Characterisation of Titanium Alloy Processed By Constrained Bending and Straightening Severe Plastic Deformation
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Date
2020
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The University Of Johannesburg
Abstract
Most Severe Plastic Deformation (SPD) processes lack both the capability for continuous process and the homogeneity of tailored material properties in processed samples. These challenges have limited the adaptation of SPD technology to process titanium alloys for biomedical and structural applications. This thesis presents microstructural and mechanical characterization of Ti6Al4V titanium alloy processed by a Constrained Bending and Straightening (CBS) SPD technique. The proposed CBS method was intended for a continuous process of titanium sheets with improved magnitude and homogeneity of the entailed strain, hardness and tensile properties. The CBS process tool was designed and fabricated in the University of Johannesburg workshop. The tool was used to process Titanium Alloy Grade 5 (Ti6Al4V) sheets at a combination of (2, 4, 6) passes (N) and (6 mm, 12 mm) feeds (F) designated as N2F6, N4F6, N6F6, N2F12, N4F12 and N6F12. Sub-samples were cut from the processed sheets. The samples were prepared, and their respective microstructural analysis, hardness and tensile tests were performed. A numerical model for the CBS process was built and simulated with ABAQUS Standard Finite Element Analysis (FEA) method. The model was used to predict the magnitude and the homogenity of the Effective Plastic (EP) strain, the tensile yield strength and the hardness of the material. The simulation results were validated with the experimental data. The experimental results showed that vi comparing with the As Received (AR) samples, the processed samples showed a decrease in the average grain size from 10µm to 3µm, together with the formation of new finer subgrains. The tensile strength, the yield strength and the hardness of the material increased by 29.3%, 33.5% and 24.4%, respectively. The values of these material properties at the F6 feed were higher than those at the F12 feed. The simulated results showed that the maximum mean EP strain of 2.87 induced in the material correlated with the highest strain homogeneity that corresponded to the lowest Coefficient of strain Variation (CV) of 18.9%. A comparison of the results from both methods on the yield strength and the hardness showed a direct correlation at the N2 and the N4 passes. A relative inverse correlation was observed at the N6 pass due to the saturation of the material hardening and the onset of yielding. Results from this study have quantitatively shown that the CBS method performed well the intended work. However, this method needs desirable improvement at a commercial level before it is viable as an alternative method for the continuous production of the titanium alloy sheets enhanced with the homogeneous microstructural and mechanical properties
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This thesis was published in the year 2020