Browsing by Author "George, Sarah"

Now showing 1 - 13 of 13
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    Open Access
    Correlation between homogenisation parameters and property evolution of AA3104 CBS after hot finish rolling
    (2025) Lebakeng, Khethisa; George, Sarah
    AA3104 can-body stock (CBS) is produced through a series of thermomechanical processing stages, including direct chill (DC) casting, homogenisation, hot rough rolling, hot finish rolling, and cold rolling. Homogenisation is an important phase that controls the growth of intermetallic particles (IMPs) and dispersoids, affecting mechanical performance and texture evolution in following rolling stages. This study investigates the impact of different homogenisation procedures on the microstructure and mechanical properties of AA3104 during lab-scale hot rough rolling and plane strain compression (PSC) testing, which simulates hot finish rolling. Industry supplied DC-cast AA3104 ingot material was exposed to various homogenisation procedures at temperatures ranging from 500°C to 610°C to produce differences in IMP morphology and dispersoid dispersion within the structures. The homogenised samples were subjected to lab-scale hot rough rolling to achieve strain and IMP breakdown in line with that of industrial rough rolling. PSC testing on a Gleeble 3800 was utilised to simulate industrial hot finish rolling conditions. The microstructural evolution was investigated using light microscopy, scanning electron microscopy (SEM) in conjunction with energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) for grain and texture characterisation. The findings show that homogenisation temperature has a significant influence on IMP fragmentation, dispersoid refinement, which then has a marked effect on recrystallisation behaviour and texture development. The findings show that the 600°C/520°C condition resulted in coarse dispersoids (127 nm) with low number density (8.93 particles/µm²), the 560°C/520°C condition produced intermediate sized dispersoids (104 nm, 19.85 particles/µm²), and the 520°C condition generated fine, closely spaced dispersoids (56 nm, 55.92 particles/µm²). The cube texture intensity was found to increase with dispersoid density, with the 520°C sample displaying the strongest cube texture (~9.04), followed by 560°C/520°C (~7.5) and 600°C/520°C (~4.8). the 560°C/520°C homogenisation resulted in the largest recrystallised grains. While the 520°C homogenisation contained the finest dispersoids, the EBSD maps showed a fully recrystallised structure with recrystallised grains being smaller than the 560°C/520°C homogenisation. This is attributed to a combination of insufficient pinning pressure to fully retard recrystallisation as well as IMP topology effects on PSN and the overall recrystallisation kinetics, resulting in a smaller grain size than anticipated. There was no recrystallisation in the as-cast protocol, demonstrating that the dispersoids play an essential role in recrystallisation. None of the high temperature homogenisation practices resulted in sufficiently fine dispersoids to retard recrystallisation. These findings emphasise the significance of adjusting homogenisation parameters to alter microstructural features, such as IMPs and dispersoids for better hot rolling performance and final property and microstructural development. The findings do illustrate the importance of a more detailed interrogation of the role of these critical features in the rolling process. This should be achieved on a laboratory scale initially. The experimental protocol for this work used a fully lab scale simulation approach, which showed successful alignment with initial partial industrial samples. The microstructures of the 560°C/520°C material were compared to samples where only the finish rolling was simulated (on industrially supplied transfer bar after industrial homogenisation and rough rolling). The grain size and cube texture intensity of the two approach showed similarities in grain size and cube texture intensities. This indicates an equivalent thermomechanical processing route to achieve these similar microstructures. This is a successful first step in the validation of a full through process simulation of processing on a laboratory scale.
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    Open Access
    Development and implementation of an opensource DIC acquisition system, focusing on temporal synchronization with the Gleeble 3800
    (2025) Vos, Maxwell; George, Sarah
    The Gleeble 3800 is a thermomechanical processing simulator with the ability to perform high temperature, high strain rate deformation experiments. The standard Gleeble 3800 uses electrical resistance heating for temperature control and a closed loop hydraulic actuator for displacement control. In order to extract greater accuracy and control of simulated processing parameters, the incorporation of a synchronised Digital Image Correlation (DIC) capability with equipment such as the Gleeble 3800 has become essential. Current commercial DIC systems offer limited versatility, particularly with regards to integrating DIC and thermal imaging on the Gleeble 3800 in addition to being financially prohibitive for many researchers. Current open-source acquisition solutions lack the specifications required for most DIC applications and custom solutions are overly complex for many DIC users. This research therefore aimed to develop an open-source DIC acquisition system for materials research applications that fulfil these shortcomings. The DIC Capture system addresses these limitations by using high quality hardware that interfaces with user orientated software, thus facilitating complex material testing protocols at a reduced cost. The DIC Capture system comprises of a hardware data acquisition (DAQ) board, full field camera system and control computer. The DAQ is controlled by an Arduino Nano ESP32 that sends camera trigger times, voltage values from the ±10V ADS8584S analogue to digital converter (ADC) and sync times with the Gleeble 3800. The camera trigger frequency follows a predetermined sequence and is iterated through a command in the Gleeble Script Language (GSL) program, which controls the Gleeble 3800, thus allowing for event-based frame-rate changes. The Python based software displays and saves images from the DIC cameras, synchronizing all data received from the DAQ board and any other image recording sources. The DIC Capture system can produce high quality DIC data sets and integrates with the Gleeble 3800 for high temperature DIC applications. The DIC capture system has been successfully tested in three different case studies across a range of testing conditions.
