Corrosion Behavior of 2024 Aluminum Alloys Structure produced by Wire Arc Additive Manufacture A dissertation submitted to the University of the Manchester for the degree of MSc by Research In the Faculty of Engineering and Physical Sciences 2017 SIHAN TAN School of Materials Corrosion and Protection Centre Table of Contents List of figures ............................................................................................................................. 3 List of tables .............................................................................................................................. 5 Abstract ..................................................................................................................................... 6 Declaration .................................................................................................................................... 7 Copyright Statement ..................................................................................................................... 8 Acknowledgement ........................................................................................................................ 9 1 Introduction ........................................................................................................................ 10 2 Literature review ................................................................................................................. 12 2.1 History of AM .............................................................................................................. 12 2.2 Metal AM .................................................................................................................... 14 2.3 Additive manufacture of Aluminium alloys ................................................................ 15 2.4 Microstructure development of AM produced Aluminium alloys .............................. 16 2.4.1 Microstructure under AM ................................................................................... 17 2.4.2 Development of the WAAM ................................................................................ 20 2.5 Corrosion behaviour of Aluminium alloys................................................................... 25 2.5.1 Localized corrosion ............................................................................................. 27 2.5.2 The initiation and propagation of pitting ............................................................ 28 2.5.3 Composition ........................................................................................................ 28 2.5.4 Pitting corrosion .................................................................................................. 32 2.5.5 Intergranular corrosion ....................................................................................... 33 3 Experimental Methods........................................................................................................ 36 3.1 Materials preparation Methods.................................................................................. 38 3.1.1 Surface preparation ............................................................................................ 38 3.1.2 Grain boundary structure ................................................................................... 39 3.1.3 Ultramicrotomy ................................................................................................... 41 3.2 Immersion test and OCP ............................................................................................. 42 3.3 Electrochemistry ......................................................................................................... 43 3.4 Characterization .......................................................................................................... 44 3.4.1 Optical Microscopy ............................................................................................. 44 3.4.2 Scanning electron microscopy (SEM) .................................................................. 44 3.4.3 Electron backscattered diffraction (EBSD) .......................................................... 45 1 4 Results and Discussion ........................................................................................................ 47 4.1 Microstructure of WAAM produced 2024 structure .................................................. 47 4.1.1 Grain structure analysis ...................................................................................... 48 4.1.2 Porosity distribution............................................................................................ 54 4.2 Corrosion behaviour of the WAAM produced AA2024 .............................................. 56 4.2.1 Compositional analysis and second phases distributions ................................... 56 4.2.2 Understanding the electrochemical process ...................................................... 62 4.2.3 Corrosion onset mechanism ............................................................................... 65 4.2.4 Intergranular corrosion ....................................................................................... 69 5 Conclusions and Suggestions for Future Work ................................................................... 71 5.1.1 General corrosion morphology ........................................................................... 71 5.2 Conclusions ................................................................................................................. 74 5.3 Further work ............................................................................................................... 75 5.3.1 Analysis of the electrochemical potential in the different regions..................... 75 5.3.2 IGC propagation in both HAZ and Deposited zone. ............................................ 75 5.3.3 TEM study ........................................................................................................... 76 Reference .................................................................................................................................... 77 Word Count: 13152 2 List of figures Figure 2.1: A CAD file for a 3D printed cup which is showing a layered structure with a uniform thickness of each layer 6.............................................................................................................. 13 Figure 2.2: Relationship between the steps in the welding process and the resultant microstructure/properties of the final product41. ...................................................................... 17 Figure 2.3: The layers developed under TIG-CMT is labelled in (a) which is showing 4 regions, which are welded metal (WM), Partially melted zone(PMZ), Heat affected zone(HAZ) and Base metal(BM) (b) and (c) are representing the dendrites formed at the bottom(c) and in the centre of the welded metal layer(c)42. ........................................................................................ 18 Figure 2.4: Microstructure developed under common metal AM techniques. (a) Laser engineered net shaping (LENS) produced CoCrMo which is deposited on wrought CoCrMo substrate6, (b) Layered structure of UAM produced AA30036, (c) structure developed under SLM which is showing banding structure16, (d) microstructure of the single bead formed AA606116. .................................................................................................................................... 20 Figure 2.5: Microstructure of AM produced AA2024 with the metal drop jetting method (a) of each metal drop (b) after deposition40. ...................................................................................... 21 Figure 2.6: CMT electrical transient for molten metal deposition stage (a) illustrating the arching phase and s/c phase c) for conventional CMT welding c) for CMT-PA23,47. ................... 23 Figure 2.7: Setup for WAAM process including a robotic hand for the wire feed and depositing position above the substrate24. .................................................................................................. 24 Figure 2.8: Schematic of oxide morphology of aluminium metal under moisture environment50,51. ......................................................................................................................... 26 Figure 2.9: Potential-PH diagram representing the stable form of aluminium under various potential and PH50. ...................................................................................................................... 26 Figure 2.10 Image (a) revealing the scanning electron microscopy (SEM) of clustering of S phase and θ phase. Meanwhile (b) showing SEM of an α phase surrounded by S phase. (c) and (d) images are obtained with transmission electron microscope (TEM). It exhibits both the rod- shaped dispersoids distributed in the matrix and precipitates which habit along the grain boundary. .................................................................................................................................... 29 Figure 2.11 precipitation route in AA2024 ................................................................................. 32 Figure 2.12 Schematic of intergranular corrosion in AA2024-T3. As the presence of dispersoids free zone along the GB and preference precipitation along the grain boundary, the GB becomes susceptive. Serve corrosion observed within the GB compared with the rest region. ...................................................................................................................................................