Control of texture and earing in aluminium alloy AA 8011A-H14 closure stock

Journal Pre-proof Control of texture and earing in aluminium alloy AA 8011A-H14 closure stock Olaf Engler, Johannes Aegerter, Dirk Calmer PII:

S0921-5093(20)30054-X

DOI:

https://doi.org/10.1016/j.msea.2020.138965

Reference:

MSA 138965

To appear in:

Materials Science & Engineering A

Received Date: 29 October 2019 Revised Date:

9 January 2020

Accepted Date: 15 January 2020

Please cite this article as: O. Engler, J. Aegerter, D. Calmer, Control of texture and earing in aluminium alloy AA 8011A-H14 closure stock, Materials Science & Engineering A (2020), doi: https:// doi.org/10.1016/j.msea.2020.138965. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Rolled Products Research & Development Bonn D-53117 Bonn

Ewald A. Werner, Prof.

Date: 11.11.2019

Editor Materials Science and Engineering A Our Contact: Dr. Olaf Engler Phone: +49 228 552 2792 E-Mail: [email protected]

MSEA-D-19-05823 Authors Contribution

• O. Engler: Conceptualization, Supervision, Writing- Original draft preparation, Writing- Reviewing and Editing • J. Aegerter: Investigation, Data curation, Visualization • D. Calmer: Investigation, Data curation, Visualization

Authors contribution.docx

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Control of texture and earing in aluminium alloy AA 8011A-H14 closure stock Olaf Engler *, Johannes Aegerter, Dirk Calmer # Hydro Aluminium Rolled Products GmbH, Research and Development Bonn, P.O. Box 2468, D – 53014 Bonn, Germany #

now with Hydro Aluminium Rolled Products GmbH, Technical Customer Service Foil, Grevenbroich, Germany

Abstract The development of crystallographic texture during the thermo-mechanical processing of aluminium sheet results in the formation of pronounced plastic anisotropy, including the well-known earing phenomenon. In the present study we track the evolution of texture, microstructure and the resulting earing profiles in Al alloy AA 8011A during down-stream processing to final-gauge sheet in temper H14. This processing, which includes two interannealings, followed by a mild final temper rolling pass, was designed for providing minimum earing for earing-critical packaging applications. Besides the experimental characterization of microstructure, texture and earing along the standard process chain, means to optimize the earing behaviour are addressed. This includes changes in the homogenization practice, possible omission of one of the two interannealings and variation of the interannealing gauge. The resulting earing properties are discussed in the light of the interplay of deformation and recrystallization with the resultant texture changes along the process chain.

Keywords: Rolling; Inter-annealing; Recrystallization; Texture; Anisotropy; Earing

* Corresponding author. Tel.: +49 228 552 2792; fax: +49 228 552 2017. E-mail address: [email protected] (O. Engler)

1

Introduction

Aluminium alloy AA 8011A (ISO AlFeSi(A)) is a non-heat treatable Al wrought alloy containing about 0.8% Fe and 0.7% Si which is used for a number of foil and packaging applications [1,2]. Compared to Si-free foil alloys of the 8xxx series, most notably AA 8079, the extra Si in AA 8011A provides some extra strength and ductility required for specific packaging applications [3,4]. For foil applications the material is usually recrystallization annealed at temperatures of typically about 350 °C, resulting in the fully soft annealed O temper which is characterized by maximum formability [5,6]. If more mechanical strength is required, AA 8011A can be work-hardened by a significant degree through cold working, typically by cold rolling, to the strain-hardened tempers H1x. Back annealing at elevated temperatures causes recovery, which is accompanied by a reduction in strength and increase in ductility to the tempers H2x. Both work hardening and softening of Al alloy sheet is commonly accompanied by the occurrence of significant plastic anisotropy, including the well-known earing phenomenon. It is well documented that plastic anisotropy is attributed to the formation of preferred crystallographic orientation, or texture, during the thermo-mechanical production of the sheet (e.g. [7–11]). In cups which are deep-drawn from soft annealed, recrystallized Al sheet often four ears at angles of 0°/180° and ±90° to the former rolling direction are observed. In contrast, cold rolled sheet with a pronounced rolling texture commonly reveals ears at the four positions ±45° to the rolling direction, with the height of the ears, and hence the amount of earing, generally increasing with the level of cold rolling. In Al alloys for packaging applications the occurrence of overly strong earing can cause major problems, e.g. during production of Al containers, beverage cans or closures. Hence, for such earing-critical products the earing level must be limited, which is commonly accomplished by controlling the final texture in such a way that it is comprised of a mixture of rolling and recrystallization texture components [12]. Accordingly, the respective ±45° and 0°/90° earing behaviour is balanced, resulting in mild earing profiles which are suitable for packaging applications. In the present paper we show how texture and earing can be controlled during industrial processing of AA 8011A-H14 sheet which is used e.g. for closures of non-carbonated beverages. Processing of AA 8011A-H14 closure stock usually includes two interannealings before a light final rolling pass, commonly referred to as temper rolling [1]. This process leads to thin-gauge sheet with medium strength and, most notably, weak textures with the desired balanced earing profiles suitable for packaging products requiring minimum earing [13–15]. First, we track the evolution of microstructure, texture and the resulting earing profiles along the standard process chain. Furthermore, means to optimize the earing behaviour through modifications of the processing route are addressed, including variation in the homogenization practice, possible omission of one of the two interannealings and variation of the interannealing gauge. The resulting earing properties are discussed in the light of the interplay of deformation and recrystallization with the resultant texture changes along the 2

