Corrosion Behavior of 2024 Aluminum Alloys Structure Produced by Wire Arc Additive Manufacture

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

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

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...... 35 Figure 3.1: (a) and (b) are photo of the given samples, the surface area exhibiting waviness while the lines representing for the deposited wire of the neighbouring layers are aligning to opposite direction. In (c) the dashed line representing each cut on the given materials to obtain a specimen with a length of 2cm and width of 1.5cm for later testing...... 37 Figure 3.2: Schematic diagram for the setup of the electropolishing including a thermometer to monitor the temperature of the solution, pure aluminium sheet as cathode, the ice bath applied for cooling down the solution, magnetic stirrer and hot plate are applied to mix the solution properly, an external power source to apply an external voltage to drive the reaction...... 40 Figure 3.3 Mechanism of the electropolishing: how the chemical etching working by uniformly dissolution of the roughness bit presenting on the surface...... 41 3

Figure 3.4: Schematic requirement for using ultramicrotomy...... 42 Figure 3.5: (a) showing a workflow of the EBSD, showing the EBSD is composed of both imaging and grain orientation mapping at the same time, and computer software merge two result together give a coloured grain structure. The colour exhibiting on each grain and their neighbouring grains represent the orientation distribution. Picture (b) showing the formation of Kikuchi Band from incident of transmitted beam on the sample71 ...... 46 Figure 4.1: The grain structure distribution in the direction perpendicular to the interface (a) showing the heat affected zone (b) deposited zone (c) dendrite zone (d) equiaxed grains...... 48 Figure 4.2 Ten grains are chosen randomly from the deposited zone and HAZ, this plot showing the distribution of the diameter of each grains taken from deposited zone (—blue) and HAZ (---red)...... 49 Figure 4.3: Variation in the grain morphology along the y-axis, (a) revealing the microstructure at the left side above the previously deposited layer. Image (b) is the microstructure taken from the position about 2mm away from(a), dashed line in (a) and (b) representing the interface between the deposited zone and HAZ ( c) displayed the diameter in both x-aixs and y- axis of grains within HAZ in position (a) and position( b). (d) and the (e) revealing the grain distribution within the deposited zone under an increased magnification in both position (a) and (b)...... 52 Figure 4.4: Schematic drawing showing the process during wire deposition...... 53 Figure 4.5: Pore distribution on the surface of the portions which are taken from (a) upper left corner (b) lower left corner (c) lower right corner of a big given sample. In (d), pores with size greater than 50μm are counted and record. as from each of the portion (a), (b) and (c), there are 9 points are taken, (e) showing the position of these nine points...... 55 Figure 4.6: (a) Scanning electron micrograph of the WAAM AA2024 alloy structure showing the interdendritic intermetallics and EDX spectra obtained at the locations indicated as A and B in

(a), revealing Al2Cu phase and Al2MgCu phase at the grain boundaries...... 58 Figure 4.7: (a) revealing the interdendritic intermetallics in Backscatter electron mode. And a corresponding EDX spectrum mapping is conducted on this region. Picture (b), (c), (d), (e) and (f) are displaying the mapping of element manganese, aluminium, magnesium iron and copper element...... 59 Figure 4.8: (a) revealing the interdendritic intermetallics in Backscatter electron mode. And a corresponding EDX spectrum mapping is conducted on selected region. Picture (b), (c), (d) are displaying the mapping of element aluminium, magnesium and copper element...... 60 Figure 4.9: Electron Backscattered Diffraction (EBSD) analysed euler colour diagram of the deposited zone within WAAM produced 2024 alloy, exhibiting uniform fine size equiaxed grains...... 61 Figure 4.10: Anodic polarization curve of WAAM produced AA2024 (with a scanning rate of 0.1mV/s)observed under the (red) de-areated 3.5% NaCl solution and (black) under areated 3.5%NaCl solution...... 63 Figure 4.11: Anodic polarization line obtained for AA2024 sample in 3.5% NaCl solution (a) AA2024-T357 (b) WAAM produced AA2024. Three regions identified which are (i) pitting initiation stage; (ii) pitting propagation and (iii) transpassive region observed in WAAM produced AA2024...... 64

Figure 4.12: Scanning electron image in SE mode revealing the (a) which consisting of one isolated S phase (b) showing isolated θ phase clustering of IMs after 10 minutes immersion testing...... 67 Figure 4.13: Scanning electron microscopy in SE mode revealing (a)De-alloying feature of the clusters from different position after an hour immersion testing. Images were taken at (a) within in equaixed zone (b) next to an open pore...... 67 Figure 4.14: Scanning electron microscopy showing the corrosion product propagation morphology next to pores after 10 minutes immersion testing in the 3.5% NaCl solution. (a) showing the formation of thin layer of corrosion product within a 20μm pore (b) revealing the propagation of corrosion product from the bottom of 10μm pores...... 68 Figure 4.15: Scanning electron microscopy examination of a circle of corrosion product and its surrounding information, (a) Secondary electron mode imaging for the tomography of the selected region and (b) Backscattered electron mode revealing the corrosion propagation from the pit ‘A’...... 70 Figure 4.16: Scanning electron microscopy revealing the cross section of WAAM produced AA2024 after 20hrs immersion in 3.5% NaCl solution. (b) illustrates the intergranular corrosion from the corroded surface and (a) revealing the feature within the corroded interdendrite intermetallics under IGC...... 70 Figure 4.17: Scanning electron microscopy revealing the cross section of WAAM produced AA2024 after 20hrs immersion in 3.5% NaCl solution. (a) Revealing the feature within the corroded interdendrite intermetallics under IGC. (b) Illustrates porous structure left on the original S phase...... 70 Figure 5.1: 'banded’ product accumulation developed with time...... 71 Figure 5.2: optical microscopy image of the sample experienced 30 minutes immersion test in 3.5% NaCl solution, these images are taken along the line which is perpendicular to the building directions...... 72 Figure 5.3: Optical microscopy images showing the evolution of the corrosion morphology on the surface of the WAAM produced AA2024 after immersing in 3.5% NaCl solution for (a-b) 30 minutes (c-d) 1 hour and (e-f) 2hours. Image are taken for both HAZ (a,c,e) and in equaixed zone(b,d,f) revealing a different corrosion morphology on them...... 73

List of tables Table 2.1: Category of AM processes, including their detailed working principle and advised purpose of each type of 3D printing...... 14 Table 3.1: The composition of the given WAAM produced AA2024...... 36

Abstract Wire arc additive manufacture (WAAM) is a novel manufacturing method for producing Aluminium alloy structures. Processing of high strength Aluminium Alloy is very difficult owing to their good thermal conductivity, reflectivity and poor weldability. In this work, a detailed investigation on microstructural modification and corrosion behaviour of the WAAM produced AA2024 alloy structure, which is supplied by The University of Cranfield, is conducted.

The grain structure of the WAAM produced AA2024 was analysed by etching and microscopy, revealing a unique layered structure within WAAM structure. Later, energy dispersive X-ray (EDX) analysis was conducted on the layered structure to determine the distribution of second phases and the associated elemental segregation.

The corrosion behaviour of the WAAM structure was assessed by immersion testing and anodic polarization in 3.5% NaCl solution. Laterally, characterisation of the surface topographical feature and the internal microstructure of the original structure and the corroded structure were conducted by scanning electron microscopy (SEM) to determine the key microstructural features responsible for corrosion initiation and propagation and to understand the corrosion mechanism in WAAM structure.

It is found that the unique banding structure of the WAAM produced AA2024 alloy structure is separated as heat affected zone (HAZ), deposited zone, columnar dendrite zone and equiaxed dendrite zone, characterized by grain size, the shape of grain, and dendrite distribution in each zone.

By comparing the change of surface topography and the second phase morphology with different corrosion testing time, it was found that the heat affected zone (HAZ) has the highest localized corrosion susceptibility compared to other zones in the WAAM 2024 alloy. Localized corrosion in the WAAM 2024 alloy structure develops in the form of both pitting and intergranular attack. The high intergranular corrosion susceptibility of WAAM 2024 alloy structure is associated with the S-phase and θ-phase interdendritic intermetallics.

Declaration No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institution of learning.

2017 SIHAN TAN

Copyright Statement The following four notes on copyright and the ownership of intellectual property rights must be included as written below:

i. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii. copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance Presentation of Theses Policy You are required to submit your thesis electronically Page 11 of 25 with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii. The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and tables (“Reproductions”), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv. i Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see v. http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=2442 0), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (se http://www.library.manchester.ac.uk/about/regulations/) and in The University’s policy on Presentation of Theses

Acknowledgement

I would like to express my heartfelt appreciation and gratitude to my supervisors, Prof. Xiaorong Zhou for his invaluable guidance and great support throughout every stage in one-year MSc by Research Study.

Thanks for everyone in the Lab and D floor for the helping and encouragement during this academic year.

I would like to express my thanks to my parents for their love and support in my whole life.

2017 SIHAN TAN

1 Introduction 3D printing technique is favoured by the manufacturers for its one-off shape forming ability1–6. In industry aspect, one-off manufacture would have a sufficient reduction in required raw materials, time and energy therefore cost. It offers accessibility for the complex shape assemblies. The shaping progress is initiated simply by running a CAD file on the computer, and printing is a fully automatic process6.

Titanium alloy in medical application is a successful example of AM in the commerce market7,8 owing to the freedom of customization. In aerospace materials, the most important term is the flight-to-buy ratio. Aluminium is the one of the most important members in transportation alloy from a century ago. It is demonstrated from its strength to weight ratio. Aluminium alloys also exhibit good corrosion resistance ability and have a relatively low cost comparing with other competitors.

