Research Articles (Materials Science and Metallurgical Engineering)

Permanent URI for this collectionhttp://hdl.handle.net/2263/1699

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Effects of strain and inter-pass time on the restoration behavior of 436 stainless steels
(Springer, 2025-11) Salojee, Muhammed Yusuf; Siyasiya, Charles Witness; Annan, Kofi Ahomkah; Moema, Joseph S.; u13271050@tuks.co.za
Ferritic stainless steels (FSSs) are viable alternatives to nickel containing austenitic stainless steels. The AISI 436 FSS grade is fully ferritic at all temperature ranges, and therefore, grain refinement can only be achieved through controlling inclusions during solidification and/or recrystallization. In this work, the effects of strain and inter-pass time on the restoration behavior of 436 were systematically investigated using the Gleeble 1500 thermomechanical processing simulator. The aim of the study was to promote the accumulation of strain by delaying recrystallization during roughing rolling simulation to improve the driving force for recrystallization during finishing rolling simulation at lower temperature. Three roughing rolling schedules were designed, i.e., the first simulated the existing industry practice (Control), the second with increased inter-pass time (INT) and the last with both increased inter-pass time and strain per pass (INT + S). Roughing and finishing were each simulated in three-passes, amounting to a total of six-passes. The SEM-EBSD results revealed that the Control and INT schedules resulted in higher strain localization, particularly around particles. On the contrary, the increased inter-pass times and strains (INT + S) led to less strain accumulation. Annealing after roughing rolling simulation led to the nucleation of fine grains in the Control, which suggests that delaying finishing rolling would promote recrystallization and the breakdown of the cast structure.
Microstructure and mechanical properties evaluation of high Ti-V microalloyed steel after coiling process at different temperatures in a hot strip rolling simulation process
(Springer, 2025-08) Jafarpour Rezaei, Seyed Reza; Siyasiya, Charles Witness; Tang, Zhenghua; Moema, Joseph
Today, reducing greenhouse gas emissions is a crucial concern, making the development of steel alloys with optimal desired properties and reduced weight an urgent priority, especially in automotive industries, as it leads to decreased fuel consumption, hence the continued interest in developing high-strength low-alloy steels (HSLA). In this research, the impact of coiling temperature on the final microstructure and mechanical properties of Ti-V HSLA microalloyed steel was investigated. The Gleeble 1500 was used to simulate the hot rolling and coiling processes. The results indicated that reducing coiling temperatures led to a transition in microstructures from polygonal ferrite and pearlite to acicular ferrite and bainite. The optimal coiling temperature for achieving high diffusion rates of microalloying elements and optimal precipitation kinetics to form nanosized precipitates was determined to be 650 °C, where the smallest precipitates were observed of 30 nm. However, the specimen subjected to a lower coiling temperature of 550 °C exhibited the highest yield stress (781 MPa), ultimate tensile strength (971 MPa), and hardness (324 HV), which were attributed to microstructural characteristics such as high dislocation density and a finer grain size of 3 µm compared to 6 µm at 700 °C.
Investigating the use of sugar alcohol as an alternative non-toxic binder to conventional coal tar/coal tar pitch for use in taphole clay
(Göller Verlag, 2025-09) Cameron, Izak Jian-Pierre; Garbers-Craig, Andrie Mariana
Currently, the main purpose for obtaining alternative binders for use in taphole clay is to reduce the exposure to harmful polycyclic aromatic hydrocarbons (PAH) associated with conventional coal tar (CTht) binders. Some advances have been made such as using lower-PAH alternative binders or phenolic resole resins (resin-bonded) in taphole clays. The use of non-toxic binders has become exceedingly difficult due to the versatility of CTht in taphole clay. This study investigated a combination binder system that consisted of a sugar alcohol (research-grade glycerine) and phenolic resole resin as a potential non-toxic binder for use in platinum smelting taphole clay. The binder system was characterized according to its composition by means of Fourier-transform infrared spectroscopy and the PAH content (16-EPA-PAH) was determined using targeted gas chromatography mass spectroscopy. Flow behaviour of the binder was determined based on rotational rheology, and the volatilization and rheological thermal stability of the binders were assessed by means of thermogravimetric analysis and thermorheology, respectively. The effect of mixing these binder constituents on the cross-linking behaviour of the phenolic resin was evaluated using differential scanning calorimetry. The combination binder had a higher average mass loss and lower carbon yield compared to conventional CTht, but with a lower total PAH content, making it a more health-friendly alternative.
