Analysis And Simulation Of Thermophysical Process In Selective Laser Sintering

Rapid prototyping & manufacturing (RP&M) as material additive manufacturing process makes a significant breakthrough in manufacturing philosophy, which is different from traditional material removal manufacturing processes e.g. metal cutting processes. Selective laser sintering (SLS) which is related to many subjects such as mechanical, material, powders sintering, laser processing, heat transfer is an important branch of RP&M. Seveal fundamental problems including interaction between powders and laser beam, 3D finite element analysis of SLS process, adaptive slicing based on material thermophysical performance are deeply investigated in this dissertation.The characteristics of powders packing are analysed. Drawing on the ideas of collective rearrangement model and Monte Carlo stochastic simulation method, the powders packing model in SLS is proposed. The packing of the same size particles and bidisperse particles are simulated by using the model. The packing results can provide for the theoretical study on the interaction of powders and laser beam. Then, the mechanism of absorption of powders for the laser beam is disscused and the laser ray tracing (LRT) model is established. The transmission and penetrating behaviors of the laser beam within the powders are simulated by the LRT model developed. Combining the powders packing model and the LRT model, the influences of laser and powder performance on the absorption are studied. At the same time, the laser penetration depth and sintered zone are simulated and the calculated results match with that from experiments.A 3D finite element model for calculating the evolution of temperature and thermal stresses is proposed. The model allows for the heat loss through convection and radiation, the non-linear behavior of the thermophysical performance of powders, phase change, Gaussian laser distribution, as well as the radical change of thermophysical performance during the powder-to-solid transition. The scanning patterns are optimized by using the FE model. The highest temperature and the smallest temperature gradient can be arrived by using the Hilbert scanning pattern. Compared with the traditional"S"scanning pattern, the Hilbert scanning pattern can make the maximum distortion decrease by 55.7 percent and the compressive strength increase by about 20 percent through checking the SLS parts of resin sand.Direct slicing based on Pro/E software is proposed according to analysis of slicing strategy based on STL file. The strategy avoids the slicing errors caused by conversion from 3D CAD to STL file. The slicing contour of 3D CAD model can be arrived through the safe, efficient, accurate direct slicing method. Adaptive slicing strategy based on thermophysical performance is proposed. The strategy considers a few of aspects including geometric features of 3D CAD model, SLS processing parameters, as well as thermophysical characteristics etc. An effort is made for pushing geometric adaptive slicing to physical adaptive slicing.A SLS virtual experimental system is developed by integration with the research fruits obtained herein. The system can simulate the SLS machining process from geometric and physical level, including 3D dynamic display of the laser scanning system, spreading powder system, temperature evolution. Using FE tools, temperature, stresses and distortion in SLS process can be caculated. The system will be benefit for the optimization of SLS process.