Improvement of tensile ductility of heavily rolled and recovery annealed
aluminum alloy sheet
N. S. Lee 1,*, Jyun-Hao Chen 1, T.Y. Tseng 2, J. R. Su 2 P. W. Kao 1
1
Department of Materials and Optoelectronic Science, National Sun Yat-Sen
University, Kaohsiung, Taiwan
2
Steel & Aluminum Research and Development Department, China Steel
Corporation, Kaohsiung, Taiwan
Keywords: Aluminum; tensile behavior; anisotropy
Abstract
For non-heat treatable aluminum alloys, strength can be achieved by heavy cold
working, but poor ductility is often accompanied. In order to restore some ductility, a
low temperature recovery annealing may be applied, in which the annealing is carried
out at a temperature below the onset of recrystallization. The recovery annealing often
produces yield drop, which is followed by flow localization, early onset of necking,
and poor ductility. In this work, Al-Fe-Mn alloy sheets produced by heavy cold rolling
(98%) followed by recovery annealing were investigated. Anisotropic tensile tests
were carried out with stress axis along the directions of 0o, 45o, and 90o from rolling
direction. It was observed that the presence of Mn in solid solution could improve
both the tensile strength and ductility of the recovery annealed aluminum alloy sheet.
Microscopic observations were also performed and related to the tensile properties.
Introduction
Aluminum alloys of thin gauge are desirable to reduce the weight and cost for
structural applications. It is always a challenge to produce alloy with enhanced the
strength and maintaining reasonable ductility. For non-heat treatable aluminum alloys,
strength can be achieved by heavy cold working, which is related to the refinement of
substructure resulted from large plastic deformation. But poor ductility is often
accompanied heavy cold working. Recovery annealing, which is carried out at a
temperature below the onset of recrystallization, may be applied to restore some
ductility. However, the recovery annealing often produces yield drop, which is
associated with flow localization, early onset of necking, and poor ductility [1-5].
These tensile behaviors may be related to the ultrafine grained structure. Yu et al. [2]
reported that an evident transition of tensile deformation behaviors appeared in
aluminum as the grain size reduced from micrometer to submicrometer range.
Recently, Huang et al [6] in a study of UFG aluminum indicated an effect of the
structural scale on fundamental mechanisms of dislocation-dislocation and
dislocation-interface reactions. They demonstrated that annealing reduced the
generation and interaction of dislocations.
The early occurrence of tensile instability in UFG alloys could be attributed to the
high flow stress and low work hardening rate [2-4]. It was suggested [7] that the poor
tensile ductility of UFG alloys may be improved by reducing the dynamic recovery
rate, which may be achieved via proper alloying, decreasing deformation temperature,
or increasing strain rate. It was reported [8] that Mn addition to 1xxx alloy can
enhance the strength and hardening rate. The objective of the present work is to study
the effect of Mn addition on the tensile strength and ductility of the recovery annealed
aluminum alloy sheet. For comparison, high purity aluminum (99.99%) was also
studied. The alloy studied was partially replace the Fe in 1050 Al alloy by Mn. The
Al-Mn-Fe alloy used was produced by continuous casting (CC). Since the maximum
solid solubility of Mn in aluminum is about 1.82 wt% [9], the high cooling rate of
continuous casting can keep significant amount of Mn in solution.
Experimental
The chemical composition (in wt%) of the Al-Mn-Fe alloy is 0.234 Fe, 0.229 Mn,
0.054 Si, 0.017Cu, 0.008 Ti, and bal. Al. The high purity aluminum used was
produced by direct chill casting. Both materials were cold rolled from 6 mm to ~0.10
mm (rolling reduction 98%). In the following text, the Al-Mn-Fe alloy and pure
aluminum will be denoted as CC and 4N, respectively. For recovery annealing, the
cold-rolled sheet was annealed at different temperature for 3 h. The annealing
temperature was selected to meet the strength requirement of H26 temper. The
annealing temperatures are 245oC and 260 oC for CC, and 150 oC and 210 oC for 4N.
Tensile tests were performed at room temperature using Sintech10/GL machine with
three different strain rates, 1.7×10-5, 1.7×10-3, and 1.7×10-1s-1, which will be denoted
as A, B, and C, respectively. Anisotropic tensile test was carried out with stress axis
along the directions of 0°, 45°, and 90° from the rolling direction. Phillip CM-200
transmission electron microscope (TEM) operate at 200kV was used to examine the
microstructure of RD-TD plane.
Results and discussion
Transmission electron microscopy (TEM) was used to reveal the microstructure of
both materials in different conditions. For 4N specimen (Fig. 1), the as-rolled (AR)
condition has the typical cold rolled structure with subgrain size about 0.9 to 1.2 m
and high density of dislocations distributed. Annealing at 150oC reduces dislocation
density and causes growth of subgrains, which have an average size about 1.36 m.
After annealing at 210oC, the microstructure is a mixture of recrystallized grains
dispersed in a matrix of partially recovered structure as shown in Fig. 1c. The CC
specimen in AR condition shows dense dislocations and an average subgrain size of
about 0.6 to 0.9m (Fig. 2a). After annealing, the dislocation density is reduced (Fig.
2). The average subgrain size is 0.85 and 1.08 m for annealing at 245oC and 260oC,
respectively.
Figure 1. TEM images showing the microstructure of 4N specimen in (a) as-rolled, (b)
150oC annealed, and (c) 210oC annealed condition.
Figure 2. TEM images showing the microstructure of CC specimen in (a) as-rolled, (b)
245oC annealed, and (c) 260oC annealed condition.
The tensile stress-strain curves for 4N specimens are shown in Fig. 3. It shows evident
anisotropic behavior and strain rate effect. In both 0o and 90o direction, increasing
strain rate improves the strength and the ductility. The improvement of ductility can
be related to the enhanced strain-hardening rate resulted from increasing strain rate.
Annealing reduces the strength but restore the strain-hardening rate and ductility in
both 0o and 90o direction. Except for testing at high strain rate (C), specimens tested
in 45º direction exhibit poor ductility. As tested in 45º direction and at lower strain
rates (A, B), the ductility of annealed specimens is even lower than that of the AR
state.
Figure 3. True stress-strain curves for 4N specimens tested in three tensile directions
at different strain rates: (a) 1.7×10-5 s-1 (A) (b) 1.7×10-3 s-1 (B) (c) 1.7×10-1 s-1 (C).
Fig. 4 shows the tensile stress-strain curves for CC specimens. In general, both
strength and ductility increase with increasing strain rate. Annealing reduces the
strength but restore the strain-hardening rate and ductility. The CC specimens also
exhibit anisotropic tensile behavior, but it is less evident than that observed in 4N
specimens. Similar to 4N specimens, CC specimens tested along the 45º direction
have lower strength than those tested along 0º and 90º directions. Furthermore,
specimens tested in 0º direction show the highest strain hardening rate and those
tested in 45º direction exhibit the lowest one.
Figure 4. True stress-strain curves for CC specimens tested in three tensile directions
at different strain rates: (a) 1.7×10-5 s-1 (A) (b) 1.7×10-3 s-1 (B) (c) 1.7×10-1s-1(C).
From observations of the surface of 210oC annealed 4N specimen during tensile test,
the development of intense shear bands in specimens tested along 45o direction were
observed (Fig. 5a). However, numerous fine slip lines were found in specimens tested
along 0o or 90o direction (Fig.5b). It is then suggested that the poor ductility of 210oC
annealed 4N specimens tested in the 45o direction might be attributed to the flow
localization associated with intense shear banding. For CC specimens, fine slip lines
distributed throughout the gauge length were found in specimens until final necking
occurred, irrespective to the testing direction.
Figure 5. OM observation of the surface of the 210oC annealed 4N specimen during
tensile test (strain rate: 1.7×10-3 s-1) along the directions of (a) 45° (b) 90° from the
rolling direction.
Even though the tensile behavior for the specimens studied shows strain-rate evident
dependence on strain-rate. However, the strain rate sensitivities determined are in the
range of 0.004 – 0.012. Therefore, the onset of plastic instability (necking) during
tensile test can be explained mainly based on the Considère criteria, which can be
expressed by
  