The influence of chloride ion concentration on the product layer of Ti microalloy steel under simulated marine atmospheric conditions
(Springer, 2025) Hu, Jie; Wang, Chuang-wei; Siyasiya, Charles Witness; Tang, Zheng-hua
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Sodium-oxide fluxed aluminothermic reduction of manganese ore for a circular economy : Cr collector metal application
(MDPI, 2025-09-18) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Aluminothermic reduction is gaining renewed interest as an alternative processing route for the circular economy. Aluminium is produced electrochemically in the Hall–Héroult process with minimal CO2 emissions if electricity is sourced from non-fossil fuel energy sources. The Al2O3 product from the aluminothermic reduction process can be recycled via hydrometallurgy, with leaching as the first step. NaAlO2 is a water-leachable compound that forms a pathway for recycling Al2O3 with hydrometallurgy. In this work, a suitable slag formulation is applied in the aluminothermic reduction of manganese ore to form a Na2O-based slag of high Al2O3 solubility to effect good alloy–slag separation. The synergistic effect of added chromium metal as a collector metal is illustrated with an increased alloy yield at 68%, from 43% without added Cr. The addition of small amounts of carbon reductant to MnO2-containing ore ensures rapid pre-reduction to MnO. This approach negates the need for a pre-roasting step. The alloy and slag chemical analyses are compared to the thermochemistry-predicted phase chemistry. The alloy consists of 57% Mn, 18% Cr, 18% Fe, 3.4% Si, 1.5% Al, and 2.2% C. The formulated slag exhibits high Al2O3 solubility, enabling effective alloy–slag separation, even at an Al2O3 content of 55%.
The effect of hot corrosion on mechanical properties of the tin-doped titanium aluminide alloy
(Springer, 2025-01) Magogodi, Steven; Mathabathe, Maria Ntsoaki; Rikhotso-Mbungela, Rirhandzu; Vilane, Velile; Siyasiya, Charles Witness; Bolokang, Amogelang Sylvester
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Effects of phosphorus on the corrosion characteristics of weathering steel in industrial environments
(Springer, 2025-10) Gan, Le; Lei, Yicong; Jiang, Yu; Siyasiya, Charles Witness; Tang, Zhenghua
Please read abstract in the article.
Comparative analysis of the subsidence of solid polyetheretherketone (PEEK) and 3D printed lattice titanium interbody fusion cages
(EDP Sciences, 2023-12) Moolman, Frans J.; Kat, Cor-Jacques; Du Plessis, Anton; Khodja, Malika
Spinal interbody fusion cages are commonly used to treat various spinal conditions, but their traditional manufacturing methods have limitations in customization and fitting. With the advancement of 3D printing, it is now possible to design and manufacture interbody fusion cages with previously unachievable features and structures. Southern Medical™ is investigating the technical feasibility of 3D-printed cages based on their existing designs and exploring the new features and capabilities enabled by additive manufacturing (AM). The mechanical performance in the subsidence of the 3D-printed devices will be compared to their existing devices as one of the feasibility points for the additively manufactured implants. A gyroid structure is used as the inner lattice of the structures. To investigate the performance of the cages with the new gyroid lattices, subsidence testing according to the ASTM F2267 methods was conducted to compare existing cages to the 3D-printed cages. The 3D printed devices outperformed the PEEK counterparts with a higher test block stiffness of 0.81 kN/mm compared to 0.55 kN/mm.
Numerical analysis of static and dynamic behaviour in an optimized Ti6Al4V Cervical fusion cage produced via additive manufacturing
(Springer, 2025-09) Bougadouha, Oumayma; Frija, Mounir; Moolman, Frans J.; Khodja, Malika; Fathallah, Raouf
Cervical intervertebral body fusion devices act as space holders between two vertebrae. The cages are used to offer support and stability after surgery. Additive manufacturing is considered an efficient technology for producing medical implants. Ti6Al4V is widely used in the biomedical sector due to its mechanical and chemical characteristics. In this study, a numerical topology optimisation was performed to obtain a porous cage with a lattice structure. The objective was to design biomimetic, lightweight, and customized cages. A numerical study was conducted to determine the performances of the designed porous cages produced by selective laser melting with Ti6Al4V. The numerical design was validated by comparing the numerical results to the experimental one done by a South African company specializing in cervical fusion cage manufacturing. Static and dynamic tests were performed according to ASTM F2067-22 standards to identify the static and dynamic characteristics of the cage under study with the chosen lattice structure and solid fraction. As a result, the lattice structure has an important effect on the residual stress value. The Gyroid lattice structure type with a 45% solid fraction had shown the best performances for the studied cage. According to the ASTM standard and after the static and dynamic tests, the designed cage is accepted since it has passed the tests.
