Secondary aluminium alloys and innovative metallurgical processes for sustainable automotive applications

In recent years, the request for Al alloys in all industrial sectors, especially in the automotive industry, has grown constantly to produce light components with low environmental impact without performance loss. The production of primary Al is very polluting and energy expensive. Instead, the secondary Al requires only about 5% of the energy of the primary process. However, the use of recycled material introduces high level of impurities into the alloys that need to be considered, since they could lead to the formation of intermetallic precipitates that could affect the performance and mechanical properties of the final products. These topics were studied using a CALPHAD approach, using the computational software Thermocalc and Pandat. Currently, HPDC processes are among the most used in foundries and allow to obtain light thin-walled components of complex shapes, using alloys of the Al-Si-Mg family, characterized by low melting temperatures, good fluidity for mould filling, and good surface finish, which is a basic requisite for plating. However, often the components produced using HPDC processes need subsequent heat treatments to optimize their mechanical properties. Furthermore, these treatments are usually carried out in plants external to the foundry and require the transport of considerable weights by road. Consequentially, they have significant impact on the economic and environmental aspects of the process. The first topic of the Ph.D. is related to the elimination of T5 heat treatment on some components prepared using an AlSi10Mg commercial alloys, to reduce the environmental impact of production. I evaluated the effect on the microstructure and mechanical properties of the addition of a nucleating agent (AlTi5B) and a eutectic modifier (Sr). In the second part of the Ph.D., I studied the effect of the composition for two different series of alloys, AlSi10Mg(Fe) and AlSi9Cu3(Fe), and the production process, HPDC and TOPCAST, on the amount of intermetallic compounds that are formed. I performed EBSD and SEM-EDS analysis to measured experimentally the % wt. of the phases of interest. The result was compared with the values predicted by applying the Scheil solidification model, or thermodynamic equilibrium, using Thermo-Calc software. Then, the effect of the composition of AlSi9Cu3(Fe) alloys on the curve of formation temperature of the primary -Al particle and the solidus curve was studied. In these regions of the phase diagrams, we have the coexistence of the solid and the liquid. However, on industrial scale, with several kilograms of molten metals for each casting, the control of temperatures during the processes is very complex, and highly dependent on the correct application of protocols by production operators. Currently this requirement can be met by using low silicon alloys (max. 7 wt.% Si), however, during this Ph.D. we study the possibility of use of secondary aluminium alloys with a higher silicon content (9-10% wt.). Finally, the correlation between the viscosity of the semi-solid material and its solid fraction was studied, to evaluate the possibility of introducing an on-line monitoring system during the industrial preparation of the slurry for the RheoMetal process. In conclusion, an overall summary of the results obtained in the thesis is drawn along with some future perspectives. During my Ph.D., I observed several times how the breadth of the composition range allowed for the same commercial secondary alloy can lead to very different microstructures and quantities of phases present. Therefore, it is essential to be able to estimate, upon the arrival of a new batch of an alloy, how it will behave, to intervene with corrections to the composition when appropriate. Recycled alloys are a key component in more eco-sustainable processes, but they also present numerous issues that must be carefully managed.