Browsing by Author "Machio, Christopher N"
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Item Analysis of the cold compaction behaviour of TiH2-316L nanocomposite powder blend using compaction models(Trans Tech Publications, 2015-07) Machio, Christopher N; Machaka, Ronald; Shabalala, T; Chikwanda, HKThe paper captures the effect of structure and the applicability of compaction models using the cold compaction of a TiH2-SS316L composite powder prepared by high energy mechanical milling. The composite blend was cold pressed uniaxially to pressures of up to 1250MPa. The compressibility of the composite blend was evaluated by fitting the experimental data to the most commonly used compaction models of Heckel, the Kawakita-Lüdde, the Cooper- Eaton, the Ge, and the Panelli-Filho compaction equations. Among the models, the Kawakita-Lüdde and Cooper-Eaton models fitted the experimental data very well with a good correlation (the correlation coefficient greater than 0.99) throughout the entire pressure range under investigation. The nature and mechanisms responsible for the densification during cold compaction are discussed. The Heckel, Ge, Panelli-Filho, and Cooper-Eaton model analysis showed that the dominant compaction mechanisms for composite blend were rearrangement of particles followed by elastic and plastic deformation. The results are discussed by way of a comprehensive model intercomparison study of the cold compaction behaviour using existing models.Item Characterization of mechanically alloyed f.c.c. Ti–Mg–based powders(Elsevier, 2010-05) Machio, Christopher N; Nyabadza, D; Sibanda, V; Chikwanda, HKFace centred cubic Ti – 10 and 20 Mg and Ti-6Al-4Mg (wt. %) alloy powders have been produced by high energy ball milling, under argon, of elemental powder mixtures containing a process control agent (PCA). Milling leads to a multi-disperse particle size distribution without affecting the particle morphologies. This affects the thermal stability of the powders and the green densities of compacts. The green densities varied in tandem with the d10 sizes of the powders and went through a minimum at 20hrs’ milling. Milling for 20 hours could be detrimental to powder consolidation and further processingItem Consolidation of titanium hydride powders during the production of titanium PM parts: The effect of die wall lubricants(Elsevier, 2015-11) Machio, Christopher N; Machaka, Ronald; Chikwanda, HKThe effects of die wall lubricants on the cold compaction of titanium hydride powder are studied. Three commonly-used die wall powder metallurgy lubricants – zinc stearate, Acrawax® C dispersion and Mirror Glaze® – are compared. The influence of each lubricant on the cold compaction behaviour of titanium hydride powder was assessed with respect to the green density and strength, green compact ejection force, and the occurrence of green compact cracking. In addition, green compacts were sintered under vacuum at various sintering temperatures in order to determine the effect of the lubricants on oxygen content, phase compositions, microstructures and hardness of the sintered components. The results indicate that there is an immediate improvement in the cold compaction process when lubrication is used. The results further indicate that the lubricity of the lubricants is similar and most of the monitored variables are insensitive to the die wall lubricant used, the microstructures of the sintered components included. However, the use of Acrawax®dispersion resulted in more instances of green compact cracking, while zinc stearate tended to increase the oxygen content of sintered specimens due to the tentative decomposition of zinc oxide, a residual of the high temperature decomposition of zinc stearate.Item Corrosion behaviour of WC-VC-Co hardmetals in acidic media(Elsevier, 2010-09) Konadu, DS; Van der Merwe, J; Potgieter, JH; Potgieter-Vermaak, S; Machio, Christopher NThe effect of increasing vanadium carbide (VC) content on the corrosion behaviour of tungsten carbide – 10 wt% cobalt hard metals was investigated in 1 M hydrochloric (HCl), and sulphuric (H2SO4) acids solutions. Increasing VC content makes the open circuit potential (OCP) in the test solutions more negative than the base alloy. Specimens exhibited pseudo passivation in all the test solutions. Increasing VC led to decreasing corrosion current density. However, the corrosion current densities during chronoamperometric tests were lower for 0 wt% VC. XRD and Raman spectroscopy showed that hydrated WO3 formed in the surface films of all specimens in hydrochloric acid (HCl), while hydrated vanadyl sulphate also formed for higher VC content specimens in sulphuric acid (H2SO4).Item Corrosion