Improving performance at Rustenburg Base Metals Refiners copper tankhouse : operational review and embracing fundamentals
(Southern African Institute of Mining and Metallurgy, 2025-06) Naidoo, Kalin; Pelser, M.; Hagemann, J.; Sole, Kathryn C.
The copper tankhouse at Rustenburg Base Metals Refinery is an essential component of the Anglo American Platinum value chain. The tankhouse uses starter-sheet technology that was adopted in the 1980s, so dependence on manually intensive labour and overall operational excellence has a significant impact on process performance. An efficient, stable, and sustainable electrowinning process contributes to minimising energy consumption and reducing operational costs, and enables improved product quality and increased throughput. Historic performance of this tankhouse has often been suboptimum over prolonged periods. An in-depth operational review of performance from 2018-2024 was carried out to understand challenges, risks, and high-impact factors that contribute to poor performance. Implementation of adherence to fundamental operational basics identified from global best practice, such as the frequency and quality of cell cleaning and maintenance, preventing backlogged harvesting or maintenance, preserving cell-top furniture conditions and integrity of electrodes, preventing short circuits and poor current distribution, and maintaining a high level of operational discipline, has since contributed to an era of exceptional performance. Root-cause analysis fault trees pertaining to poor current efficiency and high scrap rate are summarised. Current efficiency has significantly improved since 2021, now consistently exceeding 88%. The copper scrap rate, previously believed to be limited to a minimum of 4%, achieved a historic monthly low record of 2.06% in 2023. Chemical and physical quality has considerably improved, and cathodes exhibit minimal morphological defects.
Comparison of Denver and Leeds laboratory flotation cells : effect of particle size and hydrodynamics
(Southern African Institute of Mining and Metallurgy, 2025-05) Tshazi, Mfesane; Leal, L.S.; Naude, Natasia; mfesane.tshazi@up.ac.za
In this paper, the authors performed experiments in the Denver and Leeds laboratory flotation devices1 at the University of Pretoria at various particle sizes to evaluate the performance of the devices. Quartz was used in a single mineral system at discrete sizes fractions, -25 µm, +2545 µm, +45-75 µm, and +75-106 µm. Hydrodynamic analysis, based on dimensionless power and Reynolds numbers, indicated that the Leeds cell required higher power input to achieve comparable flow conditions. Specifically, the Leeds cell exhibited an average power number of 1.03, whereas the Denver cell averaged 0.77 within the same impeller speed range of 1000 rpm-1500 rpm. For comparative flotation performance, impeller speeds were calibrated, resulting in operating speeds of 1200 rpm for the Denver cell and 1400 rpm for the Leeds cell. The cells performed similarly at two coarser-sized fractions. However, some deviations were observed in the finer particle size range. The -25 (fine) fraction initially achieved a recovery of only 15%. An additional reagent dosage was required to enhance the recovery of this fraction significantly. These findings highlighted the influence of particle size on flotation recovery. For all experiments, the Denver laboratory flotation cell outperformed (defined by higher recovery) the Leeds device, while still operating at a lower impeller speed. This advantage can be attributed to its impeller-stator design and air dispersion features, effectively overcoming slurry resistance and resulting in superior flotation performance compared to the Leeds cell.
Laser metal deposition of TiB/Ti6Al4V composites with first-scale network microstructure
(Sage, 2024-09) Lekoadi, Paul Masekuru; Tlotleng, Monnamme; Siyasiya, Charles Witness; Masina, Bathusile Nelisiwe
This study investigated the effect of in-situ alloying of Ti6Al4V and TiB2 during laser metal deposition for microstructure, hardness and tensile properties enhancement. It was found that the addition of TiB2 at powder feed rates of range 0.1–0.3 rpm resulted in the formation of TiB whiskers with columnar and equiaxed network morphologies, in a lamella α + β matrix. By increasing TiB2 at 0.4–0.5 rpm, a complete dissolution of the equiaxed and columnar network morphologies were achieved. The dissolution led to a random distribution of formed TiB whiskers, resulting in enhanced hardness and ultimate tensile strength (UTS), with the 0.5 rpm giving the highest hardness and UTS values of 511 ± 13 HV and 1745 MPa, respectively.
