Laser Melting (LM) is an Additive Layer Manufacturing (ALM) process used toudproduce three-dimensional parts from metal powders by fusing the material in a layerby-udlayer manner controlled by a CAD model. During LM, rapid temperature cycles andudsteep temperature gradients occur in the scanned layers. Temperature gradients induceudthermal stresses which remain in the part upon completion of the process (i.e. residualudstresses). These residual stresses can be detrimental to the functionality and structuraludintegrity of the built parts.udThe work presented in this thesis developed a finite element model for the purpose ofudinvestigating the development of the thermal and residual stresses in the laser melting ofudmetal powders. ANSYS Mechanical software was utilised in performing coupledudthermal-structural field analyses. The temperature history was predicted by modellingudthe interaction of the moving laser heat source with the metal powders and baseudplatform. An innovative ‘element birth and death’ technique was employed to simulateudthe addition of layers with time. Temperature dependent material properties and strainudhardening effects were also considered. The temperature field results were then used forudthe structural field analysis to predict the residual stresses and displacements.udExperiments involving laser melting Ti-6Al-4V powder on a steel platform wereudperformed. Surface topography analyses using a laser scanning confocal microscopeudwere carried out to validate the numerically predicted displacements against surfaceudmeasurements. The results showed that the material strain hardening model had a directudeffect on the accuracy of the predicted displacement results.udUsing the numerical model, parametric studies were carried out to investigate the effectsudof a number of process variables on the magnitude of the residual stresses in the builtudlayers. The studies showed that: (i) the average residual stresses increased with theudnumber of melted powder layers, (ii) increasing the chamber temperature to 300°Cudhalved the longitudinal stresses. At 300°C, compressive stresses appeared on the Ti64udsurface layer, (iii) reducing the raster length from 1 mm to 0.5 mm reduced the averageudlongitudinal stress in the top layer by 51 MPa (0.04σy), (iv) reducing the laser scanudspeed from 1200 mm/s to 800 mm/s increased the longitudinal stress by 57 MPaud(0.05σy) but reduced the transverse stress by 46 MPa (0.04σy).
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