Department of Mechanical and Mechatronic Engineering
Permanent URI for this community
Browse
Browsing Department of Mechanical and Mechatronic Engineering by browse.metadata.advisor "Becker, T. H."
Now showing 1 - 4 of 4
Results Per Page
Sort Options
- ItemChemical etching investigation on surface finish and fatigue behaviour of laser powder bed fusion produced Ti-6Al- 4V.(Stellenbosch : Stellenbosch University, 2021-12) Malcolm, J. S.; Becker, T. H.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: An experimental methodology which implemented the practice of chemical etching was developed with the aim of reducing the surface roughness of laser powder bed fusion (LPBF) produced Ti-6AI-4V and thereby, improving the high cycle fatigue (HCF) performance. Hydrofluoric-nitric acid solutions, HF-HNO3, were used in the chemical etching experiments. The concentration of the HNO3 acid was fixed at 20 % while the concentration of the HF acid was varied to alter the solution concentration. The surface roughness of the LPBF produced Ti-6AI-4V samples were measured using a skidded contact profiler instrument. The subsequent chemical etching investigations revealed that an equivalent reduction in surface roughness can be achieved using either a low 2 % HF acid, or a high 12 % HF acid etching solution concentration. The solution temperature investigation demonstrated that the mass removal rate of a chemical etching process can be increased with raising solution temperature, albeit at the expense of process control. From the insights obtained from the chemical etching investigations, the recommended solution to reduce the surface roughness is a 2 % - 4 % HF acid concentration in conjunction with a solution temperature that is below room temperature. Etching durations of 60 minutes and longer allow for the minimum surface roughness that chemical etching as-built (AB) LPBF samples can be achieved. The internal pores in LPBF produced samples become exposed at the surface during the chemical etching process which limits the average surface roughness, Ra, from reducing to values significantly lower than Ra ≈ 2 μm. The HCF testing conducted was tension-tension cyclic uniaxial testing with a stress ratio, R, of R = 0.1. The LPBF produced Ti-6Al-4V fatigue samples were separated into four groups: the AB samples and then the three remaining sample groups which were chemically etched in a 4%HF20%HNO3 solution for 5, 15 and 90 minutes, respectively. The fatigue samples which received the 90 minute etch had both the lowest average surface roughness, Ra = 1.84 ± 0.03 μm, and the largest improvement in fatigue performance with reference to the AB sample’s fatigue behaviour. All the fatigue failed samples had crack initiation sites at the surface of the sample. The fabrication process-induced surface roughness acted as crack initiation sites for the AB and 5 minute etch sample groups. Chemical etching caused the controlling defect responsible for fatigue crack initiation to change from surface roughness to internal defects which were brought to the surface by the chemical etching mechanism. The opened subsurface defects acted as stress raisers which caused the fatigue crack initiations for the 15 and 90 minute etch sample groups.
- ItemFatigue crack growth rate threshold of laser powder bed Fusion Ti-6AI-4V(Stellenbosch University, 2021-12) Dhansay, N. M.; Becker, T. H.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Typically, producing Ti-6Al-4V through the laser powder bed fusion (LPBF) technique, results in the material having large residual stresses and martensitic microstructure. These stresses and microstructure have been shown to result in Ti-6Al-4V having poor fatigue properties. However, the insight into the fatigue failure mechanisms caused by the residual stress and microstructure has been limited. LPBF is one of many additive manufacturing (AM) techniques in which parts are built in a layer- wise manner with the use of powdered material and consolidated through high power laser melting. This allows for complex geometries and previously impossible geometries to be manufactured with minimal material wastage. Many industries are aware of the potential in this manufacturing technique and have shown interest in it becoming a viable option for the manufacturing of some of their components. In particular, the use of LPBF produced Ti-6Al-4V is of interest to the aerospace and biomedical industries. This is because Ti-6Al-4V is already well established in many existing industries. For LPBF produced Ti-6Al-4V to be a viable option in industry, researchers need to have an adequate understanding of the material and the implications for its mechanical properties. Fatigue property investigations have largely focused on the fatigue life approach (crack initiation) and Paris regime (region II, crack propagation). However, in recent years, the near-threshold regime (region I, crack propagation) has become of interest, albeit limited in approach. The consensus within literature shows that the large tensile residual stresses, martensitic microstructure and porosity results in poor fatigue properties. Unfortunately, the insight into the fatigue fracture mechanisms brought about by the residual stress and microstructure is not yet well established. Furthermore, the near-threshold fatigue crack growth rate regime (FCGR) experiences crack closure mechanisms which result in premature near-threshold values. The majority of the near-threshold investigations on LPBF produced Ti-6Al-4V do not account for the crack closure mechanisms and therefore produce premature results. As a result of the low crack growth rates achieved in the near-threshold regime, a window into observing the fatigue crack initiation mechanisms is obtained. More specifically, how the fatigue crack initiation mechanisms are influenced by residual stress, martensitic microstructure and changing microstructural morphology. Literature has shown that porosity acts as crack initiation sites and reduce the fatigue life of a component. Furthermore, surface and near-surface porosity have been shown to have a more severe impact on fatigue life than internal porosity. It is through using the near-threshold FCGR approach in which one can calculate the allowable pore size under operational loads. Using the load-shedding technique to obtain near-threshold FCGRs, the results showed anisotropic behaviour dependent on residual stress levels and R-ratios. The fatigue fracture mechanisms were predominantly governed by transgranular quasi-cleavage mechanisms. Furthermore, the fracture is directed by PBG morphology which results in anisotropic crack- closure effects. In addition, primary α lath orientation is governed by the crystallographic texture of the PBG which influences the mechanisms of crack propagation. With the increase in grain size, the presence of β improves the near-threshold FCGRs in the duplex anneal (DA) condition due to the superior plastic flow abilities more than in the as-fabricated and stress relief conditions. This study investigates the near-threshold FCGRs of LPBG produced Ti-6Al-4V in the as- fabricated, stress relief and bi-modal conditions in three build orientations. In addition, crack closure mechanisms are accounted for by implementing variable R-ratio testing. This research presents the influencing mechanisms of residual stress and microstructure on fatigue behaviour.
