Additive Manufacturing of Aluminium-Metal Matrix Composite developed through Mechanical Alloying OMOTOYOSI HELEN FAMODIMU (B.Eng., MSc.) A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy September 2016 This work or any part thereof has not previously been presented in any form to the University or to any other body whether for the purposes of assessment, publication or for any other purpose (unless otherwise indicated). Save for any express acknowledgements, references and/or bibliographies cited in the work, I confirm that the intellectual content of the work is the result of my own efforts and of no other person. The right of Omotoyosi H. Famodimu to be identified as author of this work is asserted in accordance with ss.77 and 78 of the Copyright, Designs and Patents Act 1988. At this date, copyright is owned by the author. Signature………………………… Date………………………………. 1 Abstract Laser melting of aluminium alloy - AlSi10Mg has increasingly been used to create specialised products in aerospace and automotive applications. However, research on utilising laser melting of Aluminium matrix composites in replacing specialised parts have been slow on the uptake. This has been attributed to the complexity of the laser melting process, metal/ceramic feedstock for the process and the reaction of the feedstock material to the laser. Thus an understanding of the process, material microstructure and mechanical properties is important for its adoption as a manufacturing route of Aluminium Metal Matrix Composites. The effect of the processing parameters (time and speed) on embedding the Silicon Carbide onto the surface of the AlSi10Mg alloy was initially investigated in Phase 1 and 2 of the research. The particle shape and maximum particle size for each milling time and speed was analysed in determining a suitable starting powder for the Laser Melting phase. An ideal shape and size for the composite powder was obtained at 500 rev/min when milled for 20 mins. The effects of several parameters of the Laser Melting process on the mechanical blended composite were investigated. Single track formations of the matrix alloy, 5% Aluminium Metal Matrix Composites and 10% Aluminium Metal Matrix Composites were studied for their reaction to the laser melting in Phase 3. Subsequently in Phase 4, density blocks were studied at different scan speeds and step-over for surface roughness, relative density and porosity. These were utilised in determining a process window to fabricate near fully dense components. Phase 5 of the research focused on microstructural and mechanical properties of the laser melted matrix alloy using the normal parameters for the matrix alloy and the modified LM parameters for the composite powders. Test coupons were built in one orientation and some coupons were heat-treated to initiate precipitation-hardening intermetallics in the matrix and composite. This study investigates the suitability of the mechanical alloying as a novel method of producing feedstock material for the LM process. This research further explores the interaction of the composite powders with the laser until suitable process parameters were obtained. Furthermore, the fractography, mechanical and microstructural evolution of the Al/SiC composite, with different percentage volume reinforcement manufactured by the LM and subsequently heat treated, was explored for the first time. 2 Table of Contents Additive Manufacturing of Aluminium-Metal Matrix Composite developed through Mechanical Alloying ............................................................................................................................................ 1 Abstract ........................................................................................................................................... 2 Table of Contents ............................................................................................................................ 3 Acknowledgements .......................................................................................................................... 8 Conferences ..................................................................................................................................... 9 List of Acronyms ........................................................................................................................... 10 List of Definitions .......................................................................................................................... 11 Chapter One .................................................................................................................................. 12 Introduction ................................................................................................................................... 12 1.1 Introduction ................................................................................................................... 12 1.2 Research Justification .................................................................................................... 13 1.3 Aim and Objectives ........................................................................................................ 13 1.4 Overview of the thesis .................................................................................................... 14 Chapter Two .................................................................................................................................. 15 Literature Review of Additive Manufacturing ............................................................................. 15 2.1 Introduction ................................................................................................................... 15 2.1.1 Additive Manufacturing for Metal Systems .......................................................... 15 2.1.2 Advantages of Additive Manufacturing for Metal Systems .................................. 16 2.1.3 Disadvantages of Additive Manufacturing ............................................................ 17 2.2 Laser Technology in Additive Manufacturing - Powder Bed Systems ......................... 18 2.2.1 Laser Melting (LM) of metal powders ................................................................... 19 2.2.2 Laser Melting Process in Powder Bed Systems ..................................................... 19 2.2.3 Laser Melting Parameters ...................................................................................... 21 2.3 Selective Laser Melting/Laser Melting for Metallic alloys and Metal Matrix Composites (MMCs) ...................................................................................................................................... 25 2.3.1 Material /Powder properties for Metallic Alloys ................................................... 26 2.3.2 Process optimisation and Track development ....................................................... 26 3 2.3.3 Selective Laser Sintering/Selective Laser Melting of Metal Matrix Composites .. 28 2.4 Summary and Critique of the Chapter .......................................................................... 29 Chapter Three ............................................................................................................................... 31 Materials ........................................................................................................................................ 31 3.1 Introduction to Materials utilised in research ............................................................... 31 3.2 Aluminium and its alloys ............................................................................................... 31 3.2.1 Properties and Metallurgy of AlSi10Mg ................................................................ 32 3.2.2 Effect of alloying elements ...................................................................................... 33 3.2.2.1 Major Alloying Elements .................................................................................... 33 3.2.2.2 Minor Alloying Elements .................................................................................... 34 3.2.2.3 Microstructure Modifiers – .................................................................................... 34 3.2.2.4 Impurity Elements .............................................................................................. 34 3.2.2.5 Al-Si Phase Diagram ........................................................................................... 35 3.2.2.6 Addition of Magnesium to Aluminium-Silicon alloys ........................................ 36 3.2.3 Powder Metallurgy of Aluminium Alloys .............................................................. 38 3.2.3.1 Gas Atomisation.................................................................................................. 38 3.2.3.2 Mechanical Alloying (MA) ................................................................................. 39 3.2.3.3 Laser Melting for Aluminium alloys .................................................................. 44 3.3 Silicon Carbide (SiC) ..................................................................................................... 45 3.4 Composites – Aluminium alloy/SiC Metal Matrix Composites (MMCs) ..................... 46 3.4.1 Particulate Reinforced Aluminium Metal Matrix Composites ............................. 48 3.4.2 Wettability and Chemistry of the Al-alloy and SiC interface