Investigation of the mechanisms affecting corrosion susceptibility of wrought aeronautical aluminium alloys Al-Cu-Li (AA2198) and Al-Cu-Mg (AA2024) for different pre-stretching levels
(Elsevier, 2025-03) Charalampidou, Christina Margarita; Pretorius, Christiaan C.E.; Salojee, Muhammed Yusuf; Karousos, Dionysios; Khodja, Malika; Mostert, Roelf Johannes; Alexopoulos, Nikolaos D.; roelf.mostert@up.ac.za
Aluminium (Al) alloy sheets are usually stretched to manufacture aircraft structures with complex geometries. The corrosion susceptibility of AA2198 Al alloy is examined as a function of the extent of pre-stretching, using a wide range of advanced microscopy, electrochemical techniques, and tensile mechanical testing. Intergranular corrosion attack manifested in sub-surface secondary cracking for the 1.5 % pre-stretching level. A mechanism of deformation-induced chemical heterogeneity is believed to be responsible for the resulting intergranular corrosion, in which δ′-phase nucleation following pre-stretching allows for the removal of matrix Li within highly deformed grains, creating a micro-galvanic coupling between neighbouring grains. At pre-stretching levels of 4.0 % and higher, a transition to transgranular corrosion was observed, whilst the electron backscattered diffraction results indicated that recovery occurs at these pre-stretching levels. At 7.5. % pre-stretching level, the charge transfer resistance values were essentially increased due to the segregation of Li to sub-grain boundaries, allowing for more matrix Li removal and, hence, to the more rapid transgranular attack. On the contrary, the level of corrosion degradation of the tensile properties of AA2024-T3, although being more severe than that of the AA2198 alloy, is not essentially influenced by varying the extent of pre-stretching.
Thermomechanical control of microstructure and precipitation in vanadium microalloyed steel: influence of finish rolling and coiling temperatures
(Wiley, 2024-04) Wu, Zhangqin; Yang, Wen; Tang, Zhenghua; Siyasiya, Charles Witness; Zhang, Jianhua
Six hot compression tests are conducted using the Gleeble3500 thermomechanical simulator to investigate the microstructural evolution and precipitation behavior in low-C–Mn V-microalloyed steel. The specimens are subjected to hot isothermal compression deformation of 87%. The optical microscopy and transmission electron microscopy using carbon extraction replica method are used to characterize the microstructures and precipitation after the simulated thermomechanical controlled process and coiling. The results indicate that increasing the finish rolling temperature benefits the refinement of ferrite grains but has little influence on the refinement of the precipitates. It is also observed that lower coiling temperatures (CTs) promote the formation of fine precipitates. When the CT is 500 °C, the average precipitate size is found to be 86 nm. Furthermore, it is found that the CT significantly influences the nucleation sites of the precipitates inter alia, the matrix, interphase, grain boundaries, and dislocations. As expected, at higher CTs, nucleation is predominantly on the defects rather than the matrix.
Empirical modelling of 2205 DSS flow curves using strain-compensated Arrhenius rate-type constitutive model
(Nature Research, 2024-09-28) Gonya, Elvis M.; Siyasiya, Charles Witness; Makhatha, Mamookho E.
This work predicts, hot flow curves of 2205 DSS using strain-compensated Arrhenius rate-type constitutive model. Twenty-five (25) × Ø10 diameter × 15 mm height cylindrical samples were hot compressed at a temperature between 850 and 1050 °C at an interval of 50 °C and strain rates between 0.001 and 5 s−1, using Gleeble 1500D. After the tests, corrected flow curves were plotted followed by computation of deformations constants at various deformation conditions using steady state stress. The values of the constants were (α = 0.009708, Q = 445 kJ/mol and n = 3.7) and seemed comparable to the previous studies of DSS. Steady state predictive model was then constructed using the calculated constants and showed a reasonably good accuracy with low value of MARE = 7.78%. Furthermore, calculated strain compensated Arrhenius rate type model was used to predict flow curves at various deformation. The model had a good estimation of flow curves of flow curves at 900–1050 °C across all strain rates as reflected by MARE = 5.47%. A notable discrepancy between predicted and experimental flow stress was observed at 850 °C and across all the strain rates. A model refinement using generalised reduced gradient improved the accuracy of the model by 34.7% despite deformation conditions at 850 °C and low strain rates (0.01/ 0.1) s−1 showing minimum improvement. Further modification of Z-parameter by compensating for the strain rate improved the accuracy of the model at 850 °C/0.01 s−1/0.1 s−1. Lastly, a comparison of the current model with the other non-linear model showed that the latter was more accurate in estimation of flow curves since it relied on characteristics flow stress points controlled by underlying active deformation mechanisms.
