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Trimet: Sophisticated Alloys with a High Level of Recycled Material

Thursday, February 24, 2022

Next to energy-efficient drive systems, lightweight construction materials are a key technology in the transport sector when it comes to reducing the environmental impact through increased resource efficiency. Various aspects of sustainability in mobility and the minimizing of CO2 emissions during the entire product life cycle are of particular interest in the automotive industry. An increasing focus in this respect is the outlay during production of components where up to 95 % of emissions can be saved through the use of recycled aluminium when compared to primary material use. Consequently, the demand in the industry for aluminium alloys with a high level of recycled material has increased sharply. High-strength wrought and cast secondary aluminium alloys can make an important contribution to climate targets. Trimet Aluminium SE/DE is involved in a number of research projects endeavoring to make materials of this kind with a positive life cycle assessment available.

In order to develop wrought aluminium alloys that conserve resources, notable companies along the entire supply chain have joined forces to create “Green-Al-Light” (www.green-al-light.de), an initiative supported by the German Federal Ministry for Economic Affairs and Climate Action. The aim of this project is to establish an optimized process chain, particularly for high-strength automotive structural components using recycled aluminium. The focus here is on the increased use of End-Of-Life (EOL) scrap material from the automotive industry and a holistic approach to all process steps and their correlations.

Existing alloys manufactured on the basis of primary aluminium are also subjected to continuous further development by Trimet. The scrap mix available on a daily basis which can be used for a variety of purposes imposes limits on the production of secondary alloys. The addition of alloy elements cannot be exactly controlled in terms of quantity and quality. This vagueness can have a major impact on the properties of alloys. Iron and copper are the most significant accompanying elements in this respect, as current technology is incapable of removing these efficiently from the melt. AlSi alloys react particularly sensitively to increased fractions of iron and copper. Consequently, they are produced as primary alloys with the lowest possible fractions of these elements for optimum mechanical and technological properties. This entails a need to use end-of-life aluminium fractions that are as high as possible during production. Research needs to break new ground to address these needs. To date, the development of aluminium alloys focused on optimum mechanical and technological properties, meaning “recycling variants” of standard primary alloys are largely unknown.

Trimet investigates effective mechanisms during the controlled addition of iron and copper fractions through simulation and experimental methods. Phase diagrams, cooling curves and phase fractions relating to the alloy elements examined are calculated in this context with the aid of CALPHAD software. Trimet produces and examines selected alloy variants on a laboratory and pilot scale on the basis of these findings. These investigations adhere to the principle of a holistic approach, with microscopic structural analyses, XRD and REM characterization phases, grain sizes, pore distribution and dendrite arm spacing, tensile tests determining mechanical properties and analyses determining corrosion properties and, depending on the alloy, castability, flow properties, hot cracking tendency and shrinking. Trimet conducts all investigations in its own laboratories and test facilities. The fundamental challenges of the different projects include the maintenance of some product requirements, regardless of sustainability considerations. In addition to surface qualities, corrosion resistance and other characteristics, the costs involved must also be considered. 

Trimet addresses demands regarding cost and energy efficiency with the new Trimal®-38 (AlSi8ZnMn) die casting alloy. With a magnesium content of maximum 0,01 %, the material does not require heat treatment to achieve the desired mechanical properties to, for example, resist crash loads. Trimet has also proven that technical and ecological properties need not be a contradiction. As a recycled variant of the proven Trimal-05 (AlSi10MnMg) primary alloy, the company has developed the trimal-04 alloy and established it successfully on the market.

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