# Texture and Mechanical Anisotropy of Ultrafine-Grained Aluminum Alloy AA6016 Produced by Accumulative  Roll Bonding

Authors: Werner Skrotzki; Ingwar Hünsche; Juliane Hüttenrauch; C.-G. Oertel; Heinz-Günter Brokmeier; Heinz Werner Höppel; Irena Topic
Journal: Texture, Stress, and Microstructure
 (2008)

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## Abstract

The texture of ultrafine-grained Al alloy AA6016 produced by accumulative roll bonding (ARB) has been measured by neutron diffraction. The starting texture consists of a strong cube component. During ARB, this texture breaks down and a texture typical for rolling of face-centered cubic metals with high stacking fault energy develops. The texture after 8 ARB cycles is characterised by theβ-fiber with the Cu component dominating. Moreover, the rotated cube component is formed. This component is typical for simple shear, which takes place during rolling on the surfaces of the sheets. Based on the Taylor factor and calculated Lankford parameter, the mechanical anisotropy of the advanced metal sheets is discussed.


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## Body

## 1. Introduction

During recent years, bulk
ultrafine-grained (UFG) materials, produced by severe plastic deformation (SPD)
and with grain sizes in the range of 1μm down to 100 nm, have received
considerable scientific attention (e.g., [1]). Compared to their conventional grain-sized
counterparts, the UFG microstructure leads, beside many other enhanced
properties in UFG materials, to an extraordinary high-strength and enhanced
ductility [2]. From
the technical point of view, ARB is a highly promising process because it
incorporates the production of new UFG materials and because it can be easily
integrated into existing
industrial process chains. Due to their significantly
higher specific strength, UFG materials have a high light-weight construction
potential for automobile and aircraft industry, while their enhanced ductility
may contribute to good metal sheet formability using conventional techniques
such as bulge testing or deep drawing. As high strength and light weight
structural components, the use of UFG materials can lead to weight reduction
and resource conservation.While much is known about texture formation in SPD materials
produced by equal channel angular pressing (e.g., [3]), texture measurements on
face-centered cubic (FCC) metals subjected to accumulative roll bonding (ARB) are less
frequent. ARB, which was originally introduced by Saito et al. (Figure 1), is a
process where the rolled sheet is cut, polished, brushed, stacked, and then roll
bonded [4–6]. By repeating
this procedure, very high strains can be introduced sucessfully into different
metals and alloys, and as a result, significant structural refinement can be
achieved. Observations of the structural evolution during ARB have shown the
formation of a lamellar structure at high strains, indicating a similarity to
what takes place during conventional rolling [7]. Furthermore, texture
formation should be comparable with that of the conventional rolling. However,
because of repeated stacking of sheets containing a texture gradient
characterized by plane strain deformation in the centre and a high portion of
shear in the surface layer of the sheets, marked differences are expected. Moreover,
there are small zones heavily treated by brushing in order to accomplish a good
bonding.Figure 1
Principle of the accumulative
roll bonding process [4].The present project aims at studying the formation of texture
thoroughly in AA6016 as a function of ARB cycles. This alloy is often used in
the automotive industry for body panels. Therefore, based on the Taylor factor and the calculated Lankford
parameter, the mechanical anisotropy of the advanced metal sheets is discussed.