of WC-VC-Co hardmetal in neutral chloride containing media(Hindawi Publishing Corporation, 2013-01) Machio, Christopher N; Konadu, DS; Potgieter, JH; Potgieter-Vermaak, S; Van der Merwe, JCorrosion is an important surface degradation process in some, if not all, applications of tungsten-carbide-(WC-) cobalt- (Co-) based hardmetals. Applications like tools for machining of metals and for wear resistance in the mining industry expose the WC-Co to fluids that can be corrosive, leading to a reduced useful life. The Co binder is the least corrosion resistant constituent, and efforts to improve WC-Co corrosion resistance have involved altering the chemical composition of the binder by introducing more corrosion resistant elements. Nickel, and chromium especially, and recently, ruthenium have been shown to markedly improve the corrosion resistance of WC-Co-based hardmetalItem Effect of process control agent (PCA) on the characteristics of mechanically alloyed Ti-Mg powders [Conference paper](Southern African Institute of Mining and Metallurgy, 2010-10) Machio, Christopher N; Chikwanda, HK; Chikosha, SThis article reports results of a study to determine the effect of process control agent (PCA) on the characteristics of Ti-Mg powders during milling. It has been shown that a 2% increase in PCA content leads to up to a 40% increase in yield of the milled powder but reduces the kinetics of the mechanical alloying process. The introduction of 4% PCA decreases the mean powder particle size by up to 30%, while 6% PCA increased the mean particle size by up to 230%. The characteristics of the milled powders will be discussed in relation to the role of PCA in the milling process.Item Effect of process control agent (PCA) on the characteristics of mechanically alloyed Ti-Mg powders [Journal article](Southern African Institute of Mining and Metallurgy, 2011-03) Machio, Christopher N; Chikwanda, HK; Chikosha, SThis paper reports the results of a study to determine the effect of process control agent (PCA) on the characteristics of Ti-Mg powders during milling. It has been shown that a 2% increase in PCA content leads to up to a 40% increase in yield of the milled powder but reduces the kinetics of the mechanical alloying process. The introduction of 4% PCA decreases the mean powder particle size by up to 30%, whereas 6% PCA increases the mean particle size by up to 230%. The characteristics of the milled powders will be discussed in relation to the role of PCA in the milling process.Item The effect of vanadium and nickel on the microstructure and transformation temperatures of Ti50Pt50 alloy(2022-11) Motsai, Tebogo; Makhatha, E; Camagu, Sigqibo; Machio, Christopher N; Daswa, Pfarelo; Radingoana, Precious; Motsi, GlendaSignificant research has been done to produce shape memory alloys that have good shape memory properties and high martensitic transformation temperatures. The Ti50Pt50 alloys have been found to have high transformation temperature of around 1050 however, they exhibit negligible shape memory properties. The solid solution strengthening, and improved shape memory properties could be enhanced by ternary alloying. Therefore, this work investigates the effect of varying V and Ni contents, in the range of 6.25 to 12.5at%, on the austenitic and martensitic transformation temperatures, and hardness of the equi-atomic Ti50Pt50 alloy. Arc melting followed by casting and solution heat treatment was carried out to produce the alloys. As-produced alloys were characterized by using scanning electron microscopy, differential scanning calorimetry and hardness testing. The microstructures showed high volume fraction of second phases formed in the TiPtV alloy compared with Ti50Pt50 and TiPtNi alloys. The multiple phases formed in the TiPtV alloys could be the cause of high hardness values observed in these alloys as compared withTi50Pt50 and TiPtNi alloys. Thermal transformation studies revealed that TiPtV alloys exhibit transformation temperature close to Ti50Pt50 alloy, in contrast with TiPtNi alloys. TiPtNi alloys thermal behaviour was improved by solution heat treatment.Item Mechanical alloying and sitering of TI - 10WT.% MG powders(2009-06) Machio, Christopher N; Nyabadza, D; Chikwanda, HK; Phasha, M; Sibanda, VMA Ti-10wt.