling of end-of-life vehicles. Minerals Engineering 19, 433–449;\n10.1016/j.mineng.2005.08.014 (2006).\n3. Nakajima, K. et al. Thermodynamic analysis of contamination by alloying elements in\naluminum recycling. Environmental Science & Technology 44, 5594–5600;\n10.1021/es9038769 (2010).\n4. Graedel, T. E. et al. What Do We Know About Metal Recycling Rates? Journal of\nIndustrial Ecology 15, 355–366; 10.1111/j.1530-9290.2011.00342.x (2011).\nb. Rewrite the sentences from lines 57-59.\nAuthors: Thank you, we split the sentences as follows (lines 66-68)\nThis contradicts the common rule in metal recycling. It posits that the purer the input materials\nthe better their properties. A reason for this behaviour is that many tramp elements form brittle\nintermetallic phases (IMPs) in Al18.\nc. Suggest using “million metric tons (MT)” instead of “Mio. t”\nAuthors: We changed the unit as suggested in the lines 29, 51, 52, 55.\nd. Replace “für” with “for” at line 224\nAuthors: Thank you, we replaced it.\ne. Suggest using “utilized/utilizing/specialized/homogenize” instead of\n“utilised/utilising/specialized/homogenise”, etc.\nAuthors: We unified the spelling to British English (utilising etc.), as in the title the word\n“aluminium” instead of “aluminum” is used.\nf. I guess ND means “non-deformed”, is that correct? Please spell out when using it for the first\ntime.\nAuthors: The acronym ND was used in a former version; we removed ND from the caption of\nFig. 2 and in line 385.\nThank you very much for submitting nice paper to Nature Communication. I have carefully read\nthe manuscript. This paper contains very interesting technical concepts. Unfortunately,\nhowever, I have decided that it is appropriate to reject this paper due to the following reasons:\nAuthors: We thank the reviewer for their opinion on our paper. In the following, we have\naddressed all comments in this following response. In the points where we disagree with the\nreviewer, appropriate modifications have been made into the paper, increasing its scientific\nquality and making the discussion of the new results even more robust. Thank you.\n1) It does not state a clear conclusion as an academic paper, and is judged to merely\nrestate the basic concepts presented in their previous paper (Patrick Krall, Irmgard\nWeißensteiner, Stefan Pogatscher: Recycling aluminum alloys for the automotive industry:\nBreaking the ource-sink paradigm, Resources, Conservation and Recycling, 202 (2024),\n107370). If this paper had been submitted to this journal first, the decision might have been\ndifferent; that is, this paper merely reaffirms the basic concepts of the previous paper and\npresents the results of some case studies.\nAuthors: We respectfully disagree with the assertion that our paper lacks a clear academic\nconclusion. The last two paragraphs of our manuscript draft show the conclusions of our paper,\nwhich we again summarise herein for the reviewer:\n• Homogenisation and rolling refine structure, boosting ductility and strength.\n• DC casting conditions enable scalable production of high-performance alloys.\n• Mixed ELV scrap can be directly upcycled without sorting or primary dilution.\n• Optimised heat treatment and pre-straining deliver superior mechanical properties.\n(we also emphasized this in an additional comment in line 292 to 294)\n• Resulting alloys match or surpass automotive grades, showing strong robustness.\nMoreover, our present research differs significantly from the previous work cited (Krall\net al., 2024). For instance, in present study, we addressed on industrially relevant\ncooling rates typical of DC casting, rather than focusing solely on rapid solidification\nas in the previous paper. This makes our new recycling approach directly applicable to\nindustrial processes. Additionally, we consider contamination arising from the shredding\nprocess (e.g., Fe impurities), rather than theoretical mixtures of pure aluminum alloys.\nprocessing route that enables rapid hardening under industrial conditions (e.g., 20 minutes\nduring the paint bake process). This results in yield strength values that are double those\nreported in the previous study. This aspect we also emphasize in an amended version of the\nmanuscript (line 244, line 268 to 272 and line 281 to 282). Moreover, we provide a detailed\nunderstanding of nano-scale structure evolution via TEM, APT and through kinetic analysis\n(given in detail in the SI). While the paper builds upon the expertise acquired in our previous\nworks – as is the case with any scientific publication – the specific aspects addressed here,\nsuch as, industrially relevant conditions, contamination levels, the development of a rapid\nhardening, and the mechanistic insights were never previously covered.