The purpose of AM is to provide an alternative manufacturing method which ismore economical and environmentally friendly and high freedom in the designing aspect4. In previous studies related to metal AM, less focus on aluminium alloys besides Al-Mg- Si which is commonly produced via casting9–18. In studies on Al Alloy AM, it was figure out that the density problem is very significant for most aluminium alloys. Poor density which would degrade the mechanical properties. In laser-based AM, owing to high reflectivity13,19 and poor flowability11 of aluminium alloy powder, it suffers very serve porosity problem. William and his co-workers from the Cranfield University1,5,20–25 have worked on wire arc additive manufactured (WAAM) in AM produced high-strength aluminium alloys and have successfully demonstrated the AM of aluminium alloys.

There are two classes of high strength aluminium alloys: 2xxx series Al-Cu-Mg alloy, and 7xxx series Al-Zn alloys. Both are favoured in the construction of aircraft. Al-Cu-Mg alloy is commonly used in manufacture in early 1980s26. However, the development of new aerospace materials, including various light ceramic materials, Al-Cu-Mg alloy has been gradually replaced by 3rd gen 2xxx Al Alloy: Al-Cu-Li. Instead of the strengthening mechanism of them, the study on the corrosion of Al-Cu-Mg alloy still retains interest from many researchers, not only for the maintenance of the current aircraft in service.

As they have complex microstructure over most of the aluminium alloys, especially the AA2024-T3, therefore it is an excellent material for teaching& learning purpose27,28.

As the corrosion behaviour is highly related to its microstructure, which is influenced by its composition, ageing condition, fabrication condition etc. This study is designed to understand the general corrosion behaviour of the wired arced additive manufactured (WAAM) produced AA2024 by three steps:

• Examination of microstructure • Electrochemical property • Link to its fabrication condition

During the WAAM, the microstructure developed was not yet studied in detail, the complex thermal cycle induced was one challenge in the understanding of the microstructure. And the linkage of the formed microstructure with the fabrication is conducted for the future optimisation of the WAAM processing parameters.

Later, the Literature review, previous work on the relationship between corrosion mechanism and some interesting microstructure feature would be discussed. Some well-accepted or debated mechanisms would be displayed to reveal the history of the AM of aluminium alloys and the corrosion behaviour of it. Results and discussion Section, the observed microstructure of the WAAM-produced AA2024 is discussed. In addition, the specific corrosion behaviour of it would be talked later: including the electrochemistry behaviour, morphology of the corroded product under the optical and SEM images, and comparing with the previous literature. Apart from them, the microstructure developed under WAAM would be linked with the resultant corrosion behaviour.

2 Literature review

2.1 History of AM Additive Manufacture (AM) also known as 3D printing or rapid prototyping. It has many synonyms but these three are most frequently used. It was developed from early 1990s until nowadays29.

The origin of AM is opposing with the conventional materials removal methods30. The core idea is ‘creating a shaped object with a highly automation and freedom of design by adding of solidified materials layer by layer31’. In any type of AM processes, there are three factors that determine the origin of the technique : materials and its forms (it could be in powder feed, wire feed or even sheet), solidification mechanisms, and the energy source input4. These three factors are co-related and lead to a specific microstructure of the product. However, apart from them, there is a demand for the research related the processing parameters and the output microstructure. One of the big problem associated with AM is the qualification, the output microstructure is different with machines and also the parts2,4.

The idea of 3D printing was firstly going real since the invention of stereolithography apparatus (SLA), which allows the printing of three-dimensional object from a digital CAD file. Afterward, other processes such as 3d printing and selective laser sintering (SLS) are developed. The photo-polymerized materials are favoured as it could be solidified with a shine of laser while remains most of the function4,32.

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.

Forward to 1990s, the AM method attracts the market for its quick one-off forming ability, it is ideal for prototyping or as a concept model which did not consider the functional property. In the 1990s, it was at a rapid developing stage of AM, including the introduction of the powder form materials and the selective laser sintering technology give a new approach for metal or ceramic AM. Therefore, it is possible to apply AM on any categories of materials (polymer, metal, ceramic, composite and multi-materials). And the introduction of the powder form materials and the selective laser sintering technology give a new approach for metal AM. At the initial AM market, the materials selection is limited by the melting point of the material. Therefore, at an early stage of AM, the polymer is favoured over other materials. However, From the 2010s, metal AM has caught the attention from industries for its possibility of net-shape-fabrication and further potential to enhance the freedom of the design in the field.

2.2 Metal AM

There are three printing systems which are concluded by E.herderick4 for their different purposes.

• Enclosed chamber allows elevated

temperature Powder Bed AM • Laser or Electron beam used • Reservoir of metal powder • A levelling system to low down the stage for fresh powder

• Nozzle to inject powder, melted to deposit materials

• Inert gas required to avoid Laser Powder injection AM oxidation of weld pool • Stage is fixed and laser head movable for further deposition • Good for repair and cladding, long processing time.

• Use other types of energy input: electron beam welder, arc welding Free form Fabrication equipment or ultrasonic energy • Good for large part which requires lower accuracy

Table 2.1: Category of AM processes, including their detailed working principle and advised purpose of each type of 3D printing.

Each has their specific advantages and also representing three important roles of AM on the way toward the wider industrialization: 1) Small and complex geometry assemblies (PBA); 2) Repairing and cladding (LPJ) and 3) low-cost big volume component(FFF)4.

Nowadays, after two decades continuous researches on metallic AM, the available technologies, quality of the output product and the number of available materials are increased.

Gibson6 has divided AM technologies into seven groups: vat polymerization, materials jetting, binder jetting, metal extraction, powder bed fusion, sheet lamination, directed energy deposition. The higher temperature materials6 are favoured for the future AM, with smaller feature size. For some materials made for bio-applications, bio-applicability is considered8,33. Some frequently used metallic examples2,16,34–37: stainless steel, nickel, refractories etc. for aluminium alloys, Al-Si alloys is most frequently applied in AM, few are conducted on 6061 alloys and 3003.

2.3 Additive manufacture of Aluminium alloys

Additive manufacture (AM) is a technique base on the layered manufacture, so the microstructure is different between the layer and their interface. Laser, electron beam and welding techniques are frequently used as a source to input heat to the sample.

As mentioned before, the research on the AM of Al-Si alloy is popular and mature. It was due to the small difference between liquidus and solidus line38. Therefore its melting point and solidification point are close. A satisfied crack free structure could be obtained. It is easier to process them with AM comparing with high strength aluminium alloys. Move to the application of AM of Al alloys, there are several difficulties need to overcome to obtain a dense product with a good quality..

1. Susceptivity to hot cracking for high strength aluminium alloys, poor weldability during the printing.39 2. Un-sufficient oxide removal after each metal layer addition could lead to the porosity as well 38 3. Limited development in laser applied AM, as good reflectivity of aluminium alloys. Laser is light as well, while the aluminium powder is very reflective, less energy are used to melted the metal powder.

4. Poor flowability of molten aluminium lead to significant void formation (significant problem with powder feedstock) 5. Difficulties in producing high quality metal powder9 It would require a high purity, high flowaibility of powder(round shape) to ensure a good property of the final product.

Reviewing of previous research stages on AM produced Al alloy, there were serval works made on high strength aluminium alloys.Bartkowiak38 and his co-workers have successfully made 2xxx and 7xxx aluminium alloys by laser selective melting method. The output product displays superfine microstructure. Average grain diameter is less than 1µm. While Liu40 has successfully made the AA2xxx by the metal drop method.

2.4 Microstructure development of AM produced Aluminium alloys

As introduced, multiple techniques available for AM production will give different microstructure in the end. Their final quality, mechanical properties and chemical properties are fully determined by their microstructure.

Figure 2.2 shows parameters during the manufacturing stage, they are linked to the resultant microstructure. The thermal cycle and weld pool shape would affect the size and shape of heat affected zone and the distortion contained in the layer. Solidification process, the cooling rate would determine the size of the grain, therefore would relate the strength and fatigue of the product. With a high cooling rate, finer grains achieved, therefore better strength and the risk of fatigue is increased as the raised population of grain boundary presenting in its microstructure. Phase transformation is very sensible content, It includes many parameters: heat input, cooling rate and also diffusion in both liquid phase and solid phase,

Figure 2.2: Relationship between the steps in the welding process and the resultant microstructure/properties of the final product41.

2.4.1 Microstructure under AM From the presenting literature, the microstructure formed under AM are classified into the following zones: fine grains within the deposited layer, re-melted or re-crystallized zone and heat affected zone found the region. Sometimes oxide layers discovered between them as well.

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. This layered structure exhibits a significant difference in mechanical properties21,42. Stress at deposited interface exhibits significantly lower strength. Figure 2.3 shows the welded microstructure after TIG-CMT which means tungsten inert gas welding combined with cold metal transfer technique. A detailed discribution of CMT would be given in 2.4.2.2. Within Figure 2.3 (b), (c); it was found the columnar dendrites formed close to the substrate while normal dendrites formed in the centre of the welded metal. The thermal gradient is very steep at the interface; therefore, these dendrites grew along the thermal gradient.

2.4.1.1 Porosity Content

Porosity is one of important problem found in the AM. The possible formation mechanism is given and explained in detail below

1. In early 3D printing stage, the quality of the metal product is not satisfying the market. The common binding theory is usually liquid phase sintering mechanism29; For high melting point metals. The balling effect43,44 is one critical cause for porosity and sometimes de-lamination. This problem frequently rose for early laser sintering. 18

2. Non-successful oxide removal also leads to porosity6. As they acted as heterogenous sites for nucleation of pores. 3. In AM requiring the shielding gas, it is possible for the entrapment of gas bubbles in the molten metal5,23. Source of hydrogen is also found in the microstructure containment in the feedstock or moisture from the environment. This entrapment of hydrogen gas appears more critical for aluminium alloys owing to the high solubility of hydrogen in molten aluminium.

