Research Publications/Outputs

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The adsorption of halogen molecules on Ti (110) surface
(2021-01) Tshwane, David M; Modiba, Rosinah; Govender, Gonasagren; Ngoepe, PE; Chauke, R
Adsorption of halogen on the metal surface has received much attention due to its technological applications and major relevance for material surface processing, corrosion protection and etching. In this work, first-principle approach was used to investigate the interaction of halogen molecules on Ti (110) surface. The present results revealed that adsorption of the halogen molecule is exothermic and occurs by dissociation bonding. The HF molecule was found to be more thermodynamically stable than the HI molecule. In addition, our results revealed that the adsorption of halogen ions on Ti (110) surface is energetically favourable than the adsorption of halogen molecule. The possible adsorption sites were tested, and the top site position was found to be the most favourable followed by the hollow and bridging site for both halogens. Furthermore, the results showed the linear relationship between adsorption energy strength and charge transfer. Also, the density of states and charge density difference was studied to investigate the electronic interaction. The charge redistribution showed an electron depletion on Ti atom and charge accumulation on the halogen region.
Adsorption-desorption of F2 diatomic molecule on Ti (100) surface at different coverages
(2021-12) Tshwane, David M; Modiba, Rosinah; Govender, Gonasagren; Ngoepe, PE; Chauke, HR
Fluorine molecules and ions are used as an etchant for metal surface processing. The presence of fluorine significantly influences the electrochemical behaviour on a metal surface, which has major relevance for etching, corrosion, electro-catalysis and galvanic deposition processes. Although the fluorine ions play an important role in metal surface etching, the studies remain limited and unclear, especially at the atomistic scale. In this work, density functional theory is used to investigate the structural and electronic properties of F2 diatomic molecule adsorption on Ti (100) surface at different coverages. Results revealed a dissociative adsorption mechanism of F2 on the Ti (100) surface. Adsorption energy analysis of F2 on Ti (100) surface denotes an exothermic process. Moreover, increasing F2 coverage resulted in the formation of TiF4 and Ti2F6 molecules on the surface. In addition, the calculated heat of formation for TiF4 was found to be more favourable than Ti2F6 species. Calculated desorption energies for TiF4 and Ti2F6 is 11.73 eV/atom and 9.04 eV/atom, suggesting non-spontaneous.
Challenges in machining of advanced materials
(CRC Press, 2022-05) Mathabathe, Maria N; Bolokang, Amogelang S; Gajrani, KK; Prasad, A; Kumar, A
Contemporary ways of employing and combining existing materials are a requisite for meeting tomorrow’s engineering applications. This is upscaled by the potent attributes of remaining competitive by keeping costs effective while advancing the fabrication of new material technologies. Thus, moderation of the tool condition and machining process, for example, is becoming successively critical to achieving more eminent yields, exceptional machine automation, better product quality, and reduced labor-intensive costs. Owing to the intricate nature of advanced materials, such as ceramic matrix composites, their brittle and heterogeneous structure and their anisotropic mechanical and thermal behavior render a variety of material removal mechanisms that result in surface defects. Therefore, this chapter’s objective is to focally evaluate how dissimilar machining techniques influence the machined surfaces of advanced materials. Furthermore, the study aims to pioneer the fundamental technologies and trendsetting development for machining process monitoring, namely, contemporary industrial practice inclusive of general workpiece surface integrity parameters, fluid consignment systems, wheel preparation options, and machine tool design/alignments. Similarly, academic research on the conventional and nonconventional machinability of advanced materials in the aerospace industry is deeply investigated. This is achieved by assessing new, various material characterization methods to identify and quantify the mechanical and thermal surface/subsurface damages and highlight their governing removal/grinding-process mechanisms.
