Quantitative Classification and Assessment of Modification in Hypoeutectic Aluminum-Silicon Alloys

170 Pages·2015·11.51 MB·English

Quantitative Classification and Assessment of Modification in
Hypoeutectic Aluminum-Silicon Alloys

Dissertation
zur Erlangung des Grades
des Doktors der Ingenieurwissenschaften
der Naturwissenschaftlich-Technischen Fakultät III
Chemie, Pharmazie, Bio- und Werkstoffwissenschaften
der Universität des Saarlandes
von

Paulo Eliseo Rossi

Saarbrücken
10.09.2015
Tag des Kolloquiums: 02.12.2015
Dekan: Prof. Dr.-Ing. Dirk Bähre
Berichterstatter: Prof. Dr.-Ing. Frank Mücklich
Prof. Dr. mont. Christian Motz
Vorsitz: Prof. Dr.-Ing. Stefan Diebels
Akad. Mitarbeiter: Dr.-Ing. Frank Aubertin

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“I found that I could say things with color and shapes that I couldn’t say in any other
way.”
Georgia O'Keeffe

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Acknowledgments
First of all, I want to express an enormous gratitude to my supervisor and main advisor,
Prof. Dr.-Ing. Frank Mücklich, for his encouragement, guidance and support.
I am very thankful to Prof. Dr. mont. Christian Motz for co-refereeing this dissertation.
I acknowledge his dedication evaluating this work.
I want to thank my group leader, Michael Engstler, for his efficient guidance and
support. I owe special thanks to Kai Rochlus, Adrian Thome and Nils Harste for their
collaboration in preparing and imaging samples and acquiring and computing data. I am
very thankful to Dr. Flavio Soldera, Dr. Sebastián Suarez, Nicolás Souza, Mathias Linz,
Federico Miguel, Federico Lasserre, Jenifer Barrirero, Simon Staudacher, Anastasia
Kruglova and Dr. Dirk Schnubel (Fa. Nemak) for the contributions in several aspects of
this work including proofreading, discussions and advice. I thank all my colleges and
friends from the Chair of Functional Materials for the wonderful work atmosphere and
support.
The present work has been performed within the framework of a project about the
influence of trace elements on Al-Si-Mg alloys (Project AiF-no. 17204 N). The project
was carried out together with researchers from companies or research centers
specialized in aluminum casting alloys. I am very grateful for the disposition to open
discussion and efficient collaboration of the project partners. I owe special thanks to
those partners who took a round robin test, which was an essential part of this work. I
acknowledge the efficient collaboration of the Institute of Casting Technology in
Düsseldorf, Germany (Institut für Gießereitechnik, IfG) and the Foundry Institute
(Gießerei-Institut, GI) at the RWTH Aachen University (Rheinisch-Westfälische
Technische Hochschule Aachen, RWTH Aachen) in Aachen, Germany, for the
cooperation. In particular, I thank Thomas Herfurth, from the IfG, for providing
samples, micrographs and measurement of mechanical properties and Veronika Groten,
from the GI, for contributing with samples and micrographs.
I acknowledge the financial support of said project (Project AiF-no. 17204 N) from the
German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) and the
German Federation of Industrial Research Associations (Arbeitsgemeinschaft
industrieller Forschungs-vereinigungen, AiF).
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Part of this work was carried out in cooperation with the Metals Division at the
Bariloche Atomic Centre, Bariloche, Argentina (Centro Atómico Bariloche, CAB). I
gratefully acknowledge Dr. Mario Moreno from the CAB for providing measurements
of mechanical properties.
I am very grateful to the German Academic Exchange Service (Deutscher
Akademischer Austauschdienst, DAAD) and the Roberto Rocca Education Program for
scholarships.
I am very thankful to Jessica Hilschmann, my family and friends for the constant
support, love and company.
And thank you, dear reader, for your interest. Feel free to contact me.
Paulo Rossi
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Abstract
Aluminum-silicon cast alloys, as those considered in this study (Al-7%Si and Al-7%Si-
0.3%Mg alloys), are widely used to manufacture technical cast products, e.g. in the
automotive industry. The mechanical properties of the castings depend on the eutectic
microstructure which, in turn, depends on the chemical composition and manufacturing
processes. A transition from a coarse eutectic structure to a finer one is called
modification. The modification can be induced, for instance, by increasing the
solidification cooling rate, by adding strontium and/or by increasing the alloy purity.
The assessment of the modification relies usually on visual comparison, thus being
ambiguous. This work suggests innovative tools to evaluate the microstructure based on
quantitative image data and it develops an objective method to assess the modification.
To ensure the representativeness of this method, it has been developed in collaboration
with thirteen experts from different research institutions and companies. The assessment
tools are applied to study the quantitative correlation between process settings,
microstructures and casting properties, which are of major importance for the research
and development of the studied alloys.

