Ultrafine-grained metal sheets produced using the accumulative roll bonding process for light-weight structures [Elektronische Ressource] / vorgelegt von Irena Topić

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• Publié par: friedrich-alexander-universitat_erlangen-nurnberg
• Publié le: 01 janvier 2008
• Nombre de lectures: 50
• Langue: English
• Poids de l'ouvrage: 15 Mo

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Ultrafine‐grained Metal Sheets produced using the 
Accumulative Roll Bonding Process  
for Light‐Weight Structures 
 
 
 
 
Der Technischen Fakultät der 
Universität Erlangen‐Nürnberg 
zur Erlangung des Grades 
 
 
DOKTOR‐INGENIEUR 
 
 
 
 
 
 
 
 
 
 
 
 
vorgelegt von 
 
Irena Topić 
 
 
 
Erlangen 2008 
 
 
  
 
 
 
 Herstellung von ultrafeinkörnigen Blechen  
mittels des kumulativen Walzprozesses 
für den Leichtbau 
 
 
 
 
Der Technischen Fakultät der 
Universität Erlangen‐Nürnberg 
zur Erlangung des Grades 
 
 
DOKTOR‐INGENIEUR 
 
 
 
 
 
 
 
 
 
 
 
 
vorgelegt von 
 
Irena Topić 
 
 
 
Erlangen 2008 
 
 
  
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Als Dissertation genehmigt von 
der Technischen Fakultät der 
Universität Erlangen‐Nürnberg 
 
 
 
 
 
Tag der Einreichung:  24.11.2008 
Tag der Promotion:   15.04.2009 
Dekan:   Prof. Dr.‐Ing. Johannes Huber 
Berichterstatter:   Prof. Dr. rer. nat. Mathias Göken 
    Prof. Dr.‐Ing. Marion Merklein  
 
 
 
 ABSTRACT
Over the last decade, nanocrystalline and ultrafine-grained (UFG) materials with a grain size
of less than 1 µm have aroused considerable interest due to their superior mechanical
properties in terms of strength and/or ductility compared to conventionally grained materials.
As such, they have a strong potential for prospective engineering applications for structural,
high durability components in automobile, aerospace and medical industry. In this work,
different materials such as commercial purity aluminium AA1050, aluminium alloy AA6016,
oxygen free copper, titanium and niobium were processed by the Severe Plastic Deformation
(SPD) technique called Accumulative Roll Bonding (ARB) in order to produce an ultrafine-
grained microstructure and improve the mechanical properties. One of the biggest advantages
of the ARB process in comparison to other SPD methods such as Equal Channel Angular
Pressing (ECAP) or High Pressure Torsion (HPT) is that it is a continuous process, which can
be incorporated in industry to produce large scale UFG metal sheets. During the ARB
process, the metal sheet surfaces are wire brushed in order to remove the oxide layer, stacked
on top of each other and rolled together with a thickness reduction of 50 %. The metals sheets
bond together during rolling and the procedure can then be repeated any number of times. The
material is subjected to very high plastic, shear deformation and the UFG microstructure starts
to develop after approximately 4 ARB cycles.

This study focuses primarily on the ARB processed commercial purity aluminium AA1050
and the technically relevant aluminium alloy AA6016. Ultrafine-grained Al metal sheets are
especially interesting for light weight construction in the automobile industry due to their high
specific strength. In order to qualify the accumulative roll bonding process for these purposes
detailed investigations on microstructural evolution, mechanical properties and sheet metal
forming using bulge tests and cup drawing tests have been carried out and investigated. Sheet
metal joining is one further technologically important issue, which places a challenge upon
UFG aluminium sheet materials. Friction Stir Welding (FSW) was found to be a desirable
joining technique for UFG materials, since it provides excellent mechanical properties and
retains the fine grained microstructure.

During the course of this study, the ARB process was adapted and optimised for every
materials system and the quality of the sheets was improved. The ARB process was
significantly shortened and it became more robust. The deformation during rolling became
more homogeneous, cracking of the edges was eliminated and crack propagation was
suppressed. These factors cumulatively contributed to less material waste during the process.
The quality of the surface was considerably improved and the sheet thickness became more
homogeneous. The contribution of a four-high rolling mill was especially manifested in terms
of the final width of metal sheets. Irrespective of the process parameters, rapid grain
refinement and significantly higher hardness and strength with increasing number of ARB
cycles, in comparison to the CG counterpart were observed for all materials. The ARB
processed materials are microstructurally anisotropic and they develop a characteristic ß-fibre
texture with a Cu component.

UFG Al sheets showed promising sheet metal forming potential under biaxial stress state
conditions and under tension-compression conditions, which occur during cup drawing
experiments. Generally, UFG aluminium samples rolled up to 4 ARB cycles showed a good
compromise between strength, elongation to failure, minimal sheet thinning and earing during
deep drawing cup tests. However, deep drawing cup tests showed that the metal sheet
formability significantly increases at elevated temperatures. Furthermore, the UFG materials
confirmed that their enhanced strain rate sensitivity can be advantageously used in order to
achieve higher formability.

The UFG AA1050 and AA6016 sheets were successfully friction stir welded. Although a
drop in hardness is measured in the nugget for both materials, the hardness is comparable to that of the CG counterparts and is not considered to be a technological limitation. However,
bulge tests and cup drawing tests both confirmed limited formability, which appears to be
governed by the amount of deformation and strength of the nugget.

Even though there is still very limited amount of research regarding the formability and direct
applications of UFG sheets, their potential should not be underestimated. The production of
UFG materials can become commercialised and cost effective, and it could become possible
to control the mechanical properties of materials by processing rather than by alloying. In the
meantime, the big technical potential of ARB processed materials was also recognised by the
aluminium manufacturers. Future interests are closely related to superplastic forming,
accumulative roll bonding of magnesium alloys for lightweight structural components, as well
as accumulative roll bonding of IF-steel. Thus, the innovation potential of the UFG materials
for advanced applications in engineering is high, and the requirements for producing such
materials are becoming more and more economically feasible.



KURZFASSUNG
In den letzten Jahren ist das Interesse an nanokristallinen und ultrafeinkörnigen Werkstoffen
enorm gestiegen. Ultrafeinkörnige (engl.: ultrafine-grained UFG) Werkstoffe mit einer
Korngröße von etwa 100 nm bis 1000 nm besitzen außergewöhnliche und für die technische
Anwendung vielversprechende mechanische Eigenschaften im Vergleich zu Werkstoffen mit
konventioneller Korngröße. Durch die hohe spezifische Festigkeit haben UFG-Werkstoffe
insbesondere im Bereich der Konstruktionswerkstoffe und des Leichtbaus ein großes
Anwendungspotenzial. Im Rahmen dieser Arbeit wurden verschiedene Werkstoffen
konventioneller Korngröße wie technisch reines Aluminium (AA1050), die
Aluminiumlegierung AA6016, hochreines Kupfer, Titan und Niob mit dem sogenannten
kumulativen Walzprozess (engl.: Accumulative Roll Bonding ARB) umgeformt. Mit diesem
Verfahren lassen sich hauptsächlich flächige Bauteile mit ultrafeinkörnigen Mikrostrukturen
erzeugen. Einer der größten Vorteile dieses Prozesses im Vergleicht zu den anderen
Hochverformungsprozessen wie z.B. Equal Channel Angular Pressing (ECAP) oder High
Pressure Torsion (HPT), ist dass er verhältnismäßig leicht in bestehende Walzanlagen
integriert werden kann und so großflächige ultrafeinkörnige Bauteile hergestellt werden
können. Grundlage des ARB-Prozesses ist es, dass ein Blech