(1)

 
  
where  and  are true flow stress and true strain, respectively. The flow stress
increases significantly by refinement of grain size to submicrometer range. However,
the strain-hardening rate is not enhanced rather suppressed by UFG structure because
of increasing rate of dynamic recovery [3]. As a result, plastic instability (necking)
occurs at the very early stage of tensile deformation in the UFG materials, which
results in limited uniform elongation. Increasing strain rate may reduce the rate of
dynamic recovery and thus enhance the strain-hardening rate, and consequently
improve tensile ductility. By comparing the results of CC with 4N specimens, one can
find the beneficial effect of alloying addition. It is believed that the enhanced
strain-hardening rate in CC specimens is mainly due to significant amount of Mn in
solid solution, which slows down the recovery of dislocations. Anisotropic tensile
behavior may be related to cold-rolling texture of aluminum [10] and/or strain path
change [11]. More works are needed to further clarify the cause of anisotropic
behavior.
Summary
In this work, Al-Fe-Mn alloy (CC) and high purity aluminum (4N) sheets produced by
heavy cold rolling (98%) followed by recovery annealing were investigated.
Anisotropic tensile tests were carried out with stress axis along the directions of 0o,
45o, and 90o from rolling direction. Significant anisotropic behavior and strain rate
effect were found in 4N specimens, while the anisotropic behavior was less evident in
CC specimens. The poor ductility of recovery annealed 4N specimens in the 45o
direction might be attributed to the flow localization associated with intense shear
banding. It was suggested that the presence of Mn in solid solution could improve
both the tensile strength and ductility of the recovery annealed aluminum alloy sheet.
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