process chain.

2 2.1

Experimental Material The material investigated in this study was Al alloy AA 8011A, containing about 0.8% Fe

and 0.7% Si. The material was produced on industrial scale in the AluNorf rolling mill in Neuss, Germany, including direct-chill (DC) casting of large rolling ingots, a homogenization cycle, followed by break-down and tandem hot rolling (Fig. 1(a)). The hot strip was coiled and cooled down to room temperature, before being cold rolled in several passes to final gauge. In the present study two different homogenization practices were tested. In the standard process the ingots are heated to 600 °C and soaked for several hours at this temperature before hot rolling commences. This homogenization cycle involving high temperature soaking will be referred to as practice “HT” hereafter. Several test coils were subjected to a two-stage homogenization, which is known e.g. from the production of beverage can stock [16]. This two-stage homogenization practice called “2S” consisted of a homogenization for a couple of hours at 600 °C, followed by a controlled cooling down to a sub-homogenization temperature of 500 °C prior to hot rolling. In both cases, viz. homogenization “HT” and “2S”, hot strip with a thickness of 4.5 mm was produced. Samples taken from the different hot strips were then cold rolled in several passes on a two-high laboratory rolling mill to a final gauge of 0.2 mm. Following the standard process for low-earing applications [1] the sheet was subjected to two interannealing treatments at 350 °C, which were conducted at intermediates thicknesses of 1.0 mm and 0.32 mm, respectively (Fig. 1(b)). Thus, the final temper rolling consisted of a rolling pass from 0.32 to 0.2 mm, i.e. a thickness reduction of 37.5%, which resulted in low earing grade sheet in the required H14 strength regime. In the present study it was further tested to omit the first interannealing, which means that hot strips with both homogenization practices were cold rolled to final gauge with only one interannealing at 0.32 mm (Fig. 1(c)). Finally, for the two-stage homogenization practice “2S” it was probed to raise the interannealing thickness to 0.34 mm in order to optimize the balance between rolling and recrystallization texture components and, therewith, improve the earing behaviour of the final sheet in temper H14.

2.2

Characterization methods Samples for microstructural analysis and mechanical tests were taken from the hot strips,

before and after intermediate annealing, and at final gauge (H14). Mechanical properties of the sheets were determined by conventional tensile tests according to the international standard ISO 6892-1. Standard ISO tensile specimens were prepared parallel the rolling direction of the

3

material and tested using a screw-driven testing machine Zwick Z100 and a tactile extensometer with an original gauge length of 50 mm (hot strips 80 mm). Two to three parallel tests were conducted to ascertain the statistical relevance of the data. It is noted that according to EN 485-2 sheet from alloy AA 8011A-H14 in the thickness range 0.2 to 0.5 mm must reach a tensile strength Rm between 120 and 170 MPa, a yield strength Rp0.2 exceeding 110 MPa and an elongation at fracture, A50mm, exceeding 1%. Earing was analysed by means of cup drawing tests following the guidelines of norm EN 1669. For sheets with thicknesses in the range from 0.2 to 1 mm blanks with a diameter of 60 mm were drawn with a 33 mm punch, resulting in a drawing ratio of ~1.8. The blanks were lubricated with lanolin (wool wax) to minimize friction. The cupping tests were conducted on a hydraulic sheet metal forming machine BUP 200 from Zwick with blank holder forces ranging from 4 to 12 kN. From the thick hot strips larger cups were produced, with a punch and blank diameter of 50 mm and 100 mm, respectively. For a quantitative assessment of the resulting earing profiles, the cup heights h(α) were determined in steps ∆α of 1° with a mechanical set-up from Huxley Bertram Engineering Ltd., Cottenham, UK. Here, α indicates the angle with respect to the former sheet rolling direction, RD. In order to minimize experimental scatter the earing profiles of minimum three cups were averaged. Owing to the orthotropic symmetry of rolled sheet, the crystallographic texture and the resultant earing profiles are principally symmetric with respect to the rolling geometry. Accordingly, the earing profile can be symmetrized (“mirrored”) with regard to the rolling direction (RD) and transverse direction (TD) of the sheet. For quantification and comparison of earing results often the percentage of earing, or mean earing value, Z, is specified, which is defined as: (1) where the values

