Department of Mechanical and Mechatronic Engineering

Permanent URI for this community

https://scholar.sun.ac.za/handle/10019.1/109

Browse

Now showing 1 - 13 of 13

Blending of powders for in-Situ Alloying of Ti-6Al-4V laser powder bed fusion
(Stellenbosch : Stellenbosch University, 2021-03) Parker, Benjamin Stuart; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Novel powder blending techniques for Ti-6Al-4V powder mixtures, using master-alloy (MA) and blended elemental (BE) powders as opposed to the industry standard of pre-alloyed (PA) Ti-6Al-4V powder,have been developed for laser powder bed fusion(LPBF), in particular direct metal laser sintering ®(DMLS®).The quality of a LPBF component is a result of both the metal powder characteristics and the LPBF process parameters. This study is limited to the metal powder characterisation of such blends, focussing on typical powder characterisation metrics of morphology, particle size distribution(PSD), flowability, apparent and skeletal density, and the angle of repose(AoR). A relatively novel metric of spreadability is a key focus in this study as it directly represents the mechanics of the LPBF recoating process. A spreadability test rig was commissioned in this work and obtained spreadability metrics of percent coverage and spread density.Moisture accumulation on stored metal powders has an adverse effect on flowability and spreadability of metal powders and was shown to significantly improve through air drying.It was found that MA or BE powder blends have spreadability characteristics similar to PATi-6Al-4V,with the standard metrics of particle morphology,PSD, flowability, and AoR having the most significant correlation to spreadability.This indicates that powders that: are more spherical, have a narrow PSD span, are highly flowable, and have a low AoR, have a better spreadability.
Design of tissue leaflets for a percutaneous aortic valve
(Stellenbosch : University of Stellenbosch, 2009-03) Smuts, Adriaan Nicolaas; Scheffer, C.; Blaine, Deborah; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
In this project the shape and attachment method of tissue leaflets for a percutaneous aortic valve is designed and tested as a first prototype. Bovine and kangaroo pericardium was tested and compared with natural human valve tissue by using the Fung elastic constitutive model for skin. Biaxial tests were conducted to determine the material parameters for each material. The constitutive model was implemented using finite element analysis (FEA) by applying a user-specified subroutine. The FEA implementation was validated by simulating the biaxial tests and comparing it with the experimental data. Concepts for different valve geometries were developed by incorporating valve design and performance parameters, along with stent constraints. Attachment techniques and tools were developed for valve manufacturing. FEA was used to evaluate two concepts. The influence of effects such as different leaflet material, material orientation and abnormal valve dilation on the valve function was investigated. The stress distribution across the valve leaflet was examined to determine the appropriate fibre direction for the leaflet. The simulated attachment forces were compared with suture tearing tests performed on the pericardium to evaluate suture density. In vitro tests were conducted to evaluate the valve function. Satisfactory testing results for the prototype valves were found which indicates the possibility for further development and refinement.
Development and characterisation of a carbon fibre reinforced MAX phase composite material.
(Stellenbosch : Stellenbosch University, 2017-12) Nel, Jan Heimriks; Blaine, Deborah; Sigalas, Iakovos; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: A carbon fibre reinforced MAX phase ceramic matrix composite (CMC) was produced in prepreg form and sintered using spark plasma sintering (SPS). MAX phase ceramics is a group of ternary ceramics exhibiting a combination of advantageous properties of both metals and ceramics. The CMC prepreg was designed with a carbon fibre weave acting as reinforcement and a MAX phase (Ti2AlC) acting as the matrix. Polymethyl methacrylate (PMMA) was used to coat the fibres and Ti2AlC powder combination to achieve a flexible and robust prepreg. Ti2AlC powder was prepared by attrition milling in a liquid medium. The particle size distribution was measured using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The mean particle size was reduced from 16 cm to 275 nm, allowing infiltration of the powder into a fibre weave. The effect of the attrition milling on the elemental composition of the powder was evaluated using energy dispersive spectroscopy (EDS) and powder X-ray diffraction (PXRD). Electrophoretic deposition (EPD), vacuum infiltration, and pressure infiltration were evaluated for ceramic infiltration into a carbon fibre weave. The ceramic infiltration was investigated by examining polished cross-sections using optical microscopy. Prepreg layers were combined and thermal debinding was performed at 400 °C to remove the PMMA coating. The CMC was sintered using SPS at 20 MPa and 1400 °C to create a 30 mm CMC disc. The sintered CMC had a density of 2.04 g/cm3 and open porosity of 16 %. The microstructure of the CMC was evaluated using optical microscopy, SEM, and X-ray computed tomography (CT). The discs were fractured using the ball on three balls (B3B) test method to determine the strength and mechanical response. EDS and SEM analysis was employed to evaluate diffusion between carbon fibres and Ti2AlC matrix. Aluminium diffused from the matrix into the fibres, resulting in the formation of Al4C3 in the carbon fibre and TiC in the matrix surrounding the fibre.
