Effect of Oscillating Field on Microstructure of Twin-roll Cast 7075 Aluminum Alloy

Rare Metal Materials and Engineering Volume 43, Issue 12, December 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(12): 2937-2942.

ARTICLE

Effect of Oscillating Field on Microstructure of Twin-roll Cast 7075 Aluminum Alloy Su Xin,

Zhang Aiping,

Xiao Yao,

Li Mengnan,

Xu Guangming

The Key Laboratory of Ministry of Education for Electromagnetic Processing of Materials, Northeastern University, Shenyang 110819, China

Abstract: The morphology and the hardness of 7075 Al alloy during oscillating twin-roll casting (TRC) process at 670 °C were investigated. It was observed that compared with that by traditional process, the dendritic grains of the strips by oscillation field TRC process were severely broken, refined and equiaxed. Segregation bands at the surface and the center of 7075 strips were dramatically reduced in electromagnetic oscillatory field conditions, even completely disappeared in alternating oscillatory condition. The transverse and the rolling direction HV hardness of 7075 strips manufactured by traditional process were 1104 and 1356 MPa, respectively. Upon applying the electromagnetic field, the maximum hardness of 1615 MPa was obtained in alternating oscillating TRC process. Besides, the properties of strips will be further improved with increasing of magnetic field. No matter in what conditions, the refinement of electromagnetic field to strips will increase at elevated temperature. Key words: 7075 aluminum alloy; half-wave oscillation; alternating oscillation; twin-roll casting (TRC)

Due to low density, high strength, good processibility and welding performance, Al-Zn-Mg-Cu alloys become very attractive lightweight and high-strength structural materials [1,2] and have been widely used in both aviation and civilian industries [3], and considered as one of the most important structural alloys in recent years. Cu content, an important alloying element in 7075 Al alloy is usually 2.5wt%~5wt%. Al-Cu strengthening phases are generated, resulting in the increase of hardness and anti-stress corrosion [4,5]. Due to low capital investment, simply direct operation and hot rolling to manufactured thin strips, twin-roll casting (TRC) process is greatly attracting global metal producer to fabricate fine microstructure sheets [6]. So far, the literatures on morphology and mechanical behavior of oscillating TRC strips are very scant [7-9]. In addition, reducing TRC resource consumption is of a significant commercial benefit [10]. So in this paper, 7075 alloy strips produced by half-wave and alternating oscillatory TRC processes, were studied in detail using continuous TRC casters and optical microscope (OM).

1

Experiment

The 7075 strips used for observing were manufactured in the Key Laboratory of Ministry of Education for Electromagnetic Processing of Materials in Northeastern University. The composition of 7075 Al alloy is listed in Table 1. All tests were carried out during the oscillatory TRC process at the pouring temperature of 670 °C. The half-wave current of 170 A is the determined industrial frequency current that was converted by rectifier. Meanwhile alternating oscillating field consisted of AC of 386A and exciting current of 100 A. These processes can provide the cooling needed for solidification and the rolling necessary for mechanical reduction. The schematic diagram of 7075 alloy oscillating TRC process is shown in Fig.1 The strips used for observing had a thickness of 5 mm, and Table 1 Nominal composition of 7075 aluminum alloy (wt%) Fe

Si

Cu

Mg

Zn

Cr

Mn

Al

0.05

0.32

1.88

2.92

5.59

0.24

0.22

Bal.

Received date: December 14, 2013 Foundation item: Project Supported by Leading Group Office of Guangdong-Hong Kong Bidding for Key Areas of Critical Breakthroughs (2009Z025) Corresponding author: Xu Guangming, Ph. D., Professor, School of Materials and Metallurgy, Northeastern University, Shenyang 110819, P. R. China, Tel: 0086-24-83681758, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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DC power supply Coil

Metallic Pinch roll RollCasting melts Electrode Tundish casting roller sheet stock AC power supply

Fig.1 Schematic diagram of 7075 alloy oscillating TRC process

the samples with a size of 300 mm×150 mm×5 mm were prepared by the conventional metallographic techniques prior to full annealing at 360 °C for 3 h by air cooling. The microstructure of the strips was investigated by Lycra metallographic microscopy after etching in hydrofluoric acid solution for 25 s at room-temperature. Hardness test was performed on a Vicker hardness meter under a load of 300 g for 25 s. All values were obtained randomly at both transverse and rolled direction of samples.

