Influence of Cu Addition on the Structure, Mechanical and Corrosion Properties of Cast Mg-2%Zn Alloy M. Lotfpour, M. Emamy, C. Dehghanian, and K. Tavighi (Submitted September 11, 2016; in revised form March 18, 2017) Effects of different concentrations of Cu on the structure, mechanical and corrosion properties of Mg- 2%Zn alloy were studied by the use of x-ray diffraction, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, standard tensile testing, polarization and electrochemical impedance spectroscopy (EIS) measurements. The average grain size of the alloy decreased from above 1000 lm to about 200 lm with 5 wt.% Cu addition in as-cast condition. Microstructural studies revealed that Mg-2Zn- xCu alloys matrix typically consists of primary a-Mg and MgZnCu and Mg(Zn,Cu) 2 intermetallics which are mainly found at the grain boundaries. The results obtained from mechanical testing ascertained that Cu addition increased the hardness values significantly. Although the addition of 0.5 wt.% Cu improved the ultimate tensile strength and elongation values, more Cu addition (i.e., 5 wt.%) weakened the tensile properties of the alloy by introducing semi-continuous network of brittle intermetallic phases. Based on polarization test results, it can be concluded that Cu eliminates a protective film on Mg-2%Zn alloy surface. Among Mg-2%Zn-x%Cu alloys, the one containing 0.1 wt.% Cu exhibited the best anti-corrosion prop- erty. However, further Cu addition increased the volume fraction of intermetallics culminating in corrosion rate enhancement due to the galvanic couple effect. EIS and microstructural analysis also confirmed the polarization results. Keywords corrosion behavior, Cu addition, mechanical proper- ties, Mg-2%Zn alloy, structural refinement 1. Introduction Recently, Mg alloys have been lured many attentions in light-weight industry by demonstrating high strength and stiffness to density ratio, excellent surface properties, reason- able cost, good noise damping capacity and perfect machin- ability (Ref 1). In contrary, there are several drawbacks which limit their use, such as: difficult processing, low strength and ductility at ambient temperature and high reactivity leading to unacceptable level of corrosion in many environments (Ref 2). Mg-Zn alloys are attracted great interest in as-cast condition as they possess better corrosion resistance properties in comparison with other Mg alloys (Ref 3-6). For improving the mechanical properties of such Mg alloys, several elements like Zr (Ref 7, 9), Y (Ref 7, 11), Gd (Ref 8), Ca (Ref 9) and Cu (Ref 10) were added to Mg-Zn alloys. Moreover, Ag (Ref 12), Ca (Ref 13), Si (Ref 13) and RE (Ref 14) were used to enhance the corrosion resistance of this class of material. Cu effects on Mg-Zn alloys have been also studied by several researchers. Cu additions could increase the castability, ductility and strength of Mg-Zn alloys (Ref 15-17). Cu could also enhance the age hardening response of these alloys in both T 6 and T 4 heat treatment conditions (Ref 10, 18). Zhu et al. (Ref 19) investigated the effect of Cu additions on ZK60 alloy and showed that the presence of 0.5-1 wt.% Cu causes better tensile properties after age hardening. Due to the low stacking fault energy (SFE) of Cu, among other elements used in Mg-Zn alloys, Cu could encourage the twining mechanism in Mg alloys for improving mechanical properties (Ref 20). On this subject, Cu addition to ZK60 cast alloy could effectively eliminate the intragranular segregation and refine the grains and change the intergranular brittle fracture to more ductile fracture when the amount is 0.5-1 wt.% (Ref 21). In Mg-Zn-Cu-Al-Mn high strength alloy, Cu addition was found to be effective on ductility enhancement by the formation of divorced eutectic MgZnCu phase (Ref 22). Also, Golmakaniyoon et al. have shown that ZC63 cast alloy exhibited dendritic cast structure with interphases at interdendritic regions, which was shown in other works either(Ref 23, 24). To the very best of our knowledge, few reports have been published around the Mg alloys containing low percentage of Zn; therefore, this is the aim of this research to study the effects of Cu addition on the microstructure, mechanical properties and corrosion behavior of Mg-2%Zn alloy. 2. Experimental In current work, six Mg-2%Zn alloys with different copper levels (0, 0.1, 0.5, 1, 3 and 5 wt.%) were prepared. Different amounts of Cu, in the form of Mg-20wt.% Cu master alloy, were added into molten Mg-2%Zn alloy at 750 °C. Melting process was carried out in graphite ceramic crucible via a high- frequency induction furnace. Molten Mg-Zn alloy was pro- tected by an appropriate gas coverage (CO 2 + 5%SF 6 ). The melt was stirred gently by a graphite rod to satisfy its complete homogeneity. It was held at 750 °C for about 5 min and finally cast into cylindrical steel mold that had been preheated up to 250 °C (as shown in Fig. 1a). M. Lotfpour, M. Emamy, C. Dehghanian, and K. Tavighi, School of Metallurgy and Materials, College of Engineering, University of Tehran, P.O. Box 14395-731, Tehran, Iran. Contact e-mail: emamy@ut.ac.ir. JMEPEG ÓASM International DOI: 10.1007/s11665-017-2672-0 1059-9495/$19.00 Journal of Materials Engineering and Performance