Development,testing and fluid interaction simulation of a bioprosthetic valve for transcatheter aortic valve implantation
(Stellenbosch : Stellenbosch University, 2012-12) Kemp, Iain Henry; Scheffer, C.; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Bioprosthetic heart valves (BHVs) for transcatheter aortic valve implantation (TAVI) have been rapidly developing over the last decade since the first valve replacement using the TAVI technique. TAVI is a minimally invasive valve replacement procedure offering lifesaving treatment to patients who are denied open heart surgery. The biomedical engineering research group at Stellenbosch University designed a 19 mm balloon expandable BHV for TAVI in 2007/8 for testing in animal trials. In the current study the valve was enlarged to 23 mm and 26 mm diameters. A finite element analysis was performed to aid in the design of the stents. New stencils were designed and manufactured for the leaflets using Thubrikar‟s equations as a guide. The 23 mm valve was manufactured and successfully implanted into two sheep. Fluid structure interaction (FSI) simulations constitute a large portion of this thesis and are being recognized as an important tool in the design of BHVs. Furthermore, they provide insight into the interaction of the blood with the valve, the leaflet dynamics and valve hemodynamic performance. The complex material properties, pulsating flow, large deformations and coupling of the fluid and the physical structure make this one of the most complicated and difficult research areas within the body. The FSI simulations, of the current valve design, were performed using a commercial programme called MSC.Dytran. A validation study was performed using data collected from a cardiac pulse duplicator. The FSI model was validated using leaflet dynamics visualisation and transvalvular pressure gradient comparison. Further comparison studies were performed to determine the material model to be used and the effect of leaflet free edge length and valve diameter on valve performance. The results from the validation study correlated well, considering the limitations that were experienced. However, further research is required to achieve a thorough validation. The comparative studies indicated that the linear isotropic material model was the most stable material model which could be used to simulate the leaflet behaviour. The free edge length of the leaflet affects the leaflet dynamics but does not greatly hinder its performance. The hemodynamic performance of the valve improves with an increase in diameter and the leaflet dynamics perform well considering the increased surface area and length. Many limitations in the software prevented more accurate material models and flow initiation to be implemented. These limitations significantly restricted the research and confidence in the results. Further investigation regarding the implementation of FSI simulations of a heart valve using the commercial software is recommended.
Hollow carbon nanospheres: a structural integrity investigation.
(Stellenbosch : Stellenbosch University, 2018-03) Du Toit, Jessica; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Hollow carbon nanospheres (HCNSs) have high strength, thermal and electrical conductivities which allow for applications in electrochemical capacitors, lithium ion battery anodes and catalyst supports. When used as a catalyst support, a HCNS encapsulates a nanoparticle, preventing sintering and increasing the catalyst lifespan. The structural integrity of the HCNS is of importance since failure of the HCNS shell will result in the sphere no longer fulfilling its purpose. The aim of this investigation was to evaluate the structural integrity and to link synthesis conditions to the structural integrity of HCNSs. HCNSs were synthesised using two different coating methods: chemical vapour deposition (CVD) and resorcinol formaldehyde (RF) treatment. The synthesis variables significantly affected the spherical shell. At best, the CVD synthesis method produced only partial spheres. Unlike the CVD method, the RF method successfully produced HCNSs with whole, unbroken shells. A bulk powder compaction testing method was developed for the nanospheres where the Heckel yield pressure, a qualitative powder parameter, was extracted from fitting the Heckel equation to the experimental data. The Heckel yield pressures for the silica nanospheres showed a clear decrease with increasing sphere diameter. An inverse relationship between Young’s modulus and nanosphere diameter is reported in literature for both polystyrene nanospheres and amorphous HCNSs. Additionally, a proportional correlation between Young’s modulus and Heckel yield pressure is reported in literature. This relationship extended to include a similar relationship between nanosphere diameter and failure stress. Therefore, the size dependency of Heckel yield pressure for the silica nanospheres studied here is supported. In this investigation, the Heckel yield pressure was used as a qualitative parameter to determine the structural integrity of the nanospheres.
