Research Articles (Materials Science and Metallurgical Engineering)

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

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Sodium-oxide fluxed slag design, phase chemistry and thermochemistry calculations for aluminium recycling from aluminothermic reduction of manganese ore
Coetsee, Theresa; De Bruin, Frederik Johannes (MDPI, 2026-06-20)
Please read abstract in article.
Role of hydrogen embrittlement on resistance to crack extension of 2024 aluminium alloy for different artificial aging conditions under short corrosion exposure times
Alexopoulos, Nikolaos D.; Lymperaki, Paraskevi; Salojee, Muhammed Yusuf; Kurup, Vinod; Rezaei, Seyed R.J.; Mostert, Roelf Johannes (Elsevier, 2026-06)
The effect of precipitation sequence for various heat treatments simulating natural aging and subsequent corrosion exposure on cracking resistance (fracture toughness) of AA2024-T3 is investigated. Artificial aging heat treatment of fracture toughness specimens was investigated for three different temperatures and for all possible heat treatment conditions, including under-, peak- and over-aged tempers. Fracture toughness Kcr decreases continuously with isothermal aging up till peak-aged condition, showing the maximum Kcr decrease of approximate 36% for the lowest investigated temperature. A small recovery was noticed for the over-aged condition. The short, 2 h exfoliation corrosion exposure time, was selected to corrode the pre-heat-treated specimens and introduce hydrogen. Surface pits, intergranular corrosion and associated secondary cracking were noticed for some aging conditions and a pick-up of diffusible hydrogen was observed in all cases. The highest fracture toughness decrease (− 25%) due to corrosion exposure was noticed for the under-aged condition that was attributed to the GPB zones formation. The isothermal aging temperature proved to be extremely influential on the corrosion-induced fracture toughness decrease, with the lowest investigated temperatures associated with the strongest embrittlement. The highest Kcr decrease per investigated aging temperature was noticed at the under-aged condition and for all aging temperatures, namely −18% for the 170 °C, −16% for the 190 °C and − 12% for the 210 °C, respectively. The lowest fracture toughness decrease due to corrosion exposure was noticed for the peak-aged specimens, where S΄΄ precipitates, fully coherent with the matrix, are formed. For these conditions, surface pits, intergranular corrosion and associated secondary cracking were not prevalent. The effect of secondary cracks and pits on the primary crack tip stress field in fracture toughness tests, is explored and shown to be crucial in understanding the Kcr – embrittlement of microstructures susceptible to corrosion pits and intergranular corrosion. Corrosion exposure on extreme over-aged specimens plays a non-important role on the fracture toughness decrease. Diffusible hydrogen contents and spectra were determined for the various aged conditions. Bulk hydrogen levels were found to be relatively low, and no correlations were observed between hydrogen and fracture toughness reductions.
Comparison of α-ferrite to γ-austenite transformations in 9%Cr steel alloys measured by neutron powder diffraction, dilatometry, and differential scanning calorimetry
Sentsho, Zeldah N.; Pistorius, Pieter Georg Hendrik; Venter, Andrew M.; Hester, James R.; Maynard-Casely, Helen E. (Springer, 2026)
Please read abstract in the article.
Machine learning-enhanced constitutive modeling and hot deformation behaviour of Ti-stabilized AISI 321 austenitic stainless steel
Nkhoma, Richard; Mwale, Vincent; Ngonda, Tiyamike; Siyasiya, Charles Witness (Elsevier, 2026-04)
Please read abstract in the article. HIGHLIGHTS • ML–Arrhenius hybrid model developed for AISI 321 hot deformation. • Random Forest predicts full flow curves with high accuracy. • ML smoothing improves stability of Arrhenius parameters. • Power dissipation efficiency and flow instability map defines optimal DRX hot-working conditions. • Framework enhances constitutive modelling of Ti-stabilized steels.