and

correspond to the average values of all ears and all troughs

of the cup, respectively. The microstructure of the different samples was studied by optical metallography. Longitudinal sections of the sheets were prepared and polished according to standard metallographical techniques. The grain structure was revealed by anodical oxidation for 1 to 2 min at 20 V at ambient temperature in 2.5% fluoroboric acid (HBF4) and subsequent investigation under polarized light. Second-phase particles were analysed in polished specimens either by optical metallography or by scanning electron microscopy (SEM). Furthermore, the specific electrical resistivity, ρ, was measured in liquid helium, i.e. at a temperature of 4.2 K, in order to attain information on the amount of solute elements in the Al matrix. More details on microstructure and microchemistry characterization of two different AA 8xxx foil alloys, including AA 8011A, were given in a recent paper by Lentz et al. [17]. The crystallographic texture of the various samples was analysed by means of standard X-ray diffraction techniques [18]. From four incomplete pole figures ({111}, {200}, {220},

4

{311}), the three-dimensional orientation distribution functions (ODFs) f(g) were calculated by means of the series expansion method assuming orthotropic sample symmetry [19]. The orientations g were expressed in form of the Euler angles {ϕ1, Φ, ϕ2} following Bunge’s notation. The ODFs were corrected for the lack of the odd-order C-coefficients by the so-called positivity method [20] and finally plotted in the form of iso-intensity lines in three characteristic sections with ϕ2 = 45°, 65° and 90° of the Euler orientation space.

3

Experimental Results

3.1

Characterization of the hot strips The microstructures and textures of the differently processed AA 8011A hot strips is

presented in Figs. 2 and 3, respectively. Moreover, Table 1 provides a summary of the specific electrical resistivity at 4.2 K, ρ, as well as the results from the uniaxial tensile tests, including yield strength, Rp0.2, ultimate tensile strength, Rm, uniform elongation, Ag, and elongation at fracture, A80mm. Finally, the percentage of earing, Z, is given together with the position(s) of the main ears.

Table 1. Mechanical and earing properties of the AA 8011A hot strip samples. ρ [µΩ·cm]

Rp0,2 [MPa]

Rm [MPa]

Ag [%]

A80mm [%]

Z [%]

position

Condition “HT”

0.243

96

124

13.3

22.8

8.8

45°

“2S”

0.200

63

104

21.7

32.1

2.2

0°/45°/90°

Fig. 2(a) shows the microstructure of the hot strip produced with the standard homogenization cycle “HT”. Evidently, the hot strip is not recrystallized, but revealed a layered microstructure where the grains were strongly elongated along the hot rolling direction (see [21]). The texture of this hot strip, shown in Fig. 3(a), displayed a pronounced texture fibre running from the Cu-orientation {112}<111> at (ϕ1,Φ,ϕ2) = (90°,30°,45°) through the S-orientation {123}<634> at about (57°,33°,65°) to the Bs-orientation {011}<211> at (35°,45°,90°) through orientation space. This texture is the characteristic β-fibre rolling texture which is typical of heavily rolled Al sheet (e.g. [8,11,15,16]). Accordingly, the hot strip disclosed pronounced earing with ears forming at ±45°; the percentage of earing, Z, was as high as 8.8% (Table 1). These findings of a non-recrystallized hot strip are in accord with the rather high strength but low elongation values obtained in the tensile tests. The microstructure of the hot strip produced with the two-step homogenization practice “2S” gives good evidence of partial recrystallization, most likely during the fairly long cooling period of the hot strip from coiling temperature down to ambient temperature (Fig. 2(b)). Accordingly, the texture of this hot strip consisted of a combination of the rolling texture 5

