3D analysis of macrosegregation in twin-roll cast AA3003 alloy

    3D analysis of macrosegregation in twin-roll cast AA3003 alloy ˇ akov´a, Mariia Zimina, Stefan Zaunschirm, Johann KastMichaela Slap´ ner, Jan Bajer, Miroslav Cieslar PII: DOI: Reference:

S1044-5803(16)30125-5 doi: 10.1016/j.matchar.2016.04.023 MTL 8240

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

9 November 2015 31 March 2016 29 April 2016

ˇ akov´ Please cite this article as: Slap´ a Michaela, Zimina Mariia, Zaunschirm Stefan, Kastner Johann, Bajer Jan, Cieslar Miroslav, 3D analysis of macrosegregation in twin-roll cast AA3003 alloy, Materials Characterization (2016), doi: 10.1016/j.matchar.2016.04.023

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3D analysis of macrosegregation in twin-roll cast AA3003 alloy

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ˇ akov´ Michaela Slap´ aa,∗, Mariia Ziminaa , Stefan Zaunschirmb , Johann Kastnerb , Jan Bajera , Miroslav Cieslara a Charles

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University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b University of Applied Sciences Upper Austria, Stelzhamerstrasse 23, 4600 Wels, Austria

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Abstract

Twin-roll cast aluminium alloys have a high potential for industrial applications. However, one of the drawbacks of such materials is an inhomogeneous

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structure generated by macrosegregation, which appears under certain condi-

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tions in the center of sheets during solidification. Segregations in AA3003 alloy form as manganese, iron and silicon rich channels spread in the rolling direction. Their spatial distribution was successfully detected by X-ray computed

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tomography. Scanning electron microscopy was used for a detailed observation of microstructure, morphology and chemical analysis of the segregation. Keywords: Aluminium alloys, Twin-roll casting, Macrosegregation, Scanning electron microscopy, X-ray computed tomography

1. Introduction

1.1. Twin-roll casting Twin-roll casting (TRC) is a casting technique, which provides a relatively high solidification rate (∼500 K/s). During TRC molten metal is poured be5

tween two water-cooled rotating rolls, which are designed to fill both a heat ∗ Corresponding

author ˇ akov´ Email addresses: [email protected] (Michaela Slap´ a), [email protected] (Mariia Zimina), [email protected] (Stefan Zaunschirm), [email protected] (Johann Kastner), [email protected] (Jan Bajer), [email protected] (Miroslav Cieslar)

Preprint submitted to Materials Characterization

March 31, 2016

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exchanger and a rolling function. The metal solidifies on rolls into a strip that is further work-hardened by the rolls. Besides that, in contrast to other rapid solidification techniques, TRC allows production of large amounts of strip ma-

The high solidification rate during TRC results in a microstructure refine-

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terial and combines both fast solidification and rolling in a single step.

ment, formation of finely distributed primary particles and high solid solution supersaturation [1, 2, 3], which is considerably higher than in materials obtained

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by conventional direct-chill casting (DC) [4, 5].

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1.2. Segregation

TRC strips can suffer from macrosegregation in a form of central eutectic segregates. The formation of these defects usually leads to premature fracture

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and reduced strength and can influence the fatigue behavior [6]. The influence of casting speed, cooling rate, temperature of the melt and other parameters on

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the final microstructure is very significant (e.g. [7]). However, the establishment of the appropriate casting conditions varies for different materials and is time

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consuming. Therefore, the study of the center line segregation formed after TRC is of great interest since it can be useful for optimization of casting conditions. Yun et. al. [8] reported that central macrosegregations are cylindrical low

melting point regions oriented in the casting direction and appear as a result of 25

combined solidification and rolling process. Such channels formed in the central plane have almost constant spacing and are at least an order of magnitude larger in size than dendritic grains in their vicinity [9, 10]. The probability of formation of the segregation increases with decreasing strip thickness or decreasing load and with increasing amount of alloying elements [8].

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Segregated channels reveal predominantly eutectic features. This suggests that a hot rolling component of the TRC process squeezes the mushy zone in between the already solidified skins that become thicker as they move into the roll gap. The liquid, which is enriched in alloying elements, is thus forced to the opposite side of the casting direction [9]. With advancement of the solidification

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front, the concentration of solutes in liquid phase gradually increases. The liquid 2

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is trapped when the two solid skins weld together at a so-called Kiss point, and well defined solute-rich channels form. The higher is the casting rate the shorter are these channels.

alloys and showed that AA3003 alloy has a long freezing range, which makes

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Birol [9] focused in his work on solidification path of several aluminium

the alloy more prone to macrosegregation. Solubilities cS of manganese and iron in aluminium solid solution are similar. However, the solubility of iron in

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liquid aluminium cL is much higher than the solubility in solid aluminium – cS /cL ∼ 0.02 [11]. Hence, the concentration of iron in liquid increases rapidly during solidification, forming a concentration gradient of iron content along the

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strip thickness – the iron concentration is the highest near the Kiss point [10]. Solubilities of manganese in both liquid and solid aluminium are comparable,

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1.3. Al-Mn alloys

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therefore no such gradient forms during solidification.