2.4.1.2 Layered microstructure developed under different AM techniques

The laser used AM method has serval disadvantages: 1) high cost 2) limitation of metal materials selection45. Reminding the FFF techniques which including the application of welding equipment, or ultrasonic equipment which satisfy the requirement. Ultrasonic welding AM is one solid-sintering technique, means no melting of the metal feedstock required, was one potential competitor in metal AM. Its layered structure is shown in Figure 2.4(b).

Another relatively new-coming technique is Wire +arc additive manufacture (WAAM), which is a typical Directed energy deposition(DED)6 method by applying current arc to deposit the metal. The description of DED is applying energy to melt the feeding materials, materials are bond together tightly while forming. It was considered similar to casting, however, a quicker cooling rate is applied and as layered deposition, the microstructure would be changed under the thermal cycle achieve a finer grain size.

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.

2.4.2 Development of the WAAM

2.4.2.1 Metal drop: In early application, laser sintering was used for metal AM, the laser is utilized to melt the metal into liquid phase, then sintering them together. However, as the result of the viscosity of molten metal and balling phenomenon, the resultant product is not reasonable for market implantation. Other methods are using the addition of the binder, or some impurity element for chemically binding.

The idea of the metal drop is different from the WAAM, its working principle is jetting of molten metal (which are molten and kept in a container). The research direction is focused on the improvement of the jetting methods. Orme and co-workers40 were 20

working on depositing droplets of molten metal on the substrate and previously deposited layers. It was proven the microstructure is more identical compare with common casting process, and no post-processing for the printing equipment is required (for example: In LPBF Cleaning of powder bed was required at the end of the processing). The porosity is also eliminated within metal drop method, as Yamaguchi46 has proven an enhanced density with finer size droplet deposition. One major difficulty is the droplet production. Ideally, all droplets are deposited at a steady rate and identical size on one plane.

Figure 2.5: Microstructure of AM produced AA2024 with the metal drop jetting method (a) of each metal drop (b) after deposition40.

2.4.2.2 CMT welding

Aluminium alloy is a trick to process in welding as well. It has a low melting point around 500°C. However, result from its good thermal conductivity and high heat of fusion. Aluminium alloys usually require a temperature over 600°C during welding. The implementation of high temperature increases the risk of weld burn or breaking the printing system47.

Cold metal transfer (CMT) welding is specifically suitable for aluminium alloys owing to its low heat input, and a good control during the welding process. There are three developed CMT welding called: CMT advanced (CMT-A), CMT pulsed (CMT-P), CMT

advance pulsed (CMT-PA) for advanced applications. During CMT welding, the process is divided into two stages: welding phase and short circuit phase41,48.

i) Welding phase: During the welding phase, the current arc initiates to the feed electrode followed by melting of the tip and is transferred to weld pool. Afterward, the current reduced to maintain the liquid bridge and force the electrode to the weld pool ii) Short circuit phase: after the detection of the electrode in the weld pool, a short circuit occurs. Accompanied by a significant reduction in current. Meanwhile, a backward force applied to draw the electrode back results in the break of the liquid bridge followed by detachment of metal drop.

Developed technique for CMT is invented for progress which has a higher requirement for the energy input or control. In CMT-A, the polarity is switched to negative during short circuit phase. The use of pulsed current in CMT-P demonstrates the possibility for higher energy input and the shortened period for the heat input stage. IN CMT-PA, both features are combined and lead to better operation accompanied with the improved product quality (for example reduced porosity, refined grain structure). Figure 2.6 showing the variation of current/voltage with time, showing the different procedure in arcing phase and s/c phase between conventional CMT (a) CMT-P (b) and CMT-PA (c)

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.

During the arcing phase in CMT-PA, few pulses existed in the arcing phase in one weld cycle. Each pulse is representing the formation of one metal droplet. Therefore, more efficient deposition is reached with the increase of pulse in arcing phase.

2.4.2.3 WAAM

Figure 2.7: Setup for WAAM process including a robotic hand for the wire feed and depositing position above the substrate24.

WAAM has alternated CMT welding into 3D printing, it has been successfully applied to various metals including Ti-6Al-4V, stainless steel, nickel alloys and few Al alloys including high strength A2024 Alloy5. Figure 2.7 showing the general setup for the WAAM process. Shielding gas is applied protecting workpiece from oxide formation during the deposition. A power source used to control the feeding of the wire and supporting current arc initiation to the workpiece. A robot hand moves in freedom to deposit molten metal on the substrate. The mechanism of the process is the same as described in 2.4.2.2 CMT welding section.

The WAAM was suggested by Williams in 2011. During past five years, there are few important developments made to updating the technique to the current industry demand. Some significant improvements are listed below:

i) Using of PAV-CMT to reduce the porosity content 23 ii) Optimized processing parameter to reduce the porosity 24 iii) Elimination of pores with rolling after each deposition of a layer combined with heat treatment.20 iv) Shot-peening to improve the density on the surface 24

v) New welding progress instead additive layer to improve the flexibility of design.

In WAAM, CMT welding is preferred for Al alloys owing to its 1) Low heat input 2) low spatter during welding 3) excellent control of metal deposition process. 4) Ability to achieve high density object.

Advantages and disadvantages of WAAM are listed below:

+Low cost

+High deposition rate within a range of 0.5-4kg/hr49

+No limit for the large component

+Outstanding Mechanic properties (porosity etc.)

+Less waste of the feed materials

+No restricted requirement for the printing environment

-Hard to manufacture complex object

-Poor accuracy (welding resolution is about 2mm) 49

These features enable the WAAM could be used in industries aiming at rapid fabrication of big volume assemblies.

2.5 Corrosion behaviour of Aluminium alloys

Aluminium alloys is favoured in industry demonstrating from its great weight-strength ratio. Its good corrosion resistance is demonstrated by its ability to form strongly- cohesive oxide onto the surface. The oxide morphology is shown in Figure 2.8. Its oxide layer is divided into two parts: a thin Al2푂3 barrier layer on the bare metal. Above the barrier layer, an extra porous hydroxide layer Al2푂3 · 3퐻2푂 is developed while in contact with moisture environment. The thickness of the addition of them is about 10nm.

Figure 2.8: Schematic of oxide morphology of aluminium metal under moisture environment50,51.

As mentioned before, aluminium alloys have a good corrosion resisting ability52. This is the result of the stability of Al under a neutral environment. According to the Figure 2.9, aluminium metal is in passive region under the neutral environment (neutral PH, ambient temperature, no external voltage).

3+ − Al Al2푂3 AlO2

Figure 2.9: Potential-PH diagram representing the stable form of aluminium under various potential and PH50.

Refer to the thermodynamic stability of the sample in different PH and potential. Pourbaix diagram under each temperature is established in figure 2.9 to relate the corrosion with environmental factors. The aluminium is found corrosion active while the PH is either below 4 or greater than 9. (a) PH<4, Al3+ is stable under in the environment; − (b) PH>9, AlO2 ion is stale, Therefore, under those conditions, the metal is not able to retain its solid metal and tends to dissolve to the solution. When c) PH is around 7 the cohesive oxide formed, insulating the contact of the metal base with the external solution51. a) Al = Al3+ + 3e− (1) − + − b) Al + 퐻2푂 = AlO2 + 4H + 3e (2) + − c) 2Al + 3 퐻2푂 = Al2푂3 + 6H + 6e (3) These three equations are active under different value of the PH: a) PH<4 b) PH>9 and (c) PH around 7.

2.5.1 Localized corrosion

Figure 2.9 explained the reason behind the outstanding corrosion resistance of aluminium to uniform corrosion. However, localized corrosion is more disastrous for passive alloys. Thelocalized corrosion is the result from the difference in chemical potential at different portions of the materials.It occurs with either the variation of the external envrionment or the ‘defects’ contained within itself, the localized corrosion could be caused with variation of oxygen concentration,the presence of second phase compounds in alloys or defects within the oxide layer. In AA2024, Pitting corrosion and intergranular corrosion are frequently observed and studied. However, the onset mechanism of the localized corrosion is still in debate53,54. In aluminium alloys, pitting corrosion is preferentially attacking ‘weak’ spot, for example, second phases particle. So the corrosion mechanism of a given AA2024 sample could relate to alloy composition27, ageing treatment26 and the manufacturing path55.

2.5.2 The initiation and propagation of pitting

In AA2024, second phase compounds are favoured as heterogeneous sites to the initiation of pits56. In pure aluminium, pit is initiated from the base of mechanical defects or the flaws on the oxide film57.

2.5.3 Composition

AA2024 alloys have been called Duralumin and 24S back to 1980s during the early developing stage57. The composition of the alloy is updated by years. It was also reported the electrochemical response is differenT57. The localized corrosion for aluminium alloys is mastered by the development of micro-galvanic cells. Copper is the major alloying element in Al-Cu-Mg alloy, enhances the risk of corrosion to the alloys. However, the presence of copper improves the formability of the alloy.

2.5.3.1 Constituents

There are three dominant types of constituents in the structure57: I. Intermetallic compounds (θ phase, S phase and Al-Cu-Mn-Fe-(Si) phase and inclusion compounds. II. dispersoids III. hardening precipitates Their distribution and electrochemical properties are closely linked to the corrosion mechanism.

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 particle surrounded by S phase. (c) and (d) images are obtained with transmission electron microscope (TEM). They exhibit both the rod-shaped dispersoids (c) distributed in the matrix and precipitates(d) habiting along the grain boundary62,63.

2.5.3.1.1 Coarse constituents: The coarse constituents or commonly called intermetallic (IM) compounds, their size are classified less than 0.5μm in AA2024-T3. It is the type of constituent which is the most strongly contributed to the corrosion, due to the size of them. These coarsen constituents are formed during the ingot formation stage and are very stable57. Under subsequent cold-rolling, the grains are elongated57,58 and aligned with rolling direction. Meanwhile, coarse constituents are broken down into smaller pieces and re-distributed in the sheet. In AA2024-T3, there are three common types of IM, which are S phase

(Al2MgCu), θ phase (Al2Cu), and Al-Cu-Fe-Mn-(Si).

