How to design Alloys for Function and Footprint

Do you know what is the most significant contributor to a casting’s CO2 footprint?

The majority of the emissions come from the alloy. The smelting process from bauxite via alumina to aluminium requires a lot of electrical power. It takes around 17.000 kWh of electricity to manufacture 1 ton of pure aluminium. So, it comes down to the electricity that the carbon footprint of an alloy is. According to the International Aluminium Institute, worldwide, 50% of the energy comes from coal power plants. Around 10% comes from natural gas plants. A third comes from hydro power, and around 6% from other renewable energy sources. Only 0.5% comes from nuclear power plants.

60% of all new or primary aluminium comes from burning stuff, which generates a lot of greenhouse gas emissions. In addition, mining operations, transport, and electrodes also emit a lot of CO2.

Are Secondary Alloys the solution?

When using secondary alloys to make new alloys, the energy-intensive primary route seems unnecessary. However, specifications add a twist. Aluminium is a non-noble metal. Processing routes for steel and copper alloys are unusable for aluminium, as the aluminium would be removed instead of the impurities.

So, every element in the aluminium scrap will be in the final alloy. The vast majority of casting scraps are made of high copper and iron alloys like 226 (46000, AlSi9Cu3) or A380 (AlSi8Cu3). You can easily remelt them to make the same alloy. However, the demand for parts goes down constantly. For high-performance alloys for structural body-in-white parts or thermal management applications, these scrap streams are unusable. The specifications limit iron and copper contents just above their natural limits.

However, there are other scrap streams that are perfectly usable for these high-performance alloys, and they help reduce the carbon footprint a lot. The global average carbon footprint is around 16 kg CO2e per kg Al, and the best primary alloys are around 4 kg CO2e per kg Al. The average primary alloy is around 1.9 kg CO2e per kg Al. That is a significant reduction in emissions.

Everything is perfectly fine with these secondary alloys, except that all OEMs want their parts made with increasing percentages of high-quality post-consumer scraps. So, everybody is running towards the same exit, which causes the scrap prices to explode. It will become the norm that secondary high-performance alloys are more expensive than primary ones, as suitable scrap is traded at a premium.

How to break out of the Spiral?

The easy answer to a complex problem is to change the alloy. Many foundries hate this solution because it adds complexity to their processing. However, sometimes real change is needed. When choosing an alloy that utilises scrap streams unsuitable for the current alloys, you can reduce the carbon footprint far below 1 kg CO2e per kg Al. You can also design the alloys to enable new properties.

Take a look at the heat sink below. It is the back of a massive 5G antenna for Ericsson. It features high, angled fins tilted toward one another to generate turbulence in the airflow. To get these fins cast, you need an alloy with excellent castability. Good castable alloys have high silicon contents and insufficient heat conductivity for application. The heat conductivity of the aluminium is the limiting factor in the range of these antennas. Range becomes the deciding factor when you need a network for whole countries.

For this part, an AlSi2.5FeMg was designed and selected for this application. The alloy has a carbon footprint of 0.36 kg CO2e per kg Al, which results in just 9 kg CO2e for the whole casting from the alloy side. It saves 91 kg CO2 per casting made with the best primary alloy and 391 kg CO2 per casting made with the global average. In this quick calculation, additional antennas with lower range are not considered.

In High-Pressure Die-Casting, the AlSi2.5FeMg alloy is uncastable. However, it works flawlessly in Rheocasting, as you can see in the picture. The heat conductivity in the casting (not a test plate) reaches up to 198 W/mK. To put this into perspective, that is just 10% below pure aluminium.

Conclusion

Yes, changing an alloy causes complexity for the foundries, but it is the best solution to enable unseen properties. As a side note to the foundries, staying with commodity alloys means staying with commodity parts and commodity pricing.

Specialised alloys in combination with Rheocasting allow design owners to expand part functionality and foundries to break out of the price war. Let’s discuss what Rheocasting can do for your part spectrum. Schedule a Free Consultation Call down below.