Effect of low frequency alternating magnetic field on hot tearing susceptibility of Mg-7Zn-1Cu-0.6Zr magnesium alloy
Graphical abstract
Introduction
Due to their high ductility, good strength (Unsworth, 1989) (Luo and Perquleryuz, 1994) and excellent mechanical properties (Chen et al., 2015), Mg-Zn-Cu ternary alloys have been widely employed for a wide variety of applications, such as aerospace, transportation and electronics industries. Zhu et al. (2011) found that the twinning mechanism in Mg-Zn-Cu ternary alloys is facilitated by the presence of Cu element, resulting in superior mechanical properties. Moreover, Wang et al. (2013a) and Lotfpour et al. (2017) found that Cu element significantly influences the grain refinement and forms separate eutectic MgZnCu phase in Mg-Zn-Cu alloys. However, the application of Mg-Zn-Cu ternary alloys has been limited due to their high hot tearing susceptibility.
Cracking during solidification is regarded as a major defect of an alloy, which is called hot tearing for casting alloys and solidification cracking for welding alloys (Kou, 2013), (Hamadellah et al., 2017), (Campbell, 2003). Eskin and Katgerman (2007) and Uludag et al. (2018) reported that the hot tearing is caused by the contraction of the mushy zone, restricted shrinkage and lack of feeding. Liu and Kou (2017) found that the hot tearing usually occurs at the terminal stage of solidification, where solid fraction (fs) is close to 1. Moreover, Liu and Kou (2017) also reported that the hot tearing phenomenon usually occurs along grain boundaries due to the presence of a limited amount of liquid between grains, which is insufficient to maintain bonding to resist the contraction force. Recently, several research groups have investigated the mechanism of hot tearing by using a wide variety of characterization techniques and theories. For instance, Kou (2015) found that the hot tearing is affected by: (1) the lateral separation of grains under the action of contraction force; (2) the lateral growth of grains and (3) the presence of liquid along the grain boundaries. It should be noted that the lateral grain growth and presence of liquid along the grain boundaries limit the hot tearing phenomenon, whereas the lateral separation of grains due to the contraction force facilitates the hot tearing. Eskin et al. (2004) have observed that the inter-dendritic separation level and solid feeding level (fs > 0.9) significantly influence the hot tearing behavior due to a large solidification range of both levels. Cao and Kou (2006) investigated the influence of solidification pathway, eutectic content, secondary phase and solidification range on hot tearing by constrained rod casting (CRC) mold. The results reveal that narrow solidification range and high eutectic content lead to low hot tearing susceptibility (HTS). Zhou et al. (2018) and Wang et al. (2018) studied the influence of secondary phase and mold temperature on HTS of Mg-Zn-Cu ternary alloys by T-shape mold and demonstrated that HTS of Mg-7Zn-xCu-0.6 Zr alloy decreased with increase in secondary phase content and mold temperature. Zhang et al. (2019) reported that HTS of Mg-2Zn-(3 + 0.5x)Y-xAl alloys firstly decreased and then increased with the addition of Al. Davis et al. (2018)explored the influence of grain refinement on HTS of AZ91D magnesium (Mg) alloy by using CRC mold. Their results indicated that HTS is related to average grain size and grain refinement decreased the HTS. A similar conclusion was obtained by Lin et al. (2007), who found that HTS of aluminum (Al) alloys decreases with grain refinement. Their results showed that HTS of casting alloy is sensitive to grain refinement if the grain size is smaller than 200 μm. Moreover, it was demonstrated that HTS can be effectively decreased by elemental addition, controlling the mold temperature and grain refinement.
In recent years, alternating magnetic field (AMF) has gained focused research attention due to its desirable influence on the microstructure and properties of casting alloys (Zhang et al., 2016), (Fu and Yang, 2012). For instance, Liu et al. (2015) demonstrated that AMF can accelerate the aging precipitation process and improve the microstructure and mechanical properties of AA2219 Al alloy. Dong et al. (2018) have indicated that the dendritic arm spacing can be altered by varying the magnetic field and that tensile strength and elongation can be enhanced by using directional solidification. To the best of our knowledge, the influence of AMF on HTS of Mg-based alloys has not been reported yet. Therefore, this work evaluated the potential of AMF to decrease the hot tearing susceptibility of Mg-7Zn-1Cu-0.6 Zr alloy.
Section snippets
Experimental procedure
Herein, the investigated alloys were prepared by using pure magnesium (Mg, 99.9 wt. %), zinc (Zn, 99.9 wt. %), copper (Cu, 99.9 wt. %) and Mg-30 %Zr master alloy, where the Mg-30 %Zr master alloy was utilized to refine the grain size of casting alloys. The chemical composition of the alloys was determined by inductively coupled plasma (ICP), as shown in Table 1. The studied alloys were melted in a steel crucible at 720 °C under the protection of SF6/N2 atmosphere. The molten alloy was held at
Influence of low-frequency AMF on HTS
Fig. 3 shows the macroscopic images and corresponding microstructure of the hot spot section of the as-prepared alloy (Alloy I, Fig. 3d and e) and AMF-treated alloy (Alloy II, Fig. 3f and g). The schematic illustration of the macroscopic sample preparation is shown in Fig. 3a–c. It can be clearly observed that the hot tearing occurred in the hot spot section due to stress concentration which was independent of AMF treatment (Fig. 3d and e). Moreover, Fig. 3(e) shows that the Alloy-II was almost
Influence of AMF on the microstructure
It is well known that the electrical resistivity of a liquid is much higher than that of its solid counterpart at the melting temperature. For instance, the electrical resistivity values of pure Mg and Cu liquids at the melting temperature are 274 nΩ‧m (Avedesian and Baker, 1999) and 215 nΩ m (Davis, 2001), respectively, whereas the electrical resistivity values for solid pure Mg and Cu at the same temperature are 154 (Avedesian and Baker, 1999) and 110 nΩ m (Davis, 2001), implying that the
Conclusions
The influence of low-frequency alternating magnetic field (AFM) on hot tearing susceptibility (HTS) of Mg-7Zn-1Cu-0.6 Zr alloy was systematically studied in this paper. The main conclusions from the current study can be summarized as follows:
- 1)
Under the influence of AMF, strong forced convections occurred in the molten metallic pool in opposite directions, leading to continuous liquid flow of the molten metallic pool during the solidification. The forced convection increased the number of free
CRediT authorship contribution statement
Ye Zhou: Conceptualization, Data curation, Writing - original draft. Pingli Mao: Conceptualization, Methodology. Zhi Wang: Data curation, Validation. Le Zhou: Software, Validation. Feng Wang: Software. Zheng Liu: Conceptualization, Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the financial support from High level innovation team of Liaoning Province (XLYC1908006). Innovation Talent Program in Science and Technology for Young and Middle-aged Scientists of Shenyang (No.RC.180111), Project of Liaoning Education Department (No. LQGD2019002 and LJGD2019004), Liaoning nature fund guidance plan (No. 2019-ZD-0210), and Liaoning Revitalization Talents Program (No. XLYC1807021 and 1907007).
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