In This Issue
Spring Bridge on the US Metals Industry: Looking Forward
March 29, 2024 Volume 54 Issue 1
In this issue of The Bridge, guest editors Greg Olson and Aziz Asphahani have assembled feature articles that demonstrate how computational materials science and engineering is leading the way in the deployment of metallic materials that meet increasingly advanced design specifications.

Current Challenges and Opportunities for the Aluminum Transformation Industry in the United States

Monday, April 8, 2024

Author: Timothy J. Warner, Bill Allemon, Craig B. Lewis, and Guillaume Bes

A major focus of ongoing technology development is enhancing energy efficiency and reducing carbon emissions in the production and use of aluminum products.

The major challenge for the metals industry today is responding to society’s need to reduce its environmental impact, in particular by reducing greenhouse gas (GHG) emissions and by reducing the quantity of landfill waste as a result of missed recycling opportunities. Aluminum is intrinsically well adapted to this challenge: its combination of low density and good mechanical properties makes it possible to design and manufacture lightweight structures at costs acceptable for high-volume requirements such as automotive applications (Aluminum Association 2021a). In addition, the fact that aluminum alloys, appropriately segregated, can be fully recycled into themselves is an enabler for a circular economy, as evidenced by the recycling of aluminum beverage cans within the same application (Aluminum Association 2021b; Aluminum Association 2022).

Warner et al fig 1.gifA macroscopic life cycle analysis of a typical aluminum transformation[1] business today is presented in figure 1. Averaged over multiple applications, the data in figure 1 indicate that upstream primary metal production contributes the most to GHG emissions (i.e., ­bauxite refining to alumina [Al2O3], followed by smelting to produce commercially pure aluminum). This is due to the high energy consumption of both the smelting process and the production and consumption of carbon anodes used in the smelting process. The major smelters are actively working on both these fronts by ensuring that their energy sources emit as little GHG as possible (e.g., by preferring renewable or nuclear power to fossil-fuel ­power sources) but also by developing so-called inert anodes (Wang and Xiao 2013) that obviate the CO2 emission due to consumption of the anode and the high energy requirement for consumable carbon anode production.

The next highest contribution to the embedded GHG content of aluminum products comes from the transformation of the aluminum (i.e., the remelting and alloying of aluminum to produce solidified ingots or billets and their subsequent processing by rolling or extrusion to foil, sheet, plate, or profiles).

There are two GHG-emission-reducing contributions indicated in figure 1: 1) “Avoided emissions,” which represent the impact of light weighting of structures by replacing heavier structures, particularly in the transport industry, and 2) The impact of recycling of aluminum scrap, which is both intrinsically desirable to reduce ­levels of landfill and an enabler for a significant reduction in the energy cost (and thus the GHG emissions) of the metal input into the transformation process (see figure 2).

This article focuses on the challenges and opportunities in increasing recycling, reducing energy use during aluminum transformation, and light weighting of consumer products. The opportunities are both environmental and economic: increasing the rate of recycling reduces metal input costs, reducing energy use has a direct financial impact on the bottom line of a plant, and light weighting reduces transport costs.

Warner et al fig 2.gifRecycling

The US aluminum transformation industry already inputs roughly 50% of externally recycled scrap into its process (Aluminum Association 2022). Increasing this level is key to reducing the amount of aluminum that ends up in landfills (currently about 50% of the volume of aluminum transformed every year [Aluminum Association 2022]) and to reducing the industry’s overall level of GHG emissions. Improving the level of reuse within the same application (i.e., ensuring closed loops within the same alloy family) will also be necessary in order to ensure sustainable recycling loops.

A primary reason why aluminum ends up in landfills today is because it is mixed with different materials such as steel, plastics, and cardboard and, as a result, cannot easily be remelted to make new aluminum products. However, even if the aluminum were to be separated from the other materials, that would still not suffice to enable recycling in high-performance wrought alloy applications such as automotive or aerospace, because such applications require different property balances and therefore composition-processing combinations. Nearly 400 distinct wrought alloy compositions in eight different series corresponding to different major alloying additions are registered with the Aluminum Association (Aluminum Association 2018), each with their own specific processing requirements and property balances. Separation and sorting of scrap, in many cases into exploitable alloy groupings, are therefore key for future volumes of ­recycling in the aluminum wrought industry.