Influence of powder particle size distribution on press-and-sinter titanium and Ti-6Al-4V preforms
(Stellenbosch : Stellenbosch University, 2016-03) Bosman, Hendrik Ludolph; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: This research focusses on the press-and-sinter manufacturing process through which titanium powders are employed to produce dense titanium and the Ti-6Al-4V alloy; more specifically, the influence of particle size distribution (PSD) on the densification behaviour and material properties are investigated. Commercially pure (CP) titanium powders of -100 and -200 mesh sizes were blended in various proportions and used to conduct compressibility and sintering studies. To produce Ti-6Al-4V, a -200 mesh 60Al-40V master alloy (MA) powder was additionally blended with the CP titanium powders. Powders and powder blend were characterised using scanning electron microscopy and laser diffraction. A vast array of specimens was produced while varying the following production parameters: aspect ratio, compaction pressure, sintering time and sintering temperature. Aspect ratios of cylindrical specimens were varied to produce thin disks (1:3), as well as square (1:1) and long (3:2) cylinders. Compaction pressures were varied from 200 MPa to 600 MPa using double action compaction. Sintering was conducted under high vacuum (<10-4 mbar, or better) with sintering temperatures ranging from 1000°C to 1300°C; typical holding times were two hours, with certain specimens being re-sintered to four, and up to six hours. From the results of the compressibility and sintering studies, a baseline densification pathway was elected: compaction at 400 MPa followed by sintering at 1300°C for two hours. This allowed meaningful comparison of the behaviour of different powder blends. Several CP titanium and MA Ti-6Al-4V powder blends of known weight compositions were considered by creating a model using the precursor powder PSD data to predict the blended powder PSDs. A few promising CP and MA blends were prepared and specimens were produced according to the elected baseline process. The densification behaviour was studied at each process step. Densification trends similar to those indicated in literature for bimodal powder blends were found for the CP titanium blends; however, the effect of the MA powder alloying addition was dominant in the case of the MA Ti-6Al-4V blends’ densification behaviour. Mechanical properties were tested using three point bending and Vickers hardness (HV10), respectively. Transverse rupture bar specimens were pressed (400 MPa) and showed either brittle or ductile fracture after being sintered for two hours at either 1000°C or 1300°C, respectively. The thermal conductivity of specific specimens was measured and showed that the thermal conductivity of sintered titanium is lower than that of the equivalent wrought material. The sintered microstructure of various specimens was investigated to gain insight into differences in pore structures among the blend compositions. A vast range of densification results has been put forth from which to extract data for future research. Recommended future work would include: the procurement of tooling for tensile test specimens, a redesign of the thermal conductivity experimental setup, and the addition of fine -325 mesh CP titanium powders to widen the range of PSDs achievable.