The first principles and experimental study of equilibrium and non-equilibrium low-temperature phases of Nb50Ru50 alloy
Nkomo, Duduzile; Mostert, Roelf Johannes; Phasha, Maje (Springer, 2025-12)
The equilibrium and non-equilibrium low-temperature phases of Nb50Ru50 alloy have been investigated using ab initio calculations (thermodynamic, elastic, and electronic properties) in conjunction with experimental microstructural and phase analyses. Low-temperature phases in Nb50Ru50 alloy originate from the transformation of tetragonal L10 phase at 750 °C, which potentially makes it useful for designing high-temperature shape memory alloys. Current results showed that the monoclinic phase P2m is the most energetically stable under equilibrium conditions, and this is complemented by the absence of negative frequencies in the phonon dispersion curves. However, negative frequencies were observed on the orthorhombic phase Cmmm, indicating that it is a metastable martensite phase. Therefore, two low-temperature phases of Nb50Ru50 have been determined, namely P2m and Cmmm. This forms the basis for further designing of the alloy with superior shape memory properties since P2m is softer and exhibits lower symmetry compared to the brittle L10 phase.
Hot deformation characteristics and constitutive modeling of Si-Doped γ-TiAl intermetallic alloy
Ellard, John Jimmy M.; Mathabathe, Maria Ntsoaki; Siyasiya, Charles Witness; Bolokang, Amogelang Sylvester (Springer, 2026-04)
Please read abstract in the article.
Predictive modeling of stress intensity factors in composite-repaired cracked aluminum plates : a finite element-based computational framework
Zouambi, Leila; Fekirini, Hamida; Bouafia, Farida; Khodja, Malika (Taylor and Francis, 2026-03-11)
The utilization of bonded composite patches for repairing fatigue-related damage in metallic structures has been acknowledged as an efficient and cost-effective approach, especially within the aerospace sector. However, accurate prediction of stress intensity factors (SIFs) in such repairs remains challenging for structural engineers. This work examines the mechanical behavior of a cracked AA2024T3 plate repaired with a carbon/epoxy composite patch under Mode I loading using finite element analysis (FEA). The study incorporates numerical results to develop a new computational model for precisely determining the repaired plate’s SIF. The developed predictive model, derived directly from FEA data analysis, demonstrates high accuracy with deviations ranging from 0.0006 to 0.0037 when compared to the finite element results. The model successfully predicts repair effectiveness and quantifies the patch’s influence on stress concentration near the crack tip. This computational framework offers a reliable method for forecasting repair efficacy and serves as a practical tool for structural engineers in the analysis and optimization of composite patch repairs. The high-precision model enables improved design decisions for structural repair applications.
Mechanistic analysis of fatigue crack propagation in AA7075-T6 aluminum alloy : three-stage growth behavior and damage tolerance implications
Zouambi, L.; Fekirini, H.; Moller, Heinrich; Khodja, Malika (Springer, 2025-11)
Please read abstract in the article.
Thermodynamic assessment of reactions in the sodium-oxide fluxed aluminothermic reduction of manganese ore with Si, Cr, and Cu collector metals
Coetsee, Theresa; De Bruin, Frederik Johannes (MDPI, 2026-02)
This study investigates the reaction thermodynamics of the sodium oxide-fluxed aluminothermic reduction of pyrolusite-based manganese ore under self-propagating high-temperature synthesis (SHS) conditions, using Si, Cr, and Cu as collector metals. The experimental results are compared with thermochemical equilibrium calculations using FactSage 7.3 thermochemistry software. Experimental mixtures were prepared with controlled additions of aluminium, sodium silicate, calcium oxide, and collector metals and heated to the ignition temperature in a muffle furnace preheated to 1350 °C. The resulting alloys and slags were analysed for bulk composition. Collector metals significantly influence alloy carbon saturation and manganese recovery. The individual reaction’s Gibbs free energy values and the gas–slag–metal equilibrium were calculated. Discrepancies between the experimental and equilibrium-predicted results highlight the kinetic factors of SHS processes, particularly with respect to aluminium uptake and manganese volatilisation. The main difference is the alloy’s aluminium uptake. The difference between the calculated and experimental aluminium levels is, in part, due to the higher partial oxygen pressure predicted in the gas–slag–metal equilibrium calculations, compared with that of the likely Al–Al2O3 governing reaction equilibrium. Short-circuiting of aluminium to the alloy is also a possible contributing factor. The findings provide insights into optimising feed formulations and process parameters for improved manganese recovery.