Aluminum beverage cans demonstrate both the potential and some of the ongoing challenges of ­recycling in the United States. Used beverage cans (UBCs) are already recycled at extremely high rates in some states (e.g., 85% in Oregon) with the creation of well-­developed circular scrap loops. However, at a national level (see figure 3), more than 50% of UBCs ended up in landfills in 2022. Although current UBC availability exceeds the recycling capacity of US rolling plants, planned domestic can stock recycling capacity increases will ensure that UBC demand will exceed ­supply within the next five years unless the rate of recovery of UBCs increases nationally.

Warner etal Fig 3.gif

The opportunities to increase the level of recycling are different for each major application of aluminum. In the aerospace market, given the level of machining required to make highly complex airframe parts, the ­largest ­recycling volumes and opportunities are in the so-called “pre-consumer scrap” generated by the aerospace machine shops. Aluminum scrap flows in the automotive market, currently low but building rapidly, trailing the recent substantial increases in aluminum alloy volume in auto body applications, are the object of industry and academic studies to better predict and exploit the future scrap loops (Zhu et al. 2021).

Warner et al Fig 4.gif

Coming back to the beverage can example, different strategies could be used to keep the end-of-life (EOL) ­aluminum scrap in the beverage can life cycle:

1) Selective collection: depositing cans in dedicated ­recycling containers is the most efficient way to have high collection rates and a high quality of collection. This process only requires a simple sorting step, removing the remaining steel cans, as consumers have already made the effort to segregate aluminum cans from other waste.

2) Global waste/scrap collection: recyclable ­domestic food packaging (mixed cardboard, plastic bottles, ­plastic food packaging, cans, etc.) can be collected in the same container. In this case, multiple high-­efficiency sorting steps are needed to extract materials one by one. This path is easier for the consumer as there are not multiple collection bins, but it is more expensive. The quality of recovery is limited by the performance of the sorting technologies and by the joining technologies used (mechanical fastening, welding, hemming, etc.).

Warner et al Fig 5.gif

For automotive EOL recycling, there are two competing strategies to recover value from the mixed Al-Mg and ­Al-Si-Mg wrought aluminum alloys: full car shredding versus selective dismantling (Fick 2021). As a domestic car is a complex product where multiple materials are con­nected, current EOL recovery is dominated by auto ­shredder companies. Their main target is a high shredding rate, but the product generated (see figure 4) remains a mix of aluminum alloys that requires multiple high-speed sorting types of equipment, including x-ray transmission (XRT) and laser-induced breakdown spectrometry (LIBS). The current bottleneck of this strategy is the limited throughput of the sorting equipment (50-100t/h for a car shredder versus 1-10t/h for sorting machines). With the electrification of the automotive industry, more and more aluminum is being used in cars; new dismantling strategies, perhaps coupled with part design for recycling, could help to concentrate wrought alloy scrap. With such selective collection, cross-contamination between ­materials is limited, and the quality of wrought alloy recovery is improved. Though car dismantling is currently highly labor-intensive, its high potential ensures that it remains on the radar for EOL recycling.

Energy Use during Transformation

Although the emissions associated with their primary aluminum content currently dominate in aluminum wrought products (for Constellium in 2021, these represented 77% of total GHG emissions per ton, see figure 1), many aluminum manufacturers have established CO2 emission intensity reduction goals of 20%–30% for their own processes by 2030 (see, for example, Carbon Disclosure Project 2023), aligned with US manufacturers across other sectors.

With the largest contributor being the reheating of metal, including the melting and reheating of primary metals and scrap (“casting” and “recycling” in figure 5), the area of greatest focus is reducing the use of natural gas. This will be accomplished through near-term efficiency improvements and longer-term replacement with alternative fuels and technology. The following discussion considers the available options with existing furnace technologies, such as electrification, alternative fuels, and overall process efficiency approaches.

