Selective laser melting of aluminium alloys : numerical modelling and material properties characterisation
Loh, Loong Ee
Date of Issue2014
School of Mechanical and Aerospace Engineering
Selective Laser Melting (SLM) is a particular type of powder-based AM techniques that is capable of fabricating complex functional metallic components with high densities. In general, SLM produced parts have finer microstructure and better mechanical properties than conventionally manufactured components due to the rapid solidification process occurring during SLM fabrication. However, the fabrication process is complex and to have better control over it, it is necessary to understand the physical phenomena occurring during the SLM fabrication process. Important process parameters have to be carefully selected for a successful fabrication. Currently, the optimal set of process parameters is determined experimentally via trial and error and is dependent on the user’s experience. As such, time and material wastage is inevitable. The development of a novel FE model for the SLM process can allow users to identify the optimum set of process parameters through the simulated melt penetration, melt width, mass of material melted and evaporated. In this thesis, a numerical heat transfer model with an effective method to achieve volume shrinkage and vaporisation is proposed. The simulated melt penetration and melt width are validated experimentally. Detailed discussions on the progression of the melt pool and the rate of temperature change have been made. Parametric study is conducted under different process parameters based on the novel FE model and the relationship between the process parameters with the melt dimensions, cooling rate, ratio between material melted and evaporated is established. A single Gaussian laser beam is commonly used in current SLM fabrication processes. However, overheating of the laser beam causes material evaporation and in certain circumstances, this wastage becomes significant. The numerical and novel theoretical studies on the double laser beam system have been conducted and derived in this thesis respectively, aiming to further reduce material wastage in SLM due to material evaporation. Results from the theoretical solution and numerical simulation are compared and are in good agreement. Three different scanning strategies for single as well as double laser beam systems are investigated and their performances compared. The results showed that the double beam scanning strategy can effectively reduce the mass of powder evaporated in the SLM process. The microstructure, mechanical properties and fracture surfaces of SLM fabricated AlSi10Mg parts under different heat treatment conditions, with and without remelting, have been characterized and compared with those of casted parts. This allows a better understanding on the effect of microstructural change on the mechanical properties. Results have shown that the microstructure changes with heat treatment and mechanical properties under different fabrication conditions differ due to the porosity, residual stress, as well as difference in microstructures.