MATERIALS TRANSACTIONS
Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Electromagnetic Field Induced Structure Transition of Aluminum Alloys during Direct Chill Casting
Lei LiQingfeng ZhuJianzhong Cui
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2017 Volume 58 Issue 8 Pages 1134-1137

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Abstract

The structure transition induced by electromagnetic fields during the horizontal direct chill casting of 3004 aluminum alloy was investigated. Results show that the coarse columnar grains were transformed to medium-fine columnar and equiaxed grains with a transition region. Crystallographic analysis indicates that some arms were firstly detached from the coarse columnar dendrite and then formed fine grains with little change of orientation. With the increase of the electromagnetic fields, the grains growing from the later-detached arms exhibited a greater orientation deviation. When the electromagnetic fields approached the set value, more arms were detached and new grains showed totally random orientations. The structure transformation is related to the fragmentation of the dendrite arms by the forced convections.

1. Introduction

Since the last century, several techniques, such as EMC (Electromagnetic casting),1,2) CREM (Casting, Refining, ElectroMagnetic process),3) and LFEC (low frequency electromagnetic casting),46) have been developed for the fabrication of high-quality aluminum alloys during direct chill (DC) casting processes. A remarkable effect of these techniques on the structure is grain refinement, which has been extensively investigated. In the CREM process, it was reported that the average grain size in 2214-51 aluminum alloy was significantly decreased from 3 to 0.18 mm.3) In the LFEC process, the macro/microstructures of 2xxx and 7xxx series aluminum alloys could be effectively refined.4,5)

In the investigations, the macro/microstructures without and with an electromagnetic field (EMF) were usually studied separately. There are very few studies on the transition region where the EMF is switched on or off. A study on this region is of fundamental interest, which can help us understand the details about the structure transformation. In our previous work, we investigated the transition region in horizontal direct chill (HDC) cast 3004 aluminium alloy with the EMFs in a switching “on→off” order. The structures showed a transition from a mixture of randomly aligned equiaxed and fine columnar grains to well aligned coarse columnar grains. A crystallographic analysis indicated that only the equiaxed grains with the preferred crystallographic direction <100> parallel to the temperature gradient could grow into coarse columnar ones.7) However, how the regularly aligned coarse columnar grains were gradually transformed to equiaxed ones with the application of the EMFs in a switching “off→on” order (an opposite structure transition process to that shown in our previous work) has not been studied from a crystallographic point of view, despite an analysis on the modification of surface quality and subsurface layers of the ingot.8) The main purpose of this work is to show the details about the structure transition during this process, along with a crystallographic analysis on the orientation evolution of the grains. Meanwhile, the structure transition mechanism was also discussed briefly.

2. Experimental Procedure

A 3004 aluminum alloy (Al-1.3Mn-1.1Mg-0.5Fe-0.2Si in mass%) ingot was produced by a HDC casting technique. Figure 1 schematically shows the casting configuration. The equipment mainly consists of tundish, flow passage, heating apparatus and mold. Two induction coils were arranged to surround the flow passage and mold, respectively, which were made of 60-turn water-cooled copper tubes. The 3004 aluminum alloy was melted in an induction furnace and then poured into an electrical resistance furnace at 760℃. After degassing, slag-removing and refining, the melt was introduced into the tundish at 710℃ to cast a ϕ100 mm ingot at a speed of 140 mm/min. The first half of the ingot was cast conventionally. The second half was cast with the application of combined EMFs, which were produced by inputting 200 A and 50 Hz alternating currents into the two coils simultaneously.

Fig. 1

Schematic diagram of the HDC casting configuration.

A length of ingot containing the EMF transformation region was cut longitudinally (along the casting direction) for macrostructure observation after polishing and etching. Some small specimens were further cut for microstructure observation with Leica DMR optical microscope. After a further electro-polishing, EBSD (Electron Backscattered Diffraction) auto scanning was performed on one of the specimens for crystallographic analysis on a Zeiss ULTRA PLUS FE-SEM, equipped with an Oxford-HKL Channel 5 system.