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Kurzfassung
Aluminium-Silizium-Gusslegierungen, wie Al-7%Si und Al-7%Si-0.3%Mg Legie-
rungen, die in dieser Arbeit untersucht werden, finden häufig Anwendung bei der
Herstellung von technischen Gussprodukten, z. B., für die Automobilindustrie. Die
mechanischen Eigenschaften des Gussteils hängen von der eutektischen Mikrostruktur
ab, die wiederum abhängig von der chemischen Zusammensetzung und den
Herstellungsverfahren ist. Eine Gefügeveränderung der eutektischen Struktur von einer
groben hin zu einer feineren, wird Veredelung gennant. Die Veredelung kann
hervorgerufen werden, z. B., durch die Erhöhung der Abkühlgeschwindigkeit bei der
Erstarrung, durch Zusatz von Strontium und/oder durch zunehmende Reinheit der
Legierung. Üblicherweise wird die Auswertung der Veredelung durch visuellen
Vergleich vorgenommen und ist dadurch subjektiv. Der vorliegende Beitrag schlägt
eine innovative Lösung vor, um die Mikrostruktur auf der Grundlage quantitativer
Bilddaten auszuwerten, und somit die Veredelung objektiv zu beurteilen. Um
Repräsentanz gewährleisten zu können, wurde die Studie unter Einbeziehung von
dreizehn Experten aus verschiedenen Forschungsinstitutionen und
Industrieunternehmen entwickelt. Die objektive Beurteilung wurde eingesetzt, um den
quantitativen Zusammenhang zwischen Prozesseinstellungen, Mikrostruktur und
mechanischen Eigenschaften zu studieren, welcher für die Forschung und Entwicklung
der Legierungen von großer Bedeutung ist.

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Contents:
Introduction ..................................................................................................................... 1
Organization of this text ................................................................................................. 2
Nomenclature used in this work .................................................................................... 3
1. STATE OF THE ART ............................................................................................. 4
1.1. Aluminum-Silicon alloys ..................................................................................... 4
1.2. Modification ......................................................................................................... 4
1.2.1. Introduction to modification ........................................................................... 4
1.2.2. Theories explaining the modification ........................................................... 11
1.2.3. The influence of modification on the mechanical properties ....................... 15
1.2.4. Available methods to assess the modification .............................................. 18
1.2.4.1. Subjective methods of classification ......................................................... 18
1.2.4.2. Objective methods of classification .......................................................... 19
1.3. Quantitative Microstructure Parameters ........................................................ 20
1.3.1. Dendrite parameters ...................................................................................... 21
1.3.2. 3D Basic particle parameters ........................................................................ 21
1.3.3. Density of the particle features and stereological equations ........................ 23
1.3.4. The Euler number ......................................................................................... 24
1.3.5. Further size and shape parameters ................................................................ 26
1.4. Quantification of Homogeneity ......................................................................... 27
1.5. Microstructure classification ............................................................................ 31
1.6. 3D microstructure representation .................................................................... 32
2. ADDRESSED PROBLEM AND GOAL OF THIS WORK .............................. 36
2.1. Problem ............................................................................................................ 36
2.2. Objective .......................................................................................................... 39
2.3. Strategy ............................................................................................................ 39
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3. METHODOLOGY ................................................................................................ 40
3.1. Description of samples ....................................................................................... 40
3.2. Microstructure imaging and image data ......................................................... 43
3.3. Subjective Assessment: The Round Robin Test .............................................. 44
3.4. Quantitative microstructure parameters ......................................................... 45
3.5. Quantitative data analysis ................................................................................. 46
3.6. Mechanical properties ....................................................................................... 49
4. RESULTS AND DISCUSSION ............................................................................ 52
4.1. Homogeneity ....................................................................................................... 52
4.1.1. Applying the Lorenz curve and the Gini coefficient to quantify the
homogeneity ................................................................................................................ 52
4.1.2. The region homogeneity ............................................................................... 54
4.1.3. The object homogeneity ............................................................................... 56
4.1.4. Constructed homogeneities .......................................................................... 57
4.1.5. Remarks of the section ................................................................................. 58
4.2. Quantitative classification and assessment of modification ........................... 59
4.2.1. Subjective assessment: The round robin test ................................................ 59
4.2.2. Objective data from the micrographs ........................................................... 65
4.2.3. Analysis ........................................................................................................ 67
4.2.3.1. Particle classification and Percentage of Modification (PM) ............... 67
4.2.3.2. Microstructure classification ................................................................. 75
4.2.3.3. Objective- vs. subjective microstructure classification ......................... 78
4.2.3.4. Degree of modification (M) .................................................................. 81
4.2.3.5. Comparison of M with the subjective assessment ................................ 82
4.2.3.6. Summary of results, including the Modification Level (ML) ............... 83
4.2.4. Remarks of the section ................................................................................. 86
4.3. Comparison between Sr- and quench-modification ....................................... 87
4.3.1. Comparison in 2D ......................................................................................... 87
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4.3.2. Comparison in 3D ......................................................................................... 88
4.3.3. Remarks of the section ................................................................................. 90
4.4. Correlation between microstructure, process settings and mechanical
properties ....................................................................................................................... 91
4.4.1. Small Punch Test (SPT) ............................................................................... 91
4.4.2. Impact properties .......................................................................................... 93
4.4.3. Remarks of the section ............................................................................... 100
5. CONCLUSIONS AND OUTLOOK .................................................................. 102
Appendix: List of micrographs .................................................................................. 106
References .................................................................................................................... 147
Curriculum Vitae ........................................................................................................ 159

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