2.5.3.1.2 Mg-Si Apart from these, Mg-Si is also reported in some literatures28. It was appearing as the result of silicon impurity presenting in the bare materials. It is negligible in BSE mode. The attribution of Mg-Si in corrosion has not been reported before. One accepted explanation is the constituent fall off before the active corrosion59.

2.5.3.1.3 S phase

Al2MgCu phase is strongly contributed to the corrosion mechanism of AA2024 materials. As its large population in AA2024 and its extinct potential difference57,60 with the matrix. It was reported that the corrosion was starting with the de-alloying of Mg and Al from

61 the Al2MgCu followed by pit nucleation. They are tending to diffuse out the S phase while Cu is diffusing in the S phase. The opposite diffusion direction of Mg/Al and Cu will eventually result in a doughnut shape in the original position. The porous structure copper-rich compound is remaining in the centre, while the periphery region is copper depleted and etched out in the end. Three micro-galvanic cells formed between copper depleted periphery, copper-enriched compound and the Al matrix. Re-deposition of nano-copper particles on the surface is observed for a longer induction time62. Following reaction are occurring Mg Mg2++2푒− (4)

Al Al3++3푒− (5)

2푒−+Cu2+ Cu (6)

2.5.3.1.4 θ phase

Al2Cu is second the most phase presenting in AA2024-T3 alloy which is found

63 contributed to the 15~20% of all constituents under T351 condition . Al2Cu phase is considered less active compared with S phase. As it is a cathodic second phase, the reaction starts at the periphery of the compound. It is demonstrating from aluminium dissolution and results in a rise in PH inside the trenches60. Corrosion is a sensitive process as the change in environment would be reflected in the corrosion rate simultaneously. Within trenches, the rise in PH

accelerates the dissolution of aluminium in the bottom of trenches, therefore, more serve corrosion in the trench.

2.5.3.1.5 Al-Cu-Fe-Mn-(Si )

Al-Cu-Fe-Mn-(Si) is found attributed to 10% of IMs within AA2024-T3. In early literature, a delayed quenched of the 80s would lead to the nucleation of S phase from Al-Cu-Fe- Mn-Si phase leave a shell of Al-Cu-Fe-Mn-(Si) around the S phase64. If these two phases are found in the cluster, the corrosion rate of S phase is increased comparing with isolated S phase. Its chemical composition is unidentified, as it is very sensible with the composition of the alloy. Therefore, many possible stoichiometric compounds available depending on the impurity content in the material.

2.5.3.2 Dispersoids and hardening precipitates

Dispersoid is formed under ingot formation stage as well57, and hardening precipitation is formed under manual ageing treatment. The distribution of them is controlled to obtain optimized mechanical and chemical properties.

28 In AA2024, the frequently observed dispersoids is Al20푀푛3 퐶푢2 , which generally displayed as short rods with a size between 200-500nm. Main hardening precipitates are S’ and θ', both are ellipse-shaped and generally less than 200nm65. These two sorts of constituents contribute to the intergranular corrosion which would be described later, as IGC is not directly related to the attack by aggressive Cl− ions compared to coarse constituents.

Heat treatment of AA2xxx is completed by three steps i) Solution treatment to dissolve all alloying element in the molten Aluminium matrix. ii) Formation of the supersaturated solution after rapid cooling maintains the supersaturated solid solution. iii) Afterward, precipitation occurs at ambient temperature or elevated temperature, and the detailed evolution of the microstructure is described in Figure 2.11. Decomposition of GB zone followed by nucleation and further diffusion of nanoscale precipitates on heterogeneous sites66.

Figure 2.11 precipitation route in AA2024

2.5.4 Pitting corrosion

It is known that the pitting corrosion is divided into three stages i) initiation of metastable pit ii) metastable pit growth and iii)stabilization of pits51,67. Good corrosion resistance of aluminium is owing to the ability of immediate formation of cohesive oxide. Oxide protects the bare material from the external environment. The poor conductivity of the oxide film is insulating which limiting the transfer of electrons through the thin oxide film50.

The pitting of aluminium is usually occurring at the small defects beneath under the surface. In AA2024-T3, the initiation results from the weaker oxide film formed upon the second phase compounds and this type of sites could be the heterogeneous sites for pit nucleation.

2.5.4.1 Initiation:

Sloti53 have reviewed the presenting principles of pitting corrosion, he introduced three well-accepted pitting initiation mechanisms: penetration mechanism, film breakdown mechanism and adsorption mechanism.

Under penetration mechanism, the metal ions are moving outward under a significantly high electric field, the aggressive ion for example chlorides are able to penetrate through 32

the oxide film, As the incorporation of anions within the film gave rise to conductive paths and act as catalyst promoting the egress of metal cations. This results to the accumulation of voids (as the metal ions moving outward) at metal-film interface then disable the passive film. Breakdown mechanism predicts the presence of blistering, slow healing oxide, accumulation of vacancies or other defects which gives the poor continuity of the oxide film. The adsorbed aggressive ions sufficiently reduce the surface tension of the oxide film as the result existing defects would grow into active cracks and creating a bridge between aggressive ions and the bare metal. If defects on the surface are negligible, the absorption mechanism is dominant. Start with adsorption of aggressive ions at the surface of the film. Oxygen acted as a catalyst promoting the metal ions travel toward the electrolyte. Meanwhile, the aggressive ions are travelling toward the metal. Complexion formed on the surface. Pores left behind result from the movement of metal ions toward the electrolyte. As pores generated during the travelling of metal ions, the passive film is getting thinner until it was completely disabled.

2.5.4.2 Metastable/stable pit growth

Selective dissolution of Mg or Al from Al2MgCu weaken the oxide film. Destabilization of the oxide film is followed by the propagation of metastable pits. During pitting, the environment becomes more serve inside pits, including the decrease of PH accompanied with an accumulation of aggressive ions inside the pit. If the equilibrium between to complicating equations is moving toward the corrosion, the metastable pit is transferred to the stable pit. If equilibrium is moving toward the oxide formation, the metastable pit would have diminished with the formation of oxide on it. Matrix around pits serve as the cathode to react with H+ in the solution, it causes the evolution of hydrogen accompanied with the pit stabilization.

2.5.5 Intergranular corrosion

AA2024 is also suspect to intergranular corrosion after heat treatment. To obtain the maximum strength of the alloying, heat treatment is unavoidable, as it is contributed to most of the strength in the alloy. Meanwhile, dispersoid free zone developed around S phase or Al-Cu-Fe-Mn-(Si) phase.

During the natural ageing treatment, either natural ageing or artificial ageing, the cluster of Cu or Cu and Mg formed from the GB zone, then the precipitation of S’ and ϴ’ are preferentially formed on dislocations or Grain boundary (GB). They are acted as a heterogeneous site for the nucleation of precipitates. Therefore, Cu element is diffusing inward the GB, from the surrounding. The accumulation of copper element within the GB leads to a copper depletion zone around the GB. However, in the peak aged AA2024, it was seen by Luo58, that there is no Cu depletion zone adjacent to GB due to the lack of S’ and θ’ precipitate along the GBs.

Overall, the precipitation of S’ and θ’ phase consumes a certain amount of copper. As diffusion rate of copper element is limited in AA2024. The area adjacent to grain boundary is displayed as copper depleted zone and dispersoid free zone. GB exhibits extinct electrochemical behaviour compared with the matrix. Knight68 has shown that there is about -220 mv comparing with AA2024-T3 sample in 0.6M NaCl solution, which makes the GBs are anodically corrosion active.

Figure 2.12 Schematic of intergranular corrosion in AA2024-T3. As the presence of dispersoids free zone next to the GB and preference precipitation along the grain boundary, the GB becomes susceptive. Serve corrosion observed within the GB ( corrosion event is shown with black colour).

3 Experimental Methods

The AM produced AA2024 is supplied by the Cranfield University; An examed elemental composition is given in table 3.1.

Composition (wt%) Si Fe Cu Mn Mg Zn Cr Ti Zr V AA2024 0.0031 0.106 4.39 0.78 1.6 0.0216 0.171 0.110 0.0991 0.019

Table 3.1: The composition of the given WAAM produced AA2024.

a b

Figure 3.1: (a) and (b) are photo of the given samples, the surface area exhibiting waviness while the lines representing for the deposited wire of the neighbouring layers are aligning to opposite direction. In (c) the dashed line representing every cut on the given parts to obtain specimens with a dimension of 2cm x1.5cm for later testing.

3.1 Materials preparation Methods The given materials are labelled with their position for the lateral experiment. The sample was cut by cutting machine with a speed of 2000um/min and a feeding rate of 0.01m per second. A scratch free surface is needed for electrochemical test and the characterisation to confirm the reliability of later corrosion test.

3.1.1 Surface preparation

As shown in the figure 3.1, the rough surface is seen from the given AM-produced sample, and this needs to be removed by grinding. The sample is then grinded with 80, 120, 240, 400, 800, 1200, 2400 and 4000 grit carbide grinding papers. The sample was rotated 90 °C anti-clockwise before moving to the higher grit. After grinding, sample was washed with de-ionised water.

Afterward, polishing is applied, until a mirror finish surface achieved. In this step, 3μm, 1 and 0.25 μm diamond paste are applied on their corresponding polishing cloth. The alcohol-based lubricant is used with an interval of 15s during polishing. The sample was then washed with soap, ethanol to washing off the containment from the previous level of diamond paste. Afterward, the sample is dried under a stream of cool air prior to switching to the next level of polishing.

There is another comparing sample which is a sheet of AA2024-T3 which was naturally aged plus cold rolling. The only this sample is grinded with 1200, 2400, 4000 before polishing.

After polishing sample is washed ultrasonically in acetone for 10-15 minutes, following by washing subsequently in water, water, ethanol, eventually was dried under a stream of cool air. Unused samples are stored in the box half filled with silica balls to avoid the exposure to air or moisture.