2

Results and Discussion

2.1 Microstructure Fig.2 and Fig.3 show microstructures at the upside of 7075 strips through TRC process without and with oscillating a

electromagnetic field. We can see that dendritic grains are elongated along the rolled direction and small scale segregation is distributed on the surface of strips randomly. Fig.2a and 3a show the coarse dendritic arms. In contrast with conventional condition, dendritic grains are broken, refined, equiaxed and their random orientations are seen obviously. In particular in Fig.2c and 3c, traces of dendritic grains almost disappear. The fragmentation of dendritic grains is much more obvious resulting from electromagnetic stirring force and microstructures become more uniform. Fig.4 and Fig.5 show central morphologies of 7075 strips manufactured without and with oscillating electromagnetic field. When solidification occurs at the central part of the strip, thermal transport property weakens. As a result, equiaxed grains take the dominant presence in corresponding region. From these OM images, we know clearly that dendritic grains are coarse, and their irregularity structures are remarkable in conventional condition. After using oscillating electromagnetic field, dendrites receive further refinement. No matter in terms of grains refinement or structural uniformity, alternating TRC process is the optimum way of producing 7075 thin strips. Fig.6 and Fig.7 show the microstructures of undersurfaces of 7075 alloy strips in both transverse and rolled direction. The stronger the cooling of molten metal coming into contact with bottom mill, the finer the structure in corresponding b

c

100 μm Fig.2 Microstructures of 7075 strips at top surface in transverse direction: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

a

b

c

100 μm Fig.3 Microstructures of 7075 strips at top surface in rolled direction: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

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a

b

c

100 μm

Fig.4 Central microstructures of 7075 strips in transverse direction: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

a

b

c

100 μm Fig.5 Central microstructures of 7075 strips in rolling direction: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

a

b

c

100 μm Fig.6 Microstructures from undersurface of 7075 strips in transverse direction: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

a

b

c

100 μm Fig.7 Microstructure from undersurface of 7075 strips in rolled direction: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

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regions[11-13]. At the same time, the alloying elements content was higher than that of other parts of strips [6,10,14,15]. Hence, the gradual refinement of dendritic grains is in order of conventional, half-wave oscillating and alternating oscillating TRC processes. It is noteworthy that best structural uniformity of 7075 strips is obtained by alternating oscillating TRC process; in the contrary, the uniformity of structure generated in conventional condition is the worst. 2.2 Macrosegregation Massive macrosegregation with various phases generated by several additional elements during solidification can be found in 7075 alloy strips. Heat transfers from molten metal to the chilled surfaces of rolls during solidification [16,17]. Phase grows preferentially against total heat transfer so that coarse columnar grains are finally formed. High concentration solutes along with impurities molten alloy are pulled to the center of strips gradually by columnar grains growing and traction. Eutectic phases with low-melting point are squeezed tightly, flowing along fine pores to the hot part of strip center, and then solidified [18]. The distribution of macrosegregation at the surface and the center of strips was investigated and is shown in Fig.8 and Fig.9. Segregation bands are presented obviously wherever the strips is, and central segregation bands are squeezed dramatically so that structural stratification in corresponding region is turned up. After half-wave oscillating TRC process, a

in Fig.8b and Fig.9b, central segregation bands are replaced into fine equiaxed grains. Stratification phenomenon almost disappears. Fig.8c and Fig.9c show that segregation bands are completely vanished after using alternating oscillating electromagnetic field. 2.3 Influence of relative geomagnetic intensity on hardness of Al alloy strip Fig.10 shows OM images of 7075 strips through alternating oscillating TRC process with different intensities. From Fig.7, fragmentation of dendritic grains is very serious and structural uniformity is further improved upon increasing of exciting current from 100 A to 300 A. Reasons for uniformity above are Lorentz force, which will be increased as the amplification of electromagnetic field. Dendritic grains are broken and wrecked. At the same time, the structure of 7075 strips is obviously refined and homogenized. Due to the structural uniformity, the mechanical properties of the strip can be optimized accordingly. The Vickers hardness of 7075 strips under different magnetic intensity are listed in Table 2. The result shows that upon increasing of magnetic intensity, the hardness of strips is further increased[19]. The maximum hardnesses in both transverse direction and rolled direction are about 1376 and 1756 MPa, respectively. The difference in hardness between transverse direction and rolled direction depends on rolling process. Anisotropy of the rolling force c

