Design of Al-Mg-Si-Mn alloys with Zn, Cr and Sc additions with unique strengthening response Design von Al-Mg-Si-Mn-Legierungen mit Zn-, Cr- und Sc- Zusätzen mit besonderem Verfestigungsverhalten Der Technischen Fakultät / der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades DOKTOR‐INGENIEUR vorgelegt von Oleksandr Trudonoshyn, M.Sc. aus Bila Tserkva, Ukraine 1 Als Dissertation genehmigt von der technischen Fakultät der Friedrich‐Alexander Universität Erlangen‐Nürnberg Tag der mündlichen Prüfung: 15.06.2020 Vorsitzender des Promotionsorgans: Prof. Dr.-Ing. habil. Andreas Paul Fröba Gutachter/in: Prof. Dr.‐Ing. habil. Carolin Körner Prof. Dr.‐Ing. Karsten Durst ii ACKNOWLEDGMENTS First of all I would like to thank Prof. Dr.-Ing. habil Carolin Körner for giving me an opportunity to provide current research under her supervision and for the scientific freedom. It is a great honor to work under her supervision. I would also like to thank Peter Randelzhofer for the comprehensive help in all areas of my life during the project period. I would like to express my gratitude to my colleagues from WTM FAU, for creating a great friendly working environment, their technical support and helps. I would also like to thank Nicklas Volz from the Department of Materials Science & Engineering (WW1) University Erlangen- Nürnberg for the help with the TEM investigations and Sebastian Rehm for the help with the partly automated image analysis. I am grateful to Prof. Dr.-Ing. Karsten Durst (Head of Physical Metallurgy Department, Institute of Materials Science, TU Darmstadt) for the willing and motivated second appraisal, as well as his expert discussion contributions to the dissertation. I would also like to thank Prof. Dr. rer. nat. Mathias Göken for the chairmanship of the examination committee. Many thanks to my first supervisor Prof. K. Mykhalenkov, who inspired me to plunge into the world of science as well as all detractors, who motivated me to work harder. Finally, I owe a deep sense of gratitude to my family (N. Trudonoshyna, I. Trudonoshyn, A. Trudonoshyn), all my friends (especially O. Kalashnikova, K. Pryhornytska, Plokhotniuk bros., the Nykonenko family, T. Nekrasov, O. Vergeles, the Vashchuk family, the Petryna-Druzhchenko family, captain Baranov, E. Gershevich, A. Ryabinin, I. BUzhanska) for their constant encouragement throughout my research period. Current research have been founded by German Academic Exchange Service (DAAD) and was done in cooperation with Olena Prach (PhD student from TU-Darmstadt, DAAD scholarship holder). I would like also to thank Olena Prach for the discussion and comprehensive help not only in case of the research project but throughout the entire period of our friendship. iii iv ABSTRACT An excellent combination of the properties of Al, high strength-to-weight ratio, good formability, good electrical mass conductivity, unique corrosion behavior, and recycling potential make it the essential material for different fields of application. The growing demand for more fuel-efficient and ecological vehicles to reduce energy consumption and air pollution is a challenge for the transport sector. Al is the second most used material of the total weight of the car. Each 1 kg of Al is able to replace about 2 kg of steel or cast iron, more and more types of car parts and components are produced from Al. The automotive industry in Europe has tripled the average amount of Al used in cars during the last three decades. A unique feature of Al alloys is that they can be cast by all known casting technologies. The high pressure die casting (HPDC) is the most useful casting technique; about 50% of the total amount of the castings from light alloys are produced by this method. The aluminium-intensive car body structures have a great demand of the thin-wall HPDC structural parts. However, the currently available HPDC aluminium alloys do not meet all the requirements of car bodies. First of all, the most commonly used aluminium alloys have insufficient levels of ductility that is essential for joining casting parts with sheets and extruded parts. Basically, the consumer properties of HPDC alloys are determined by alloy composition, defect levels and microstructure. Al-Mg-Si alloys are well known as alloys capable of providing an excellent combination of high strength and high ductility levels. The Al-Mg-Si system always attracts the attention of researchers since its solidification and precipitation processes are very complex and sensitive to chemical composition. However, the existing studies on the Al-Mg-Si alloys are mainly focused on the wrought alloys (with low Mg and Si content) and hyper-eutectic casting alloys. Therefore, the development of high-strength, high-ductility Al-Mg-Si based alloys for the HPDC process can be very valuable for improving the quality of automotive components. The present study was carried out to the alloy design for the HPDC process in order to satisfy the requirement of mechanical properties, in particular, ductility for the application in automotive body structure. The effects of Sc, Cr and Zn on the solidification and microstructural evolution and the mechanical properties of hypoeutectic Al-5.7Mg-2.6Si-0.6Mn base alloy have been investigated by the combination of thermodynamic calculations and the experimental validation. A comprehensive literature review of the features of the structure and mechanical properties of commercial Al-Mg-Si wrought and cast alloys is given in Chapter 1. In this chapter also an extensive review of the strengthening methods and especially of the precipitation strengthening as the main one for Al-Mg-Si alloys is represented. Fundamental differences v between cast and wrought alloys of the Al-Mg-Si system that allow additional alloying of the cast Al-Mg-Si alloys to enhance the precipitation strengthening effect were established. In addition to a literature review on the structure and properties of the studied system, Chapter 2 provides an overview of the analysis of the Al-Mg-Si phase diagram, as well as the possible effects of various alloying elements and their concentrations on the studied system. To select alloy compositions for research, phase diagrams and also solidification curves of alloys were calculated by Thermo-Calc software with TCAl2:Al-alloys v2.1 database. Chapter 4 deals with the detailed characterization of the microstructure with scanning electron microscopy, the structure of α-Al dendrites with transmission electron microscopy and common (hardness, tensile test) and local (microhardness) mechanical properties of the studied alloys in as-cast state. Chapter 4 describes the effects of Sc, Cr and Zn on the solidification behavior, microstructural changes, the relationship between the mechanical properties and the microstructure. In addition to the formation of new intermetallics, the Zn addition leads to the formation of nanosized strengthening precipitates in the α-Al dendrites. Zn-containing precipitates can form even in the as-cast state that promote significant strengthening effects. Chapter 5 deals with the detailed characterization of the alloys after heat treatment. Two types of heat treatment were applied to the studied alloys: artificial aging from as-cast state (one- step heat treatment) and solution treatment with subsequent artificial aging (two-step heat treatment). Cr-containing alloys did not show any significant differences in the mechanical properties in comparision to the base alloy. Sc-containing alloys showed the most prominent increase in strength after one-step heat treatment. Zn-containing alloys showed the most interesting combination of the properties after two-step heat treatment. Differences in the character of changes in the mechanical properties were explained based on the TEM results and the strengthening mechanisms in the designed alloys. It has been shown that in the states in which alloys have the highest values of hardness and strength, the structure of α-Al dendrites contains a significant amount of nanoscale precipitates. Current research was founded by German Academic Exchange Service (DAAD) and was done in cooperation with TU-Darmstadt. vi ZUSAMMENFASSUNG Die hervorragende Eigenschaftskombination von Al, hohe spezifische Festigkeit, gute Verformbarkeit, gute elektrische Leitfähigkeit, einzigartiges Korrosionsverhalten und das Recyclingpotenzial machen es zum unverzichtbaren Material für viele Anwendungsbereiche. Die wachsende Nachfrage nach effizienteren und umweltfreundlicheren Fahrzeugen zur Reduzierung des Energieverbrauchs und der Luftverschmutzung ist eine wesentliche Herausforderung für den Industriebereich Automotive. Al ist das am zweithäufigsten verwendete Material gemessen am Gesamtgewichts des Autosmobils. Je 1 kg Al können etwa 2 kg Stahl oder Gusseisen ersetzt werden, mehr und mehr Autoteile und -komponenten werden aus Al hergestellt. Ein einzigartiges Merkmal von Al-Legierungen ist, dass sie mit allen bekannten Gusstechnologien gegossen werden können. Druckgießen (HPDC) ist die gebräuchlichste Gießtechnik, mit diesem Verfahren werden ca. 50% der Gesamtmenge der Leichtmetall-Gussteile hergestellt. Für aluminiumintensive Karosseriestrukturen besteht ein hoher Bedarf an dünnwandigen Druckguss-Bauteilen. Die derzeit erhältlichen Aluminium-Druckgusslegierungen erfüllen jedoch nicht alle Anforderungen für Karosseriebauteile. Vor allem weisen die am häufigsten verwendeten Aluminiumlegierungen ein unzureichendes Maß an Duktilität auf, was aber für das Verbinden von Gussteilen