3.1.2 Grain boundary structure

After the mechanical polishing, most of the micro-scratches (thickness greater than 0.25 μm) are removed from the surface. However, if interested in grain boundary distribution information under a small magnification, it would require more advanced surface treatments. In this case, due to the structure of the sample, either electropolishing or etching could be chosen for this purpose.

3.1.2.1 Electropolishing

It is the electrolytic polishing. The electrolyte is made-up from 20% of perchloric acid and 80% of ethanol. The temperature of the solution is maintained less than 10 °C for the safe operation (the beaker is placed in ice bath during the solution preparation and the electropolishing). Whole process was conducted in the fume broad. The mixing of solution is classified as very explosive if mix rapidly; therefore, very slow addition of solution was conducted while the temperature of the solution is monitored,

The operation procedure of the electropolishing is written below

i) the sample is immersed in the acetone for 5 minutes and dried in a stream of cold air ii) Configuration: Voltage set to 20V, cathode and anode are connected to the power source iii) Place the sample in the electrolyte for 20 seconds while switching on the power source. iv) Rinse in water, water and ethanol following by drying under cold air

Figure 3.2: Schematic diagram for the setup of the electropolishing including a thermometer to monitor the temperature of the solution, pure aluminium sheet as cathode, the ice bath applied for cooling down the solution, magnetic stirrer and hot plate are applied to mix the solution properly, an external power source to apply an external voltage to drive the reaction.

The surface was required to be polished to 1 μm prior to electropolishing. The prepared surface is not flat if looking under high magnification as 1µm class scratches still present on the surface. As presented in figure 3.3, the area in contact with the solution is ‘dissolved’ by the solution, and the amount ‘dissolved’ is depending on the chemical composition of the part, and their activity. Electropolishing is usually applied to obtain a high degree of flatness of the surface. In addition, this was commonly used before Electron backscattered diffraction (EBSD) which is a technique could map the GB orientation.

Figure 3.3 Mechanism of the electropolishing: how the chemical etching working by uniformly dissolution of the roughness bit presenting on the surface.

3.1.2.2 Etching

Etching is one method for seeing the GB of the metallic sample. The Keller’s solution is commonly used for 2xxx aluminium alloys. Similar with electropolishing, however, the etching solution is more aggressive than electrolyte and is preferentially attack the GB aggressively. The Keller’s solution is made up from (95ml de-ionised water, 2.5ml nitric acid, 1.5ml hydrochloric acid and 1 ml hydrofluoric acid).

The etching is conducted in the fume board as well. The sample is immersed in the Keller’s solution for about 5 seconds, and then it was washed with a lot of water to dilute the corrosive solution on the surface. Afterward, sample rinsed with soap, ethanol, and finally dried under a stream of cool air.

3.1.3 Ultramicrotomy

Ultramicrotomy is one traditional method for preparing nm thickness TEM sample. However, it was also used in preparing the cross section for SEM sample in few years. In the present study, the ultramicrotomy is used to obtain a high quality cross-section and with a detailed edge information. Firstly, a small slice is cut from the sample by hacksaw. 41

Later, grind the slice into a small tip as presenting in the figure 3.4 prior to doing the ultramicrotomy.

Figure 3.4: Schematic requirement for using ultramicrotomy.

Firstly, the right and left side of the sample is held at 30° to the grinding paper to prepare the tiny tip which containing the interested site. Firstly, the glass was cut into a small triangle shape with very sharp edges, then it is used as glass knife. The approach (travelled distance of the knife after one press of “upward” key) is set as 1um to 2um during the cut by glass knife. The glass knife is set parallel to the tip first, then the knife is set 30° to the left following by 30° to the right side. Cut by the glass knife, is to adjust the height and width of the tip, the goal is to have a top with width of 150-200um, and height of 30-50μm. Afterward, the diamond knife replaced the glass knife, and the approach is reduced to 0.1μm. The knife is set at 0° to the tip, 10 to 20 cuts are made by the diamond knife to obtain a clean tip with the scratch free surface.

3.2 Immersion test and OCP

The prepared surface is first, cover with lacquer-45 to cover unprepared surface, and the edge was protected by the wax. This was done to avoid the entrance of the NaCl solution which might associate with crevice corrosion. This would distribute the value seen. Wax was prepared by mixing of Beeswax and colophony with a ratio of 4:1. The

mixture is heated on the hot plate with steady temperature of 200°C. This wax can solidify immediately after applying on the sample

In this study, the 3.5% sodium chloride solution (3.5% NaCl) is used as electrolyte. Few immersion periods are selected for monitoring the tomography change and the corrosion process happened in the micro-level. The sample is immersed in the freshly made sodium chloride solution for 10 minutes, 30 minutes, 1 hour, 2hours, 10 hours 15hours and 20 hours. And the corresponding OCP is recorded. The corrosion product is generally removed by polishing with 0.25um diamond paste followed by washing in acetone before later characterization.

3.3 Electrochemistry

The preparation in this experiment is the same, however, in this electrochemistry, the only 1cm2 surface is exposed, others are painted with lacquer-45 and the edge are protected by the wax. The salortron analytical potentiostat is used to measure the change in current and voltage during the polarization. The potentiostat is connecting to compute. The sample was set in a three-terminal cell connection with Silver/Silver chloride Reference electrode and platinum wire counter electrode. Before the polarization, the working electrode (sample) is immersed in NaCl 3.5% solution under 25±5 °C for half an hour to stabilize the open circuit potential.

The anodic polarization is conducted under two conditions: one is de-aerated and the other aerated. The de-aerated condition is obtaining by bubbling of nitrogen gas in the solution for 1 hour to 2 hours. The anodic polarization is usually conducted in the 3.5% NaCl solution.

For WAAM-2024 sample, its scanning rate is 3.5%v/s and the scanning start from -20mv to the OCP and stop at the 300 above the OCP tested. For AA2024-T3, the scanning rate is 0.15mv/s, and was scanned from -50mv to the OCP to the +500mv above the OCP.

3.4 Characterization

3.4.1 Optical Microscopy

Optical microscopy is applied for obtaining the GB distribution of the etched or electropolished materials, the contrast is rose from the different height difference within the sample. A sufficient etching could supply enough contrast between grain boundary and the grain body.

3.4.2 Scanning electron microscopy (SEM)

Scanning electron microscope is one important technique in the study of materials science. In this study, two SEM: Zeiss EVO 50 and Zeiss FEG-SEM Ultra 55 are used, with are connected to the computer. The corresponding analytical software is INCA.

In a typical SEM, a magnetic field is applied as an electromagnetic lens, and the emitted electrons, they travel through the lens and convert, then form a small tip on the sample. This tip can scan over the surface of the sample. The size of the tip determines the spot size, which determines the resolution of the SEM.

There are two modes available in SEM: Secondary electron (SE) mode and backscatter electron (BSE) mode. SE mode is good for the low voltage, the incident electron extracted secondary electrons just 5 nm underneath the surface. These low energy SEs give information on the surface topography. BSE mode requires a higher voltage to extract the BSEs located at about 250nm under the surface. Therefore, collected BSE would give more compositional or crystallographic information in the image. In addition, in BSE mode, Energy dispersive X-ray spectroscopy (EDX) could be conducted on the sample to identify the chemical composition at one spot and their ratio as well.

However, in SE mode, if applying a high voltage, a scattering of BSE would distribute the signal cause the blurring of the image. A careful selection of voltage, working distance, and adjusting the stigmatisation will give the best quality of the image. The last term is

the correction of lens defect. It was reported that after the exposure of surface under electron beam, ‘charging’ effect might occur—carbon deposition57,63,69. Therefore, a gentle polishing is required to remove it.

3.4.3 Electron backscattered diffraction (EBSD)

EBSD is the only technique which could easily identify and map the orientation of grains in the selected area. The electron beam is scanned over the surface, and BSE is collected by points on the image. The sample is rotated 60° to the BSE detector70.

The working principle is dominant by the low energy loss inelastic scattering70. Incident beams enter the crystalline materials and are diffusely scattered in all directions, there for when that scattered beam reach a plane and elastically scattered(in all directions) to give strong, reinforced beam. Each family of plane gives two cones of beam radiation on each side.This cone cut the phosphorus screen give two Kikuchi line. This pair of lines will form a band corresponding to crystallography information71.

The EBSD detector collects the backscattered electrons which are deflected by lattice planes, the exit surface of the specimen will give a clear Kikuchi pattern. Figure 3.5 illustrates the mechanism of obtaining an EBSD diagram including setup of sample within the SEM chamber, the analysis of Kikuchi band at each point on the surface, and computer combines all the data give a coloured EBSD map. Projection of Kikuchi band on a sphere which representing a pole figure. On a sphere, each Kikuchi band representing a crystalline plane in a single grain; each interaction point representing a type of symmetry; by calculating angles between convergence angle to separate phase etc72. Detector collect pole figure on each point on the diagram, then the colouring is given by Euler map, firstly a reference direction (normal direction etc.) was chosen. Then the direction of grains would be found relative to the reference direction. In which the grain orientation with of <001>, <111>, <101> would be represented by the red, blue and green colour72. Therefore, The colour distribution equal to the grain orientation on a selected area.

Figure 3.5: (a) showing a workflow of the EBSD, showing the EBSD is composed of both imaging and grain orientation mapping at the same time, and computer software merge two result together give a coloured grain structure. The colour exhibiting on each grain and their neighbouring grains represent the orientation distribution. Picture (b) showing the formation of Kikuchi Band from incident of transmitted beam on the sample71

4 Results and Discussion

4.1 Microstructure of WAAM produced 2024 structure

WAAM is developed from 2011, however, most of the works are conducting on the improvement of the microstructure (elimination of porosity content, increase the density, and mechanical strength, or searching for suitable post-processing)5,20,23,24. However, the information on the more detailed microstructure analysis is not enough for understanding the corrosion behaviour of it. For a better understanding the corrosion behaviour, the WAAM produced microstructure is investigated first. Information on grain size distribution and identification of the intermetallic compounds (IM) is required to conduct on the sample prior to any corrosion test.