\n2) The authors' discussion focuses on some of the mechanical properties of aluminum (tensile\nstrength stress-strain property etc.). Actually they are showing very interesting results. On the\nother hand, the properties that practical metallic materials must satisfy are not limited to\nmechanical properties; physical and chemical properties (electric conductivity, corrosion\nresistance etc.) are also extremely important. These properties are highly dependent on\nchemical composition, so that it is essential to correlate the relationship between each property\nand composition. Such detailed information is best presented in a materials science journal\nrather than this journal.\nAuthors: We understand the reviewer’s comment regarding the importance of physical and\nchemical properties, in addition to mechanical properties. In our study, we focused on the\nmechanical properties, as these are the most critical for structural applications, particularly in\nautomotive manufacturing. Specifically, properties such as strength, elongation, and strain\nhardening are essential for processes like deep drawing and stretch forming. We have\ndemonstrated in our research that the strain hardening behaviour of our upscaled alloys is\nexcellent. While we acknowledge the significance of properties such as corrosion resistance\nand electrical conductivity, these are often more application-specific. For example, corrosion\nresistance is less critical for inner car body components, and mitigation strategies (i.e. coatings,\ncladdings etc.) are already in place to be further studied to expand the field of potential\napplications. To address this, we have added a comment in line 291 noting that corrosion\nbehavior will be explored in future studies. Similarly, while electrical conductivity is important\nin certain contexts, it is less relevant for structural car parts, and there are known methods to\nenhance it when necessary (e.g. Boron melt treatment or overaging). We agree that a\ncomprehensive investigation of the relationships between chemical composition and further\nspecific properties is valuable. However, we believe that such detailed studies are better suited\nfor materials science journals, as the reviewer suggests. Our current work represents a\n7\nfoundational step, and we view this exploration as a natural progression for future research in\nboth academia and industry.\n3) The biggest problem in recycling aluminum is contamination by the uncontrollable alloying\nelements. This problem has been discussed by many researchers (Reuter, Nakajima, Graedel\netc.). but has rarely been cited. This is a topic that is appropriate for the Nature Communication.\nThank you for recognising that our research topic is appropriate for Nature Communications.\nThe essential solution of this problem lies in controlling or removal of the alloying elements in\nsecondary aluminum, which are present in much higher concentrations than in other base\nmetal materials like steel. It must be said that the technology presented in this paper is merely\na temporary solution (I understand the importance of a temporary solution). The secondary\naluminum alloy developed by the authors will eventually be recycled again, which\nwill undoubtedly result in further enrichment of alloying elements. The authors should provide a\nsolution to this problem.\nAuthors: We thank the reviewer for highlighting this critical issue in aluminium recycling. We\nhave now cited the relevant works by Reuter, Nakajima, and Graedel (References 2–4, line\n34) to acknowledge the broader context of this challenge. We recognise that the research\nherein represents a temporary solution, our paper is timely considering the state-of-the-\nart. Given the urgency of this issue, we believe that our present solution address the\nrecycling challenges for the upcoming decades (~20 years). The concept presented in our\nwork aims at closing the recycling loop for Al alloys. As the use of Al in passenger vehicles\ncontinues to grow, driven by high-purity wrought and cast alloys, the compositions we propose\nrepresent an upper limit for vehicle compositions. Over time, these compositions are expected\nto dilute, as a 100% recycling rate is impossible to be achieved due to economic and energetic\nconstraints. Additionally, the increasing demand for Al will further mitigating the enrichment of\nalloying elements in the next years to come. We fully agree with the reviewer that, beyond\naddressing this current recycling problem, it is essential to prevent such issues from arising\nagain in the future. To