Investigation of copper as collector metal in sodium-oxide fluxed aluminothermic reduction of manganese ore
Coetsee, Theresa; De Bruin, Frederik Johannes (MDPI, 2026-01)
Please read abstract in the article.
DC ferrochrome smelting : the arcing zone and its influence on energy transport and exergy dissipation
Oterdoom, Harmen; Reuter, Markus; Zietsman, Johannes Hendrik (Springer, 2025-02)
Phenomena between the electrode tip, the arc attachment zone (AAZ), and the bath below the AAZ—taken together as arc attachment volume (AAV)—have a significant impact on the performance and efficiency of DC arc furnaces. This paper investigates the phenomena in the AAV in detail. Phenomena are analyzed with reference to industrial scale furnace viz. a Kazakh DC ferrochrome furnace. The impact of the various reactions in the AAV under different conditions both from an energy and exergy flow perspective is analyzed. The temperature, carbon in feed (thus the carbon to feed ratio called RC), and slag from the slag bath are investigated as the significant variables affecting phenomena in the AAV. Results show that alloy can become the reductant to fume specifically Mg(g), SiO(g), Cr(g) and even Fe(g) from slag containing MgO SiO2, FeOx and Cr2O3. The implications and resulting mass and energy transfer by these fumes can play a significant role in understanding open-bath ferrochrome smelting better as well as the energy balance of the arc and its impact on overall furnace efficiency. The irreversibility of the fuming reactions and reoxidation of metal vapor has a significant negative impact on furnace operation if the temperature in the AAV is not managed well due to poor operation regarding charging of feed. This implies not only that the average slag and metal temperature are of importance, but especially the actual AAV temperature is crucial. To fully understand the effect of chemical potential of the gases coming from the AAV, an exergy analysis uniquely showed that the fumes can be essential to reduce energy consumption if managed well, especially if the chemical potential energy can be harnessed in other zones within the furnace. This investigation is also relevant to other open-bath processes with fumable oxides, for example in smelting of ferronickel, titania slag, direct reduced iron, or even iron ore. Understanding the reaction mechanisms in the AAV in detail will be significant to push efficiencies of these applications to their thermodynamic, kinetic, and technological limits to ensure that their green metallurgical impact is fully realized.
Performance evaluation of boron/epoxy and carbon/epoxy composite patches for extending service life of damaged AA7075-T6 aircraft components
Zouambi, Leila; Fekirini, Hamida; Moller, Heinrich; Khodja, Malika (Taylor and Francis, 2026)
Despite extensive theoretical predictions, no systematic experimental comparison between boron/epoxy and carbon/epoxy patch repairs has been conducted under standardised conditions, creating a critical knowledge gap in aerospace structural maintenance. This study provides the first rigorous experimental validation of these competing repair technologies under identical ASTM E647 protocols. AA7075-T6 specimens (400 × 160 × 2 mm3) with 6 mm rivet holes and 1 mm starter cracks were repaired using single-sided composite patches and subjected to constant amplitude fatigue loading (22 kN, 10 Hz, R = 0.1). Crack propagation monitoring revealed boron/epoxy patches achieved 96,000 cycles versus 78,000 cycles for carbon/epoxy systems, a quantified 23% performance advantage. Stress intensity factor analysis demonstrated up to threefold reductions compared to unrepaired specimens, with boron/epoxy exhibiting superior crack growth retardation (da/dN rates 40% lower at equivalent ΔK values). Post-fatigue residual strength testing confirmed enhanced load-bearing capacity in both repair configurations. These benchmark experimental data validate two decades of theoretical predictions whilst establishing the first standardised comparative database for evidence-based material selection in aircraft repair programmes. The quantified performance metrics provide essential validation data for regulatory compliance and structural integrity assessments in aerospace maintenance applications.