β-fibre orientations Cu, S and Bs together with the cube orientation {001}<100> at (0°,0°,90°) (Fig. 3(b)). The mechanical strength is lower and the elongation values are larger than in the hot strip produced with homogenization practice “HT” (Table 1), which also agrees with the observation of partial recrystallization. The deep drawn cups showed a weak mixed earing profile with eight ears at 0°, 45° and 90°, the percentage of earing was quite low, with Z = 2.2%. The differences in the recrystallization behaviour are presumably caused by the different precipitation states resulting from the two different homogenization cycles “HT” and “2S”. As discussed in more detail in Ref. [17], soaking at high temperature during homogenization practice “HT” will lead to a strong supersaturation of the alloy elements Fe and Si in the Al matrix, but minimum volume of disperoids. During subsequent annealing – or during the cooling of the coiled hot strip – these elements in supersaturated solid solution will strongly interfere with the progress of recrystallization [22–24], resulting in a largely non-recrystallized microstructure (Fig. 2(a)). The cooling period from the soaking temperature of 600 °C to the sub-homogenization temperature of 500 °C in the two-step homogenization practice “2S” was introduced to precipitate a large fraction of the alloy elements Fe and Si in the form of fairly coarse AlFeSi disperoids [17], which inhibit recrystallization much less than solute elements. Accordingly, a two-step homogenization practice facilitates recrystallization during the subsequent processing steps (Fig. 2(b)). The lower concentration of alloy elements in solid solution in the hot strip produced with the two-step homogenization practice “2S” is reflected in the reduced specific electrical resistivity of ρ = 0.200 µΩ·cm, compared to the hot strip produced with the single-step homogenization practice “HT”, with ρ = 0.243 µΩ·cm (Table 1).

3.2

Standard processing – homogenization practice “HT” and two interannealings

As outlined in Sec. 2.1, the standard processing of AA 8011A-H14 sheet for packaging applications involves two interannealings, one at 1.0 mm and a second one at 0.32 mm. Hence, the microstructural development during down-stream processing is characterized by the interchange of rolling passes and recrystallization during interannealing with its concomitant changes in texture. Fig. 4(a) shows a micrograph of the cold rolled sheet at 0.32 mm thickness before the second interannealing, revealing an as-deformed microstructure with strongly elongated grains. Accordingly, the material had a well-developed β-fibre rolling texture comprised of the Cu, S and Bs orientations together with some intensities of the cube orientation which is presumably retained from the previous interannealing (Fig. 5(a)). Because of the preceding rolling pass the material had a quite high strength of Rp0.2 = 161 MPa at low elongation values and showed pronounced ±45° earing with Z = 6.6% (Table 2). Soft annealing at intermediate gauge gave rise to recrystallization of the material. Fig. 4(b) shows a fully recrystallized microstructure consisting of more or less equi-axed grains with an average size of 16 µm. The resultant recrystallization texture was rather weak, consisting of the cube orientation together with some intensities at the former S-orientation (Fig. 5(b)). These

6

orientations, commonly referred to as R-orientation, are attributed to growth of recrystallization nuclei that inherit the former rolling texture orientations [8,25,26]. Such mixed cube + R textures are the typical recrystallization textures of cold rolled, soft annealed Al-Fe-Si alloys (e.g. [27–30]). After the interannealing the material was very soft, with Rp0.2 = 42 MPa and large elongation values, and showed a mild earing profile with four ears at 0° and 90° and a percentage of earing of Z = 1.8% (Table 2). The temper rolling pass to final gauge (i.e. temper H14) was again characterized by an elongation of the microstructure (Fig. 4(c)). Note that the aspect ratio of the individual grains was rather small, about 3:1, which agrees well with the low level of rolling reduction during the final temper rolling pass. Simultaneously, the rolling texture orientations Cu, S and Bs sharpened while the cube orientation of the recrystallization texture slightly decreased (Fig. 5(c)). Accordingly, the earing profile switched to 45° earing with a slight increase in Z to 2.0% (Table 2). The materials strength increased considerably to Rp0.2 = 146 MPa, by contrast, which is in the range specified for alloy AA 8011A in temper H14 (see above). Table 2. Mechanical and earing properties of AA 8011A, produced with homogenization practice “HT” and two interannealings. State

Rp0,2 [MPa]

Rm [MPa]

Ag [%]

A50mm [%]

Z [%]

position

as-rolled (0.32 mm)