Various alloying elements are added into aluminium alloys to improve their

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properties. The most common in AA3003 series alloys, which find their application mainly in automobile industry, are manganese, iron, silicon and copper. These elements are known to have strong influence on the precipitation behavior [12, 13]. Iron and silicon greatly decrease solubility of manganese in solid 55

solution and accelerate precipitation of intermetallic compounds. Copper can enhance decomposition of the supersaturated solid solution [14]. Two main phases which are frequently observed in Al-Mn-Fe-Si alloys are or-

thorhombic Al6 (Mn,Fe) and cubic α-Al12−15 (Mn,Fe)3 Si1−2 [15, 16, 17]. Several morphologies of these particles were described – faceted polyhedra, dendrite, 60

elongated rods, Chinese script, branched skeleton [18]. The main factors influencing the morphology and crystallographic phase are chemical composition and cooling rate [19]. Size and distribution of intermetallic particles have fundamental role in controlling the alloys microstructure evolution during processing [20, 21] and re-

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crystallization as they can both stimulate or impede the grain growth [16, 22]. 3

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To minimize their negative effects on functional properties of final product the efforts are focused on the possibility to control their formation [18].

In twin-roll cast Al-Mn-Fe-Si sheets the primary particles located in the

casting direction and their distribution is finer. Towards the central line the

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vicinity of the strip surfaces are nearly spherical, exhibit strong alignment in

solute levels decrease and coarser intermetallic particles are encountered [2]. This inhomogeneity cannot be reduced by homogenization treatment. Sun et.

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al [15] showed that various shape particles are the same phase which has no fixed orientation relationship to the Al matrix. They are polygonal plates which appear to be of various form due to a different angle of view.

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1.4. X-ray computed tomography

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The microstructural analysis is conventionally carried out in 2D by means of light optical microscopy and scanning electron microscopy. For information in

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3D subsequent cuts through the sample are analyzed. However, such techniques are destructive and rather time consuming. Sectioning by a focused ion beam in

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scanning electron microscope provides high resolution images but is applicable only on limited volumes of materials. An example of nondestructive method of material characterization is X-ray

computed tomography (XCT), which has become very important technique for 85

3D characterization of materials [23]. It is widely used for detection of cracks and pores, evaluation of parameters of cellular materials and visualization of intermetallic phases in metals [24]. XCT utilizes an X-ray source, a turntable as well as a digital detector (Figure 1). A sample is placed on a rotary table between the X-ray source and the

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detector. A projection image is generated at various angular positions, hundreds of projection images are needed to reconstruct a 3D dataset with a mathematical algorithm. The generated volumetric dataset consists of volumetric pixels, so called voxels, which feature a grey value corresponding to the density and the atomic number of the elements within that voxel [24].

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The aim of the present study is to evaluate the morphology of central 4

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tional techniques and by X-ray computed tomography.

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2. Experimental

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macrosegregation in twin-roll cast AA3003 aluminium alloy by both conven-

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Experimental aluminium alloy from AA3003 series with composition: 1.0 wt.% Mn, 0.2 wt.% Fe, 0.6 wt.% Si, 0.2 wt.% Cu and 0.2 wt.% Zr was twin-roll cast in

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industrial conditions. The melt was cast from temperature 720 ◦ C between two water-cooled rolls with diameter 600 mm. The casting speed was 600 mm/min and the final thickness and width of the strip were 8.5 mm and 1 m, respectively.

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The microstructure of the central segregation after casting was investigated by means of light optical microscopy, scanning electron microscopy and X-ray computed tomography.

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For observation in light optical microscope (LOM) Olympus GX51 and in

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scanning electron microscope (SEM) FEI Quanta 200 FX FEG samples were cut in all three perpendicular directions – rolling (RD), transverse (TD) and normal (ND) and mechanically ground on SiC papers and polished by diamond

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suspension. SEM was used for observation in back scattered electrons (BSE) at 10 kV, for chemical analysis by energy dispersive spectrometry at 15 kV (EDS) operated by EDAX Genesis software and for grains visualization by electron back scatter diffraction at 10 kV (EBSD) equipped with EDAX EBSD camera 115

and OIM software.

X-ray computed tomography measurements were carried out using a Nan-

otom 180NF desktop device from General Electric with a 2316x2316 Hamamatsu flat panel detector and a 180 keV nano-focus tube with transmission target and an equipped external liquid cooling system to stabilize measurements. The scan 120

was performed with 95 keV acceleration voltage using 210 µA and a 0.1 mm copper prefilter, achieving a voxel size of (5.25 µm)3 . Reconstruction was done with the software GE phoenix datos|x 2 reconstruction 2.2.1 using a 3x3 inline median filter and a beam hardening correction. For visualization and evaluation the software VGStudio MAX 2.2 was used whereby the surface near region

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was excluded during evaluation to reduce influence of artefacts. For noise re-

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duction a 9x9x9 median filter was applied, the starting value for the advanced surface determination was chosen to be µ+3.5 σ. µ is the expected value of the

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Gaussian distribution from the matrix material, the standard deviation is σ.