Existing Technologies

Starting with technology upgrades, a few common themes emerge. Price competition and CO2 emission reduction demands have caused aluminum manufacturers to deploy commercially available technologies on their gas-fired furnaces, including improved burner technologies, magnetic stirring, and waste heat recovery.

One example is the use of regenerative furnaces. This design extracts heat from exhaust gases, storing the energy in a medium within each of two burners working alternately. While one burner is firing, the other burner captures energy from the furnace’s exhaust gases, which it then uses to pre-heat combustion air when it is that burner’s turn to fire. Each burner fires for approximately 2–3 minutes before the process is reversed. Regeneration can recover up to 85% of the energy that is typically lost as waste heat (Kermeli et al. 2016).

Non-contact stirring of the furnace melt charge ­increases melt rates and reduces cycle time, thereby increasing ­energy efficiency. The technology uses an electro­magnetic device, permanent magnets, or a combination of both to improve flow within the melt. ­Easily retrofitted to existing ­furnaces, these devices improve furnace yield by 15% or more, while reducing natural gas consumption by approximately 5%. Other benefits include reduction of waste materials, elimination of melt surface burn, and ­homogeneity of the final product ­(Kermeli et al. 2016).

Generic waste heat recovery devices on furnace exhaust flues are a well-established technique to improve energy efficiency. The energy recovered may be used for heating in other industrial processes, such as pre-heating lubricants in nearby rolling mills or pre-heating and drying aluminum scrap, thereby reducing consumption of ­natural gas.

Electrification

Electrification of industrial heating and other processes is currently much discussed (e.g., DOE 2022), ­primarily driven by government agencies, consultants, and technology solution providers.

Electric-powered solutions to all the major reheating operations in the aluminum transformation process are available and used in the industry. However, their applications have historically been limited by the high cost of electricity compared with the equivalent energy supplied as gas (see below) and by some scaling issues. For example, induction furnaces are used to remelt Al-Cu-Li alloys, for safety reasons in particular (Singh and Gokhale 2014). However, the maximum remelting furnace size commercially available today is roughly 20 tons, compared with more than 100 tons for a gas-fired furnace. Achieving the same production capacity would not only imply investments in many more furnaces but also a different workshop layout, with correspondingly high capital expenditure.

Although feasible in principle, substantial electrification would also raise questions around the net effect on carbon footprint, grid capacity, and reliability, as well as the cost effectiveness of technology conversion.

For example, with most electricity in the Midwest still generated from coal, the reduction in GHG emissions due to electrification at the aluminum manu­facturers’ facilities in this region would be more than offset by the increase in emissions at the utilities (Wang and Xiao 2013). In other parts of the country, such as the South and Southeast, where a higher percentage of power comes from carbon-free sources (hydropower and nuclear, respectively), the issue is the capacity of the local grid. Greatly increasing electrical demand would require investment in primary distribution, a financial burden on industrial and end-use customers.

The greater cost of electricity versus natural gas per equivalent energy unit is a fundamental challenge. Despite the rapid increase in solar power generation capacity, the continued fall in the price of photovoltaic generation (US Energy Information Administration 2023a), and the continued maturation of utility-scale energy storage technologies (US Energy Information Administration 2023b), electricity still cannot compete with the price of natural gas at the time of writing. However, the future does look bright. The US Energy Information Administration estimates the levelized capital cost of photovoltaic electricity generation in 2028 to be $24.08 per megawatt-hour, much lower than the future cost of modern coal-generated power at $57.73 (Statista 2023).

Fuel Switching

Replacing the use of natural gas with less carbon-intensive fuels is another logical step toward decarbonization. ­Trials are underway globally to evaluate reducing ­natural gas consumption by replacing air with oxygen in its combustion, as well as its complete replacement with hydrogen.

