Materials Science and Engineering A336 (2002) 249–262

Texture control by thermomechanical processing of AA6xxxAl–Mg–Si sheet alloys for automotive applications—a review

Olaf Engler *, Jurgen HirschVAW aluminium AG, Research and De�elopment, P.O. Box 2468, D-53014 Bonn, Germany

Received 25 April 2001; received in revised form 5 December 2001

Abstract

The properties of Al-alloys for car body applications are largely controlled by microstructure and crystallographic texture ofthe final sheets. In this paper, the impact of texture on formability and, in particular, on surface appearance of the sheets isreviewed. The paper summarizes the principles of microstructure and texture evolution during the main steps of the thermome-chanical processing of age-hardenable Al–Mg–Si sheets (6xxx series alloys). The most important parameters that may be usedto modify the textures and hence to improve the resulting properties are outlined. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Aluminum 6xxx alloys; Rolling; Recrystallization; Texture; Formability; Ridging

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1. Introduction

The demands for weight reduction in automotiveconstruction have led to increasing interest in sheetsmade from aluminum alloys for autobody applicationsso as to increase fuel efficiency and reduce vehicleemissions (e.g. [1–3]). In this regard the interest inheat-treatable 6xxx series alloys has increased markedlyin the last years [4–9]. Heat-treatable alloys achievetheir final strength only during the paint bake cycle ofthe final automotive construction. Thus, they combinethe good formability of the solution-treated state (T4temper) with the increased service strength of the agehardened state (T6 or T8 temper). These alloys are wellsuited for automotive skins where high dent resistanceis required.

Alloys of 6xxx series contain magnesium and silicon,both with and without additions of copper. Comparedto other Al-alloys, including the 5xxx and 2xxx seriesalloys, 6xxx sheets stand out by a combination of goodformability, good corrosion resistance and satisfactorystrengthening potential during paint bake cycles atsufficiently high temperature. The potential 6xxx alloys

that are in use for autobody sheets include AA6009,AA6010, AA6016, and AA6111; recently, alloyAA6181A was introduced for recycling aspects (Table1). Among these alloys, US carmakers favor the higher-strength alloy AA6111, while European car companiesprefer the high formability alloy AA6016.

In commercial production of AA6xxx sheets thematerial goes through a specific thermomechanicaltreatment before reaching the final gauge. Such a se-quence is shown schematically in Fig. 1. Property con-trol depends on most of these process steps individuallyas well as in their rather complex interaction. Thealloys are DC-cast as large ingots with dimensions upto 600 mm thick, 2 m wide and 4–9 m in length (Fig.1a). The ingots are scalped on their (later) rollingsurfaces in order to remove surface blemishes. In prepa-ration for the hot rolling, the ingots are preheated to atemperature between 480 °C and 580 °C, in a cyclewhich may last up to 48 h (Fig. 1b). During thepreheating the material is homogenized, short-rangeintercellular segregation (coring) is reduced and solublephases in the material are dissolved. The hot ingots arethen transferred to the rolling line which, in modernproduction lines, typically consists of a reversiblebreakdown mill (Fig. 1c), followed by a high-speedmulti-stand tandem mill (Fig. 1d). In the breakdownmill the ingots are reversibly rolled in a number of

* Corresponding author. Tel.: +49-228-552-2792; fax: +49-228-552-2017

E-mail address: olaf.engler@vaw.com (O. Engler).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0921 -5093 (01 )01968 -2

O. Engler, J. Hirsch / Materials Science and Engineering A336 (2002) 249–262250

Table 1Chemical composition (wt.%) of the most important AA6xxx alloys used for automotive sheets

TiMg ZnAlloy MnFeCuSi

0.4–0.8 0.6–1.0 0.15–0.60AA 6009 �0.5 0.2–0.8 �0.25 �0.10.2–0.8�0.50.15–0.600.8–1.20.6–1.0AA 6010 �0.1�0.25

�0.5�0.21.0–1.5AA 6016 0.25–0.6 �0.2 �0.2 �0.150.1–0.450.6–1.1 �0.40.5–0.9 �0.10.5–1.0AA 6111 �0.15

�0.25�0.3�0.40.6–1.0 0.15–0.5AA 6181A �0.250.7–1.1

passes to a transfer gauge of 25–40 mm. The multi-stand hot rolling reduces the thickness of the slab in 3or 4 steps of approximately 50% thickness reduction toa strip with a thickness between 3 and 6 mm at adefined exit temperature. The strain rates duringtandem rolling may approach levels in excess of 100s−1. The hot band is coiled and allowed to cool beforeit is cold rolled to its final gauge of around 0.8–1.2 mm(Fig. 1e).

