MODELLING OF THE EFFECTS OF ENTRAINMENT DEFECTS ON MECHANICAL PROPERTIES IN AL-SI-MG ALLOY CASTINGS By YANG YUE A dissertation submitted to University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Metallurgy and Materials University of Birmingham June 2014 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract Liquid aluminium alloy is highly reactive with the surrounding atmosphere and therefore, surface films, predominantly surface oxide films, easily form on the free surface of the melt. Previous researches have highlighted that surface turbulence in liquid aluminium during the mould-filling process could result in the fold-in of the surface oxide films into the bulk liquid, and this would consequently generate entrainment defects, such as double oxide films and entrapped bubbles in the solidified casting. The formation mechanisms of these defects and their detrimental e↵ects on both mechani- cal properties and reproducibility of properties of casting have been studied over the past two decades. However, the behaviour of entrainment defects in the liquid metal and their evolution during the casting process are still unclear, and the distribution of these defects in casting remains difficult to predict. An algorithm developed by Reilly, named the oxide film entrain- ment model (OFEM), could be used to predict the formation and distribu- tion of entrainment defects in castings. The model was integrated into the computational fluid dynamics (CFD) software FLOW-3D, and applied face normals of free surface and their interaction to capture the entrainment of surface films. In this research, validation and development of the OFEM were carried out. Preliminary simulations were run to investigate the entrainment crite- ria defined in the OFEM and to understand the capability of the model. The initial validation of OFEM used previous reported experimental data on the fatigue and tensile properties of castings, but did not succeed owing to the quality of the data. Then modelling of three common entrainment mech- anisms in fluid flow, namely plunging jet, return wave and rising jet, were conducted, and the predicted defect quantities in the samples were com- pared with the bending strengths of the castings from the Al-7Si-0.4Mg alloy. Directly observation of transient flows in moulds by real-time X- ray radiography showed good correlation between real filling scenario and simulation results. However, the attempt to establish a quantitative rela- tionship between the predicted quantities of the entrainment defects and the mechanical properties of the real castings, were adversely a↵ected by shrinkage porosity found in the castings. A further validation eliminated the influence of shrinkage porosity on the casting properties. The tensile strength of the cast test bars from this validation was compared with ei- ther the number of defects, or the defect concentration within the bars i obtained from the simulation. A general relationship between the mechan- ical strength of the cast test bars and the quantity of estimated defects was apparent. The validation test for OFEM algorithm has been conducted, but strong correlation between the modelling prediction and the experimental results have not been achieved yet. A series of modelling work undertaken in this research highlighted the potential of the method as an indicator of the en- trainment severity in di↵erent mould designs. Comparison of the Weibull moduli obtained from the modelling and experimental results also showed that the model had the potential to predicted the reliability of castings. The e↵ects of di↵erent modelling conditions on the modelling results were discussed, and some useful courses were suggested, such as proper settings of the particle properties should be used, to increase the accuracy of future simulations. This research also assessed the behaviour of entrainment defects in the liquid state and during solidification. The samples that contained entrain- ment defects were scanned using ultra-fast synchrotron X-ray radiography. The defects were directly viewed at both room temperature before melting and in a fully liquid state. The reconstructed images showed three di↵erent pore morphologies, namely entrained pores, tangled double oxide films and closed cracks. The morphology evolution of the entrainment defects were studied, which showed that the oxidation of the internal atmosphere was the main driving force of the pore shape changes, but has less e↵ect on the morphology change of tangled double oxide films. The reconstructed three- dimensional images gave direct evidence of the mechanism of morphological change of defect and the interaction between entrainment