Premature ageing of a blast furnace taphole clay containing resole resin and liquid pitch as binder
Garbers-Craig, Andrie Mariana; Cameron, Izak Jian-Pierre ; Ramjee, Shatish (Southern African Institute of Mining and Metallurgy, 2025-07)
The cause of reduction in workability and increased ageing of a blast furnace taphole clay was examined. The investigated taphole clay contained 60 mass% alumina, with phenol-formaldehyde resole resin and liquid pitch as the binder system. The workability and Marshall extrusion pressure of as-manufactured clay samples were evaluated to identify the extent of ageing of the clay. The wettability of all the raw materials was investigated to confirm compatibility between the dry raw materials and both the resin and liquid pitch, while the dry raw materials were analysed using XRF, XRD and SEM-EDS. The characterisation of the resin and liquid pitch, as well as the analysis of their interaction, was performed using viscosity measurements, Fourier-transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. Analyses confirmed a chemical interaction between the resole resin and liquid pitch, where the chemical structure of the resole resin changed when mixed with pitch, preventing the typical curing behaviour of the resin. After ageing of the resin and liquid pitch mixtures, the resin prematurely cross-linked, causing the curing process to move to lower temperatures, i.e., an earlier onset of curing. The premature cross-linking of the resole resin was likely due to secondary amines present in the liquid pitch that acted as a catalyst for the resin curing process. This reduction in curing temperature after ageing was confirmed by an increase in binder viscosity, which was the primary cause of the reduced workability, increased ageing and increasing Marshall extrusion pressure of the taphole clay.
Comparative analyses of the mechanical and microstructural properties of screws manufactured by cutting, extrusion and deep rolling techniques
Newlands , Pearline Ami; Kpakpo Addo, Edgar Nii; Ampah-Essel, John Ekow; Ardayfio, Beatrice; Asante, Joshua Tuah; Nyankson , Emmanuel; Agyei-Tuffour, Benjamin (Taylor and Francis, 2025-10-13)
This article evaluates the influence of different fabrication techniques, extrusion (ET), cutting (CT), and deep rolling techniques (DRT) on the microstructural and mechanical behavior of medium carbon steel screws for fastening applications. It involves mechanical, microstructural, and compositional analyses of an identical medium-carbon steel composed of chromium (Cr), vanadium (V), niobium (Nb), silicon (Si), manganese (Mn), and carbon (C). It also considers the effects of fabrication method on the fatigue performance, hardness, and fracture characteristics of the screws. The analytical studies considered constitutive equations; whereas, the numerical approaches used finite element analyses (FEA) to corroborate the fatigue stress distributions in the threads of the screws manufactured by ET, CT and DRT. The results showed that the microstructures had area fractions of ∼0.03, ∼0.20 and ∼0.23 for DRT, CT, and ET, respectively, owing to the respective phase structures high in carbon in the steels. The DRT significantly reduced martensite area fraction and refined grain structure, leading to improved ductility and fatigue performance. Mechanical testing showed that DRT screws had the highest fatigue limit (109 MPa), while CT screws had the greatest surface hardness (∼467 HV). The FEA showed a DRT sample with improved fracture toughness (∼300 MPa√mm) and a slower rate of crack growth. The stress distributions and crack growth under loading in the FEA analyses corroborate the experimental trends. The results suggest that the DRT is particularly beneficial for screw components subjected to cyclic stresses and fatigue-critical applications.
Numerical analysis of mechanical modifications induced by laser shock peening on AA2024-T351 for aeronautical structures
Righi, Sabrina; Fekirini, Hamida; Mebarki, Hichem; Polese, Claudia; Khodja, Malika (Aeronautical and Astronautical Society of the Republic of China, 2025-06)
This work explores the impact of Laser Shock Peening (LSP) on the mechanical state of AA2024-T351 specimens used in the aerospace industry. Numerical simulations, conducted using ABAQUS/Explicit and based on the Johnson-Cook behavior model, examine the residual stresses and plastic deformations induced by a single circular laser impact. The results reveal that LSP, with a peak pressure of 3 GPa and a pulse duration of 25.8 ns, induces surface hardening and anisotropic compressive stresses, enhancing fatigue resistance. In-depth analysis shows that damage is concentrated around the impact center, remaining below 0.03. This study highlights the importance of optimizing LSP parameters to maximize its benefits while minimizing damage, thus contributing to the improved durability of aeronautical structures.
