Department of Mechanical and Mechatronic Engineering

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

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

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    Critical powder characteristics for high quality laser powder bed fusion
    (Stellenbosch : Stellenbosch University, 2024-02) De Waal, A; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The acceptance of Additive Manufacturing (AM) as a reliable manufacturing technology in South Africa and globally, is still limited. This is due to concerns about replicability and variations in quality. Efforts to enhance AM standards aim to address these challenges by focusing on understanding raw metal materials. Characterizing and comprehending powder materials pose significant challenges for the AM community, especially in technologies utilizing a powder bed. Powder behaviour during spreading is a relatively new research area that is not fully understood. Academia and industry practitioners are actively exploring these concepts to enhance AM's quality, reliability, and replicability. This research aims to identify and validate critical metal powder characteristics that include the powder morphology, particle size distribution, moisture content, flowability and various densities of the powder, and their relation to efficient powder spreadability characteristics that effectively describe the powder layer quality. Furthermore, the relationship of these characteristics to consistently building high-quality final parts using the AM technique known as laser-powder bed fusion (L-PBF) must be established. The results obtained in this project aid the Collaborative Programme in Additive Manufacturing (CPAM) strategy that is focused on identifying L-PBF as a reliable and accepted manufacturing technique in South Africa as well as globally. Standardised and customised powder characterisation testing was conducted on different powders that included two spherical powders that are designed for LPBF, namely, commercially pure titanium (CP Ti) and Ti-6Al-4V, as well as a CP Ti powder that is produced by the hydride-dehydride process (HDH), that displays an angular particle shape and is not typically used for L-PBF. The results from the powder characterisation experiments successfully identified critical powder characteristics that influence powder spreadability and, consequently, the as-built part quality. Thus, the impact of powder characteristics on the spreadability of the powder and the resultant direct effect on the powder layer quality is demonstrated, emphasizing the importance of aligning spreading parameters with the critical powder characteristics that include powder flowability, packing ability, and settling rate for consistent, high-quality powder layers. A criteria system that validates whether a powder is suitable for L-PBF, based on the powder characteristics, was identified, and used to determine whether a mixture of the spherical and HDH CP Ti powders is useable for L-PBF as a more cost-effective solution. It was determined that a 90:10 mass ratio mixture of the spherical to HDH CP Ti powders delivered suitable spreadability results for L-PBF.
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    Evaluation of the methacrylamide/ N’N,-methylenebisacrylamide binder system for gel-casting of titanium powder.
    (Stellenbosch : Stellenbosch University, 2023-03) Erasmus, W; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The discrete element method (DEM) has demonstrated potential as a design tool for assessing cohesionless (dry) materials when accurate input parameter values are specified. However, the calibration processes establishing precise contact model parameters and their values for bulk materials, with cohesion present, remain somewhat novel. Accordingly, the behaviour of Cohesive materials was examined at the bulk level. To this end, moisture was employed to induce material cohesion in three different sand grades. Concurrently, the DEM approaches for simulating the latter were investigated, with two strategies emerging for calibrating the cohesive parameters depending on the level of cohesion in the material. To this end, the prevailing non-cohesive contact models were acquainted with their bulk calibration procedures. Moreover, the most favourable cohesive contact models were analysed, and their potential to replicate the bulk cohesive response of the moistened sand was examined. These included the Full Johnson, Kendall & Roberts (JKR), Simplified Johnson, Kendall & Roberts (SJKR), liquid-bridge and generic linear cohesive models. Subsequently, the linear cohesive model’s non-cohesive contact parameters were calibrated for six numerical particle upscaling factors, utilising the established calibration techniques. The particle-wall friction coefficient, damping coefficients and, significantly, the particle-particle friction coefficient remained constant as the particles were upscaled. However, the calibrated particle densities and contact stiffnesses increased with particle upscaling. Additionally, the linear cohesive model’s cohesive contact parameters were calibrated for the sands at four degrees of pore saturation, namely 0 % (dry), 5 %, 10 % and 15 %. The latter established the maximum tensile/rupture force (F ) and rupture distance (D ) at each upscaling factor. Moreover, the calibrated non-cohesive parameters could be kept constant for the test material in dry and wet conditions. The upscaling factors translated to ratios of 1.0 to 4.0 for the largest (. – . mm, "rough") sand grade, 2.5 to 9.9 for the middle (. – . mm, "medium") sand grade and 16.5 to 60.0 for the smallest ( : . – . mm, "fine") sand grade. Unique F and D combinations for the "rough" and "medium" sands were obtained by combining (superimposing) the numerical replications of a vertical displacement angle of repose test, a draw-down test’s shear angle and the centroid elevation of the rotating drum’s cohesive particle bed, resulting in the first calibration strategy for establishing the cohesive contact parameters for mildly cohesive materials. In addition, a centrifuge was also utilised to analyse the materials’ slope angle at elevated lateral g-forces. A combination of the latter with the vertical displacement angle of repose was used to obtain unique F and D parameter values for the "fine" sand (through the superimposition of the F and D response surfaces) and resulted in the second calibration