Effect of milling speed on the formation of Ti-6Al-4V via mechanical alloying
(2018-10) Daswa, Pfarelo; Gxowa, Zizo; Monareng, Mathibe JI; Mutombo, Kalenda
Ti-6Al-4V alloys are well known for good corrosion resistance, excellent surface oxide biocompatibility and high specific strength. This paper investigates the influence of milling parameters on the formation of Ti-6Al-4V alloy processed using mechanical alloying. A mixture of CP Ti and master alloy (60Al-40V) powder were milled using a high energy ball mill (HEBM) in order to produce a mechanically alloyed Ti-6Al-4V alloy powder. Milling was carried out at a constant milling time of 5 hours at varying speeds of 800, 900, 1000 and 1100 rpm in order to investigate the influence of speed on the formation of Ti-6Al-4V alloy. Scanning electron microscopy with energy dispersive analysis (EDX) and X-ray diffraction (XRD) were performed on the mechanically alloyed powder to investigate the chemical homogeneity and the formation of alpha (a) and beta (ß) phase during the mechanical alloying of CP Ti and 60Al- 40V.
The effects of quaternary alloying additions on the TiAl alloy: Preferential site occupancy, interfacial energetics to physical parameters
(2021-08) Mathabathe, Maria N; Modiba, Rosinah; Bolokang, Amogelang S
The effect of Nb, Sn, Mn, and Si at a concentration of (0.3-2 at. %) on -TiAl based alloys’ preferential occupancy, interfacial energetics and mechanical properties were investigated by density functional theory (DFT) encompassed by the typical gradient estimation configuration and substantiated by experimental work. The site occupancies, elastic constants, shear and bulk modulus, B/G ratio, Poisson's ratio, hardness and universal anisotropy are systematically elucidated. The results indicated that Mn adamantly occupies Ti sites, while the Nb, Sn and Si occupy Al sites, with no significant influence on the alloy composition in-terms of their site preference. Interfacial energy of /a2-M system is the prerequisite energy to generate an interface from bulk materials. The stability criteria were satisfied attributable to Cij’s values greater than zero. The overall mechanical properties of the substitutional solid solution-TiAl alloy exhibited improved ductility. The alloy was produced by vacuum arc melting, with subsequent annealing to achieve homogeneity. The TEM results demonstrated that the a2/ interface boundaries yielded the -TiAl {111} <110>||a2-Ti3Al (0001)<110> orientation relationship.
The electrochemical corrosion behaviour of Ti48Al-2Nb-0.7Cr-0.3Si alloy in 3.5%NaCl
(2022-11) Magogodi, Steven M; Mathabathe, Maria N; Bolokang, Amogelang S; Siyasiya, C
Titanium aluminides (TiAl) have attracted industrial interest, particularly for high temperature applications owing to their promising mechanical properties. Due to the application of these materials to the marine environment it is therefore important to understand their performance under such conditions. In this study, the corrosion behaviour of Ti-48Al-2Nb-0.7Cr-0.3Si alloy produced by vacuum arc melting was investigated using the potentiodynamic polarization method. The corroded surface and Ti-Al phases were investigated by scanning electron microscopy (SEM), optical microscopy, and X-ray diffraction (XRD). Hardness was examined by a Vickers micro hardness tester with a diamond indenter. Pitting was observed to occur within the surface defects and caused severe corrosion.
Grain structure orientational change in Ti6Al4V alloys induced by sea water quenching and novel stress relief annealing process
(2023-03) Bolokang, Amogelang S; Mathabathe, Maria N; Motaung, DE; Arendse, CJ; Swart, HC
We report on the microstructures and properties of Ti6Al4V alloys, which were achieved upon quenching in sea water medium with potential high cooling rate. The Ti6Al4V alloys were quenched at 1000 and 1100 °C, respectively. Moreover, the effect of post-quenching annealing performed at 900 °C was analyzed. As a result, the quenched alloy experienced surface thermal stress, due to rapid cooling and thermal shock, due to exposure to high temperature annealing. The alloy quenched at 1000 °C developed equiaxed grain structure after annealing, while the 1100 °C-quenched alloy generated irregular shaped lamellae structures. TEM analysis for the 1000 °C-quenched samples revealed the a'-martensite decomposed into the equilibrium a + ß phases. Moreover, the 1100 °C-quenched Ti6Al4V alloy revealed an a'-acicular martensitic structure.