Sodium oxide-fluxed aluminothermic reduction of manganese ore with synergistic effects of C and Si reductants : SEM study and phase stability calculations
Coetsee, Theresa; De Bruin, Frederik Johannes (MDPI, 2025-07-28)
Aluminothermic reduction is an alternative processing route for the circular economy because Al is produced electrochemically in the Hall–Héroult process with minimal CO2 emissions if the electricity input is sourced from non-fossil fuel energy sources. This circular processing option attracts increased research attention in the aluminothermic production of manganese and silicon alloys. The Al2O3 product must be recycled through hydrometallurgical processing, with leaching as the first step. Recent work has shown that the NaAlO2 compound is easily leached in water. 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 carbon and silicon reductants with aluminium is illustrated and compared to the test result with only carbon reductant. The addition of small amounts of carbon reductant to MnO2-containing ore ensures rapid pre-reduction to MnO, facilitating aluminothermic reduction. At 1350 °C, a loosely sintered mass formed when carbon was added alone. The alloy and slag chemical analyses are compared to the thermochemistry predicted phase chemistry. The alloy consists of 66% Mn, 22–28% Fe, 2–9% Si, 0.4–1.4% Al, and 2.2–3.5% C. The higher %Si alloy is formed by adding Si metal. Although the product slag has a higher Al2O3 content (52–55% Al2O3) compared to the target slag (39% Al2O3), the fluidity of the slags appears sufficient for good alloy separation.
Permanent cathode technologies in copper electrowinning : development and status
Naidoo, Kalin; Sole, Kathryn C. (MDPI, 2025-09)
The replacement of copper metal starter-sheet cathodes with the use of permanent cathode technology, in which the target metal is plated onto an inert blank template, has enabled significant benefits in the copper electrowinning process. These include the application of significantly higher current density, which reduces tankhouse footprint and increases process intensity per unit area; improved operator safety with less reliance on manual electrode handling; and the implementation of process automation and robotics. Cathodes of >99.99% chemical purity and with a smooth and aesthetic surface morphology are consistently produced. This review considers the evolution and development of the permanent cathode process, its commercial adoption across the global copper industry, and the current technology status.
Evaluation of binderless briquettes as potential feed for the electric arc furnaces at Barro Alto, Brazil
Mogalanyane, Johnny Obakeng; Naude, Natasia; Garbers-Craig, Andrie Mariana (MDPI, 2025-07)
Barro Alto processes nickel laterite ore using rotary kilns and six-in-line rectangular electric arc furnaces. This study evaluated the briquetting of ferronickel ore to reduce kiln fines, improve furnace charge permeability, and enhance process safety. Binderless briquettes were produced from screened ore at two size fractions (−6.3 mm and −12.5 mm), with moisture contents of 16% and 24%, cured under closed and open conditions. The physical and metallurgical properties of the briquettes were assessed using ISO standard tests. The results confirmed successful agglomeration of the ore into binderless briquettes. Screening the run-of-mine (ROM) ore improved the feed quality, increasing the NiO grade from 2.0% to 2.2% in the −6.3 mm fraction. The briquettes from the −6.3 mm ore at 16% moisture exhibited the highest green strength (559 N). Higher moisture content reduced the briquette strength and increased both the reduction disintegration and decrepitation indices. The decrepitation index increased from 0.33% to 0.61% for the −6.3 mm briquettes when the moisture increased from 16% to 24%. The reduction levels were 33.4% and 39.2% for −6.3 mm and −12.5 mm briquettes with 16% moisture, respectively. This study concludes that optimal performance was achieved using −6.3 mm ore, 16% moisture, and open curing, thereby balancing reduction efficiency and mechanical stability.
Effects of strain and inter-pass time on the restoration behavior of 436 stainless steels
Salojee, Muhammed Yusuf; Siyasiya, Charles Witness; Annan, Kofi Ahomkah; Moema, Joseph S. (Springer, 2025-11)
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
Jafarpour Rezaei, Seyed Reza; Siyasiya, Charles Witness; Tang, Zhenghua; Moema, Joseph (Springer, 2025-08)
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.

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