Selecting Cast Alloy Alloying Elements Suitable for a Circular Society

Abstract

Resource efficiency, energy usage, and carbon footprint drive the need to use aluminium alloys to manufacture lightweight components. The current paper targets the effects of alloy composition on the heat associated with remelting from a material circularity perspective. Si as an alloying element increases the required heat in the recycling cycle. Limiting the Si content in cast materials can reduce the energy needed in the recycling process by 20%, leading to significant gains in energy usage and CO₂ emissions from gas heated furnaces and fossil fuel-generated electricity.

1. Introduction

Resource efficiency, energy usage and carbon footprint related to global warming issues were drawn out early in the Charney report [1]. Both energy usage and CO₂ footprint pose problems primarily resulting in legislation with stricter regulations and requirements on emissions [2]. Aluminium alloys play a central role in manufacturing lightweight components. Weight reduction has a central role in reducing energy usage and greenhouse gas emissions from transport due to underlying factors such as automobile dependence and public transport systems needs [3]. Keeping in mind that the CO₂ footprint depends a lot on the usage of fossil fuels for electricity generation and material production, Serrenho et al. [4] concluded that during the transition to a fossil-free electricity generation, the weight reduction of vehicles could produce more significant cumulative emission savings by 2050 than those resulting from electrification itself. The effective recycling methods available, make aluminium alloys a preferred choice for a circular society [5].

Jarfors et al. [6] concluded in a recent paper that the preferred elements based on their contribution to strength with a minimum of impact through embodied energy, CO₂ footprint and water usage were Mn and Zn, with Cu and Si sharing third place in the ranking, followed by Cr, La, Ce, and Mg. Including element cost, the order was Mn, Zn, Si, and Cu on a shared fourth place with La, and Mg and Cr sharing sixth place followed by Ce. The main elements in the current alloying systems would be Si, Mg, Cu, and Zn. Elements such as Fe are either impurities or necessary for castability [7]. These results should be seen in the light of primary production. For a circular society, the choice of equipment influences the energy efficiency and material losses for each cycle, whereas in-process recycling also substantially contributes to increased energy usage, impacting energy consumption and CO₂ footprints significantly, especially for a foundry [8,9].

One of the main contributing factors to the energy involved in the recycling process is the latent heat of fusion for melting, $\mathsf{\Delta }{H}_f$. This value depends on the density and bond strength of the element. The average value for the 92 first elements in the periodic system is 308 kJ/kg. Aluminium has 397 kJ/kg making it slightly higher than average. Each element in an alloy will contribute with its own melting energy to the alloy’s complete heat of fusion through the Kopp–Neuman rule of mixture, Equation (1) [10]. This is a common way for property estimation in simulations.

$$\mathsf{\Delta }{H}_f=\frac{1}{100}{\displaystyle \sum }_{i=1}^{i=N}{c}_i\Delta H_f^i$$

(1)

where $c_i$, the concentration of element i, (%) $\mathsf{\Delta }{H}_f^i$, is the heat of fusion of element i (kJ/kg). Four elements differ from most other elements, plotting the respective heats of fusion. C and B are commonly found in grain refiners and used in minimal additions, Figure 1. This is also true for Be that in some regions around the world is present in minute amounts in gravity die-casting alloys such as EN-AC42xxx or similar to protect Mg from oxidation during melting. On the other hand, Si plays a central role in recycling and is used in large quantities to transform wrought materials into high-pressure die-casting alloys such as EN-AC46xxx alloys. Si is one of the elements with a high heat of fusion, Figure 1.

Figure 1. Latent heat of fusion versus element atomic number.

Die-casting alloys represents a smaller tonnage than high-pressure die-casting alloys being a casting process with higher productivity [7].

The current paper targets the effects of alloy composition on the heat of fusion effects on the remelting of aluminium alloys, focusing on the effect of the Si content taking an approach to circularity aspects and energy efficiency. This approach was taken since energy drives the CO₂ footprint in the recycling of aluminium alloys.