In order to obtain both maximum age hardeningresponse and good formability, the material is thensolution treated. The sheet is unwound from the coiland passed through a continuous annealing line (Fig.1f). In this line the material is rapidly heated to temper-atures between 500 and 570 °C to dissolve the harden-ing phases and then quenched to retain thecorresponding alloying elements in solid solution. Dur-ing the heating period recrystallization of the as-de-formed microstructure takes place. After the finalanneal the sheets are leveled, possibly pre-aged forstabilization and improved age hardening response [10],and generally pre-lubricated or pre-coated before beingsupplied for blanking and stamping. The final in-servicestrength of the manufactured parts is achieved after theforming operations through age hardening, preferablyduring the final automotive paint baking cycles, whichconsist of annealing for 20–30 min at temperaturesbetween 160 and 200 °C.

The plan of the present paper is as follows. InSection 2, we will highlight the importance of thecrystallographic texture on the properties of the sheetsin the final T4 temper. After a short review on textureanalysis in Al-sheet alloys, we will summarize the evolu-tion of texture and microstructure during the mainsteps of the thermomechanical processing of 6xxxsheets (Section 3). The characteristic texture changeswill be outlined and illustrated by means of new exam-ples obtained in industrial production facilities. The keyparameters that may be used to modify the resultingtextures will be addressed. Furthermore, the character-istic textures observed in the 6xxx series alloys arecompared to the well-established texture evolution instandard not-heat treatable Al-alloys, like Al–Mg–Mnalloys of the 3xxx and 5xxx series (Section 4).

2. Texture and formability of AA6xxx sheet alloys

The formability of standard steel sheet is generallysuperior to that of Al-automotive alloys, as indicatedby forming limit diagrams and other formability tests,including limiting dome height and Erichsen depth (e.g.[11]). This is in part due to the principle difference inthe crystallographic texture between (fcc) aluminumand (bcc) steel sheets. Thus, in general, formability canbe improved by a proper texture control. It is wellestablished that texture affects the plastic anisotropy ofthe final recrystallized sheets like the wall thicknessreduction (R-value) and the earing properties duringdeep drawing. Furthermore, texture controls the form-ing behavior in terms of affecting forming limit dia-grams [12,13], although these correlations are not yetcompletely clarified. It has repeatedly been describedthat a pronounced Goss orientation {0 1 1}�1 0 0� inthe recrystallization texture leads to poor formability[14–17]. By contrast, there is a discord as to how thecube orientation {0 0 1}�1 0 0�, the typical texture

Fig. 1. Schematic diagram illustrating the typical steps of thermome-chanical processing of AA6xxx sheet alloys.

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Fig. 2. On the appearance of ridging in conventionally processedAA6016 sheet (T4 temper): (a) optical micrograph showing strongtexture banding; (b) waviness, W, (smoothed roughness profile) after20% tensile deformation perpendicular to the rolling direction, fortwo textures with different sharpness, f(g)max, illustrating the influ-ence of texture on ridging.

ferred to as ridging or roping—has been linked to thepresence of bands of similar orientation in the sheets[18–20]. This is illustrated in Fig. 2a, which shows anoptical micrograph with strong texture banding inAA6016 in condition T4 after anodical oxidation. InFig. 2b, the corresponding roughness plot after 20%tensile deformation perpendicular to the rolling direc-tion is displayed, which reveals the ridging tendency ofthis particular material.

Bands of similar crystallographic orientation will de-form collectively and thus tend to form elevated ordepressed band-like regions. Because of the typicallyvery strong cube recrystallization texture of materialthat reveals ridging, the occurrence of ridging has beenattributed to the existence of cube bands in the recrys-tallized state (e.g. [19]). More recently, however,Baczynski et al. [20] have stressed the importance of theGoss orientation in provoking ridging. On the otherhand, the characteristic cube recrystallization texture isusually accompanied by strong rotations about therolling direction i.e. toward Goss (e.g. [21]). Thus, withregard to the ridging phenomenon, a discriminationbetween cube and Goss orientation may be unneces-sary. In a more general context this means that thepresence of a pronounced cube recrystallization texture,including the typical rotations toward Goss, is detri-mental to the use of Al-sheets by promoting the occur-rence of paint-brushes lines. Vice versa, in order toavoid ridging, the texture sharpness of the sheet in T4temper needs to be weakened using appropriate pro-cessing schedules.