Investigation into the production and application of porous titanium within the biomedical field
(Stellenbosch : Stellenbosch University, 2014-12) Van Zyl, Willem Heber; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Department of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: In this study, commercially pure titanium foam was produced using space holder powder metallurgy techniques. Titanium foam is attractive as a scaffolding material for bone replacement and implants in the body. The porous morphology of the foam promotes osteogenesis, while the mechanical behaviour of the foam is closer to that of bone, which has an elastic moduli range of 5 - 40 GPa. Titanium foam was manufactured from powder mixtures of commercially pure titanium (CPTi) powder mixed with 41.4 wt% ammonium bicarbonate (ABC) powder and 1.45 wt% polyethyl glycol (PEG) powder. In this study, two CPTi powders with different particle size distributions, < 75 μm (-200 mesh, designated TiAA) and < 200 μm (-100 mesh, designated TiG), were mixed with the space holder ABC powder, that had been sieved into specified particle size ranges. The size ranges of space holder material studied were: 0 - 710, 250 - 425, 425 - 560, and 560 - 710 μm. This allowed foams with different large or macropore distributions to be produced from the different mixtures. The mixtures were uniaxially compacted at 100 MPa into transverse rupture bars. The ABC and PEG was then removed by thermal debinding in air for 5 hours at 100 °C and 1 hour at 330 °C each, consecutively. The debound samples are then sintered under high (10-6 mbar) vacuum on yttria-stabilised zirconia substrates, heating at 5 °C/min to 1200 °C, with a 2 hour hold at temperature. The microstructures of the different foams were evaluated by examining the polished samples using light optical microscopy. Three point bend tests were conducted on the sintered bars in order to determine the flexural strength and flexural modulus of the different foams. The produced foams had a relative density range between 37.5 - 62.5 % and average macro pore size range between 300 - 500 μm. The foams were found to have an elastic modulus similar to that of bone, 2 - 7 GPa. Finally, the mechanical properties of the foams were compared to known open foam mechanical models and other research projects. It was found that: (i) changes in either metal or space holder powder influences the sintering behaviour of metal foams, (ii) sintered titanium foams with similar densities but different macro/micropore size distributions have different mechanical responses to stress and (iii) the Ashby-Gibson model, based on foam density alone, gives a rough estimate of mechanical properties for the titanium foams studied, but does not capture variations due to pore size distribution.
Machining of powder metal titanium
(Stellenbosch : University of Stellenbosch, 2011-03) Sobiyi, Kehinde Kolawole; Blaine, Deborah; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: The purpose of this study is to investigate the machinability of commercially pure (CP) titanium, manufactured using the press-and-sinter PM process. To this end, CP titanium powder (-200 mesh) was compacted and sintered in vacuum (10-4 torr) for two hours at 1200°C. Small cylindrical samples were compacted at pressures varying from 350 to 600 MPa in order to determine the compressibility of the powder. Following these tests, four larger stepped-cylinder samples were compacted at pressures close to 400 MPa and sintered under similar conditions. These samples had sintered densities varying between 3.82 and 4.41 g/cm3. They were used to evaluate the machinability of the sintered titanium using face turning machining tests. The samples were machined dry, using uncoated carbide (WC-Co) cutting tool. Cutting speeds between 60-150 m/min were evaluated while keeping the feed rate and depth of cut constant at 0.15 mm/rev and 0.5 mm, respectively. The final machined surface finish and the tool wear experienced during the face turning machining tests were monitored in order to evaluate PM titanium’s machining performance. This study showed that it is possible to use the press-and-sinter PM process with CP titanium powder, with a particle size of less than 75 μm (-200 mesh), to manufacture sintered titanium. However, particle shape influences the compressibility of the powder and pressing parts of larger volume, such as the machining test sample shape, is challenging when using such small particle size powder. Processing conditions, such as compaction pressure, sintering temperature and sintering time, influence the sintered density. Results from the machinability tests show that tool wear increases with a decrease in the porosity of the sintered titanium. A more porous sintered material has both lower strength and thermal conductivity. As these factors have opposing effects on the machinability of materials, it is concluded that the strength of the sintered titanium has a stronger influence on its machinability than the thermal conductivity. The cutting tool wear was uniform but showed indications of crater wear. The machined surface of the denser parts had minimal defects compared to less dense parts. Chip shape is long for the dense parts, and spiral for the less dense parts. The chips formed were all segmented, which is typical for titanium. The machinability of the sintered CP titanium was compared to that of wrought titanium alloys. As expected, it was found that the machinability of the sintered titanium was poor in comparison.