%Mg powder alloy has been produced by mechanical alloying. Elemental powders of Ti and Mg were ball milled in a Zoz-Simoloyer CM01 for 16 and 20 hours under argon. Mechanical alloying was followed by XRD, SEM and particle size analysis. Test specimens of the milled powders were produced by cold compaction and sintering (under argon). The milling resulted into a fcc TiMg solid solution for both milling durations. Sintering leads to a partial decomposition of the fcc TiMg to hcp TiMg and Ti.Item Microstructures and thermal behaviour of Ti31.75V18.25Pt50 and Ti25V25Pt50 potential shape memory alloys(2022-11) Motsai, Tebogo M; Makhatha, ME; Camagu, Sigqibo T; Machio, Christopher N; Daswa, Pfarelo; Radingoana, Precious M; Motsi, Glenda TThe binary Ti50Pt50 alloy exhibits high transformation temperatures (1050°C) and has gained a great deal of attention due to its potential for high temperature applications in the automotive and aerospace industries. However, this binary system has negligible shape memory effect. Shape memory effect is the property of a material to recover to its initial shape following heating while retaining residual deformations throughout an inelastic loading/unloading cycle. Improved shape memory properties could be achieved by ternary alloying Ti50Pt50. A study reported on partial substitution of Ti with 6.25 and 12.5 at. % V in the equi-atomic TiPt resulted in increased austenite and martensitic transformation temperature. This study is a follow-up to investigate the impact of higher V contents (18.25 and 25 at. %) on microstructures and the thermal behaviour of the alloys. Samples were prepared by arc melting of blended powders, and characterised for microstructures, thermal behaviour and hardness. The as-cast microstructure of Ti31.75V18.25Pt50 revealed a matrix of a single phase (Ti, V) Pt and second phases comprising TiO and (Ti2O) + Pt, while that of Ti25V25Pt50 showed a matrix of (Ti,V) Pt with TiO and Ti2O. Solution heat-treated Ti31.75V18.25Pt50 showed a matrix of the single phase (Ti, V) Pt with second phases of (Ti, V) Pt, while Ti25V25Pt50 showed a matrix of (Ti, V) Pt with second phases of (Ti, V) Pt, TiO, TiO2 and Ti2O. Increasing vanadium from 18.25 to 25 at. % led to a decrease in transformation temperature in both the as-cast and solution heat-treated conditions. The transformation temperature increased after solution heat treatment. The enthalpy change during transformation ( H) decreased with higher vanadium content. Hardness values increased with vanadium content increase in both as-cast and heat-treated conditions.Item Modelling and optimization of surface roughness during AISI P20 milling process using Taguchi method(2019-06) Tlhabadira, I; Daniyan, IA; Machaka, Ronald; Machio, Christopher N; Masu, L; VanStaden, LRSurface roughness Ra is a parameter normally used to indicate the level of surface irregularities during machining operations. This work aims to model the cutting process, correlate and optimise the critical process parameters using the Taguchi method during the milling operation of AISI P20 in order to reduce surface roughness. The Autodesk Fusion 360 (2.0.5357) was employed for modelling the stress, displacement and thermal behaviour of the cutting tool and work piece under different cutting conditions. The experimental plan was based on Taguchi’s technique including L9 orthogonal array with three factors and three levels for each variable and studying the contribution of each factor on surface roughness. The Taguchi method was used to study the effect of process parameters and establish correlation among the cutting speed, feed and depth of cut with respect to the major machinability factor, surface finish. The machining parameters evaluated in this study are the depth of cut (d), spindle speed (N) and cutting feed (fm) while the response factor measured is surface roughness. The physical experiments were conducted on M200 TS material on a DMC 635 V DMG ECOLINE, Deckel Maho Germany, Siemens 810D, 3-Axis, CNC vertical milling machine using carbide inserts and the surface roughness was measured using the Mitutoyo SJ–201, surface roughness Machine. The statistical analysis of both the numerical and physical experiments brought about the development of a mathematical model and optimum solutions for the evaluation of surface roughness during the milling process with high degree of correlation with experimental values thus validating the developed model.Item Phase transformations during sintering of mechanically alloyed TiPt(south African Institute of Mining and Metallurgy (SAIMM), 2010-10) Nxumalo, S; Chikwanda, HK; Machio, Christopher NA TiPt alloy was produced by mechanically alloying the desired quantities of titanium and platinum. The resultant TiPt alloy powder was cold pressed to produce green bodies. Several sintering conditions were used to sinter this alloy. It was observed that TiPt phase formed together with other phases such as Ti solid solution Ti(Pt), Ti3Pt and Pt5Ti3. The quantities of the different phases depended on the sintering conditions used.Item Separation of DR synthesized Ti-Mg Alloy by