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Inclusions and Hydrogen and Their Effects on the Quality of Direct Chill Cast and Flat Rolled Aluminium Alloys for Aerospace Applications

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Inclusions and Hydrogen and Their Effects on the Quality of Direct Chill Cast and Flat Rolled Aluminium Alloys for Aerospace Applications Inclusions and Hydrogen and Their Effects on the Quality of Direct Chill Cast and Flat Rolled Aluminium Alloys for Aerospace Applications By Alexander James Gerrard A thesis submitted to the college of Engineering and Physical Sciences of The University of Birmingham For the degree of Doctor of Engineering School of Metallurgy and Materials College of Engineering and Physical Sciences University of Birmingham Birmingham B15 2TT United Kingdom September 2014 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties.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 Flat rolled Al alloys manufactured using the Direct Chill (DC) casting process are used in safety critical applications and therefore need to be of the highest possible quality. The presence of inclusions and pores can be damaging to the mechanical properties, so the liquid metal is cleaned using a mixture of inert Ar and reactive Cl gases before casting. These gases are bubbled up through the molten metal and remove inclusions, (such as oxides), by agglomeration and floatation, and dissolved H, (that is responsible for porosity within in the casting), by diffusion and floatation of the gas out of the melt. This project is comprised of three sections, with the central theme aimed at better understanding the formation and control of inclusions and porosity in DC cast and hot rolled Al alloys. The first section explores the aetiology of defects in DC cast and hot rolled Al plate, found using ultrasound. Inclusions extracted from 7050 alloy plate, (typically 2.3% Cu, 2.5% Mg, 6.2% Zn, 0.12% Zr), were extracted and examined using scanning electron microscopy (SEM). The inclusions were composed of two phases. A second type of defect found in thin gauge, (<45 mm), 7475 alloy plate, (typically 1.5% Cu, 2.3% Mg, 6.7% Zn, 0.21% Cr), was established to be a coarse grain structure, caused by excessive widening of large cross section ingots during hot rolling. The second section aimed to provide a better understanding of the role of Cl on degassing. Ar and Ar and Cl containing gas bubbles were trapped in solidifying Al alloys. Gases were extracted using a mass spectrometer and the composition was established to be a mixture of H2, N2 and H2O. The metal-gas interface was then examined using Scanning Electron Microscopy (SEM). The effects of Cl on the degassing performance of a Spinning Nozzle Inert Floatation (SNIF) degassing unit were then approached statistically using ALSCAN. While different 7xxx alloys appeared to be influenced by Cl differently, with average improvement or reduction in degassing efficiency ranging from +8 to -5%, the degassing of 7xxx alloys was, on the whole, not significantly affected by the presence of Cl. Finally, 20 kg melts of Al and Al alloys were doped with H and subsequently degassed, and the reduction in H over time was measured using ALSPEKH, over a range of 0.35 ml/100g to 0.15 ml/100g. The degassing rate of a 7010 alloy (typically 1.7% Cu, 2.3% Mg, 5.2% Zn, 0.13% Zr) appeared to be reduced the most when Cl was present, compared to 5083 alloy, (typically 4.5% Mg, 0.7% Mn, 0.15 Cr), and pure Al. In the third section the LECO H determination device was used to degas Al specimens to very low levels, (<0.005 ppm H), in order to simulate the H absorption into double oxide film defects. Such defects have been proposed to act as reservoirs into which H can diffuse and promote the formation of H gas porosity. Degassed specimens were melted under air or N2 atmosphere, so that when cracks formed in the surface oxide layer either Al2O3 or AlN formed. After subsequent exposure to pure H2 gas the specimens that formed AlN were found to have absorbed more H than those heated in air, because the AlN was porous compared to fully dense alumina. The research and work presented within therefore provides a better understanding of: the formation of specific defects in wrought Al alloy plate, the influence of Cl on the degassing of Al alloys, and the process of diffusion of H into double oxide film defects as they react with the surrounding melt. I For my family; Linda, Tony and Michael. II Acknowledgements I wish to express by warm and sincere thanks to my friends and colleagues at the University of Birmingham and Alcoa Europe for their enthusiasm, motivation, friendship and support throughout my EngD. It has been both challenging and rewarding, and has undoubtedly changed my life for the better. I would like to acknowledge and thank the EPSRC and Alcoa for financial support, and for providing sponsorship, access to equipment and training and the opportunity to work in a thrilling technical and industrial environment. I would like to acknowledge the teaching and administration staff at the University of Birmingham, and the various technicians who have assisted me at various points throughout my studies. In particular I would like to thank Adrian Caden for his technical knowledge, skills and assistance with experiments at the University of Birmingham. I would like to thank fellow PhD students Elizabeth Hinton and Richard Sheridan for taking the time to help with experimental work at various points throughout this project, and I would also like to thank my numerous colleagues in the Remelt, Plate, Quality and Technical departments at Alcoa Europe, in particular, my industrial supervisors Neil Merill and Alex Morris for their guidance and support. I would also like to thank my friends and family for keeping me (more or less) sane over the last five years. Without them I simply would not have made it. Finally I would like to express my deep and sincere gratitude to my supervisor, Bill Griffiths, for giving me this opportunity. He has provided guidance, knowledge, resource, motivation and without his support it would not have been possible to complete this work. It has been an honour to work under his supervision. III Table of Contents Table of Contents Abstract .................................................................................................................................................... I Acknowledgements ................................................................................................................................ III Table of Contents ................................................................................................................................... IV List of Figures ....................................................................................................................................... VIII List of Tables ....................................................................................................................................... XVII Chapter 1 Introduction ........................................................................................................................... 1 Chapter 2 literature review ................................................................................................................... 10 2.1 Wrought aluminium alloys: ......................................................................................................... 10 2.1.1 Direct chill (DC) casting of aluminium alloys: ....................................................................... 11 2.2 Molten metal quality: ................................................................................................................. 14 2.2.1 Solubility of hydrogen in aluminium and its alloys .............................................................. 15 2.2.2 Hydrogen in liquid aluminium .............................................................................................. 21 2.2.3 Oxidation of aluminium alloys ............................................................................................. 24 2.3 Porosity in aluminium castings ................................................................................................... 31 2.3.1 Hydrogen gas porosity ......................................................................................................... 31 2.3.2 Nucleation and growth of gas porosity ................................................................................ 42 2.4 Inclusions in aluminium alloys .................................................................................................... 45 2.4.1 Double oxide film defects .................................................................................................... 51 2.5 Hydrogen and inclusion removal from molten aluminium alloys ............................................... 60 2.5.1 Hydrogen removal ............................................................................................................... 61 2.5.2 Bubbles and solid phases in molten aluminium .................................................................. 65 2.5.3 Removal of inclusions and undesirable metallic elements from aluminium. ...................... 69 2.5.4 Filtration ............................................................................................................................... 70 2.5.5
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