3. Results and Discussions

Figure 2 shows the macrostructures in the longitudinal section of the ingot containing the EMFs transformation region. As can be observed, the structures of the alloy are noticeably affected by the combined EMFs. Without the EMFs, the structure consists of coarse columnar α-Al grains. When the EMFs are applied, it is finally transformed to medium-fine columnar and equiaxed grains in the upper and lower parts of the ingot, respectively. Between these two regions, there also exists a mixture of fine columnar and equiaxed grains, as roughly sandwiched between the two V-shaped dividing lines. The distributions of the medium-fine columnar and equiaxed grains with the EMFs indicate a structure inhomogeneity throughout the ingot. This can be observed more clearly in Fig. 3, where the microstructures corresponding to positions 1–5 denoted in Fig. 2 are shown, respectively.

Fig. 2

Macrostructures in the longitudinal section of the ingot containing the EMFs transformation region. The V-shaped dividing lines separate the structures of coarse columnar, mixture and medium-fine columnar grains.

Fig. 3

Microstructures corresponding to positions 1(a), 2 (b), 3 (c), 4 (d) and 5 (e) denoted in Fig. 2, respectively.

During the conventional casting, the growth of the grains highly depends on the temperature gradient that should be perpendicular to the solid/liquid interface, roughly denoted by the left V-shaped dividing line in Fig. 2. When one of the crystallographically equivalent <100> directions of the initial α-Al grains is approximately consistent with the temperature gradient, they will grow preferentially along this specific direction and finally form the columnar morphology (see more details in Ref. 7)). However, when the combined EMFs are applied, their growth will be heavily disturbed by the forced convections in the melt. The basic principle of inducing the forced convections can be described by the electromagnetic body force F:3,4)   

\[ F = J \times B = \frac{1}{\mu} (B\nabla) B - \frac{1}{2\mu} \nabla B^{2} \](1)
where B and J are the magnetic induction intensity and current density generated in the melt, respectively, and μ is the permeability of the melt. The first term of the equation is responsible for the production of the forced convections in the melt.

It is well known that convections promote the fragmentation of dendrites. So far, both mechanical breakdown9) and remelting of dendrite arms10) have been proposed as the mechanisms of dendrite fragmentation. Some recent investigations show that the latter should be the true mechanism to induce detachment of dendrite arms.1113) Due to the liquid flow in the mush zone, the solute from the solidification front is transported to the roots of secondary and/or high-order arms. The accumulation of the solute lowers the melting point of the solid/liquid interface. Moreover, the convections can also bring the high-temperature melt to the roots of the arms. Both the accumulated solute and the high-temperature melt facilitate the remelting of the arms at the roots. As a consequence, the arms are detached from the dendrites. Some of the detached arms surviving dissolution turn into the embryos of new grains. Due to such multiplication effect, the structure is greatly refined by the application of the EMFs.

However, the HDC casting is strongly affected by gravity. Compared with the melt close to the bottom mold, the melt in the upper side has a lower undercooling and then a lower surviving rate of the detached arms. This is because the contacting pressure between the bottom mold and ingot is small. In this case, these detached arms may grow into medium-fine columnar grains along the temperature gradient due to the lack of sufficient grains that inhibit their growth ahead of them. On the contrary, the detached arms in the lower melt possess a high surviving rate, and can finally form a fine equiaxed structure (see more details in Ref. 7)).

