Composition Homogenization Evolution of Twin-Roll Cast 7075 Aluminum Alloy Using Electromagnetic Field

Rare Metal Materials and Engineering Volume 44, Issue 3, March 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(3): 0581-0586.

ARTICLE

Composition Homogenization Evolution of Twin-Roll Cast 7075 Aluminum Alloy Using Electromagnetic Field Su Xin1,

Liu Tan2,

Xu Guangming1

1

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

2

College of Information Science and Engineering, Northeastern University, Shenyang 110819, China

Abstract: Many microstructural defects such as well-developed dendritic crystal and severe segregation have been found in 7075 aluminum alloy strips by continuous twin-roll casters. Morphology and phase composition of 7075 alloy strips were tested in this paper. Electromagnetic field of 0.13 T was applied during a twin-roll casting (TRC) process to study the difference in solute element distribution compared to conventional process. The most soundest method by which the microstructure of slabs was greatly refined and equiaxed, and segregation bands narrowed down efficiently, was an alternating oscillating process with industrial frequency current and frequency of 386 A and 50 Hz, respectively. In addition, t (AlZnMgCu) phase with high density was formed at (sub) grain boundaries, which was refined and homogenized consecutively in accordance with direct chill (DC) electromagnetic field, half-wave oscillating field and alternating oscillating field; moreover, netlike precipitates completely disappeared at oscillating TRC processes. Key words: morphology; twin-roll cast (TRC); 7075 aluminum alloy; electromagnetic field

The continuous twin-roll aluminum alloy strip casters have been researched and improved in China for 40 years. Today, China has become a large country in the field of manufacturing the continuous twin-roll casters and stands side by side with Italy (FATA Hunter) and France (Pechiney Aluminum Engineering)[1,2]. China also stands as the predominant role in the field of aluminum alloy strip production by this kind of casters[3]. The continuous TRC aluminum alloy strip casters have a great many advantages including simple structure, low investment, short construction period, convenient maintenance and management, low cost, simple technology to master, stable and reliable working, product quality up to the relevant national standards, etc[4]. In particular they adapt to equipment updating for private aluminum strip casting enterprises in developing country[5,6]. Traditional methods on grain refinement that relay on appending Alloy Grain Refiner to aluminum alloy melt, including Al-Ti, Al-Ti-B, Al-Ti-C and so on, were replaced by TRC methodology, the productions of which were significantly

optimized in term of quality and processing technical performance. The 7XXX series alloys are a kind of ultra-high strength aluminum alloy, widely used in both aviation for the construction of plane structures such as wings and fuselages and civilian industries because of their excellent specific strength, and welding performance[7,8]. They have been considered as a new era of lightweight and high-strength structure materials[9]. Because the percentages of solute atoms are high as well as TRC process is a non-equilibrium solidification process, solute atoms are unavoidably gathered at (sub) grain boundaries during solidification process so as to form crystalline phase[10]. The number, size, distribution and morphology of these crystalline phases have a great influence on Al alloy performance. In terms of the 7XXX series alloys, the Al6CuMg4 and Al2Mg3Zn3 phases exist in a wide homogeneity range even in the respective ternary systems, and in the quaternary system the homogeneity region of the mutual solid solution is rather vast as well[11,12]. The phases mentioned which were mutually soluble

Received date: March 18, 2014 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 © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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are collectively called t phase. The lattice parameter of t phase is varying from 1.415 up to 1.471 nm. The quaternary solution between atoms compounds AlMgCu and MgZn2 is defined as the M phase with a hexagonal structure with approximate lattice parameters a=0.518 nm and c=0.852 nm. Another solid solution with a cubic structure which is formed by compounds Al5Cu6Mg2 and Mg2Zn11 is designed as phase. The lattice parameter is a=0.831~0.855 nm[13]. The distribution of the phase region in solid state has been given by Mondolfo in 1976 with the exception of the AlZn phase[14]. High element content and wide solidification range in 7XXX series alloys and technical features of TRC act as the predominate cause in TRC process. The amount of experimental data on commercial alloys of the 7XXX series show that they contain at least one of the two phases, m or t. But the interaction of electro-magnetic TRC process-induced phase and varying manufacturing processes has not been mentioned in previous work[15,16]. In the paper, the effect of different TRC processes on the formation of dendritic crystal and phase was investigated and elucidated in order to clarify differences of microstructures and types and compositions of phases under different TRC conditions.