3. Evolution of texture and microstructure during theprocessing of AA6xxx sheet alloys

3.1. Determination and representation of texture data

For texture analysis several pole figures were mea-sured by standard X-ray diffraction techniques [22] andused to compute the three-dimensional orientation dis-tribution functions (ODF) f(g) by the series expansionmethod [23]. The ODFs were corrected with respect tothe so-called ghost error following the method of Luckeet al. [24]. All ODF calculations were performed underthe assumption of orthotropic sample symmetry asdefined by the rolling direction, RD, the transversedirection, TD, and the normal direction, ND, of therolled sheets. Therefore, ODF representation wasconfined to the familiar subset of the Euler space with0°� (�1, �, �2)�90°. Table 2 lists the Miller-indices,{h k l}�u � w�, and Euler angles, �1, �, �2, of the mostcommonly observed orientations of rolled and recrys-tallized 6xxx sheets.

An important consideration in analyzing the texturesof 6xxx sheets— like in many other rolled sheet prod-

component of recrystallized Al-sheets, affects formabil-ity (see Refs. [12,15,17]). Both experimental investiga-tions and FEM simulations conducted by Bryant et al.have shown that a weak, widely scattering cube recrys-tallization texture significantly increases the limitingdome height [14], yet this difference is not readilyapparent from the forming limit diagrams.

Finally, texture, more precisely, the local distributionof specific grain orientations (i.e. the microtexture) hasbeen identified as the main reason for the appearance ofthe so-called paint-brush lines on the sheet surfaceduring forming operations. These defects may serve asa strain concentration during forming, which wouldlimit the sheet formability [18]. Much more severely,these effects are still visible in the automotive compo-nent after painting. Hence, such parts are objectionableand not generally usable for exterior automotive appli-cations. This phenomenon—which is commonly re-

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ucts— is the occurrence of variations in texture andmicrostructure through the thickness of the sheets.Therefore, pole figures were systematically determinedat different through-thickness layers of the rolled speci-mens. For this purpose, the layer of interest was pre-pared by careful grinding and polishing, followed by anetching treatment in dilute sodium hydroxide (NaOH)so as to remove the demolished surface layers. In thefollowing, the position of the layer within the plate orsheet along the normal direction is indicated by theparameter s which is defined by s=2�t/t0 (t0, sheetthickness; �t, distance from the center). Thus, s= +1and s= −1 denote the upper and lower surface of thesheet, and s=0 identifies the sheet center at mid thick-ness. The layer that is most representative of the aver-age sheet texture, and hence of the overall sheetproperties, is the quarter thickness (s=0.5). Thus, ifnot explicitly stated otherwise, all textures shown in thispaper are taken from this intermediate layer.

3.2. Hot rolling— transfer slab

Hot rolling is a key step in the production processthat may contribute significantly to the development ofthe properties of the final sheet. The starting material,conventional DC-cast ingots, comprises a fairly coarsegrained cell structure (Fig. 3a). During the solidifica-tion, very irregularly shaped coarse Al(Fe,Mn)Si-con-stituent particles with a size of 2–20 �m and a spatialdensity of 103–104 mm−2 form at the grain boundariesof the initial cell structure (Fig. 4a). (In view of the factthat these primary particles reduce the Si-contents ofthe matrix, the alloys must contain excess Si to enablethe maximum precipitation of the stochiometric Mg2Sihardening phase.) Since the grain size is controlled bygrain refinement, the crystallographic texture of theas-cast material is almost random.

As illustrated on Fig. 1, the DC-cast material isreheated and then subjected to a series of hot deforma-tions, including numerous rolling passes in a reversing

breakdown mill followed by multi-stand hot rolling.After the breakdown rolling, the microstructure typi-cally comprises slightly elongated recrystallized grainswith a size of the order of 200 �m (Fig. 3b). Because ofthe high deformations and temperatures involved in thebreakdown rolling, the primary phases break up anddevelop a more spherical shape (Fig. 4b).

Fig. 5 shows the texture of the transfer gauge mate-rial. The ODF given in Fig. 5a was derived from asample taken in the short transverse plane (i.e. TD/ND)and subsequently rotated in the standard sheet plane(RD/TD). Thus, this ODF represents the integratedtexture over the full plate thickness. In agreement withthe recrystallized microstructure, a quite well definedcube recrystallization texture with strong scatter,mainly about the ND, prevails. However, in contrast tothe deformation of thin sheet, the roll gap geometryduring breakdown rolling is such that the resultingdeformation field is much more non-uniform. The arcof contact is usually much less than the plate thickness.Frictional effects— induced by the work rolls—as wellas temperature gradients contribute to inhomogeneousdeformation. In the center-plane, where the deforma-tion state is fairly symmetric, the texture, after recrys-tallization, is characterized by a symmetric cubeorientation (Fig. 5b). Away from the center-plane, incontrast, shear strains develop as a consequence of theboundary conditions [25]. This essentially results in arotation of the plane strain rolling texture orientationsabout the TD (Fig. 5c). Further homogeneous deforma-tion of the rotated cube orientation may lead to furtherND-rotations towards the cube orientation, thus sup-plying potential nuclei for the cube recrystallizationtexture [26]. As a matter of fact, the hot rolling textureof the plate surface displays a cube texture with strongrotations about both TD and ND (Fig. 5c). As pointedout by Nes et al. [27], a sufficiently large number ofcube nuclei must be present in the microstructure of thetransfer slab in order to obtain strong cube textures inthe hot band.

