Analysis of the Mechanical Properties of Secondary AlZnMg(Cu) Die-Cast Alloys
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Copyright Ó 2020 The Author(s) https://doi.org/10.1007/s40962-020-00510-6
Abstract In this work, the development of AlZnMg(Cu) alloys was performed by thermodynamic simulations and experimental tests. With calculations, the solidification processes and phase compositions could be predicted. As secondary casting alloys, the selected compositions of AlZn4Mg3Cu(Fe) and AlZn5Mg4Cu have higher contents of alloying elements, such as iron and silicon, and were successfully processed with a cold-chamber die casting machine. In addition, an energy- and time-saving optimization of the material properties was performed by a modified heat treatment (T6*) with a total time of only 12 h. The alloys
were evaluated based on the characterization of their mechanical properties and the analysis of their microstructures. In particular, the elimination of cost-intensive alloying elements and the application of a modified heat treatment method clearly show the potential of AlZnMg(Cu) die-cast alloys for future industrial use.
Introduction
possible, but the phase is usually specified with MgZn2.6 In this phase, zinc can be substituted by aluminum and copper, but high copper content has a destabilizing effect.6,7 In addition, the cubic T-(Al2Mg3Zn3), orthorhombic S-(Al2CuMg), and, at high zinc levels, cubic Z-(Mg2Zn11) phase can occur. Both g- and T-phases form a quasi-binary eutectic with aluminum. In general, the occurrence of these phases depends on the concentration ratio of Zn/Mg. At a ratio Zn/Mg [ 2, the formation of g-(MgZn2) predominantly occurs; at Zn/Mg B 2, T-(Al2Mg3Zn3) dominates.8 For a long time, only MgZn2 was investigated for precipitation strengthening. However, in most commercially used AlZnMg(Cu) alloys, both g- and T-phases occur. The Zn/ Mg ratio and combined Zn ? Mg content are crucial for the final properties of AlZnMg(Cu) alloys. With a Zn ? Mg content of 2–5 wt.%, little or no hardening effect is present, whereas good castability is given only up to a Zn ? Mg content of approximately 6 wt.%. Although high Zn ? Mg contents lead to high strengths, they also increase the susceptibility to stress corrosion cracking.10
The development of sustainable production methods is prompting resource-saving ways to produce load-bearing lightweight components. In this context, the use of secondary aluminum offers attractive opportunities, as up to 95% of the energy required for primary production can be saved.1 Secondary alloys fulfill the requirements in die casting and enable a cost-efficient production of complex components. Furthermore, AlZnMg(Cu) wrought alloys reach tensile strengths above 700 MPa; hence, these alloys satisfy the mechanical requirements in structural components.2 The alloying elements zinc, magnesium, and copper with a proportion of 0.5–2 wt.% form complex intermetallic phases that control the material properties and enable high hardenability and high strengths.3,4 First, the g-(MgZn2) phase must be mentioned, which is a hexagonal Laves phase in which extremely dense packings are formed due to favorable
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