This work provides an overview of the aluminum (Al) recycling process, from the scrap upgrading to the melting process. Innovations and new trends regarding the Al recycling technologies are ...highlighted. Aluminum recycling offers advantages in terms of environmental and economic benefits. The presence of deleterious impurities in recycled Al alloys is increasing and this is the main drawback if compared to primary alloys. The continuous growth of undesired elements can be mitigated by different technologies, preliminary operations and treatments, and by the optimization of the melting process. Downgrading and dilution are possible solutions to reduce the rate of impurities, but they are not sustainable if the final use of Al alloy continuously increases. The main objectives in the development of the Al recycling are shown and discussed. In particular, the evolution of preliminary treatments of the scrap, as sorting, comminution and de-coating, is reported and a review of the melting technologies is also presented. However, the choice of performing preliminary operations to the melting stage, thus improving the operating conditions during the furnace running, is a trade-off between costs and process efficiency.
The effects of Gd content on the solidification path and microstructure of an AlSi7Mg0.3 alloy have been studied. Two different Gd levels, 0.1 and 0.5 mass%, respectively, have been investigated ...while the material has been solidified at low (0.2 °C s
−1
) and high (1.3 °C s
−1
) cooling rates. Computer-aided thermal analysis and metallographic techniques have been used to study the solidification and microstructural evolution of the alloy at different Gd contents and cooling rates. The results show how Gd has no effect on the grain refinement at both cooling rates. The eutectic structure is unaffected at Gd level of 0.1 mass%. A concentration of 0.5 mass% Gd promotes the precipitation of the gadolinium phosphide (GdP) phase instead of aluminium phosphide (AlP) compounds, thus suppressing the eutectic plateau during the solidification and serving as nucleant for the GdAl
2
Si
2
phase. The eutectic Si crystals solidified at low cooling rate are refined at 0.5 mass% Gd content, while the morphology of Si crystals is only partially modified at high cooling rate.
A modified fibrous-like eutectic structure strongly improves the final mechanical properties of Al–Si foundry alloys, especially ductility. Beside widely used commercial eutectic modifiers such as Sr ...and Na, lanthanides appear to be a possible alternative in the eutectic Si modification process for hypoeutectic Al–Si casting alloys. All lanthanides have similar physical and chemical properties, such as density, melting point, and fading phenomena, which have been compared in the present review. They also show atomic radii close to the optimal atomic radius for a modifying agent. However, the microstructural evolution of eutectic Si is strictly related to the specific element and content of lanthanides, whose abundance, reserve, mining, production and market situations have been preliminary evaluated in this paper. The eutectic modification mechanisms induced by lanthanides are not well-discussed and clarified yet. The advantages and disadvantages of individual addition of lanthanides for chemical modification of hypoeutectic Al–Si alloys have been here critically reviewed. The use of lanthanides for eutectic Si modification in Al–Si alloys has been discussed from both effectiveness and economical point of views. Nowadays, the actual cost and their efficiency seem to make lanthanides still far to be used in foundry industry for commercial and large-scale applications.
This research aimed to study the formation and distribution of oxide-related defects in the gravity die casting process of an AlSi7Cu0.5Mg alloy by using experimental and numerical investigations. ...Metallographic and image analysis techniques were conducted to map the distribution of oxide inclusions inside the casting at the microscopic level. Numerical simulations were used to analyse the filling and solidification stages, and to foresee the turbulence of the melt and the formation of the oxide defects. The results show that most of the defects were correlated with the oxide layers or bubbles entrained inside the liquid metal. The accuracy of the numerical code in simulating the metal fluid-dynamic behaviour and the heat transfer was verified, and the results were in agreement with the experimental findings. The numerical distribution of defects was consistent with the experimental results, proving that the model successfully predicted the formation of oxide-related defects.
The evolution of sludge particles in a secondary high-pressure die-cast AlSi9Cu3(Fe) alloy has been investigated over three levels of iron (0.80, 1.00, 1.20 wt.%) and manganese (0.25, 0.40, ...0.55 wt.%), and two levels of chromium (0.06, 0.10 wt.%). Metallographic and image analysis techniques have been used in order to quantitatively evaluate the morphological and dimensional variations of sludge with different Fe, Mn and Cr contents. The results indicate that any increase of the Fe, Mn and Cr level promotes the formation of coarser sludge, with polyhedral and star-like morphologies. Fe and Mn produce an increase on the number of primary Fe-rich particles, while Cr promotes the formation of a large number of secondary Fe-rich particles. No relationship is revealed between the morphological variation of sludge and the Fe, Mn or Cr contents, as well as between the sludge factor and the Fe:Mn ratio. In the analyzed range of composition, the design of experiment methodology and the analysis of variance have been used in order to develop statistical models that accurately predict the average size and number of sludge particles in the AlSi9Cu3(Fe) diecasting alloys.
•The increase of the Fe, Mn and Cr level promotes the formation of coarser sludge.•The sludge content is more related to Fe and Mn levels.•The size and content of sludge can be described by linear regression models.•The regression models consider the Fe, Mn, Cr levels and their interactions.•The sludge fraction can be estimated from the sludge factor by linear relationship.
