Theses and Dissertations (Metallurgical Engineering)

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Browsing Theses and Dissertations (Metallurgical Engineering) by Author "Mendonidis, P., Prof."

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    Characterisation of Ti6Al4V lattice structures produced by direct metal laser sintering for possible biomedical applications
    (Vaal University of Technology, 2022) Raselabe, Kabelo Matome; Baloyi, N.; Mendonidis, P., Prof.
    The use of Ti6Al4V lattice structures as a new detergent to the stress shielding phenomenon caused during implantation has been growing. The stress shielding phenomenon is caused by the inhomogeneous transfer of load between the implant and the surrounding bone. This is mainly due to the implants containing modulus (110 GPa) being higher than that of the surrounding bone (2-20 GPa). Due to high cost and tedious process involved in manufacturing lattice structures using conventional technique, as a result of the complexity in lattice structure’s geometry, a process known as additive Manufacturing is highly recommended. Additive manufacturing processes like the use of Direct metal laser sintering (DMLS) process to create three-dimensional (3D) parts, layer by layer using computer-aided design (CAD) allows to freely build parts with geometric complexities In this dissertation, the DMLS process was used to manufacture Ti6Al4V lattice structures with Young’s modulus like that of cortical bone or within the range of 2-20 GPa. These structures were manufactured at different build orientations (Horizontal and Vertical) and strut sizes (1 and 1.5 mm). The Ti6Al4V lattice structures post-manufacturing were subjected to stress-relieving (SR) post-treatment of 650 ºC under argon atmosphere for 2 hours and 950 ºC under vacuum (VHT) atmosphere for 2 hours. The purpose of stress-relieving is to remove the residual stresses caused by the thermal gradients during manufacturing. While the vacuum heat treatment is for promoting ductility. Under these conditions (stress relieving and vacuum heat-treatment) the lattice structures were then analysed for microstructure, hardness, compression, and corrosion. For stress relieved lattice structures, DMLS's Ti6Al4V SR lattice structures possessed α' martensite and prior β grains along the build orientation. When comparing the mechanical properties of vertical and horizontal lattices, it was discovered that vertical lattice has higher hardness, compressive strength (324.1 MPa), and ductility (12%), as well as a lower Young's modulus (1.5 GPa). Vertically built lattices corroded at a faster pace than horizontally built. To change the microstructure and characteristics of Ti6Al4V lattices, vacuum heat treatment at 950 °C was performed on the SR lattices. The microstructure study demonstrated a change of α' to α+β lamellar type structure. Furthermore, the VHT enhanced ductility (18-29%) and demonstrated a Young modulus value of 2.8-3.3 GPa which is within the range for the biomedical area. The corrosion rate of horizontally built lattices was lower than that of vertically built lattices. It was then concluded that, the DMLS Ti6Al4V lattice structures yielded a Young's Modulus within the range necessary for cortical bone replacement. Which in turn mean that the Ti6Al4V lattice is more likely to be accepted in the biomedical area than fully solid/dense DMLS made Ti6Al4V, which ranges from 110 – 120 GPa. Other properties of Ti6Al4V lattice structures demonstrate that VHT lattices have higher hardness than SR lattices and that VHT lattices corrode less than SR lattices. This ultimately led to both 1 and 1,5 mm strut sizes, the Horizontally formed lattice was more corrosion resistant than the Vertically created lattice.
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    Determination and improvement of wear and abrasion properties of Glass Reinforced Plastic (GRP) piping for slurry applications
    (Vaal University of Technology, 2014-09) Van der Schyff, A.; Mendonidis, P., Prof.
    Glass Reinforced Plastics (GRP) are composite materials comprising a resin matrix reinforced with glass fibers, and are well known for their excellent corrosion resistance and high strength-to-weight ratios. However, their use in slurry transporting pipes is uncertain because the wear resistance of GRP pipe materials is not well documented due to the difficulty encountered in standardizing mechanical properties of GRP pipes. The aim of this project was to identify the main variables that influence the wear and abrasion properties of GRP pipes and the differences of abrasion properties between the continuous and discontinuous filament wound pipes that are manufactured by Amitech SA. In order to achieve the aim of the project, the mechanisms of wear in ductile and brittle matrix GRP composites were investigated. The Pin-on-disk and Jet-impact tests were identified as the best abrasion tests suited for testing GRP composites. Data collected from these tests indicated that the resin type, curing, additive content and- type, fiber content and manufacturing methods are factors affecting wear resistance. It was further found that hard surfaces provided better resistance to abrasive wear, whereas ductile matrices performed better under erosive wear.