Multi-Scale Multi-Physics Modeling of Laser Powder Bed Fusion Process of Metallic Materials With Experiment Validation

Yi Zhang, Purdue University

Abstract

Laser Powder Bed fusion (L-PBF), also known as Selective Laser Melting (SLM) or Direct Metal Laser Sintering (DMLS), is the primary additive manufacturing (AM) technique for rapid manufacturing metallic parts. It is known as a solution for the fabrication of complex geometry parts and saving the cost of tooling when comparing with traditional manufacturing. Although there are some experimental studies and a few modeling efforts are conducted for a variety of metallic materials, there is still a lack of thorough understanding of the process-property relations in the L-PBF process, which hinders the wide application of the L-PBF technique. Numerical simulation can be helpful for a better understanding of the process-property relationship and have the potentials for design optimization of the L-PBF process. However, due to the complicated physical phenomena at varying length scale during the L-PBF process, development of a comprehensive model for the L-PBF process is one of the major challenges. The goal of this thesis is to develop a physics-based multi-scale modeling framework to simulate the L-PBF process. The modeling framework developed in this thesis can be used as a design and optimization tool for the future L-PBF process. Models at multiple length scales, including molecular dynamics (MD), discrete element model (DEM), finite element model (FEM), and coupled fluid dynamics (CFD) and cellular automata (CA) model are presented, with each model providing unique process-property information for the process. A new MD model is developed for the L-PBF process. The sintering kinetics at the atomistic level is revealed. Atom diffusion mechanism, diffusion activation energy at different regions of the metal particle is studied. Further, the tensile test of the sintered structure is simulated in MD, and the mechanical response of the sintered structure at different heating rates is studied. The results show that the mechanical properties of the laser sintered parts can be improved through increasing laser heating rate. A novel DEM model is formulated which is capable to simulate the particular nature of complete PBF process, including powder deposition, laser heating, and recoating. The role of processing parameters, including laser power, scan speed, hatch spacing, is investigated. The DEM results show that increasing laser power and reducing scan speed and pitch size can increase the temperature of the powder bed. Moreover, a new coupled thermo-mechanical FEM is presented. The material model includes temperature-dependent material properties, liquid-solid phase transition, and laser adsorption properties. The layer-by-layer additive manufacturing mode is implemented using the "birth-death" element. The FEM successfully predicts L-PBF part distortion and cracking. To understand the microstructure of the L-PBF parts, a novel method that couples CFD powder melting and CA solidification is developed. The grain shape, size, orientation of the laser-scanned region is predicted using this method. The results show that with increasing of laser scan speed, the grain size decreases. The misorientation angle of the columnar grains increases with scan speed. To sum up, a multi-scale, multi-physics modeling framework for the L-PBF process is developed. The multi-scale modeling results are validated and compared with experimental measurements. The model can provide a computational tool for metal additive manufacturing process design and optimization.

Degree

Ph.D.

Advisors

Han, Purdue University.

Subject Area

Engineering|Mechanical engineering|Materials science

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