A primary reason why aluminum ends up in landfills today is because it is mixed with different materials such as steel, plastics, and cardboard and, as a result, cannot easily be remelted to make new aluminum products.

Oxygen enrichment in furnace applications is well established in the iron and steel foundry industry and is used to increase production throughput. The underlying principle of oxygen-fuel technologies is to reduce GHG emissions by displacing nitrogen in the furnace, greatly reducing NOx generation as well as reducing the quantity of natural gas used and therefore of CO2 emitted. However, in an aluminum furnace, the use of oxygen requires precise control due to strict temperature requirements and to eliminate the potential for chemically damaging the production batch.

The use of hydrogen to fully replace natural gas is getting much press (Marocco et al. 2023; Deloitte 2023), primarily due to its reduction of GHGs, enabling the production of “green aluminum.” Again, the reduction of the overall carbon footprint rests solely on the emissions intensity (emissions factor) of the electricity used to produce the hydrogen.

The two principal challenges facing the aluminum industry today are reducing its own GHG emissions and increasing recycling rates.

We have identified three main challenges with hydrogen as a fuel for the aluminum industry (Hyinheat 2023). First, the safety aspect: due to the characteristics of hydrogen, the risk of leaks is greater than with natural gas, and the explosivity limit is higher. Secondly, the impact of moisture: after hydrogen combustion, the amount of water vapor in the exhaust fumes is very high. R&D activities are ongoing to determine the impact of this atmosphere on the quality of our products and on the lifetime of our infrastructure (furnaces, pipes, etc.). The last, but not least, challenge is the supply chain: as discussed previously, if the power used for hydrogen production is from a conventional carbon-intensive grid, the environmental footprint is worse. If power is supplied by carbon-free electricity, the discussion pivots in hydrogen’s favor on an emissions-only basis. Depending on furnace configuration and application, the use of carbon-free generated hydrogen could reduce CO2 emissions by up to 90% (LMA 2023). It should be noted that, as hydrogen gas is a much smaller molecule than natural gas, existing national infrastructure and plant-level distribution systems are not designed to safely transport the fuel.

Energy Efficiency

Regardless of the energy source and the carbon footprint of the electrical grid, energy-efficient production and usage continue to be a consistent area of focus. The trident of energy efficiency incorporates technology upgrades, best practice replication, and continuous improvement actions.

Industrial facilities have historically focused on individual capital project improvements for plant infrastructure such as compressed air, pumps, motors, fans, and lighting. Some have conducted best practice sharing and opportunity identification workshops, such as the Energy Treasure Hunt process. However, these singular actions do not instill a comprehensive process of continuous improvement specific to energy efficiency and lowering emissions reductions.

Various standardization and best practice efforts provide guidance and encouragement to the industry to improve its practices holistically. Although more popular in Europe than in the United States, the ISO 50001 Energy Management Standard (ISO 2018) is gaining traction to help US manufacturers establish robust energy-specific continuous improvement programs.

To encourage the use of the standard, the DOE’s Better Plants initiative created the 50001 Ready Program (DOE 2023), with a suite of tools and guides to supplement the succinct content found in ISO documentation. The DOE also provides both remote and onsite training to help implement 50001 and technical analytical tools developed by the agency.

In addition, the EPA’s ENERGY STAR program, which predates ISO 50001, provides complimentary tools, training, and technical best practices for energy management programs (US Environmental Protection Agency 2023).

Avoiding GHG Emissions through Light Weighting: Past and Future Approaches

Enabling weight reductions for transportation applications has long been a focus of the aluminum industry. Reduced weight in such applications translates directly into reduced energy consumption during the usage phase and, in general, into reduced GHG emissions.

Materials scientists can contribute to light-weighting structures by developing materials that meet the mechanical requirements of the application but have lower densities, as is the case with the Al-Cu-Li alloys developed over the last couple of decades and used especially in the aerospace industry (Lequeu et al. 2010; Warner 2006). In addition, increasing materials properties, such as strength, enables reductions in part thicknesses with concomitant weight reductions for the relevant component.