Table 2Miller indices and Euler angles of the most important orientations of Al and Al-alloys after rolling and after recrystallization (approximated)

Designation Euler anglesMiller Indices{h k l}�u � w�

�1 � �2

C {1 1 2}�1 1 1� 90° 30° 45° Rolling{1 2 3}�6 3 4� 59°S 34° 65° Textures

Components0°/90°{0 1 1}�2 1 1� 45°B 35°{0 1 1}�1 0 0� 0°Goss 45° 0°/90°

Cube {0 0 1}�1 0 0� 0° 0° 0°/90°CubeRD {0 1 3}�1 0 0� 0° 22° 0°/90° Recrystallization

{0 0 1}�3 1 0� 22°CubeND 0° 0°/90° TextureR 60°{1 2 4}�2 1 1� Components53° 36°

45°{0 1 1}�1 2 2� 0°/90°65°P

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Fig. 3. Evolution of the grain structure during the thermomechanical processing of AA6016 sheets.

3.3. Hot rolling—hot band

During the following multi-stand hot rolling, theprocess parameters change drastically. The rolling tem-perature drops, whereas strain and strain rates increasegreatly. Accordingly, the microstructure of the hotband, especially at the mid thickness position (s=0), iscomprised of a highly elongated band structure charac-teristic of a deformed material (Fig. 3c). While theconstituent phases are largely unaffected by hot rolling,additional secondary particles with size of the order of1 �m may form (Fig. 4c). These secondary phases mayinclude small Al(Fe,Mn)Si precipitates with size 50–500 nm, excess Si particles with sizes of up to several�m and plate-like Mg2Si precipitates with lengths (i.e.diameters) of up to 1 �m (e.g. [28]). In contrast to theprimary constituents, some of these secondary parti-cles, especially Mg2Si, can re-dissolve during the fur-ther thermomechanical treatment. Accordingly, sizeand density of the secondary phases strongly dependon the details of the thermomechanical processing, aswill be addressed later in more detail.

The cube recrystallization texture of the transfer slab

is transformed into a typical fcc (copper-type) rollingtexture. In such textures, most orientations are assem-bled along the so-called �-fiber, that runs through theEuler angle space from the C orientation{1 1 2}�1 1 1� through the S orientation {1 2 3}�6 3 4�to the B orientation {0 1 1}�211� (Fig. 6a, Table 2; seeRef. [29]).

Because of the above-mentioned changes in processparameters—reduced deformation temperature, in-creased strain and strain rate— the energy stored in thematerial upon deformation increases. Dynamic recrys-tallization is not typically observed in commerciallyprocessed Al-alloys. However, recrystallization cantake place between passes and, particularly, during thecooling period after deformation is completed. Thisso-called post-dynamic recrystallization accounts forthe more or less strongly developed cube orientation{0 0 1}�1 0 0� that frequently accompanies the �-fiberrolling texture orientation in the hot band textures (e.g.Fig. 6a). Fig. 6b displays a subset of Fig. 6a, namelythe �2=45° ODF section. This section comprises mostof the characteristic fcc rolling and recrystallizationtexture components, viz. the cube, C and B orienta-

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tions (Table 2). The relative amount of the cube orien-tation with regard to the rolling texture orientationscritically depends on the processing parameters. Forinstance, an increase in strain and strain rate and/or adecrease in deformation temperature all enlarge thestored energy. Accordingly, the potential for post-dy-namic recrystallization increases. Vice versa, however,at hot band end temperatures below the solubility limitfor the Mg2Si phase the formation of Mg2Si precipi-tates may strongly impede post-dynamic recrystalliza-tion, which is reflected in a weakened cube texture e.g.(Fig. 6c). Deformation and/or temperature gradientsaccompanying the practical hot rolling operations maylikewise lead to differences in the degree of post-dy-namic recrystallization (Fig. 6d). Nonetheless, the over-all texture gradients, including the tendency for formingthe rotated cube (shear) orientation {0 0 1}�1 1 0�, gen-erally diminish because of the high thickness reductionsin excess of 80% that are typically achieved duringmulti-stand hot rolling.