Aluminum recycling is a promising solution to environmental and economic issues. Secondary aluminum production will rise in the near future; however, the process is not without challenges. Some of ...the major concerns during remelting of aluminum are the metal losses due to the oxidation of the molten metal and the removal of impurities from the metal bath. The current study summarizes the latest progress in the use of solid salt fluxes for secondary aluminum production and the treatment of molten metal. The chemistry of solid fluxes has been reviewed, with a correlation to their main chemical and physical characteristics, such as density, fluidity, wettability, and reactivity. An overview of the main types of solid fluxes is also provided, with a particular focus on their functions and applications. The efficiency of solid fluxes relies on several factors, including but not limited to the fluxes’ chemical composition and physical properties, flux amount, processing temperature, and flux morphology. The effect of salt fluxes in delivering satisfactory metal cleanliness and sufficient metal recovery has been summarized according to the main flux’s properties.
The early stages of eutectic solidification in a copper-containing 7xxx series aluminum alloy (AA 7068 or AMS 4331) were studied using the two-thermocouple computer-aided thermal analysis (CATA) ...technique. A feature was detected on the cooling rate curve at the equilibrium solidus temperature of the alloy which persists until the peak of the subsequent final eutectic solidification. Detailed analysis of the temperature difference between the wall and the center of the thermal analysis sample, together with examination of the eutectic solidified on the walls of porosities and a study of the eutectic nucleation on the basis of the non-classical theory of adsorption heterogeneous nucleation, indicated how the feature can be related to the faceting of the atomic structure of the solid/liquid (S/L) interface. The solidification of the remnant liquid after the faceting transition at the equilibrium solidus point depends on the interfacial undercooling and proceeds
via
either primary phase re-nucleation or secondary phase nucleation by adsorption. The eutectic solidification is affected by the presence of the primary phase which acts like an adsorbent.
The effects of Sr addition and pressure increase on the microstructure and casting defects of a low-pressure die cast （LPDC） AISi7Mg0.3 alloy have been studied. Metallographic and image analysis ...techniques have been used to quantitatively examine the microstructural changes and the amount of porosity occurring at different Sr levels and pressure parameters. The results indicate that an increase in the filling pressure induces lower heat dissipation of the liquid close to the die/core surfaces, with the formation of slightly greater dendrite arms and coarser eutectic Si particles. On the other hand, the increase in the Sr level leads to finer microstructural scale and eutectic Si. The analysed variables, within the experimental conditions, do not affect the morphology of eutectic Si particles. Higher applied pressure and Sr content generate castings with lower amount of porosiW. However, as the filling pressure increases the flow of metal inside the die cavity is more turbulent, leading to the formation of oxide films and cold shots. In the analysed range of experimental conditions, the design of experiment methodology and the analysis of variance have been used to develop statistical models that accurately predict the average size of secondary dendrite arm spacing and the amount of porosity in the low-pressure die cast AISiTMg0.3 alloy.
Phase Change Materials (PCMs) are materials that release or absorb sufficient latent heat at a constant temperature or a relatively narrow temperature range during their solid/liquid transformation ...to be used for heating or cooling purposes. Although the use of PCMs has increased significantly in recent years, their major applications are limited to Latent Heat Storage (LHS) applications, especially in solar energy systems and buildings. PCMs can be classified according to their composition, working temperature and application. Metallic PCMs appear to be the best alternative to salts and organic materials due to their high conductivity, high latent heat storage capacity and wide-ranging phase change temperature, i.e., melting temperature and chemical compatibility with their containers. This paper reviews the latest achievements in the field of low-melting point metallic PCMs (LMPM-PCMs), i.e., those with melting temperatures of less than 420 °C, based on Zn, Ga, Bi, In and Sn. Pure LMPM-PCMs, alloy LMPM-PCMs and Miscibility Gap Alloy (MGA) LMPM-PCMs are considered. Criteria for the selection of PCMs and their containers are evaluated. The physical properties and chemical stability of metallic PCMs, as well as their applications, are listed, and new application potentials are presented or suggested. In particular, the novel application of metallic PCMs in casting design is demonstrated and suggested.
This paper introduces and examines a new generation of chillers for judicious control of the solidification structure of cast metals and alloys. This innovative chiller makes use of the absorption of ...the latent heat of melting of a Phase Change Material (PCM) incorporated in the chiller. In this work, Al-4.5 wt%Cu melt was cast in sand molds fitted with a traditional solid steel chiller as well as this new type of chiller consisted of a steel container filled with a given amount of pure zinc as PCM. Effects of the PCM fitted chiller on the thermal history, solidification and structure of the castings were studied by experimental and computer simulation investigations. The optimum casting parameters and mold and chillers dimensions were selected using ProCast simulation software. Use of the PCM fitted chiller resulted in faster columnar-equiaxed transition, 27 %, 54 % and 40 % reduction in the length of the columnar zone and the primary and the secondary dendrite arm spacings, respectively, and about 30 % improvement in the hardness. Thermal gradients and cooling rates at different points of the PCM fitted casting were generally more than those in the traditionally chilled casting. Use of the PCM fitted chiller eliminated the formation of the feathery grains and reduced the formation of the undesired Fe-rich phases close to the chiller due to different thermal history of the castings. Segregation pattern and change of Growth Restriction Factor (GRF) along the castings were studied and related to the cooling and growth conditions experienced by each casting.
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•A new generation of chillers for tailoring the solidification structure of alloys was introduced.•Effects of using pure Zn, as a PCM, on solidification structure of Al-4.5 wt%Cu was investigated.•Columnar to Equiaxed Transition (CET) was accelerate due to calculated melting of the PCM.•Formation of undesirable feathery grains was prevented using the new chiller.•Similarity criteria and thermal history data were obtained from ProCast simulation results.
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