161

172

1.7

3.7

6.6

45°

interannealed

42

102

26.8

34.3

1.8

0°/90°

H14

146

151

0.8

1.8

2.0

45°

3.3

Modified processing – homogenization practice “HT” and one interannealing

The development of microstructure and texture during down-stream processing with one intermediate annealing (at 0.32 mm) is presented in Figs. 6 and 7, respectively; Table 3 summarizes the resulting mechanical data and earing results. Since the first interannealing at 1.0 mm was skipped, the level of cold rolling from hot strip (4.5 mm) to intermediate gauge (0.32 mm) was much higher, viz. almost 93% as opposed to 68% between the first and second interannealing in the standard route (Fig. 1). Accordingly, the micrograph of the sheet rolled to intermediate thickness showed a heavily layered as-deformed microstructure (Fig. 6(a)); the rolling texture was much sharper with merely small intensities of a retained cube texture (Fig. 7(a)). This is also reflected in an increased strength of Rp0.2 = 190 MPa as well as pronounced 45° earing with Z exceeding 13% (Table 3). After soft annealing at intermediate gauge the microstructure (Fig. 6(b)) looked very similar as in the standard route (Fig. 4(b)), although – due to the increased rolling degree – the grain size of 12 µm was slightly smaller. The texture of the interannealed strip again revealed a mixed texture with intensities of the cube recrystallization texture and some retained rolling texture orientations (Fig. 7(b)). Compared to the textures of the standard process (Fig. 5(b)) the

7

retained rolling texture orientations prevailed, however, resulting in mild 45° earing with Z = 2.1% (Table 3). The microstructure at final gauge, i.e. in temper H14, likewise resembled the material of the standard processing, with a characteristic elongated grain structure; the aspect ratio increased slightly to about 3.5:1 (Fig. 6(c)). The H14 sheet showed a mixed texture comprised of rolling and recrystallization texture components (Fig. 7(c)) where, again, the rolling texture components were noticeably more pronounced than in the standard process (Fig. 5(c)). This resulted in increased 45° earing with Z = 5.6% (Table 3). Table 3. Mechanical and earing properties of AA 8011A, produced with homogenization practice “HT” and one interannealing. State

Rp0,2 [MPa]

Rm [MPa]

Ag [%]

A50mm [%]

Z [%]

position

as-rolled (0.32 mm)

190

216

1.9

4.2

13.4

45°

interannealed

45

111

23.9

29.0

2.1

45°

H14

153

158

1.0

1.8

5.6

45°

3.4

Modified processing – homogenization practice “2S”

As addressed in Sec. 3.1, the main effect of the different homogenization cycles are the strong differences in the amount of recrystallization with the resultant changes in microstructure (Fig. 2) and texture (Fig. 3) obtained after hot rolling. In this section we will summarize the impact of the two-step homogenization practice “2S on the subsequent development of microstructure, texture and earing during processing of sheet in temper H14. Again, two down-stream routes with two intermediate annealings (at 1.0 mm and 0.32 mm) and one intermediate annealing (at 0.32 mm) were tested (Fig. 1) and compared to the results obtained for the standard high-temperature homogenization practice “HT”. It turned out that the microstructure evolution of the hot strip with homogenization practice “2S” strongly resembled the results obtained for homogenization practice “HT” with two or one interannealing(s), which were presented in Figs. 4 and 6, respectively. That is to say, the grain elongation after cold rolling and the grain size after interannealing were primarily controlled by the number of interannealings, whereas the preceding homogenization practice had no obvious effect. The texture evolution likewise resembled the pattern obtained for homogenization practice “HT”, with its characteristic changes of rolling and recrystallization texture components (Figs. 8 and 9). However, because of the stronger cube texture in the hot strip (Fig. 3), the textures of the samples produced with homogenization practice “2S” were always shifted towards stronger intensities of the cube orientation at the expenses of the rolling texture orientations Cu, S and Bs. This holds for all samples investigated, with one or two interannealings, from hot strip all the way down to final gauge (H14). Accordingly, the earing results underwent a general shift from ±45° earing towards 0°/90°

8

earing, too. Comparison of the earing data for homogenization practice “2S” in Table 4 with the results obtained for homogenization practice “HT” in Tables 2 and 3 gives good evidence that in all cases where 45° ears dominated in the latter, earing was improved, in that the Z values noticeably decreased. Vice versa, for the single example of an interannealed sheet with 0°/90° earing and Z = 1.8% (Table 2), earing increased to Z = 2.9% (Table 4). In cases where homogenization practice “HT” produced weak 45° ears, samples with homogenization practice “2S” showed similarly low Z values, yet either at the four positions 0° and 90° or with eight ears at 0°, 45° and 90° (Table 4). It is noted that the strength of the samples produced with homogenization practice “2S” was slightly lower (by a few MPa) than obtained for the sheets with homogenization practice “HT”, but still well within the limits for alloy AA 8011A-H14 (see above). Table 4. Mechanical and earing properties of AA 8011A, produced with homogenization practice “2S” and one or two interannealings. Route 2 interannealings

1 interannealing

3.5

State

Rp0,2 [MPa]

Rm [MPa]

Ag [%]

A50mm [%]