4.1.1 Grain structure analysis

4.1.1.1 Perpendicular to the deposited layer

Figure 4.1: The grain structure distribution in the direction perpendicular to the interface (a) showing the heat affected zone (b) deposited zone (c) dendrite zone (d) equiaxed grains.

Figure 4.1 showing the microstructure perpendicular to the deposited layers including (a) revealing the coarse grain size and some fine equiaxed grain is embedded within this region, (b) mainly columnar grains with decorating of fine equiaxed grains (c) dendrites (d) small equiaxed grains. Red arrows indicate some interdendrites presenting in the (c).

Distribution of the size of grains from HAZ or Deposited zone

Figure 4.2 Ten grains are chosen randomly from the deposited zone and HAZ, this plot showing the distribution of the diameter of each chosen grain taken from deposited zone (—blue) and HAZ (---red). Then give a overview of the possible size of grains presenting in these two zone..

In Figure, 4.2, the distribution of the diameter of grains within the zone (a) and (d) in Figure 4.1 are put together, ten grains from each zone were taken randomly and diameter of each grain is worked out. The average diameter in zone (d) is 42.22μm and is 69.7μm in zone (a). It is showing a very small size difference, as the average grain size in HAZ is about 1.5 times bigger than that in the deposited zone.

In WAAM produced 2024 structure, there are four types of zones can be separated from each other:

4.1.1.1.1 Heat Affected zone(HAZ):

HAZ which is the previously deposited layer, the grains are grown under the induced heat from the newly deposited layer. However, the heat input is smaller than its melting

point. For these grains are only allowed to grow a bit. Size of the grains is ranged from 50μm to 89μm. Small equiaxed grains around 10-30μm are decorating within this zone.

If considering the similarity of WAAM with casting, the previously deposited layer acts as a cold mould, at this zone, but with the same melting point with the deposited metal73,74.The HAZ developed at the cold mould close to the interface. In the term of understanding the microstructure evolution during casting, thermal gradient term G is introduced73,74. Thermal gradient describes the heat difference between two interfaces and the rate of heat transfer between them.

4.1.1.1.2 Deposited Zone (DZ):

This zone comprises of very fine equiaxed grains which have an average diameter of 13.2μm, and the columnar grains grow perpendicularly from the layer of HAZ. It was comprised of both superfine grains and columnar grains. Under the effect of the supercooling from the direct contact with the ‘cold mould’, superfine grains are formed under rapid cooling of them. They nucleated heterogeneously on the previously deposited layer. Another type is columnar grains in (b), very steep thermal gradient lead to a directional growth of the grain from the cold mould.

4.1.1.1.3 Columnar Dendrite zone (Mushy Zone)74:

Above the deposited zone, it was made up from the heavy dendrite GB, also some interdendrites presenting as the result of segregation. At this zone, the thermal gradient is lowered down, and the cooling rate remains fast, therefore at this region, there is a combination of interdendrites and columnar grains and equiaxed grains. The cooling effect from the cold mould is not enough for the uniform solidification. Instead, the low thermal gradient left this zone with a co-existence of melt and dendrites. The solidification of the second phase is completed by diffusion under solid phase, or with the viscous melt. Therefore, the interdendrite structure is seen in Figure 4.1 and indicated with red arrows. This microsegregation phenomenon results in a susceptive 50

zone, as the accumulation of alloying element, clusters of IMs, and higher Cu concentration is found within this zone.

4.1.1.1.4 Equiaxed grain zone (EZ):

After the allowed cooling time, the portion at the top of the deposited layer can grow and solidify into equiaxed grains in a range of 26μm to 60μm. At this zone, the thermal gradient is lower than other regions, dendrites could relax and grow into equiaxed grains with the function of time.

4.1.1.2 Along the depositing direction

Along the line which is parallel to the welding direction, it was seen that not all portion exhibiting the clear layered microstructure in 4.1.1.1. In some portion, the deposited zone is not presenting, and HAZ did not show much difference from the EZ, only undergo very little growth of the grains.

Figure 4.3: Variation in the grain morphology along the y-axis, (a) revealing the microstructure at the left side above the previously deposited layer. Image (b) is the microstructure taken from the position about 2mm away from(a), dashed line in (a) and (b) representing the interface between the deposited zone and HAZ . Here the size of grain is presented with a measurement of the diameter in both x-aixs and y-axis. Table (c) showing the measured size of grains within HAZ in both position (a) and position( b). Chosed grains are outlined with red dashed line, grain a, b are found in deposited zone at position (a) while c,d are from HAZ in position (b). (d) and the (e) revealing the grain distribution within the deposited zone under an increased magnification in both position (a) and (b)..

In Figure 4.3, the black line indicating the interface between the HAZ and the deposited layer, (b) is taken only about 2mm away from each (a). These two types of the structure are usually presenting close to each other. In 4.3 (a), it was showing a coarser grain size in both HAZ and DZ compare with that in 4.3(b). There are two grains from each were chosen from (a) and (b), this was indicated in figure 4.3 (c). Meanwhile, the size of the columnar grains in (a) is wider than them in (b). The deposited zone which is just above the HAZ is shown in the (d) and (e). It is clear that under the same scale, the grain displayed in (d) is much larger than that in (e).

Figure 4.4: Schematic drawing showing the process during wire deposition.

During the WAAM procedure, the molten wire is deposited on the previously deposited layer also the wire which is deposited next to it (Figure 4.4). Gu 21as make a simulation showing the heat distribution on the surface of the sample during WAAM. It was showing a significant heat energy at the depositing point, however, was radiating to the surrounding, therefore, the pre-deposited wire gained extra heat energy for growing, and in a different thermal gradient, therefore, the morphology is different in the nearby region.

Its complex microstructure is dominated by the thermal cycle during the WAAM progress, some portion underwent re-heating from the solidified or solidifying condition comparing with the presenting literatures6,19, the known microstructure for AM produced metal. In CMT-welding area, Liang42 have discovered multiple zones around the welding interface including weld zone, fusion zone, partly melted zone and HAZ zone. However, the partially melted zone is not presenting in the microstructure of WAAM produced AA2024. It was suggested that in WAAM produced AA2024, there is the low heat input profit from an improved function in CMT-PA, and possibly the similarity of the ‘base metal’ and deposited metal.

4.1.2 Porosity distribution

In AM produced aluminium alloys, the discussion of porosity term is necessary, as the high solubility of hydrogen gas in the molten aluminium, and very low for solid aluminium similar with welding, therefore there is higher opportunity to entrap shielding gas or hydrogen containment in the supply wire23. Insufficient oxide removal also would lead to the formation of pores.

The removal of coarse constituents during polishing could also lead to certain porosity in which diameter is determined by the size of IMs. Cong23,24 has found the pores are accumulating along the interface layer(HAZ), as the hydrogen bubbles are trying to escape from the deposited layer. However, due to the high cooling rate, most of them are entrapped along the interface, very close to the HAZ. However, it is known that during the WAAM, a small WFS/TS ratio(wire feed speed)/ TS (transfer speed) ratio 24 determines the porosity content. When WFS/TS is 10, the pores are distributed uniformly in the sample. When it is greater than 20, it was reported the concentration of the pores along the interface of deposition, representing a steep thermal gradient. To explain this, if a small WFS ratio is applied, the corresponding thermal gradient is also reduced result in limited diffusion rate of bubbles.

Figure 4.5: Pore distribution on the surface of the portions which are taken from (a) upper left corner (b) lower left corner (c) lower right corner of a big given sample. In (d), pores with size greater than 50μm are counted and record. as from each of the portion (a), (b) and (c), there are 9 points are taken, (e) showing the position of these nine points.

Figure 4.5 shows the pore distribution of the surface of the sample at different positions. 4.5(a) is taken from the left-upper region, 4.5(b) is taken from the left-down region, and c is taken from the right-down region. These areas are selected and recorded respectively. The figure 4.5(e) showing positions of nine different spots which are chosen to take images from the sample (a), (b) and (c). (d)-1, 2, 3 are showing the number of the pores greater than 50µm at those positions.

As mentioned before, the removal of IMs could result in pores as well, to separate them from the pores formed during metal formation, only pores greater than 50µm are countered. As in Rebecca’s research75, she has found in AM produced stainless steel, the pores which are greater than 50μm are active to corrosion. Figure 4.5 has proven a random distribution of pores over the whole surface of WAAM produced

AA2024, there are some pores with a diameter of over 100μm might formed under the immigration of neighbouring hydrogen bubbles during the solidification process.

4.2 Corrosion behaviour of the WAAM produced AA2024

In section 4.1, the microstructure and the chemical condition of the WAAM produced AA2024 is discussed. In Literature review, it was highlighted that the corrosion behaviour is very dependent on the internal microstructure, including the grain boundary size, distribution and the composition of the IM compounds.

The major idea is the clustering of the IMs and the concentration of Cu content at different portion. With the lack of the TEM technique, the influence of the dispersoids and the precipitation are negligible in this study. There are few focus areas which are: corrosion morphology in different scale and immersion condition is discussed, initiation mechanism of pitting corrosion and intergranular corrosion and a comparison of the general corrosion resistance with the AA2024-T3.

4.2.1 Compositional analysis and second phases distributions

Decomposition of the eutectic phase along the GB is observed under SEM. The Figure 4.6 showing the GB next to pre-existed pores within deposited zone, and EDX is performed to find chemical composition of GB.

In Figure 4.6 the EDX point analysis is performed at point A and B which are from the bright and dark region within the GB. It shows the brighter region is comprised of θ phase while the darker region is comprised of S phase.