1

Experiment

Experiments were carried out in the Key Laboratory of Ministry of Education for Electromagnetic Processing of Materials in Northeastern University. The samples of TRC 7075 alloy were manufactured by a continuous TRC caster. The nominal composition of the alloy is Al-2.69wt%Mg-5.65 wt%Zn-1.83wt%Cu-0.45wt%Si. The technical parameters of continuous TRC caster are listed in Table 1. All the roll-casting processes were made at the pouring temperature of 670 °C without and with electromagnetic field. Aluminum alloy melt was poured in TRC continuous caster with exciting current and industrial frequency current of 100 and 386 A, respectively. Extremely complex and simple experimental conditions of traditional process, static MF process, half-wave oscillatory process and alternating oscillatory process were obtained. Incidentally, half-wave current with 170 A was determined as industrial frequency current which was converted by rectifier. Meanwhile, alternating current was defined as industrial frequency current. Rolled strips with 5 mm thickness were cool-rolled by twin-roll irreversible mill until the thickness reduction was up to 1 mm. The samples were subjected to stress relieving annealing immediately at 410 °C for 3 h. After that, hot samples were furnace-cooled till room temperature. Table 1

2

Results

2.1 Optical micro morphology Fig.1 shows optical microstructure of 7075 alloy strip produced by TRC casters with rolling speed of 1.5 m/min. Massive misorientation crystal nuclei are generated because of ultra-cooling velocity (up to 102~103 °C/s) near rolls as shown in Fig.1a. Coarsened dendritic crystals are distributed throughout the entire alloy strip. In addition, because of gravity, molten alloy atoms contacting with bottom roll are much closer, while large air cleft has turned up near the surface of topping roll. In Fig.1b, the grain size is refined and the microstructure becomes homogeneous. All these phenomena illustrate that dendritic crystal arms are avianized a bit in the static MF condition. Having the strip applied to half-wave oscillating field, as shown in Fig.1c, dendritic crystals are broken, refined, equiaxed and their orientation irregularly are seen obviously. Grains receive further refinement. Fig.1d reveals that microstructure equiaxed by alternating oscillating TRC processes has the best uniformity. According to solidification characteristics, obvious segregation bands could be easily formed at the centre of strips. Intermediate stratification will be found in partial serious macro-segregation regions which finally would have a bad influence on strip application. Fig.2a shows that rigorous segregation phenomenon occurs at the center of rolled strips during the traditional TRC process. a

b

c

d

100 μm

Technical parameters of twin-roll caster

Roll diameter/ Roll length/ Roll speed/ Roll pressure/ Material mm mm m·min-1 kN 500

Grain structure was observed by Lycra optical microscopy (OM). Samples for OM observations were mechanically polished etched in a 1% hydrofluoric acid solution for 20 s at room-temperature. Microstructure observations and energy spectrum analysis curve measurements were made using SSX-550 Scanning Electron Microscope (SEM) instrument in the NEU Research Center for Analytical Sciences.

500

0~7

0~70

Gun steel

Fig.1

Microstructures at the center of 7075 strips: (a) common condition, (b) static MF condition, (c) half-wave oscillating condition, and (d) alternating oscillating condition

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Segregation bands are presented obviously and numerously

400

AlKa

a

300

100

Element Mg Al Cu Zn

0

2

4

b

6

8

10

12 14

Energy/keV Fig.3

Surface morphology (a) and phase compositions (b) at grain boundaries in traditional process