2. Methodology

This research was designed following the post-positivistic in the sense that the axiology originating from the drive towards sustainability generates a bias, but within the boundaries of energy efficiency, the approach should warrant generalised and verifiable results. The identification of critical elements for energy-efficient circular alloy element selection and alloys design will thus be general and verifiable. Based on the current understanding, the latent heat of fusion, typical alloy composition, and relative energy requirements associated with melting and remelting are analysed. This was taken as a critical indicator of the required energy of material undergoing recycling in a sustainable and circular materials process. A route forward is thus proposed. The recycling of aluminium will also embody trace elements that affect properties and processability. These are not considered in the current study as they only have a minor impact on the heat content.

3. Results and Discussion

The results and discussion chapter is divided into four sections. The first section compares the estimate using Equation (1) with some computational thermodynamics using Thermocal^TM. The second section analyses the latent heat for fusion for a number of commercial wrought alloys, and the third part is a similar analysis for cast alloys. The fourth part analyses how to resolve the energy deficiencies found in current recycling and suggests processing routes and alloy families for an improved circularity. Based on the energy hysteresis in remelting, critical elements to consider for circularity are analysed. The different main elements and the associated families are collated in Table 1 [11].

Table 1. Wrought and cast alloys [11].

Alloy Family	Alloying Elements	Alloy Family	Alloying Elements
1xxx	Al (pure)	1xx	Al (pure)
2xxx	Al-Cu	2xx	Al-Cu
	Al-Cu-Mg		Al-Cu-Mg
	Al-Cu-Li	3xx	Al-Si-Mg
3xxx	Al-Mn		Al-Si-Cu
4xxx	Al-Si		Al-Si-Cu-Mg
5xxx	Al-Mg	4xx	Al-Si
6xxx	Al-Mg-Si	5xx	Al-Mg
7xxx	Al-Zn	7xx	Al-Zn-Mg
	Al-Zn-Mg	8xx	Al-Sn-Cu-Ni
	Al-Zn-Mg-Cu
8xxx	Al-Li-Cu-Mg

3.1. Energy Requirements for Wrought Aluminium Alloys

The latent heat of fusion variation in the wrought alloy families is displayed as averages over the alloy families (Figure 2) and with the data collated in Appendix A (Table A2). Two families differ from the others, 4xxx and 7xxx. The 4xxx has a significantly higher heat of fusion originating from the heat of fusion of Si. The 7xxx family has a lower heat of fusion owing to the Zn content. Based on this, the inclusion of the 4xxx alloy in a circular material approach would significantly increase the energy in each cycle.

Figure 2. Latent heat of fusion for common wrought alloy families.

3.2. Energy Requirements for Cast Aluminium Alloys

Repeating the analysis for the cast alloys using the same procedure as the wrought alloys shows that the situation is different in the cast alloy families. Compared to the wrought alloy families, Si content is more common in the cast alloys, see Figure 3. Both the 3xx and 4xx families have more significant amounts of Si and, therefore, higher fusion heat. Similar to the wrought materials 7xxx family, the 7xx also contains Zn and, as such, displays lower heat of fusion.

Figure 3. Latent heat of fusion for common cast alloy families.

3.3. Critical Elements to Consider in Circular Manufacturing

Das [12] studied the element mix and accumulation in scrap and made the following observations.

• Not uncommonly recycled wrought metal matches existing wrought alloys reasonably well, primarily in the 3xxx and 6xxx series.
• Individual lots of scrap can have relatively wide varying compositions, particularly related to higher Cu and Zn contents.
• Cast alloy scrap composition can vary greatly, and there is little similarity to wrought scrap. Both Cu and Zn are commonly higher.
• Mixtures of wrought and cast scrap are troublesome due to higher Si, Cu, and Zn concentrations.

Cu and Zn may cause issues in processing. Cu drives hot cracking and stress corrosion cracking. Zn has a high vapour pressure and affects processability and is limiting applications in terms of elevated temperature and low-pressure applications. Ref. [13] In terms of heat of fusion, increased Zn concentration is only beneficial, and Zn is not an issue from a circularity perspective. Neither is there any significant effect on the heat of fusion from Cu.

Accepting the possibility that scrap commonly can match existing alloys, scrap sorting is critical to avoid downgrading of metals, which can be supported by closed-loop recycling [14]. The 4xxx family has high Si levels which is unwanted from a circularity perspective. This type of alloy can only be recycled within its own family or used to prepare high Si concentration cast alloys. Neither route is preferred for an energy effective circular materials environment.