In Figure 4.7, and EDX mapping is conducted. This EDX mapping reveals the distribution of Cu, Mg, Fe and Mn elements in the selected area. In Mn and Fe mapping, those portions have the strongest contrast indicates the presence of Al-Cu-Mn-Fe phase.

In Figure 4.8, the EDX mapping conducted on a ‘peanuts’ shaped compound. It is suggested as a cluster of S phase and θ phase. Few black dots presenting of the centre of S phase without any contrast been seen in the EDX mapping.

In the figure, GB in the sample is decorated by clustering of S phase and θ phase. This micro-segregation phenomenon is investigated for small volume per deposition and a low heat input during the CMT printing. It was believed a high cooling rate is reached during solidification but is not enough for rapid cooling of the entire deposited part 22. During the cooling, α matrix which is in the centre of a dendrite solidifies first and leaves most of the copper and magnesium element at the periphery. Some round IM compounds observed in the grain body as the result of diffusion of alloying elements under re-heating by the wire addition at the side. In the conventional process, homogenization is followed to relieve the segregation of the second phases on GB. However, for WAAM produced AA2024, it would also lead to coarsening of the grain results in a loss in mechanical strength.

From the Figure 4.6, 4.7 and 4.8, it was found the population of the S phase is greater than that of θ phase, and a relatively low amount of Al-Cu-Fe-Mn phase also is detected. As S phase is formed prior to θ, therefore, Cu is firstly consumed to form S phase, then θ phase76. α matrix is decorated with a little amount of dissolved Cu, Mg Fe and Mn element which could be seen in 4.7 and 4.8.

Figure 4.6: (a) Scanning electron micrograph of the WAAM AA2024 alloy structure showing the interdendritic intermetallics and EDX spectra obtained at the locations indicated as A and B in (a), revealing Al2Cu phase and Al2MgCu phase at the grain boundaries.

a) c)

d) e) f)

Figure 4.7: (a) revealing the interdendritic intermetallics in Backscatter electron mode. And a corresponding EDX spectrum mapping is conducted on this region. Picture (b), (c), (d), (e) and (f) are displaying the mapping of element manganese,a) aluminium, magnesium iron and copper element.

c) d)

Figure 4.8: (a) revealing the interdendritic intermetallics in Backscatter electron mode. And a corresponding EDX spectrum mapping is conducted on selected region. Picture (b), (c), (d) are displaying the mapping of element aluminium, magnesium and copper element.

Figure 4.9: Electron Backscattered Diffraction (EBSD) analysed euler colour diagram of the deposited zone within WAAM produced 2024 alloy, exhibiting uniform fine size equiaxed grains.

For 2024-T351 alloy, the energy contained in the grain is important, as they have undergone cold-rolling for dislocation strengthening, dislocations lines are introduced in the sample, and accommodating within grains. Therefore, the rise in the energy of the grain, it has a strong effect on the intergranular corrosion, as IGC is favoured for high energy grains58.

However, the WAAM produced AA2024 structure theoretically similar with casted AA2024 structure. The as-cast structure confirms the lack of the dislocation within the grain. In Figure 4.9, EBSD diagram of the sample showing the preferential orientation of grains and their distribution on the scanning area. In Figure 4.9, the colour of neighbouring grains is from completely different classes; therefore they are orientated

far different from the neighbouring grains. These features represent the random orientation of the AM produced sample in the deposited layer. This random orientation could be the result of the non-flat surface of the pre-deposited layer.

4.2.2 Understanding the electrochemical process

4.2.2.1 OCP Electrochemistry is one typical method on understanding corrosion behaviour of the sample. In WAAM produced AA2024, the OCP varied between -700 to -820 mV (vs silver/silver chloride reference electrode). This value is obtained from more than ten OCP tests in 3.5% NaCl solution.

The corrosion potential AA2024-T351 in 3.5% NaCl could be varied from -682mv to - 555mv (vs SCE)57,60. This lower OCP value is believed because of distribution of second phase on GB, segregation of alloying elements or lack of thermomechanical process conducted on the workpiece. This indicates worse corrosion behaviour of WAAM produced AA2024 than AA2024-T3.

4.2.2.2 Anodic polarization

Figure 4.10: Anodic polarization curve of WAAM produced AA2024 (with a scanning rate of 0.1mV/s) observed under the (red) de-areated 3.5% NaCl solution and (black) under areated 3.5%NaCl solution.

These two curves in Figure 4.10 represent the anodic polarization in the both aerated 3.5% NaCl solution and de-aerated 3.5% NaCl solution. The bubbling of nitrogen gas significantly reduces the dissolved oxygen in solution. Two curves give a different corrosion potential and higher current density on the sample, but the pitting potential is almost the same57. However, it was found that in aerated polarization, only the most active site is shown61, and the maximum current density is higher than that in de-aerated condition.

The polarization test accelerating the corrosion process while sweeping external voltage in anodic direction. It is a man-made progress, therefore, breakdown potential would increase with scan rate.

In AA2024-T3, there are two significant changes observed in the curve indicating two different corrosion processes. E1 appearing at -660 mV was representing the de-alloying process of S phase followed by nucleation of pits; E2 appears at -600 mv representing the onset of the intergranular corrosion.

iii B A ii

Figure 4.11: Anodic polarization line obtained for AA2024 sample in 3.5% NaCl solution (a) AA2024-T357 (b) WAAM produced AA2024. Three regions identified which are (i) pitting initiation stage; (ii) pitting propagation and (iii) transpassive region observed in WAAM produced AA2024.

Figure 4.11 (a) showing the anodic polarization curve observed for AA2024-T3 in 3.5% NaCl solution, there are two breakdown potentials appearing in the (a). It results in the significant difference of potential between pitting corrosion and IGC, the IGC occurs following the activation of the pitting. The curve in 4.11(a) only occurred for T3 or T3+ conditioned AA2024. As the propagation mechanism of IGC and pitting corrosion are significantly different61. The pitting potential is low (de-active) while IGC is active at a high potential.

In 4.11(b), this breakdown potential is pinned to the dissolution of S phases, followed by the propagation of metastable pits. There are some current transits observed while sweeping the potential anodically (before 퐸푝푖푡 is reached). These anodic current transits are showing initiation of metastable pits with subsequently re-passivation. The potential keep rising as the accumulation of corrosion product which confirms an environment for stabilization of the pits.

4.2.2.2.1 Stabilization of the pit

In figure 4.11(b) the feature before point A showing propagation of metastable pits. it would be stabilized once the corrosion product formed accompanied with the evolution of 퐻2 gas. Since the environment in the bottom of pit is different from the surface. Solution potential gradient formed between the pit and solution. Chloride ions are tending to migrate into the pit due to the difference in solution potential while aluminium ion or magnesium ions are migrating outward the pit.

At Point A, the stabilized pit is initiated accompanied with destabilization of passive film. From A to B the pitting corrosion is dominant. The current density observed rises sharply with a small change in voltage in region ii.

4.2.2.2.2 Transpassive Dissolution

Keep sweeping the voltage anodically, the current density reduced but still increase steeply. After point B, it is transpassive region in which the reaction is demonstrated by dissolution of the passive film. Pitting corrosion is no longer dominant while the oxide film is no longer protective. It usually observed with hydrogen evolution as well.

If reversing the external voltage, a passivity region could be identified which is could result from the expansion of the pit has reached a threshold, once large pit formed while it grows in three-dimensional. The external voltage is not enough to support the further expansion; the dissolution rate would reduce gradually while the metal is passivated again.

4.2.3 Corrosion onset mechanism

Scanning electron microscopy imaging performed after a sufficient immersion period, during this section, the onset of corrosion could be classified into two types: de- alloying of S phase, or from the pre-existed pores.

4.2.3.1 De-alloying of θ/S phase and clustering of IMs

In Figure 4.15, S phases have shown sufficient de-alloying phenomenon, it left a porous structure of remnant Cu-rich particle in the centre of the original position and the periphery is etched out. In the SEM images, the S phase, θ phase and Al-Cu-Fe-Mn phases are occupied as clustering together along GB. In the clustering, while in the attractive environment, the S phase occurs dissolved prior to others. There are following features are seen in the cluster of them after 10 minutes immersion, there are some black spots and lines representing the de-alloying and the trenching of the periphery. Apart from them, few clustering of IMs exhibits porous structure only at the location of the S phase.

The Figure 4.14 exhibits the corrosion morphology of cluster of IMs (in figure (a)) isolated θ phases and S phase after 10 minutes immersion in 3.5% NaCl solution:

In the cluster: S phase exhibits porous structure, leave the remnant of non-attacked clean θ phase and Al-Cu-Fe-Mn phase sitting next to porous remnant. Meanwhile, some isolated S phase and θ phase which is just next to the clustering are corroded.

Isolated S phase: It exhibits both de-alloying and the trenching out around the periphery of the compound. De-alloying occurs earlier than trenching. In S phase, the trenching is a result of the reaction between Cu-depleted zone, matrix and Cu remnant.

Isolated θ phase: de-alloying occurs in the centre of the θ phase without the trenching around it.

57 The S phase is more active to corrosion, as the potential of Al2MgCu phase is -830mV

57,60 under deaerated 0.1 NaCl solution and Al2Cu phase is -484mV . Which is nearly 400 mV negative than Al2Cu phase. Comparing the corrosion features within the isolated S phase &θ phase and cluster of IMs. It was figured out the S phase in the cluster has corroded faster than isolated while θ remaining un-attacked in clustering. Therefore, it shows coupling between the S phase (low potential) and θ phase (high potential), which

results in the more serve corrosion of S phase region. Boag has shown a detailed work on the coupling between second phases in cluster 60.

In Figure 4.16, more serve de-alloying features occurs after an hour immersion test, in 4.16 (a) the remnant in the centre is isolated from the surrounding due to the trenching around it. Meanwhile, in 4.16(b) many fragments of IMs are observed in within the GB which ought to be the θ phase or Al-Cu-Fe-Mn phase.