strategy for highly cohesive materials. Consequently, the F was found to scale cubically with numerical particle size, whilst the D was scale invariant. A minor increase of D with material pore saturation was also found. However, the bulk cohesive response was insensitive to the magnitude of D , but its facilitation of an attractive tensile branch greatly enhanced the bulk cohesive response of the wet material. The validity of the calibrated parameter sets was evaluated by comparing the bulk cohesive flow of the sands across a conveyor transfer point. To this end, an impact plate was placed in the path of the feeding stream, flowing off the conveyor. Accordingly, the maximum (peak) forces exerted on the material boundary during bulk flow and the residual forces (material weight) after bulk flow were analysed. Additionally, qualitative comparisons of the impact plate’s physical and predicted cohesive pile formations were made. Only the three smallest scale factors provided enough resolution to accurately replicate the largest sand grade’s peak and residual forces. Furthermore, for the "medium" sand, the three smallest scale factors accurately replicated the peak and residual forces for the wet cases. For the "medium" sand’s non-cohesive dry case, the three smallest scale factors were accurate for the peak forces, whereas only the smallest scale factor was accurate for the residual forces. The "fine" sand grade’s forces were accurate for the five smallest upscaling factors when moisture-induced cohesion was present, whilst only the peak forces were accurately replicated with the three smallest scale factors for the non-cohesive case. Consequently, the degree to which particles may be upscaled increased as cohesion increased the characteristic lengths of the bulk material region being replicated. The latter translated to a minimum numerical resolution of four particle diameters for simulating the peak forces of particle-boundary interactions, whilst a resolution of eight particle diameters was required to replicate material build-up. Also, the qualitative replication of the material’s pile formation was sensitive to accurately representing the physical material’s particle size distribution, whilst quantitative measurements appeared insensitive to the latter. Moreover, employing the JKR, SJKR, or liquid-bridge contact models did not improve the modelling of the bulk cohesive behaviour.
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    Fracture mechanics-based fatigue life assessment of additively manufactured Ti-6Al-4V
    (Stellenbosch : Stellenbosch University, 2024-02) Macallister, N; Becker, Thorsten Hermann ; Blaine, DC; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: This dissertation presents a study on fracture mechanics-based fatigue life assessment for Additively Manufacturing (AM). The mature laser powder bed fusion (LPBF) process with the Ti-6Al-4V alloy in particular is selected for study, as it is well suited to the South African context with regard to economic climate, strong AM relationships and abundant mineral titanium reserves available. Furthermore, the Ti-6Al-4V alloy is a staple of aerospace, automotive and biomedical industries which are amongst the largest promoters for using AM technology, and for whom fatigue characterisation remains a prevalent topic as many end-use applications are intended for cyclic loading. Though significant research in fatigue behaviour exists, the conundrum of reliably certifying fatigue life in AM parts persists. This problem stems from the complex relation between AM print parameters, build orientation, surface roughness, inherent defects, residual stresses, meso- and microstructure; and establishing reportable fatigue strength baseline values required by industry. Moreover, as the AM environment promises saving in cost and time, full fatigue testing schemas are undesirable. As such, alternate damage-tolerant methods are becoming increasingly popular, where adopting fracture mechanics-based frameworks accompanied by limited or non-destructive testing could aid in certification. For this purpose, the dissertation first presents a novel version of the fatigue predictive NASGRO model where parameters are established that are unique to LPBF produced Ti-6Al-4V meso- and microstructures. In establishing these parameters for LPBF produced Ti-6Al-4V, the influence of process inherent microstructure, residual stress, and orientational dependant meso- structure is considered through examining near-threshold in combination with steady-state fatigue crack growth rates. The analysis shows that the descriptors of material constraint are sensitive to build orientation and microstructure. Furthermore, the effect of residual stresses is observed to not be severe. In this a clear effect of build orientation and meso- and microstructure is established for selecting NASGRO model parameters. Secondly, the proposed NASGRO formulation is translated into a comprehensive novel damage-tolerant fracture mechanics-based model to estimate fatigue life. Non-uniform defect populations, typical of AM material, in terms of size, shape and location are captured through X-ray tomography and surface profilometry and used as inputs modelled as equivalent crack lengths. The fatigue strength estimations are shown to be sensitive to fatigue crack growth rate threshold parameters and short crack growth mechanic descriptions. Furthermore, by introducing multiple crack initiations, the fatigue estimates are shown as distributions and are sensitive to defect number. Finally, sub-size specimen testing is investigated as a potentially elegant solution to accompany fatigue life assessments for threshold validation. Where results show inconsistent near-threshold fatigue behaviour linked to the microstructure. In this, considering unique meso- and microstructural features of LPBF produced Ti-6Al-4V, the domain and suitability in using sub-size specimens for fatigue crack growth rate threshold testing is discussed. Overall, this dissertation walks the path required in establishing reliable damage tolerant fatigue life estimation approaches for LPBF produced Ti-6Al-4V. Providing fundamental insights into interactions of fracture mechanic mechanisms and descriptions necessary for reliably modelling fatigue behaviour, therefore contributing to the developing frameworks and philosophies in AM to help in certification of fatigue performance of LPBF produced Ti-6Al-4V components.