In conventional processing lines, the hot band is thencoiled and allowed to cool down to ambient tempera-ture. During this cooling period heavy precipitation offine dispersoids, mostly Mg2Si precipitates, may occur.Fig. 7 shows a TEM micrograph of the cooled hotband, showing several dispersoids of 0.1–0.5 �m to-

gether with a multitude of finely dispersed Mg2Siplatelets. As will be addressed later, these dispersoidsmay strongly interfere with the progress of recrystalliza-tion. Accordingly, control of the thermomechanical hotrolling is an essential tool in optimizing the texture and,ultimately, the formability of 6xxx sheet material.

3.4. Cold rolling

After the cold rolling, the sheets comprise a highlyelongated, deformed microstructure (Fig. 3d). Therolling texture of the hot band sharpens significantly(Fig. 8a, see Fig. 6a), whereas the through-thicknesstexture gradients diminish further. Nonetheless, thesheet center usually comprises a stronger B orientationthan the outer layers. In order to illustrate the rollingtexture gradients in more detail, it is very convenient toplot the maximum orientation densities f(g) along the�-fiber versus the corresponding Euler angle �2 (e.g.Fig. 8b). Though the overall texture features remainconstant, it is obvious that with increasing distancefrom the sheet center, i.e. with increasing parameter s,the C orientation increases at the cost of the Borientation.

The thickness reduction applied in commercial coldrolling is typically of the order of 70–80%. Though this

Fig. 4. Evolution of the precipitation state during the thermomechanical processing of AA6016 sheets.

O. Engler, J. Hirsch / Materials Science and Engineering A336 (2002) 249–262 255

Fig. 5. Texture of the transfer gauge material: (a) ODF of the integral texture as derived from the short transverse section (ND/TD); (b) {1 1 1}pole figure derived at the center layer (s=0); (c) {1 1 1} pole figure derived at the surface (s=1).

reduction is evidently sufficient to increase the sharp-ness of a rolling texture that pre-exists in the hot band(Fig. 6), it is not enough to get rid of a very stronginitial cube texture that may exist in the hot band. Inwhat follows we will illustrate this point which becomesimportant with a view to hot band annealings so as tomodify the final recrystallization textures (see below).Fig. 9a shows the texture of a hot band after anadditional solution annealing in the form of the �2=0°and 45° ODF sections. The texture consists of a sharpcube orientation with characteristic RD-rotations;rolling texture orientations are negligible. The texturesof the 2 mm (50%) and 1 mm (75%) cold rolled sheetsare given in Fig. 9 b and c. Although the cube intensitydecreases significantly (26�12�7), it still has the max-imum texture intensity, while the �-fiber sharpens onlyslowly. It is noted that this effect is masked by the high

symmetry of the exact cube orientation [22] which tendsto feign overly large sharpness. Considering the actualvolume fractions of the textures in Fig. 9 would putmore emphasis on the �-fiber orientations.

3.5. Recrystallization

3.5.1. Recrystallization textures in 6xxx alloysThe final step of thermomechanical processing of

6xxx sheets consists of a heat treatment in a continuousannealing line (Fig. 1f). Since this solution heat treat-ment is required to achieve a re-dissolution of thesecondary phases that have precipitated during thevarious preceding steps of thermomechanical processing(Figs. 4 and 7), it has to be carried out at a very hightemperature, preferably in excess of 540 °C. Accord-ingly, the solution treatment is accompanied by recrys-

O. Engler, J. Hirsch / Materials Science and Engineering A336 (2002) 249–262256

tallization, leading to a typical recrystallized mi-crostructure consisting of fine, slightly elongated grainswith a size of 20–30 �m (Fig. 3e).

The textures observed in recrystallized 6xxx sheetsreported in the literature are essentially similar in thatthey are composed of the same groups of orientations,

Fig. 6. Texture of the hot band. (a) Hot band texture showing some recrystallization (cube) texture intensities besides the rolling texturecomponents C, S and B (s=0.5); (b) �2=45° section of the ODF in (a); (c) texture of a hot band rolled with a lower finishing temperature,showing less cube texture (s=0.5; �2=45° section); (d) texture of the center layer of the hot band shown in (a), (b) (s=0; �2=45° section).