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    Open Access
    The effect of laser shock peening and shot peening on the fatigue performance of aluminium alloy 7075
    (2017) Becker, Alexander; Tait, Robert B; George, Sarah
    It has been well established that most fatigue cracks initiate from stress concentration sites found on the surfaces of components subject to cyclic fatigue loading. The introduction of residual compressive stresses into the surface layers of components, through various means including shot peening and laser shock peening, can result in local residual compressive stresses which provide a resistance to both crack initiation and propagation, thus leading to an increase in the fatigue life of the components. The effects of both laser shock peening (LSP) and conventional shot peening (SP) on the fatigue properties of both 7075-T6 and 7075-T0 aluminium round bar test specimens were investigated and compared by means of cyclic 3-point bend fatigue testing. This investigation focused on the role that the peening induced microstructure, surface morphology and hardness had on the fatigue life of the test specimens. It was found that both the laser shock peening and shot peening processes substantially increased the fatigue lives compared to unpeened AA7075-T6. The laser shock peening process more than doubled the fatigue life of the specimens and the shot peening process increased the fatigue life by approximately 1.6×. No discernible hardening effects could be determined in the laser shock peened specimens. However, the shot peening process resulted in a distinct hardened region within the surface layers of the AA7075-T6 specimens which was attributed to the longer pressure duration of the shot peening process which results in greater plastic deformation. It was also shown that polishing the shot peened and laser shock peened specimens after their respective peening procedures resulted in a significant increase in fatigue life. Polishing after peening resulted in a 3.4× fatigue life increase in the shot peened test specimens (T6 condition) and a 5.4× fatigue life increase in the laser shock peened test specimens (T6 condition). This result highlights the role that surface roughness plays in component fatigue life. Furthermore, the increase in the average fatigue life of the polished test specimens shows that the depth of the residual compressive stresses induced by the peening processes were deep enough to allow for surfaces layers to be removed from the test specimens without any detrimental effect to the overall average fatigue life of the components. The result also suggests that the magnitudes of the residual stresses induced by the laser shock peening process being greater than those of the shot peening process. The main difference between the peening treatments was demonstrated as originating from the surface roughening effects of the two peening procedures. The laser shock peening process only slightly increased the surface roughness of a polished AA7075-T6 test specimens. The shot peening process severely affected the surface roughness of the test specimens, creating many potential crack initiation sites. The AA7075-O test specimens (annealed) showed no overall improvement in their fatigue life, regardless of the mechanical treatment received. The increased ductility of the specimens during the 3-point bending fatigue process led to stress relieving of the peening induced compressive stresses. The specimens were however still fatigued to failure. This enabled the analysis of the effect of the peening induced surface roughness to be analysed. It was found that the shot peened and laser shock peened surface roughness values were significantly higher than the roughness values of the T6 specimens owing to the increased ductility and thus workability of the test specimens. These increased surface roughnesses resulted in the shot peened test specimens failing before the laser shock peened specimens. Both sets of peened specimens failed before the "as machined" and polished test specimens highlighting the role that their induced surface roughnesses had on their fatigue lives. The cross-sectional microstructures of the peened samples in each material condition showed varied changes in the microstructure of the treated aluminium alloy. There was evidence of a large degree of plastic deformation near the surface of shot peened specimens in both material conditions. However, there was limited evidence of changes to the grains structure of the laser shock peened specimens, in both material conditions. In addition, the ability of the laser shock peening process to recover fatigue life in damaged components was also investigated. This brought into question whether the laser shock peening process can be used on a partially fatigued component at the point of crack initiation, in an attempt to further improve the fatigue life of the component. It was found that the laser shock peening of the cracks initiated in fatigue life recovery process did little to effectively recover fatigue life in the damaged components. A degree of life extension was present as cracks re-initiated after a few thousand cycles and was attributed to crack tip closure. This closure led to a general reduction in the fatigue crack growth rate when compared to laser shock peened/polished test specimens fatigued at the same stress.
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    Open Access
    The effect of stabilization heat treatment on AA5182 aluminium alloy
    (2013) Morrison, Graham K; George, Sarah; Knutsen, Robert D
    AA5182 aluminium alloy is used for the manufacturing of can ends for beverage cans. The alloy selection for this part is based on the formability of the material and its resistance to softening over time. Owing to the intricate design of the can end opening tab, it is vital that the material maintains its strength during its shelf life. The mechanical properties of the AA5182 aluminium alloy are dependent on the microstructural evolution of the alloy during processing and forming. Al-Mg alloys, like AA5182, can undergo a low temperature heat treatment, which has the effect of stabilizing the microstructure and minimizing the subsequent recovery processes during and after coil coating. The effects of these heat treatments have been investigated in order to understand the effectiveness of the stabilization heat treatment on the AA5182 alloy. This study investigates various stabilization heat treatment temperature profiles, and then aims to characterize the microstructural evolution of the material during a simulation of the coil coating practice that the material is exposed to as the final step in the rolling mill operation.