dissolution of waste by-products in acid media(2010-06) Mushove, T; Chikwanda, HK; Ndlovu, S; Machio, Christopher NThe objective of this work is to isolate synthesized Ti-Mg alloy powder through leaching out of MgO and any unreduced oxides of Titanium. Leaching media investigated are H2SO4, HCl and organic acids ascorbic acid/oxalic and L-cysteine /ascorbic acids in equi-molar concentrations. The use of the test leaching media will be counter balanced by the need not to dissolve the Ti-Mg alloy formed during.Item Separation of DR synthesized Ti-Mg Alloy by dissolution of waste by-products in acid media(2010-06) Mushove, T; Chikwanda, HK; Ndlovu, S; Machio, Christopher NThe objective of this work is to isolate synthesized Ti-Mg alloy powder through leaching out of MgO and any unreduced oxides of Titanium. Leaching media investigated are H2SO4, HCl and organic acids ascorbic acid/oxalic and L-cysteine /ascorbic acids in equi-molar concentrations. The use of the test leaching media will be counter balanced by the need not to dissolve the Ti-Mg alloy formed during.Item Ti-Mg alloy powder synthesis via mechanochemical reduction of TiO2 by elemental magnesium(2009-04) Mushove, T; Chikwanda, HK; Machio, Christopher N; Ndlovu, SThis paper reports the preliminary results of an investigation on the synthesis of a Ti-Mg alloy powder through mechanochemical processing of TiO2 and Mg powders. TiO2 was mixed with elemental Mg according to a nominal stoichiometric composition with 15% excess Mg. The powder mixture was mechanically milled in a Simoloyer high energy ball mill for 5 different durations. Contamination was minimised by processing under a high purity argon atmosphere. Changes in phase composition were studied by XRD techniques. TiO2 was reduced, as shown by the formation of MgO. The extent of the reduction, as indicated by XRD peaks’ intensities, increased with milling time. XRD spectra of powders milled for 24 hours revealed virtual disappearance of TiO2 peaks and there was no evidence of elemental Ti. The lattice parameter of the resulting Ti metal was larger than that of elemental Ti. This implies that the Ti was alloyed with free Mg to produce Ti-Mg alloy powder. The lattice parameter increased with increasing milling time.Item Ti-Pt Alloys form mechanical milling(Microscopy society of Southern Africa (MSSA), 2009-12) Nxumalo, S; Chikwanda, HK; Machio, Christopher NThe high temperature ß-TiPt phase has been shown to undergo a martensitic transformation upon quenching to room temperature. The martensitic transformation results from a transformation of the parent B2 cubic structure to the martensitic B19 orthorhombic structure at a temperature of approximately 1000oC. The martensite phase results in shape memory effect being observed in this alloy at this temperature. Other alloys such as TiNi and TiPd have also been investigated for the martensitic transformation and shape memory effect. The initial alloys used in the studies were arc melted then cast for TiPd and arc melted then cast or mechanically alloyed for TiNi. The shape memory effect in these alloys has been observed to occur at temperatures of 520oC maximum in these alloys. A few researchers have studied the martensitic transformation in TiPt alloys using arc melted cast samples. In this work high temperature shape memory alloys are targeted using powder metallurgy as a processing route.Item Use of equimolar cysteine/ascorbic acids to recover MCP synthesized Ti(Mg) alloy(Southern African Institute of Mining and Metallurgy, 2010-10) Mushove, T; Chikwanda, HK; Machio, Christopher N; Ndlovu, SDissolution of waste by-products of mechanochemical processing (MCP) synthesis of Ti(Mg) alloy, from TiO2 and 15 wt.% excess Mg, was conducted in equimolar cysteine/ascorbic acids. The synthesized alloy is inherently mixed with MgO and other oxides of Ti and the objective was to leach out the by-products of the MCP process and hence recover a pure isolated Ti-Mg powder. The dissolution of particulate polycrystalline MgO and TiO2, and Ti-20Mg solid solution (obtained from milling blended elemental powders) was also investigated to note how they individually behaved in the leach reagent. The study indicated that separation was possible although with considerable contamination from MgO and TiO2 and inadvertent co-dissolution of Ti(Mg) alloy. Further work on sequential dissolution of the by-products using equimolar cysteine/ascorbic acids and inorganic acids is recommended. Equimolar cysteine/ascorbic acid leach reagent was effective in dissolving TiO2; while inorganic acids will dissolve MgO. It is envisaged that sequential dissolution will minimise the unintended co-dissolution of the Ti(Mg) alloy.