400

AlKa

a

Element Mg Al Cu Zn

200 100

at% 22.461 55.376 9.646 12.517

b

CuKa ZnKa

b

wt% 15.726 43.043 17.657 23.574

CuLa ZnLa MgKa

Intensity/cps

300 a

wt% at% 16.981 23.049 48.596 59.419 11.140 5.783 23.283 11.749

CuKa ZnKa

200

CuLaZnLa MgKa

Intensity/cps

wherever the strips are, and central segregation bands are squeezed badly and dramatically so that stratification phenomenon in the corresponding region is turned up. Fig.2b represents that segregation bands phenomena mentioned above reduce a bit in static MF condition. The region where segregation bands already narrow enough almost looks like a line. After the half-wave oscillating TRC process, as shown in Fig.2c, central segregation bands are replaced by fine crystals. Fig.2d shows that segregation bands disappear completely wherever the strips are after the alternating oscillating TRC process. 2.2 Precipitate phase analysis Fig.3 to Fig.6 show that the surface morphologies of 7075 alloy strips manufactured without and with electromagnetic field and EDX spectrum of precipitated phases. The precipitated phases are distributed in the shape of continuous net as shown in Fig.3. However, after using electromagnetic field, the continuous netlike precipitates get weak, even in Fig.5 and Fig.6, sizes of precipitates are further diminished and their distributions are much more homogeneous. From OM images, Mg, Cu and Zn elements take majorities of components in strips. Primary compositions of precipitates (arrows in OM images) in different conditions are shown in Fig.7. Mg content decreases remarkably from 23 at% (B=0) to the minimum of 13 at% (B=0.13 T), but Cu content increases from 5 at% to 16 at%. Ruling out the interference of α(Al) matrix, atomic ratio of Mg, Cu and Zn changes approximately from 4:2:1 to 1:1:1. As solute atoms diffuse from precipitates to α(Al) matrix, the solute solid solubility increases and homogenization of alloys will be improved correspondingly. In general, T (AlZnMgCu) phase takes an overwhelming majority of composition of rolled strips.

0 c

2

4

6

8

10 12 14

Energy/keV

d Fig.4

Surface morphology (a) and phase compositions (b) at grain boundaries in static MF process (DC)

100 μm

Fig.2

Segregation bands at the center of 7075 aluminum alloy strips: (a) common condition, (b) static MF condition, (c) half-wave oscillating condition, and (d) alternating oscillating condition

3

Discussion

3.1 Influence of electromagnetic field on microstructure Aluminum alloy melt that passes through the condensing zone, based on itself static pressure, immediately forms a single metal shell, and then by the coactions of counterrotating rolls and melting super-cooling, molten metal slops

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30 Element Content/at%

a

AlKa

0

wt% 8.398 45.764 32.719 13.128

at% 12.532 61.516 18.669 7.283

Fig.7

2

4

6

8

10 12 14

Surface morphology (a) and phase compositions (b) at grain boundaries in half-wave oscillating process

b

300

100 0

FeKa

200

2

4

6

wt% 8.655 40.049 27.228 22.831

at% 13.488 56.219 16.228 13.226

CuKa ZnKa

Element Mg Al Cu Zn

CuLaZnLa MgKa

Intensity/cps

AlKa

a

400

8

10 12 14

Energy/keV Fig.6

10 5 Without

DC

Half-wave oscillation

Alternation oscillation

Precipitate contents in different condition: without and with electromagnetic field

Energy/keV Fig.5

15

Electromagnetic Field

CuKa ZnKa

100

b Element Mg Al Cu Zn

300 200

20

0

CuLa ZnLa MgKa

Intensity/cps

400

Mg Cu Zn

25

Surface morphology (a) and phase compositions (b) at grain boundaries in alternating oscillating process

into the crystal region. Finally the consolidation of latent heat derived by rotating rolls, begins. The process of metal solidification is going on by means of the nucleation and the growth of crystal nuclear in metal melt. Nucleation rate us formula during process of homogeneous nucleation can be derived as[17]:

us 

 a 3 f ( )  NSkT  GA  exp   exp   2  h  kT   kT (GV ) 

(1)

1 f ( )  (2  cos )(1  cos ) 2 (2) 4 where θ is wrapping angle of newly crystal to heterogeneous nucleation; T denotes the melt temperature; a is the nuclear shape factor; k and h denote the Boltzmann constant and the Plank constants, respectively; σ denotes interface energy. Ns is the total number of atoms on unit area; ΔGA and ΔGv are activation energy of atom jump through liquid-solid interface and volume free energy, respectively. When ΔGA increases under the effect of static MF condition, nucleation rate reduced as we know from Eq. (1). In addition, from the growth of crystal nuclear perspective, the relationship between super-cooling degree (ΔT) and the growth of crystal nuclear is: (3) R  2T 2 where μ2 is a constant value. No matter how crystal nuclear grows, its growth rate coincides with its supercooling degree. In static MF solidification process, the melting point of metal and its supercooling degree diminishes, as well as the growth rate correspondingly. Besides, the above mentioned nucleation rate is reduced, which also hinders the growth of crystal nuclear (Fig.1b). Meanwhile, grains of 7075 alloy are refined though growth rate being dropped off bit by bit[18]. As a consequence, the influence of electromagnetic field on nucleation rate alleviated is much lower than that inhibitory action of crystal nuclear growth. This makes the microstructure of 7075 alloy more meticulous under applied static MF condition. Incidentally, in constant static magnetic field, the relative movement between alloying agents, such as Al3+, Mg2+, Cu2+, and Zn2+, should occur by means of different strength of Lorentz force. In this case, the diffusibility of solutes increases[19]. With the coactions of particle mass and solute diffuseness, macro-segregation phenomenon of the strip weakens.