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    Open Access
    Evaluation of the stress ralaxation technique for measuring softening kinetics in aluminium alloys
    (2006) George, Sarah; Knutsen, Robert D
    The development of the microstructure during thennomechanical processing (TMP) is critical in determining the final properties and quality of metal strip. In the particular case where aluminium sheet is used for lithographic applications, the surface appearance after electro-etching should be devoid of any streaking or inhomogeneous discolouration. The cause of possible streaking effects can be related to poor microstructure development during TMP and often arises as a result of inadequate recrystallisation. To avoid the deleterious effects, it is important to implement the appropriate rolling conditions in order to control the processes of recovery and recrystallisation. The means by which the correct rolling conditions can be established is usually by extensive laboratory simulations and concomitant microstructural analysis. In view of the fact that this approach is often tedious, the present study has investigated the use of the stress relaxation technique to provide rapid data on the recovery and recrystallisation kinetics for commercial purity aluminium under defonnation conditions that closely simulate hot rolling operations. Stress relaxation (SR) curves have been generated for AA1200 aluminium, as well as for two magnesium containing alloys, namely AA5182 (5wt% Mg) and an experimental alloy (Al-l %Mg). Fully recrystallised microstructures were subjected to uniaxial compression in the temperature range of 300-400?C. Strains and strain rates were up to 0.7 and ls respectively. Stress relaxation was measured for intervals up to 15 minutes and the evolved microstructures were examined after fixed intervals using polarised light microscopy and electron backscatter diffraction.
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    Open Access
    Flexible Media Polishing Machine for Ti-6Al-4V Components
    (2022) de Jongh, Quintin Oliver; Kuppuswamy, Ramesh; George, Sarah
    New-age components (notably those in the bio-medical and aerospace industries) are often manufactured from hard to machine materials such as Ti-6Al-4V and tungsten carbide and have extreme surface finish requirements. Flexolap polishing offers a technique to achieve these requirements, and minimizes the disadvantages encountered by other polishing techniques. The aim of this dissertation is that of designing and developing a working Flexolap polishing machine (and associated abrasive compound) suitable for the South African manufacturing industry (and any industry with a lack of financial means and skills for advanced manufacturing). This aim was affirmed by developing models to characterize the Flexolap polishing process, as well as by using experimental analysis to verify the process viability and compatibility with Ti-6Al-4V (and subsequently, other easier to machine materials). A thorough literature review is presented with the most important conclusions of optimal polishing conditions (high hydration, high media impinging velocity and a 45º polishing angle) and applicable modelling methods (momentum, critical values, and vibrational analysis). Two models (empirical and analytical) and a supporting simulation model are presented for force control and stipulation of required conditions for ductile regime polishing to occur. A third model is presented later in the dissertation and classifies the viscoelastic nature of the designed abrasive as well as its damping effect as the compounded media is hydrated (the media-workpiece interaction becomes more underdamped). The results of all modelling processes provide proof that effective polishing can be achieved at designed working conditions (times of less than 10 minutes to desired surface roughness and contact stresses less than that for brittle failure but greater than that required to induce ductile regime polishing). The process of creating/building the Flexolap machine is described through design and development. Process calculations to support design include belt and pulley calculations, bearing calculations, steady abrasive/air flow calculations and weld strength calculations. Experimental techniques include gathering force data (using a dynamometer) and finding surface properties (using a profilometer and various microscopes). This led to gathering a large set of results which were used to study the process viability. Analysis is based on surface roughness and texture change over time (or change over other parameters), as well as the changes in force over parameters such as media hydration, media diamond concentration and media velocity. Experimental results display trends of polishing forces decreasing with an increase in hydration, while surface roughness decreases in a logarithmic manner (with a great initial decrease before reaching a convergence point). Surface texture is also shown to improve with a lesser presence of asperities and a more uniform texture overall (with greater hydration levels). Higher impinging velocities lead to lower surface roughness being achieved quicker while higher diamond concentration tends to create higher roughness at higher impinging velocities. Medium-high media hydration (30%), higher impinging velocities (31.4 m/s) and a polishing angle of 45º are proven to be the most effective polishing conditions. The study was successful in proving the viability and effectiveness of the Flexolap polishing technique while also providing pertinent experimental and theoretical data towards the further study of the process. The expected benefits the outcomes of this study provide to industry are: an easy to learn finishing process that can easily integrate into a finishing or manufacturing workshop (thus upskilling operators), an efficient and cost effective means of polishing hard to machine materials (reducing cycle time and cost), and a framework of polishing machine that can be easily adjusted to meet industry needs for example: size/shape of workpiece and automation of the process. This study was embarked on due to the lack of inexpensive and easy to operate polishing methods available in South Africa, particularly because raw titanium is often exported for processing due to the low availability of advanced manufacturing equipment for the material in South Africa as well as the low level of operator experience in advanced manufacturing machinery.