The compositions of the cast alloys 3xx and 4xx have wide variations in Si, and there are several alloys with significantly lower Si compositions. Polmear [13] and Das [12] show that the interaction between Fe and Si, Cu, and Mg is critical, and the Fe content needs monitoring and control. The Fe containing intermetallics significantly affects ductility. Furthermore, it may contain Cu and Mg and scavenge critical elements to mechanical performance, requiring extended heat treatments of loss in performance. Independent of this, reducing Si will improve energy used in the circulation of the material and allow easier management of Fe due to reduced precipitations of Fe–Si containing intermetallics.

To reduce the energy used in the circulation of the material, reducing Si is essential, but this affects the possibility of downgrading from wrought materials to cast materials. The new manufacturing routes in the automotive industry resulting from the electrification of the drives systems significantly alter the supply chain and the requirements of a green supply chain [15]. Aluminium use has grown steadily in cars, making aluminium recycling critical [16,17]. Larger castings also drive the trend towards using and developing alloys with lower Si content than currently dominating alloys [18].

The most commonly used alloys are the A380–A384 alloys, which correspond to the Al9Si3Cu alloys, but with a range of Si typically found from 8.5% up to just above 11%, see Figure 4. In Figure 4 the results from ThermoCalc^TM (v2022b, TCAL6-database) are shown. The fact that Si forms a deep eutectic means that the alloy has a large negative heat of mixing reducing the difference compared to the Kopp–Neuman estimate with increasing Si content, see Equation (1). The effect is still significant and reducing the Si content below 7% generates significant benefits. In a complex alloy there will also be other mixing effects resulting in a comparison between an idealized alloy with few elements compared to an alloy, with an alloy having a complete analysis, including more or all elements in the alloy, which reduces the heat of fusion due to mixing effects for dilute systems.

Figure 4. Latent heat of fusion for common cast alloys and to alloys adapted for circularity.

3.4. Matching Alloy and Processing to Ensure Energy Efficiency in Circular Manufacturing

The effect of reducing the Si concentration to improve alloy energy characteristics for circularity significantly hampers castability. Alloys with compositions around 6% Si are commonly used for die-casting alloys and cannot match the high-volume production required for both the automotive and telecom industries. This also includes large castings made by Tesla [18], see Figure 4.

The example by Tesla is a unique adaptation of an alloy to produce strengths and ductility and has a specific application as an automotive structural component. Further reduction of heat through Si reduction caused further issues with castability. One way to work around this problem is to use semisolid processing. Semisolid processing has demonstrated a unique ability to cast what are known as difficult to cast materials. The best alloys for semisolid processing are alloys that are approximately 4–8% Si [19,20]. Recently alloys under development for more structural applications (RheoGreen) and heat transfer applications (RheoCool) are well suited for a low energy circularity situation, see Figure 4.

4. Conclusions

In the current study, the energy consumption driven by remelting was analysed by estimating the heat of fusion for a large number of alloys and alloys families. The following conclusions can be made.

• Of the commonly used alloying elements used for Aluminium alloys B, Be, and Si contribute to the heat of fusion. The most troublesome element is Si since it is used in more significant amounts and is a common addition in the recycling process.
• In order to adapt, alloys with high Si content should be avoided, and new alloys with reduced Si and lower heat of fusion should be prioritised to achieve a low energy circulation situation for aluminium alloys.
• The wrought family 4xxx alloys should be avoided, and other alloys should be given priority.
• In the cast alloy families, the range of Si varies greatly, and there is a possibility to use alloys belonging to the 3xx and 4xx families.
• Newly developed alloys such as that by Tesla are castable using high-pressure die-casting (HPDC) but have only a limited reduction of the required heat in the recirculation.
• The further reduction also requires a process change and rheocasting where newly developed alloys such as RheoCool and RheoGreen can significantly reduce the required energy to recycle aluminium alloys.

Based on these results, it is recommended to reduce the Si content in the cast Al–Si alloys and, when required, combine this with rheocasting to facilitate a high castability. The large volume of high Si alloys remaining in circulation as engine blocks would be used for the foreseeable future. These high Si-containing alloys could be used as a source of Si and Cu where the need exists to adjust compositions from low to higher Si but not the eutectic levels, significantly impacting the heat of fusion.