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In general, when the electromagnetic pressure generated by electromagnetic oscillating field in metal melt is less than 0.1 MPa, grain refinement mainly lies on the influence of the pulling and compressing force of oscillating field. As applying simultaneously half-wave current (i) and DC magnetic field (B) on the cast-rolling zone, alloy melt would create an unidirectional Lorentz force with 200 Hz, a continuous changing density region, as is shown in Fig.1c, so that the greater the dendritic crystal arm growing from surface to center parts of the strips, the greater the moment working on columnar crystalline zone. So the coarse dendritic crystal arm could be broken and wrecked. The great amount of broken dendritic crystals are pushed to the front of solidifying interface and become newly crystal nucleus. The influence of pulling and compressing of oscillating field enhances the wetting action of metal melt to high temperature compounds of solid phase, but reduces the critical heterogeneous nucleation free energy based on melt. These two functions mentioned force a plenty of dendritic crystal wrecked and the microstructure of 7075 alloy refined and equiaxed. When applying the alternating oscillating field, generally approximately simultaneous alternating current (I) and DC magnetic field (B) on the cast-rolling zone, alloy melt would be given a dynamic Lorentz force with magnitude changing frequency of 200 Hz, and direction changing frequency of 100 Hz. So the mechanical effect of alternating oscillating electromagnetic force which is subdivided into pulling and compressing, shearing, refracting and twisting forces on new columnar crystals is further intensified, which makes dendritic broken phenomena further remarkable as well as the number of crystal nucleus large increasing[20]. Meanwhile, localized stirring action leads to a higher oscillating electromagnetic force. Under the temperature gradient (TEMPGRAD) decreasing and temperature distribution homogenizing, the amount of crystal nucleus simultaneously is produced in a vast region and the microstructure of 7075 alloy is refined and equiaxed obviously (Fig.1d). The surface temperature departing from the rolls was investigated to be about 320 to 360 °C, which could not lead to a perfect recrystallization. Residual deformation energy in the 7075 alloy strip ought to be equal to 10% to 25% of cold deformation energy[21]. 3.2 Effect of precipitates on mechanical performance The microstructure of high-strength aluminum alloy elucidates that the sizes of the second phase particles distributed on solid solution matrix are varying, containing coarse compound particles with micron size above, and fine particles of aging process and high temperature precipitation after solidification. The fine aging-precipitated phases have a strengthening effect on the aluminum alloy matrix, while coarse brittle phases reduce the fatigue life of Al alloy. The greater the volume fraction of coarse brittle phases is, the faster the fatigue crack propagation (FCP) rate of alloys is,

which was stated by Zhang Guojun. The morphology and the distribution of precipitated phases also act a significant role on material performance[22]. The small round homogeneous particles favor the improvement of plasticity. The strength of alloys has close relations with the dissolved degree of alloy elements. Because solute atoms diffuse in the 7075 alloy matrix, the solid solubility of elements increases and the strength of alloys will be improved correspondingly. Participated phases can be divided into soluble participated phase and insoluble participated phase according to solubility, and the former could be transferred into strengthening phase by means of homogenizing treatment and solution heat treatment[23]. Participated phase has a terrible effect on ductility of materials, i.e. the materials tensile ductility decreases with the increase of the percentage of phase. The size of the participated phase also has a great influence on material performances. Coarse participated particles are broken and deformed during TRC manufacturing, and then their fragments are arrayed along the direction of processing, which can form crack source easily. When the load is applied, the fracture toughness and fatigue strength of alloys are obviously reduced. Besides, the distribution of the participated phase should not be neglected. When the participated phase takes on the shape of continuous net, it becomes more brittle. Therefore, the plasticity and the strength of materials instantaneously decline. When the participated phase takes on a homogeneous, discontinuous shape, it has little influence on material strength. In summary, according to the sequence of traditional process-static MF process-half-wave oscillating processalternating oscillating TRC (Fig.3 to Fig.6), the solid solubility of elements increases, and the distribution of precipitated phases becomes more and more discontinuous, which bring better and better improvements of mechanical performances of 7075 alloy.