Gas-phase reactions in nano-strand formation from Al-Fe-Ni powder reacted with CaF2-SiO2-Al2O3-MgO-MnO-TiO2 Flux at 1350 °C : SEM study and diffusion calculations
(MDPI, 2025-03) Coetsee, Theresa; De Bruin, Frederik Johannes; theresa.coetsee@up.ac.za
Please read abstract in the article.
'New mining scheme, same old problems?' ; risk factors and health challenges in underground community mining in Ghana
(IOP Publishing, 2024-12) Aram, Simon Appah; Saalidong, Benjamin M.; Yankyera, Akwasi; Akoto, Akwasi Bamfo; Amoah, Samuel Kofi
Ghana's small-scale mining sector has long been a cornerstone of its economy and a vital source of livelihood for many communities. However, the sector is plagued by significant challenges, including environmental degradation, health and safety risks, and illegal practices. In response, the Ghanaian government introduced the Community Mining Scheme (CMS), a new mining model designed to promote local community participation in artisanal small-scale gold mining (ASGM). This study examines the workplace risk factors and associated health challenges faced by ASGM miners under the CMS, drawing on interviews with fifty underground CMS miners in Ghana. Our findings reveal that miners continue to face numerous health and safety challenges, including inadequate lighting, poor ventilation, exposure to noise and odors, unsupported or poorly supported loose materials, non-use of personal protective equipment, high dust levels, and unsafe blasting practices. Additionally, health concerns such as skin rashes, musculoskeletal injuries, and the risk of mercury poisoning persist. These challenges mirror those reported in the literature on unregulated and illegal ASGM in Ghana, indicating that existing issues remain unresolved under the CMS. To prevent the undermining of this initiative, it is crucial to adopt progressive practices, such as localized coping strategies and the recruitment of trained professionals to lead ASGM operations within the CMS. Enhancing miners' risk perception through targeted outreach and awareness campaigns is also essential. Further recommendations for mitigating safety risks and health challenges are discussed in the article.
Delta ferrite in modified 9Cr–1Mo steel weld metal
(Springer, 2025) Mahlalela, Sibusiso Samuel; Pistorius, Pieter Georg Hendrik; sibusiso.mahlalela@up.ac.za
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Element transfer behaviors of agglomerated CaF2-ZrO2 fluxes in EH36-shipbuilding steel subject to high-heat input submerged arc welding
(Springer, 2024-10) Chen, Angran; Zhang, Yanyun; Coetsee, Theresa; Kaldre, Imants; Wang, Cong
EH36-shipbuilding steel has been welded by CaF2-ZrO2 fluxes with designed ZrO2 additions. Possible chemical and electrochemical reactions have been postulated to analyze alloying element transfer behaviors. The decomposition of ZrO2 during SAW has been validated by applying the gas–slag–metal equilibrium model and the O supply capacity of ZrO2 has been quantified. For the entire compositional range, O content has been controlled within a well-maintained range from 220 to 400 ppm, and the transferred quantity of Zr content reaches to the maximum value of 120 ppm. It is further demonstrated that ZrO2 addition incurs appreciable Si loss within the weld metal.
Green binder system for taphole clays : a potential for glycerine and phenolic resin
(Elsevier, 2025-06) Cameron, Izak Jian-Pierre; Garbers-Craig, Andrie Mariana
Recent advancements in taphole clay binder development have focused on replacing toxic coal tar/pitch, which contains polycyclic aromatic hydrocarbons (PAH), with greener alternatives. In this study, three different taphole clays were prepared in the laboratory using different binder systems which include phenolic resin: 1) coal tar, 2) glycerine, and 3) petroleum waxy oil. The clays were evaluated and compared to the conventional coal tar and phenolic resin-containing clay used in platinum smelters. The evaluation methods employed included workability and extrusion pressure ageing, hardenability, strength development, and high-temperature properties, which comprised cold crushing strength after ageing at 200 °C, carbon yield, volatile organic compound concentration and apparent porosity. The results indicated that the preferred binder to replace coal tar was glycerine, as this clay retained both its plasticity and hardenability during ageing, while also attaining comparative strength development and high-temperature properties to those of the reference clay containing coal tar.

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