Deformation behaviors of twin roll cast Mg-Zn-X-Ca alloys for enhanced room-temperature formability
Introduction
Twin roll casting (TRC) is a promising process by which to fabricate strip plates, such as stainless steel and Al alloy sheets. Recently, it was reported that commercial Mg alloy sheets can be successfully fabricated through the TRC process at a low cost [1], [2]. TRC Mg alloys exhibit good mechanical properties; however, their lower formability at room temperature is a drawback when applied to automobile components due to high cost of sheet forming. Therefore, the development of highly formable, high-strength TRC Mg alloys is necessary in order to widespread the application of Mg sheets and reduce the weight of vehicles. The high formability of Mg alloys is closely related to their weaker basal textures or random textures. There have been numerous studies of texture modifications of strong basal textures that are vulnerable to room-temperature formability. One approach that changes a strong basal texture into a random texture involves some form of severe deformation (SDF), such as equal-channel angular extrusion (ECAE), differential speed rolling (DSR) or cross rolling to develop weaker basal textures [3], [4], [5]. However, these types of processes increase the production costs, and the shapes of Mg alloys with which these processes can be applied are limited. Another approach to change the texture of Mg alloys involves the addition of alloying elements such as Ca, Sr, Y or RE, among others. Yuasa et al. [6] studied the effects of additions of Ca and Sr on Mg-Zn alloys. Mg-Zn-Ca and Mg-Zn-Sr alloys exhibit weaker basal textures and high formability with index Erichsen of 7.3–8.2 mm due to the higher stacking fault energy. Chino et al. [7] studied the effects of the Y content on Mg-Y alloys, finding that Y as an alloying element could increase the activity of prismatic <a> cross slip, resulting in the formation of weaker textures, also finding that Mg-1.5Zn-xY alloys show excellent index Erichsen in the range of 5.7–9.2 mm. It has also been reported that Mg-Zn-Mn-RE alloys show weak and random basal textures with a broader distribution of basal poles toward TD and with formability at room temperature of 9.5 mm, which is nearly identical to that of Al alloys [8]. Despite the fact that they exhibit excellent room-temperature formability, they are not fabricated through the TRC process and do not have sufficient yield strength compared to that of Al alloys. Other research reported numerous correlations between the degree of formability and the r value as well as between the formability and work-hardening capacity, which is the inverse of the yield ratio [9], [10], [11]. Therefore, understanding of the effect of alloying elements on deformation behaviors affecting the formability of Mg alloys and the correlation between formability and mechanical properties, and between mechanical properties and deformation behaviors should be needed. However, the relationship among the formability, deformation behaviors and mechanical properties of Mg alloys has not yet been investigated.
In the present study, the effect of alloying elements such as Al, Y, Cu and Si on deformation behaviors of Mg-Zn-Ca alloys were investigated to develop new highly formable, high-strength TRC Mg alloys since addition of these alloying elements can be further strengthened in Mg-Zn-Ca alloys with relatively affordable price. The microstructures and mechanical properties of Mg-4Zn-X-Ca alloys were investigated by optical microscopy and in tensile and compressive tests. The texture evolution of TRC Mg-4Zn-X-Ca alloys was examined by X-ray diffraction analysis. In order to find the correlation between the formability and deformation behaviors with different compositions, VPSC simulations were carried out to calculate the contribution of the deformation modes of tension and compression deformation based on the experimental results.
Section snippets
Experimental procedures
Alloys with nominal compositions (in wt%) of Mg-4Zn-0.3Mn (Z4), Mg-4Zn-0.3Ca-0.3Mn (ZX40), Mg-4Zn-0.3Al-0.3Ca-0.3Mn (ZAX400), Mg-4Zn-0.3Y-0.3Ca-0.3Mn (ZWX400), Mg-4Zn-0.3Cu-0.3Ca-0.3Mn (ZCX400) and Mg-4Zn-0.3Si-0.3Ca-0.3Mn (ZSX400) were subjected to a TRC process on a laboratory scale. The chemical compositions of the Mg-4Zn-X-Ca alloys are listed in Table 1. The alloys were melted at 720–730 °C using a steel crucible under protective gas and were transferred to a preheated nozzle held at 650 °C,
Microstructure of Mg-4Zn-X-Ca alloys
Cross-sectional micrographs of the Z4, ZX40, ZAX400, ZWX400, ZCX400 and ZSX400 TRC alloys are shown in Fig. 1. This figure shows that all of the alloys exhibit centerline segregation, which is detrimental to the mechanical properties and the surface quality of the strip plates; this type of segregation forms along the casting direction due to the partition redistribution of solute atoms during the twin roll casting process. However, the volume fraction of the segregation is not excessive, and
Conclusions
Formability and VPSC simulations of Mg-4Zn-X-Ca alloys were done in an effort to understand the effects of different deformation behaviors on the room-temperature formability of Mg-4Zn-X-Ca alloys. Furthermore, the mechanical properties and the microstructural and texture evolution properties of Mg-4Zn-X-Ca alloys were also investigated. The main conclusions are given below.
- (1)
Mg-4Zn-X-Ca alloys showed sound microstructures in the as-cast condition with no occurrence of inverse segregation on
Acknowledgments
This research was supported by the World Premier Materials (WPM) Program funded by the Korea Ministry of Knowledge Economy, the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2015R1A2A1A01006795), and the International Research & Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2013K1A3A1A20047134).
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