this end, we are actively working on solutions for the automotive industry\nto narrow down the number of alloys in use and reduce compositional complexity by increasing\ntheir property range. For example, our work on crossover alloys (Ref: 22: On the potential of\naluminum crossover alloys. Progress in Materials Science 124, 100873; and Ref: 18: Making\nsustainable aluminum by recycling scrap: The science of “dirty” alloys. Progress in Materials\nScience 128, 100947), aims to address this issue improving future ELV recyclability. We now\naddress in line 56 to 59 of the manuscript:\nroads today in the coming years. However, designing sustainable cars and alloys22,23 to prevent\nsuch new recycling dilemmas in the future is also crucial, but unfortunately seems far from\nglobal realisation.\n23. Gaustad, G., Olivetti, E. & Kirchain, R. Design for Recycling. Journal of Industrial\nEcology 14, 286–308; 10.1111/j.1530-9290.2010.00229.x (2010).\n4) As a conclusion, the authors should be encouraged to revise and submit this paper to other\nhigher materials science journal such as Acta Materialia, JALCOM, JOM etc. as the series\npapers.\nAuthors: We respectfully disagree with the reviewer’s suggestion. While we acknowledge the\nvalue of publishing in specialized materials science journals, the goal of this work is to reach a\nbroader audience, including policymakers, industry stakeholders, and society at large, to drive\nthe paradigm shift urgently needed in this field. We believe that this journal provides the\nappropriate platform to engage with this wider community.\nThe authors describe an innovative approach for aluminium-alloy upcycling from entire end-of\nlife (ELV) vehicles. The topic is definitively of interest for the international scientific community.\nThe proposed method looks extremely interesting and of high potential impact if compared\nwith the methodologies actually in use both in industry and also mostly in research approaches.\nAuthors: We thank the reviewer for the appreciation of the high potential impact of our work.\nThe authors are requested to addo some information and discussion regarding the robustness\nof the proposed methodology at the varying (usual scattiring) of the composition scenarios and\nalso for different heat treatment and processing sequences. At the moment such aspects look\na bit implicit in the main document and should be better clarified.\nAuthors: We thank the reviewer for their comment. We now address robustness of properties\nand handling of compositional scatter.\nWe amended/added the following text passages in the revised manuscript:\nRevised manuscript lines 249 to 253: The values for the EU alloy (Supplementary Figure 4a,b)\nare only slightly lower than those of the PU alloy, although the composition differs (mainly 0.45\n% more Si and 0.6 % less Cu) pointing at compositional robustness for internal combustion\nengine vehicles.\nRevised manuscript lines 292 to 2945: While achieving this superior strength requires precise\ncontrol over the final thermo-mechanical processing, the results also indicate that a wide range\nof properties is achievable (Supplementary Figure 2).\nRevised manuscript lines 300–302: To demonstrate the robustness of our approach we\naddressed a range of alloy compositions and cooling conditions, including those achievable in\nindustrial direct chill casting processes.\nIn lines 305 to 309, we added a statement on the compositional robustness and related it to\nindustrial practise and also added references 56 and 57:\nThe comparable property range resulting from the average European car and US pickup alloys\ndemonstrate notable compositional robustness. Achieving this robustness seems feasible with\ntypical shredders processing more than 100 vehicles per hour56 and typical recycling furnaces\nhandling scrap from over 50 ELVs57.\n56. Sander, S., Schubert, G. & Jäckel, H.-G. The fundamentals of the comminution of\nmetals in shredders of the swing-hammer type. International Journal of Mineral\nProcessing 74, S385-S393; 10.1016/j.minpro.2004.07.038 (2004).\n57. Krone, K. Aluminiumrecycling. Vom Vorstoff bis zur fertigen Legierung (Vereinigung\nDeutscher Schmelzhütten, Düsseldorf, 2000)\nWith such modification the paper can be definitively considered for publication on the journal.\nAuthors: We thank the reviewer for this suggestion.\n11"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for addressing my comments on the manuscript. I am happy with the detailed responses provided by the authors.", "reviewer_3": "The required modifications were introduced in the paper. It can be accepted for publication."}, "author_response": ""}]

Decision letter