Despite the general analysis above, it is of potential interest to understand how exactly the structure evolves from the very beginning when the EMFs are applied. Figures 4(a) and (b) show the microstructures corresponding to the dashed rectangular frames A and B in Fig. 2, respectively. Prior to the occurrence of the obvious structure transformation, some fine grains can be spotted in the coarse columnar grains (as enclosed by the dashed circles). This may represent the starting of actions of the EMFs. To identify this, a crystallographic analysis is followed. Figure 5(a) shows the all-Euler orientation micrograph corresponding to the dashed rectangular frame in Fig. 4(a) (the black solid lines denote grain boundaries: > 10°). In order to facilitate the study on the structure evolution, four successive parts (I–IV) are selected. Parts I–III in the same contrast represent a coarse columnar single-crystalline dendrite. Part I shows a single orientation, while parts II and III involve some fine grains with new orientations. When the growth proceeds to part IV, the coarse columnar dendrite stops growing and is replaced by fine columnar/equiaxed grains. Figure 5(b) shows the <100> pole figures corresponding to parts I–IV (axes X and Y define the sample coordinate system), respectively. In pole figure I, the three crystallographic equivalent <100> poles reveal a typical single-crystalline character. In pole figure II, some scattered poles from the fine new grains appear. With small deviations, these new orientations are still close to those shown in part I, as indicated by the circles. In pole figure III, the orientations become more scattered, some of which deviate too much from the initial orientations. In pole figure IV, the orientations are distributed almost in the whole <100> pole figure, indicating totally random orientations of the grains. Obviously, the structures experience a progressive evolution of orientations during the transition process.

Fig. 4

Microstructures corresponding to the dashed rectangular frames (a) A and (b) B in Fig. 2, respectively. The dashed circular frames enclose the fine grains embedded in the coarse columnar grains.

Fig. 5

(a) All-Euler orientation micrograph corresponding to the microstructure in the dashed rectangular frame in Fig. 3(a), and (b) <100> pole figures corresponding to parts I–IV in (a), respectively.

The little orientation modification of the fine grains (part II in Fig. 4(a)) is from the initial detached arms from the columnar grains. Due to the convections, an impinging effect on the detached arms can be induced, so that their orientations can be modified. In this work, it takes several seconds for the EMFs to reach the set value. As the intensities of the EMFs are small at the beginning, the induced convections are weak. When some arms are detached from the dendrites, their orientations relative to the mother dendrite are less affected until the inter-dendritic liquid is completely solidified, as shown in pole figure II (Fig. 5(b)). With a further increase of the EMFs, the impinging effect by the convections on the detached arms is enhanced. It leads to more deviated orientations from the mother dendrite, as shown in pole figure III (Fig. 5(b)). When the EMFs approach the set value, the detachment and the impinging effects are greatly strengthened. The copious detached arms transported to the solidification front inhibit the growth of the coarse columnar grains, and then formed fine columnar/equiaxed grains with random orientations, as shown in pole figure IV (Fig. 5(b)).

4. Conclusion

EMFs were applied during the horizontal direct chill casting of 3004 aluminum alloy. It is found that the structure was transformed from coarse columnar grains to medium-fine columnar grains and equiaxed grains. A transition region consisting of a mixture of fine columnar and equiaxed grains existed in this transformation process. Crystallographic analysis on the transition region shows that some arms were firstly detached from the coarse columnar dendrite to form fine grains with little modification of the orientation. With increase of the EMFs intensities, the grains growing from the later-detached arms displayed a greater orientation deviation from the mother dendrite. When the intensities of the EMFs approached the set value, more arms were detached to form fine columnar/equiaxed grains to inhibit the growth of the coarse columnar dendrite with totally random orientations. The structure transformation was associated with the fragmentation of the dendrite arms due to the transportation of solute and high-temperature melt in the mush zone by the EMFs induced convections.

Acknowledgments

The work was supported by the Liaoning Provincial Natural Science Foundation of China (2015021002), the Fundamental Research Funds for the Central Universities [N150904003 and N160913002], the China Postdoctoral Science Foundation (2015M570250), the Northeastern University Postdoctoral Science Foundation (20150202) and the National Natural Science Foundation of China (51690161 and 51201029).

REFERENCES
 
© 2017 The Japan Institute of Metals and Materials
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