- ItemFeasibility of additive manufacturing for patient-specific knee replacements.(2021-12) Nortje, B. D.; Van der Merwe, J. H.; Becker, T. H.ENGLISH ABSTRACT: Osteoarthritis causes the degradation of the articular cartilage of the knee. This results in a loss of function of the joint. With a total knee replacement surgery, these articular surfaces are removed and replaced with artificial components. The ability to integrate patient-specific implant components can improve the results of a total knee replacement. The focus of this research is to investigate the feasibility of manufacturing methods required for a femoral knee replacement component for patient-specific implants. Through understanding what has been accomplished in literature, a multi-directional pin-on-flat wear tester is developed. This machine is based on the motion that is observed by the knee articulation in specific. A cross-shear ratio is designed to achieve similar wear characteristics to that seen in a knee replacement. This development is validated through the use of Co-Cr-Mo samples. The developed and implemented machine is utilised to perform an assessment of laser powder bed fusion, a powder bed based additive manufacturing technique, for use in patient-specific femoral knee replacements. Focus is placed on the use of Ti-6Al-4V and Co-Cr-Mo; where the latter is used for a benchmark to indicate the relative wear properties of the former. The average wear rates that resulted after 3 x 106 cycles were 2.58 mg/MC and 2.63 mg/MC for Co-Cr-Mo and Ti-6Al-4V, respectively. Additive manufacturing provides the ability to manufacture a near net shape component that is suited for the patient’s geometry. Orthopaedic surgeons consider the full and natural functioning of the replaced joint to be an indication of the operation’s success. The unsatisfactory performance of an implant can be attributed to the incorrect tension of the ligaments surround- ing the joint; this is often found to be a result of geometric intolerances of the standard implant sizes in relation to the patient’s natural joint. This can be rectified through the use of additive manufacturing which provides the ability to manufacture components of high geometric tolerances. A feasibility analysis is done with regards to cost and suitability of man- ufacturing processes for patient-specific femoral knee replacements. Additive manufacturing is compared to CNC machining to assess the feasibility of pur- suing a patient-specific femoral knee replacement that is manufactured from Ti-6Al-4V. The device that was developed illustrated satisfactory performance in ac- cordance to what is expected from literature. Through these tests, Ti-6Al-4V displayed suitable wear properties for use in articulating joints. The study places focus on the knee joint, but can be adapted for many other articulating joints with lower loading. It can then be shown that additive manufacturing may not be a feasible option for manufacturing based on cost, but may prove beneficial for reasons such as geometry complexities, surface finish, accuracy and extent to which the surfaces can be altered.
- ItemLaser powder bed fusion produced Ti-6AI-4V: microstructural transformations and changes in deformation behaviour through thermal treatments.(Stellenbosch : Stellenbosch University, 2021-12) Ter Haar, G.; Becker, T. H.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: The current 4th Industrial Revolution has brought about enormous potential for social-economic development in South Africa. Additive manufacturing, a key technology of the 4th Industrial Revolution, and titanium, an abundantly mined but under-developed mineral commodity, collectively inspired industrial embracing of titanium-powder additive manufacturing, and specifically, laser powder bed fusion produced Ti-6Al-4V. Manufactured part quality concerns and the limited understanding of the innovative process-structure-property interactions impede its industry acceptance. Through developing fundamental insights into interactions between the microstructure and thermal treatments, mechanical performance can be improved, tailored, and optimised. Since conventional process routes cannot be applied to additively manufactured components, novel thermal treatments need to be developed. This dissertation documents experimental research into thermal treatments of laser powder bed fusion produced Ti-6Al-4V. By developing novel insights into the material’s unique metallurgical response to thermal treatments, tensile behaviour of the material is improved. The research investigates three temperature regions for thermal treatments post-fabrication and one in-situ approach. Thermal treatments between 750 – 960 °C are used to develop insight into the microstructure’s large-scale morphological transformation. Thermal treatments at 960 °C achieve fragmentation and grain globularisation. This is followed by quenching to attain a superior bi-modal microstructure. Thermal treatments at temperatures below 650 °C are used to develop insight into the initial stages of martensite decomposition phase transformation and material stress relief. Microhardness and tensile properties revealed material embrittlement. Fine precipitates in the microstructure were identified using high-resolution transmission electron microscopy. Based on these findings, two theories for the cause of material embrittlement based on two possible transformation routes of the initial stages of martensite decomposition are proposed. Mechanical properties of laser powder bed fusion produced Ti-6Al-4V depend on build orientation. Although the unique columnar-shaped and textured prior-β microstructure is identified as a probable cause of deformation anisotropy, limited insight into the cause of anisotropic deformation exists. Thermal treatments above 975 °C are used to globularise the columnar prior-β grain morphology. Microstructural anisotropy between two orientations is quantified using electron backscatter diffraction maps, and the influence of microstructure on deformation and crack initiation is studied. Insight into the deformation behaviour and the identified relation between prior-β crystallographic texture and α-lath morphological texture is used to formulate a theory of the probable cause of material deformation anisotropy. The study lastly investigates a novel approach to thermal treatments by using high-energy process parameters to achieve in-situ heating. An iterative approach for part optimisation using non-default process parameters is undertaken. While findings indicate that an improved microstructural and residual stress state can be achieved, detrimental effects of part oxidation and part-edge bulging are also observed.