HANS – Recycling of aluminium chips

The purpose of the project was to test and evaluate methods to increase the internal use of chips in order to minimize transport and avoid additional remelting. The goal is to achieve equivalent mechanical properties to the cast component currently. The methods to achieve this goal were initially a literature study followed by briquetting trials and casting trials.

Centrifugation of chips before briquetting has not affected the final moisture content in the briquettes. Pre-drying at elevated temperature has also no effect on the final result. The pressing of briquettes drives out so much cutting fluid that no difference in the moisture content of the briquettes has been measured regardless of whether the chips have been centrifuged or not.

In experiments with the addition of briquettes to a small melt (10 kg), the amount of inclusions increased significantly as the relative amount of briquettes increased from 10% to 20%. Even in full-scale experiments with a larger melt (850 kg), a clear difference was seen in terms of inclusions between a reference melt and after the addition of about 8% briquettes. After degassing the melt with added briquettes, however, there is a considerable reduction in the amount of inclusions, both in terms of number and size.

The addition of briquettes produces a noticeable difference in density of the cast material. Degassing and proper slagging significantly reduced the amount of porous inclusions and gases, even in comparison with the reference melt without briquette addition. This is also confirmed in studies of fracture surfaces where the deviations that occur are at a microscopic level.

In the full-scale melt, no direct differences in mechanical properties between the reference melt and the melt with added briquettes have been detected, neither before nor after degassing. Based on this work, it seems that the addition of chips in the form of briquettes can be a good and manageable solution from a sustainability perspective, not negatively affecting the resulting cast material. However, further tests should be conducted to verify this, especially for material applications with higher requirements.

GRETA – a case study on die-cast secondary aluminium

In ongoing production at AGES in Kulltorp, die-cast components of a secondary aluminium alloy have been continuously taken out for mechanical testing and microstructure investigations. The aim has been to increase the understanding of whether properties vary and, if so, why. In an industrial and well-controlled process, the properties and performance of the obtained material has been analysed for comparison with the original secondary alloy and its specification to see how the material varies during production. The results showed a uniform quality of the produced castings. The hardness test also showed very even results for each component. No statistical difference between the samples could be demonstrated. This means that even if some data collected from the castings were on the edge of the desired range, the properties have not been significantly affected. In other words, there is a higher potential in recycled alloys than that reported in the standard SS-EN 1706:2021. Solidification rate, possible heat treatment and the amount of defects play a decisive role in the final properties of a cast part. By optimizing these, properties that exceed the standard can be obtained.

Evolution of microstructure in a binary Al-Cu system (Al-4.3Cu) and a commercially alloyed Al-Cu system (A205) during solution heat treatment was investigated using optical microscopy (OM), scanning electron microscopy (SEM), wavelength-dispersive X-ray spectroscopy (WDS), and differential scanning calorimetry (DSC). The diversified coarseness of the microstructure was initiated by controlling the solidification rate. Different solution treatment temperatures were applied to identify a proper solutioning temperature. The larger microstructural scale required an increased solutioning temperature and prolonged holding time to obtain homogenized solutes in the α-Al matrix. The diffusion of Cu primarily controlled the solution heat treatment process. A diffusion-based model was applied and calibrated to determine the dissolution rate of an Al2Cu particle in the matrix. The model operates on a similar time scale with the experimental results for the Al-4.3Cu and A205 alloys with various microstructural scales, different chemical compositions, and at different solution treatment temperatures. Three-dimensional (3D) reconstructed images from SEM images and energy dispersive spectroscopy (EDS) map of elements showed that TiB2 particles shield the Cu-rich phases in the boundaries of α-Al grains, presumably acting as a physical barrier to the diffusion of Cu solutes toward α-Al grains. The model also suggests that the effective diffusion coefficient of Cu in Al, in the presence of TiB2 particles, reduced by a factor of 2.0–2.5 in the A205 alloy compared with the binary Al-Cu alloy. © 2020 by the authors.

In order to design and produce high-quality castings with reliable performance, the effect of the melt handling and melt quality during different processing stages needs to be understood and controlled, and numerical methods to provide correct input data to structural analyses of castings enabled. This paper aims to investigate tensile properties, in particular tensile toughness, of a secondary high-pressure die casting (HPDC) aluminium alloy with different levels of defects and imperfections. The melt, which was transported in liquid state from the smelter to the foundry, has been sampled after different holding times by casting into Y-blocks. Tensile testing was performed, and the levels of defects and imperfections were characterized using measurements of porosity, bifilm index, density index, sludge factor and the amount of iron-rich intermetallics. Two different quality indices have been evaluated, and a method to apply the results in simulations of damage in a casting, containing defects, subjected to load is demonstrated.

Al-Si-Cu alloys are the most widely used materials for high-pressure die casting processes. In such alloys, Fe content is generally high to avoid die soldering issues, but it is considered an impurity since it generates acicular intermetallics (β-Fe) which are detrimental to the mechanical behavior of the alloys. Mn and Cr may act as modifiers, leading to the formation of other Fe-bearing particles which are characterized by less harmful morphologies, and which tend to settle on the bottom of furnaces and crucibles (usually referred to as sludge). This work is aimed at evaluating the influence of sludge intermetallics on the fatigue behavior of A380 Al-Si-Cu alloy. Four alloys were produced by adding different Fe, Mn and Cr contents to A380 alloy; samples were remelted by directional solidification equipment to obtain a fixed secondary dendrite arm spacing (SDAS) value (~10 µm), then subjected to hot isostatic pressing (HIP). Rotating bending fatigue tests showed that, at room temperature, sludge particles play a detrimental role on fatigue behavior of T6 alloys, diminishing fatigue strength. At elevated temperatures (200◦C) and after overaging, the influence of sludge is less relevant, probably due to a softening of the α-Al matrix and a reduction of stress concentration related to Fe-bearing intermetallics. 

The primary precipitation of Fe-rich intermetallics in AlSi9Cu3(Fe) type alloys is studied for different Fe, Mn, and Cr contents and cooling rates. Differential scanning calorimetry, thermal analysis, and interrupted solidification with a rapid quenching technique were used in combination in order to assess the nucleation temperature of sludge particles, as well as to follow their evolution. The results revealed that the sludge nucleation temperature and the release of latent heat during sludge formation are functions of Fe, Mn, and Cr levels in the molten alloy (i.e., the sludge factor, SF) and cooling rate. Moreover, it can be concluded that sensitivity to sludge formation is not affected by cooling rate; i.e., a decrease in the SF will reduce sludge nucleation temperature to the same extent for a higher cooling rate as for a lower cooling rate. The sludge formation temperature detected will assist foundries in setting the optimal molten metal temperature for preventing sludge formation in holding furnaces and plunger systems.