Structural light weighting in automotive applications will remain of interest even with the expected transition to battery electric vehicles (Hart et al. 2023). Largely because of range considerations, the aluminum content of today’s battery electric vehicles is greater than their internal combustion engine counterparts of similar size and mission. Despite expected improvements in battery cost and storage density, aluminum light-weighting solutions are expected to remain attractive, both from the point of view of their potential to reduce energy consumption and from an economic point of view, for at least the next decade.

There is currently strong interest in the additive manufacturing of aluminum parts, as additive manufacturing techniques enable the direct fabrication of parts that are of complex shapes. Comparisons between the CO2 emissions of the fabrication of a given part via conventional or additive manufacturing are very unfavorable for additive manufacturing, largely because of the high energy intensity of additive processing (Ingarao et al. 2018). Nevertheless, the design freedom associated with additive manufacturing can result in significant part weight reductions and in very substantial reductions in the number of parts required for a given component (Faludi and Van Sice 2020). Particularly in applications whose use-phase emissions are very sensitive to weight reduction, such as aerospace, this can result in overall life cycle benefits for the additive manufacturing of some specific components, for example, small-scale but complex satellite ­components (Ingarao et al. 2018).

Another current trend with high potential is the concurrent optimization of materials, their processing, and part design. A recent example is Tesla’s “megacasting” (Visnic 2020), which exploits the scrap available from the sheet metal used in a car assembly plant in a cast alloy part, in which the alloy, the process, and the design have been co-optimized.

As discussed above, historical aluminum alloy development for high-performance applications has resulted in the development of a large number of alloys with very tight compositional specifications and highly optimized processing conditions. As a result, the preferred approach to incorporating increased quantities of recycled metal is to separate and/or sort scrap for recycling in the same alloy. In many cases, a rationalization of the number of alloys used in the market would be beneficial. Where efficient separation and/or sorting is not possible or economically viable, the aluminum industry is looking into making optimum use of the corresponding mixed alloys (Aramburu et al. 2021; Raabe et al. 2022). Two main routes are being assessed: 1) The exploration of the new compositional spaces that are generated by mixing existing alloys, facilitated by innovative alloy design ­methods using either artificial intelligence techniques (Menou et al. 2019) or combinations of physics-based modelling approaches (Xiong and Olson 2015), or both, and 2) The exploration of alloy design and processing techniques that are “impurity tolerant” (Raabe et al. 2022)(i.e., techniques that allow the incorporation of larger quantities of impurity elements by reducing their detrimental impacts). A third route, alloy purification in the liquid state, is the subject of active research, but no fully viable technology is available yet.

Conclusions

The two principal challenges facing the aluminum industry today are reducing its own GHG emissions and increasing recycling rates. Much work has already been done on both fronts, but major opportunities remain.

Regarding the industry’s direct emissions, energy efficiency initiatives are proving effective. Substantially lower-emission heating technologies exist or are in development, but to be effective in reducing overall GHG emissions, these will require concomitant investments in energy infrastructure.

The aluminum industry already has a strong track record of recycling. Given the intrinsically excellent ­recyclability of aluminum alloys, there are many opportunities to further extend and develop recycling, which the industry is addressing aggressively, but the engagement of other stakeholders will be critical to promoting full circularity (e.g., state and federal support to increase deposit rates by introducing deposit regulations and customers’ and suppliers’ engagement to recycle end-of-life vehicles).

The traditional light-weighting role of aluminum alloys will remain important going forward, but it will need to be achieved within the constraints of low GHG emissions and high recycled content.

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[1]  Aluminum transformation designates the remelting, rolling, or extrusion and thermo-mechanical treatment of aluminum. Upstream of this is the production of “primary aluminum,” from the mining of bauxite to the smelting of aluminum metal.

About the Author:Timothy J. Warner is scientific director, Bill Allemon is global energy best practices leader, Craig B. Lewis is vice president, strategic transformation, and Guillaume Bes is metal and recycling corporate leader, all at Constellium.