Design and characterization of innovative Al-7Si-Mg casting alloys with Er and Zr additions

The more stringent requirements for fuel consumption optimization and efficiency increase for environmental reasons are constantly pushing industries and scientific institutions to develop stronger and more microstructurally stable Al alloys, to substitute steels and cast irons in vehicles. A general-purpose family of Al alloys used in the transportation industry for structural applications is that of hypoeutectic Al-Si casting alloys and, more specifically, of A356 (Al-7Si-0.4Mg). Literature review showed that microstructural characteristics and mechanical properties at room and high temperature of A356 are found to be strongly influenced by suitable additions of alloying elements. Thus, tuning the chemical composition of Al-Si-Mg casting alloy is the most promising approach to improve their characteristics, possibly expanding their applicability. In particular, suitable additions of rare earth Er improved the room temperature mechanical properties of A356 alloy, due to the formation of L12 Al3Er strengthening dispersoids. Combined additions of Er and Zr to pure Al caused strong improvements in its mechanical properties and thermal resistance, due to the precipitation of Al3(Er, Zr) L12 strengthening dispersoids. The improved thermal resistance of ternary Al-Er-Zr alloys is caused by the coarsening resistance of the strengthening dispersoids, which in turn is found to be a result of the sluggish diffusivity of Zr in Al. Despite the promising effects of Er and Zr, the state of the art on this topic is limited and many questions have not been solved yet. Thus, the present PhD research is intended to have both a scientific and an industrial relevance. From a scientific point of view, this work is aimed at expanding the state of the art regarding the influences of Er and Zr on commercial Al-Si-Mg alloys. In particular, the focus is on characterizing, understanding and modelling the effects of Er and Zr additions on eutectic Si modification, on the age-hardening response and microstructural stability, on the mechanical behaviour (at room and high temperature) and, finally, on the corrosion resistance of Er- and Zr-containing alloys, based on A356. More specifically, in the first part of the work, the designed alloys were characterized from a microstructural point of view by means of optical microscope, SEM and STEM analyses and from a mechanical point of view at different temperature and after different exposure times at high temperatures. From a microstructural point of view, additions of Er caused a modification of the eutectic Si morphology, which in the as-cast state passed from plate-like to fibrous. Further, the addition of Er hindered the formation of Fe-containing intermetallic compounds usually found in commercial Al-Si-Mg alloys. On the other hand, different classes of intermetallic compounds containing Er formed, according to the chemical composition of the alloy. Further additions of Zr did not cause relevant modifications in the eutectic Si morphology with respect to alloys with only Er; as intermetallic compounds are concerned, the intermetallic compounds already present in Er-containing alloys were slightly enriched in Zr. Additions of 0.59 wt.% Zr caused the formation of pro-peritectic (Al, Si)3(Zr, Ti) intermetallic compounds which acted as inoculants for Al dendrites, changing their morphology from elongated to globular. From a mechanical point of view, suitable Er and Zr additions increased the strength and ductility of A356 and its resistance to prolonged high temperature exposure. In particular, among the alloys containing only Er, additions of 0.3 wt.% of this element (named E3) resulted in the best combinations of ductility and strength, due to a modified eutectic morphology, a low amount of embrittling intermetallic compounds and the presence of Al3Er coarsening-resistant strengthening dispersoids. Among the alloys containing Er and Zr, the one with 0.3 wt.% Er and 0.59 wt.% Zr additions (named EZ35) showed the best ductility and the highest mechanical properties, due to a modified eutectic morphology, relatively low amount of embrittling intermetallic compounds, globular morphology and Al3(Er, Zr) strengthening dispersoids. Considering the results obtained from tensile tests and preliminary microstructural observations, A356, E3 and EZ35 were selected for the studies performed in the rest of the work. The three alloys were studied by means of SEM micrographs on deep etched specimens subjected to different Solution Heat Treatment (SHT) times, in order to characterize the influence of high temperature thermal treatment on the evolution of eutectic Si morphology and on intermetallic compounds. The mechanisms underlying eutectic Si morphology evolution in the three alloys were identified and successfully modelled using thermodynamic and kinetic models. STEM micrographs were taken and DSC tests performed on samples in different thermal treatment conditions, in order to assess the influence of Er and Zr on the age hardening response of A356. In particular, it emerged that Al3Er and Al3(Er, Zr) strengthening dispersoids precipitated during SHT, suggesting that conventional SHT could be optimized for modified alloys. Al3Er and Al3(Er, Zr) also precipitated during subsequent aging. finally, the presence of Er and Zr reduced the activation energy for precipitation of Mg- and Si-containing β’’ and β’ dispersoids. To conclude the first part of the thesis, MatCalc simulation software was used and thermodynamic and kinetics models applied to a ternary Al-7Si-0.4Mg alloy, to study the influence of thermal treatment on the evolution of its microstructure and mechanical properties. Simulations started with the identification of the phases formed during solidification, following the Scheil approach. Subsequently, the model alloy was subjected to 5h solution heat treatment at different temperatures, in order to study the dissolution kinetics of intermetallic compounds and the ability of the software to describe the influence of temperature on the process. The model alloy was then subjected to quenching at different quenching rates and subsequently aged at 200 °C for 3h. Scope of this part of the analysis was to simulate the quench sensitivity of Al-7Si-0.4Mg alloy and the influence of quench rate on its mechanical property after aging. Simulated results were validated using experimental results from a commercial A356 alloy. It was shown that thermodynamic and kinetics models implemented in MatCalc can be used to effectively predict the influence of complex thermal treatments on the evolution of microstructure and mechanical properties of cast Al-Si-Mg alloys. The second part of the thesis was devoted to a characterization of the corrosion resistance and electrochemical characteristics of A356, E3 and EZ35 in simulated sea water (3.5 wt.% NaCl aqueous solution). Short- and long-term corrosion resistance was studied by means of linear polarization tests, electrochemical impedance spectroscopy and microstructural characterization after different immersion times (1h, 1 day, 4 days, 1 week, 2 weeks and 4 weeks). Further, scanning Kelvin probe force microscopy was used to measure the electrochemical potential difference of intermetallic compounds with respect to Al, quantifying microgalvanic coupling. E3 was the alloy showing the best corrosion resistance, thanks to a relatively low amount of intermetallic compounds with a small electrochemical potential difference with respect to the matrix and to a modified eutectic Si, which reduced microgalvanic coupling. On the other hand, the high electrochemical potential difference of intermetallic compounds in alloy A356 with respect to Al and the elongated morphology of eutectic Si induced a more intense microgalvanic coupling, reducing the corrosion resistance of the alloy. The addition of Zr to the chemical composition of E3 reduced the corrosion resistance of EZ35. The higher amount of intermetallic compounds with slightly higher electrochemical potential difference with respect to Al and the globular morphology of Al dendrites were considered as responsible for the reduced corrosion resistance of the alloy. After a proper understanding of the influences of Er and Zr on the microstructural, mechanical and electrochemical characteristics of A356, alloys with suitable chemical composition could be proposed on an industrial scale, in order to improve the performances of commercially used Al-Si-Mg alloys and expand their applicability.