O. Engler, J. Hirsch / Materials Science and Engineering A336 (2002) 249–262 257

Fig. 7. TEM micrograph of the coiled hot band, showing several dispersoids of size 0.1–0.5 �m plus numerous finely dispersed plate-like Mg2Siprecipitates.

viz., the cube orientation, sometimes accompanied bystrong scattering or rotations, together with intensitiesof the former rolling texture S orientation—commonlyreferred to as R component—and the P orientation{0 1 1}�1 2 2� (Table 2). As an example, Fig. 10a showsthe recrystallization texture of a laboratory-processed6xxx alloy in which all characteristic recrystallizationtexture orientations are present with about equal inten-sity. However, the intensity and volume fraction of thedifferent recrystallization texture components criticallydepends on the details of the preceding thermomechan-ical processing in so far that the cube texture may bemuch more pronounced (Fig. 10b) or, vice versa, maybe so weak that it no longer forms a distinct peak (Fig.10c).

3.5.2. Nucleation of the cube and R orientationThe recrystallization textures of most rolled Al-al-

loys, including the 6xxx series alloys, are characterizedby the cube orientation with strong scattering about theRD towards the Goss orientation (Fig. 10a and b). It iswidely established by now that nuclei with cube andRD-rotated cube orientations form in band-like struc-tures which are already present in the as-deformedmicrostructure, the so-called cube bands (e.g. [30–32]).During the subsequent growth, grains with the exactcube orientation prevail, which is caused by their favor-able growth conditions into several components of thedeformation texture by means of compromise growtheffects [31,33].

Besides the cube bands, the grain boundaries mayserve as nucleation sites. Nucleation at the grainboundaries proceeds by growth of subgrains on one

side of a preexisting grain boundary into the deformedmatrix on the other side beyond this boundary. Conse-quently, this mechanism leads to orientations similar tothe rolling texture [34,35]. During the subsequentgrowth, R oriented grains prevail to the disadvantageof other, competing rolling texture orientations, sincethey stand out by a fast growing orientation relation-ship to the other three symmetrically equivalent compo-nents of the S orientation in the rolling texture [33].

3.5.3. Recrystallization in the presence of particlesIn two-phase, particle-containing Al-alloys, recrystal-

lization is further influenced by the precipitation state.Large particles with sizes larger than 1 �m can promoterecrystallization by introducing additional nucleationsites. This particle stimulated nucleation (PSN) takesplace in the so-called deformation zones that formaround the particles through the interaction betweenslip dislocations and particles [36]. The resulting recrys-tallization textures are typically very weak and oftenappear to be almost random. Accurate ODF analysis,however, has established the occurrence of some char-acteristic intensities of the so-called P orientation{0 1 1}�1 2 2� and a significant rotation of the cubeorientation by 20°–30° about the ND towards{0 0 1}�3 1 0� (Fig. 10c) [31,37].

Small particles like the Mg2Si precipitates in Fig. 7strongly impede the progress of recrystallization (Zenerdrag). Under extreme circumstances— large volumes offinely dispersed particles and very high strains—allmotion of high-angle grain boundaries is suppressed,such that the entire dislocation energy that is stored inthe microstructure during the preceding deformation

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Fig. 8. Texture of the final gauge. (a) Texture of the cold rolled sheetshowing a typical rolling texture characterized by the �-fiber rollingtexture (s=0.6); (b) intensity variations along the �-fiber for variousthrough-thickness layers, indicated by parameter s.

can merely be released by extended recovery reactions,which is referred to as continuous recrystallization orrecrystallization ‘in situ’ [35,38,39]. In less severelypinned cases the Zener inhibition may affect differentrecrystallization texture components in a different man-ner which is attributed to the different microstructuralcharacteristics of the corresponding nucleation sites. Inparticular, it has been shown that dispersoids mayselectively inhibit PSN, which, in turn, gives rise to apronounced cube recrystallization texture notwithstand-ing that large particles are present in the material[37,40,41].

The ability of large particles to act as nucleation sitesdepends on the particle size, �, the driving pressure forrecrystallization, pD, and the Zener drag due to disper-soids, pZ [42]. Considering further that the size of thedeformation zone, �, is about twice of that of theparticles, �, it follows that only particles with a size inexcess of �* will be able to initiate PSN:

�*=12

�crit=2�GB

pD−pZ

(1)

(�GB, specific grain boundary energy). It follows fromEq. (1) that for an increasing Zener drag, pZ, the criticalnucleus size, �crit, increases. For PSN, �crit will eventu-ally exceed the size � of the constituent particles presentin a given material, so that PSN can no longer occur.Nucleation at the cube bands is less affected by thedispersoids, by contrast, since the subgrains within thecube bands are easily able to exceed the critical nucleussize even in case of a strong Zener drag [37].