## 2. Experimental Details

In this investigation, sheets of the aluminum alloy AA6016
with an original thickness of 1 mm were used. The chemical composition is
listed in Table1. Prior to roll bonding, the material was a solution, heat
treated at 520°C for 1 hour and subsequently quenched in water. Prior to
rolling, the sheets were prewarmed in a furnace to 230°C for 4 minutes. The ARB process, optimized with regard to thermal stability and
interlamellar bonding [6], was applied using a four-high rolling mill
(Carl Wezel, Mühlacker) at a workpiece temperature of 230°C to small strips of
material, which had a width of 80–100 mm and a length of approximately 300 mm.
The thickness reduction was set nominally to 50% per rolling pass. After each
pass (cycle), the surfaces were wire brushed in order to remove the oxide layer
and the sheet was folded at half of the length and then the rolling process was
repeated (Figure 1). A maximum of 8 ARB cycles, corresponding to εvonMises=6.4, were applied which lead to a sheet with 28=256 bonded
layers, each having a thickness of 3.9 μm. After each cycle, the bonded metal
sheets were air cooled.Table 1
Chemical composition of aluminum alloy AA6016.
wt.%SiCuFeMnMgCrZnTiOtherAlAA60161.0–1.50.520.50.20.25–0.60.10.20.150.15BalanceThe microstructure of the ARB processed AA6016 after
different number of cycles was observed using scanning and transmission
electron microscopy (SEM: Zeiss DSM 965, TEM: Philips CM 200). Sheet
cross-sections for SEM observation and TEM foils after mechanical grinding were
prepared using the standard Struers electrolyte A2 at 20 V at about −10°C and 22 V at room temperature,
respectively.Texture
measurements were done by neutron diffraction on a stack of eight sheets with a
size of 10 mm× 10 mm. Because of their high
penetration depth, neutrons allow bulk texture measurements. The
orientation distribution function (ODF) was calculated from the measured pole figures (200, 220, 111)
by using the harmonic method with a maximum series expansion coefficient of 22
[8, 9]. The Euler
angles are in the Bunge notation [10].To discuss the texture-induced plastic anisotropy, the
Taylor-Bishop-Hill theory of crystal plasticity was used to calculate the
Taylor factor and the Lankford parameter [11–13].
The Taylor factor M describes the correlation between applied
stress σxx and resolved shear stress τ according to(1)M=σxxτ
with x being parallel to the tensile axis. In
polycrystalline metals, the Taylor factor of the aggregate is a superposition
of the Taylor factors of each orientation Mi weighted by its volume fraction fi:(2)Mres=∑ifiMi.
To calculate M,
the Taylor Program written by Van Houtte [14] was used. For room temperature
deformation, the {111}〈110〉 slip systems were taken.For discussion of the yield stress, it is necessary to
examineM as a function of the Lankford parameter(3)r=dεyydεzz,
where dεyy and dεzz are incremental strains in y and z directions, that is, within and normal to
the sheet plane, respectively. For plane strain deformation with dεzz = 0, r becomes infinite. Therefore, it is more appropriate to use ρdescribing the ratio of width contraction to
tensile extension:(4)ρ=−dεyydεxx.
Assuming volume constancy (dεxx+ dεyy+ dεzz = 0) the Lankford parameter
becomes(5)r=ρ1−ρ.
Minima in the M(ρ) curves
belonging to minimum Taylor energy give information on the most likely
deformation mode during tension [15–19]. Thus ρat minimum M yields r.
To get information on the deep drawing properties of rolled sheets, the
coefficient of normal anisotropy(6)〈r〉=14(rRD+2r45°+rTD)and
its variation
(7)Δr=12(rRD−2r45°+rTD)
(with RD = rolling direction, TD = transverse direction, and 45° = direction at 45° to RD
in the rolling plane) are important. For good deep drawing, 〈r〉 should be large, thus only leading to a small
thickness reduction, and Δr should be zero leading to isotropic drawing
conditions within the sheet plane.


