Equal Channel Angular Pressing (ECAP)

262 Pages·2005·7.63 MB·English

Equal Channel Angular Pressing (ECAP) of
AA6082: Mechanical Properties, Texture and
Microstructural Development
by
Jens Christofer Werenskiold
A thesis submitted to
The Norwegian University of Science and Technology (NTNU)
in partial fulfilment of the requirements for the degree of
Doktor Ingeniør
Trondheim
Oktober 2004
NTNU
Norwegian University of
Science and Technology
Doctoral theses
for the degree of Doktor Ingeniør
Faculty of Natural Science
and Technology
Department of Materials Technology
IMT-report 2004:63.
ISBN 82-471-6481-7 (electronic)
ISBN 82-471-6482-5 (printed)
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I. PREFACE
This work has been carried out at the Norwegian University of
Science and Technology, (NTNU), Faculty of Natural Science and
Technology, Department of Materials Technology. The research was carried
out as part of the COMPFORM project and funded by the Norwegian
Research Council and Hydro Aluminium AS.
This work has given me the opportunity to work in a new field of
materials technology at NTNU and has been very educating. Hopefully, the
initiation of this work will result in new insight in the field of severe plastic
deformation (SPD) in coming years.
The thesis consist of a general introduction to equal channel angular
pressing (ECAP), aluminium alloys and deformation mechanisms, followed
by a detailed strain analysis of ECAP’ed materials, mechanical properties
and finally a study of the microstructural and textural development resulting
from equal channel angular pressing.
The main results presented in this thesis have been published (chapter
5), or are in the process of being published (chapter 6 and 7), in
international journals and conference proceedings.
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II. ABSTRACT
This thesis deals with the concept of ECAP applied to a commercial Al-
Mg-Si alloy (AA6082.50). A detailed analysis of the strains introduced by
ECAP in a single passage through the die has been made, based on direct
measurements on partially pressed samples.
Further, the mechanical properties of ECAP’ed samples have been
investigated. The effect of initial material temper and post-ECAP artificial
aging was investigated in addition to the effect of strain accumulation and
processing route.
Finally, a detailed study of the textural and microstructural development
was made. The continuous evolution of texture and microstructure was
followed through the ECAP deformation zone up to an accumulated strain
of 2 (2 passes) by route A, and linked to strain measurements from the same
zone.
Strain measurements on sectioned samples have validated the plane
strain assumption for ECAP. The shear angle has been measured and some
typical features of ECAP have been corroborated, i.e. friction and material
temper affect the strain distribution, the strain homogeneity and the work-
piece corner angle, friction being the most significant here. Also, new
conclusions have been drawn. The analysis of material element deformation
histories along path lines reveals that ECAP can be interpreted as the
combination of shearing and stretching (i.e. tension and/or compression).
Furthermore, the final shear strain angle obtained in ECAP appears to be
friction and material temper independent in the zone of homogeneous
deformation.
The 6082 alloy has been successfully processed by ECAP at room
temperature to strains (cid:304)=6 to (cid:304)=8. The alloy has been pressed in the
solutionized, T4, homogenized and soft annealed states. The highest tensile
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strength was obtained from the solutionized material, followed by T4,
homogenized and soft annealed. This behaviour is linked to the solid
solution content prior to ECAP and the potential for dynamic precipitation
during ECAP processing.
The tensile elongation to failure drops to a constant level between 4%
and 9% after ECAP and is highest for the soft annealed and lowest for the
solutionized material. The ductility in the solutionized material can,
however, recover to ~18% elongation to failure (i.e. an increase by a factor
2-3) by low temperature heat treatment with only a small drop in tensile
strength.
Soft annealed and ECAP’ed material has been compared to cold rolling
to similar strains. The tensile strength response to accumulated strain is
similar, but the ductility and post uniform deformation is different.
However, the ECAP’ed material can be processed to higher strains and,
thus, achieving higher strength.
The tensile yield strength behaviour of the homogenized and ECAP’ed
material can be described by a simple relation to the grain size and the
fraction high and low angle boundaries.
The typical texture components related to ECAP of aluminium, pressed
by route A, have been identified. The typical ECAP texture starts to develop
already at ~25% strain and increases in intensity during the first pass. In the
second pass, two of the stable texture components continue to increase in
intensity, while the other texture components decrease.
The microstructural development during the first pass is dominated by
deformation banding leading to grain-subdivision. The average linear
intercept distance (grain size) is reduced from ~80μm to ~4μm for high
angle boundaries and from ~10μm to ~0.7μm for low angle boundaries.
During the second pass, the linear intercept distance is further reduced to
~1.8μm for high angle and ~0.3μm for low angle boundaries.
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Deformation twins are observed during the second pass and are believed
to play an important role in severe plastic deformation when the grains
reach the sub-micron or nano-metre scale.
The deformation banding have been explained in terms of the low
energy dislocation structure (LEDS) theory, and has been shown to be an
important mechanism in the early stages of grain subdivision, and is further
believed to be the main source of high angle grain boundary formation by
grain subdivision down to a grain size of approximately ~0.6μm, when
other deformation mechanisms may be energetically more favourable.
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III. ACNOWLEDGEMENTS AND CONTRIBUTIONS
This research was carried out as part of the COMPFORM project and
funded by the Norwegian Research Council and Hydro Aluminium AS.
I would like to thank my main supervisor, Prof. Hans Jørgen Roven for
his great contributions and stimulating discussions and for motivating me
during this work.
I would like to give a special to thank Dr. Stéphane Dumolin for his
contribution to chapter 5: Strain analysis in ECAP. His involvement in
strain measurements and aid in mathematical and continuum mechanical
calculations is greatly acknowledged.
I would also like to thank Mr. Hans Bjerkaas for fruitful discussions
regarding EBSD analysis, Mr. John Rasmus Leinum for help with XRD
texture measurements, Mr. Håkon Atle Nesbø for his contributions to
chapter 6 in performing the post ECAP heat treatments and mechanical
testing, and Mr. Simen Olsen for sharing my office and lots of interesting
discussions the last years.
Finally, I would like to thank my one and only Laila Beate Kjønstad for
her patience, encourage and support over the past years.
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