Z [%]

as-rolled

156

170

1.6

3.7

2.5

0°/45°/90°

interannealed

41

99

28.4

36.6

2.9

0°/90°

H14

138

144

1.8

3.4

1.3

0°/45°/90°

as-rolled

180

212

1.9

5.0

8.0

45°

interannealed

39

103

25.2

30.4

4.2

0°/90°

H14

146

154

1.2

2.1

3.0

0°/90°

position

Modified interannealing gauge to improve earing

The texture effects caused by the interplay of rolling and recrystallization along the various process chains discussed in the previous sections are summarized in Fig. 10 by means of the texture intensities, f(g), of both the cube orientation and the β-fibre rolling texture orientations. As regards the β-fibre orientations, simply the maximum texture intensity along the rolling texture β-fibre was taken. Deformation evidently sharpens the rolling texture orientations at the expenses of the cube orientation, which took place during the cold rolling to intermediate gauge and during the final temper rolling pass. Note that in the standard process route with two interannealings the texture effects up to the first annealing are not shown, which explains why in Fig. 10(a) the rolling texture before the second interannealing was weaker than in the hot strip. Vice versa, recrystallization during interannealing led to a significant increase in the intensity of the cube orientation at the expenses of the β-fibre rolling texture orientations. However, the latter did not disappear completely, but after the interannealing treatment some β-fibre orientations remained. These remnant β-fibre orientations with intensities ranging from 3 to 7 correspond to the R-orientation, which is frequently observed in the recrystallization textures of cold rolled, soft annealed Al-Fe-Si alloys (e.g. [25–30]). Apparently, the higher level of recrystallization in the hot strip produced with the two-step

9

homogenization practice “2S” generally led to larger intensities of the cube orientation throughout the entire process chain, although these differences diminished towards the final gauge material. Comparison of the texture results presented in Fig. 10 with the earing data in Tables 2–4 suggests that minimum earing is achieved when the maximum intensity of the β-fibre orientations slightly exceeds that of the cube orientation. This holds for the process routes with two interannealings, irrespective of the homogenization practice, where very low earing values with Z ≤ 2.0% were achieved. In contrast, in the sheet produced with homogenization practice “HT” and one interannealing the intensity of the β-fibre orientations was about twice as large as that of the cube orientation (Fig. 10(a)), resulting in a percentage of earing as high as Z = 5.6%. Interestingly, the sheet produced with homogenization practice “2S” and one interannealing revealed equal intensities of β-fibre and cube orientations (Fig. 10(b)). This material showed borderline earing behaviour with 0°/90° ears and Z = 3.0%. Fig. 11 summarizes the resulting earing profiles in temper H14 for the four different processing routes, viz. two different homogenization cycles combined with one or two interannealings. For each processing route the symmetrized earing profile h(α) is plotted as a function of the in-plane angle α. It is seen that for homogenization practice “HT” 45° earing dominated (red curves), while for homogenization practice “2S” the 0° and 90° ears prevailed (blue curves). For both homogenization practices earing was stronger when the first interannealing at 1.0 mm was omitted. In conclusion, the standard processing (i.e. “HT” with two interannealings) gave good earing properties, while in the processing with only one interannealing 45° earing became too strong. For the altered homogenization practice “2S” the earing was generally lower. With one interannealing borderline properties with 0°/90° earing and Z = 3.0% were achieved, while the conventional two interannealings gave rise to a very smooth profile with Z as little as 1.3% (Table 4). The experiments conducted in the present study have confirmed that an increasing rolling reduction during the final temper rolling pass will generally lead to a stronger rolling texture and, in turn, to stronger 45° earing at the expenses of 0°/90° earing. Hence, it seemed promising to repeat the processing with homogenization practice “2S” and one interannealing using a slightly increased interannealing gauge in order to get a more balanced earing profile at final gauge (H14). Thus, we performed a similar set of rolling and annealing experiments with an interannealing thickness of 0.34 mm as opposed to 0.32 mm as in the previous experiments. Thus, the temper rolling pass was increased from 37.5% to 41%. The resulting final-gauge properties are summarized in Table 5, the earing profile is further included in Fig. 11 (in green). Evidently, the earing has indeed decreased with respect to the material with the original intermediate gauge (Fig. 11, blue); the strength increased marginally. Thus, for a process route with only one interannealing a slight increase in interannealing thickness will probably improve the earing quality, provided the two-step homogenization practice “2S” is applied.