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    Open Access
    Investigating effects of interpass time during hot finish rolling on the evolution of texture in AA3104 Can Body Stock
    (2025) Chaole, Nkopo; George, Sarah; Hanief, Nasheeta
    During the hot rolling of aluminium strips, inter-pass time variability along the strip length leads to non-uniform microstructure and texture development, which impacts material properties. This study investigated the microstructure and texture evolution in AA3104 can body stock (CBS) during hot finish rolling by simulating multi-pass, high-temperature plane strain compression tests on a Gleeble 3800. Two scenarios were simulated: Position A, with a longer inter-pass time between the first two passes and a shorter time before the third pass, and Position B, with reversed timings. Post-deformation, samples were annealed and analysed using Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD). Results indicate Position A exhibits smaller and larger grains, while Position B shows uniform grain size distribution. Texture analysis reveals stronger Cube texture in Position A and Goss and P texture dominance in Position B. These variations highlight the role of inter-pass time in recrystallisation behaviour, influencing grain structure and texture during hot rolling.
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    Open Access
    Investigation into the surface modification of Ti-6Al-4V to facilitate antimicrobial ionic silver integration for use in implantable orthopaedic devices
    Vazirgiantzikis, Iosif; George, Sarah
    Malignant bone tumours often require a patient to make the choice between limb salvage surgery and complete amputation. The Ti-6Al-4V alloy is the material of choice for implantable orthopaedic devices as it provides a favourable combination of biocompatibility, corrosion resistance and mechanical properties. The only drawback of titanium is that, owing to its bio-inertness, living tissue struggles to attach, creating an opportunity for bacterial adhesion. The “race for the surface” is the term given for the competition between living tissue and bacteria to colonise the implant surface. If bacterial adhesion occurs at a higher rate than tissue adhesion, the chance of infection rises significantly. It has been shown that there is an opportunity to give tissue adhesion the edge by slowing down the initial colonisation of the implant surface by free-swimming bacteria. Silver has a relatively low toxicity level of 28 mg/kg in the body. Current research has focussed mainly on reducing bio-inertness and improving the antimicrobial properties of titanium via the incorporation of silver. In general, the studies conducted on antibacterial surfaces are limited to testing the final sample directly in contact with bacteria, with no attempt to measure silver release rate profiles. The research in this dissertation aimed to investigate methodologies for the incorporation of silver into a modified surface of Ti-6Al-4V in order to facilitate an antimicrobial effect for use in orthopaedic implants. The methodologies investigated were: anodic oxidation of Ti-6Al-4V, followed by silver ion exchange; Ag-doped TiO2 fused to the surface of Ti-6Al-4V via anodic oxidation; and Ag ion implantation into anodically oxidised and polished Ti-6Al-4V. The generated surfaces and sub-surfaces were characterised microstructurally via SEM, FIB, TEM and AFM and chemically by RBS, XRD, AAS and EDS. Ag+ release rate investigations were conducted with the use of ICP-MS. This study was limited to the use of two anodising electrolytes (i.e. 0.5M H2SO4 and 2.1M H3PO4) and altering the AgNO3 concentration (0.05 - 5.0M) and Ag implantation dosage (0.4 - 1.2x1017 ions/cm2 ), where applicable to the method. Results from the Ag ion exchanged samples showed that, microstructurally, the surface produced via anodising in 0.5M H2SO4 and 2.1M H3PO4 were different in terms of pore morphology, Ra, pore homogeneity across the surface and crystal structure. Sub-surface analysis via FIB/TEM found that the ca. 200nm thick TiO2 samples all contained silver nanoparticles (AgNPs). Samples anodised in 0.5M H2SO4 produced an anatase crystal structure, whilst those anodised in 2.1M H3PO4 produced rutile crystal structures. Silver uptake by samples anodised in 0.5M H2SO4 showed decreases in Ag absorption at high (5.0M) AgNO3 ion exchange concentrations, relative to low (0.05M) concentrations. The opposite effect was observed for samples anodised in 2.1M H3PO4. Ag+ release curves corroborated the absorption data by displaying the same trends in terms of Ag+ release post ion exchange. It was concluded that it was a combination of diffusion bottlenecking and higher reactivity of the anatase phase formed during anodising in 0.5M H2SO4 with Ag+ versus the rutile phase that led to these trends. Synthesis of TiO2 powders showed that increasing the AgNO3 concentration (0.05-5M) resulted in AgTiO2 powders with increasing Ag content. Ag-TiO2 powder was successfully fused to the surfaces via anodic oxidation in 0.5M H2SO4 and 2.1M H3PO4 at 100V. Ag-TiO2 powder fused preferentially in areas where downward pressure was present. Microstructurally, the sub-surfaces produced an anodic oxide approximately 200nm thick, to which a significantly thicker, AgNP-containing, TiO2 was attached. XRD data indicated additional Brookite (020) peaks, owing to the presence of the attached Ag-TiO2 powder on the surfaces. Ag-TiO2 powders attached via 0.5M H2SO4 showed a higher overall Ag+ release at all investigated powder concentrations (0.48 - 76.93 wt% Ag) versus those attached via 2.1M H3PO4. This was concluded to be due to the anatase phase produced by 0.5M H2SO4 having greater oxidative power, thus accelerating oxidative dissolution