Dissimilar metal joining of ZK60 magnesium alloy and titanium by friction stir welding
Highlights
► Dissimilar joint of Mg–Zn–Zr alloy and pure Ti has been successfully made by FSW. ► Zn and Zr as alloying elements formed the reaction layer with Ti at the interface. ► Tensile strength of the joint with the layer was higher than without any layer. ► Alloying elements had beneficial effect to improve the strength of Ti and Mg joint.
Introduction
Magnesium alloys have the lowest density among the commercially available alloys, but their corrosion resistance is poor [1], [2]. Titanium alloys also have low density, but its corrosion resistance is very high [3]. Hence, there are urgent needs to achieve dissimilar joining between magnesium and titanium so that these alloys can be applied to reduce the weight of various products. Most of the works about dissimilar joining of magnesium alloys and other metals have been performed on magnesium and aluminum joint [4], [5], [6]. However, titanium and magnesium do not form a solid solution and any compounds [7]. This means that it is difficult to join pure titanium and pure magnesium. In previous work, we have reported that AZ-type magnesium alloy (Mg–Al–Zn alloy) and AMCa-type magnesium alloy (Mg–Al–Ca alloy), which contain aluminum as an alloying element, can be successfully joined to pure titanium, because an aluminum-rich thin layer formed at the joint interface by reaction with titanium, and the tensile strength of the joint interface was improved by this aluminum-rich layer [8], [9]. Thus, the content of aluminum of the magnesium alloy affected the interfacial microstructure and the tensile strength of the dissimilar joint.
Among magnesium alloys, Mg–Zn–Zr alloy has a higher tensile strength than Mg–Al–Zn alloys. Therefore, Mg–Zn–Zr alloy such as ZK60 has been used in airplane and automobile products. However, it was difficult to fusion weld this alloy because of high susceptibility to the solidification cracking at the weld metal, which were caused relatively high Zn content in Mg–Zn–Zr alloy [10]. Hence, friction stir welding (FSW) is a solid state joining process and a joining temperature is lower than that in the fusion welding process. Therefore, FSW is suitable joining process for a Mg–Zn–Zr alloy [11]. In addition, Zn and Zr have a strong affinity with titanium and are expected to act as the similar effect of aluminum to form the reaction layer at the joint interface. However, no study has been reported on the characteristics of the dissimilar metal joint interface between titanium and Mg–Zn–Zr alloy. This study has been conducted by focusing on the effect of Zn and Zr of alloying elements on the microstructure and the tensile strength of the dissimilar metal joint of ZK60 Mg–Zn–Zr alloy and titanium by FSW in composition with pure magnesium and titanium joint.
Section snippets
Experimental procedures
Chemical compositions of titanium and Mg–Zn–Zr alloy were shown in Table 1. Commercially used Mg–5.5 mass% Zn–0.57 mass% Zr alloy (ZK60) and 99.5 mass% magnesium (Mg) were joined to commercially used 99.5 mass% titanium (Ti) by FSW. In ZK60 alloy, Zn is soluble in magnesium, and Zr partly forms the compound with Zn [12]. The joining conditions are presented in Table 2 and a schematic of the plates used in FSW is shown in Fig. 1. The probe was inserted in the ZK60 side and the probe edge was
Results and discussion
Fig. 2 shows the tensile strength of Ti and ZK60 joints at different probe offsets of 1.0 and 1.5 mm in comparison with those of the Ti and Mg joints for comparison. The tensile strengths of the Ti and ZK60 joints increased with increasing the probe offset and reached to 237 MPa, at 1.5 mm offset, which was 69% of that of ZK60 base metal, and much higher than that of Ti and Mg joint, 135 MPa the probe offset 1.5 mm, though, in the Ti and Mg joint, little change was observed in the tensile strength
Conclusion
The effect of alloying elements of ZK60 Mg–Zn–Zr alloy on the microstructure of the dissimilar joint interface with titanium and the joint strength in comparison with pure magnesium and titanium has been investigated. Zn and Zr of alloying elements formed a thin reaction layer with titanium at the joint interface by friction stir welding. The fracture of the joint by tensile test occurred mainly in the stir zone of Mg–Zn–Zr alloy and partly at the joint interface. The tensile strength of the
Acknowledgement
This work was performed under the Cooperative Research Program of the Institute for Joining and Welding Research Institute, Osaka University.
References (16)
- et al.
Scripta Mater.
(2004) - et al.
Mater. Charact.
(2004) - et al.
Mater. Sci. Eng. B
(2009) - et al.
Mater. Sci. Eng. B
(2010) - et al.
J. Electrochem. Soc.
(1990) - et al.
Adv. Eng. Mater.
(2003) - et al.
J. Electrochem. Soc.
(1961) - et al.
Adv. Eng. Mater.
(2006)