Investigation into the thermal behaviour of the B2–NiAl intermetallic alloy produced by compaction and sintering of the elemental Ni and Al powders
(2019-11) Camagu, Sigqibo T; Mathabathe, Maria N; Motaung, David E; Muller, TFG; Arendse, CJ; Bolokang, Amogelang S
A cubic B2–NiAl alloy was synthesized by mixing of elemental Ni and Al powders followed by cold compacting and sintering of some samples at 750 °C and other samples at 1300 °C. The alloys sintered at 1300 °C exhibited a brittle B2–NiAl structure easily crushable into powder, while the 750 °C sintered alloys were not nearly as brittle. Electron Backscatter Diffraction (EBSD) results show that the B2–NiAl alloy sintered at 750 °C for a longer time (120 h) has larger grain size associated with the <111>-oriented grains encountering high mobility boundaries than the alloy sintered at 750 °C for a shorter time (48 h). Samples morphologies were analyzed using the Scanning Electron Microscope (SEM). Structural development of the alloy was studied via the x-ray diffraction (XRD) technique. The B2–NiAl intermetallic developed a thin scale of stable Al2O3 alloy due to oxidation in air at 750 °C for 120 h. As a result, further oxidation on the sample's surface was restricted, except the traces of Al2O3 formed via intergranular oxidation transformed into a metastable monoclinic oxynitride phase due to nitrogen (N) contamination.
Laser cladding - a modern joining technique
(Elsevier, 2021-04) Bolokang, Amogelang S; Mathabathe, Maria N
The shroud shelve life of turbine blades depends on wear resistance. Thus, the wear of blade shroud shelves’ contact area ascertains the overall service life of gas turbine engines (GTE). Moreover, research shows that the life service of turbine blades may be enhanced by plasma cladding in the form of wire. However, plasma cladding has its limitations such as the fact that it’s performed manually and administers non-uniformity of the coating. As a result, these limitations can be overcome by laser cladding coating technique, which is accomplished by direct injection of cladded powder into the melt pool. The current chapter confers a review on laser cladding technology which is a modern joining technique to fulfill the requirements of many industrial applications. The intention is to facilitate researchers by providing comprehensive knowledge about this technology to encourage them to explore it by future research and innovation efforts. The significant merits and operational areas of these approaches are also discussed in this chapter.
Overageing characteristics of alloy A356 and Al-Mg-Si casting alloys
(2019-03) Daswa, Pfarelo; Moller, H; Govender, Gonasagren
Al-Si-Mg casting alloys, such as Al-7Si-0.3Mg alloy A356, are heat treatable and can be precipitation hardened to the T6 temper condition. However, Al-Mg-Si casting alloys (5xx series) are generally not considered to be heat treatable. These 5xx series castings are known for good castability and good resistance to corrosion, especially in marine environments. This paper investigates the extent to which 5xx series alloys could possibly be artificially aged. The influences of artificial ageing time on the overageing characteristics of both Al-Mg-Si and A356 casting alloys have been studied. A356 aluminium alloy castings were produced using the CSIR rheo-high pressure die casting process (R-HPDC). Al-Mg-Si alloys were cast using permanent mould casting. The rate of overageing of these alloys is of importance for potential higher temperature applications. The overageing characteristics of Al-Mg-Si and A356 aluminium alloys have been investigated at an artificial ageing temperature of 190oC for ageing times up to 128 hours. It is shown that the rate of overageing of Al-Mg-Si casting alloys is lower than for alloy A356. This could possibly result in the use of these alloys in applications at temperatures that are higher than where alloy A356 can be employed. It also allows the possibility of using the 5xx series alloys as an alternative to other Al-alloys for R-HPDC applications.