of the AgNPs. RBS data corroborated these trends. Relative to their Ag ion exchange counterparts, the Ag-TiO2 samples had a lower Ag+ release at 0.05M and 0.5M AgNO3 concentrations. However, at 5.0M AgNO3 the Ag-TiO2 samples had a higher Ag+ release. This was the trend irrespective of the anodising electrolyte. Both the anatase and rutile TiO2s showed a reduction in Ra post Ag ion implantation and the polished Ti6Al4V samples showed an increase in Ra. This was due to preferential erosion of areas with high free surface energy. In the case of both TiO2s these were “high points” in the oxide and for polished Ti6Al4V these were the grain boundaries. Both TiO2s were amorphised during ion implantation. All ion implanted TiO2 showed the presence of AgNPs within the first 50nm of the surface. These AgNPs increased in size as the implantation dosage was increased. Polished Ti6Al4V showed no AgNP formation but EDS mapping confirmed that the silver was also located 50nm within the surface. TiO2 Ag+ release was similar for both implantation dosages because the surfaces had been supersaturated at the low dose, thus an increase in implantation dose had no significant effect on further silver uptake. The release rates were also similar between the oxides because of amorphisation. Polished Ti6Al4V showed an increase in Ag uptake and Ag+ release when the implantation dose was increased. RBS results corroborated the observed Ag+ release results. In comparison, both the ion exchanged samples and the Ag-TiO2 fused samples showed performances in similar ranges of Ag+ release. The Ag-TiO2 samples showed a greater degree of tailorability of the Ag+ release, whereas the ion exchanged samples showed a lesser sensitivity to an increase in AgNO3 concentration. Ag ion implanted samples showed an order of magnitude lower Ag+ release relative to the other studied methods. In comparison to literature, all ion exchanged and Ag-TiO2 samples had the potential to have a 100% antimicrobial effect (AE). Ion implanted oxides had a 55-100% potential, while the polished Ti6Al4V had a 55% AE at low dose and a 100% AE at high dose. In order to achieve maximum silver ion release and the associated antimicrobial effect, the technique of Ag-TiO2 fused to the surface using the 2.1M H3PO4 and 0.5M H2SO4 electrolytes yielded the best results, with a silver ion release of 550 and 600 ppb respectively over two weeks. This technique also satisfied the research aim, in that the methodology offered a combination of tailorability of silver release and commercial scalability.
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    Open Access
    The multi-pass deformation of Ti-6Al-4V
    (2017) Sikhondze, Bridget Gcinaphi; George, Sarah; Knutsen, Robert D
    Ti-6Al-4V is a two phase alloy used in the aerospace, military and biomedical industries. The thermomechanical processing (TMP) of Ti-6A-4V involves the breakdown of the ingot microstructure at temperatures above the beta transus (Tβ) into fine lamellar colonies. Subsequent hot deformation is carried out at temperatures within the two phase region to produce a microstructure that is either equiaxed or bimodal. However, to avoid the formation of shear bands, voids or cracks within the final microstructure, this secondary fabrication is carried out at slow speeds, and these render the TMP of titanium a time consuming and expensive process. This project aims to investigate the possibility of using a steckel mill for the TMP of Ti6Al4V. A steckel mill is a single stand reversing mill with 2 coiler furnaces on either side. These furnaces allow for the annealing of strip metal in between passes thus keeping the strip at a high temperature throughout processing. In this way, reversing passes can occur indefinitely until a desired gauge thickness is achieved. A steckel mill therefore represents a cheaper and faster method to produce metal sheet or plate. Ti-6Al-4V cylinders with a diameter of 10mm and length of 15mm were uniaxially compressed in the Gleeble 3800. Hot compression was carried out isothermally in a 3 pass schedule at temperatures of 850°C, 950°C and 1050°C. The strain was kept constant at 0.48 and strain rates of 1/s and 10/s employed. Interpass times corresponding to a strain rate of 10/s were 44s and 77s, whilst those corresponding to a strain rate of 1/s were 324s and 712s. Statistical analysis in the form of analysis of variance (ANOVA) was used to determine the parameter most influential on the microstructural evolution of Ti-6Al-4V and the Taguchi method used to identify the optimum parameters suitable for the TMP of Ti-6Al-4V using a steckel mill. Three successive passes at 850°C, 1/s resulted in a microstructure consisting of coarse, deformed grains and some finer recrystallised grains. The influence of a low strain rate was such that it promoted recrystallisation at this temperature, while that of interpass time brought about recrystallisation and grain growth. At the same temperature and a strain rate of 10/s, less recrystallisation, together with a heavily deformed microstructure was observed. This was due to the heterogeneous distribution of strain which was a consequence of the high strain rate used. At 950°C, at both 1/s and 10/s, 1 pass resulted in a bimodal microstructure. With subsequent passes, the amount of equiaxed alpha was observed to increase. This increase was a result of a strain induced phase transformation (SIT) from beta to alpha at high temperatures. The extent of this transformation increased with an increase in strain rate. Therefore, after 3 consecutive passes at 10/s, a fully alpha (though heavily deformed) microstructure was formed. A subsequent post deformation heat treatment would lead to recrystallisation of these grains and a microstructure consisting of refined