Mechanical behaviour of additive manufactured lattice structures
(Stellenbosch : Stellenbosch University, 2020-03) Tshikwand, Georgino Kaleng; Blaine, Deborah; Du Plessis, Anton; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Lattice structures are open-cell, strut-based structures made up of unit cells that are tessellated in 3 orthogonal directions. Depending on the physical design of the unit cell, the lattice structure can be designed for customized stiffness, strength, and specific strain energy absorption. This allows for the design of lightweight, load-bearing structures, suitable for functional engineering applications. 5 X 5 X 5 octet-truss and diamond lattice structures of a specified relative density were designed using equations that relate the relative density to the strut dimensions. Computer-aided design (CAD) models of the structures were created and used to produce these lattice structures of Ti6AL4V using the additive manufacturing (AM) technique: laser powder bed fusion (L-PBF). Finite element analysis (FEA) was used to simulate the uniaxial compression of the lattice structures, yielding predictions for the stress distribution in the lattice struts, and allowing for the prediction of the deformation and failure modes. 3D solid and 1D beam elements were used for this purpose. A prediction of the global mechanical properties and deformation mechanism of the lattice structures was established for both types and compared. A comprehensive flowchart describing the FEA approach taken in order to predict the mechanical behavior of L-PBF lattice structures is provided. Mechanical uniaxial compression of the as-built lattice structures was conducted. Global mechanical properties were determined from the load-deformation data. The progressive collapse of struts under the load was analyzed in order to categorize the failure as stretch- or bending-based. Micro-computed tomographic (μCT) analysis of the as-built structures showed that the struts were thicker as compared to the CAD structures. Thus new CAD models were created with the strut thickness correlating to the actual average dimension. This resulted in improved prediction of the mechanical response of the as-built structures. A comparison of the FEA predictions and the experimentally measured mechanical properties of the lattice structures was carried out. It was determined that both the 3D solid and 1D beam structures predicted the actual mechanical properties of the octet-truss lattice structure within an error margin of less than 25 %. However, for the diamond lattice structure, only the 3D solid structure predicted the actual mechanical properties with an error margin of less than 20 %. A study of the stress distribution across individual struts in the structure was used to explain the deformation mechanisms observed in the respective lattice structures. The octet-truss structure was found to deform by a combination of 45° and 135° shear bands caused by the stretching of the horizontal struts in those planes, whereas the diamond lattice structure was found to deform by 45° shear bands caused by strut bending.
Next generation high temperature gas reactors : a failure methodology for the design of nuclear graphite components
(Stellenbosch : Stellenbosch University, 2015-03) Hindley, Michael Philip; Blaine, Deborah; Groenwold, A. A.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: This thesis presents a failure evaluation methodology for nuclear graphite components used in high-temperature gas reactors. The failure methodology is aimed at predicting the failure of real parts based on the mechanical testing results of material specimens. The method is a statistical failure methodology for calculating the probability of failure of graphite components, and has been developed and implemented numerically in conjunction with a finite element analysis. Therefore, it can be used on any geometry and load configuration that can be modelled using finite element analysis. The methodology is demonstrated by mechanical testing of NBG-18 nuclear grade graphite specimens with varying geometries under various loading conditions. Some tests were developed as an extension of the material characterisation, specifically engineered to assess the effect of stress concentrations on the failure of NBG-18 components. Two relevant statistical distribution functions, a normal distribution and a twoparameter Weibull distribution are fitted to the experimental material strength data for NBG-18 nuclear graphite. Furthermore, the experimental data are normalised for ease of comparison and combined into one representative data set. The combined data set passes a goodness-of-fit test which implies the mechanism of failure is similar between data sets. A three-parameter Weibull fit to the tensile strength data is only used in order to predict the failure of independent problems according to the statistical failure methodology. The analysis of the experimental results and a discussion of the accuracy of the failure prediction methodology are presented. The data is analysed at median failure load prediction as well as at lower probabilities of failure. This methodology is based on the existence of a “link volume”, a volume of material in a weakest link methodology defined in terms of two grouping criteria. The process for approximating the optimal size of a link volume required for the weakest link failure calculation in NBG-18 nuclear graphite is demonstrated. The influence of the two grouping criteria on the failure load prediction is evaluated. A detailed evaluation of the failure prediction for each test case is performed for all proposed link volumes. From the investigation, recommended link volumes for NBG-18 are given for an accurate or conservative failure prediction. Furthermore, failure prediction of a full-sized specimen test is designed to simulate the failure condition which would be encountered if the reactor is evaluated independently. Three specimens are tested and evaluated against the predicted failure. Failure of the full-size component is predicted realistically but conservatively. The predicted failure using link volume values for the test rig design is 20% conservative. The methodology is based on the Weibull weakest link method which is inherently volume dependent. Consequently, the conservatism shows that the methodology has volume dependency as experienced in the classic Weibull theory but to a far lesser extent.
