Investigation of residual stresses in the laser melting of metal powders in additive layer manufacturing

Abstract

Laser Melting (LM) is an Additive Layer Manufacturing (ALM) process used to produce three-dimensional parts from metal powders by fusing the material in a layerby- layer manner controlled by a CAD model. During LM, rapid temperature cycles and steep temperature gradients occur in the scanned layers. Temperature gradients induce thermal stresses which remain in the part upon completion of the process (i.e. residual stresses). These residual stresses can be detrimental to the functionality and structural integrity of the built parts. The work presented in this thesis developed a finite element model for the purpose of investigating the development of the thermal and residual stresses in the laser melting of metal powders. ANSYS Mechanical software was utilised in performing coupled thermal-structural field analyses. The temperature history was predicted by modelling the interaction of the moving laser heat source with the metal powders and base platform. An innovative ‘element birth and death’ technique was employed to simulate the addition of layers with time. Temperature dependent material properties and strain hardening effects were also considered. The temperature field results were then used for the structural field analysis to predict the residual stresses and displacements. Experiments involving laser melting Ti-6Al-4V powder on a steel platform were performed. Surface topography analyses using a laser scanning confocal microscope were carried out to validate the numerically predicted displacements against surface measurements. The results showed that the material strain hardening model had a direct effect on the accuracy of the predicted displacement results. Using the numerical model, parametric studies were carried out to investigate the effects of a number of process variables on the magnitude of the residual stresses in the built layers. The studies showed that: (i) the average residual stresses increased with the number of melted powder layers, (ii) increasing the chamber temperature to 300°C halved the longitudinal stresses. At 300°C, compressive stresses appeared on the Ti64 surface layer, (iii) reducing the raster length from 1 mm to 0.5 mm reduced the average longitudinal stress in the top layer by 51 MPa (0.04σy), (iv) reducing the laser scan speed from 1200 mm/s to 800 mm/s increased the longitudinal stress by 57 MPa (0.05σy) but reduced the transverse stress by 46 MPa (0.04σy).