equiaxed grains the result. After 1 pass at 1050°C, at either 1/s or 10/s, a Widmanstätten microstructure was formed. However, after 3 consecutive passes at 1/s, the microstructure remained mostly Widmanstätten whilst at 10/s, a bimodal microstructure was formed. The combination of a high strain rate, low interpass times, sequential strain imparted on the sample as well as the high temperature at which the compressions were carried out, elevated the extent to which a strain induced phase transformation from alpha to beta proceeded. Statistical analysis using ANOVA and the Taguchi method revealed that a schedule of 3 passes performed at 1050°C, 1/s and the corresponding interpass times as being the optimum parameters for the TMP of Ti-6Al-4V during steckel mill rolling. Analysis of the microstructural evolution across all 3 temperatures, however, showed that 3 passes carried at 950°C, 10/s, with interpass times of 44s and 77s, as being the optimum parameters. Steckel mill rolling of Ti-6Al-4V has thus been confirmed as a feasible process for the production of Ti-6Al-4V sheet material.
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    Open Access
    The study of intermetallic particles in aluminium alloy AA3104 can-body stock during homogenisation
    (2017) Magidi, Livhuwani Tessa; George, Sarah; Knutsen, Robert D
    Aluminium alloy AA3104 is commonly used for the manufacture of beverage can bodies. This alloy has excellent formability and strength properties. The evolution of the AA3104 microstructure and intermetallic particles during thermo-mechanical processing (TMP) has a direct impact on quality parameters, which influence the formability of the material during beverage can deep drawing and wall ironing. These parameters are earing, tear-off and galling resistance. During homogenisation of AA3104 direct chill (DC) ingot, there is a phase transformation from β-Al₆(Fe,Mn) orthorhombic phase to the harder α-Alₓ(Fe,Mn)₃Si₂ cubic phase. Phase transformation occurs by diffusion of Si and Mn, where diffusion of Mn determines the rate of transformation. The presence of the α-phase intermetallic particles is crucial for galling resistance, thus improving the formability of the material. Ideal galling resistance requires 1- 3% total volume fraction (VF) of intermetallic particles, 50% of which should be the harder α- phase. The homogenisation treatment variables, such as temperature, as well as the effect of the intermetallic particle VFs with the correct β to α ratio is investigated. The aim of this research is to characterise intermetallic particles in the as-cast condition and investigate the evolution of particles as a result of a two-step homogenisation treatment, where the primary step temperature was varied between 560⁰C and 580⁰C, and the secondary step was performed at 520⁰C. The characterisation process involves particle phase identification using compositional and morphological analysis. A particle extraction setup is then used to extract intermetallic particles from the bulk specimen by dissolving Al matrix in dry butanol and those particles are analysed. The evolution of volume fraction of particles and their distribution is then investigated using light microscopy, image analysis, XRD and the Rietveld method. The SEM micrographs show a larger quantity of smaller, more closely dispersed intermetallic particles at the edge of the ingot, compared to those at the centre. The β-Al₆(Fe,Mn) phase is more geometric in shape, while the α-Alₓ(Fe,Mn)₃Si₂ phase comprises isolated areas of Almatrix within the particle centres (Chinese-script like). The phases are distinguished based on morphological identification using SEM and compositional identification using EDS, where Si content within the α-phase is used to differentiate between the phases. XRD patterns with the Rietveld method show the presence of β and α as the major phase particles within the homogenised specimens near the edge and at the centre. Phase quantification using 2-D analysis and particle extraction shows more α-phase near the edge and less α-phase at the centre. The two techniques agree in trend but differ in values. The particle extraction analysis is more trustworthy than 2-D particle analysis, where error is suggested to arise during thresholding in 2-D microstructural analysis. Additionally, homogenisation at 580°C/520°C yields more α-phase than homogenisation at 560°C/520°C both near the edge and at the centre of the ingot. Important observations emerge from this study: (i) Microstructural [two-dimensional (2-D)] and particle extraction [three-dimensional (3-D)] techniques agree when it comes to microstructural qualification and tend to slightly differ on particle quantification (value obtained from both techniques), (ii) both techniques show the presence of α and β phases, as well as reveal the morphological differences within the particles, (iii) both techniques show similar trends of high amount of β-phase during as-cast and an increase in α-phase after homogenisation due to phase transformation. Additionally, phase quantification reveals that more α-phase near the edge and less α-phase at the centre, and (iv) homogenisation at 560°C/520°C yields α-phase VF which is closer to the desired β→α ratio of 50% compared to homogenisation at 580°C/520°C. Therefore, homogenisation at 560°C/520°C is the better homogenisation treatment temperature option. Furthermore, both 2-D microstructural analysis and particle extraction analysis are reliable techniques that complement each other when qualitatively and quantitatively studying the evolution of intermetallic particles in aluminium alloy AA3104 canbody stock during homogenisation. However, particle extraction analysis has been shown to have a higher accuracy, thus is deemed more reliable.