Sinter infiltration of TI-6A1-4V
(Stellenbosch : Stellenbosch University, 2020-03) Govender, Preyin; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Ti-6Al-4V is a widely used titanium alloy within the aerospace and medical industry with powder metallurgy (PM) becoming a fast growing industry within. The press-and-sinter technique is one such PM process that can be used to manufacture parts. In order to improve mechanical properties, dimensional tolerances and machinability, ferrous PM press-and-sintered parts are often infiltrated with molten Cu during the sintering heat treatment process. This project investigates the feasibility of infiltrating sintered Ti-6Al-4V compacts with molten Al, in order to improve its properties. Two Ti-6Al-4V powder blends were mixed, namely a blended elemental (BE) blend consisting of elemental Ti, Al and V powder in a 90:6:4 wt% ratios and a master alloy (CPTi+MA) blend consisting of commercially pure Ti (CPTi) powder with a master alloy (MA) powder of Al and V mixed in a 90:10 wt% ratio. These powder blends were compacted in 10 mm right cylinders with a relative green density (ρg) of approximately 75%. The compacts were sintered under high vacuum at temperatures of 1100 ºC and 1200 ºC, respectively, to achieve relative sintered densities (ρs) of < 92%. At ρs > 92 %, open pore channels close off preventing infiltration. Infiltration disks were compacted from pure Al spherical powder. The mass of the disks was calculated by taking the residual porosity of the sintered Ti-6Al-4V compacts, and providing enough molten Al to infiltrate the pores exactly. Infiltration took place under a nitrogen atmosphere at various temperatures between 700 °C - 900 °C, above Al melting point (660.6 ºC) to ensure melting, and for various dwell times. Slices of wrought Al bar stock were also evaluated for infiltration. Neither the Al powder compacts nor the wrought Al slices melted, with the result that all attempts to infiltrate the Ti-6Al-4V with Al failed. Characterization of the sintered and infiltrated samples was performed. Optical microscopy as well as energy dispersive X-ray spectroscopy (EDS) analysis was used to view the microstructures and elemental distribution in the microstructures, respectively. From these analyses, it was confirmed that infiltration with Al did not occur for any of the samples. CPTi+MA samples showed, on average a change in relative density from 74.5% to 90.2%, while BE samples showed negligible change from 73.6% to 74.9% relative density. The microstructure of both blends were observed with the CPTi+MA having a ɑ+β microstructure; while the BE having a predominantly ɑ-microstructure. EDS imaging for the CPTi+MA samples showed a fairly homogenous elemental distribution for both sintering temperatures. The BE blends showed an inhomogenous distribution at both sintering temperatures. Large pores were visible as Al particles melted and diffused into Ti and V, leaving high Al concentrations surrounding the pores. The results indicate that Ti and V diffuse into the Al disks during the infiltration heat treatment process, raising the melting temperature of the Al disks. By observing the concentration of Ti in Al disks, we see on the Ti-Al phase diagram that an intermetallic may have formed, thus increasing the melting temperature above the temperatures used for infiltration.