10

Table 5. Mechanical and earing properties of AA 8011A, produced with homogenization practice “2S” and one interannealing at higher thickness, at final gauge (H14). Route

Rp0,2 [MPa]

Rm [MPa]

Ag [%]

A50mm [%]

Z [%]

position

Interannealed at 0.34 mm

148

155

1.5

2.9

2.1

0°/45°/90°

4

Discussion

It is well known that the plastic anisotropy of Al alloys is intimately linked to the formation of crystallographic texture during the various steps of thermo-mechanical processing of Al sheets – homogenization, hot and cold rolling and possible intermediate and/or final annealing (e.g. [7–16]). Accordingly, in technical Al sheet products earing is commonly controlled by means of producing mixed textures that comprise rolling and recrystallization texture components to balance the respective ±45° and 0°/90° earing behaviour [12]. In the present study it is demonstrated how texture control can be utilized during industrial processing of Al alloy AA 8011A-H14 closure stock in order to minimize earing through the interplay of rolling and recrystallization with the resulting textural effects. Closure stock is usually produced with two interannealings, followed by a light temper rolling pass to condition H14 [1]. The evolution of microstructure, texture and the resultant earing properties in alloy AA 8011A along this processing route was described in Sec. 3.2. In brief, cold rolling to intermediate gauge generally leads to the formation of a β-fibre rolling texture (Fig. 5(a)), which is accompanied by an increase of 45° earing. Recrystallization during inter-annealing typically leads to a cube recrystallization texture together with medium intensities of a retained rolling texture, or R orientation (Fig. 5(b)) [25–30], which is characterized by ears under 0° and 90°. The final-gauge properties are then achieved through a light rolling pass to medium strength (H14). This final temper rolling pass of the order of 30-40% thickness reduction sharpens the rolling texture orientations at the expenses of the recrystallization texture orientations (Fig. 5(c)), yet the rolling reduction is not large enough to significantly degrade the latter. Accordingly, the final-gauge H14 material comprises balanced cube and rolling texture orientations and, in consequence, the desired light earing profile suitable for products requiring minimum earing properties (Table 2; Fig. 11) [1,13–15]. The above example has demonstrated that texture and earing properties at final gauge depend on the interplay of recrystallization during the two interannealings and the deformation applied during cold rolling before the first interannealing, between the two interannealings and after the second interannealing. Therefore, all changes to the process chain that affect the texture evolution will inevitably either improve, or deteriorate, the final-gauge earing properties. In the present study we have tested to skip the first interannealing (Fig. 1(c)). However, this process route led to overly large 45° earing (Table 3; Fig. 11), which is attributed to the sharpening of the rolling texture orientations after the remaining interanneal (Fig. 7(b)) and, most notably, before the interanneal (Fig. 7(a)). Hence, this process route is not suitable for

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producing low earing AA 8011A-H14 sheet. The results presented in Sec. 3.4 imply that the amended homogenization practice “2S” has the capability of improving the earing properties of AA 8011A-H14 closure stock. Apparently, partial recrystallization of the hot strip (Fig. 2(b)) with its weakened mixed texture (Fig. 3(b)) had a beneficial effect on earing throughout the entire process chain down to final gauge (H14), see Fig. 11 and Table 4. In all intermediary states the balance between rolling and recrystallization texture components was shifted towards the latter and, as a consequence, the earing results showed an analogous trend from ±45° earing towards 0°/90° earing. For the process route with two interannealings earing values Z of as little as 1.3% were obtained, which was the best result achieved throughout this study. Processing with one interannealing gave borderline earing properties with Z = 3.0%, which is still a substantial improvement with respect to the route with homogenization practice “HT” and one interannealing (Z = 5.6%, Table 3). A thorough comparison of the final-gauge earing profiles obtained with one interannealing disclosed that homogenization practice “HT” produced 45° earing, while the sheet produced with homogenization practice “2S” revealed 0°/90° earing. Hence, for homogenization practice “2S” it seemed meaningful to raise the interannealing thickness in order to strengthen the rolling texture orientations and, therewith, get a more balanced earing profile at final gauge (H14). The resulting earing properties indeed improved somewhat with respect to the original intermediate gauge (Table 5, Fig. 11). In conclusion, a change in homogenization from the one-step, high temperature practice “HT” to the two-step practice “2S” has great potential of improving the earing properties of AA 8011A-H14 closure stock. Omission of the first interannealing is more critical, however. For the original homogenization practice “HT” it is not advised since the earing values increased too much (Table 3, Fig. 11). Homogenization practice “2S” generally led to better earing properties and hence the process route with one interannealing – preferably performed at a slightly raised intermediate gauge – may yield sufficiently low earing for packaging applications. It must be noted however that the state of the partially recrystallized microstructure of the hot band will be subjected to fairly large process scatter. Slight batch-to-batch changes in chemical composition or minor process variations in coiling temperature or alike will inevitably lead to different levels of partial recrystallization which, in turn, will feed through to the final-gauge earing properties. Hence, although omission of the first interannealing is certainly of commercial interest, a process route with two interannealing treatments will stabilize the final-gauge properties and hence help maintaining the constant, very low level of earing required for AA 8011A-H14 closure stock.