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    The effect of the build orientation and heat treatment on the fatigue and fracture properties of a directed energy deposited nickel-based superalloy (Inconel 718)
    (2020) Paul, Mikyle; George, Sarah
    This project focused on determining the effect of build orientation and heat treatment on the fatigue crack growth rate and fracture toughness properties of the nickel-based superalloy, Inconel 718 that has been manufactured by additive manufacturing technique, directed energy deposition (DED). Three different build orientations namely XY, XZ and ZX, in both the as-built and heat-treated conditions were tested through a combination of comparative fatigue crack growth rate (FCGR) and fracture toughness (FT) tests. A heat treatment protocol was applied to the material to relieve residual stress from the building process to homogenise the microstructure and to introduce precipitation hardening to the material for strengthening. This heat treatment protocol consisted of a solution treatment at 1200 °C for 2 hours followed by a water quench. Thereafter an ageing procedure was conducted at 650 °C for 16 hours followed by air cooling. The Paris equation for each build orientation and material condition was determined with the aid of a crack measurement technique that was developed in this project. This technique was designed to significantly improve the measurement accuracy of the fatigue crack during testing. The developed crack measurement system consisted of a digital camera and stereomicroscope that were mounted to the ESH servo-hydraulic testing machine with a rig that was designed and manufactured during this project. The FCGR and FT tests were conducted on compact tension (CT) specimens and the ASTM E647 and ASTM E1820 test standards were used as guidelines to test the specimens respectively. FCGR tests were conducted at a load range of 7.2 kN and a stress ratio of 0.1. The results indicated that there was a significant variation in the fatigue performance with respect to the material's build orientation. The fatigue crack path encountered several deviations during testing, resulting in inconsistent Paris data. FT tests were conducted at a cross-head speed of 2 mm/min (in the range of 0.8 to 2.0 MPa√m/s). The material exhibited an elastic-plastic behaviour and therefore, a J-integral fracture toughness analysis was undertaken. Jmax values (the value of J at the maximum load) were obtained for each build orientation and material condition, and compared. It was seen that the average Jmax values for each build orientation differed, indicating that the build orientation affected the fracture toughness properties of the material. The applied heat treatment protocol homogenised the microstructure of the material and induced precipitation hardening. This was confirmed with the aid of light microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy, where a high concentration of niobium-rich Laves phase was almost completely eliminated by the heat treatment. The positive effects of the heat treatment on the material were noticed during the FCGR tests as the fatigue crack growth resistance and fatigue life of the material increased. The gradient of the Paris curve, m, was seen to increase from approximately 5 to approximately 8. This effect was also noticed during the FT tests where a uniform increase in Jmax of approximately 50 kJ/m2 was recorded for each build orientation. It was concluded that a post manufacture heat treatment is essential for residual stress relief and homogenisation to occur in IN718 manufactured by DED. The material's response to fatigue is not uniform when in the as-built state for all build orientations due to the presence of the niobium-rich Laves phase. The material displays superior mechanical properties after the heat treatment protocol in terms of fatigue crack growth resistance and fracture toughness when compared to the as-built condition. Furthermore, it was also seen that the build parameters such as scanning width and hatch spacing affected both the fatigue and fracture properties of the material and must therefore also be taken into consideration when producing the material. A successful crack measurement technique and apparatus was also developed that greatly increases the accuracy of Paris data obtained. The XZ orientation was seen to possess the most uniform fatigue and fracture properties amongst the three orientations tested in this project in both the as-built and heat-treated conditions.