3. Results and discussion

Twin-roll casting produces a material with an inhomogeneous structure.

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Grains near the strip surface are elongated and inclined towards the rolling direction. In the central part of the strip grains are more equiaxed. However,

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they are still slightly inclined and merge at the central line where the solidification path ended (Figure 2). Their inclination is ∼20◦ towards the rolling direction. For more details about the grain structure see [25].

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3.1. Microstructure

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During solidification of the strip the two solidification fronts are coming from the surface and the melt is enriched in alloying elements. When the two solid-

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ification fronts finally meet, solute rich segregation of primary particles forms. Such segregations have a form of long channels stretching along the rolling direction. Overviews of their distribution from light optical microscope and scanning electron microscope are given in Figures 3 and 4. The length of each segregation is several mm in the rolling direction, which is several times larger than the average grain size [26]. The mutual spacing between each segregation in the 145

transverse direction is (500 ± 100) µm. However, they are not aligned in one plane but are distributed in a band with the width of approximately 1 mm in the central part (Fig. 3a and 5a). The density of the channels is in the magnitude of 10 mm−2 . In average, their width is 60 µm and their height is 100 µm. Detailed images (Fig. 5b and 5c) from SEM reveal that the size and shape

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of primary particles forming the segregation are not homogeneous. Some of the segregations contain coarse primary particles with irregular shape and diameter of around 20 µm. Usually they are not continuous but contain aluminium spots inside them. 6

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A majority of segregations is composed of two types of particles. Both types can be recognized in more detail in Fig. 6a. Larger particles have irregular shape and size 2-5 µm. Smaller particles are elongated and form ”fan shaped”

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structures. These two types of particles are spatially divided into groups which

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are closely adjacent to each other (see the right part of Figure 5c). The size of these groups reaches 100 µm in section perpendicular to the normal direction in 160

larger segregations. The different morphologies (blocky, dendritic) of primary

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particles coexisting next to each other were also reported in other aluminium

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alloys, e.g. [15, 16, 14, 18]. 3.2. Chemical analysis

EDX analysis was performed on several segregations in order to investigate their composition. Examples of distribution maps of aluminium and main alloy-

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ing elements in AA3003 alloys – manganese, iron, silicon and copper – are shown

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in Figure 6. In BSE the segregation appears as bright spots in a dark aluminium matrix (Fig. 6a). EDX analysis proved that the segregation is composed mainly

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of Mn and Fe located generally in the same particle (Fig. 6c and 6d). Silicon is also partially present in the Mn- and Fe-containing particles but

mainly appears in their interspace in clusters with size comparable to these particles (Fig. 6e).

Copper containing particles (Fig. 6f) were observed adjacent to the larger

particles rich in Mn and Fe. This is in accordance with observation of Jaradeh 175

[14] who reported in 3003 alloy that most of the Al-Mn-Fe elongated precipitates were decorated by phases rich in copper. Although maps in Figure 6 suggest that Mn and Fe are dominant in the segregation, EDS point analysis shows that the dominant element is still Al. The composition of larger particles fluctuates around 65 wt.% Al, 16 wt.% Mn,

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11 wt.% Fe and 8 wt.% Si. Majority of these particles was identified by EBSD as cubic α-Al(Mn,Fe)Si phase with a = 1.264 nm [27] which is one of the two main phases usually observed in Al-Mn-Fe-Si type alloys [28]. 7

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3.3. X-ray computed tomography X-ray computed tomography enables detailed observation of the spatial ar-

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rangement of the segregation. The resulted distribution is depicted in Figure 7 for all three principal directions and also in Figure 8 in a general observation an-

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gle. XCT clearly confirms that the segregation forms long channels oriented in the rolling direction in the central part of the strip. The direction is consistent with the one reported by Yun at. al. [8].

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Figure 7b documents the fan shape distribution of segregations and also the inclinations of the segregations in the rolling direction by ∼20◦ which corre-

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sponds to the orientation of grains near the central part of the strip (Fig. 2). Figure 7a clearly shows the inhomogeneity of the segregation along the transverse direction – fan shaped smaller particles with larger blocky ones.

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4. Conclusion

In modified twin-roll cast AA3003 aluminium strips central segregations form

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during solidification. They have a shape of channels elongated in the rolling direction with the length up to several mm and thickness in the order of 100 µm. 200

The primary particles forming the segregation occur in different sizes and are of α-Al(Mn,Fe)Si phase. X-ray computed tomography is a quick and non-destructive method which

provides information about the presence and spatial distribution of the segregation. Scanning electron microscopy is suitable for detailed observation of 205

morphology and phases identification of the segregations.

Acknowledgement ˇ P107-12-0921, GAUK 227085 and The financial support of grants GACR ˇ M.Z., J. B. and M.C. SVV-2015-260213 are gratefully acknowledged by M.S., S.Z. and J.K. thank FFG, the governments of Upper Austria and Styria for the 210

support of the K-Project ZPT+.

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Graphical Abstract

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Macrosegregations in twin-roll cast sheets stretch along the rolling direction. X-ray computed tomography is an effective tool for visualization of the segregation. The segregations copy the shape of grain boundaries.