b

100 μm Fig.8 Macrosegregation at the surface of 7075 strips: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

a

b

c

100 μm

Fig.9 Macrosegregation at the central of 7075 strips: (a) without electromagnetic field, (b) half-wave oscillating field, and (c) alternating oscillating field

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a

b

a

b

c

d

100 μm

Fig.10 Microstructures of 7075 aluminum alloy strips in different magnetic field: (a) IMF=100 A, surface; (b) IMF=300 A, surface Table 2

HV hardness of the strips under different magnetic intensity in transverse (T) and rolling (R) direction (MPa)

100 μm

Direction

Traditional

Half-wave oscillation

Alternating oscillating

Higher oscillating

T

1104

1279

1339

1376

(b) Tcast=690 °C, rolled direction; (c) Tcast=670 °C, transverse

R

1356

1419

1615

1756

direction; (d) Tcast=690 °C, transverse direction

leads to the structural difference [20]. 2.4 Influence of pouring temperature Fig.11 show the structures of strips in transverse direction and rolling direction during oscillating TRC processes. The pouring temperature is raised from 670 °C to 690 °C. In Fig. 11b and Fig.11d, dendritic grains are much finer at 690 °C. By comparison, the microstructure formed at 670 °C seems more rough. Macrosegregation of 7075 strips during oscillating TRC process at both 670 and 690 °C is shown in Fig.12. In general, local segregation is further improved at 690 °C, resulting in the increase of hardness. a

c

b

d

100 μm Fig.11 Microstructures of strips through half-wave oscillating TRC process: (a) Tcast=670 °C, rolled direction; (b) Tcast=690 °C, rolled direction; (c) Tcast=670 °C, transverse direction; (d) Tcast=690 °C, transverse direction

Fig.12 Macrosegregation of 7075 strips through alternating oscillating TRC process: (a) Tcast=670 °C, rolled direction;

At higher pouring temperature, the melt flows more frequently. Furthermore, oscillatory field is amplified largely. Because physical heat during solidifying process is released greatly at higher temperature, subcooling of solid-liquid interface reduces [19,21], leading to high nucleation rate, massive equiaxed grains and grain refinement. Columnar grains formed during solidifying process are broken and drawn to solid-liquid interface, so that broken dendrites fill the vacancies on low nucleation rate [22]. Therefore, the regularity of grains gets to be improved after increasing of the pouring temperature.

3

Conclusions

1) After half-wave oscillating TRC process, the dendritic grains of 7075 alloy are refined, and even broken. During alternating TRC process, massive equiaxed grains are distributed regularly. The microstructure of strips has the best uniformity. 2) Central segregation bands are squeezed tightly during TRC process, resulting in centralized stratification. However, after using oscillating field during TRC process, central segregation of bands weakens greatly and even disappears completely. 3) The increase in hardness of strips results in fine and homogeneous structure during oscillating TRC process. The HV hardnesses of traditional TRC strips on transverse direction and rolling direction are 1104 and 1356 MPa, respectively. While the maximum hardnesses obtained using oscillating field are 1339 and 1615 MPa, respectively. 4) The pouring temperature is an important factor for strengthening alloys. No matter in what conditions during

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TRC process, the morphology and the mechanical property will consistently be optimized as the pouring temperature increases.

10 Birol Y. Journal of Materials of Processing Technology[J], 2009,

Acknowledgments: There, the author thanks Dr. Xu Guangming for

12 Godard D, Ardhambault P, Aeby-Gautier E. Acta Materialia[J],

his technical support and critical review of the manuscript. I also thank Dr. Li Jintao, Liu Wen and Chen Ming for assistance with twin-roll casting experiment and electron microscopy.

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