Figure 4.12: Scanning electron image in SE mode revealing the (a) which consisting of one isolated S phase (b) showing isolated θ phase clustering of IMs after 10 minutes immersion testing.

Figure 4.13: Scanning electron microscopy in SE mode revealing (a)De-alloying feature of the clusters from different position after an hour immersion testing. Images were taken at (a) within in equaixed zone (b) next to an open pore.

4.2.3.2 Initiation around the pore

Remind the high population of pores on the as-polished sample (in Figure 4.3). As known, most of the metastable pits die due to the rapid self-healing ability of the aluminium alloy oxide film50. Only some of the pits which have been successful acidified and with a high concentration of Cl− ions in the bottom of the pit could survive and become stable pits. However, as the sample has sufficient porosity developed on the surface during the either printing or polishing stage.

When the pore which has a sufficient depth, there is a lower concentration of oxygen at the bottom of the pore. The potential difference arises between the bottom of the pore and the metal surface. Pore acts as a ‘bowl’ for the aggressive ions, If the depth is smaller than 10μm, it is reported that the passive film would automatically form on pit75. Therefore, during the immersion test, pores in Figure 4.16 could act like a ‘stable pit’. The shape of it allows the accumulation of Cl− and hydrolysis reaction. So, the pre- existed pore acting as a heterogeneous site for the nucleation of the stable pits increases the opportunity for a successful pitting around them.

a b

Figure 4.14: Scanning electron microscopy showing the corrosion product propagation morphology next to pores after 10 minutes immersion testing in the 3.5% NaCl solution. (a) showing the formation of thin layer of corrosion product within a 20μm pore (b) revealing the propagation of corrosion product from the bottom of 10μm pores.

4.2.4 Intergranular corrosion

In this study, it was recognized that most of second phases are all habiting on the GB. Additionally, most of IMs are clustered. The WAAM produced AA2024 suffers serve intergranular corrosion due to the segregation of second phases on GB. In the heat treated AA2024-T3 sample, the activation of the intergranular corrosion is the formation of the dispersoids free zone around GB, S phase and copper depletion zone near the GB57,65 and the precipitates which is sitting on the GB are found undergo de-alloying as well60. Distinct electrochemical potential investigated between GB and its periphery.

In Figure 4.18, images are taken around the corrosion site ‘A‘ after 2 hours immersion testing. in SE mode, there is a stable pit is protected by aluminium hydroxide while in BSE mode, it reveals the corrosed portion which exhibiting a darker colour. The corrosion is proven propagating from the stable pit continuously.

In Figure 4.19, cross-section sample is prepared by ultratomy, porous structure left. (as in 4.20(b)) Meanwhile, in the cluster, the θ phase and the Al-Cu-Fe-Mn phase remain unattacked.

It was believed the onset of IGC in WAAM produced AA2024 is the same as pitting mechanism: de-alloying of the IMs cluster. It was suggested that with an extended time, the pit grows three-dimensionally along the GB as they are acting as anodic path for access to further attack. The GB is suspect to be attacked as they are composed of second phases which have different chemical potential with matrix and is favoured by aggressive ions.

Figure 4.15: Scanning electron microscopy examination of a circle of corrosion product and its surrounding information, (a) Secondary electron mode imaging for the tomography of the selected region and (b) Backscattered electron mode revealing the corrosion propagation from the pit ‘A’.

Figure 4.16: Scanning electron microscopy revealing the cross section of WAAM produced AA2024 after 20hrs immersion in 3.5% NaCl solution. (b) illustrates the intergranular corrosion from the corroded surface and (a) revealing the feature within the corroded interdendrite intermetallics under IGC.

Figure 4.17: Scanning electron microscopy revealing the cross section of WAAM produced AA2024 after 20hrs immersion in 3.5% NaCl solution. (a) Revealing the feature within the corroded interdendrite intermetallics under IGC. (b) Illustrates porous structure left on the original S phase.

5 Conclusions and Suggestions for Future Work

5.1.1 General corrosion morphology

The corrosion solution chosen for this study is 3.5% NaCl. As-polished samples are immersed in the solution for 10 minutes, 30 minutes, 1 hour, 2 hours, 10 hours and 20 hours.

There is ‘white band’ formed and accumulated after sample was immersed in 3.5% NaCl for more than 1 hour. During first 30 minutes, the white band starts to develop on the surface of the sample.

1cm 1cm 1cm

a b c

Figure 5.1: 'banded’ product accumulation developed with time.

The ‘white rust’ aluminium hydroxide formed along the HAZ of the sample. The chemical composition of it is 퐴푙2(푂퐻)3

3+ + 퐴푙 + n퐻2O  2퐴푙(푂퐻)푛 +n 퐻 (7)

Hydrolysis reaction listed above is accompanied with the dissolution of Mg and Al in the bottom of the pit cause the acidification inside the pit. The corrosion product piles up around the pit around it with evolution of 퐻2. The presence of corrosion product would

protect the stabilization of the pit. Eventually, the corrosion rate has increased rapidly during the pit propagation stage.

As shown in Figure 4.12, during the induction of sample in the 3.5% NaCl solution, at first hour, the white rust formed on the surface. With a prolonged time, the oxide film above the deposited layers is found transferred from transparent to pale yellow colour while white rust piling up at the interfaces.

Figure 5.2: optical microscopy image of the sample experienced 30 minutes immersion test in 3.5% NaCl solution, these images are taken along the line which is perpendicular to the building directions.

As stated in Figure 4.13, these interface regions showing a darker (white rust propagation) colour under the optical microscope, and have a high population of pores than the rest area.

These pores are stable pits evolved during immersion test. Therefore, it suggested pits propagation starts preferentially along the HAZ. It exhibits different corrosion susceptivity at the interface and deposited layer. The possible reason for the susceptivity is might because the big cathode area/anode area ratio in the HAZ where the matrix is the cathode, and the second phases sitting on the GB acting as anode in the reaction.

Figure 5.3: Optical microscopy images showing the evolution of the corrosion morphology on the surface of the WAAM produced AA2024 after immersing in 3.5% NaCl solution for (a-b) 30 minutes (c-d) 1 hour and (e-f) 2hours. Image are taken for both HAZ (a,c,e) and in equaixed zone(b,d,f) revealing a different corrosion morphology on them.

In Figure 4.14, the evolution of surface morphology under optical microscopy with time. It was found, the pitting corrosion occurs first within the HAZ. In Figure 4.14(a-b) while few pits observed in the deposited layer, the IGC is seen in HAZ. And in Figure (c-d) and (e-f) the corrosion product is preferentially propagated at HAZ which means more serve 73

corrosion at this region. The width of the corrosion product is increased as the result of the evolution of hydrogen gas. It was believed that the suspecitivity of HAZ result in that it was corroded first and with a higher corrosion rate than the deposited layer. The differential cell built between these two zones, therefore the corrosion products are accumulated at the HAZ.

5.2 Conclusions

In the present study, the microstructure and corrosion behaviour of wire arc addictive manufactured (WAAM) 2024 aluminium alloy structure are investigated.

Microstructure:

A layered structure is observed in wire arc addictive manufactured 2024 aluminium alloy structure. Each layer consists of four different zones, including: heat affected zone (HAZ), deposited zone, columnar dendrite zone and equiaxed dendrite zone.

Porosity with typical dimension ranging from 20 µm to approximately 200 µm are present in the WAAM structure. The distribution of porosity is relatively uniform with slightly high population density observed in the HAZ.

The WAAM results in two different types of interdendritic intermetallics in the alloy, namely: S-phase (Al2MgCu), and θ-phase (Al2Cu) phase, which are mainly distributed along the grain boundary network. Further, the S-phase (Al2MgCu), θ-phase (Al2Cu) and Al-Cu-Fe-Mn phase intermetallics are also observed within the grain interior.

Overall, the microstructure of WAAM structure is very similar with that produced by casting. Due to the smaller volume for each solidification process and the relatively high cooling rate during WAAM, relatively fine grains are observed in the WAAM structure.

Corrosion:

Increased amount of corrosion product was observed at the HAZ of each layer in the alloy after immersion testing in 3.5 wt. % NaCl solution, suggesting the higher localized corrosion susceptibility of HAZ in WAAM 2024 alloy relative to other zones.

High population density of porosities in HAZ may lead to its high localized corrosion susceptibility since the occluded environment of porosity provides the necessary chemical condition (low pH and high Cl-) for localized corrosion development.

Localized corrosion in the WAAM 2024 alloy structure develops in the form of both pitting and intergranular attack.

The high intergranular corrosion susceptibility of WAAM 2024 alloy is attributed to the formation of S-phase and θ-phase interdendritic intermetallics, which are present along the grain boundaries. Selective dissolution of Mg and Al from S-phase results in Cu-rich remnant. Along with θ-phase, the Cu-rich remnants act as effective cathode to promote the anodic dissolution of aluminium in the periphery, resulting in intergranular corrosion of the alloy.

5.3 Further work

5.3.1 Analysis of the electrochemical potential in the different regions

Especially the HAZ, deposited metal zone and mushy zone. The micro-segregation within the mushy zone representing a higher concentration of Copper. This might result in some change in the electrochemical potential. Kevin probe could be used to test the electrochemical potential within each layer.

5.3.2 IGC propagation in both HAZ and Deposited zone.

The analysis of IGC could be conducted with a very short time immersion, and then use longer time, as their propagation rate and the initiation time are different. In addition, a further analysis could have conducted on the continuous sites or discontinuous sites 75

on the corroded surface, the corrosion site could be a cluster of Al and Cu underneath the surface. A further ultramicrotomy could be used to obtain the cross section of this site.

5.3.3 TEM study

With the application of TEM, the identity of corrosion site could be figured with Kikuchi band. It requires preparing a TEM foil off the corroded surface. Additionally, heat treatment could be conducted on the sample, and the distribution of the precipitates and dispersoids could be found nearby the GB.

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