Thus, although the large constituent particles are ingeneral basically unaffected by the preceding ther-momechanical history, their efficiency in initiating re-crystallization through PSN greatly depends on theprecipitation state of the Mg2Si dispersoids—size, vol-ume and, in particular, degree of dispersion. The mate-rial whose final recrystallization texture is shown in Fig.10b was produced by using standard processingparameters as outlined beforehand. Here, the disper-soids that form during the coiling of the hot strip (Fig.7) efficiently suppress PSN, so that recrystallization isdominated by the cube oriented grains. In the exampleshown in Fig. 10c the material was pre-treated so as toprecipitate Mg2Si in the form of particles that were toocoarse to exert a significant Zener drag. Accordingly,PSN could prevail, leading to the characteristic PSNrecrystallization texture [43]. Obviously, Fig. 10a repre-sents an intermediate state with limited Zener drag,resulting in a mixture of cube and PSN recrystallizationtexture orientations. Means on how to vary the disper-sion state and, hence, optimize the textures and proper-ties of the final recrystallized sheets through changes inthe thermomechanical processing will be presented in asubsequent paper [44].

O. Engler, J. Hirsch / Materials Science and Engineering A336 (2002) 249–262 259

Fig. 9. (a) Texture of the solutionized hot band, showing a strong cube recrystallization texture; (b) texture after 50% cold rolling; (c) texture after75% cold rolling (s=0.5; �2=0° and 45° sections).

Fig. 10. Variety of recrystallization textures in AA6xxx series alloys after the solutionizing annealing (T4 temper, s=0.5). (a) Texture of athermomechanically treated material, showing cube, P and R orientations with about equal intensities; (b) conventionally processed material,showing a strong cube recrystallization texture; (c) material pre-treated so as to coarsen Mg2Si particles, resulting in a characteristic weak PSNrecrystallization texture (see text for details).

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4. Comparison of the texture evolution in 6xxx seriesalloys with that of conventional non-heat treatableAl-alloys

The overall texture evolution observed in the present6xxx series alloys is in line with results from otherAl-alloys that are produced according to comparableproduction cycles, like the conventional non-heat treat-able 3xxx and 5xxx series alloys used e.g. for beveragecans and can ends, respectively (e.g. [45–47]). The maindifferences are caused by the Mg2Si particles that mayreadily precipitate during the thermomechanical pro-cessing of 6xxx alloys in dependence on the details ofthe time/temperature history.

Whereas the transfer slab textures after re�ersing hotrolling (at temperatures �400 °C) are very similar asthose observed in corresponding transfer gauge of non-heat treatable alloys (Fig. 5), the hot strip discloses asignificant impact of the Mg2Si particles. Most non-heat treatable alloys containing Mg readily recrystallizeat the typical hot mill exit temperatures, so that coiledhot strip of these alloys usually undergoes completerecrystallization during the cooling period (self anneal-ing). In 6xxx alloys, in the same temperature regimeMg2Si particles form (Fig. 7), which impede the pro-gress of recrystallization. Accordingly, for hot bandcoiling temperatures between 300 and 350 °C the 6016hot band textures display significant amounts of rollingtexture orientations (Fig. 6a and b); at hot band coilingtemperatures below 300 °C the hot strip is almostcompletely unrecrystallized (Fig. 6c). In order toachieve full recrystallization, much higher temperaturesare required.

The textures forming during cold rolling of a 6016hot band that was fully recrystallized by applying anadditional hot band anneal strongly resemble thoseobserved in conventional Al-alloys (Fig. 9), while sheetsprocessed according to standard practices (i.e. withunrecrystallized hot band) showed a much strongerrolling texture (Fig. 8). This difference can readily beattributed to the texture prior to cold rolling, i.e., thehot band texture. Recrystallized hot bands typicallydisplay a fairly mild cube texture that, upon furthercold rolling, will slowly rotate towards the characteris-tic rolling texture �-fiber. The unrecrystallized hotband, by contrast, already contains a pronouncedrolling texture (Fig. 6). During additional rolling themain texture characteristics will remain unchanged,whereas the overall texture will sharpen further (Fig. 8).

Recrystallization during the final solutionizing cyclehas a great impact on texture and properties of theresulting sheet. In general, the recrystallization texturesof 6xxx series alloys are composed of the same compo-nents that are well established from other Al-alloys(Fig. 10a): the cube orientation that evolves from thecube bands, the R orientation nucleating at the former

grain boundaries and the ‘random component’ due toPSN [31]. With a view to the nucleation sites, it shouldbe noted that the cube orientation is typically accompa-nied by pronounced RD scatter towards Goss (e.g. Fig.10b). As for nucleation through PSN, although theresulting textures are very weak, they do display somecharacteristic features, viz., a significant rotation of thecube orientation about the ND plus characteristic in-tensities of the P component (Fig. 10c).

