Aluminum is a versatile material that is used in many areas thanks to its combination of high strength and low weight – from construction and mobility to the packaging industry.
However, the production of pure aluminum is extremely energy-intensive. In the so-called Bayer process, aluminum oxide is extracted from bauxite, producing toxic red mud as a waste product, which poses a significant environmental burden. In view of declining natural ore deposits, aluminum recycling is becoming increasingly important to ensure the sustainable use of the material.
Aluminum recycling: Can the raw material be recycled indefinitely?
Although aluminum and its alloys can theoretically be recycled an infinite number of times through pure melting, this approach has its limitations. Particularly in the case of chips from industrial machining, the large surface-to-mass ratio during melting leads to so-called “burn-off.” This causes a significant portion of the material to burn and be irretrievably lost to the value-added process. Chemicals are used in industry to prevent this burn-off, but their handling and disposal is also problematic for the environment.
Solid-state recycling of aluminum: Processes for sustainable aluminum recycling
An innovative approach to avoiding these losses is solid-state recycling (see Figure 1). In this process, chips are compacted and reshaped below the material-specific melting temperature so that no “burn-off” occurs. The aim is to restore the properties of conventional aluminum through extreme forming.
A well-known process is hot extrusion, in which pre-compacted chips are pressed through a taper under high pressure and at temperatures ranging from 400 to 540 °C. During this forming process, the oxide layers that naturally form on aluminum surfaces break down. The increased temperature allows diffusion processes to take place, enabling a material bond (welding) between the chips. However, oxide layers and residues from machining—such as cooling lubricants—hinder this process.
Extrusion alone does not fully exploit the potential of solid-state recycling, as the type of forming process results in significant differences in material properties. The Institute of Forming Technology and Machines (IFUM) is therefore researching various alternative processes to further improve the performance of this recycling approach. These include
- direct sintering using the FAST (Field Assisted Sintering) process – see Figure 2,
- the ECAP (Equal Channel Angular Pressing) process – see Figure 3
- and the MAF (Multi Axial Forming) process – see Figure 4.
Aluminum chip recycling through FAST direct sintering
In conventional sintering processes, powdered materials are first pressed into green compacts and then heated in a furnace below their melting temperature for several hours. This allows diffusion to form a sufficient bond between the particles. However, this process can have a negative effect on the microstructure and takes several hours.
A further development is field-assisted sintering technology (FAST). Here, powder or chips are compacted in a tool between two punches in a vacuum chamber (Figure 2). At the same time, a pulsed direct current is introduced into the material via electrodes. Due to the high electrical resistance, the particles or chips heat up directly in the material. At the same time, the punches exert pressure on the sample, so that compaction and heating overlap. This combined effect breaks down the oxide layers on the chips and enables the creation of a material bond.
Another advantage of the FAST process is its flexibility: it can be carried out at material-specific solution annealing temperatures. If the material is then quenched directly from the sintering heat, a metastable, soft state is created in hardenable alloys. This can be specifically work hardened by subsequent cold forming. In addition, artificial aging leads to a further increase in strength. For example, by applying this process route, the tensile strength of chip-based samples of the EN AW 6060 alloy can be increased by more than 40% compared to conventional solid material, which represents an additional significant advantage of this route.
IFUM is currently investigating how different aluminum alloys in chip form behave in the FAST process. The aim is to develop a common thermomechanical heat treatment route that is suitable for different alloys. This process is intended to produce semi-finished products with optimized properties from waste chips, which can then be used for components. The ultimate goal is to create a modern, high-performance material from a by-product of machining that does not have to undergo the loss-prone, energy-intensive process step of remelting.
Recycling aluminum using the ECAP process
Equal channel angular pressing (ECAP) is a process in which a workpiece is pressed through a channel with a constant cross-section (Figure 3). The channel is angled so that the material is subjected to high deformation during the pressing process without reducing the cross-section.
ECAP is used to refine the grain structure of metals through plastic deformation at elevated temperatures. For the recycling of aluminum chips, loose chips are first pressed into briquettes, brought to process temperature in a furnace, and then placed in a preheated tool. Using a hydraulic press, the briquettes are pressed through the channel and severely deformed in the process. This process causes oxide layers to break up and a material bond to form between the chips.
A particular advantage of ECAP lies in its repeated application: with each process stage, the structure is further refined and the homogeneity of the structure in the sample increases. In combination with common heat treatments, this allows strengths to be achieved that are significantly higher than those of conventional primary materials. ECAP thus opens up great potential for the production of high-performance materials from recycled aluminum chips.
Aluminum recycling through multi-axial forming (MAF)
Multi Axial Forming (MAF) is a special forming process in which materials are formed not only in one direction, but systematically across multiple axes. While conventional processes are usually limited to a single main direction of stress, MAF combines repeated forming in different directions.
For the recycling of aluminum chips, this means that chips pressed into briquettes are deformed multiple times at elevated temperatures. The high temperatures reduce the forces required and promote diffusion processes, while the multiple forming operations repeatedly break up the oxide layers. In this way, a homogeneous material composite is built up step by step.
With an increasing number of forming cycles (Figure 4), the structure of the material becomes much more refined and homogeneous, so that the original chip boundaries are no longer visible. The investigations at IFUM show that the secondary aluminum produced in this way has a very fine microstructure and excellent mechanical properties. After six forming cycles of various alloys, tensile tests showed a 20% increase in strength compared to values reported in the literature for a comparable condition. MAF thus represents a powerful alternative to energy-intensive melting.
IFUM is continuing to research which of the processes is best suited for which applications during the current project period and potentially in follow-up projects.
