Effects of Er and Zr Additions on the As-Cast Microstructure and on the Solution-Heat-Treatment Response of Innovative Al-Si-Mg-Based Alloys

Abstract

The microstructure of Al-Si-Mg alloys strongly depends on their chemical composition and on the heat treatment that is performed during production. The influence of solution heat treatment and the additions of Er and Zr on the microstructure of gravity cast A356 (Al-7Si-0.4Mg) are investigated. The as-cast microstructure is characterized by means of grain size measurements, morphology of eutectic Si as well as morphology, area fraction and chemical composition of the intermetallic compounds. The morphology of eutectic Si is found to be unstable with respect to high temperature exposure during solution heat treatment. The evolution mechanisms are described using thermodynamic and kinetic models for the investigated alloys and validated using optical and SEM micrographs. The effects of high temperature exposure during solution heat treatment on the amount and morphology of the intermetallic compounds is also characterized using optical and SEM micrographs.

High Temperature Behavior of Al-7Si-0.4Mg Alloy with Er and Zr Additions

“…The other Er-containing sets of particles were characterized by the concurrent presence of Fe or Mg, as clearly shown by elemental maps in Figure 9. The morphology of some of these Fe-or Mg-containing particles was similar to that observed in the E3 alloy in the metallographic sample (Figure 8b) and in the gauge length of the crept specimen (Figures 7 and 9) and suggests that they could be the residuals of the particles with complex composition and morphology observed also in the as-cast condition and partly modified during the solution treatment [46]. Figure 9 also includes some Zr-containing particle with the size of the order of one micron, some of which are also iron-containing.…”

“…The chemical compositions of the three alloys measured by glow discharge optical emission spectroscopy (GDOES) are shown in Table 1. They are close to those of ingots with the same nominal composition that were used for heat treatments optimization and tensile characterization [30,45,46]. Bars of about 12 × 12 × 100 mm 3 size were sampled from the mid-low part of each cylinder.…”

“…Thus, the morphology of Si in these alloys was more related to the as-cast microstructure than to further exposure at high temperature. Further, the area fractions of coarse intermetallic phases for the A356, E3 and EZ35 alloys, which were 0.5, 0.9 and 1.3 vol% in as-cast alloys, after 5 h SHT reduced to 0.3, 0.6 and 1.1 vol% [46]. The Mg-containing π-Al 8 Mg 3 FeSi 6 Chinese script and Mg 2 Si phases show the most significant changes during the 5 h SHT.…”

“…In PA conditions, the α-Al phase at the dendritic core is flanked by Si particles and by coarse phase containing Er and Zr in the modified alloys and also combinations of Fe, Cu and Mg and minor alloying additions in the reference alloy. In as-cast conditions, the eutectic Si length was longer and with higher aspect ratio for A356 (about 27 µm and 10.8, respectively), but shows a rapid decrease of both morphology parameters within the first dozen of minutes at 540 • C, reaching respectively 9.2 µm and 4.2 after 5 h at 540 • C. On the other hand, E3 and EZ35 form coral-like eutectic Si structures, resulting in shorter and lower aspect ratio Si particles in metallographic sections (of the order of 4 µm and 1.65, respectively) [46]. The 5 h SHT only slightly increased the average eutectic Si particle length, while decreasing the aspect ratio to about 1.35 for both alloys.…”

“…The microstructural features and aging behavior of the three alloys in the T6 condition will be here summarized anticipating both creep test and fractographic analyses results. The authors of the present paper have presented and discussed them in detail elsewhere [46].…”