From Fig. 10 it appears that intensity and volumefraction of the various recrystallization texture compo-nents—especially of the cube orientation—criticallydepend on the details of the preceding thermomechani-cal processing. Finely dispersed Mg2Si particles stronglyinterfere with the progress of recrystallization and,hence, play a key role in controlling strength and typeof the final recrystallization texture. It has been shownthat fine dispersoids may selectively inhibit PSN to theadvantage of the cube recrystallization texture [40,41].Thus, an increasing volume of finely dispersed Mg2Siparticles may shift the recrystallization texture from onebeing entirely controlled by PSN (Fig. 10c) over anintermediate case (Fig. 10a) to one where PSN is com-pletely suppressed, resulting in a pronounced cube tex-ture (Fig. 10b).

5. Summary and conclusions

It is well established that the formability of AA6xxxsheets for car body applications is largely controlled bythe crystallographic texture of the sheets in final T4temper. In general, a too strong recrystallization texturemust be avoided so as to improve formability and, inparticular, to prevent the formation of paint-brush lines(ridging; see Fig. 2). The present paper shows how thetexture evolves during the thermomechanical processingof 6xxx series alloy sheets from the pre-heating all theway down to the final recrystallized state. Also, meanshave been addressed that my be used to modify theresulting textures. In particular, control of the state ofMg2Si precipitation offers a valuable tool in controllingthe texture and, therewith, improve the ultimate proper-ties of the sheets.

Regarding alloy composition, elements like Fe andMn form large constituents particles. These particlesare important in that they tend to weaken the recrystal-lization texture and refine the grain size in the final T4state by providing nucleation sites for PSN. Moreover,Mn-bearing particles restrict grain growth upon thefinal solutionizing treatment that is typically carried outat temperatures of as much as 520–540 °C. However,too high concentrations of such elements must beavoided since they degrade formability.

During homogenization, non-dissolvable constituentparticles are spherodized and soluble phases in the

O. Engler, J. Hirsch / Materials Science and Engineering A336 (2002) 249–262 261

material, mainly Mg2Si, are dissolved. Obviously, therate of homogenization critically depends on annealingtime and temperature, with temperatures in excess of540 °C being necessary to dissolve most Mg2Si. Labo-ratory scale experiments have shown that pre-heating atlower temperatures results in incomplete dissolution ofMg2Si [44]. Hence, the volume of the—harmful—Mg2Si dispersoids that may form during the subsequentthermomechanical processing is reduced, resulting inweaker recrystallization textures [43] and, consequently,improved formability and surface appearance (no ridg-ing). However, lowering of the homogenization temper-ature reduces the age-hardenability of the material [8]which counteracts the favorable effect of a weakenedtexture.

Hot rolling parameters like rolling speed and exittemperature control the level of energy stored duringhot deformation and, therewith, the progress of recrys-tallization during the cooling period of the coiled hotstrip (Fig. 6). Furthermore, precipitation of Mg2Si atexit temperatures below say 350 °C may strongly im-pede post-dynamic recrystallization of the hot band(e.g. Fig. 7). Both the degree of recrystallization and theprecipitation state of the coiled material may be alteredthrough an additional annealing treatment [40,41,44].Thus, hot band anneals represent an efficient way incontrolling texture and property of the final sheet incondition T4.

The main variable in cold rolling is the strength of theresulting rolling texture. Here, the initial texture, that isthe hot band texture prior to the cold rolling, and thestrain applied during cold rolling are of importance(Figs. 8 and 9). Among other factors, the rolling texturestrength relates to the sharpness of the final recrystal-lization texture. Thus, hot band annealing (see above)or intermediate annealing may be applied to weakenthe rolling texture and, consequently, the T4 recrystal-lization texture, resulting in improved formability[48,49].

The mechanism of recrystallization during the finalsolution annealing and the resulting textures and grainsizes are governed by the precipitation state. Largevolumes of finely dispersed Mg2Si-particles selectivelyinhibit PSN and, therewith, favor the formation of astrong cube recrystallization texture (Fig. 10) [40,41,44].Thus, in conclusion, control of the state of Mg2Siprecipitation during the entire chain of thermomechani-cal processing is required so as to achieve a weak finalrecrystallization texture with good formability andgood surface appearance (no ridging effects) in 6xxxsheets.

Acknowledgements

Stimulating discussions with Dr E. Brunger, Dr S.

Keller, Dr D. Wieser (all VAW aluminium AG), Prof.A.J. Beaudoin (Univ. of Champaign-Urbana, IL) andDr J.D. Bryant (Reynolds Metals) are gratefullyacknowledged.

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