Browsing by Author "Anderson, Lucas Steven"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Evaluating measurement techniques: establishing a testing framework for residual stress in selective laser melted Ti-6Al-4V
    (Stellenbosch : Stellenbosch University, 2017-12) Anderson, Lucas Steven; Becker, Thorsten Hermann ; Westraadt, Johan; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Residual stress (RS) plays an important role in the mechanical performance of components. Due to the manufacturing process involved in Selective Laser Melting (SLM), high RS is generated within the produced components. These stresses can increase component failure rates either during the manufacturing phase or in service. An investigation was performed into the capabilities of various stress measurement techniques for the application of measuring the RS distribution in SLM produced Ti-6Al-4V (Ti64). This investigation will be used as the basis for creating a testing framework for further studies involving the RS distribution in SLM produced Ti64. The stress measurement techniques were identified and reviewed with respect to the following: stress scale measurable, the stress tensor produced, measurement type, measurement penetration into SLM produced Ti64 and the achievable stress resolution in Ti64. Three techniques were selected for further evaluation, namely: neutron diffraction (ND), X-ray diffraction (XRD) and stress relaxation coupled with Digital Image Correlation (DIC). SLM produced Ti64 specimens built with nine combinations of build layer thickness and exposure strategy were used as test specimens. ND was used to resolve the macro-stress distribution along a plane running through the depth of the tested specimens and XRD was used to measure both near surface stress and, combined with electro-polishing, the stress distribution through individual build layers. The development of a technique – using focused ion beam (FIB) micro-milling and DIC displacement mapping – for the measurement of the residual stress at the layer scale, was also initiated. ND was capable of performing volumetric stress distribution measurements through the full depth of the specimens. Long testing durations and limited accessibility limits its application to RS measurements in SLM produced Ti64. A reduced analysis domain should be used in future testing to allow for more stress orientations to be scanned. The XRD technique, coupled with electro-polishing, was capable of resolving the in-plane stress distribution through individual build layers. The use of the sin2ψ method simplifies the calculation of the stress components. Care should be taken when interpreting the results obtained at the surface as high surface roughness can lead to erroneous stress results. Due to equipment failure the FIB-DIC technique could not be investigated fully. A validation test showed that the technique was able to resolve the in-plane strain components resulting from stress relaxation to a depth of ~20 μm. Further work on this method will include testing on SLM produced Ti64 specimens. The influence of build layer thickness and exposure strategy on RS was also investigated. An increase in the build layer thickness resulted in a decrease in the stress component magnitude and gradient at both the component scale and at the layer scale. The exposure strategy influences the homogeneity of the stress distribution. A uni-directional exposure strategy produces an approximately uni-axial stress distribution at the component scale, whereas the use of two or more laser vector directions results in an approximately bi-axial stress distribution at the component scale. The stress distribution at the layer scale remains uni-axial regardless of the exposure strategy used.