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    Open Access
    The effect of the surface condition of Aluminium ingot (AA3003) during roll bonding with clad Aluminium alloy (AA4045) to form an Aluminium brazing material
    (2020) Mutsakatira, Innocent; George, Sarah
    Hulamin is the leading producer of aluminium products in South Africa. One of the products made at Hulamin is the aluminium brazing sheet. The aluminium brazing sheet is made from two aluminium alloys, AA3003 and AA4045. The main alloying element in the 3XXX series alloy is manganese and the main alloying element in the 4XXX series alloy is silicon. An aluminium brazing sheet is manufactured during an industrial process called “accumulative roll bonding”, where AA4045 is termed “the clad” and AA3003 “the core”. The two materials are stacked together with the core sandwiched between two clad layers. Before the materials are stacked together, they undergo surface preparation. At Hulamin, the surface roughness of the core is kept at 10 µm and the surface roughness of the clad at 1 µm. After surface preparation, the stacked material is put into a hot rolling mill, where it undergoes reduction through several passes until it reaches the desired gauge. The aim of this project is to determine the effect of the surface roughness of both the clad and the core on the quality of the bond after roll bonding. While the relevant literature specifies that an increase in surface roughness increases bond strength, the current set surface finishes being implemented at Hulamin have been obtained through trial and error, with no validated experimental work to support them. This research aims to find the optimum surface finish in order to streamline the process of surface preparation. A design matrix was constructed based on the surface finish being used at Hulamin, where the core was at 10 µm and the clad at 1 µm. Fourteen surface conditions were formulated and three tests were performed on each surface condition. The samples were manually ground on different grit papers to an average surface roughness of 0.5 µm, 1 µm and 3 µm for the clad and 7 µm, 10 µm, 15 µm and 25 µm for the core. Simulation of the hot rolling at the University of Cape Town's (UCT's) Centre for Materials Engineering (CME) laboratory was achieved using plane strain compression testing (PSC) on the Gleeble 3800. The PSC sample geometry of 30 mm x 50 mm x 10 mm was achieved by stacking a 5 mm sample from the clad liner plate and a 5 mm thick sample from the as-cast core material. To simulate the hot roll bonding the tests were run at 450 ºC at a strain rate of 1.5 s-1 . The test parameters were obtained from the Hulamin mill log data. In order to assess the strength of the bond, post PSC test, tensile shear testing was performed on specimens wire-cut from the gauge of the deformed PSC sample. The tensile shear specimens were designed according to ASTM D3165. The tensile shear tests were performed on a Zwick Universal Testing machine, in conjuction with single-camera Digital Image Correllation (DIC). The purpose of the DIC was to monitor the strain localisation at the interface. The tensile test was run at 0.0012 mm/min at room temperature. The shear test results confirmed that surface roughness played a major role in the bond strength formed between these two dissimilar materials. It was found that the Hulamin benchmark surface preparation, set at 10 µm and 1µm, could be improved by increasing the surface roughness of the core to 15µm while keeping the clad surface finish constant. The rolling direction (RD) of the specimen was cut, mounted and polished for microstructural feature characterisation, using light microscopy and scanning electron microscopy (SEM) with backscattered electron (BE) imaging. In order to characterise the bond further, energydispersive X-ray spectroscopy (EDS) was performed across the interface of the samples to show the diffusion of Si. Microstructural analysis revealed that a poor bond resulted in the presence of large voids, while a high integrity metalurgical bond contained very small voids. Also, a good metallurgical bond allowed for the diffusion of Si across the bond, although these results were qualitative because diffusion of Si across the interface is largely time- and temperature-dependent. Combined strain and microstructural results showed that finer surface roughnesses yielded poorer bonds because of minimal frictional force and that rougher surface finishes also yielded poorer bonds, owing to larger troughs on the surface of the material that led to void formation at the interfaces, which in turn caused sites of delamination. There had to be an optimum surface finish that existed between the two alloys where the finish would obtain a metallurgical bond that was of optimum strength. Should this optimum finish be exceeded, the strain level would inevitably increase during tensile shear testing, with the induced voids increasing in size and Si diffusion across the interface decreasing, thereby indicating a compromise in the quality of the bond. It was found that the Hulamin benchmark surface preparation, set at 10 µm and 1µm, could be improved by increasing the surface roughness of the core to 15µm while keeping the clad surface finish constant. The findings of this research could be of significant value to Hulamin in the improvement of the quality and cost of the end product under consideration.
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    The effects of chemical composition, solidification rate, and homogenization on the intermetallic particle microstructure of AA8XXX aluminum alloys for battery foil applications
    (2025) Maluleka, Tshepo; George, Sarah
    The growing demand for high-performance lithium-ion batteries, driven by the electric vehi-cle (EV) market, necessitates the development of current collectors that offer both superior electrical conductivity and enhanced mechanical properties. AA1XXX series aluminum al- loys, commonly used in battery applications, are limited by their strength and ductility. As an alternative, AA8XXX aluminum alloys, particularly those based on the Al-Fe-Si system, show promise due to their improved strength. However, their intermetallic particle (IMP) microstructure needs to be understood for optimal performance. This research investigates the impact of chemical composition, cooling rate, and homoge- nization processes on the IMP microstructure of AA8021 and AA8079 alloys, which are poten-tial candidate alloys for this application. Using 2D and 3D analytical approaches supported by analytical tools including light microscopy (LM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD). The study identified three primary morphologies: plate-shaped, feathery/skeletal, and needle-shaped, corresponding to Al3Fe, AlmFe, and Al6Fe phases. The results reveal that higher Fe content in AA8021 leads to a higher volume fraction of finer IMPs, while higher Si content in AA8079 enhances the stability of Al3Fe and reduces the presence of AlmFe. Further- more, cooling rate significantly influences IMP morphology and phase stability, with higher cooling rates favoring the formation of finer, metastable phases at the surface. Homogeniza-tion treatments induce phase transformations from metastable Al6Fe and AlmFe to the stable Al3Fe phase, improving the uniformity and distribution of IMPs.