## 3. Results and Discussion

The starting material is totally recrystallized with equiaxed grains
with sizes of about 20–30μm (Figure 2(a)). During ARB, an
ultrafine-grained lamellar structure develops (Figure 2(b)). After 3-4 cycles, a
steady-state width of the lamellae of about 200 nm is reached
(Figures 2(c), 2(d)) [6]. A more detailed analysis of the microstructural evolution by electron
backscatter diffraction (EBSD) is under way.SEM image of the grain structure in the initial state
(a) and after 2 ARB cycles (b) (reconstructed from an orientation map) with low-
and high-angle boundaries shown as thin and thick lines, respectively. TEM
bright-field images of the lamellar subgrain and grain structure after 4 (c) and
8 ARB cycles (d), respectively. (Horizontal: rolling direction RD, vertical: normal
direction ND.)
(a)(b)(c)(d)The starting texture consists of a strong cube component,
spreading out to form a〈100〉 fiber parallel to ND and a minor Goss
component (Figures 3, 4(a), and 4(c)). During ARB, this texture breaks down and
a texture typical for rolling of face-centered cubic (FCC) metals with high-stacking fault energy develops. The
texture is characterised by the β-fiber with the Cu component dominating (Figures 3 and 4(b)).
Moreover, the rotated cube component is formed. This component typical of simple shear
during rolling develops in the surface layer of the sheets. The shown textures
are the bulk textures of the entire sheet thickness. Thus, with these textures, it is possible to calculate the bulk properties
of the ARB sheets. Calculations of the mechanical anisotropy of the sheets are
given in Figure 3.Texture of ARB AA6016 after (a) 0, (b) 2, (c) 4, (d) 6, and (e) 8 cycles displayed as ODF
sections atφ2 = 45° with
intensities given in multiples of a random distribution (m.r.d.). Key figure
shows the position of the main texture components.Intensity along different fibers: (a)α-fiber, (b) β-fiber, and (c) ND-fiber.
(a)(b)(c)The calculations using the Taylor model were done with a
systematic variation of the strain tensor [19, 20]. The variation of the strain
tensor describes different deformation modes (e.g., ρ = 0 and 1; plane
strain, ρ = 0.5: axially
symmetric deformation) which are directly related to the Lankford parameter (see
(3)). The textures measured for each sheet were discretized into 1000 single
orientations. The denoted Taylor factors M are the volume weighted means of the Mi factors calculated for these
sheet orientations (see (2)). The minimum M value characterises the minimum deformation
energy and the deformation mode is given by ρat minimum M (Figure 5). The Taylor
calculations were done with the full and relaxed constraints (FC and RC) models. Strong rolling leads to flattening of grains in the rolling plane and elongation
in the RD. Therefore, in case of RC, the so-called pancake model was used, that
is, the shear strains, εzx and εzy, in the rolling plane were relaxed.Figure 5
Taylor factorM calculated with the FC Taylor model as
a function of strain mode for tensile deformation of an ARB sheet (8 cycles) in
different directions. Arrows mark the minima in the M(ρ) curves.Figure6 shows the Taylor factor as a function of the number
of cycles. In the initial state (cube recrystallisation texture), M45° is higher than MRD and MTD. After ARB cycling, giving rise
to the typical FCC rolling texture development, this relation is reversed. The M values calculated with the FC Taylor model are
somewhat higher than those calculated with the RC model. Moreover, in the RC case MTD is slightly smaller than MRD. As M is directly related to the flow stress (1)
there is flow stress anisotropy between RD/TD and 45°, which becomes reversed with
ARB cycling.Taylor factorM at minimum ρ as a function of the number of cycles, (a) FC and (b) RC Taylor models.
(a)(b)Figure7 shows the Lankford parameter as a function of the
number of cycles. Similar to the anisotropy of M, in the initial state, rRD and rTD are higher
than r45°. After ARB, this relation is
reversed. As r is related to ρ(3) this
means that with ARB cycling the strain mode during tension changes. The results
obtained with the FC and RC models are in qualitative agreement. A strong
increase of simulated r45° values with rolling degree has
also been reported by Park [21]. However, in this case a strong discrepancy
with experiment exists.Lankford parameterr calculated for tensile deformation in
different directions as a function of the number of cycles, (a) FC and (b) RC
Taylor models.
(a)(b)The deep drawing conditions of the sheets can be judged from the magnitudes of〈r〉 and
Δr.The larger
the〈r〉, the less the sheet
thinning takes place, while a decrease of the norm of Δr leads to less earing.As seen in Figure8, 〈r〉 increases
steadily with increasing the number of cycles, while Δr decreases
and changes its sign. Thus with regard to sheet thinning, the high-cycle ARB
state is best suited. In contrast, the low-cycle ARB state should not result in
any earing. As Δr changes its sign with increasing the number
of ARB cycles, earing is shifted from the RD/TD positions (Δr positive) in the initial state to the 45° positions (Δr negative) in the high-cycle ARB state.Coefficient of normal anisotrop〈r〉 (a) and its variation Δr (b) as a function of the number of cycles
calculated with the FC and RC Taylor models.
(a)(b)In conclusion,
the results obtained here reveal that low-cycle ARB material may be optimal for
deep drawing, while they also show good strength and ductility. Although high-cycle
ARB leads to higher strength and ductility [6], desirable drawing properties
are not expected due to earing. It should be pointed out that the
considerations made above are based on standard room temperature deformation
mechanisms. As discussed in literature, high temperature deformation mechanisms
like grain boundary sliding, coble creep, and thermally activated annihilation
of dislocations should be considered for UFG aluminium materials even at low
homologous temperatures [2, 22, 23]. From the above-made considerations, it is
suggested that for obtaining a good compromise between strength and deep
drawing ability, the sheets should be rolled between 3 to 5 ARB cycles. In this
context, it should be also mentioned that recent results on ARB processed
AA6016 alloy (up to 8 cycles) show very promising formability under the complex
loading conditions in a bulge test. The observed formability of UFG AA6016 is
comparable to the conventional grain-sized counterparts, but the UFG material exhibits
higher burst pressures [24]. However, in comparison to bulge testing slightly
different material behavior may be expected during deep drawing due to a different
stress state.Nevertheless, starting with a cube texture low-cycle ARB may be optimal for deep drawing with
simultaneously fulfilling good strength and ductility. Measurements of the Lankford
parameter confirming the calculations presented here are under way. It should
be mentioned that similar calculations were already performed on rolled
molybdenum sheets and stand in good agreement with the experimental results [25].
However, it may be remarked that small deviations of the calculated and
measured mechanical parameters in [25] may be partly attributed to the effect of grain shape, latent hardening,
and dislocation structure [26, 27], all of which have not yet been taken into
account.



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*Source: 328754-2008-03-12.xml*