This paper presents a study showing the efficacy of the inherent strain method and modifications thereof to predict residual stresses within selective laser melted components. Cubic specimens with variations in hatch rotations were produced, and the residual stress state was simulated and measured by employing the neutron diffraction technique. Variances in the simulated and observed values of stress in the samples were investigated to show the efficacy of the isotropic, orthotropic, and thermo-mechanical simulation methods. The simulations indicated a suitable prediction of the residual stress with the incorporation of hatch rotation, but as being less adept at resolving the residual stress of the components with no hatch rotation. The most accurate simulation results in the horizontal stress directions were seen for the 90° hatch rotation specimen, for which the average difference between the measured and simulated values were below 65 MPa for all simulation types. The simulations largely over-predicted Z direction compressive stresses; however, the thermo-mechanical simulation type predicted this stress with an average difference of 116, 98, and 72 MPA for the 0°, 67°, and 90° hatch rotations respectively.
This paper presents an evaluation of the tensile properties and microstructural characterization of Ti6Al4V alloy manufactured with three different processing routes; traditional wrought processing, investment cast and Laser Engineered Net Shaping (LENS). Tensile specimens were machined from each process and tensile tested at room temperature. Fractured specimens were characterized using light optical microscopy, stereo microscopy, microhardness and Scanning Electron Microscopy (SEM) to investigate the microstructural morphology and the structural hardness variation. The investment cast Ti6Al4V alloy microstructure revealed large equiaxed grains containing various orientated lamellar colonies. The additive manufactured microstructure revealed long columnar grains with Widmanstatten α’ martensite laths and retained β grain boundaries. While the wrought Ti6Al4V microstructure was observed as smaller equiaxed grains with large colonies of fine lamellar and transformed β. Additive manufactured specimens had higher yield strength, ultimate tensile strength and hardness compared to the investment cast and wrought manufactured specimens.
Thermal inconsistencies inherent in the selective laser melting manufacturing process generate residual stress that may exceed the yield strength of the material resulting in deformation and cracking of components. This may however be mitigated through a stress-relief annealing process. The purpose of this study is to investigate the effect of stress-relief anneal time on the residual stress in selective laser melted Co-Cr-Mo components by means of simulation and neutron diffraction techniques. Thin-walled samples were manufactured and subjected to heat treatment at 1065°C for different holding times (0.25, 1, 2 and 3 hours) as well as a sample heat treated at 750°C for 1 hour. Evaluation of the residual stress reveals that the heat-treated samples have significantly lower residual stress as compared to an as-built (non-heat treated) control sample. Results indicate that the range of annealing times investigated produce near-identical residual stress values. Higher annealing temperatures however result in lower residual stresses. Comparison of simulated and neutron diffraction results indicate anisotropic behaviour of the material which can be attributed to the layer-wise addition of material.
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