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5

Summary and conclusions

In the present paper we track the evolution of texture, microstructure and earing properties in Al alloy AA 8011A closure stock during down-stream processing to final-gauge sheet in temper H14. This process, which includes two interannealings, followed by a mild final temper rolling pass, was designed for providing minimum earing properties for earing-critical packaging applications. Furthermore, means to optimize the processing route were studied, including utilization of a different homogenization cycle, omission of one of the two interannealings and variation of the interannealing gauge. The resulting earing properties were discussed in the light of the interplay of deformation and recrystallization with the resultant texture changes along the process chain. A change in homogenization from the original one-step cycle with high soaking temperature to a two-step practice with a lowered sub-homogenization temperature generally led to improved earing properties. For the original homogenization practice omission of the first interannealing, although commercially attractive, produced overly high earing. For the two-step homogenization processing with one interannealing – preferably at a slightly raised intermediate gauge – good earing properties were achieved, in contrast. However, a process route with two interannealings, despite being more costly, will facilitate producing AA 8011A-H14 closure stock with constant, very low earing properties. Of course, changes in the process chain may also affect the mechanical properties. Minor variations in strength at final gauge could indeed be correlated to the different levels of recrystallization as a function of the homogenization practice applied and, most notably, to the level of cold rolling as a function of the number of interannealings. However, for all process routes discussed the final-gauge properties laid well within the specification of alloy AA 8011A-H14.

Acknowledgements The authors are indebted to their former colleagues Dr. L. Löchte und Dr. G. Wegmann for valuable discussions.

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Figure Captions Fig. 1. Scheme of the production of Al alloy AA 8011A sheet with low earing, (a) production of the hot strip, (b) processing with two interannealings, (c) processing with one interannealing. Fig. 2. Microstructure of the two different AA 8011A hot strips, produced (a) with homogenization practice “HT” and (b) with homogenization practice “2S” (optical metallography, longitudinal sections, magn. 25×). Fig. 3. Textures of the two different AA 8011A hot strips, produced (a) with homogenization practice “HT” and (b) with homogenization practice “2S” (ODF sections ϕ2 = 45°, 65° and 90°). Fig. 4. Evolution of the microstructure during processing of the AA 8011A sheet, produced with homogenization practice “HT” and two interannealings; (a) cold rolled to second intermediate gauge (0.32 mm), (b) interannealed, (c) cold rolled to final gauge, i.e. temper H14 (optical metallography, longitudinal sections, magn. 200×). Fig. 5. Evolution of the texture during processing of the AA 8011A sheet, produced with homogenization practice “HT” and two interannealings; (a) cold rolled to second intermediate gauge (0.32 mm), (b) interannealed, (c) cold rolled to final gauge, i.e. temper H14 (ODF sections ϕ2 = 45°, 65° and 90°). Fig. 6. Evolution of the microstructure during processing of the AA 8011A sheet, produced with homogenization practice “HT” and one interannealing; (a) cold rolled to intermediate gauge (0.32 mm), (b) interannealed, (c) cold rolled to final gauge, i.e. temper H14 (optical metallography, longitudinal sections, magn. 200×). Fig. 7. Evolution of the texture during processing of the AA 8011A sheet, produced with homogenization practice “HT” and one interannealing; (a) cold rolled to intermediate gauge (0.32 mm), (b) interannealed, (c) cold rolled to final gauge, i.e. temper H14 (ODF sections ϕ2 = 45°, 65° and 90°). Fig. 8. Evolution of the texture during processing of the AA 8011A sheet, produced with homogenization practice “2S” and two interannealings; (a) cold rolled to second intermediate gauge (0.32 mm), (b) interannealed, (c) cold rolled to final gauge, i.e. temper H14 (ODF sections ϕ2 = 45°, 65° and 90°). Fig. 9. Evolution of the texture during processing of the AA 8011A sheet, produced with homogenization practice “2S” and one interannealing; (a) cold rolled to intermediate gauge (0.32 mm), (b) interannealed, (c) cold rolled to final gauge, i.e. temper H14 (ODF sections ϕ2 = 45°, 65° and 90°). Fig. 10. Evolution of the texture intensities, f(g), of cube orientation and β-fibre rolling texture orientation during processing of the AA 8011A sheets, produced (a) with homogenization practice “HT” and (b) with homogenization practice “2S” (see text for details). Fig. 11. Earing profiles h(α) in temper H14 for the different processing routes (symmetrized); see text for details.

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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: