Licentiate Thesis On the Influence of Si on Anodising and Mechanical Properties of Cast Aluminium Alloys Baiwei Zhu Jönköping University School of Engineering Dissertation Series No. 019 • 2017 Licentiate Thesis in Materials and Manufacturing On the Influence of Si on Anodising and Mechanical Properties of Cast Aluminium Alloys Dissertation Series No. 019 © 2017 Baiwei Zhu Published by School of Engineering, Jönköping University P.O. Box 1026 SE-551 11 Jönköping Tel. +46 36 10 10 00 www.ju.se Printed by Ineko AB 2017 ISBN 978-91-87289-20-0 ABSTRACT The combination of two cost-effective processes, i.e. casting and anodising, would be an interest for the aluminium component applications. However, there are some obstacles in the application of anodising on cast Al alloys. The challenges mostly relate to the alloying elements especially Si and the surface quality. With the development of casting process, cast aluminium alloys with low Si content can be casted, and a complex geometry component with reasonably good surface finish can be achieved. This study aims to identify the influence of Si on anodising and mechanical properties of Al-Si alloys. In this study, six Al-Si alloys with three different Si level and two different Sr level were investigated. Sr acts as a modifier to change the morphology of Si particles. The directional solidification technology was used to vary the microstructure coarseness by controlling the cooling rate to study the influence of Si level, Si particle morphology and cooling rate on mechanical properties, oxide layer formation and corrosion protection performance in cast Al-Si alloys. This study has observed that Si has a significant influence on anodising. During anodising, Si particles are anodised at a lower rate than the Al phase. The presence of Si particles in eutectic phase make the oxide layer locally thinner and more defected due to the low oxide growth rate in eutectic phase. This study observed the presence of residual metallic Al phase beneath or between Si particles. Due to their presence and their geometry, Al can be shielded by Si particles and prevented from oxidation. Si particles also act as a key role in the corrosion protection of oxide layer in Al-Si alloys. The corrosion attack propagates along Si particles as well as oxide defects to the Al substrate. It is found that the morphology of Si particles has a significant influence on the oxide layer formation and corrosion protection performance of the oxide layer on cast Al- Si alloys. A substantially improvement the corrosion resistance of anodised layer on Al-Si alloys is attributed to the morphology change from interconnected flakes to disconnected Si fibres when Sr is added, with less oxide defects and better oxide distribution. The Si level governs the mechanical properties of Al-Si based alloys. An increase of Si content in Al alloys improves the mechanical properties such as ultimate tensile and yield strength as well as hardness of the materials, but decreases the ductility. However, an increase of Si level in Al alloys decreases the thickness of oxide layer, and thereby, the corrosion protection of the oxide layer is deteriorated. Keywords: Al-Si based casting alloys, anodising, corrosion protection, mechanical properties SAMMANFATTNING Kombinationen av två kostnadseffektiva processer, gjutning och anodisering, är av intresse för tillämpning på aluminiumkomponenter. Det finns dock hinder för tillämpning av anodisering på gjutna aluminiumlegeringar. Utmaningarna relaterar till effekten av legeringselement, i synnerhet Si, och komponentens ytkvalité. Med utvecklingen av gjutprocesser kan aluminiumlegeringar med lågt Si-innehåll gjutas, och komponenter med komplex geometri med förbättrad ytkvalité kan uppnås. Denna studie syftar till att identifiera påverkan av Si på anodisering och mekaniska egenskaper hos Al-Si-baserade legeringar. I denna studie undersöktes sex Al-Si-legeringar med tre nivåer av Si och två nivåer av Sr. Tillsatser av Sr leder till modifikation av morfologin hos Si-partiklar. Med hjälp av tekniken ”riktad stelning” varierades stelningshastigheten för att studera sambanden mellan halten och morfologin av Si, mikrostrukturens grovlekoch dess inverkan på mekaniska egenskaper, samt oxidskiktets bildning och korrosionsbeständighet hos gjutna Al-Si-legeringar. Denna studie visar att Si har ett betydande inflytande på anodisering r. Under anodisering, anodiseras Si-partiklar i en lägre takt än Al-fasen. Närvaron av Si-partiklar i eutektisk fas bidrar till ett lokalt förtunnat oxidskikt med fler defekter på grund av den låga oxidtillväxthastigheten i eutektisk fas. Denna studie observerade förekomsten av kvarvarande metalliska Al fasen under eller mellan Si-partiklar. På grund av Si-partiklarnas närvaro och geometri, kan Al skärmas från oxidation. Si-partiklar har även en nyckelroll i korrosionsskydd av oxidskiktet i Al-Si-legeringar. Korrosionsangreppet propagerar längs Si-partiklar samt oxiddefekter till Al- substrat. Morfologin av Si-partiklar har funnits ha en betydande inverkan på oxidskiktets bildning och korrosionsskydd hos gjutna Al-Si-legeringar. En väsentlig förbättring av korrosionsbeständigheten i anodiserat skikt hos Al-Si-legeringar tillskrivs förändring av kiselns morfologi från sammankopplade flingor till osammanhängande fibrer när Sr tillsätts, med mindre oxiddefekter och bättre fördelning av oxid. Si halten påverkar demekaniska egenskaperna hos Al-Si-baserade legeringar. En ökning av Si- innehållet i Al-legeringar förbättrar de mekaniska egenskaperna såsom brott och sträckgräns samt hårdhet hos materialen, men minskar duktiliteten. En ökning med Si halt i Al-legeringar minskar dessvärre tjockleken hos oxidskiktet, och därigenom, försämrar oxidskiktets korrosionsskydd. Nyckelord: Al-Si-baserade gjutningslegeringar, anodisering, korrosionsskydd, mekaniska egenskaper ACKNOWLEDGEMENTS I would like to express my sincere appreciation to: My supervisor, Associate Prof. Caterina Zanella, for her continuous support and guidance of my research and study for her immense knowledge, patience and motivation. Prof. Salem Seifeddine and Prof. Peter Leisner, my supervisors, for the valuable comments and advices and for giving me the opportunity to have a fun journey. Prof. Anders E.W. Jarfors for helpful discussion and useful comments. Associate Prof. Per O.Å. Persson and Associate Nils-Eric Andersson, for helping me with microstructure analysis and useful comments. Dr Michele Fedel and Prof. Flavio Deflorian, for helpful and useful comments. Dr Ehsan Ghassemali and Jörgen Bloom, for helping me with the microscopy analysis. Toni Bogdanoff, Esbjörn Ollas, Peter Gunnarsson and Lars Johansson, for helping me with the experimental works. The KK-stiftelsen (The Knowledge Foundation), for financial support. The industrial partners involved in the RheoCal project, COMPtech AB and Ahlins I HABO AB, for good collaboration. All my colleagues and friends in the Department of Materials and Manufacturing, Jönköping University, for creating an excellent working environment and for all fun we have had in these two and half years. Finally, I would like to gratefully and sincerely thank my family, for providing me with support, patience and love to push myself to a higher level. Baiwei Zhu (ᵡ᷿፤) Jönköping 2017 SUPPLEMENTS The following supplements constitute the basis of this thesis: Supplement I B. Zhu, P. Leisner, S. Seifeddine, A.E.W. Jarfors; Influence of Si and cooling rate on microstructure and mechanical properties of Al-Si-Mg cast alloys. Presented in ASST 2015, May 17th-21st, Madeira, Portugal. Published in Surface and Interface Analysis, 2016, 48: pp. 861-869. B. Zhu was the main author. P. Leisner, S. Seifeddine, A.E.W. Anders contributed with advice regarding the work. Supplement II B. Zhu, S. Seifeddine, P.O.Å. Persson, A.E.W. Jarfors, P. Leisner, C. Zanella; A study of formation and growth of the anodised surface layer on cast Al-Si based on different analytical techniques. Presented in eastForum 2015, Jun 25th-26th, Lund, Sweden. Published Materials and Design, 2016, 101: pp. 254-262. B. Zhu was the main author. P.O.Å assisted with transmission electron microscopy work. S. Seifeddine, A.E.W. Anders, P. Leisner and C. Zanella contributed with advice regarding the work. Supplement III B. Zhu, M. Fedel, N-E. Andersson, P. Leisner, F. Deflorian, C. Zanella; Effect of Si content and morphology on corrosion resistance of anodised cast Al-Si alloys. Submitted to the Journal of Electrochemical Society B. Zhu was the main author. M. Fedel assisted with electrochemical impedance spectroscopy work. N-E. Andersson assisted with focused ion beam-scanning electron microscopy work. P. Leisner and F. Deflorian contributed with advice regarding the work. C. Zanella contributed with results analysis and advice regarding the work. TABLE OF CONTENTS CHAPTER 1: INTRODUCTION ............................................................................................................................. 1 1.1 BACKGROUND ................................................................................................................................................................................... 1 1.2 CAST ALUMINIUM ALLOYS ......................................................................................................................................................... 1 1.3 ANODISING OF CAST ALUMINIUM ALLOYS ....................................................................................................................... 4