Elsevier

Journal of Crystal Growth

Volume 548, 15 October 2020, 125827
Journal of Crystal Growth

Study on the transitional structures of 7075 aluminum alloy ingot after switching off a low-frequency electromagnetic field in the horizontal direct chill casting

https://doi.org/10.1016/j.jcrysgro.2020.125827Get rights and content

Highlights

  • We studied transitional structure after switching off electromagnetic field.

  • Equiaxed grains with field are changed to columnar/feathery ones without field.

  • Columnar grain is attributed to weakening of convection after switching off field.

  • Multiple twin relationships exist between adjacent feathery grains.

  • Quasicrystal-enhanced nucleation mechanism is responsible for feathery grains.

Abstract

The transitional structures of 7075 aluminum alloy ingot after switching off a low-frequency electromagnetic field (LFEF) in the horizontal direct chill casting was investigated. The results showed that the equiaxed grains with the LFEF are firstly transformed to columnar grains and then to feathery grains without the LFEF. Also, there exists a transitional region consisting of relatively fine equiaxed grains between the equiaxed grains and the columnar grains. Crystallographic analysis indicated that only the relatively fine equiaxed grains with ideal orientations can grow into columnar grains. Moreover, there exist multiple twin relationships between some adjacent feathery grains, which have a near-common or common 〈1 1 0〉 direction. The equiaxed grains with the LFEF result from the forced convection. The relatively fine equiaxed grains and the columnar grains are attributed to the weakening of the forced convection when the LFEF is switched off. For the feathery grains, it is of interest to find that the quasicrystal-enhanced nucleation mechanism is responsible for their formation under high thermal gradient although it has been reported for twinned equiaxed grains under nearly nil thermal gradient in the literature.

Introduction

In recent years, applying an electromagnetic field (EMF) to the various solidification processes of metals has achieved increasing attention due to its strong ability to modify the macro/microstructures [1], [2], [3], [4], [5], [6], [7], [8]. Direct chill (DC) casting process, as the most important casting technique of aluminum alloys, has also extensively involved EMFs to improve the qualities of the ingots. On the basis of the EMFs, techniques of electromagnetic casting (EMC) [9], [10], Casting, Refining, Electromagnetic process (CREM) [11], and low-frequency electromagnetic casting (LFEC) [12], [13], [14], [15], [16] have been developed. In general, the application of an EMF to the DC casting of aluminum alloys has the advantages of refining grains [9], [10], [11], [12], reducing macrosegregraion [13], [14], restraining cracks [15] and improving surface qualities of ingots [16], [17], etc.

To accurately compare the effects of an EMF, it is common practice to apply it to one half of the ingot and not to the other half. The main difference in the way that an EMF is applied among various studies is whether the EMF is applied to the first or second half of the ingot. When studying the effects of the EMF, the macro/microstructures in the two halves are usually observed separately. In reality, whether the EMF is on or off in the middle of casting, it will take a period of time to reach the next steady-state casting process. In this period of time, a transitional region is usually formed. However, little attention has been paid to this region, which generally plays a crucial role in determining the subsequent development of the macro/microstructures during the DC casting process. We have ever studied the transitional regions produced when combined EMFs (alternating currents were input into two coils surrounding the flow passage and the mold to provide the so-called combined EMFs) were applied in both the “on → off” [18] and “off → on” orders [19] during the horizontal DC casting of 3004 aluminum alloy. In the former case, there existed a transitional region consisting of fine equiaxed grains, and some of them grew into coarse columnar grains. In the latter case, there was a transition region consisting of a mixture of fine columnar and equiaxed grains, and it was found that few arms firstly detached from the coarse columnar dendrites and then formed fine grains. These results indicate that it is of fundamental importance to investigate the transitional regions where the EMF is switched on or off during the DC casting process of aluminum alloys.

In our previous work [20], we applied a LFEF to the first half of the horizontal DC cast 7075 aluminum alloy ingot and then switched it off in the second half. The result showed that the structure underwent a significant change from equiaxed grains with the LFEF to columnar and feathery grains without the LFEF. However, we didn’t focus much on the details of the transitional structures, especially in terms of crystallography. Further, we didn’t explore deeply into the mechanism of the structural transformation, especially the formation mechanism of the feathery grains. In this work, these aspects on the transitional structures will be studied systematically.

Section snippets

Experimental

A 7075 aluminum alloy ingot (Al-5.6Zn-1.4Cu-2.7 Mg-0.23Cr-0.3Fe-0.25Si (wt%)) was produced by a horizontal DC casting technique in this work. Fig. 1 schematically shows the casting configuration. To impose a LFEF in the casting process, a 60-turn induction coil, made of a copper tube with cooled water flowing through, was arranged around the mold. The casting process was as follows. The 7075 aluminum alloy was melted in an induction furnace at 760 °C and then transferred to an electrical

Macro- and microstructures

Fig. 2 shows the macrostructure in the longitudinal section of the 7075 aluminum alloy ingot containing the region where the LFEF is switched off. Clearly, the structure of the alloy undergoes important changes. With the LFEF, the structure consists of coarse and fine equiaxed α-Al grains in the upper and lower parts of the ingot, respectively. After the LFEF is switched off, it is firstly transformed to coarse columnar grains and then to feathery grains.

For more structural details with the

Discussion

When the 100 A/20 Hz alternating current is input into the induction coil, a time varying LFEF will be generated and thus a current J induced in the melt within the mold during the horizontal DC casting process. Due to the interaction between the LFEF and the induced current, the melt is subjected to an electromagnetic body force F (the so-called Lorentz force), which can be expressed as [11], [12]F=J×B=1μBB-12μB2,where B is the magnetic induction intensity, J the induced current density

Conclusions

LFEF was switched on and then off during the horizontal DC casting of 7075 aluminum alloy. It is found that

  • (1)

    the structure is transformed from equiaxed grains with the LFEF to columnar grains and then to feathery grains without the LFEF.

  • (2)

    a transitional region consisting of relatively fine equiaxed grains emerges between the equiaxed grains and the columnar grains. In this region, the grains with their 〈1 0 0〉 axis approximately parallel to the normal of the local solidification interface can grow

CRediT authorship contribution statement

Lei Li: Methodology, Writing - original draft. Qingfeng Zhu: Conceptualization, Supervision, Writing - original draft. Yubo Zuo: Investigation, Formal analysis. Jianzhong Cui: Resources.

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.

Acknowledgments

This work was supported by the National Natural Science Foundation of China [grant numbers 51874092, 51690161 and 51674078]; and the Fundamental Research Funds for the Central Universities [grant number: N180905009].

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