Process and materials design via statistical modeling for Inconel-625/tungsten carbide wear-resistant composite coatings fabricated by laser direct metal deposition technique
(2023-03) Olakanmi, EO; Hoosain, Shaik E; Lawal, SA; Pityana, Sisa L
It is critical that coating’s microstructural characteristics are designed to meet its wear-resistance functional requirement. To the best of our knowledge, no study had related carbide dissolution ratio (CDR) in laser deposited Inconel 625 composite coating and its microstructural parameters to its wear performance. This study explores how laser processing and materials parameters influence CDR, microhardness (MH) and wear resistance (measured in terms of volume of materials loss: VML) of fibre-laser deposited composite coatings by employing response surface methodology (RSM). The nature of inter-relationship between the CDR, coating’s microstructural parameters (average mean free path and size of retained particles, and MH) as well as VML was explored to determine appropriate process and materials parameters to optimise wear resistance of the coatings. CDR increases with laser energy density while MH and wear resistance increase with laser energy density up to a threshold of 19.70J/mm2. Above the energy density threshold, MH and wear resistance reduce. Inconel 625 content varies directly with CDR and VML while it has an indirect relationship with MH. The shielding gas flow rates have no effect on micro-hardness whereas it varies indirectly with CDR and VML. A fully consolidated coating characterised with uniformly distributed retained reinforcement’s particle size of 40 µm; mean free path of 30 µm within the Inconel 625 matrix; MH = 852 HV0.5; and CDR = 77.08% has the most desirable wear resistance (VML = 9.42mm3) when processed with laser energy density (19.70 J/mm2), Inconel content (70wt%) and shielding gas flow rates (6.00 l/min). This study provides new insight, for coating manufacturers, on how CDR and microstructural parameters can be manipulated as laser process and materials variables are altered in designing most desirable wear resistant composite coating.
Semi-solid casting of pure magnesium
(2019-01) Curle, Ulyate A; Wilkins, Jeremias D
Semi-solid processing works on the principal of a solidification temperature interval of a substance. The substance is heated to a temperature within this interval so that there exists a related solid-liquid fraction ratio. The substance with this phase structure is then shaped by a forging or casting process. It has been stated before that it is impossible to semi-solid process and cast pure metals or eutectic alloys due to their thermodynamic temperature invariance, meaning that there is no temperature interval. It was demonstrated recently that it is possible to semi-solid casting high purity aluminium (Curle UA, Möller H, Wilkins JD. Scripta Materialia 64 (2011) 479-482) and the Al-Si binary eutectic (Curle UA, Möller H, Wilkins JD. Materials Letters 65 (2011) 1469-1472). The working principal is that there exists a time interval during thermal arrest during which solidification takes place with a solid-liquid fraction ratio until all the liquid is consumed upon cooling. The aim with this work is to demonstrate that pure magnesium can also be rheo-high pressure die cast (R-HPDC) with the system developed at the CSIR in South Africa. Magnesium is notoriously difficult to cast due to the thermal properties of magnesium. The metal was poured into a cup, processed for about 6 seconds after which it was HPDC into a plate. The microstructure of the casting consists of a structure that was solid and a structure that was liquid during thermal arrest at the time of casting.
Surface characterization of the cyclically oxidized -Ti-48Al-2Nb-0.7Cr alloy after nitridation
(2019-08) Mathabathe, Maria N; Govender, Gonasagren; Siyasiya, CW; Mostert, RJ; Bolokang, Amogelang S
Surface properties of the nitrided -Ti-48Al-2Nb-0.7Cr intermetallic alloy after cyclic oxidation was characterized. The nitride coating was successfully deposited after annealing of the Ti-48Al-2Nb-0.7Cr intermetallic alloy at 900 °C under nitrogen atmosphere. As a result, the nitride layer on the Ti-48Al-2Nb-0.7Cr alloy significantly improved its cyclic oxidation and the best performance was recorded at 900 °C after 900 h (h). Atomic force spectroscopy (AFM) analysis confirmed the surface multi-layers with surface roughness and a spectral root mean square (RMS) amplitude of 165 nm. Microstructures were analysed employing scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS) for micro-analysis. Phase analysis was conducted using X-ray diffraction (XRD).

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