4

Conclusions

1) Dendritic crystals of strips by stress relieving annealing are refined, but can’t be wrecked in static MF condition. However, the electromagnetic oscillating field process can make these dendritic crystals broken, future refined and equiaxed. Furthermore, the influence of alternating oscillating TRC process is better than that of half-wave oscillating process. 2) In the four processes mentioned above, the latter two have a less segregation at central part of alloy strip. After applying alternating oscillating field, segregation bands disappear. 3) The solute solid solubility could be enhanced by electromagnetic field process, and the increasing sequence is as follows: traditional, static MF, half-wave oscillating and alternating oscillating TRC processes. The main precipitated phase is t phase, its composition is never changed while its

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content slightly decreases. The size of the precipitated phase also reduces as well as the continuity of its distribution weakens resulting in the improvement of the mechanical performances of 7075 alloy.

References 1 Kim J H, Yeom J T, Lee D G et al. Journal of Materials Processing Technology[J], 2007, 187-188: 635 2 Chen Z Y, Xu S Q, Dong X H. Acta Metallurgica Sinica[J], 2008, 21(6): 451 3 Sun Naiyu, Patterson B R, Suni J P. Materials Science and Engineering A[J], 2006, 416: 232 4 Gao Peng, Zhou Tietao, Xu Xiaoqing et al. Rare Metal Materials and Engineering[J], 2013, 42(1): 6 (in Chinese) 5 Godard D, Ardhambault P, Aeby-Gautier E. Acta Materialia[J], 2002, 50(9): 2319 6 Birol Y. Journal of Materials of Processing Technology[J], 2009, 209: 506 7 Modi O P, Saxena M, Prasad B K et al. Corrosion[J], 1998, 54: 129

Materials and Engineering[J], 2011, 40(1): 63 (in Chinese) 11 William J C, Starke E A. Acta Materialia[J], 2003, 51(19): 5777 12 Liu Xiaoyang, Pan Qinglin, He Yunbin et al. Materials Science and Engineering A[J], 2009, 500: 150 13 Yin Zhimin, Pan Qinglin, Jiang Feng et al. Scandium and Its Alloys[M]. Changsha: Central South University Press, 2007: 421 (in Chinese) 14 Wang W M, Bian X F, Qin J Y et al. Metall Meter Trans A[J], 2000, 31(9): 2163 15 Li C, Wu Y Y, Li H et al. Acta Mater[J], 2011, 59(3): 1058 16 Chen Xuehai, Chen Kanghua, Dong Pengxuan et al. Rare Metal Materails and Engineering[J], 2013, 42(2): 273 (in Chinese) 17 Gremend M, Allen D R, Rappaz M et al. Acta Mater[J], 1996, 44( 7 ): 2669 18 Xu C L, Jiang Q C. Materials Science and Engineering A[J], 2006, 437(2): 451 19 Qiao Shengru, Li Yanli, Li Yun et al. Rare Metal Materials and Engineering[J], 2009, 38(4): 570 (in Chinese) 20 Bassler B T, Hofmeister W H, Bayuzick R J. Materials Science and Engineering A[J], 2003, 342(1-2): 80

8 Prasad B K. Wear[J], 2002, 252: 250

21 Misra A K. Metallurgical Transaction A[J], 1986, 17: 358

9 Haga T, Ikawa M, Wtari H. Journal of Materials Processing

22 Moldovan P, Popescu G , Miculescu F. Journal of Materials

Technology[J], 2006, 172: 271 10 Chen Yuqiang, Yi Danqing, Pan Suping et al. Rare Metal

Processing Technology[J], 2004, 153-154: 42 23 Li X, Starink M J. Materials Science Forum[J], 2000, 311: 1071

586