defects and mi- croporosity. ii To my family iii Acknowledgements Iwouldliketoexpressmysinceregratitudetobothmysupervisors,Prof. Nick Green, for his kind guidance and encouragement in the first two year of my research, and Dr. Bill Griffiths, for his constructive criticisms and continuous support throughout the course of this work. Although they only shared a fraction of their wisdom, I benefited a great deal from their insight and knowledge. Without their help and great patience, this work could not have been accomplished. IthankthefinancialsupportfromtheSchoolofMetallurgyandMaterials at University of Birmingham for my program, without which I would never start my PhD here. IwishtoacknowledgeDr.CarlReilly,whoinitiatedthemodellingmethod used in this study and introduced it to me, and Dr. Jean-Christophe Gebe- lin, who o↵ered technical assistance for the modelling work; the technicians, Adrian Caden and Peter Craemer in the Foundry, and David Price in the Mechanical Testing Lab, who provided a wealth of experience to facili- tate the experimental work contained within this thesis. Their assistance is critical to this work. I thank Julie Fife in Swiss Light Source (SLS) at Paul Scherrer Institut (PSI) in Switzerland for her help on conducting synchrotron X-ray experiments. IamalsogratefultomycolleaguesinSchoolofMetallurgyandMaterials, especially, Dr. Hang Wang and Dr. Jianglin Huang, for their continuously support both in academics and after-hour life, and friends at University of Birmingham, Mofei Guo, Xi Liu, Guangxiong Wang, Yang Yang and others, their “distractions” made my PhD life in Birmingham varied and colourful. The last mention must go to my family: my father YUE Zhihong and my mother ZHANG Huiping. Their unconditional support and love in this as with every endeavour are what made my success possible. Now and always, they are everything to me. iv Preface This dissertation is submitted for the degree of the Doctor of Philosophy at the University of Birmingham. It describes research carried out in School of Metallurgy and Material Science between October 2009 and September 2013, under the supervision of Dr. W.D. Griffiths and Prof. N.R. Green. Except where appropriately referenced, this work is original and has not been submitted for any other degree, diploma and other qualification. It does not exceed 50,000 words in length. Parts of this dissertation have been published or submitted for publication in: Y. Yue, W.D. Griffiths and N.R. Green, ”Modelling of the e↵ects of • entrainment defects on mechanical properties in Al-Si-Mg alloy”, in Materials Science Forum, Vol.765, pp.225-229, 2013. Y. Yue, W.D. Griffiths, J.L. Fife, and N.R. Green, ”In-Situ Char- • acterization of Entrainment Defects in Liquid Al-Si-Mg Alloy,” in 1 International Conference on 3D Materials Science,2012,pp.131–136. Y. Yue and N.R. Green, ”Modelling of Di↵erent Entrainment Mech- • anisms and Their Influences on the Mechanical Reliability of Al-Si Castings,” in IOP Conference Series: Materials Science and Engi- neering, 2012, vol. 33, paper No.012072 Yang Yue September 2013 Contents List of Figures x List of Tables xxiii 1 Introduction 1 1.1 Objectives of This Work . 2 1.2 Thesis Layout . 3 2 Literature Review 5 2.1 Entrainment Defects . 5 2.1.1 Formation of Entrainment Defects . 8 2.1.2 Evolution of Entrainment Defects . 11 2.1.2.1 FurlingandUnfurling . 11 2.1.2.2 Relationship between Entrainment Defects and Porosity 13 2.1.3 E↵ects on the Mechanical Properties of Castings . 17 2.1.4 Healing of Entrainment Defects . 20 2.2 Computational Modelling of Entrainment Defects . 21 2.2.1 Background . 22 2.2.2 Integrated Modelling of Entrainment Defects . 25 2.2.2.1 Cumulative Entrained Free Surface Area Method . 25 2.2.2.2 Cumulative Surface Scalar Method . 26 2.2.2.3 Air Entrainment Model . 28 2.2.2.4 Dimensionless Number Criterion . 29 2.2.2.5 Bubble Entrainment Model . 34 2.2.3 Modelling of Discrete Defects . 36 2.2.3.1 Modelling of Reoxidation Inclusion in Steel . 37 2.2.3.2 Modelling of the Folding Mechanisms . 39 2.2.3.3 Modelling of Oxide Entrainment . 41 2.2.4 Modelling of Oxide Film Deformation . 45 2.2.5 Validation of Modelling . 47 2.3 X-Ray Tomography of Materials and Defects . 48 2.3.1 Introduction of X-ray Tomography . 49 2.3.1.1 Principles of X-ray Tomography . 49 2.3.1.2 Tomography Mode and Radiation Source. 53 2.3.1.3 Artefacts in X-ray Tomography . 55 vi CONTENTS 2.3.2 The Application of X-ray Tomography . 56 2.3.2.1 3D Visualisation and Image Analysis . 57 2.3.2.2 Quantitative Characterisation . 57 2.3.2.3 In Situ Experiments . 58 2.3.2.4 Simulation of Tomography Data . 59 2.3.2.5 X-ray Tomographical Study of Entrainment Defects .