Slurry formulation for gel-cast titanium
(Stellenbosch : Stellenbosch University, 2020-04) Piek, Jacques; Blaine, Deborah; Sigalas, Iakovos; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: In this study, commercially pure titanium (CPTi) parts were gel-cast. Gel-casting is a ceramic forming technology developed in the early 1990’s. Titanium is popular in the aerospace and biomedical industries for its excellent corrosion resistance, high mechanical strength, high strength-to-weight ratio and excellent biocompatibility. A novel process for titanium slurry gel-casting was developed, studying the sedimentation behaviour of a methacrylamide (MAM)/methylene bisacrylamide (MBAM) and an Isobam® polymer binder system, respectively. Factors influencing the sedimentation behaviour of titanium particles in a binder are the monomer content, monomer:cross-linker ratio, dispersant content, slurry mixing time and solid loading of the slurry. An optimum slurry was developed with 20 wt% monomers, at a 6:1 MAM:MBAM ratio, with dispersant content of 0.8 wt% ammonium hydroxide (NH4OH). CPTi powder with a particles size distribution of 15-45 μm was used at a solid loading of 55 vol%. Stokes Law was used tosuccessfully suspend the powder particles in the cast slurry to obtain an evenlydense microstructure. The slurry was cast into a resin 3D printed rectangular bar-shape mould, polymerized at 60 ˚C for 2 hours and dried in air at room temperature for 12 hours. Thermal gravimetric analysis (TGA) was conducted on the dried samples to determine the temperatures where the various binder constituents debind. Binder burnout was achieved by heating the dried parts to 400 ˚C at 1 ˚C/min and holding for 30 min, before presintering the parts at 650 ˚C for 30 min to obtain handling strength. The parts were vacuum sintered at 1200 ˚C for 2 hours at a heating rate of 10 ˚C/min. The shrinkage measured from cast to sinter, was 10.4 % and 9.03 % in the length and width of the rectangular bars, respectively. Optical microscopy was used to study the sintered microstructure of the gel-cast parts, finding an evenly dense microstructure. Scanning electron microscopy (SEM) was used to study the fracture surfaces of the tensile test specimens, confirming that only intermediate sintering has taken place. Energy dispersive spectroscopy (EDS) was used to determine the elemental composition of the sintered microstructure, observing that carbon and oxygen contamination has taken place. Finally, the mechanical properties were evaluated: a yield- and ultimate tensile strength of 323 MPa and 378 MPa, respectively, and a hardness value of 60 HRBW, which is 86 % of wrought.
A Study of copper infiltration for conventional ferrous powder metallurgy.
(Stellenbosch : Stellenbosch University, 2018-12) Scholtz, Arno Paul; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
ENGLISH ABSTRACT: Powder metallurgy (PM) describes a group of manufacturing technologies whereby fine metallic powders are consolidated to create engineering components. The most widely used PM technology is the press-and-sinter process, used to manufacture ferrous alloys. Ferrous PM is frequently sinter-infiltrated with copper (Cu) in order to improve the materials machinability. During sinter-infiltration, molten copper infiltrates the inherently porous PM structure, filling the pores and creating a more dense, less porous material. Residual Cu, from incomplete infiltration, is a common defect in copper infiltrated PM parts. This project investigates the parameters of sinter-infiltration that are critical to successful infiltration and the production of high-quality Cu infiltrated ferrous PM alloys. Common reasons for poor infiltration include incomplete delubrication prior to sintering, and inadequate furnace atmosphere and temperature control. Both of these aspects can introduce impurities into the material during the sintering process, which inhibits efficient infiltration and sintering. This study focuses specifically on the influence of delubrication and furnace control on the sinter-infiltrated material quality. A proprietary copper-infiltrated ferrous PM alloy, with specified processing parameters, is used for this study. A systematic evaluation of both the delubrication and copper-infiltration processes is presented. In order to perform this evaluation, it was necessary to implement control of the furnace temperature and atmosphere. As such, an extensive overhaul of the furnace at Stellenbosch University, with particular attention given to both the electrical and process gas flow systems, was performed and the details are presented in this study. The results of the study indicate that in order to successfully perform good quality sinter-infiltration of the proprietary alloy with copper, different process gas atmospheres are required in the furnace for the delubrication and sintering steps, respectively. During delubrication, a reducing atmosphere of 80:20 vol% N2:H2 with a dewpoint of 8 °C must be maintained. During sintering an atmosphere composition of 80:20 vol% N2:H2 must be maintained with a low dewpoint of -35 °C. For the cooling of the samples an atmosphere composition of 95:5 vol% N2:H2 was used. Furthermore, a detailed metallographic analysis of the consecutive stages of copper infiltration, over the temperature range of 1070 to 1130 °C was conducted. The results of which show how the interaction between Cu and Mn in the alloy significantly influences the melting and infiltration behaviour. This research provides valuable insight into the high sensitivity of the copper infiltration process to furnace atmosphere control, as well as the advantage of understanding the detailed interaction between different alloying elements in the PM material during copper infiltration. Insight leads to the ability to control the process and to design for good quality copper infiltration during sintering.

Browse

Browsing Department of Mechanical and Mechatronic Engineering by browse.metadata.advisor "Blaine, Deborah"

Results Per Page

Sort Options