The Secondary Alloy Dilemma

In the presentation about sustainable castings based on the Audi Space frame, I discussed the price of carbon emissions. The conclusion was that using secondary alloys would make a better world. But as usual, if you look in closer detail, it is not that simple.

First of all, the origin of the scrap is differentiated. There are post-industrial and post-consumer scraps. Only the post-consumer scraps are rated at 0 kg CO2 per kg Al. For post-industrial scrap, the carbon footprint of the production where it comes from has to be accounted for.

Secondly, there is a differentiation of aluminium alloys in use. There are sheet metals and casting alloys. Let’s look at the sheet metal first. They all have a low amount of alloying elements. These alloying constituents mostly don’t exceed 1%. If it is higher, most of the time, it’s magnesium, iron, or silicon. Aluminium casting alloys have a higher silicon content to use the better castability of the aluminium-silicon eutectic system. Silicon contents between 7 and 12% are widely used.

Modern casting alloys for automotive applications have a high purity. They need that purity to fulfil structural tasks and absorb a lot of energy in case of a crash. Therefore, they need high strength and/or high ductility. Elements like iron, copper, and zinc have restricted specifications to achieve that. In HPDC, often used AlSi10MnMg, the iron content is usually capped at 0.15%. For copper a maximum of 0.05% is allowed. Some OEMs only allow 0.03% copper. Zinc is usually capped at 0.05%.

So, not many sheet metals are usable for producing casting alloys. Their high iron contents limit their usability as a scrap source. Also, a lot of silicon has to be added to the alloy. The carbon emission of pure silicon is between 4 and 6,5 kg CO2 per kg Si. This is a carbon uplift of around 0.5 to 0.6 kg CO2 per kg Al just from the silicon.

Then let’s look at casting alloys. Ingate and overflow systems are directly remelted. But they are internal scraps and do not count. Other high purity alloys can be cleaned and well recycled. Aluminium wheels are a prime example of that. But they have the same requirements and have the scrap in a closed loop for themselves. It is nearly impossible to get them on the market.

If you currently take a car from the scrap yard and recycle the castings into new structural castings, you will have a bad time. Structural castings are quite new. They started first in 1994. Most castings in 20-year-old and older cars are made from an alloy called 226 or AlSi9Cu3. From the naming alone, you cannot have that near your high-performance alloys.

With that in mind, the sources of scrap are limited. A good alloy supplier who has a strong supply network on the secondary market can deliver a high-purity alloy. Even there, the amounts are limited, so secure your quantities now.

If you need help navigating through that jungle of secondary alloys, let’s build a sustainability strategy together that boots your enterprise value. Schedule a free consultation down below to learn the first steps.